Graphene Composite Materials 9789811276781

Table of contents :
Cover
Half Title
Graphene Composite Materials
Copyright
Preface
About the Authors
Contents
List of Figures
List of Table
1. Introduction
1.1 Structure, Properties, and Preparation Methods of Graphene
1.1.1 Structure and properties of graphene
1.1.2 Preparation method of graphene
1.2 Introduction to Composite Materials
1.2.1 Basic principles of composite materials
1.2.2 Application and development of composite materials
1.3 Overview of Graphene-Reinforced Composites
References
2. Graphene-Reinforced Metal Matrix Composites
2.1 Introduction to Graphene-Reinforced Metal Matrix Composites
2.2 Graphene-Reinforced Powder Superalloy Composites
2.2.1 Introduction to superalloys
2.2.2 History of powder superalloys
2.2.3 Graphene-reinforced powder superalloys
2.2.4 Preparation of graphene-reinforced powder superalloy
2.2.4.1 Research on dispersion of graphene
2.2.4.2 Influence of graphene content
2.2.4.3 Process route for the fabrication of graphene-reinforced powder superalloy
2.2.5 Microstructure and properties of graphene-reinforced powder superalloys
2.2.5.1 Microstructure of powder superalloy
2.2.5.2 Microstructure of graphene-reinforced powder superalloys
2.2.5.3 Physical properties
2.2.5.4 Mechanical properties
2.2.6 Reinforcing mechanism of graphene
2.3 Graphene-Reinforced Aluminum Matrix Composites
2.3.1 Introduction of graphene-reinforced aluminum matrix composites
2.3.2 Preparation of graphene-reinforced aluminum matrix composites
2.3.2.1 Powder mixing
2.3.2.2 Metal-forming process
2.3.3 Microstructure and properties of graphene-reinforced aluminum matrix composites
2.3.4 Reinforcing mechanism of graphene
2.3.4.1 Thermal mismatch strengthening
2.3.4.2 Grain refinement strengthening
2.3.4.3 Dislocation strengthening
2.3.4.4 Stress transfer strengthening
2.4 Graphene-Reinforced Titanium Matrix Composites
2.4.1 Introduction of graphene-reinforced titanium matrix composites
2.4.2 Preparation of graphene-reinforced titanium matrix composites
2.4.2.1 Dispersion method of graphene
2.4.2.2 Densification process of GrTMCs
2.4.3 Interface optimization of GrTMCs
2.4.4 Properties of GrTMCs
2.4.4.1 Mechanical properties
2.4.4.2 Tribological behavior
2.4.4.3 Thermal conductivity
2.4.5 Reinforcing mechanism of graphene
2.4.6 Potential applications and prospects
2.5 Graphene-Reinforced Copper Matrix Nanocomposites
2.5.1 Mechanical mixing
2.5.2 Adsorption mixing
2.5.3 Chemical synthesis
2.5.4 In-situ CVD synthesis
2.6 Graphene-Reinforced Magnesium Matrix Composites
2.7 Applications and Trends in Graphene-Reinforced Metal Matrix Nanocomposites
2.7.1 Applications of graphene-reinforced metal matrix nanocomposites
2.7.1.1 Aviation, aerospace, and weapon systems
2.7.1.2 Automobile industry
2.7.1.3 Electronic and electrical field
2.7.2 Constraints for the applications of graphene-reinforced metal matrix nanocomposites
2.7.2.1 Cost factor
2.7.2.2 Preparation process
2.7.2.3 Disjointed industrial chain
2.7.3 Development trend of graphene-reinforced metal matrix nanocomposites
2.7.3.1 Structural complexification
2.7.3.2 Integration of structure and function
References
3. Graphene-Reinforced Resin Matrix Composites
3.1 Introduction to Graphene-Reinforced Resin Matrix Composites
3.1.1 Overall performance characteristics of resin matrix composites
3.1.2 Molding methods of resin matrix composites
3.1.2.1 Hand lay-up molding
3.1.2.2 Injection molding
3.1.2.3 Resin transfer molding
3.1.2.4 Filament winding
3.1.2.5 Pultrusion molding
3.1.3 Development of graphene-reinforced resin matrix composites
3.2 Graphene-Reinforced Resin
3.2.1 Principle of graphene-reinforced resin
3.2.2 Preparation method of graphene-reinforced resin
3.2.2.1 Solution mixing
3.2.2.2 Melt blending
3.2.2.3 In-situ polymerization
3.2.3 Applications of graphene-reinforced resin matrix composites
3.2.3.1 Preparation of GO-modified epoxy resin
3.2.3.2 Mechanical properties of carbon fiber-reinforced composites
3.3 Toughening of Epoxy Matrix Composites by Graphene
3.3.1 Toughening of matrix resin
3.3.2 Surface modification of carbon fiber
3.3.3 Graphene macrostructure-reinforced resin matrix composites
3.3.4 Applications of graphene-toughened resin matrix composites
3.4 3D Printing of Graphene Resin Matrix Composites
3.4.1 Introduction to 3D printing technology
3.4.2 3D printing process of graphene resin matrix composites
3.4.2.1 Inkjet printing
3.4.2.2 Fused deposition modeling (FDM)
3.4.2.3 Stereolithography (SLA)
3.4.2.4 Selective laser sintering (SLS)
3.4.3 Applications of 3D-printed graphene resin matrix composites
3.4.3.1 Electronics
3.4.3.2 Energy
3.4.3.3 Biomedical
3.4.3.4 Aerospace
3.4.4 Graphene polyetheretherketone matrix composites for 3D printing
3.4.4.1 Introduction
3.4.4.2 Preparation and processing of 3D printed graphene/PEEK composites
3.4.4.3 Preparation and molding of chemically modified graphene/PEEK composites
3.4.4.4 Application prospect
3.5 Graphene Sandwich Composites
3.5.1 Overview of sandwich composites
3.5.2 Graphene sandwich composites and applications
References
4. Graphene Rubber Matrix Composites
4.1 Overview of Graphene Rubber Matrix Composites
4.2 Preparation Method of Graphene Rubber Matrix Composites
4.2.1 Solution mixing method
4.2.2 Latex mixing method
4.2.3 Mechanical mixing
4.2.4 In-situ polymerization
4.2.5 Other methods
4.3 Graphene-Reinforced General-Purpose Rubber Composites
4.3.1 Graphene/natural rubber composites (GNR)
4.3.2 Graphene/styrene-butadiene rubber composite (GSBR)
4.3.3 Graphene/isobutylene-isoprene rubber composites (GIIR)
4.3.4 Graphene/butadiene rubber composite (GBR)
4.3.5 Graphene/ethylene propylene diene monomer rubber composite (GEPDM)
4.3.6 Graphene/nitrile butadiene rubber composite (GNBR)
4.3.7 Graphene/carboxylated nitrile rubber composite (GXNBR)
4.4 Graphene-Reinforced Special Rubber Composites
4.4.1 Graphene/silicone rubber composite (GSR)
4.4.2 Graphene/fluoroelastomer composite (GFKM)
4.4.3 Graphene/acrylic rubber composite (GACM)
4.4.4 Graphene/styrene-butadiene-styrene thermoplastic rubber composite (GSBS)
4.4.5 Graphene/hydrogenated nitrile rubber composite (GHNBR)
4.5 Properties of Graphene-Reinforced Rubber Composites
4.5.1 Mechanical properties
4.5.2 Fatigue endurance
4.5.3 Damping performance
4.5.4 Thermal behavior
4.5.5 Electrical properties
4.5.6 Electromagnetic shielding performance
4.5.7 Media resistance performance
4.5.8 Gas barrier properties
4.5.9 Tribological property
4.5.10 Other properties
4.6 Conclusion
References
5. Graphene Composite Coating
5.1 Introduction to Graphene Composite Coating
5.2 Graphene-Based Polymer Coating
5.2.1 Surface modification of graphene
5.2.2 Anti-corrosive coating
5.2.3 Thermally conductive coating
5.2.4 Electrically conductive coating
5.2.5 Other coatings
5.3 Graphene-Based Inorganic Coating
5.3.1 Metal composite coating
5.3.2 Non-metallic composite coatings
5.4 Composite Coatings of Graphene and Other Nanomaterials
5.4.1 Composite coating of graphene and 0D nanomaterials
5.4.2 Composite coating of graphene and 1D nanomaterials
5.4.3 Composite coating of graphene and 2D nanomaterials
References
Index

Citation preview

Graphene Composite Materials

Graphene Composite Materials

Sijia Hao • Cheng Yang • Yubin Chen Beijing Institute of Aeronautical Materials, China & Beijing Institute of Graphene Technology Co. Ltd., China

World Scientific

Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

Library of Congress Cataloging-in-Publication Data Names: Hao, Sijia, author. Title: Graphene composite materials / Sijia Hao, Cheng Yang, Yubin Chen, Beijing Institute of Aeronautical Materials, China & Beijing Institute of Graphene Technology Co. Ltd., China. Description: Hackensack, NJ : World Scentific, [2023] | Includes bibliographical references and index. Identifiers: LCCN 2023019861 | ISBN 9789811276781 (hardcover) | ISBN 9789811276798 (ebook for institutions) | ISBN 9789811276804 (ebook for individuals) Subjects: LCSH: Graphene. | Graphene--Composition. Classification: LCC TA455.G65 H36 2023 | DDC 620.1/15--dc23/eng/20230516 LC record available at https://lccn.loc.gov/2023019861

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. 〈〈石墨烯复合材料〉〉 Originally published in Chinese by East China University of Science and Technology Press Copyright © East China University of Science and Technology Press. 2020 Copyright © 2023 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher.

For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. For any available supplementary material, please visit https://www.worldscientific.com/worldscibooks/10.1142/13424#t=suppl Desk Editors: Aanand Jayaraman/Steven Patt Typeset by Stallion Press Email: [email protected] Printed in Singapore

Preface

Composite materials are a large category of new materials that have developed rapidly since the middle of the 20th century. Their outstanding properties have been favored and have been applied in many high-end fields such as aviation and aerospace. Graphene was discovered in the early 21st century, and it quickly became a rising star in materials science due to its exceptional physical and chemical properties. Therefore, the fabrication of graphene composite materials, which exhibit much improved performance as compared to the sum of excellent properties originating from each component, has come to be expected by a majority of scientific researchers and is also the inevitable responsibility of scientists and technologists. The scope of this book includes scientific and technological details along with the present-day industrial approach and needs. The book consists of four sections: graphene-reinforced metal matrix composites, graphene-reinforced resin matrix composites, graphene rubber composites, and graphene composite coatings; the research background, research achievements, and possible applications in the corresponding fields of each section are also described. This book is suitable for physicists, chemists, material scientists, and engineers, building a link between their languages, a link that is necessary for the future development of graphene composite materials. This book is our humble effort to present the state of the art of graphene composite materials research intended for various applications. We have tried to place these developments in scientific and technical contexts to assess the likelihood of uptake of these technologies and their relevance v

vi Graphene Composite Materials

in world’s pressing needs for energy, miniaturization, communication, transportation, and health. This book is intended for new entrants and active researchers in the field of graphene science and technology in industry and academia, government officials responsible for research and innovation, entrepreneurs and industrialists venturing into applications of graphene, and students and interested lay persons. We hope that all readers will find great enrichment and understanding as they explore the pages of this book. Suggestions for improvement will be gratefully received. We gratefully acknowledge all collaborators who have contributed to the development of this book: Zhidong Ren, Jing Xu, Yue Xing, Shaojiu Yan, Xudong Wang, Xiaofeng Wang, Guangbao Mi, Zhengtao Su, and Jing Li. In addition, we are very grateful to Fujiao Ma from ECUST for her invaluable help with the preparation of the manuscript. Finally, we would like to thank our families for their full support and understanding as we could not have succeeded in such a long and demanding task without support from them.

About the Authors

Sijia Hao has been a Senior Engineer at the Research Center of Graphene Applications at AECC Beijing Institute of Aeronautical Materials since 2015. He has led a research group at Beijing Institute of Graphene Technology Ltd. since 2020. Prior to joining BIAM, he received his Ph.D. from the Tokyo Institute of Technology in 2012 under the supervision of Toshiaki Enoki and conducted post-doctoral research at Tsinghua University. He has actively worked on the research and development of graphene nanocomposites for a variety of applications. He has coauthored and translated several books on graphene and other carbon nanomaterials. Cheng Yang is a Professor and Doctoral Supervisor at AECC Beijing Institute of Aeronautical Materials. She currently leads a research group on graphene and functional materials. She has actively worked on the preparation of graphene nanosheets, functional graphene, and graphene macrostructures. She had done a lot of work on the function and structure of integrated graphene microwave absorbers, graphene resin matrix composites, and environmental applications of graphene. Yubin Chen is a Senior Engineer at AECC Beijing Institute of Aeronautical Materials. He obtained his Ph.D. in 2015 from Beijing University, College of Chemistry and Molecular Engineering. He has published a number of technical standards, research-based reports, journal papers, and patents on graphene research. His areas of interests include graphene films, foam, and graphene composite materials. vii

Contents

v

Preface About the Authors

vii

List of Figures

xi xxv

List of Tables Chapter 1 Introduction

1

Chapter 2 Graphene-Reinforced Metal Matrix Composites

13

Chapter 3 Graphene-Reinforced Resin Matrix Composites

149

Chapter 4 Graphene Rubber Matrix Composites

227

Chapter 5 Graphene Composite Coating

269

Index

311

ix

List of Figures

Chapter 1 Figure 1.1. Figure 1.2. Figure 1.3. Figure 1.4. Figure 1.5. Figure 1.6. Figure 1.7. Chapter 2 Figure 2.1. Figure 2.2. Figure 2.3. Figure 2.4. Figure 2.5.

Structure and energy band structure of graphene. Scheme of preparation methods of graphene. Schematic diagram of the principle of liquid phase exfoliation method. Schematic diagram of graphite oxide and its reduction to prepare graphene. Schematic diagram of epitaxial graphene on silicon carbide single-crystal substrate. Schematic diagram of chemical vapor deposition method. Large-area graphene grown on copper foil. Schematic diagram of (a) argon atomization and (b) PREP methods for powder making. Process route of powder superalloy parts. Morphology of superalloy powder (a) before and (b) after planetary ball milling. Microstructure of planetary ball-milled superalloys with different graphene loadings. (a) Without graphene; (b) with 0.15 wt% graphene. EPMA analysis of planetary ball-milled superalloy with 0.15 wt% graphene.

xi

2 4 5 6 7 7 8

22 23 26 26 27

xii Graphene Composite Materials

Figure 2.6. Figure 2.7. Figure 2.8. Figure 2.9. Figure 2.10. Figure 2.11. Figure 2.12. Figure 2.13. Figure 2.14. Figure 2.15. Figure 2.16. Figure 2.17. Figure 2.18. Figure 2.19. Figure 2.20. Figure 2.21. Figure 2.22. Figure 2.23. Figure 2.24. Figure 2.25. Figure 2.26.

Morphology and energy spectrum of planetary ball-milled alloy carbide. Morphology of superalloy powder particles after low-temperature ball milling. Morphology of superalloy after low-temperature ball milling. Energy spectrum analysis of long strips in low-temperature ball-milled superalloys. Morphology of superalloy with graphene dispersed by mechanical stirring. Wet mixing method of ultrasonic + mechanical stirring. Graphene/powder superalloy particles after wet mixing method. Final microstructure of the wet mixed superalloy. Tensile properties of superalloys with graphene loadings of 0.005–0.3 wt%. High-temperature creep properties of superalloys with graphene loadings of 0.005–0.3 wt%. Process route for the fabrication of graphenereinforced powder superalloys. Graphene-reinforced powder superalloy capsules and testing specimens. Relationship between grain size and mechanical properties of U720. Grain structure of FGH95 superalloy. Grain structure of FGH96 alloy. Morphology of γ′ phase of FGH95. Morphology of γ′ phase of FGH96. Relationship between the yield strength of René88DT and the size and volume fraction of γ′ phase. Microstructure of the as-extruded alloy. (a) Parallel to the extrusion direction. (b) Vertical to the extrusion direction. Graphene in as-extruded alloy. Graphene in the microstructure of heat-treated alloy.

27 28 29 29 30 30 31 31 34 34 35 36 37 37 38 38 39 39 40 41 42

List of Figures xiii

Figure 2.27. Figure 2.28.

Figure 2.29.

Figure 2.30. Figure 2.31. Figure 2.32. Figure 2.33. Figure 2.34. Figure 2.35. Figure 2.36. Figure 2.37. Figure 2.38. Figure 2.39. Figure 2.40. Figure 2.41. Figure 2.42. Figure 2.43.

Graphene nanosheets and curved zigzag grain boundaries in grains. Morphology of graphene nanosheets with different sizes. (a) Small-sized (1 μm) graphene nanosheets; (b) large-sized (25 μm) graphene nanosheets. Some physical properties of superalloys with and without reinforcement of graphene. (a) Thermal diffusivity; (b) enthalpy; (c) thermal conductivity; (d) average specific heat at constant pressure. Comparison of high-temperature creep properties of superalloy with and without reinforcement of graphene. Comparison of thermal compression deformation resistance of superalloys with and without reinforcement of graphene. Morphology and TEM diffraction pattern of graphene before incorporation. Graphene morphology after incorporation. 3D morphology of graphene in graphenereinforced superalloy. FIB-SEM images of graphene-reinforced superalloy. Energy spectrum scan of the interface between graphene and matrix. Filtering image of interface between matrix and graphene. Graphene nanosheets at the tear ridge of tensile fracture. Graphene nanosheets at the dimples of tensile fracture. Differential scanning calorimetry (DSC) curve of graphene/pure aluminum composite powder. SEM morphology of graphene/pure aluminum composite powder ball milled for 20 h. Raman spectra of graphene/aluminum mixed powder with 2.0 wt% graphene loading under various milling durations and pristine graphene. Schematic diagram of spark plasma sintering (SPS).

42

42

43 45 46 47 47 48 48 49 50 50 51 56 59 60 62

xiv Graphene Composite Materials

Figure 2.44. Figure 2.45. Figure 2.46. Figure 2.47. Figure 2.48. Figure 2.49. Figure 2.50.

Figure 2.51.

Figure 2.52. Figure 2.53. Figure 2.54. Figure 2.55. Figure 2.56.

Schematic of friction stir processing (FSP). Optical micrographs of (a) aluminum matrix nanocomposites with 0.1 wt% graphene loading and (b) pure aluminum. XRD patterns of pure aluminum, 1.0 wt% MWNT/Al composite, and 0.1 wt% graphene/Al composite. XRD patterns of GR/2024 nanocomposites with different graphene contents. TEM images of GR/2024 nanocomposite with 0.15 wt% graphene. Fracture morphology of GR/2024 nanocomposites with 0.5 wt% graphene. SEM images of (a) graphene attached to Al powder using a planetary mill at 100 rpm (graphene is marked by an arrow), (c) graphene embedded and dispersed in Al powder using an attrition mill at 500 rpm, (b) and (d) display the magnified images of (a) and (c), respectively. TEM images of the graphene in the hot-rolled GR/Al nanocomposites with 0.3 vol% graphene (graphene is marked by white arrow) observed on the (a) RD (rolling direction)–TD (transverse direction) plane, (b) ND (normal direction)–RD plane. In (c) and (d), graphenes (marked by red lines) are given. (e) Between the graphenes (marked by white arrow), highly deformed regions (marked by circles) are observed at 6% deformation. Comparison of SEM images of Al flake surface with and without adsorbed GO nanosheets. (a) Structure of the graphene and GO, (b) FTIR of the GO and reduced GO by rapid heating to 550°C in a flowing Ar atmosphere. Basic process route for preparing GrTMCs. SEM morphology of graphene in titanium alloy powder. SEM morphology of graphene nanosheets/titanium alloy powder after wet stirring and mixing.

63 66 67 67 68 69

70

71 72 73 83 84 85

List of Figures

Figure 2.57. Figure 2.58. Figure 2.59.

Figure 2.60. Figure 2.61. Figure 2.62. Figure 2.63. Figure 2.64. Figure 2.65. Figure 2.66. Figure 2.67. Figure 2.68. Figure 2.69. Figure 2.70.

Figure 2.71. Figure 2.72.

xv

SEM morphology of GO/titanium alloy powder mixed by wet ball milling. 86 SEM morphology of GO loaded with metal particles. 88 The digital images and low-magnification microstructure of the GrTMCs billet prepared by HIP. TN2 has no addition of GO; TO2 is GrTMCs with 0.15 wt% GO. 89 Microstructure and morphology of GrTMCs billet prepared by HIP. (a) Without graphene; (b) with 0.15 wt% GO. 90 Digital images of GrTMCs slab prepared by HIP and rolling deformation process. 90 The experimental process of GrTMCs by SPS and some fabricated samples. 91 Microstructure of Ti60 titanium alloy sintered at (a) 1000°C and (b) 1150°C. 92 SEM image of Ti60 titanium alloy incorporated with 0.3% GO. (a) Matrix; (b) lamellar structure. 93 Raman spectra of GO in initial powder and sintered composite samples. 94 XRD results of GrTMCs sample after SPS process. 95 Schematic diagram of laser sintering for preparing GrTMCs. 96 Relationship between nanoparticle size and mechanical properties of titanium matrix nanocomposites. 98 DSC results of the interface reaction between titanium alloy powder and carbon. 99 The interface morphology of graphene and titanium alloy matrix in GrTMCs after isothermal forging. (a) TEM image; (b) HRTEM image of area A in (a); (c) EDS result of graphene in (a). 100 Comparison of room-temperature tensile properties of GrTMCs prepared by HIP. 102 Comparison of tensile strength of GrTMCs fabricated by SPS under different process conditions. 102

xvi Graphene Composite Materials

Figure 2.73. Figure 2.74. Figure 2.75. Figure 2.76. Figure 2.77. Figure 2.78. Figure 2.79. Figure 2.80. Figure 2.81. Figure 2.82. Figure 2.83. Figure 2.84.

Figure 2.85. Figure 2.86.

Comparison of high-temperature tensile properties of GrTMCs prepared by HIP. Room-temperature friction and wear properties of GrTMCs prepared at different sintering temperatures. High-temperature tribological properties of GrTMCs sintered at 1150°C. Influence of graphene addition on the tribological properties of GrTMCs. (a) Friction coefficient; (b) wear rate. SEM morphology of GrTMC tensile fracture. The reinforcing mechanism of graphene during the tribological test of GrTMCs. (a) Before test; (b) after test. Development history of high-temperature titanium alloys in China. Schematic representation of fabricating graphene-copper artificial nacre. SEM images of graphene/copper composite powder prepared by mechanical ball milling. (a) Low magnification; (b) high magnification. Graphene coated by silver by electroless plating. (a) XRD pattern; (b) SEM image; (c) TEM image. Schematic illustration for the production of graphene/copper matrix composites by electrostatic adsorption process. SEM images of composite powder. (a), (b) Graphene/copper composite powder; (c), (d) GO/copper composite powder; (e), (f) graphene agglomeration in graphene/copper composite powder. Mechanical properties of graphene-reinforced copper matrix composites. (a) Compression curves; (b) tensile curves. Tensile fractures of pure copper and graphene/ copper composites. (a) Pure copper; (b), (c) GO/copper composites; (d), (e) graphene/ copper composites.

103 104 105 106 107 108 110 113 115 116 117

118 119

119

List of Figures

Figure 2.87.

Figure 2.88.

Figure 2.89.

Figure 2.90.

Figure 2.91.

Figure 2.92. Figure 2.93. Figure 2.94. Figure 2.95.

The preparation and characteristics of graphene/ copper composites. (a) SEM image of graphene; (b) TEM image of graphene; (c) AFM image of graphene; (d) SEM image of graphene/copper composite powder; (e) TEM image of graphene/ copper composite powder; (f) particle size distribution of copper powder. Preparation and characteristics of graphene/ copper oxide composite. (a) SEM image of graphene/copper oxide composite powder; (b) SEM image of graphene/copper composite powder; (c) digital image of composite powder and bulk; (d) XRD results of graphene/copper and graphene/copper oxide composite powders; (e) Raman spectroscopy of composite powder and bulk composites. Mechanical properties tests of composites. (a) Tensile stress–strain curve; (b) yield strength and fracture elongation; (c) elastic modulus and hardness of the composite. Relevant characteristics of graphene/copper composite powders. (a), (b) Raman spectra of the composite powder; (c), (d) SEM images of the composite powder in low and high magnification. Performance test of pure copper and composite materials (C0 is pure copper, C1 is 0.07 vol% Gr/Cu composite, and C2 is 0.115 vol% Gr/Cu composite). Schematic diagram of fabricating process to prepare graphene/copper composites with PMMA as solid carbon source. Characteristics of graphene/copper composite powder prepared with solid carbon source PMMA. Mechanical properties of graphene/copper matrix composites prepared by in-situ synthesis. Stress–strain curves for graphene/copper matrix composite with nano-laminated structure.

xvii

121

122

122

124

125 126 126 127 128

xviii Graphene Composite Materials

Figure 2.96.

Schematic diagram of the preparation of multi-layer graphene/magnesium matrix composites by liquid dispersion method. Figure 2.97. SEM fractured images of (a) Mg-1Al-1Sn alloy and (b) graphene/Mg-1Al-1Sn alloy. Figure 2.98. SEM morphology of AZ91 alloy powder mixed with different contents of GO. Figure 2.99. Stress–strain curves of AZ91 and rGO/AZ91 composites. Figure 2.100. TEM micrographs of 0.3 wt% graphene/ magnesium matrix composite. Figure 2.101. Room-temperature mechanical properties of pure magnesium and graphene/magnesium matrix composites. (a) Microhardness; (b) stress–strain curve. Figure 2.102. (a) Uniform graphene-copper composite powder; (b) SEM image of graphene evenly coated by copper particles; (c) digital image of bulk alloy. Chapter 3 Figure 3.1. Figure 3.2. Figure 3.3. Figure 3.4. Figure 3.5. Figure 3.6.

Figure 3.7.

Figure 3.8.

Composite structure of A380. Hand lay-up process. Schematic of resin transfer molding process. Large autoclave equipment used for composite curing and molding. (a) H81 resin film; (b) GH81 resin film; optical microscopic images of GH81 (c) before and (d) after mechanical grinding. (a) DSC curves of H81 and GH81, the heating rate is 5°C/min; (b) DSC curves of GH81 under different heating rates; (c) extrapolation of curing temperature of GH81. (a) Strength, (b) modulus, and (c) tensile fracture morphology of H81-300; (d) thermogravimetric curve of GO; and (e) tensile fracture morphology of GH81-300. The impact on epoxy toughness with different dispersion degrees of graphene.

130 131 131 132 133

133 138 152 153 155 158 164

165

168 171

List of Figures

Figure 3.9. Figure 3.10. Figure 3.11. Figure 3.12. Figure 3.13. Figure 3.14. Figure 3.15. Figure 3.16. Figure 3.17. Figure 3.18. Figure 3.19.

Figure 3.20. Figure 3.21. Figure 3.22. Figure 3.23.

Surface carboxylation of graphene and the flexural performance of reinforced epoxy resin composites. Amino functionalization of carboxyl functional groups on the peripheries of GO. Mechanical properties of polybenzimidazolegrafted graphene epoxy resin composites. Dispersion, interphase, and mechanical properties of GO-reinforced epoxy resin composites treated with different silane coupling agents. Schematic illustration of the interphase and deformation under load for PEA-grafted GO-reinforced epoxy resin composites. Properties and microstructure of 3D layered graphene aerogel-reinforced epoxy resin composites. Flowchart for the preparation of 3D graphene/epoxy composites prepared by template-directed CVD process and their performance. 3D graphene structures chemically cross-linked by polyamine and their epoxy resin composites. Compressive performance and microstructure of carbon nanofiber/rGO aerogel. Micromorphology of MWCNT/GO conductive aerogel and the electrical conductivity of its composites. Schematic diagram of typical 3D printing process for graphene/polymer matrix composites: (a) inkjet printing; (b) fused deposition modeling; (c) stereolithography; (d) selective laser sintering. Typical FDM 3D printer. Graphene 3D printing filaments and products fabricated by AECC Beijing Institute of Aeronautical Materials. Chemical structure of polyetheretherketone. GO/PEEK composite pellets via melt blending, the GO loading from left to right are 0, 0.1 wt%, 0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt%.

xix

172 173 175 176 177 182 182 183 184 185

189 191 192 199 200

xx

Graphene Composite Materials

Figure 3.24.

Figure 3.25. Figure 3.26.

Figure 3.27. Figure 3.28. Figure 3.29. Figure 3.30. Figure 3.31.

Figure 3.32. Figure 3.33. Figure 3.34.

Chapter 4 Figure 4.1. Figure 4.2. Figure 4.3.

Figure 4.4.

TEM images of GO in the ultra-thin section of melt-blended GO/PEEK composites; the GO loadings are (a) 0.5 wt%, (b) 1.0 wt%, (c) 1.5 wt%, and (d) 2.0 wt%. TGA curves of melt-blended GO/PEEK composites with different GO loadings. The digital images of 3D printing consumables prepared from neat PEEK and GO/PEEK with different GO loadings. From left to right, the loading of GO is 0, 0.1 wt%, 0.3 wt%, 0.5 wt%, 0.7 wt%, 1.0 wt%, and 1.5 wt%. Diameter distribution of 3D printing GO/PEEK filament with 1.5 wt% GO. Digital images of 3D printed parts by PEEK and GO/PEEK composite. Tensile strength of GO/PEEK composites with different GO loadings fabricated by injection molding and 3D printing. Flexural strength of GO/PEEK composites with different GO loadings fabricated by injection molding and 3D printing. SEM images of the cross-section of the impact test specimens fabricated by 3D printing: (a) neat PEEK; (b) GO/PEEK composite; (c) enlarged view of the white box in (b). 3D-printed parts fabricated by GTPEKL/PEEK composite filaments. Configuration of a sandwich composite. Electromagnetic parameters of graphene-modified honeycomb core. SEM image of rGO/NBR composite. Schematic illustration of the preparation of NR/GO and NR/GR composites. TEM image (a) GR; (b) the NR latex particles containing GR shell after 15 times dilution; (c) NRLGRS; (d) NRLGRS-TR; (c′) and (d′) are the magnified images of (c) and (d), respectively. Volume resistivity of composites as a function of GNPs and graphite content.

200 201

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207 208 210 211 230 232

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List of Figures

Figure 4.5. Figure 4.6. Figure 4.7. Figure 4.8. Figure 4.9. Figure 4.10. Figure 4.11.

Chapter 5 Figure 5.1.

Figure 5.2. Figure 5.3. Figure 5.4. Figure 5.5.

Figure 5.6.

Dielectric constant of GHNBR composites at different rGO contents. The damage accumulation (LDE) versus the number of cyclic stress–strain. SEM images of the cross-section of cryo-fractured nanocomposite (a) PANI@rGO/NR and (b) rGO/PANI/NR. A possible microwave absorption mechanism in rGO/NBR composite. Schematic diagram of the interaction between AMICl and GO. SEM images of worn surface of GO/NBR nanocomposites under dry sliding condition: (a) NBR; (b) 0.5 wt%; (c) 1.5 wt%; (d) 3 wt%. SEM of the worn surface of GO/NBR nanocomposites under water-lubricating conditions: (a) NBR; (b) 0.5 wt%; (c) 1.5 wt%; (d) 3 wt%. Digital pictures of as-prepared GO dispersed in water and 13 organic solvents through bath ultrasonication (1 h). Top: Dispersions immediately after sonication. Bottom: Dispersions three weeks after sonication. Tafel plots of bare copper and GO-coated copper substrates. A schematic of the electrophoretic deposition setup for fabricating PIHA/GO coatings. EIS spectra of (a) bare copper, (b) copper coated with only KF99, and (c) PIHA/GO-coated copper after KF-99 treatment. Dimensional characteristics of three coatings on Nickel foam surfaces: (a) PA-coated nickel, (b) PU-coated nickel, and (c) conformal coating of graphene film on an Ni foam. Microbial corrosion of three coating surfaces. (a) SEM image of biofilm on Gr/Ni, (b) MICresistant Gr/Ni anode after 30 days of MIC testing, (c) SEM image of biofilm on PA/Ni,

xxi

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xxii Graphene Composite Materials

Figure 5.7. Figure 5.8. Figure 5.9.

Figure 5.10.

Figure 5.11. Figure 5.12. Figure 5.13. Figure 5.14. Figure 5.15. Figure 5.16.

(d) corroded Ni/PA anode after 30 days of MIC experiment, (e) SEM image of biofilm on PU/Ni, and (f) corroded Ni/PU anode after 30 days of MIC experiment. Schematic of a 3D nanoarchitecture that combines CNT pillars and graphene sheets. Graphene-based film on FGO as heat spreader for hotspot. Heat-spreading performance of coating comprising graphene and FGO with different functional agents (Eph: phonon transmission, Gth: phonon thermal conductance). Anti-icing effect of FDO-GNR coating. (a) Contact angle (161°) image of a water drop on a coating surface. Photographs of the coating maintained in a −14°C environment (b) before and (c) after ice-cold water is dropped onto the coating surface. (d) Contact angle (131°) image of a water drop on a coating surface. Photographs of the coating maintained at (e) −14°C and (f) 4°C environment after ice-cold water is applied onto the coating surface. De-icing effect of FDO-GNR coating. De-icing effect of FDO-GNR coating after adding lubricant. The SEY data for stainless steel coated with graphene using various EPD biases and deposition times. Scheme and mechanism of the antimicrobial activity of graphene/AgNW hybrid coating. Images of graphene/AgNW/EVA/PET hybrid coating. Enhanced antimicrobial activity of coating by water electrolysis using a graphene/AgNW electrode. (a) Scheme diagram of the electrolysis cell. (b) Death rate versus electrolysis time; (c)–(e) Representative photograph of colony-forming units (CFU) showing various electrolysis time points, i.e., (c) 0 s, (d) 30 s,

280 281 282

283

284 285 285 286 287 288

List of Figures xxiii

Figure 5.17. Figure 5.18.

Figure 5.19.

Figure 5.20. Figure 5.21.

Figure 5.22.

and (e) 4 min. (f) Image of hyphae on the surface of a denture plate without graphene/AgNW coating. (g) Image of the surface of a denture plate with graphene/AgNW coating with 4-min electrolysis time. (h) The photograph of a usable denture coated with graphene/AgNW. The dashed boxes show the graphene/AgNW-coated areas. Mechanism of Ag/rGO coating on PET fabric modified with dopamine. Morphology of hybrid coating on PET fabric. (a), (b) Ag/rGO-coated PET fabric without pretreatment with dopamine. (c), (d) Ag/rGOcoated PET fabric with pretreatment with dopamine. Synthesis of graphene/TiO2 composite coatings through sol–gel method. (a) AFM image of a starting graphene sheet; (b) SEM image of TiO2 particles grown on a GO sheet after the first hydrolysis reaction step; (c) SEM image of TiO2 nanocrystals on GO after hydrothermal treatment in the second step. The scale bars are 100 nm. Schematic illustration for LbL self-assembly of CdS QDs/GNs, pure GNs, and pure CdS QDs. UV-vis absorption spectra of (a) pure CdS QDs and GNs assembled after five deposition cycles, and (b) (CdS QDs/GNs)n (n = 1, 5, 10, 15, 20) multilayered films with varied deposition cycles. (c) and (d) Plot of transformed Kubelka−Munk function versus energy of light for (a) and (b), respectively. (e) Stacking model between CdS QDs and GNs-PAH. (f) Absorption of (CdS QDs/GNs)n multilayered films at wavelength of 475 nm versus the number of deposition cycles. TEM images of (a) GO, (b) GNs-PAH, and CdS QD/GN composite film with (c) one and (d), (e) five deposition cycles, and (f) high-resolution TEM image of CdS QDs on the GNs-PAH with selected area electron diffraction (SAED) pattern in the inset.

289 290

291

292 293

294

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xxiv Graphene Composite Materials

Figure 5.23.

Figure 5.24. Figure 5.25. Figure 5.26. Figure 5.27.

Figure 5.28.

Figure 5.29. Figure 5.30. Figure 5.31. Figure 5.32.

(a) and (d) Transient photocurrent responses of CdS QD and GN films with the same number of deposition cycles, and (CdS QDs/GNs)n multilayered films with different number of deposition cycles. (b) and (e) Photocurrent–voltage curves and (c) and (f) electrochemical impedance spectroscopy (EIS) Nyquist plots of CdS QD film and CdS QD/GN multilayered film with the same number of deposition cycles in 0.1 M Na2S aqueous solution under visible light irradiation (λ > 420 nm). Scheme for the synthesis of CNT carpets directly from graphene. Characterization of CNT coating grown from graphene. Electrical properties of graphene and CNT carpet hybrid materials and the supercapacitor device’s characterization. Sequence of sensor fabrication process of the humidity sensor using BP/graphene heterojunction. (a) Schematic diagram of the sensor, (b) image of the full wafer of fabricated sensor, (c) optical image of single graphene chip, (d) electrospray system to deposit BP on graphene, and (e) schematic diagram of the sensor based on the BP/graphene heterojunction. Morphology and structure of graphene/BP heterohybrid coating. (a), (b) SEM images of humidity sensor based on graphene/BP hybrid, (c) TEM images, and (d) SAED of BP flakes. Transient response and estimated stability of the humidity sensor after 1 h based on (a) BP only and (b) BP/graphene heterojunction. Typical Raman spectrum of monolayer graphene, monolayer MoS2, and graphene/MoS2 heterostructure. Schematic of the wet transfer process to fabricate 2D graphene/h-BN heterostructure. Schematic of the liquid phase exfoliating method to fabricate 2D graphene/h-BN heterostructure.

295 295 296 297

299

299 300 300 301 302

List of Tables

Chapter 2 Table 2.1. Characteristics of the typical first-generation powder superalloys. Table 2.2. Characteristics of the typical second-generation powder superalloys. Table 2.3. Characteristics of the typical third-generation powder superalloys. Table 2.4. Average tensile strength of superalloys with small graphene loadings. Table 2.5. Creep properties of alloys with small graphene loadings. Table 2.6. Electrical resistivity and conductivity of superalloys with and without reinforcement of graphene. Table 2.7. Room-temperature tensile properties of graphenereinforced superalloys. Table 2.8. High-temperature tensile properties of graphenereinforced superalloys. Table 2.9. Low-cycle fatigue properties of graphenereinforced superalloys at 650°C. Table 2.10. High-temperature creep properties of graphenereinforced superalloy. Table 2.11. Parameters for thermal compression test. Table 2.12. Analysis results of long-term aging phase.

xxv

20 20 21 33 33 43 44 44 45 45 46 52

xxvi Graphene Composite Materials

Table 2.13. Production technique and corresponding mechanical properties of some recently reported graphene-reinforced aluminum matrix nanocomposites. Table 2.14. Elemental composition of Ti60 matrix and lamellar structures. Table 2.15. Room-temperature tensile properties of GrTMCs prepared by HIP. Table 2.16. High-temperature tensile properties of GrTMCs prepared by HIP. Chapter 3 Table 3.1. Characteristics of resin matrix composite molding process. Table 3.2. Mechanical properties of carbon fiber-reinforced composites. Table 3.3. Features of 3D printing processes for graphene/polymer matrix composites. Chapter 4 Table 4.1. Properties of graphene, carbon nanotubes, nanostructured steel, and polymers. Chapter 5 Table 5.1. Different kinds of covalent modifications of GO using different modifying agents, their dispersion stability in various solvents, dispersibility, and electrical conductivity. Table 5.2. Non-covalent modification of GO using different modifying agents, their dispersion stability in various solvents, dispersibility, and electrical conductivity. Table 5.3. Functionalized graphene directly from graphite using different modifying agents, their dispersion stability in various solvents, dispersibility, and electrical conductivity. Table 5.4. Comparison of the interlayer force constant per unit area in graphene/MoS2 heterostructure, graphene, and MoS2.

65 93 101 103

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229

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Chapter 1

Introduction

A composite material is a high-performance material that combines a minimum of two or more materials with different essential characteristics through specialized molding processes and manufacturing methods, in which the continuous phase is known as the matrix, and the other phases are the reinforcements. Given the different combinations of metal materials, inorganic non-metallic materials, and organic polymer materials, a variety of composite material systems can be formed. The interactions of various constituent materials produce a synergistic effect in performance, so that the comprehensive performance or certain characteristics of the resulting composite material are better than those of the original constituent materials, thereby satisfying a variety of requirements. The applications of composite materials develop quickly, and according to the ten major fields proposed by “Made in China 2025”, composite materials can play an important role in eight different fields. New reinforcements and matrices for composite materials are constantly emerging; meanwhile, the nanocomposites, smart composites, and structuralfunction-integrated composites have provided new directions for the development of composite materials. Graphene is the new wonder material, first isolated successfully in 2004, in which carbon atoms covalently bond to each other in the form of a planar structure. Graphene has many extraordinary physical and chemical properties, and it has already attracted much attention from academia and industry in recent years and become a rising star on the horizon of materials science. Taking graphene with high-performance characteristics as one extra component to improve the performance of existing composite 1

2

Graphene Composite Materials

materials, or to design various new composite materials, has become a hot issue in the field of science and engineering.

1.1 Structure, Properties, and Preparation Methods of Graphene 1.1.1 Structure and properties of graphene Graphene is a brand-new two-dimensional (2D) material prepared by two scientists, Geim and Novoselov, in 2004.1 It is a one-atom-thick layer of sp2-hybridized carbon atoms arranged in a honeycomb lattice, with a thickness of only 0.34 nm, and the adjacent atomic layers are bonded to each other by van der Waals interactions. Inside its atomic plane, individual carbon atoms form delocalized π bonds with pz orbitals, giving rise to its unique electronic properties. In contrast to the strong covalent bonds within the plane, the van der Waals interactions between individual atomic planes are weak, which makes graphene easy to exfoliate. Based on this principle, single-layer graphene or multi-layer graphene can be prepared from graphite raw materials by mechanical exfoliation. As a 2D material, there is a clear distinction between graphene and bulk graphite.2 When the number of layers is varied from multiple to few, the lattice potential field where the carbon atoms are located would change, thus giving a special electronic band structure. Its valence band (π electron) and conduction band (π* electron) meet at six vertices (i.e., K and K′ points) of the first Brillouin zone. Near these points (i.e., near the Fermi level), the electron exhibits linear energy–momentum relation, giving rise to the unique Dirac cones; therefore, the K and K′ points are also called Dirac points, as shown in Figure 1.1. Since the electron-filled

Figure 1.1.

Structure and energy band structure of graphene.4

Introduction 3

valence band and the unoccupied conduction band exhibit complete symmetry near the Dirac point, graphene has many novel physical properties, such as the quantum anomalous Hall effect. Near the Dirac point, in particular, the effective mass of electron in graphene is zero, giving massless Dirac fermion behavior.3 Due to the novel zero-bandgap semi-metallic electronic structure, graphene has extremely high carrier mobility. At present, the mobility for suspended graphene can reach 200,000 cm2/(V·s), far beyond other existing materials, and it is likely to become the next-generation material for replacing single-crystal silicon in the electronic technology.1,5 At ambient conditions, novel physical phenomena such as the quantum Hall effect and bipolar electric field effect could be observed on graphene,6 which demonstrates broad application prospects in electronics and fundamental physics. In addition, graphene also exhibits other extraordinary physical properties. Graphene shows outstanding mechanical properties with Young’s modulus ~1100 GPa and breaking strength ~130 GPa.7 It is an excellent electrical and thermal conductor,3,8 and the thermal conductivity at room temperature is up to 5000 W/(m·K). 9 It has extremely high transparency, and the visible light absorption of single-layer graphene is only 2.3%,10 and it has a huge specific surface area of ~2600 m2/g.11 These novel properties of graphene have attracted much interest in the field of scientific research and engineering. Graphene has broad application potential in transparent conductive films, flexible electronic devices, transparent electrodes, optoelectronic devices, chemical sensors, structural materials, and other fields, which has covered almost all research areas of material science.8,12,13

1.1.2 Preparation method of graphene Since the excellent properties of graphene were discovered, the preparation technology of graphene has been developed rapidly, accompanied by the needs of a large number of research studies and applications, as shown in Figure 1.2. The first isolation of graphene is realized by mechanical exfoliation, then high-temperature epitaxial growth on silicon carbide and reduction of graphite oxide are adopted to prepare graphene, and later chemical vapor deposition is utilized. After ten years of development, the preparation of graphene has made significant progress in both yield scale and crystalline quality. The industrial-scale production of

4

Graphene Composite Materials

Quality

Micromechanical exfolia on

Epitaxial on SiC

Chemical vapor deposi on Liquid phase exfolia on Reduced graphene oxide

Yield Figure 1.2.

Scheme of preparation methods of graphene.8,14

high-quality graphene is the key to the real entry of graphene into the practical field. The exfoliation method is the first method developed for isolating graphene. Graphene can be regarded as a single-layer or multi-layer graphite material, so graphene can be obtained from graphite by exfoliation.1 Geim et al. obtained graphene for the first time in 2004 by means of a manual mechanical peeling method; that is, the highly oriented pyrolytic graphite (HOPG) was repeatedly peeled off with tape, and the graphene was finally pasted on a 300-nm SiO2/Si substrate, which was then observed by optical means.1 Graphene sheets with excellent quality could be isolated by using such facial method, but the problem is that the number of layers, size, and shape of obtained graphene cannot be controlled, and the efficiency is extremely low. At present, such a method for preparing graphene is only adopted in some fundamental research studies. With the assistance of certain physical and chemical means, especially through the formation of graphite intercalation compounds to weaken the interlayer force of graphite and the following ultrasonic method, graphite can be dispersed into solution and graphene can be

Introduction 5

Figure 1.3.

Schematic diagram of the principle of liquid phase exfoliation method.15

prepared; such a method is known as liquid phase exfoliation method and is shown in Figure 1.3. Mass production of graphene can be achieved by using such method; however, substances such as surfactants must be added in the obtained graphene solution due to the need for stable and superhydrophobic graphene. In addition, a small number of residual reagents used in the intercalation process would result in certain contamination of obtained graphene; thus, the quality is lower than that obtained by mechanical exfoliation. Graphite is converted into graphite oxide by means of the oxidationreduction method, which is also a way of reducing the interlayer interaction of graphite and making it easy to disperse in the solution phase, known as the redox method. After oxidation by strong inorganic acids, huge amounts of oxygen-containing functional groups are formed on the surface of graphite, such as hydroxyl (─OH) and carboxyl (─COOH) groups; thus, its nature is switched from hydrophobic to hydrophilic, and then the resultant material can be dissolved and dispersed into aqueous solution.16,17 Such a category of graphite oxide does not have same electrical conductivity or other properties of graphene, but it also features a layered structure, and treatment with a reducing agent (such as hydrazine) is able to remove the functional groups introduced by oxidation, giving rise to restored conductivity and layered structure similar to that of graphene, as shown in Figure 1.4.18 The graphene obtained by using such a method is of very bad quality due to the damage caused by peroxidation, and it relies heavily on the reduction process. Nevertheless, it is the first time to achieve such a great yield of graphene, and conductive graphene membrane can be easily prepared by the coating method; thus, it is still of great interest to the industry.19,20

6

Graphene Composite Materials

Figure 1.4.

Schematic diagram of graphite oxide and its reduction to prepare graphene.18

Besides such top-down exfoliation processes using graphite as a raw material, bottom-up synthesis methods for chemically forming or assembling graphene have also attracted much attention. In 2006, Walt A. de Heer et al. discovered that graphene can be epitaxially grown on silicon carbide substrates by annealing, which paves a new way for the preparation of graphene.21 The principle of this method is that when silicon carbide is heated, silicon atoms on the surface escape from the lattice, leaving behind enriched carbon atoms to reconstitute the graphene. This process generally occurs at 1200–1300°C, and undergoes complex rearrangement and other processes. Between the finally formed graphene and the silicon carbide substrate, there is a carbon-rich interface layer, as shown in Figure 1.5.22 Nevertheless, since the thickness of graphene can be controlled by the heating time and temperature, the silicon carbide single-crystal substrate is generally employed; this method is still of great interest for enabling the controlled preparation of largearea high-quality graphene. However, high cost and difficulty of transferring graphene onto desired substrates limit the application of this method. Subsequently, in 2009, Ruoff and coworkers first reported that singlelayer graphene could be deposited on the surface of copper foil during low-pressure chemical vapor deposition (CVD).23 Due to the use of lowcost metal copper foil and the high quality of the obtained graphene, as well as large-area controllable preparation that can be achieved, the CVD method has quickly developed into one of the major preparation methods for graphene films, as shown in Figure 1.6.24

Introduction 7

Figure 1.5. substrate.22

Schematic diagram of epitaxial graphene on silicon carbide single-crystal

Figure 1.6.

Schematic diagram of chemical vapor deposition method.24

In 2010, Byung Hee Hong and coworkers25 succeeded in the preparation and transfer of a 30-inch (1 inch = 2.54 cm)-scale graphene film, and demonstrated its practicality in touch screens and by other means, as shown in Figure 1.7. In 2013, Ruoff et al. achieved the growth of centimeter-scale single-crystal graphene domains.26

8

Graphene Composite Materials

Figure 1.7.

Large-area graphene grown on copper foil.25

1.2 Introduction to Composite Materials 1.2.1 Basic principles of composite materials A composite material refers to the combination of two or more component materials with significantly different physical and chemical properties through specific molding processes and manufacturing methods, giving rise to new materials with diverse properties from the component materials. An important feature for composite materials is that their components maintain their original microscopic morphology and they present a state of multi-phase composition, which is distinguished from alloys and other multi-component materials. Usually, we refer to the continuous phase as the matrix, and the components of the other dispersed phases as the reinforcements. A variety of composite material systems can be tuned by differentiating the components in the composite materials that exhibit a variety of properties. In the composite materials, the interaction of various constituent materials produces a synergistic effect in performance, so that the comprehensive performance or certain characteristics of the material are improved as compared to the original constituent materials; thus, it has gained more and more popularity among practice. Such a compositing effect enables high performance and functionality of materials, and it is an important feature of composite materials that is distinguished from simple mixing of materials. As a discipline, composite materials have emerged and developed over several decades; however, the use of composite materials can be traced back to an earlier period. The symbol of the development of modern composite materials is the glass fiber-reinforced resin composite

Introduction 9

material, namely, FRP, which appeared in the 1940s. It is a category of composite materials with significantly better performance than the matrix material and has been widely used in many fields; on the other hand, it is also considered a stepping stone for mankind to systematically study composite materials and compositing effects.

1.2.2 Application and development of composite materials Composite materials have a wide scope of use and many products can be made from them, which are intensively applied in the defense industry and various fields of national economy. So far, the composite materials are primarily classified by metal matrix composites, resin matrix composites, ceramic matrix composites, rubber composites, and composite coatings. Metal matrix composites have shown exceptional comprehensive properties, but studies on them started relatively late and the cost is high. Currently, they are mainly applied in aerospace, defense industries, and other fields. Resin matrix composites have excellent performance and low cost, so they have been widely used in aerospace, automobile, ship, electronics, construction, mechanical equipment, sporting goods, and many other fields. Ceramic matrix composites exhibit excellent high-temperature performance and relatively high cost, and are generally suitable for extreme conditions. Rubber is an important elastic material, and reinforced rubber composite is another type of composite material that has been widely used. Coatings have a long history, and multi-component composite coatings have been developed rapidly and have gradually emerged in many fields.

1.3 Overview of Graphene-Reinforced Composites As a new material with extraordinary properties, novel composites with high performance are prepared by combining with graphene, and this has become a reasonable choice for broadening the applications of graphene. Recent studies have demonstrated that the introduction of graphene can improve the mechanical properties of most existing composite materials to a certain extent, and the featured electrical conductivity and thermal conductivity of graphene also provide more possibilities for graphenebased multifunctional composite materials. So far, research on graphene composite materials has achieved many accomplishments, and some composite products reinforced by graphene

10

Graphene Composite Materials

are on trial in many fields. We have reason to believe that the combination of graphene and composite materials will give rise to a new research field and yield more fruitful results.

References 1. Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., Grigorieva, I. V. and Firsov, A. A. (2004). Electric field effect in atomically thin carbon films, Science, 306 (5696), pp. 666–669. 2. Wallace, P. R. (1947). The band theory of graphite, Phys. Rev., 71 (9), pp. 622–634. 3. Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Katsnelson, M. I., Grigorieva, I. V., Dubonos, S. V. and Firsov, A. A. (2005). Twodimensional gas of massless Dirac fermions in graphene, Nature, 438 (7065), pp. 197–200. 4. Castro Neto, A. H., Guinea, F., Peres, N. M. R., Novoselov, K. S. and Geim, A. K. (2009). The electronic properties of graphene, Rev. Mod. Phys., 81 (1), pp. 109–162. 5. Du, X., Skachko, I., Barker, A. and Andrei, E. Y. (2008). Approaching ballistic transport in suspended graphene, Nat. Nanotechnol., 3 (8), pp. 491–495. 6. Zhang, Y., Tan, Y. W., Stormer, H. L. and Kim, P. (2005). Experimental observation of the quantum Hall effect and Berry’s phase in graphene, Nature, 438 (7065), pp. 201–204. 7. Lee, C., Wei, X., Kysar, J. W. and Hone, J. (2008). Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science, 321 (5887), pp. 385–388. 8. Novoselov, K. S., Fal’ko, V. I., Colombo, L., Gellert, P. R., Schwab, M. G. and Kim, K. (2012). A roadmap for graphene, Nature, 490, p. 10. 9. Balandin, A. A., Ghosh, S., Bao, W., Calizo, I., Teweldebrhan, D., Miao, F. and Lau, C. N. (2008). Superior thermal conductivity of single-layer graphene, Nano Lett., 8 (3), pp. 902–907. 10. Nair, R. R., Blake, P., Grigorenko, A. N., Novoselov, K. S., Booth, T. J., Stauber, T., Peres, N. M. R. and Geim, A. K. (2008). Fine structure constant defines visual transparency of graphene, Science, 320 (5881), pp. 1308–1308. 11. Zhang, L., Zhang, F., Yang, X., Long, G., Wu, Y., Zhang, T., Leng, K., Huang, Y., Ma, Y., Yu, A. and Chen, Y. (2013). Porous 3D graphene-based bulk materials with exceptional high surface area and excellent conductivity for supercapacitors, Sci. Rep., 3 (1), p. 1408.

Introduction 11

12. Geim, A. K. and Novoselov, K. S. (2007). The rise of graphene, Nat. Mater., 6 (3), pp. 183–191. 13. Allen, M. J., Tung, V. C. and Kaner, R. B. (2010). Honeycomb carbon: A review of graphene, Chem. Rev., 110 (1), pp. 132–145. 14. Bonaccorso, F., Lombardo, A., Hasan, T., Sun, Z., Colombo, L. and Ferrari, A. C. (2012). Production and processing of graphene and 2D crystals, Mater. Today, 15 (12), pp. 564–589. 15. Li, X., Zhang, G., Bai, X., Sun, X., Wang, X., Wang, E. and Dai, H. (2008). Highly conducting graphene sheets and Langmuir-Blodgett films, Nat. Nanotechnol., 3 (9), pp. 538–542. 16. Zhu, Y., Murali, S., Cai, W., Li, X., Suk, J. W., Potts, J. R. and Ruoff, R. S. (2010). Graphene and graphene oxide: Synthesis, properties, and applications, Adv. Mater., 22 (35), pp. 3906–3924. 17. Marcano, D. C., Kosynkin, D. V., Berlin, J. M., Sinitskii, A., Sun, Z., Slesarev, A., Alemany, L. B., Lu, W. and Tour, J. M. (2010). Improved synthesis of graphene oxide, ACS Nano, 4 (8), pp. 4806–4814. 18. Compton, O. C. and Nguyen, S. T. (2010). Graphene oxide, highly reduced graphene oxide, and graphene: Versatile building blocks for carbon-based materials, Small, 6 (6), pp. 711–723. 19. Gilje, S., Han, S., Wang, M., Wang, K. L. and Kaner, R. B. (2007). A chemical route to graphene for device applications, Nano Lett., 7 (11), pp. 3394–3398. 20. Becerril, H. A., Mao, J., Liu, Z., Stoltenberg, R. M., Bao, Z. and Chen, Y. (2008). Evaluation of solution-processed reduced graphene oxide films as transparent conductors, ACS Nano, 2 (3), pp. 463–470. 21. Berger, C., Song, Z., Li, X., Wu, X., Brown, N., Naud, C., Mayou, D., Li, T., Hass, J., Marchenkov, A. N., Conrad, E. H., First, P. N. and de Heer, W. A. (2006). Electronic confinement and coherence in patterned epitaxial graphene, Science, 312 (5777), pp. 1191–1196. 22. Poon, S. W., Chen, W., Tok, E. S. and Wee, A. T. S. (2008). Probing epitaxial growth of graphene on silicon carbide by metal decoration, Appl. Phys. Lett., 92 (10), p. 104102. 23. Li, X., Cai, W., Colombo, L. and Ruoff, R. S. (2009). Evolution of graphene growth on Ni and Cu by carbon isotope labeling, Nano Lett., 9 (12), pp. 4268–4272. 24. Muñoz, R. and Gómez-Aleixandre, C. (2013). Review of CVD synthesis of graphene, Chem. Vap. Depos., 19 (10-11-12), pp. 297–322. 25. Bae, S., Kim, H., Lee, Y., Xu, X., Park, J.-S., Zheng, Y., Balakrishnan, J., Lei, T., Ri Kim, H., Song, Y. I., Kim, Y.-J., Kim, K. S., Özyilmaz, B., Ahn, J.-H., Hong, B. H. and Iijima, S. (2010). Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol., 5 (8), pp. 574–578.

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26. Hao, Y., Bharathi, M. S., Wang, L., Liu, Y., Chen, H., Nie, S., Wang, X., Chou, H., Tan, C., Fallahazad, B., Ramanarayan, H., Magnuson, C. W., Tutuc, E., Yakobson, B. I., McCarty, K. F., Zhang, Y.-W., Kim, P., Hone, J., Colombo, L. and Ruoff, R. S. (2013). The role of surface oxygen in the growth of large single-crystal graphene on copper, Science, 342 (6159), pp. 720–723.

Chapter 2

Graphene-Reinforced Metal Matrix Composites

Metal matrix composites developed in the late 1970s are composites made of metals or alloys as matrix as well as components with different shapes and properties as reinforcement, e.g., fibers, whiskers, and nanofillers. This category of materials not only inherits excellent plasticity, electrical, and thermal conductivity from metal matrix but also shows further improved strength and modulus due to the introduction of fiber reinforcement; in the meantime, reduced density is achieved. In addition, this kind of material possesses a series of characteristics such as excellent damping, wear resistance, fatigue resistance, non-moisture absorption, nonoutgassing, and low thermal expansion. Therefore, applications of metal matrix composites used as ideal structural materials have been launched first in cutting-edge technical fields such as aerospace, aviation, and military industries. In particular, the composite materials based on light metals such as aluminum and magnesium have been the earliest investigated and utilized. Metal matrix composites reinforced by continuous carbon and boron fibers were rapidly developed first, but the complex production process and high cost of such metal matrix composites have set up certain obstacles for their research. To meet the urgent demands for materials accompanied with the continuous development of science and technology, the metal matrix composites in the past 30 years have shown broader application prospects in various aspects. Especially in the 1980s, Toyota used ceramic fiber-reinforced aluminum matrix composites for the 13

14

Graphene Composite Materials

manufacture of diesel engine pistons, which greatly promoted the research and development of metal matrix composites. Nowadays, new metal matrix composite systems composed of various substrates and reinforcements have been developed for the purpose of different structural and functional applications, making it one of important branches of composite material science. Aluminum alloy is one of the most widely used metal materials, with the characteristics of light weight, high strength, and corrosion resistance. With the continuous evolving of materials, aluminum alloys are gradually approaching the limit of material performance. As a material with exceptional physical and chemical properties, graphene is highly valued. Incorporating graphene with aluminum alloy by using graphene as reinforcement can further improve key properties such as strength and toughness on the basis of the excellent properties presented by aluminum alloy, thus meeting the rising demands for practical uses. With the elevation of turbine inlet temperature and thrust-to-weight ratio of the aeroengine, higher and higher requirements are put forward for the temperature-bearing capacity and comprehensive performance of the turbine disk material. Nickel-based powder superalloy, with characteristics of uniform composition, being free of macrosegregation, excellent thermal processing performance, and exceptional mechanical properties, has been widely used for the hottest engine components, and it is the material of choice for manufacturing turbine disk of advanced aeroengines. The integration of high strength, high toughness, high thermal conductivity, and high specific surface area of graphene, as well as high strength and excellent damage tolerance of powder superalloy, is able to improve the comprehensive mechanical properties of mature powder superalloy. By combining the advantages from both graphene and traditional powder superalloy, the comprehensive properties of powder superalloys such as strength, plasticity, fatigue performance, creep performance, damage tolerance, and other properties can be further improved. Therefore, a new kind of powder superalloy material reinforced by graphene is expected to be developed to better meet the requirements of powder superalloy used for advanced aeroengines. Titanium alloys and titanium-aluminum intermetallic compounds are often used in service environments that require high specific strength, excellent corrosion resistance, high creep resistance, and fatigue performance due to their excellent mechanical properties. With the rapid development of aerospace weapons and equipment, the operating temperature

Graphene-Reinforced Metal Matrix Composites 15

of materials is getting higher and higher, which means that the hightemperature performance of materials, especially creep properties, is becoming more and more important. The performance of titanium alloys prepared by traditional technology has approached or reached the theoretical limit. Therefore, it is necessary to explore new and transformative technologies to reinforce titanium alloys. Graphene-reinforced titanium matrix composite is a new material concept that combines the high strength, high stiffness, and high thermal conductivity of graphene nanosheets with the high damage tolerance of titanium alloy/titaniumaluminum intermetallic compounds, and it is the cutting-edge technology for titanium materials. Compared with titanium alloy matrix, the graphene-reinforced titanium matrix composite prepared with graphene nanosheets displays lower density, higher strength, and greatly improved friction and thermal conductivity, which provides a brand new solution for countering the deficiencies of thermal conductivity and wear resistance of the titanium alloy matrix. In addition, the composite materials formed by graphene and other common metals such as magnesium, copper, and iron are also popular research issues on the topics of metal matrix composites, and many beneficial results have been obtained. It is believed that with the development of these technologies, the potential of metal composite materials can be fully utilized and new application fields can be explored.

2.1 Introduction to Graphene-Reinforced Metal Matrix Composites Metal matrix composites are a class of materials that involve the combination of a continuous metallic matrix and reinforcements, which is one of the high-performance materials developed rapidly in recent years. The metal matrix composite combines excellent properties of each material component. It can not only maintain the excellent plasticity and toughness of the metal matrix but can also exhibit the characteristics of high strength, high stiffness, and high hardness of the reinforcement. At the same time, the metal matrix composites have shown great tailorability and tunability of their properties; that is, the requirements of material properties for various practical engineering applications can be satisfied to the greatest extent by manipulating the content, distribution, and interfacial bonding state of each component. Traditional ceramic particles, whiskers, or

16

Graphene Composite Materials

fiber-reinforced metal matrix composites often surpass their matrix metal alloys in properties such as strength and stiffness.1,2 Such composite materials have been widely used as structural materials in aerospace, automobile, and transportation.3 The size of the reinforcement has a significant influence on the strength, plasticity, and fracture behavior of the composite. Taking ceramic particle-reinforced metal composites as an example, both the tensile strength and elongation of the material increase with the decline of the particle size. Nanocomposites can be obtained when the particle size or grain size of the reinforcement is decreased from micrometer scale to nanometer scale, and the mechanical properties of metal matrix composites will be further enhanced.4 The emergence and rapid development of nanotechnology have spawned a large number of nanocrystalline materials.5 The application of nanotechnology in the field of materials science and engineering has opened up new opportunities and research directions for the development of nano metal matrix composites.6 The use of nanomaterials as reinforcements has unique advantages, and nano-reinforcements with excellent properties show great application potential in metal matrix composites. In recent years, graphene has attracted extensive attention as a new material with great research value and application prospects. Single-layer graphene is currently one of the thinnest and hardest materials in the world. Scientists predict that graphene will generate a profound impact on the world in the next 20 years. Single-layer or multi-layer graphene possesses many excellent physical and chemical properties, making it one of the most effective nano-reinforcements for composite materials. Graphene was first obtained by the micromechanical exfoliation method in 2004, and then tested and characterized. Subsequently, researchers have successively developed a variety of preparation methods, such as mechanical exfoliation, chemical vapor deposition (CVD), and chemical intercalation exfoliation techniques. The development of graphene preparation has greatly promoted the research and application of graphene-reinforced metal matrix nanocomposites. In recent years, reports on graphene-reinforced metal matrix nanocomposites have gradually increased. Since harsh conditions such as high temperature, high vacuum, or protective atmosphere are involved in the preparation process, and the problems of interfacial wetting and interface reaction control between graphene and the metal matrix exist during

Graphene-Reinforced Metal Matrix Composites 17

material compounding, the effective addition of graphene into the metal matrix has become a big challenge. During the preparation of graphenereinforced metal matrix composites, the main involved scientific and engineering issues include the following: (1) how to achieve effective dispersion of graphene in the metal matrix; (2) how to realize sound interfacial interaction between the graphene reinforcement and the metal matrix; and (3) how to avoid the structure of graphene from being damaged during the preparation, deformation, and heat treatment of composite materials. The traditional preparation technology of metal matrix composites mainly includes two categories: one is casting technology based on liquidstate metal, such as squeeze casting, gas pressure infiltration, stirring casting, and centrifugal casting; the other is powder metallurgy technology based on solid-state metal, including pressureless sintering, hot pressing, hot isostatic pressing, and spark plasma sintering. Casting technology has broad application and development prospects with a simple process, low cost, high production efficiency, and it is suitable for large-scale production. However, the current use of casting technology to prepare graphenereinforced metal matrix nanocomposites has led to serious problems of graphene agglomeration, so the performance of the prepared composites is often poor. Powder metallurgy technology mainly includes powder mixing, sintering, and thermal deformation process. During the powder mixing process, the uniform dispersion of graphene can be effectively achieved by means of the composite of metal powder and graphene. During the sintering process, high temperature and high pressure are employed to promote diffusion welding between metal particles and improve the interface between graphene and metal to achieve densification of the composite material. Subsequent thermal deformation treatment and heat treatment can further improve the material density and manipulate the microstructure of the composite material. Therefore, powder metallurgy technology is an effective way to prepare high-performance graphene-reinforced metal matrix nanocomposites. Different metal matrix materials exhibit different properties, and the preparation, interface control methods, and material properties of corresponded composites are not the same as each other. In the following, the latest research progress of graphene-reinforced aluminum matrix, copper matrix, nickel matrix, titanium matrix, and magnesium matrix nanocomposites will be introduced in terms of preparation process, microstructure,

18

Graphene Composite Materials

and properties, and the application and development trend of graphenereinforced metal matrix nanocomposites are summarized at the end of this chapter.

2.2 Graphene-Reinforced Powder Superalloy Composites 2.2.1 Introduction to superalloys Superalloy refers to a material that works for a long time under a certain temperature and stress, which needs to have excellent high-temperature mechanical properties, oxidation resistance, corrosion resistance, superb fatigue properties, fracture toughness, and other magnificent comprehensive properties. Nickel-based superalloys are the most important modern superalloys and have been widely used in aerospace, power industries, and other fields. Nickel is a silvery white metal with a density of 8.9 g/cm3, whose melting and boiling points are 1455°C and 2730°C, respectively. Nickel has a face-centered cubic unit cell, and its structure is relatively stable. It does not undergo allotropic transformation with the change of temperature. Nickel also exhibits an excellent performance in terms of chemical stability, and oxidation occurs only above 500°C. Nickel has strong alloying ability; even if cobalt, tungsten, niobium, aluminum, chromium, titanium, molybdenum, and other alloying elements are added, there will be basically no formation of harmful phases. Therefore, the outstanding features given by nickel-based alloys, such as high strength and high temperature stability, make it widely used in the main load-bearing components of aircraft, aeroengines, aircraft fire control systems, hot-end components of advanced propulsion systems, and hightemperature key components of other equipment. With the elevation of turbine inlet temperature and thrust-to-weight ratio of aeroengine, higher and higher requirements are put forward for the temperature-bearing capacity and comprehensive performance of the turbine disk material. Therefore, the number and concentration of alloying elements in superalloys are also increasing, which correspondingly leads to more and more complex compositions of alloys and brings two prominent problems: (1) macrosegregation of alloying elements which leads to inhomogeneous microstructure and composition; (2) poor thermal process performance such that the alloy can only be used in the as-cast state.

Graphene-Reinforced Metal Matrix Composites 19

In order to solve these two problems, nickel-based superalloys with superb performance are designed and fabricated by means of powder metallury.

2.2.2 History of powder superalloys The Birmingham Small Arms company was the first company in the world to prepare the first batch of prealloyed powder via water atomization technology, and develop Nimonic®90 and Nimonic®100 series alloys. Subsequently, the United States further developed powder metallurgy technology. In order to avoid oxidation and pollution of the powder, inert gas (or vacuum) atomization was used to prepare prealloyed powder in the 1960s, and preparation of high-purity prealloyed powder emerged. In 1969, M. M. Allen first developed Astroloy by powder metallurgy, but the performance was not ideal due to the occurrence of severe primary particle boundaries (PPB). In 1972, Pratt & Whitney successfully developed the first powder superalloy IN100 by adopting a combination of processes, i.e., argon atomization (AA) + hot compaction + hot extrusion (HEX) + superplastic forging, and this superalloy was applied to the turbine disk and compressor disk for F100 engine to massively power the F-15 and F-16 fighters. After more than 40 years of development, powder superalloys have experienced four generations.7 The first generation of powder superalloys was mainly investigated in the 1960s and 1970s, typically represented by René95 developed by GE Corporation of the United States. The first-generation powder superalloy was characterized by a high content of γ′ phase (generally greater than 45%), which is heat treated below the complete solution temperature to obtain a fine-grained structure and pursue high strength, so it is called high-strength powder superalloy. However, it shows poor crack growth resistance and durability, and the service temperature does not exceed 650°C. In addition to the René95, the first-generation powder superalloys also include IN100, MERL76, Udimet720 of the United States, and EP741NP of Russia, whose characteristics are listed in Table 2.1. The studies on the second-generation superalloys mainly began in the 1980s and developed on the basis of the first-generation powder superalloys. They are characterized by controlling the content of γ′ phase to obtain coarse grains, and the tensile strength is lower than that of the first generation, but it has higher creep strength and crack growth resistance.

20

Graphene Composite Materials Table 2.1.

Superalloy

Characteristics of the typical first-generation powder superalloys. Company and nationality

Year

Content of γ′ phase (%)

Solution temperature of γ′ phase (°C)

Density (g/cm3)

René95

GE, USA

1972

50

1160

8.26

IN100

P&W, USA

1972

61

1185

7.90

MERL76

P&W, USA

1979

64

1190

7.95

Udimet720

SMC, USA

Early 1980s

45

1150

8.10

EP741NP

VILS, Russia

1974

60

1180

8.35

Table 2.2.

Characteristics of the typical second-generation powder superalloys.

Superalloy

Company and nationality

Year

Content of γ′ phase (%)

Solution temperature of γ′ phase (°C)

Density (g/cm3)

René88DT

GE, USA

1984–1988

37

1135

8.26

N18

SNECMA, France

1984

55

1195

8.00

The second-generation superalloys can operate at temperatures of 650– 750°C, and they are called damage-tolerant alloys. The second-generation powder superalloys mainly include the René88DT developed in the United States and the N18 developed in France, and both of them have been applied in practice. For example, the René88DT is used in PW4084 and GE90 engines to power the B777 civil aircraft, and the N18 alloy is used in the M88 Turbofan engine for Rafale series fighters. Table 2.2 lists the characteristics of the second-generation powder superalloys. In order to meet the higher-performance requirements of turbine disks for new-generation aeroengines, the third-generation powder superalloys were developed in the 1980s, and their performance features both the high strength of the first-generation superalloys and the damage tolerance of the second-generation superalloys. Compared with that of the secondgeneration superalloys, the operating temperature is increased to 650– 800°C, and the content and the solid solution temperature of γ′ phase are moderate. A coarse-grained or fine-grained structure can be obtained when heat treated above or below the complete solution temperature of γ′ phase, which is suitable for the preparation of dual-structure and

Graphene-Reinforced Metal Matrix Composites 21 Table 2.3.

Superalloy

Characteristics of the typical third-generation powder superalloys. Company and nationality

Year

Content of γ′ phase (%)

Solution temperature of γ′ phase (°C)

Density (g/cm3)

René104

GE/P&W/NASA GRC, USA

1992–1999

51

1160

8.27

Alloy10

Honeywell, USA

Early 1990s

55

1180

8.35

LSHR

NASA GRC, USA

Early 1990s

60

1160

8.33

RR1000

RR, UK

Early 1990s

46

1160

8.20

dual-property disks. Typical third-generation powder superalloys include René104, Alloy10, LSHR, and RR1000, whose characteristics are shown in Table 2.3. Accompanied by further improvement of aeroengine performance, the United States has successfully developed the fourth-generation powder superalloy, i.e., the René130 developed by GE, which is being randomly tested and applied to the current largest aeroengine GE9X. Due to the complex preparation process of powder turbine disks and the high technical barrier, only few countries, such as the United States, Russia, the United Kingdom, and France, are capable of fabricating these powder superalloy components, and can independently develop powder superalloys and establish their own trademark names, such as the René series in the United States, the N series in France, and the EP series in Russia. The main process route of powder disk development in Europe and the United States is argon atomization + hot isostatic pressing + hot extrusion + isothermal forging. Different from the process routes of the Europeans and Americans, the dominant process route in Russia is plasma rotary electrode process (PREP) + direct hot isostatic pressing. The scheme of argon atomization and PREP is shown in Figure 2.1. The studies on powder superalloys in China began in the late 1970s. At present, three generations of powder superalloys have been developed: the first generation includes FGH95 and FGH97 with operating temperature below 650°C; the second-generation damage-tolerant FGH96 with operating temperature 750°C; the third-generation high-strength and damage-tolerant FGH99 with operating temperature 800°C.8

22

Graphene Composite Materials

(a)

(b)

Figure 2.1. Schematic diagram of (a) argon atomization and (b) PREP methods for powder making.7

Among them, the first-generation FGH95 and the second-generation FGH96 have been relatively mature, the third-generation powder superalloy is undergoing engineering application research, and the fourthgeneration powder superalloy is currently under pre-research. FGH95 is a precipitation hardenable superalloy with highly alloyed γ′ phase, and the volume fraction of γ′ phase is about 50%. Its yield strength is 30% higher than that of GH4169, and the operating temperature can be increased by 110°C under the same stress. The main manufacturing process of this powder disk is direct hot isostatic pressing, and it has been applied to a certain type of turboshaft engine. FGH96 alloy is the second-generation damage-tolerant powder superalloy developed in China, and the volume fraction of γ ′ phase is about 36%. The fatigue crack growth rate of this alloy at the temperatures of RT~650°C is 50% lower than that of FGH95, and the creep and fracture strengths are higher, while tensile strength decreases by less than 10%. At present, hot isostatic pressing + (hot extrusion) + isothermal forging is mainly used to prepare powder disk blanks in China, and its performance has reached the same level of similar foreign superalloys. By employing such a process, a variety of high-temperature key hot forging parts are fabricated, such as high- and low-pressure turbine disks, sealing disks, and baffles.

Graphene-Reinforced Metal Matrix Composites 23 Post-treatment of Powder Master Alloy VIM

Argon Atomization Powder

Sieving

Degassing and Decontamination

Capsule

Isothermal Forging

Hot Isostatic Pressing

Ultrasonic Nondestructive Testing Powder Disk Blank

Thermal Treatment Microstructure and Properties Testing

Hot Extrusion

Machining

Figure 2.2.

Parts

Process route of powder superalloy parts.

The development of powder superalloy in China has mainly been furthered by AECC Beijing Institute of Aeronautical Materials and Central Iron and Steel Research Institute. The main process of making powder superalloy products adopted by AECC Beijing Institute of Aeronautical Materials is argon atomization + hot isostatic pressing (+ hot extrusion) + isothermal forging, and the process route (shown in Figure 2.2) is consistent with that used in Europe and the United States. Such a process is complex and expensive, but the product has excellent comprehensive performance, long service life, and higher safety and reliability. The main process route of the products used by Central Iron and Steel Research Institute is PREP + direct hot isostatic pressing, which has a simple process, low cost, and high production efficiency.

2.2.3 Graphene-reinforced powder superalloys With the development of the aerospace industry, especially with the development of national defense weapons and equipment in the direction of high performance and high efficiency, practical engineering has higher and higher requirements for the comprehensive performance of hightemperature materials, and it is difficult for a single material to meet the actual needs; therefore, people have gradually turned their attention to new nickel-based superalloys. Carbon materials such as carbon fibers and carbon nanotubes can be used for reinforcement of nickel-based alloys.9 However, carbon materials

24

Graphene Composite Materials

such as carbon nanotubes are very easy to entangle and hard to disperse during the preparation process, and the wettability with the metal matrix is not enough, resulting in a weak interface bond with the metal matrix, which seriously affects the enhancement effect. Therefore, current studies on carbon nanotube reinforcement are focused on coatings and films, while the reports on bulk alloy materials are relatively rare. Graphene has exceptional mechanical properties and temperature resistance, making it an ideal material as reinforcement to improve the strength of materials. Compared with the existing reinforcing materials such as carbon nanotubes, graphene has three advantages: (1) the surface roughness of wrinkled graphene nanosheets, which can effectively adhere to the surrounding materials; (2) large specific surface area of the graphene, since graphene displays a planar structure, which provides more contact opportunities with the matrix material rather than tubular carbon nanotubes; meanwhile, the upper and lower surfaces of graphene nanosheet can fully contact the matrix material; and (3) since graphene has a unique geometric structure, when microcracks in the material encounter a 2D graphene nanosheet, the microcracks are deviated or forced to tilt and twist around the graphene basal plane, and such a process helps absorb the excess energy of crack propagation and expansion. The crack propagation process is more efficient for 2D planes than for materials with a high aspect ratio. Furthermore, graphene can be prepared from graphite, indicating that graphene is readily available and inexpensive to produce on a large scale. Due to the unique properties of graphene, the modification of materials with graphene is expected to obtain materials with higher performance, and graphene-reinforced composites have become important area of research in the field of graphene applications. Nickel-based powder superalloys exhibit characteristics of uniform composition, nonmacrosegregation, excellent thermal processing performance, and exceptional mechanical properties. Nickel-based powder superalloys have been widely used for high-temperature components of aeroengine, and they are the material of choice for manufacturing turbine disks of advanced aeroengines. If the high strength, high toughness, high thermal conductivity, high specific surface area, and other characteristics of graphene are combined with the high strength and high damage tolerance of powder superalloys, the comprehensive mechanical properties of powder superalloys can be further improved, such as strength, plasticity, fatigue performance, creep performance, damage tolerance, and other properties. It is expected

Graphene-Reinforced Metal Matrix Composites 25

that a graphene-reinforced powder superalloy will be developed to better meet the demand of powder superalloys for advanced aeroengines.

2.2.4 Preparation of graphene-reinforced powder superalloy 2.2.4.1 Research on dispersion of graphene In the process of studying the preparation of graphene-reinforced powder superalloy, the dispersion method of graphene is a difficult research issue. Graphene has the following characteristics: the density is low, which is quite different from the density of the metal matrix; due to the single-layer carbon atom nanosheet structure of graphene, when it is introduced into a metal matrix composite material, graphene is prone to agglomeration, and the dispersion ability of graphene is not good enough; graphene is neither hydrophilic nor lipophilic, and its reactivity is not high. In the studies on graphene-reinforced metal matrix composites, all these characteristics of graphene lead to the problem that graphene cannot be uniformly dispersed in the metal matrix. The planetary ball milling method has demonstrated its superiority in the research of Cu-based and Al-based graphene composites, and the mechanical properties of the composite materials have been significantly improved.10,11 Graphene and superalloy powder can be mixed by means of planetary ball milling to achieve improvement of their dispersibility and to prevent graphene from agglomerating in superalloy powder. The graphene/superalloy powders were wet-milled with water using a planetary ball mill for 10 h. Figure 2.3 shows the morphology of the superalloy powders before and after ball milling. After ball milling, the particle size distribution of the powder is uneven, and a portion of the powder is damaged, attached, and distributed on the larger powder particles. The 250 mesh FGH96 superalloy powders were mixed with a GO solution with a concentration of 3 g/L by planetary ball milling, and the added amount of graphene was 0.15 wt%. The milled powders with different graphene contents were subjected to hot isostatic pressing followed by isothermal forging. After that, solution-aging treatment was carried out, and the microstructure of final products is shown in Figure 2.4. In the superalloy with 0.15 wt% graphene loading after ball milling, a large number of precipitates are presented around the grain boundaries. Electron probe microanalysis (EPMA) of the alloy after introduction of graphene indicates that the interface mainly consists of carbides formed by carbon

26

Graphene Composite Materials

(a)

Figure 2.3. milling.

(b)

Morphology of superalloy powder (a) before and (b) after planetary ball

(a)

(b)

Figure 2.4. Microstructure of planetary ball-milled superalloys with different graphene loadings. (a) Without graphene; (b) with 0.15 wt% graphene.

and titanium (as shown in Figure 2.5). For the graphene alloy ingots prepared by the ball milling, the morphology of grain boundary precipitates and graphene nanosheets in the powder superalloy was further analyzed by transmission electron microscope (TEM). No single-layer/multi-layer graphene nanosheet is determined, and localized carbon agglomeration is observed, which is believed to react with the alloying element titanium, as shown in Figure 2.6. The tensile properties at room temperature and 650°C for the powder superalloy with graphene by planetary ball milling are both lower than those of the powder superalloy without graphene. The planetary ball milling method can effectively alleviate the agglomeration of graphene during the mixing, but some active elements

Graphene-Reinforced Metal Matrix Composites 27

Figure 2.5.

Figure 2.6.

EPMA analysis of planetary ball-milled superalloy with 0.15 wt% graphene.

Morphology and energy spectrum of planetary ball-milled alloy carbide.

28

Graphene Composite Materials

graphene

Figure 2.7. milling.

Morphology of superalloy powder particles after low-temperature ball

in the superalloy are prone to easily react with graphene, which will lead to the deterioration of mechanical properties. To reduce the interface reaction of graphene, it is a straightforward idea to adopt the low-temperature ball milling method. The low-temperature ball milling mixing method is ball milling of the superalloy powder under the cooling of liquid nitrogen, and the added amount of graphene is 0.1 wt%. Figure 2.7 shows the morphology of superalloy powders after low-temperature ball milling; the superalloy powders are broken into flakes, and the graphene nanosheets are transparent, which adhere to the surface of the superalloy powder. Hot isostatic pressing was performed on the ball-milled powders with different graphene loadings, and solution-aging heat treatment was carried out on the graphene alloy ingots after hot isostatic pressing. The microstructure of the final product is shown in Figure 2.8; it is clear to see that the size of the precipitated γ′ phase is small (~50 nm), and a large number of long strips with a thickness of several micrometers are precipitated at the grain boundaries. Energy dispersive spectroscopy (EDS) analysis shown in Figure 2.9 suggests that the precipitates are matrix elements such as Co, Cr, Ti, Al, and Ni, whereas no carbon is evidenced. After low-temperature ball milling, the mechanical properties of powder superalloys at room temperature and 650°C are greatly reduced. The mixing method of low-temperature ball milling addresses the problem of the reaction between graphene nanosheets and matrix elements, and also avoids the agglomeration of graphene in the matrix. However, by using such a method, great influence is generated on the superalloy powder and final microstructure, and the mechanical properties

Graphene-Reinforced Metal Matrix Composites 29

Figure 2.8.

Morphology of superalloy after low-temperature ball milling.

Figure 2.9. Energy spectrum analysis of long strips in low-temperature ball-milled superalloys.

are also deteriorated. The ball milling method indeed solves the problem of uniform dispersion of graphene, but the goal of mechanical properties is not achieved. Is there a way to ensure that the graphene is uniformly dispersed and not agglomerated, while minimizing the impact on the superalloy powder? The authors tried to directly introduce 0.15 wt% GO to 1 kg powder superalloy by mechanical stirring followed by degassing reduction at 500°C in vacuum; the final microstructure of the alloy (hot isostatic pressing + heat treatment state) is shown in Figure 2.10. The results were consistent with the previous judgment; the problem of graphene agglomeration and the reaction between graphene and the matrix really occur in the final microstructure. However, during the mixing process, the powder is less affected, and the structure of the matrix is closer to the graphene-free case. Therefore, pursuing a more effective method for dispersing graphene has become the key for introducing graphene reinforcement into the matrix. Based on this idea, the authors tried the wet

30

Graphene Composite Materials

Figure 2.10.

Morphology of superalloy with graphene dispersed by mechanical stirring.

Ultrasonic mixing of graphene/ethanol

Performance testing

Powder/graphene ultrasonic +mechanical stirring and mixing

Nickel-based alloy specimen for testing

Heat treatment

Degassing reduction

Pre-heat treatment

Hot extrusion

Hot isostatic pressing

Figure 2.11. Wet mixing method of ultrasonic + mechanical stirring.

mixing method of ultrasonic + mechanical stirring. In order to increase the specific surface area of the powder in contact with graphene, powder with smaller particle size (~270 mesh) was used for mixing, and the amount of each mixed powder was reduced. The wet mixing method of ultrasonic + mechanical stirring (hereinafter referred to as “wet mixing method”) is illustrated in Figure 2.11. After graphene and ethanol were ultrasonically mixed, they were added to the superalloy powder in batches, and ultrasonic + mechanical stirring was performed continuously. Then, the as-mixed powders were subjected successively to 500°C heated degassing reduction in vacuum, pre-heat treatment, hot isostatic pressing, hot extrusion, and heat treatment to obtain the final microstructure. The morphology of graphene/powder particles after mixing is shown in Figure 2.12, in which the transparent, thin, feather-wrinkled graphene nanosheets are uniformly dispersed, and the superalloy powder particles are less affected, indicating that the composite powders are successfully obtained. The final microstructure is shown in Figure 2.13.

Graphene-Reinforced Metal Matrix Composites 31

Graphene sheet Graphene sheet

Figure 2.12.

Graphene/powder superalloy particles after wet mixing method.12

Figure 2.13.

Final microstructure of the wet mixed superalloy.

The graphene/superalloy composite powder was successfully prepared by the wet mixing method, and the mechanical properties of the final material were improved. Based on this method, combined with the process route of traditional powder superalloy (Figure 2.2), a process route for the preparation of graphene-reinforced powder superalloys is proposed.

2.2.4.2 Influence of graphene content The introduction of graphene undoubtedly leads to a change of the composition of the powder superalloy, which will consequently affect the

32

Graphene Composite Materials

grain size, crystal dislocation, and physicochemical properties of the composite. Therefore, it is necessary to study the influence of graphene content on superalloys and determine the appropriate amount of introduced graphene. Here, basic experimental research on graphene-reinforced FGH96 powder superalloys with different loadings (0.005–0.3 wt%) was carried out. The wet mixing method was employed to prepare the powder with graphene contents of 0.005 wt%, 0.01 wt%, 0.05 wt%, 0.1 wt%, and 0.3 wt%, and the alloy ingot was subjected to hot isostatic pressing + isothermal forging + heat treatment. The mechanical property measurements were carried out on the superalloy with a final structure, and the results are as follows. In the case of a small amount of graphene loading (0.005 wt%, 0.01 wt%, and 0.05 wt%), the room-temperature tensile, high-temperature tensile, and high-temperature creep properties of the alloy are significantly improved compared with those without introduction of graphene. As shown in Table 2.4, the average tensile strength at room temperature gradually increases with the elevation of graphene loading, and the average tensile strength is increased by about 30 MPa compared with that without introduction of graphene. It is worth noting that the average section shrinkage rate is raised by 8% on the basis of elevated tensile strength. The average tensile strength at 400°C, 650°C, and 750°C is about 15 MPa, 32 MPa, and 23 MPa higher than that without graphene, and the average elongation is increased by 8%, 8%, and 15%, respectively. As shown in Table 2.5, with the increment of graphene loading, the duration for high-temperature creep residual strain to reach 0.2% gradually increases. Compared with alloy without graphene, the residual strain of the graphene-reinforced alloy after 300 h is much lower. For powder superalloys with different graphene loadings (0.005– 0.3 wt%), the tensile strength at room temperature and 650°C increases gradually with the rise of graphene loadings. The elongation at room temperature and 650°C increases first and then decreases with the increment of graphene loadings, and it approaches the highest value in the range of 0.05–0.1 wt%, as shown in Figure 2.14. The high-temperature creep properties under 700°C and 690 MPa of powder superalloys with different contents of graphene are compared and shown in Figure 2.15. When the graphene loading is less than 0.1 wt%,

Graphene-Reinforced Metal Matrix Composites 33 Table 2.4. Average tensile strength of superalloys with small graphene loadings. Temperature (°C)

Graphene content (wt%)

23

400

650

750

Table 2.5. Graphene loading (wt%) 0.005

σb (MPa)

δ5 (%)

ψ (%)

0.005

1591

23.9

38.5

0.01

1592

22.5

34.8

0.05

1594

23.2

37.3

No graphene

1562

22.4

34

0.005

1497

17.8

20.4

0.01

1504

16.9

19.3

0.05

1509

16.5

18.9

No graphene

1489

15.8

19.2

0.005

1496

26.8

25.1

0.01

1499

23.4

21.9

0.05

1498

29.5

27.6

No graphene

1477

24.9

26

0.005

1171

20

20.9

0.01

1163

20.1

19.3

0.05

1175

17

18.4

No graphene

1147

16.7

20

Creep properties of alloys with small graphene loadings. Test temperature (°C)

Test stress (MPa)

Duration (h)

Residual strain (εp%)

700

690

68

0.049

9.052083 0.01

700

690

300

0.461

68

0.003

9.84375 0.05

700

690

700

690

0.2

300

0.482

68

0.046

9.53125 No graphene

0.2

0.2

300

0.392

68

0.072

183

0.2

300

1.533

34

Graphene Composite Materials Elongation-room temperature Elongation (%)

Tensile strength (MPa)

Tensile strength-room temperature

Content of graphene (%)

Content of graphene (%) Elongation-650°C Elongation (%)

Tensile strength (MPa)

Tensile strength-650°C

Content of graphene (%)

Content of graphene (%)

Duration for the residual strain to reach 0.2 wt% (h)

Figure 2.14. Tensile properties of superalloys with graphene loadings of 0.005–0.3 wt%.

Content of graphene (%)

Figure 2.15. High-temperature creep properties of superalloys with graphene loadings of 0.005–0.3 wt%.

Graphene-Reinforced Metal Matrix Composites 35

with the increment of graphene loadings, the duration for the residual strain to reach 0.2 wt% increases gradually. When the graphene loading is 0.3 wt%, the duration decreases sharply, and the duration reaches the maximum at graphene loading of 0.1 wt%. In the case of a small content of graphene (0.005–0.05 wt%), both the tensile and creep properties of the alloy are improved with the increment of graphene loadings. When the content of graphene is raised to 0.3 wt%, the tensile plasticity and the duration for high-temperature creep residual strain to reach 0.2% are decreased consecutively, and the tensile as well as high-temperature creep properties reach the peak in the range of 0.05–0.1 wt%. Therefore, the graphene loading of 0.05–0.1 wt% is a reasonable choice. In the following experiments and research, graphene loading of 0.1 wt% is determined for powder mixing.

2.2.4.3 Process route for the fabrication of graphene-reinforced powder superalloy The process route for the fabrication of 0.1 wt% graphene-reinforced powder superalloy on the basis of the wet mixing method with ultrasonic + mechanical stirring is shown in Figure 2.16. The capsules and testing

Argon Atomization Powder

Master Alloy VIM

Sieving

Batch Mixing

Superalloy Powder

Mixing Stirring GO Nanosheets Ultrasonic Dispersion

GO Solution Wash and Dry

Ethanol Absolute

Capsuling and Seal Welding

Nondestructive Testing

Thermal Treatment

Isothermal Forging

Degassing

Hot Isostatic Pressing

Hot Extrusion

Figure 2.16.

Process route for the fabrication of graphene-reinforced powder superalloys.

36

Graphene Composite Materials

Figure 2.17.

Graphene-reinforced powder superalloy capsules and testing specimens.

specimens of typical fabricated graphene-reinforced powder superalloy are shown in Figure 2.17.

2.2.5 Microstructure and properties of graphene-reinforced powder superalloys 2.2.5.1 Microstructure of powder superalloy The microstructure of powder superalloy mainly includes grain size and γ′ phase, in addition to a small amount of carbon and boride. The grain size of the powder superalloy has a direct influence on the mechanical properties of the alloy. Figure 2.18 shows the relationship between the mechanical properties and grain size of U720 alloy. It can be concluded from the figure that the powder superalloy with fine-grained structure shows high strength and fatigue properties, while the coarsegrained structure is beneficial to improve the creep performance and crack growth resistance of the powder superalloy. For the first-generation powder superalloys, standard heat treatment is adopted to obtain a fine-grained structure, which indicates that the priority is the strength of the alloy. Figure 2.19 shows the grain structure of FGH95, and the ASTM grain size is 8–10. For the second-generation powder superalloys, heat treatment is performed to obtain a suitable coarse-grained structure, which is a

Graphene-Reinforced Metal Matrix Composites 37

Figure 2.18.

Relationship between grain size and mechanical properties of U720.13

Figure 2.19.

Grain structure of FGH95 superalloy.

requirement for the development of alloys with damage-tolerant characteristics. Figure 2.20 shows the grain structure of FGH96 developed in China, and the ASTM grain size is about 7.5. The zigzag-shaped grain boundary of FGH96 is presented, which is beneficial to further slow down the crack growth of the alloy. In general, nickel-based powder superalloys can be regarded as twophase alloys: a matrix γ phase and a reinforcing γ′ phase. Due to the difference in size, the reinforcing γ′ phase can generally be divided into primary γ′ phase, secondary γ′ phase, and tertiary γ′ phase.

38

Graphene Composite Materials

Figure 2.20.

Grain structure of FGH96 alloy.

Secondary γ ′ phase Secondary γ ′ phase

Primary γ ′ phase

Figure 2.21.

Tertiary γ ′ phase

Morphology of γ′ phase of FGH95.

The first-generation FGH95 is heat treated below the complete solution temperature of the γ′ phase, so the coexistence of aforementioned three γ′ phases are presented in the matrix. Figure 2.21 shows the morphology of the γ′ phase of FGH95 alloy. The primary γ′ phase has various shapes, with a size of 1–5 µm; the secondary γ′ phase is mostly irregular square, with a size of 50–500 nm; and the tertiary γ′ phase is spherical with a size of less than 50 nm. The second-generation FGH96 alloy is generally heat treated above the solution temperature of the γ′ phase, and the primary γ′ phase is completely dissolved in the matrix. Therefore, only secondary and tertiary γ′ phases are distributed in the matrix. Figure 2.22 shows the typical morphology of the γ′ phase of FGH96. The secondary γ′ phase is irregular square with a size of 50–300 nm; the tertiary γ′ phase is spherical with a size less than 50 nm.

Graphene-Reinforced Metal Matrix Composites 39

Tertiary γ' phase

Secondary γ' phase

Figure 2.22.

Morphology of γ′ phase of FGH96.

Figure 2.23. Relationship between the yield strength of René88DT and the size and volume fraction of γ′ phase.14

In addition to the grain size, the size and morphology of the γ′ phase also affect the mechanical properties of powder superalloys. Figure 2.23 shows the relationship between the yield strength of René88DT and the size and volume fraction of the γ′ phase. It is indicative from the figure that with the increment of the size of the γ′ phase, the yield strength of the alloy gradually decreases.

40

Graphene Composite Materials

2.2.5.2 Microstructure of graphene-reinforced powder superalloys The microstructure of the graphene-reinforced powder superalloy with a small extrusion ratio (4:1) in the hot extrusion process is shown in Figure 2.24. Due to the small extrusion ratio, the prior particle boundary (PPB) is not completely eliminated, and it is expected to be improved in the following isothermal forging process. It is clear to observe that after hot extrusion, the microstructure is uniform and the size of grains is fine (ASTM grain size 11), and there is no scenario of a large number of graphene agglomerations and carbide at the grain boundaries, which also proves that the sound dispersion of graphene in superalloys is achieved by the wet mixing method. The microstructure of the as-extruded specimen parallel to the extrusion direction shown in Figure 2.25 suggests that the

(a)

(b)

Figure 2.24. Microstructure of the as-extruded alloy. (a) Parallel to the extrusion direction. (b) Vertical to the extrusion direction.

Graphene-Reinforced Metal Matrix Composites 41

Figure 2.25.

Graphene in as-extruded alloy.

translucent thin layer of feather-like graphene nanosheets is distributed across the grain boundaries. After isothermal forging, the alloy is subjected to solution treatment at 1150°C for 2 h and followed by aging treatment at 760°C for 8 h to obtain the final microstructure. The microstructure after heat treatment is shown in Figure 2.26, the ASTM grain size is 7, and it is evidenced that the translucent thin-layer wrinkled graphene nanosheets span multiple crystal grains. Energy spectrum analysis demonstrates that the graphene nanosheets mainly consist of element carbon, and the rest are matrix elements. In addition to spanning multiple grain boundaries, graphene nanosheets can also be found within grains and at twin crystals, as shown in Figure 2.27. Therefore, classic curved and zigzag-shaped grain boundaries can be formed in the reinforced alloy, which can improve the creep properties of the material. The graphene in the reinforced alloys varies in size, ranging from 1 µm to 30 µm, and graphene nanosheets in different sizes present different morphologies in the alloy. Figure 2.28 shows the typical morphologies of small-sized (1 µm) and large-sized (25 µm) graphene nanosheets in reinforced superalloys. The small-sized graphene nanosheets are translucent and wrinkled, and the edges of these graphene nanosheets can be clearly evidenced. The large-sized translucent graphene sheets are in a wrinkled and curled state, and the multi-layer folding of the graphene nanosheets can be clearly observed.

42

Graphene Composite Materials

Figure 2.26.

Graphene in the microstructure of heat-treated alloy.

Graphene nanosheet

Zigzag grain boundary

Figure 2.27.

Graphene nanosheets and curved zigzag grain boundaries in grains.

Edge of graphene sheet

(a)

(b)

Figure 2.28. Morphology of graphene nanosheets with different sizes. (a) Small-sized (1 µm) graphene nanosheets; (b) large-sized (25 µm) graphene nanosheets.

Graphene-Reinforced Metal Matrix Composites 43

2.2.5.3 Physical properties

5.5

200

5.0

160

4.5 4.0

FGH96 FGH96+0.3wt% GR

3.5

Enthalpy (cal/g)

Thermal diffusivity (10–6 m2/s)

Comparison of thermal diffusivity, enthalpy, thermal conductivity, and average specific heat at constant pressure of powder superalloy with and without reinforcement of graphene is shown in Figure 2.29. The electrical resistance and conductivity of graphene-reinforced alloy are listed in Table 2.6. It is indicative that a small content of graphene has no obvious FGH96 FGH96+0.3wt% GR

120 80 40

3.0 200

400

600 800 1000 Temperature (°C )

0

1200

200

400

40

FGH96 FGH96+0.3wt% GR

35

1200

(b) Average specific heat at constant pressure (kg·°C )

Thermal conductivity (W/(m·K))

(a)

800 1000 600 Temperature (°C )

30 25 20 15 10

680

FGH96 FGH96+0.3wt% GR

640 600 560 520 480 440

5 200

400

600 800 1000 Temperature (°C )

1200

200

(c)

400

600 800 1000 Temperature (°C )

1200

(d)

Figure 2.29. Some physical properties of superalloys with and without reinforcement of graphene. (a) Thermal diffusivity; (b) enthalpy; (c) thermal conductivity; (d) average specific heat at constant pressure. Table 2.6. Electrical resistivity and conductivity of superalloys with and without reinforcement of graphene. Superalloy

Resistivity (µΩ·cm)

Conductivity (MS/m)

FGH96 + 0.3 wt% GR

1.28

0.78

1.287

0.78

FGH96

1.257

0.8

1.26

0.79

44

Graphene Composite Materials

effect on the thermal diffusivity, enthalpy, resistance, and electrical conductivity of the alloy.

2.2.5.4 Mechanical properties The reinforced superalloy with 0.1 wt% graphene loading was subjected to hot extrusion + hot isostatic pressing + heat treatment to obtain the final microstructure. The mechanical properties of graphene-reinforced powder superalloys are as follows: Tensile properties: The room-temperature and high-temperature tensile properties of graphene-reinforced powder superalloys are listed in Tables 2.7 and 2.8, respectively. Compared with the superalloy without graphene, the room-temperature tensile strength is increased by an average of 91 MPa (6% increment), and the yield strength is increased by 70 MPa (6.2% increment). The tensile strength at 650°C is increased by an average of 91 MPa (6.4% increment), and the plasticity is greatly increased by 24%. The tensile strength at 750°C is 48 MPa higher than the standard. Low-cycle fatigue performance: At 650°C, the cycle times of graphenereinforced superalloy specimens are much larger than the standard Table 2.7.

Room-temperature tensile properties of graphene-reinforced superalloys.

Content of graphene (wt%)

σb (MPa)

δ5 (%)

ψ (%)

σ0.2 (MPa)

0.1

1603

23.2

37.8

1193

0

1512

/

21.5

1123

Table 2.8.

High-temperature tensile properties of graphene-reinforced superalloys.

Content of graphene (wt%) 0.1

Temperature (°C)

σb (MPa)

δ5 (%)

ψ (%)

650

1522

20.8

21

Without graphene 0.1 Standard of conventional superalloy

750

1431

11

17

1168

19

20

1120

10

12

Graphene-Reinforced Metal Matrix Composites 45

requirements of conventional powder superalloys, showing excellent lowcycle fatigue performance, as shown in Table 2.9. High-temperature creep performance: The residual strain at high temperature for 68 h of graphene-reinforced superalloy is 77.5% lower than that without graphene, and when the test time is 300 h, it is 66% lower than that without graphene, demonstrating excellent high-temperature creep performance (Table 2.10). It is evidenced in Figure 2.30 that Table 2.9.

Low-cycle fatigue properties of graphene-reinforced superalloys at 650°C.

Content of graphene (wt%)

q (°C)

Waveform

f (Hz)

Re

∆t (%)

N (weeks)

0.1

650

Triangle wave

0.33

0.05

0.8

65,020

Standard of conventional superalloy

650

Triangle wave

0.33

0.05

0.8

≥5000

Table 2.10.

High-temperature creep properties of graphene-reinforced superalloy.

Graphene loading (wt%) 0.1

Without graphene

Test temperature (°C)

Test stress (MPa)

700

690

Duration (h)

Residual strain (εp%)

68

0.009

279

0.2

300

0.255

68

0.04

221

0.2

300

0.762

Rupture (630h) Rupture (569h)

Figure 2.30. Comparison of high-temperature creep properties of superalloy with and without reinforcement of graphene.

46

Graphene Composite Materials

compared with superalloy without reinforcement of graphene, the residual strain of the graphene-reinforced superalloy does not change significantly in the first 125 h, and after 150 h, the graphene-reinforced superalloy shows better creep resistance. High-temperature deformation resistance: The uniaxial thermal compression tests are carried out on graphene-reinforced superalloys according to the test parameters listed in Table 2.11, and the test results are shown in Figure 2.31. It can be evidenced that the deformation resistance Table 2.11.

Parameters for thermal compression test.

Temperature (°C)

Strain rate (s−1)

950

0.0005

0.001

0.01

0.1

0.5

1

10

1000

0.0005

0.001

0.01

0.1

0.5

1

10

1050

0.0005

0.001

0.01

0.1

0.5

1

10

1070

0.0005

0.001

0.01

0.1

0.5

1

10

1100

0.0005

0.001

0.01

0.1

0.5

1

10

1120

0.0005

0.001

0.01

0.1

0.5

1

10

1150

0.0005

0.001

0.01

0.1

0.5

1

10

500

True Stress, MPa

400

300

200

100

0 0.0

0.2

0.4

0.6

True Strain

Figure 2.31. Comparison of thermal compression deformation resistance of superalloys with and without reinforcement of graphene.

Graphene-Reinforced Metal Matrix Composites 47

of the graphene-reinforced superalloy is significantly increased compared to the conventional powder superalloy.

2.2.6 Reinforcing mechanism of graphene It can be concluded from the above results that the tensile properties, lowcycle fatigue properties, and high-temperature creep properties of graphene-reinforced powder superalloys have been significantly improved, so this section will focus on the reinforcing mechanism of graphene. The first to be concerned is the state of graphene in the reinforced superalloy, including the morphology and chemical state after its incorporation. The morphology of graphene before and after incorporation is observed at high magnification (Figures 2.32 and 2.33); it can be found that the morphology of the multi-layer graphene is consistent with that before the incorporation, which is wrinkled, bent, and folded. CT scans were performed on the reinforced superalloys to reconstruct the 3D morphology of graphene in the superalloys; as Figure 2.34 shows, the complex wrinkled morphology of graphene in the superalloy can be observed.

Figure 2.32.

Morphology and TEM diffraction pattern of graphene before incorporation.

Figure 2.33.

Graphene morphology after incorporation.

48

Graphene Composite Materials

Figure 2.34.

3D morphology of graphene in graphene-reinforced superalloy.

Figure 2.35.

FIB-SEM images of graphene-reinforced superalloy.

Graphene-Reinforced Metal Matrix Composites 49

In order to find out whether the chemical state of graphene changes in the reinforced superalloy, the reinforced superalloy was first observed by focused ion beam scanning electron microscope (FIB-SEM). The surface of the samples prepared by FIB is extremely flat, much better than the polished ones; therefore, more details can be evidenced. It is indicative from Figure 2.35 that obvious annular contrast exists at the interface between graphene and matrix in the samples, which suggests that the atomic structure and element species at the interface between graphene and matrix are quite different from those of graphene and matrix. In order to study the change of the interface, the interface between graphene and matrix was scanned by the energy spectrum (Figure 2.36); it can be found that there is interfacial diffusion between graphene and the matrix. Such a phenomenon of interfacial diffusion can also be found by filtering images of the interface between the matrix and graphene (Figure 2.37), which also demonstrates that the nature of graphene is not affected. The 2D thin film structure and wrinkled structure make graphene form a good bonding interface with the matrix, and at the same time the properties of graphene are preserved, thus the graphene is able to show its unique properties in the superalloy.

Figure 2.36.

Energy spectrum scan of the interface between graphene and matrix.

50

Graphene Composite Materials

Graphene nanosheet

matrix

Figure 2.37.

Figure 2.38.

Filtering image of interface between matrix and graphene.

Graphene nanosheets at the tear ridge of tensile fracture.

In order to study the mechanism of graphene in fracture, the tensile fracture surface morphology was analyzed. Figures 2.38 and 2.39 are the fracture morphology of graphene-reinforced superalloy stretched at 650°C. It is found that there are lots of graphene nanosheets attached to the tearing ridge, forming a good bonding interface with the matrix, and it shows an elongated shape parallel to the tearing direction. The graphene sheets located at the dimples showed irregular translucent flake morphology. The fracture morphology of the graphene nanosheet indicates that

Graphene-Reinforced Metal Matrix Composites 51

Figure 2.39.

Graphene nanosheets at the dimples of tensile fracture.

stress transfer occurs during the stretching process, and the graphene bears a part of the mechanical load. In order to study the effect of the long-term aging process on the graphene structure, the long-term aging test at 650°C for 1000 h was carried out on the reinforced superalloy with 0.3 wt% graphene loading, and the phase analysis was performed. The result is shown in Table 2.12, it can be evidenced that the mass fraction of MC (metal carbides) + M3B2 (metal borides) phase in FGH96 is about 0.39, and after long-term aging (650°C + 100–5000 h), the mass fraction of MC + M3B2 phase is decreased slightly. After 0.3 wt% graphene is introduced, the interfacial diffusion of carbon element and the matrix alloy leads to an increase in the total amount of MC + M3B2 phase, but after the reinforced superalloy undergoes an aging treatment at 650°C for a long time of 1000 h, the mass fraction of MC + M3B2 phase remains unchanged, which plays a stabilizing effect. Combined with observation and analysis of microstructure and performance in Section 2.2.5 as well as mechanism research in Section 2.2.6, the reinforcing mechanism of graphene can be summarized as follows: (1) In the reinforced superalloy, interfacial diffusion functions in graphene, but the nature of graphene is not alternated, making it fully exhibit its novel properties. Graphene acts as an ultrafine particle to reinforce the matrix, and improves the strength and toughness of the reinforced superalloy through pinning dislocations and stress transfer.

52

Condition

Ti

Nb

W

Mo

Zr

Cr

Ni

Co

B

Σ

FGH96

0.158

0.066

0.049 0.05

0.017

0.037

0.006

0.002

0.006

0.391

FGH96 + long-term aging (650°C/100 h)

0.153

0.059

0.044 0.047

0.016

0.028

0.003

0.002

0.006

0.358

FGH96 + long-term aging (650°C/400 h)

0.154

0.06

0.046 0.048

0.016

0.028

0.003

0.002

0.006

0.363

FGH96 + long-term aging (650°C/1000 h)

0.158

0.061

0.046 0.048

0.017

0.023

0.002

0.002

0.006

0.363

FGH96 + long-term aging (650°C/5000 h)

0.156

0.061

0.042 0.043

0.015

0.021

0.002

0.001

0.007

0.348

FGH96 + 0.3 wt% GR

0.486

0.176

0.102 0.07

0.032

0.023

0.004

0.002

0.004

0.899

FGH96 + 0.3 wt% GR + long-term aging (650°C/1000 h)

0.473

0.171

0.103 0.086

0.032

0.025

0.003

0.002

0.005

0.900

Graphene Composite Materials

Table 2.12. Analysis results of long-term aging phase.

Graphene-Reinforced Metal Matrix Composites 53

(2) The 2D thin film microstructure and wrinkle characteristics of graphene give rise to an excellent bonding interface with the matrix, and this bonding interface can effectively prevent dislocation transfer and crack growth, thereby improving the strength and crack growth performance of the reinforced superalloy. (3) Graphene exhibits a wrinkled morphology and a large specific surface area, which effectively prevent the growth of grains during heat treatment and play a role in fine-grain strengthening, thus improving the strength of the reinforced superalloy. (4) At the same time, high wrinkle strength and plasticity of graphene make the matrix form more obvious curved and zigzag grain boundaries, which improve the creep performance of the reinforced superalloy.

2.3 Graphene-Reinforced Aluminum Matrix Composites 2.3.1 Introduction of graphene-reinforced aluminum matrix composites Due to the advantages of low cost, low density, high specific strength, excellent ductility, and machinability, aluminum and its alloys are widely used in the fields of aviation, aerospace, automotive, and electronic industries. However, common aluminum alloys are no longer able to satisfy the needs of the rapid development of modern industrial technology. In the past few decades, researchers have made great efforts in the conventional process of research to improve the mechanical properties of aluminum alloys, including alloying element adjustment, structural design, heat treatment system, and deformation process, but it is difficult to achieve huge mechanical breakthrough of aluminum alloys. Carbon-based materials as reinforcements are able to effectively improve the strength and stiffness of aluminum and its alloys. Initially, researchers conducted a lot of studies on carbon fiber or carbon nanotube-reinforced aluminum matrix composites. With the discovery of graphene, researchers have found that compared with carbon fibers and carbon nanotubes, graphene-based materials present higher strength, higher modulus, larger specific surface area, and better elongation properties, which means that graphene-reinforced aluminum matrix nanocomposites are expected to be the next-generation aluminum matrix composites. It is worth mentioning that, compared with

54

Graphene Composite Materials

particles, whiskers, and fiber reinforcements, the unique 2D structure of graphene has a new strengthening and toughening mechanism. In addition, graphene with exceptional performance such as optical, thermal, and electrical properties, as well as nano-quantum effects, is expected to endow the aluminum alloy matrix with multifunctional properties, thereby giving rise to a new type structural-functional integrated material with lightweight, superior thermally and electrically conductive properties, and remarkable processability. In the past ten years, with the rapid development of preparation and dispersion technology of graphene and graphene-like materials, researchers have attempted to incorporate graphene into the aluminum matrix, and some pioneering works have been made. The investigations find that graphene is an excellent nano-reinforcing phase for aluminum matrix composites, and the addition of a small amount of graphene can significantly improve the mechanical properties such as tensile strength and yield strength of aluminum matrix. More importantly, some research results show that after the incorporation of graphene, the elongation of graphene-enhanced aluminum matrix nanocomposites is not reduced, whereas excellent plasticity and processability are retained. The excellent mechanical properties make graphene-reinforced aluminum matrix nanocomposites show broad application prospects in the fields of aviation, aerospace, electronics, and automotive industries.

2.3.2 Preparation of graphene-reinforced aluminum matrix composites The priority in the preparation of graphene-reinforced aluminum matrix nanocomposites is to achieve uniform dispersion of graphene reinforcements, which is difficult by means of the traditional casting processes. The composite of aluminum powder and graphene is an ideal way to prevent agglomeration of graphene and to achieve its uniform dispersion. Another challenge in the preparation of graphene-reinforced aluminum matrix nanocomposites is to suppress the chemical reaction between the aluminum alloy matrix and graphene. In a wide temperature range, the solid solubility of carbon in aluminum is quite low (e.g., after treatment at 1000°C which is far beyond the melting point of aluminum alloy, the solid solubility is only about 6 × 10–4–12 × 10–4%).15 However, both elements of carbon and aluminum are thermodynamically unstable, and the

Graphene-Reinforced Metal Matrix Composites 55

chemical reaction between them can be expressed by the following chemical equations: 4 1 C(s) + Al(l) = Al4 C3(s) ∆G10 = −89611 + 32.841T 3 3

(2.1)

4 1 C(s) + Al(l) = Al4 C3(s) ∆G10 = −89611 + 32.841T 3 3

(2.2)

4 1 4 C(s) + Al(s) = Al4 C3(s) ∆G20 = ∆G10 + ∆G20 = −75211 + 17.406T (2.3) 3 3 3 where ∆G is Gibbs free energy and T is absolute temperature. From Equations (2.1) and (2.3), from room temperature to 2000 K, the standard Gibbs free energy of the reaction between aluminum and carbon to form Al4C3 is negative. Therefore, aluminum may undergo chemical reactions in the temperature range from room temperature to 2000 K to form Al4C3 phase. In terms of kinetics, the thickness of the interface reaction layer for the reaction between carbon and aluminum satisfies the relationship of equation (2.4):   Q  t Z =  2∅ D0 exp  −  RT   

(2.4)

In the equation, Z is the thickness of the interface reaction layer, D0 is the diffusion coefficient, Q is the reaction activation energy, R is the gas constant, T is temperature, t is time, and ∅ is a coefficient that depends on the carbon concentration at the interface. As shown in equation (2.4), the thickness of the interface reaction is related to the reaction temperature and time. Higher reaction temperature and longer reaction time would give rise to a thicker interface reaction layer. Therefore, in order to avoid a strong chemical reaction between graphene and aluminum matrix, in the process of preparing the composite, the preparation temperature should be reduced as much as possible, and the residence time at high temperature should be reduced at the same time (Figure 2.40). Experiments have shown that there is little or no chemical reaction between solid aluminum and carbon (perhaps the reaction rate is so slow

56

Graphene Composite Materials 800

Temperature (°C)

temperature 600 AI melt 400

thermal flow

200

Exothermic GNFs/AI recation

0 0

60

120 Time (min)

180

Figure 2.40. Differential scanning calorimetry (DSC) curve of graphene/pure aluminum composite powder.

that it is imperceptible). But when aluminum is in a molten state, they will react with each other and give an acicular aluminum carbide (Al4C3) phase.16 The resulting A14C3 is a brittle phase and is very sensitive to moisture, which will cause the bulk material to pulverize in the atmospheric environment. Therefore, the formation of Al4C3 should be avoided during the preparation of graphene-reinforced aluminum matrix nanocomposites. In order to effectively avoid the introduction of harmful Al4C3 phases during the preparation process, the metallurgical temperature of the rapid solidification/powder metallurgy (RS/PM) process adopted is far lower than that of the smelting process, which is an ideal method for preparing graphene-reinforced aluminum matrix nanocomposites. This lowtemperature synthesis process can not only effectively control the interface between the aluminum matrix and graphene but also effectively confine the grain size of the aluminum matrix. Therefore, the aluminum matrix nanocomposites prepared by RS/PM usually have better mechanical properties. Due to its simple process and high designability, RS/PM has become an effective method to prepare graphene-reinforced aluminum matrix composites.17 This process involves mechanical mixing of graphene and aluminum powder in a ball milling jar, followed by consolidation and sintering. The consolidation process includes compression molding and cold isostatic pressing, and the sintering process includes

Graphene-Reinforced Metal Matrix Composites 57

pressureless sintering, hot isostatic pressing, hot pressing, and plasma discharge sintering. In order to further improve the material density and microstructure uniformity, secondary mechanical deformation treatments are often carried out, such as hot extrusion, hot forging, and hot rolling.17

2.3.2.1 Powder mixing It is well known that the effective dispersion of nanofillers in the matrix is the primary difficulty in the preparation of nanocomposites. The desired dispersion effect is difficult to achieve by simply mixing metal aluminum powder with carbon-based nanofillers such as graphene or carbon nanotubes. This is because the van der Waals forces between nanofillers are difficult to break through conventional mixing methods and mechanical forces. High-energy ball milling, especially planetary ball milling, is able to achieve good dispersion of graphene nanosheets in aluminum alloy powder, which is one of the most common dispersion processes in the literature. High-energy ball milling involves usually placing the mixed powder of aluminum powder and nanofiller in a certain proportion into a grinding jar containing stainless steel or ceramic grinding balls, and good dispersion can be achieved by random collision of these balls with the mixed powder. Based on different conditions of the media, high-energy ball milling is divided into wet milling and dry milling. During the ball milling process, the mixed powder undergoes repeated deformation, fracture, and cold-welding processes so as to achieve uniform mixing, and the bonding between the ball-milled powders can be achieved at the atomic scale. In the process of high-energy ball milling, fine-grained phase, supersaturated solid solution, metastable crystalline phase, even amorphous alloys, and other strengthening phases can be introduced by adjusting the milling parameters (e.g., ball-to-material ratio, milling atmosphere or solvent selection, milling revolution, and milling duration). The wet ball milling process using liquid as the dispersion medium can make the graphene nanosheets better dispersed. Therefore, in the grinding process of aluminum powder and graphene, in order to prevent the oxidation of aluminum powder and improve the dispersibility of graphene nanosheets, some organic solvents (e.g., ethanol, acetone, and NMP) or lowtemperature solvents (e.g., liquid nitrogen and liquid ammonia) are usually chosen as the ball milling medium. Sometimes, in order to improve

58

Graphene Composite Materials

the dispersibility, a certain amount of organic additive is added as surfactant during the powder mixing process. Surfactants (e.g., stearic acid, acrylic acid, and silane) are easily adsorbed on the surface of the particles during the ball milling process, thereby effectively preventing cold welding and agglomeration between particles. To improve the dispersion of graphene nanosheets, graphene powders are often subjected to ultrasonic treatment and surface modification prior to ball milling. Graphene nanosheets tend to agglomerate due to the presence of van der Waals forces; ball milling of as-sonicated graphene in a liquid medium can break up the agglomerates, and the presence of surfactants is helpful to maintain the dispersed state of graphene nanosheets. The addition of surfactants is a common method to improve the dispersion of graphene nanosheets and keep the stability of solution. Hydrophilic or hydrophobic surfactants can be physically adsorbed on the surface of graphene nanosheets to realize the dispersion of graphene nanosheets in different solutions. Repulsive force or steric hindrance can effectively prevent graphene nanosheets from agglomerating, resulting in a colloidal suspension. Sodium dodecylbenzenesulfonate (SDBS), sodium dodecyl sulfate (SDS), cetylammonium bromide (CTAB), and octylphenol ethoxylate (TritonX-100) are known to be effective to stabilize graphene nanosheets.17 In contrast to hydrophobic graphene, amphiphilic GO is easier to disperse in various solvents, so GO is often used as the raw material of graphene-reinforced metal matrix nanocomposites. The existence of a large number of organic functional groups (especially acidic groups) on the surface of GO makes it negatively charged. On the one hand, the surface of the aluminum powder loses electrons and becomes positively charged due to the friction of grinding balls in the rotating shell. On the other hand, oxidized film on the aluminum powder is removed by corrosion of acidic groups and mechanical exfoliation, and the fresh exposed surfaces are positively charged because they are prone to losing electrons. In this way, the negatively charged GO is easily adsorbed to the surface of the positively charged aluminum powder through electrostatic interaction; and under the striking with grinding balls, a tight bonding is generated between GO and aluminum powder (see Figure 2.41). Although the high-energy ball milling process can simply and effectively realize the dispersion and composition of graphene in aluminum powder, it is also easy to introduce impurities and structural defects. Figure 2.42 shows the Raman spectrum results of graphene nanosheets with different milling durations. The intensity ratio between the D and G

Graphene-Reinforced Metal Matrix Composites 59

Figure 2.41. SEM morphology of graphene/pure aluminum composite powder ball milled for 20 h.

bands is used to evaluate the degree of disorderliness/structural defect of the graphene structure.18 The research shows that the peak intensity ratio ID/IG of the Raman spectra for graphene nanosheets after ball milling is significantly improved due to the crystal defects and lattice distortions introduced by high-energy ball milling. Apart from high-energy ball milling, another method for improving the dispersibility of graphene nanosheets in aluminum matrix is to establish the adsorption effect between aluminum particles and graphene molecules. Based on the adsorption theory, researchers have proposed two strategies. The first one is to make better compatibility between aluminum particles and graphene by surface modification of aluminum particles. Highly hydrophilic polymers such as polyvinyl alcohol (PVA) are one category of the typical surface modifiers. In fact, the surface of the as-modified aluminum particles can be effectively wrapped by graphene nanosheets through hydrogen–oxygen bonding. Ultimately, the surface modifier is removed by a pyrolysis reaction during the subsequent thermal process. Because the adsorption capacity of graphene nanosheets is significantly improved, the coating of aluminum particles can be achieved very effectively through adsorption, thereby obtaining uniform dispersion of graphene nanosheets. In practice, if the surface modifier is not fully pyrolyzed, it will remain in the material as an impurity phase. To simplify the adsorption process and avoid unnecessary doping, a specific electrostatic adsorption method based on the electrostatic interaction between graphene and aluminum particles has subsequently emerged. In this method, GO nanosheets are first dispersed into an aqueous solution, and

60

Graphene Composite Materials

Figure 2.42. Raman spectra of graphene/aluminum mixed powder with 2.0 wt% graphene loading under various milling durations and pristine graphene.18

then aluminum particles are mixed and stirred continuously for a certain period of time. GO is negatively charged in aqueous solution due to the presence of ionized groups such as carboxyl and hydroxyl groups, whereas aluminum particles are positively charged due to ionization on their surface. The electrostatic attraction between GO and aluminum particles is crucial for their uniform mixing. Because the adsorption process is simple and low cost, and it does not cause damage to the graphene structure, it is widely used for compositing aluminum powder and graphene.

2.3.2.2 Metal-forming process After the graphene nanosheets are uniformly dispersed in aluminum particles, a suitable metal-forming process is essential to obtain graphenereinforced aluminum matrix nanocomposites with excellent properties. Metal forming is to form good interfacial bonding between the mixed powders by heating and pressing, and to eliminate the pores in the material to achieve a fully dense product. Pressureless sintering is one of the simplest methods to obtain graphene-reinforced aluminum matrix nanocomposites. The graphene/aluminum mixture is first cold pressed into a preform using a molding/cold isostatic pressing process, and then the

Graphene-Reinforced Metal Matrix Composites 61

preform is sintered in air or a protective atmosphere. The thermal energy applied during sintering is able to consolidate and densify the preform, which is accompanied by an increase in the grain size of the material. Atomic diffusion is the most important sintering mechanism, and sufficient temperature (over 80% of the melting point of the sintered phase) is the guarantee to achieve activated diffusion. For aluminum matrix composites, the aluminum oxide film on the outer surface of aluminum particles will hinder the diffusion of aluminum atoms. Increasing the sintering temperature is an option to promote the sintering and molding of aluminum matrix composites, but increasing the sintering temperature will lead to grain coarsening of the aluminum matrix, and even cause undesired interface reactions between aluminum and graphene. According to the Hall–Petch relation, once the grain coarsening occurs, the strength of the composite will drop significantly. The formation of the Al4C3 interfacial phase will directly consume a part of the graphene-reinforcing phase and hinder the transfer of shear stress from the matrix phase to the graphene-reinforcing phase, thereby significantly affecting the mechanical properties of graphene-reinforced aluminum matrix composites. Pressureassisted sintering is another option to obtain better densification without additional problems. On the one hand, additional pressure increases the plastic deformation and creep of the material during sintering, which is another driving force for material densification; on the other hand, the applied external pressure does not lead to grain growth in the aluminum matrix. Since the densification rate is increased by external pressure, this method can lower the sintering temperature, shorten the sintering time, and suppress grain growth. At present, a variety of pressure-assisted sintering techniques have been developed, such as hot pressing sintering, hot isostatic pressing, and spark plasma sintering. The hot pressing (HP) sintering method employs unidirectional pressure to assist sintering, which metallurgically shapes the powder in a mold under moderate temperature, vacuum, or protective atmosphere, and under certain pressure. By improving the heating method, a new rapid molding technology of HP sintering has been developed, which is also known as spark plasma sintering (SPS). The schematic of the SPS process is shown in Figure 2.43, and the experimental setup is very similar to that of HP sintering. Heating of the powder compact is provided by a pulsed current at a pulsed DC voltage, typically achieving ultra-high heating rates in excess of several hundred degrees per minute. Compared to conventional methods, the densification process of SPS is much faster, taking

62

Graphene Composite Materials

Figure 2.43.

Schematic diagram of spark plasma sintering (SPS).19

only a couple of minutes. SPS is an energy-saving, time-saving, and efficient method for sintering graphene-reinforced aluminum matrix nanocomposites. In addition, due to the lower sintering temperature and shorter sintering time, the grain coarsening of the aluminum matrix can be effectively suppressed, which makes it an ideal method for the preparation of nanocrystalline metal matrix composites. By improving the pressing method, another different pressure-assisted sintering method, the socalled hot isostatic pressing (HIP), is developed. HIP encapsulates powder compacts in a closed container for HP consolidation, and the pressure is generated by compressing inert gas. Despite the complex process and demanding equipment requirements, the properties of metal matrix composites prepared by HIP are generally excellent. Due to the high uniform pressure, the sintering temperature required for this process can be reduced by 10% to 15%, which can inhibit the grain growth of the aluminum matrix and the chemical reaction between the graphene nanofiller and the aluminum matrix; hence, it is beneficial to improve the performance of composite materials. More importantly, HIP can effectively reduce the number of pores in the material and increase the density of the material, and at the same time, the microstructure uniformity of the composite material is greatly improved. In addition to the sintering process, deformation treatments such as hot forging, hot rolling, and hot extrusion are also applied in the

Graphene-Reinforced Metal Matrix Composites 63

preparation of graphene-reinforced aluminum matrix nanocomposites to further improve the density and uniformity of the material. For example, Shin et al. containerized the mixed powder of multi-layer graphene and aluminum microparticles into a copper tube for repeated hot rolling, wherein the hot rolling temperature was 500°C.20 After rolling, the copper tube was mechanically removed, and the synthesized graphene-reinforced Al matrix nanocomposites exhibit dense structures and excellent mechanical properties. Friction stir process (FSP) developed from modification of friction stir welding has also been used to prepare graphene-reinforced aluminum matrix nanocomposites. As shown in Figure 2.44, intense heat is generated by the friction between the rotating shoulder and the workpiece to soften the machining area. A rotating pin is used to agitate the interior of material so as to generate plastic deformation and material mixing, resulting in densification, homogenization, and refinement of the microstructure. It was originally used by Mishra et al. in 1999 for the preparation of superplastic aluminum with fine grains.22 In the preparation of graphenereinforced aluminum matrix nanocomposites, Jeon et al. directly introduced GO aqueous solution to the surface of metal substrates in a colloidal form.23 During the FSP process, water is evaporated by the heat generated by the friction, and the GO is thermally reduced to graphene, which is

Figure 2.44.

Schematic of friction stir processing (FSP).21

64

Graphene Composite Materials

uniformly mixed and incorporated into the aluminum matrix. The results show that the tensile strength of graphene-reinforced aluminum matrix nanocomposites prepared by the FSP method is not ideal, but its elongation is effectively improved.

2.3.3 Microstructure and properties of graphene-reinforced aluminum matrix composites Recently, researchers have used various improved methods to prepare graphene-enhanced aluminum matrix nanocomposites with excellent performance. In the past few years, the structure and corresponding properties of graphene-reinforced aluminum matrix composites prepared by various powder metallurgy methods have also been extensively studied. Previous studies on graphene-reinforced aluminum matrix nanocomposites are summarized and listed in Table 2.13, including composition, properties, production technique, and corresponding mechanical properties. The microstructure of graphene-reinforced aluminum matrix nanocomposites is determined by the preparation process, and its properties are determined by its microstructure. Similar to other nanostructured composites, the dispersion of graphene nanosheets in the aluminum matrix, as well as the interfacial bonding strength between graphene and the aluminum matrix, can significantly affect the mechanical properties of graphene-reinforced aluminum matrix nanocomposites. The metal-forming process of graphene-reinforced Al matrix nanocomposites is also crucial to achieve excellent mechanical properties. Graphene agglomerates or pores induced by improper powder metallurgy processes are important defects in composites, which greatly deteriorate the properties of graphene-reinforced aluminum matrix nanocomposite bulks. On the other hand, the higher molding temperature used to increase the material density may lead to a chemical reaction between graphene and the aluminum matrix to give the Al4C3 phase, resulting in poor interfacial strength. In the meantime, higher forming temperature will lead to coarsening of aluminum matrix grains, which is also detrimental to material properties. Bartolucci et al. adopted powder metallurgy to prepare graphenereinforced aluminum matrix nanocomposites with 0.1 wt% graphene loading.24 The atomized pure aluminum powder and the thermally exfoliated reduced graphene nanosheets were blended in a powder mixer for

Graphene-Reinforced Metal Matrix Composites 65 Table 2.13. Production technique and corresponding mechanical properties of some recently reported graphene-reinforced aluminum matrix nanocomposites. Composition 0.1 wt% GR/Al

Production technique

Mechanical properties

Refs.

Ball milling → Hot isostatic pressing (375°C, 20 min) → Hot extrusion (550°C, extrusion ratio 4:1)

σb: 262 MPa, δ: 2.6%, HV: 99 ± 5 (as-sintered), 84 ± 5 (as-extruded)

24

0.15 wt% GR/2024 Ball milling → Hot isostatic σ0.2: 262 MPa, σb: 400 MPa, 11,25 pressing (480°C, 2 h) → Hot δ: 12% extrusion (450°C, extrusion 0.5 wt% GR/2024 σ0.2: 319 MPa, σb: 467 MPa, ratio 16:1) → T4 heat δ: 12% treatment (480°C, 30 min, water cooling) 0.3 wt% GR/Al

Mechanical stirring → Sintering σ0.2: 195 MPa, σb: 280 MPa, (600°C, 4 h) → Hot extrusion δ: 9.5% (470°C, extrusion ratio 4:1)

26

0.3 wt% GR/Al

Mechanical stirring → Sintering σb: 249 MPa, δ: 13% (580°C, 2 h) → Hot extrusion (440°C, extrusion ratio 20:1)

10

0.3 wt% GR/Al

Mechanical stirring → Hot pressing sintering (530°C, 1 h, 600 MPa)

Elastic modulus: 90 GPa, Hardness: 1.59 GPa

27

0.3 vol% GR/Al

Ball milling → Hot rolled (500°C)

σb: 370 MPa, δ: 6%

20

0.5 vol% GR/Al

σb: 410 MPa, δ: 4%

0.7 vol% GR/Al

σb: 450 MPa, δ: 3%

0.3 vol% GR/2024

σb: 510 MPa, δ: 4.5%

0.5 vol% GR/2024

σb: 630 MPa, δ: 4%

0.7 vol% GR/2024

σb: 750 MPa, δ: 3.5%

1 wt% GR/6061

Ball milling → Hot pressing sintering (630°C, 10 min, 100 MPa)

Bending strength: 320 MPa (ball milling for 10 min), 380 MPa (ball milling for 30 min), 720 MPa (ball milling for 60 min), 800 MPa (ball milling for 90 min)

18

GR/5052

FSP

σb: 192 MPa, δ: 28%

23

0.5 wt% GR/Al

Ball milling → SPS (560°C, 4 min, 30 MPa)

σb: 142 MPa, δ: 18.5%

—

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Graphene Composite Materials

(a)

(b)

Figure 2.45. Optical micrographs of (a) aluminum matrix nanocomposites with 0.1 wt% graphene loading and (b) pure aluminum.24

5 min, and then milled in a high-energy ball miller for 1 h under an argon protective atmosphere. Stearic acid (2.0%) was used as a surfactant to minimize the cold-welding effect and prevent the agglomeration of graphene nanosheets during ball milling. The consolidation process of mixed powder was performed via HIP at 375°C for 20 min, followed by hot extrusion at 550°C with an extrusion ratio of 4:1. Figure 2.45 shows the optical micrographs of polished and etched aluminum matrix nanocomposite samples with 0.1 wt% graphene loading. Though it is very difficult to make a quantitative measurement on grain size of the material after powder metallurgy and hot extrusion, the finer-grain structure of graphene-reinforced aluminum matrix nanocomposites is observed when compared with pure aluminum, which will result in the improvement of the mechanical properties. Despite a finer microstructure than the pure aluminum that will improve the mechanical properties of nanocomposite, the formation of aluminum carbide in the graphene nanocomposite still leads to a deleterious effect on mechanical properties. The existence of the aluminum carbide phase in the hot-extruded graphene-reinforced aluminum matrix nanocomposite can be confirmed by the X-ray diffraction (XRD) pattern shown in Figure 2.46, which may be caused by the high extrusion temperature. Compared with Bartolucci’s research, Yan’s team of AECC Beijing Institute of Aeronautical Materials fabricated graphene-reinforced aluminum matrix nanocomposites by the RS/PM process, where hot extrusion with a lower temperature was adopted, and atomized Al-Mg-Cu alloy

Graphene-Reinforced Metal Matrix Composites 67

Figure 2.46. XRD patterns of pure aluminum, 1.0 wt% MWNT/Al composite, and 0.1 wt% graphene/Al composite.24

Figure 2.47. contents.25

XRD patterns of GR/2024 nanocomposites with different graphene

powder and GO nanosheets were taken as raw materials.11,25 The prepared graphene-reinforced aluminum matrix nanocomposites were subjected to hot extrusion at 450°C with an extrusion ratio of 16:1. The extruded graphene-reinforced aluminum matrix nanocomposite rods were heat treated at 495°C for 0.5 h, and then quenched in water. Figure 2.47 shows the XRD pattern of graphene-reinforced aluminum matrix nanocomposites, in

68

Graphene Composite Materials

Figure 2.48. TEM images of GR/2024 nanocomposite with 0.15 wt% graphene.25

which no peaks of aluminum carbide phase are observed. At the same time, intact graphene nanosheets in the form of veils are observed in the TEM images of the extruded rods of graphene-reinforced aluminum matrix nanocomposites with 0.15 wt% graphene, as shown in Figure 2.48. In addition, good interfacial bonding between graphene and aluminum matrix can be observed by TEM which play an important role in improving the mechanical properties of the composites. Meanwhile, the evidence of the intact graphene nanosheets in the TEM image directly proves that graphene nanosheets can be successfully incorporated into the aluminum matrix without damaging its inherent structure by means of the powder metallurgy. When 0.15 wt% GR and 0.50 wt% GR are incorporated in Al-Mg-Cu alloy, the tensile strength of the alloy increases from 373 MPa to 400 MPa and 467 MPa, respectively, and the yield strength is increased from 214 MPa to 262 MPa and 319 MPa, respectively. More importantly, the plasticity of graphene-reinforced aluminum matrix nanocomposites does not decrease when the yield strength is increased by nearly 50%, i.e., a phenomenon completely different from other fillerreinforced metal matrix composites. The authors believe that this may be due to the wrinkled structure of graphene nanosheets and their good interfacial bonding with the aluminum matrix. The wrinkled graphene nanosheets are straightened during the initial plastic deformation of the graphene-reinforced aluminum matrix nanocomposite, and then the graphene is pulled out from the aluminum matrix and fractured. This special

Graphene-Reinforced Metal Matrix Composites 69

Figure 2.49.

Fracture morphology of GR/2024 nanocomposites with 0.5 wt% graphene.25

synergistic deformation mechanism of graphene nanosheets in aluminum matrix is helpful to improve the ductility of graphene-reinforced aluminum matrix nanocomposites. In addition, the pulling-out scenario of graphene nanosheets on the tensile fracture of the sample with 0.5% graphene loading is clearly evidenced in Figure 2.49. Recently, Shin et al. from Korea reported graphene-reinforced aluminum matrix nanocomposites fabricated by hot rolling.20 First, graphite flakes with a thickness of 6–8 nm were mechanically exfoliated by planetary ball milling in the presence of isopropyl alcohol at a ball-to-material ratio of 15:1. Planetary milling was performed at a rotation speed of 200 rpm for 1 h. In addition, in order to maintain ambient processing temperature in the mill jar, intermittent ball milling is used. Then, the ball-milled graphene flakes were further ball milled with aluminum powder and a process control agent of 1.0 wt% stearic acid, and a similar intermittent milling was performed at 100 rpm for 3 h. Figures 2.50(a) and 2.50(b) show the SEM images of the as-mixed powder, in which exfoliated graphene flakes with a size below 10 µm can be evidenced on the surface of aluminum. The graphene nanoflakes are thin enough that the morphology of the aluminum powder underneath the graphene can be clearly observed. In the end, the mixed powder was subjected to highenergy ball milling at 500 rpm for 6 h under the protection of argon atmosphere. Graphene is embedded and dispersed inside the aluminum matrix during high-energy ball milling, because graphene nanosheets are not observed on the surface of aluminum particles in SEM images, not

70

Graphene Composite Materials (a)

(b)

(c)

(d)

Figure 2.50. SEM images of (a) graphene attached to Al powder using a planetary mill at 100 rpm (graphene is marked by an arrow), (c) graphene embedded and dispersed in Al powder using an attrition mill at 500 rpm, (b) and (d) display the magnified images of (a) and (c), respectively.20

observed even in the magnified images, as shown in Figures 2.50(c) and 2.50(d). The sintering of the ball-milled powder is accomplished by a twostep process of compaction and hot rolling. The hot rolling temperature was 500°C, and rolling was conducted with a 12% reduction per pass, until a fully dense graphene-reinforced aluminum matrix composite was obtained. The graphene nanosheets dispersed in the aluminum matrix were observed by TEM, and the graphene aligned along the rolling direction is slightly wrinkled, as shown in Figure 2.51. The average number of graphene layer in the graphene-reinforced aluminum matrix nanocomposite is about 5, and the yield strength of the material is increased by 160% by refining grains of the aluminum matrix via high-energy ball milling. When 0.7 vol% reinforcing graphene is incorporated, the yield strength of the composites increased from 262 MPa to 440 MPa, while the elongation decreased from 13% to 3%.

Graphene-Reinforced Metal Matrix Composites 71 (a)

(c)

(b)

(d)

(e)

Figure 2.51. TEM images of the graphene in the hot-rolled GR/Al nanocomposites with 0.3 vol% graphene (graphene is marked by white arrow) observed on the (a) RD (rolling direction)–TD (transverse direction) plane, (b) ND (normal direction)–RD plane. In (c) and (d), graphenes (marked by red lines) are given. (e) Between the graphenes (marked by white arrow), highly deformed regions (marked by circles) are observed at 6% deformation.20

Wang et al. fabricated graphene-reinforced pure aluminum matrix nanocomposites based on flake powder metallurgy.10 They used GO nanosheets as the raw material, and numerous hydroxyl and epoxy groups on the surface of GO are considered to promote the dispersion and provide a stable solution. The spherical aluminum powder particles are ground into flakes via ball milling, and the obtained higher specific surface area is more suitable for adsorbing GO nanosheets. Highly hydrophilic polymer PVA was adopted as a binder, and GO nanosheets can be bonded to the flake surface of flaky aluminum particles. After treatment with 3.0 wt% PVA aqueous solution, the aluminum flakes were added to deionized water to form a powder slurry. The GO aqueous dispersion was added dropwise to

72

Graphene Composite Materials (a)

(b)

Figure 2.52. Comparison of SEM images of Al flake surface with and without adsorbed GO nanosheets.10

the flaky aluminum slurry and stirred, and then the GO/Al composite powders were obtained by filtration. Graphene nanosheets with fine wrinkles but not smooth morphology on the surface of aluminum flakes are observed by SEM (as shown in Figure 2.52), which indicates that dispersion of GO nanosheets in aluminum matrix is excellent. The binder PVA is completely decomposed after heat treatment at 550°C for 2 h in flowing Ar. Meanwhile, the GO nanosheets were effectively reduced to graphene nanosheets. Fourier Transform Infrared Spectroscopy (FTIR) provides convincing proof that GO nanosheets are effectively transformed to graphene nanosheets by thermal reduction, as shown in Figure 2.53. After thermal reduction, the characteristic bands of O─H, C═O and C─O strenching vibrations are weakened or even disappeared, while the vibration peaks of the graphene structure are enhanced. The graphene/pure aluminum composite powder was compacted into a billet and sintered in Ar atmosphere at 580°C for 2 h. Then, the consolidated graphene-reinforced aluminum matrix nanocomposites were hot extruded at 440°C with an extrusion ratio of 20:1 to obtain dense composites. Finally, the mechanical properties of graphene-reinforced aluminum matrix nanocomposites were tested. The results show that the tensile strength of the composites with only 0.3% graphene loading is increased by 62%, i.e., from 154 MPa to 249 MPa. The reinforcing effect significantly exceeds that of any other reinforcing phase. This demonstrates that graphene has great application potential as the most ideal reinforcing phase for aluminum matrix nanocomposites.

Graphene-Reinforced Metal Matrix Composites 73

(a)

(b)

Figure 2.53. (a) Structure of the graphene and GO, (b) FTIR of the GO and reduced GO by rapid heating to 550°C in a flowing Ar atmosphere.10

2.3.4 Reinforcing mechanism of graphene Graphene exhibits extraordinary properties such as exceptionally high elastic modulus and strength, large specific surface area, low density, and high thermal stability. Therefore, much attention has been paid to the research of the synthesis process and application technology in the field of graphene. Incorporating graphene nanosheets into aluminum and aluminum alloy matrix to increase the strength of aluminum matrix composites is one of the engineering applications of graphene that has been developed in recent years. It has been reported that the addition of a small amount of graphene nanofillers can significantly improve the mechanical properties of the aluminum matrix, such as tensile strength, yield strength, flexural strength, and hardness. The reinforcement efficiency of graphene is higher than that of multi-walled carbon nanotubes because of its larger surface area, which enables the introduction of more interfaces into the

74

Graphene Composite Materials

composite. The analysis and research on the reinforcing mechanism of graphene–aluminum nanocomposites can help us better understand the role of graphene in aluminum matrix, which can also provide insights into the preparation and application of graphene–aluminum nanocomposites. According to previous studies, nano-reinforcement mainly enhances the strength of the metal matrix through thermal mismatch, grain refinement, dislocation, and stress transfer strengthening mechanism. In the following, the effects of these four mechanisms on the strengthening of graphene-reinforced aluminum matrix nanocomposites will be discussed.

2.3.4.1 Thermal mismatch strengthening Since the coefficients of thermal expansion of the reinforcement and the matrix are different, the thermal deformation of the reinforcement and the matrix is not same as the temperature changes, resulting in thermal residual stress of the matrix around reinforcements. Due to these thermal residual stresses, the dislocation density of the matrix around reinforcements is raised, thereby causing a strengthening effect, i.e., thermal mismatch strengthening. However, the influence of thermal mismatch strengthening on the matrix is available within a certain region; only the matrix around the reinforcements is strengthened, and the matrix far from the reinforcement is not affected by the thermal mismatch between the reinforcements and the matrix.28 The strength of such mismach region is higher than that of the general matrix, and it plays a bearing role in the deformation process of the composite material. The thermal mismatch strengthening mechanism is in charge of strengthening of the entire composite material by means of these thermal mismatch strengthening regions. The coefficients of thermal expansion for graphene and pure aluminum are 1 × 10–6/K and 2.3 × 10–5/K, respectively, i.e., there is a 20-fold difference. In addition, graphene has a large surface area, which means a bunch of interfaces can be created between graphene and the aluminum matrix, thereby resulting in a larger thermal mismatch strengthening region. Therefore, graphene theoretically contributes a lot to the strengthening of the aluminum matrix through thermal mismatch. At present, the research on thermal mismatch strengthening of graphene-reinforced aluminum matrix nanocomposites is still relatively quite limited, and it is urgently required to carry out relevant theoretical research through mathematical modeling and computer simulation.

Graphene-Reinforced Metal Matrix Composites 75

2.3.4.2 Grain refinement strengthening Grain boundaries act as an impediment to dislocation motion. When the dislocation within the grain moves to the grain boundary, it will be difficult to continue to advance, and it will accumulate at the grain boundary. The grain boundary blocks the movement of dislocations and has a strengthening effect. Thus, grain refinement provides an important means to improve strength and toughness of composite materials. There are many slip systems in aluminum alloys, so that the barrier effect of grain boundaries on dislocation movement is more significant. Therefore, grain refinement strengthening is a predominated strengthening mechanism in aluminum matrix composites. The Hall–Petch equation (2.5) is a generally accepted theoretical model for calculating the effect of grain refinement on yield strength29:

σ = σ 0 + Kd −1/ 2

(2.5)

In the equation, σ is the yield strength, σ 0 is the lattice friction that needs to be overcome to move a unit of dislocation, d is the average grain diameter, and K is a constant. It is evidenced that the grain size has a significant effect on the yield strength of the material, and the smaller the grain size, the greater the yield strength of the material, and vice versa. In the powder metallurgy, there are hot and cold deformation processes such as ball milling, hot extrusion, and hot rolling, which will produce significant grain refinement of the aluminum matrix. The process of sintering and heat treatment of aluminum matrix nanocomposites is often accompanied by recrystallization and grain growth of the aluminum matrix. The presence of graphene has a significant effect on recrystallization and grain growth. On the one hand, graphene is able to pin the large-angle grain boundaries and impede the growth of grains. On the other hand, graphene is also capable of promoting the recrystallization and nucleation of the aluminum matrix, so that ultrafine grains can be obtained in the composite material. The utilization of the role of graphene in the aluminum matrix, as well as the design of a reasonable deformation and heat treatment process, is helpful to obtain ultrafine-grained graphene–aluminum nanocomposites. Grain refinement strengthening can not only improve the yield strength and tensile strength and other strengths of the material but can also significantly improve the plasticity such as the section shrinkage and elongation

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Graphene Composite Materials

of the material, thereby comprehensively improving the mechanical properties of the composite material.

2.3.4.3 Dislocation strengthening The higher the dislocation density in the metal, the easier it is for the dislocations to cross each other when they move; jogs and kinks are formed, resulting in dislocation entanglement which hampers the motion of dislocations and prevents any further permanent plastic deformation. This greatly increases the strength of the metal, and such a method of improving the strength of a metal by increasing the dislocation density is called dislocation strengthening. Dislocation strengthening is one of the most effective strengthening methods for metal materials. Since the dislocation theory was proposed, a lot of studies have been done on the interaction between dislocations, and great progress has been made in dislocation strengthening. The relationship between the flow stress τ (as well as the yield strength) of the metallic material and the dislocation density ρ can be expressed by the following Equation (2.6):

τ = aMGb ρ

(2.6)

In the equation, a is a material-dependent constant, M is the Taylor factor, G represents the lattice friction that needs to be overcome to move a unit of dislocation, and b is the Burger vector. One can see from Equation (2.6) that the strength of the material is proportional to the root square of the dislocation density, i.e., higher dislocation density gives rise to higher strength of the material. Metal aluminum crystallizes in a face-centered cubic structure, which presents 12 slip systems, and the chances of the dislocation motion on a slip system are relatively small, hence the dislocation density of aluminum metal is usually low. The introduction of graphene has a significant effect on increasing the dislocation density of Al matrix nanocomposites. Shin measured the dislocation densities of aluminum alloy 2024 (Al2024), ball-milled Al2024, and GR/Al2024 nanocomposites with 0.7 vol% graphene in plastically deformed and undeformed states by XRD peak broadening.20 The dislocation density of undeformed Al2024 is 1.5 × 1014 m−2, and the dislocation densities of ball-milled Al2024 and GR/Al2024 composite are 1.8 × 1014 m−2 and 3.67 × 1014 m−2, respectively. One can see that ball milling can slightly increase the dislocation density of Al2024

Graphene-Reinforced Metal Matrix Composites 77

(20% increment), whereas the dislocation density of the composites with introduction of graphene is greatly increased (145% increment). The dislocation densities of ball-milled Al2024 and GR/Al2024 composites subjected to plastic deformation are increased to 3.55 × 1014 m−2 and 8.56 × 1014 m−2, respectively, which are 137% and 471% higher than that of undeformed Al2024, respectively. It is thus clear that graphene can significantly increase the dislocation density of the matrix aluminum alloy and cause significant dislocation strengthening during the plastic deformation of the composite material.

2.3.4.4 Stress transfer strengthening Whether it is thermal mismatch strengthening, grain refinement strengthening, or dislocation strengthening, graphene exerts its strengthening effect indirectly by affecting the dislocation motion in the aluminum alloy matrix. In addition to the aforementioned three indirect strengthening mechanisms, graphene can also play a stress transfer strengthening role by directly bearing the load transferred from the aluminum matrix. Shin et al. compared the strengthening effect of carbon nanotubes and graphene on the pure aluminum matrix, and found that the short-fiber model can be used to explain the strength change of the composite materials with the contents of carbon nanotubes and graphene.20 According to the short-fiber model, the yield strength of the composite σc can be expressed as the following Equation (2.7):  S τ  σ c = Vr    m  + σ m (1 − Vr )  A  2 

(2.7)

In this equation, Vr is the volume fraction of reinforcements, S is the interface area between the reinforcement and the matrix, A is the cross-sectional area of carbon nanotubes or graphene nanosheets, τ m is the shear strength of the matrix, and σm is the yield strength of the matrix. Hence, one can see that under the premise that the reinforcements are uniformly dispersed and excellent interfaces with the matrix are formed, the yield strength of the composite material is positively correlated with the yield strength of the matrix, and the shear strength is positively correlated with the volume fraction of the reinforcements. The shear strength and yield strength of the matrix material can be improved by

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Graphene Composite Materials

indirect strengthening mechanisms such as thermal mismatch strengthening, grain refinement strengthening, and dislocation strengthening, thereby improving the yield strength of composite materials. In contrast, increasing the volume fraction of the reinforcement directly contributes to the improvement of the yield strength of the composite material through the mechanism of stress transfer strengthening. The morphological characteristics of the reinforcements also have a significant effect on the strengthening efficiency of the composites. Compared with carbon nanotubes, graphene has a larger surface area, and more interfaces with the matrix aluminum can be formed, which is beneficial to the transfer of loads. According to the calculation on the basis of the volume fraction of the reinforcement, the reinforcing effect of graphene on the matrix aluminum is three times that of carbon nanotubes. The good interfacial bonding of graphene with the aluminum matrix is the key to obtaining excellent mechanical properties. The interfacial bonding of graphene-reinforced aluminum matrix composites is facing many challenges, the most important of which is how to increase the wettability of graphene and aluminum matrix, and how to inhibit the chemical reaction between graphene and the aluminum matrix at high temperature. As for the interface wetting issue, other elements can be considered to optimize the matrix composition, and the graphene surface can be chemically treated by microwave plasma, CVD in-situ growth, or electroless plating. Since the microscopic bonding mechanism of the interface between graphene and aluminum matrix in composite materials is very complex, it is difficult to carry out systematic research experimentally, so the method of computational simulation is more and more widely used in this field. The typical process includes establishing a mathematical model and simulating the experimental process through a computer to find the best experimental plan and process, and then verifying it through the experimental results. This method can shorten the research cycle. Through a combination of theory and practice, an optimized preparation process could be formulated to obtain graphene-reinforced aluminum matrix nanocomposites with excellent properties. The reinforcement of graphene to the aluminum matrix is well recognized, but the effect on plasticity is not. Some researchers have found that the addition of graphene nanosheets is able to slightly reduce the elongation of aluminum alloys. However, it has also been reported that the

Graphene-Reinforced Metal Matrix Composites 79

addition of graphene can improve the elongation of aluminum alloys. The enhancing mechanism of graphene needs to be further studied. The mechanical properties of graphene-reinforced aluminum matrix nanocomposites are mainly determined by the properties of the matrix, the dispersion of graphene, the interfacial bonding, and the adopted preparation process. Although the ball milling process is prone to introducing impurities and damaging the structure of graphene nanosheets, it still proves to be the most effective and economical means to uniformly disperse graphene nanofillers into aluminum matrix. Suspension adsorption is a newly developed technique to disperse functionalized carbon nanotubes and graphene into aqueous solutions and adsorb onto aluminum particles. This may be a feasible and effective process to uniformly disperse graphene nanofillers into aluminum matrix without damaging or contaminating the graphene reinforcing phase. The low-temperature sintering process can effectively inhibit the interfacial chemical reaction and it is very suitable for the preparation of graphene-reinforced aluminum matrix nanocomposites, but the process parameters should be further optimized to obtain excellent interfacial bonding. During the deformation process, whether the load can be transferred from the aluminum matrix to the graphene nanosheet reinforcing phase is mainly determined by the interfacial properties. Therefore, excellent interfacial bonding is crucial for improving the mechanical properties of graphene-reinforced aluminum matrix composites. On the basis of the mixing law and the ultra-high strength of graphene, the strength of aluminum and aluminum alloys will be significantly increased by incorporating a small amount of graphene nanosheets. But, so far, the reported experimental data are still far from the theoretical calculation results. The development of the graphene-reinforced aluminum matrix nanocomposite is still in its infancy, and its process parameters, microstructures, interface reactions, and bonding states have not been fully understood, which leaves a lot of room for further improvement of its mechanical properties. In particular, the theoretical research on the strengthening and toughening mechanism of graphene-reinforcing phase is not enough. In addition, the physical properties such as corrosion resistance, electrical and thermal properties of graphene-reinforced aluminum matrix nanocomposites have been poorly studied. In the meantime, the future of the low-cost, large-scale preparation process (such as casting method) of graphene-reinforced aluminum matrix nanocomposites is still worth anticipating.

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Graphene Composite Materials

2.4 Graphene-Reinforced Titanium Matrix Composites 2.4.1 Introduction of graphene-reinforced titanium matrix composites Titanium and titanium alloys are important structural metal materials that have been developed and applied since the early 1950s. Due to their superior properties such as high specific strength, excellent corrosion resistance, and wide operating temperature, they have quickly become key materials in the aerospace field. In particular, they can be used for components and parts in aircraft and engines that require high strength and higher temperature resistance, and good weight reduction benefits are also achieved. In a sense, the amount and of titanium alloys used in the weapons and equipment have become an important indicator for the advanced level, and it greatly influences the combat readiness. For example, the amount of titanium alloys used in the fourth-generation fighter F-22 of the United States has reached 41% of the total weight. The amount of titanium alloy used in the fuselage of new civil aircraft is also increasing. In Boeing B777 aircraft, the amount of titanium alloy has reached 7% of the whole structure. In the aerospace field, titanium alloys are mainly used to manufacture pressure vessels for launch vehicles, some satellite structural components, and components that require high strength and good thermal strength in strategic missile bodies. The titanium alloys (including titanium–aluminum intermetallic compounds) can work in the temperature range from below room temperature to 900°C for long time, and for near-α titanium alloys with high temperature resistance, 600°C is considered to be the “thermal barrier” temperature; further improvement of the working temperature is limited by their performance such as creep, durability, microstructure stability, oxidation resistance, and flame retardancy.30 With the continuous development of aerospace, weaponry, and other fields, the comprehensive performance of materials also requires constant improvement. For example, although the design scheme of the overall structure of the high-pressure compressor components of advanced aeroengines can improve the thrust-to-weight ratio, it also greatly increases the bearing capacity of the components at high temperature. With the elevation of operating temperature, the high-temperature performance of materials, especially the creep properties, is becoming more and more

Graphene-Reinforced Metal Matrix Composites 81

important. The performance of titanium alloys prepared by conventional technology has approached or reached their theoretical limit. Researchers have tried to use laser cladding,31 micro-arc oxidation,32 and other surface modification technologies and alloying methods to improve the wear resistance and high temperature resistance of the titanium alloy surface, but the comprehensive mechanical properties of the titanium alloy matrix have not been effectively improved. Therefore, it is necessary to explore new and transformative technologies to reinforce titanium alloys. The development of titanium matrix composite materials through composite strengthening has become another solution. The preparation of titanium matrix nanocomposites by compounding nanoparticles into titanium alloy matrix is a major breakthrough in material design, igniting infinite imagination and creativity for designing advanced weapons and equipment. The researchers have incorporated nano-reinforcements such as ceramic particles (SiC, TiC, TiB, etc.), fibers (carbon fibers, SiC fibers), and carbon nanotubes into the titanium and titanium alloy matrix, and successfully prepared particle/fiber-reinforced titanium matrix composites. The results of the study of mechanical properties show that the incorporation of the abovementioned reinforcing phases can improve the strength of the composites, but the toughness and plasticity of the composites are impaired, which will place restrictions on the application scope of titanium and titanium alloy composites.33–38 Graphene-reinforced titanium matrix composites (GrTMCs) are a new material concept. Graphene exhibits extraordinary properties such as light weight, high strength and toughness, and large specific surface area. Combined with the high temperature resistance and fatigue resistance of titanium alloys, it is expected to prepare higher-performance titanium matrix nanocomposites, which can be categorized into the frontier field of titanium science and technology. In addition, the poor thermal conductivity of titanium and titanium alloys makes it difficult to process and form, which also restricts the application of titanium and titanium alloys in many fields. Graphene-based materials have excellent thermal conductivity, which is expected to improve the thermal conductivity and mechanical properties of titanium alloys at the same time, and graphene will become an ideal reinforcing material for titanium alloys. The 600°C high-temperature titanium alloy composite (Φ60 mm × 120 mm) incorporated with GO was fabricated by the powder metallurgy method. Compared with the alloy without GO, the microstructure of the composite with 0.3% GO is obviously refined, the average

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Graphene Composite Materials

size of the α phase is decreased by about 36%, the room-temperature yield strength and the hardness are increased by >10% and ~25%, respectively.39 The microhardness of graphene-reinforced titanium matrix nanocomposites fabricated by laser sintering reaches 450 HV, which is 1.5 times higher than that of laser-sintered pure titanium (180 HV).40 The density, microhardness, and compressive strength of graphene-Cu/Ti6Al4V composites prepared by microwave sintering reached 96.55%, 534 HV, and 1602 MPa, respectively.41 These preliminary studies suggest that graphene has potential applications as reinforcement for titanium alloy. Compared with the titanium alloy matrix, the graphene-reinforced titanium matrix composites prepared with graphene reinforcement have lower density and higher strength, and wear resistance is significantly improved. This provides new solutions for improving the structural/functional properties of the titanium matrix. If the technology of uniform dispersion and interface regulation of graphene in titanium alloy matrix can be completely conquered in the future, the use of such nanocomposites, which can withstand extreme loads at high temperatures and have thermal conductivity and other functional properties, will lead to the great leap forward of engine design and even other high-end manufacturing technology.

2.4.2 Preparation of graphene-reinforced titanium matrix composites Due to the high chemical activity of titanium and the strong interface reaction with graphene, the manipulation of the interface reaction between titanium and graphene and the uniform dispersion of them are the keys to the preparation of GrTMCs, so it is preferred that the preparation process does not undergo a molten state with low thermal loads or a duration as short as possible. Although the optimal preparation method for GrTMCs has not yet been found, previous studies have shown that the process derived from powder metallurgy technology is the most probable technical route to achieve uniform dispersion of graphene, which not only improves the performance of the alloy matrix material in many aspects but also ensures that the preparation method is feasible. It mainly involves processes such as graphene dispersion and GrTMCs densification, as shown in Figure 2.54. The following will introduce the preparation process of GrTMCs.

Graphene-Reinforced Metal Matrix Composites 83 Dispersion of graphene Atomization/PREP/ element mixing

Quality test of powder

Solvent such as ethanol solute

Sieving and batch mixing

Titanium and titanium alloy powders Mixing Stirring

Ultrasonic dispersion

Graphene solution Pretreatment and characterization of graphene

Wash and Dry

HIP

Deformation (Rolling, forging)

optional

SPS

Mixed powder of graphene and Titanium

Laser sintering Densification of GRTMCs

Figure 2.54.

Basic process route for preparing GrTMCs.

2.4.2.1 Dispersion method of graphene Graphene is mainly added to titanium alloy powder by mechanical mixing, including dry mixing without solvent or wet mixing with solvent. The dry mixing method mainly mixes graphene and titanium alloy powder through high shearing effect, which inevitably leads to an increase in temperature and is prone to damage the graphene structure. Therefore, the wet mixing method is currently the dominated preparation method of graphene and titanium alloy mixed powder, and its basic route includes graphene solution preparation, stirring/ball milling mixing, drying, and degassing. The quality of the mixed powder not only affects the sintering process but also seriously influences the dispersibility of graphene. Generally, the preparation of single-layer graphene is very difficult, and the graphene actually used is often multi-layer graphene composed of multiple atomic layers. In the process of graphene dispersion, the interlayer bonding of multi-layer graphene is tight and difficult to exfoliate. In addition, graphene is prone to agglomeration and a good interfacial bond cannot be formed between graphene and titanium powder, as shown in Figure 2.55. This will lead to uneven dispersion of graphene in the titanium powder, which is likely to cause defects in the local microstructure

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Graphene Composite Materials

Figure 2.55.

SEM morphology of graphene in titanium alloy powder.

of the material. Therefore, the use of graphene as a reinforcement presents higher requirement for the dispersion methods and process windows, and it is difficult to control. In order to improve the dispersibility of graphene, functional groups are introduced into the graphene sheets, and GO is such a derivative of graphene. By modification through incorporating oxygen-containing functional groups into the graphene substrate, the interlayer spacing of GO is increased, the van der Waals force between the layers weakens, and it is easy for graphene to disperse into a single-layer structure. At the same time, because of its amphiphilicity, GO can form a relatively uniform dispersion system in various solvents, and the exfoliation of GO sheets can be achieved by ultrasonic dispersion. During the vacuum heating of GO, the oxygen-containing functional groups will be gradually removed as heated to 190°C, and it is again reduced to a graphene-like structure. At 550°C, most of the GO is decomposed to give graphene, and the mechanical properties of GO will be gradually increased in this process.42 Wet mechanical stirring is a common process technology for mixing nanoparticles with titanium powder, which is achieved by controlling parameters such as stirring speed, duration, and temperature. For the mixing process of graphene/GO and titanium powder, on the one hand, it needs to be carried out in a solvent, and on the other hand, it needs to be stirred at a certain temperature until viscous mixed powder slurry is obtained. Figure 2.56 shows the SEM morphology of graphene nanosheets/titanium alloy powder after stirring and mixing. It can be evidenced from Figure 2.56 that under the appropriate stirring process, the graphene nanosheets are relatively uniformly dispersed in the

Graphene-Reinforced Metal Matrix Composites 85

Figure 2.56. SEM morphology of graphene nanosheets/titanium alloy powder after wet stirring and mixing.

spherical titanium alloy powder, and the interfacial bonding between the graphene nanosheets and the titanium alloy powder is excellent.39 This process exhibits the advantages of simple operation and precise control of process parameters. The obtained graphene nanosheet/titanium alloy mixed powder has a loose packing density of more than 70% of the theoretical value, which can improve the formability of subsequent powder metallurgy. It is suitable for large-scale production of graphene and titanium alloys. Therefore, this process shows broad application prospects. Taking the high activity, easy oxidation, and strict control of impurity element content of titanium powder into account, specific equipment should be used in the stirring and mixing process, and in the meantime the stirring parameters should be strictly controlled. For wet ball milling mixing, although the high-energy friction and collision between graphene and titanium powder during the mixing process may lead to damage of the graphene structure, the advantages of a simple process principle and uniform mixing make it currently preferred in the

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Graphene Composite Materials

graphene oxide

graphene oxide

Figure 2.57.

SEM morphology of GO/titanium alloy powder mixed by wet ball milling.

preparation of GrTMCs. The method has realized the preparation of graphene (GO)/titanium alloy mixed powder by controlling process parameters such as rotational speed, ball milling time, and ball-to-material ratio. Under normal circumstances, the graphene solution and titanium powder are sealed in an agate tank, the ball milling duration is 2.5–24 h, the ballto-material ratio is 5–10:1, and the rotation speed is 200–360 rpm. Ball milling can further refine the powder if the duration is over 12 h, and at the same time, the graphene will be embedded in the titanium powder and result in a bonding similar to “implantation”. The prepared powder is also called a composite powder, but it may cause a certain degree of oxidation and the introduction of impurities. Methods such as low-temperature ball milling and vacuum ball milling can be adopted to perform the mixing. Figure 2.57 presents the SEM morphology of GO nanosheets/titanium alloy powder mixed by ball milling. As shown in Figure 2.57, by utilizing the appropriate ball milling process, the pre-alloyed spherical titanium alloy powder is refined into flake powder, GO is embedded in the titanium alloy flake powder, and the bonding between them is excellent. GO

Graphene-Reinforced Metal Matrix Composites 87

nanosheets will not be precipitated when processed afterward.39 During the ball milling process, the selection and optimization of ball milling equipment are also very important since the differences in the characteristics of the ball mill jar, the grinding ball, and the graphene and titanium powder materials are quite large. The element mixing method is also a pathway for preparing graphene/ titanium-aluminum matrix alloy mixed powder by ball milling. For example, Xu et al. mixed Ti, Al, B, Nb, and Cr powders with the atomic percentage of 48:47:2:2:1, and then 3.5 wt% of multi-layer graphene with an average thickness of 40 nm and a lateral dimension of 50 µm was added.43 The mixed powder is subject to ultrasonic treatment to achieve uniform dispersion and then ball-milled. Both the ball milling jar and the grinding balls are cemented carbide, the ball-to-material ratio is 10:1, the ball milling duration is 8 h, and the rotational speed is 80 rpm. In the abovementioned mixing method of stirring or ball milling in liquid phase, ultrasonic pretreatment of graphene/GO is prior to the dispersion in a solvent such as alcohol, and sometimes a combined technology of ultrasonic and stirring is also employed. In order to further improve the dispersibility of graphene/GO and reduce the possibility of agglomeration as much as possible, it can be pretreated by loading metal particles. The nano-metal particles can react at the molecular level with oxygen-containing functional groups on the surface of GO, resulting in a metal-oxygen-carbon chemical bond which is coated on the surface of GO (Figure 2.58), thereby improving the interfacial bonding and strength between graphene and titanium powder.

2.4.2.2 Densification process of GrTMCs In the process of powder metallurgy, the loose packing powder or compact is fabricated into a dense body with a certain strength and density, and it is necessary to adhere the powder particles to each other under appropriate temperature, pressure, and atmosphere conditions, so as to improve its microstructure and performance. This process is the most basic step of powder densification, which is defined as “sintering” in ISO 3252-2019 “Powder Metallurgy-Vocabulary”. As mentioned above, in the densification process of GrTMCs, the predominated methods include hot isostatic pressing, spark plasma sintering, and laser sintering. On the basis of the sintering, the deformation process, such as rolling and forging, is able to further increase the densification and improve the microstructure.

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Figure 2.58.

SEM morphology of GO loaded with metal particles.

(1) Hot isostatic pressing Hot isostatic pressing (HIP) is a kind of short-time manufacturing process developed in the past 50 years, which integrates powder forming, sintering, and heat treatment, and it overcomes the obstacles of high sintering temperature in powder metallurgy process. The densification mechanism of sintering in this process is similar to that of hot pressing, which can be analyzed and interpreted by the basic theoretical model of hot pressing. Different from hot pressing, the pressure used in HIP is higher and more uniform, so the densification effect is more obvious; that is, lower sintering temperature can be used to achieve higher density.44 From this point of view, HIP is a suitable technology for the densification of GrTMCs. By manipulating and optimizing the process parameters of the densification process, the use of HIP is expected to realize the preparation and industrial production of large-sized GrTMCs. Titanium alloys without graphene and GrTMCs billets were prepared by the same HIP process, and the composition of the titanium alloy matrix was Ti-6Al-4V (the volume fraction of Al and V is 6% and 4%,

Graphene-Reinforced Metal Matrix Composites 89

Figure 2.59. The digital images and low-magnification microstructure of the GrTMCs billet prepared by HIP. TN2 has no addition of GO; TO2 is GrTMCs with 0.15 wt% GO.

respectively, and the rest is Ti). Figure 2.59 shows the billet samples with a diameter of 50 mm and their microstructure at low magnification. The TN2 billet on the left has no addition of graphene, and the TO2 on the right is a GrTMCs bar billet. Figure 2.60 exhibits the microstructure of TN2 and TO2 billet. As shown in Figures 2.59 and 2.60, after the HIP process at 875–950°C and 110–150 MPa, the surface of the billet with 0.15 wt% GO is smooth and clean, the low-magnification microstructure is dense, and no defects are evidenced. The microstructure is significantly refined (average size of α phase is greatly reduced) and the uniformity is better compared with the billet without graphene. On the basis of HIP, GrTMCs slabs can be prepared by the rolling deformation process. The detailed process is HIP at 900–950°C, 120– 150 MPa, and followed by rolling at 900–1000°C. The current specifications of the prepared slabs are 230 mm × 50 mm × 15 mm, with uniform thickness and excellent surface quality, as shown in Figure 2.61. In the same way, GrTMCs samples can also be prepared by HIP and forging processes. For example, the mixed powder is forged at 900–1000°C after hot isostatic pressing at 700–900°C and 120–150 MPa.45

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Graphene Composite Materials

(a)

(b)

Figure 2.60. Microstructure and morphology of GrTMCs billet prepared by HIP. (a) Without graphene; (b) with 0.15 wt% GO.

Figure 2.61. process.

Digital images of GrTMCs slab prepared by HIP and rolling deformation

(2) Spark plasma sintering Spark plasma sintering (SPS) technology is an efficient and rapid sintering method. Since the heating process of SPS is derived from the rapid heat generation of the pulse current, the densification speed of the SPS process is very high. Compared with the conventional hot pressing sintering method, the temperature of the powder changes more rapidly during the SPS process, the time of the grain growth is significantly shortened, and the size of the grain is significantly reduced, so that a material with a finer grain size can be obtained, and the properties of the material are consequently improved.46 As for the sintering mechanism, there are

Graphene-Reinforced Metal Matrix Composites 91

currently two well-recognized mechanisms: (1) Due to the effect of the pulse current, the surface ionization of the sintered material occurs and plasma is generated, the interior of the particles is exposed to form a clean new surface, where the grain boundary diffusion is promoted, and the densification speed of material is increased rapidly; (2) due to the discharge effect between the localized particles, a high temperature of several thousand or even tens of thousands of degrees is generated, and the surfaces of the melted particles contact each other and a sintering neck is formed, which then diffuses through the grain boundary to form a dense structure.47 Therefore, based on the advantages of SPS technology, this process is widely used in the preparation of GrTMCs. The current matrix materials include pure titanium, titanium alloys, and titanium-aluminum intermetallic compounds.48–50 Usually, SPS of GrTMCs is carried out in five main stages. In the first stage, a slight initial pressure is applied to the mixed powder of titanium and GO. Then, a constant pressure and a pulse voltage are applied to activate the powder surface and generate a small amount of heat in the second stage, followed by maintaining the temperature and vacuumizing the powder in the third stage. In the fourth stage, pressure is raised continuously, and DC current is applied to heat the mixed powder to the required temperature under the effect of simultaneous constant pressure. Eventually, pressure is raised to the required value and held for 4–8 min in the fifth stage; the sample is in a high incandescent state and rapidly densified, and then cooled to room temperature to obtain the final sample, as shown in Figure 2.62. During the whole process, the sintering temperature, pressure, and atmosphere conditions are the most important

Figure 2.62. The experimental process of GrTMCs by SPS and some fabricated samples.

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Graphene Composite Materials

(a)

Figure 2.63. (b) 1150°C.50

(b)

Microstructure of Ti60 titanium alloy sintered at (a) 1000°C and

parameters for the preparation of samples, and the selection of these parameters is determined by the type and size of the GrTMCs matrix. For example, the sintering temperatures of graphene/titanium, graphene/hightemperature titanium alloy, and graphene/titanium-aluminum intermetallic compound are generally 600–900°C, 900–1100°C, and 1000–1200°C, respectively. The sintering temperature of GrTMCs is 20–50°C higher than that without the addition of graphene. Figure 2.63 shows the microstructure of 600°C high-temperature titanium alloy (Ti60) prepared by SPS without adding GO. The phase transition temperature of Ti60 is about 1050°C. With the temperature increase from 1000 to 1150°C, according to the phase transformation theory, Ti60 should gradually transform from equiaxed primary α phase to needle-like β phase. Due to fast heating rate of the SPS process, the duration for the material temperature to be above the phase transition point is only 5~6 min, and the grains do not have sufficient time to finish the complete transformation from α phase to β phase, so the microstructure finally appears to be a lamellar α phase. In the case of the Ti60 incorporated with 0.3% GO, because the content of GO is low, the microstructure of the sintered sample is still dominated by Ti60. Compared with the Ti60 titanium alloy without GO under the same sintering temperature, the microstructure of the matrix material does not change significantly. As shown in Figure 2.64(a), the difference lies in a more uniformly dispersed lamellar structure for the case of the Ti60 incorporated with GO. Elemental analysis was performed by EDS, and the mass fractions of the primary elemental composition of the matrix

Graphene-Reinforced Metal Matrix Composites 93

(a)

(b)

Figure 2.64. SEM image of Ti60 titanium alloy incorporated with 0.3% GO. (a) Matrix; (b) lamellar structure.50 Table 2.14.

Elemental composition of Ti60 matrix and lamellar structures.50

Elemental composition

Ti (%)

Al (%)

Sn (%)

C (%)

O (%)

Ti60

86.2

6.3

3.7

0.3

3.5

Area 1

84.1

6.1

4.3

2.0

3.6

Area 2

2.2

0.1

0

73.5

24.2

Area 3

1.9

0.1

0

92.8

5.2

and lamellar structures are shown in Table 2.14. The elemental ratio of the matrix of Ti60 with GO is basically the same as that in the sintered sample of Ti60 without GO, which indicates that the addition of GO does not significantly increase the oxygen content in the matrix microstructure. Figure 2.64(b) is an enlarged view of the lamellar microstructure, and a distinct lamellar structure can be observed. In order to confirm the composition of the lamellar structure, the Raman spectrum of the aforementioned lamellar structure under 514 nm laser was measured (Figure 2.65); it is found that there is a certain relationship between the Raman spectra of the lamellar structure and the pristine GO. The characteristic peak of GO at 1300 cm−1 basically disappears, but the peak at 2600 cm−1 is observed which is not assigned to GO, whereas the characteristic peak at 1600 cm−1 is retained. By comparing with the standard Raman spectrum of graphene, it is found that the characteristic peaks at 1600 cm−1 and 2600 cm−1 are

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Graphene Composite Materials 3000 original powder sintered sample

Intensity

2500

2000

1500

1000

500 1000

1500

2000

2500

3000

Raman shift (cm–1)

Figure 2.65.

Raman spectra of GO in initial powder and sintered composite samples.50

exactly the G and 2D characteristic peaks of graphene. This indicates that after GO undergoes the sintering process, most of the oxygen-containing functional groups are removed, and GO is decomposed to form a structure similar to multi-layer graphene, with only a small amount of residual GO left. The EDS elemental analysis shows that the primary elements in the very center of the lamellar structure in Figure 2.64(b) are carbon (73.5 wt%) and oxygen (24.2 wt%), and the mass fraction of carbon near the edge reaches 92.8%. Therefore, it can be determined that the lamellar structure in Figure 2.64(b) originates from the introduced GO, and most of the GO is decomposed to form graphene during the sintering process, with only a small amount of residual GO remaining in the central area. Since the maximum sintering temperature reaches 1150°C, it is necessary to take account of possible chemical reactions during the sintering process. Below 1939 K, the Gibbs free energy of TiC formation by reaction between titanium and carbon can be expressed as the following equation51: ∆G = −184571.8 + 41.382T − 2.042T ln T + 2.425 × 10 −3 T 2 −

9.79 × 105 T (2.8)

Graphene-Reinforced Metal Matrix Composites 95

Intensity (a.u.)

Ti TiO2

1150°C 0.3%GO

1000°C 0.3%GO

1000°C 0% GO 30

40

50

60

70

80

2θ(°)

Figure 2.66.

XRD results of GrTMCs sample after SPS process.50

At the experimental temperature, this value varies from –142.55 kJ/mol to –147.31 kJ/mol, which indicates that the formation of TiC is a spontaneous reaction. From a thermodynamic point of view, during the process of SPS, graphene spontaneously reacts with titanium to form TiC. However, the TiC composition in the microstructure measured by XRD (Figure 2.66) suggests that this reaction is not significant in the actual sintering process. The intensities of the standard diffraction peaks of TiC at 36° and 42° in the samples with addition of GO do not increase sharply compared with the samples without GO. This indicates that in the SPS process of the samples with GO, the chemical reaction between Ti and graphene would hardly proceed, and the graphene is retained after the sintering process. (3) Laser sintering It is generally believed that the laser sintering of metal powder basically follows the mechanism of the traditional liquid phase sintering, but due to the extremely short reaction time between the high-energy laser beam and the mixed powder, the formation and solidification process of the liquid phase is very fast, and some stages of the traditional liquid phase sintering cannot be fully accomplished. Therefore, the formation of liquid phase and rearrangement of particles play a leading role in the densification

96

Graphene Composite Materials

process.52 In the initial stage, as the laser energy is injected into a sintered area, the metal binder with lower melting point in the powder layer is melted. The rigid “skeleton” of the original powder system collapses as more liquid phase grows along the grain boundaries and particle-toparticle interfaces of metal binder, resulting in the shrinkage of the powder layer and reduction of porosity. When the metal binder is completely melted, the liquid phase coats and wets the particles. Due to the capillary force exerted by the liquid phase and its own viscous flow, the particle rearrangement is accelerated, thereby further increasing the densification of the powder layer. The principle of laser sintering for metal powders provides the possibility for the preparation of graphene/metal composites. For example, GO/iron nanocomposites were prepared on the surface of 4140 alloy steel by a laser sintering process. Studies have shown that the reaction between graphene and iron matrix to form carbides is limited by the rapid melting and solidification process of laser sintering, so only a part of GO and matrix materials participates in the reaction at the interfaces.53 This research provides a technical approach for the preparation of GrTMCs by laser sintering. Figure 2.67 shows the schematic diagram of laser sintering for preparing GrTMCs. As shown in Figure 2.67, the graphene/titanium mixed powder slurry contains polyvinyl alcohol (PVA), and during the laser sintering process, PVA evaporates from the composite material through high-temperature gasification, which makes GO vertically aligned in the cross-section. Although part of graphene reacts with titanium at interfaces to form TiC, a small amount of graphene still exists in GrTMCs, which is believed to reinforce titanium. By adopting process parameters such as

Figure 2.67.

Schematic diagram of laser sintering for preparing GrTMCs.53

Graphene-Reinforced Metal Matrix Composites 97

laser frequency of 50 Hz, laser power of 80 W, spot diameter of 0.8 mm, and scanning speed of 2 mm/s, GO/Ti nanocomposites (the content of GO is 2.5 wt%) with a dense structure and high purity were prepared by laser sintering under argon protection.54 It is found that GrTMCs can be fabricated by rationally controlling the incorporated amount of graphene and optimizing the laser sintering process. However, it is very difficult to uniformly disperse high-content graphene, and the size of the prepared GrTMCs is limited.

2.4.3 Interface optimization of GrTMCs The interface, as the linkage between the nano-reinforcement and the titanium matrix, plays a crucial role in the comprehensive performance of titanium matrix nanocomposites (TMNCs). Theoretically, the nanoenhancing phase can greatly improve the mechanical properties of TMNCs (Figure 2.68), but in practice it is far from reaching its theoretical value, mainly because of the unsatisfactory interfacial bonding which is caused by easy agglomeration of the nano-enhancing phase and poor wettability with the titanium matrix. For GrTMCs, the interface area is very small, generally several nanometers to several micrometers. The interface structure and characteristics are determined by controlling the process factors and directly affect the performance of the composite material. A weak interface reaction can improve the wettability of the matrix to the nano-reinforcing phase and improve the interfacial bonding strength, and the strong interface reaction will generate a ceramic phase that leads to brittle fracture, which has a very adverse effect on the composite. Therefore, the key to the interface optimization of GrTMCs is to clarify the correlation of “process-interface-performance” by understanding the interface structure and properties. At present, the research on interface optimization of GrTMCs is still in its infancy. Since graphene and carbon nanotubes are both nanocarbon materials, the research on the interface behavior of carbon nanotubereinforced titanium matrix nanocomposites provides a good reference for GrTMCs. For example, experimental studies indicate that the carbon nanotube is more active to react with titanium than graphite; a smooth interface is produced and the interaction between the TiC dispersoids and the titanium matrix is improved. TiC dispersoids with a size of 1–6 µm are uniformly dispersed in the matrix, and the amount of TiC dispersoids increase in proportion to the addition of nanotube/graphite content. In the

Graphene Composite Materials

Elongation, %

Limits of fluidity and strength, MPa

98

Grain size, nm (1) σ0.2 in accordiance with the Hall-Petch ratio; (2) σ0.2 in accordiance with dispersed hardening; (3) σB-tensile strength; (4) relative elongation, %.

Figure 2.68. Relationship between nanoparticle size and mechanical properties of titanium matrix nanocomposites.55

case of hot extrusion at 1000°C, carbon nanotubes are more likely to react with titanium, whereas layered graphite exhibits anisotropic properties and obstructs the reaction with titanium (graphite structure with a diameter of approx. 4 µm remains); the unreacted graphite acting as crack initiation site degrades the mechanical properties of the material. This is consistent with the results of the DSC analysis shown in Figure 2.69. When the mixed powder of graphite and titanium alloy is heated to about 1100°C, the interface reaction starts to emerge and heat is released. Since graphite and titanium are in the status of incomplete contact, the temperature is slightly higher than the interface reaction in the composite material. From this point of view, the interface reaction process as well as the quantity and distribution of the products can be optimized through manipulating the process.

Graphene-Reinforced Metal Matrix Composites 99 Exothermal

Temperature (°C)

Figure 2.69. carbon.

DSC results of the interface reaction between titanium alloy powder and

For GrTMCs, the interface reaction bonding is also the dominating interfacial bonding mode. In the case of SPS and rolling at 950°C, due to the strong atomic diffusion, a weak interface reaction layer of 10 nm is formed between the graphene nanosheets and the titanium matrix, where ionic Ti-C bonds are generated.49 This nanoscale interface reaction layer helps to improve its interface strength, which in turn improves its mechanical properties. It indicates that the hot rolling can promote the dispersion of graphene nanosheets and improve the interface bonding between graphene nanosheets and the matrix, which means the interface is optimized. In the case of HIP and forging at 970°C, it is obtained from the high-resolution TEM image and energy spectrum analysis that graphene with a content of 0.5 wt% still exists in the composite material after thermal processing, the interlayer spacing is about 0.34 nm, and the structure of graphene is not damaged. However, there is also a TiC interface reaction layer between the graphene and the matrix, as shown in Figure 2.70, indicating that the isothermal forging process also improves the interfacial bonding between the graphene and the titanium alloy matrix.45 It is thus concluded that adopting an appropriate thermal deformation process indeed does not completely destroy the interface structure between graphene and the matrix, but optimizes and strengthens the interface

100

Graphene Composite Materials

(a)

(b)

(c)

Figure 2.70. The interface morphology of graphene and titanium alloy matrix in GrTMCs after isothermal forging.45 (a) TEM image; (b) HRTEM image of area A in (a); (c) EDS result of graphene in (a).

performance between them as well as making full use of the beneficial effect of TiC. It is expected that the interface structure and performance of the titanium matrix can be further improved by the manipulation of the interface structure and properties.

2.4.4 Properties of GrTMCs 2.4.4.1 Mechanical properties (1) Room-temperature tensile properties Table 2.15 shows the room-temperature tensile properties of GrTMCs fabricated by HIP under the same process conditions. It can be evidenced from Table 2.15 that the value scattering of room-temperature tensile properties for GrTMCs billets (Φ50 mm) with 0.15 wt% GO is small. Compared with the matrix without GO, the tensile strength and yield strength are increased by 92.3 MPa and 105.3 MPa, respectively. The

Graphene-Reinforced Metal Matrix Composites 101 Table 2.15. Material TN2

GrTMCs

Room-temperature tensile properties of GrTMCs prepared by HIP. 25°C

Measured value

Rm (MPa)

Rp0.2 (MPa)

A (%)

Z (%)

E (GPa)

862

788

18.5

48.6

115

861

787

18.4

48

115

861

787

19.2

48.9

114

Average value

861.3

787.3

18.7

48.5

114.7

Measured value

953

892

16.2

38.7

115

954

893

15.8

39.3

115

Average value

954

893

16.5

38.1

116

953.7

892.7

16.2

38.7

115.3

Note: The specification of the billet is Φ50 mm.

plasticity is decreased slightly, and the measured minimum elongation is 15.8%. The average value of the measured results of both samples is shown in Figure 2.71, which demonstrates that under the condition of HIP, a small amount of GO can play a significant role in reinforcing the titanium alloy matrix. On the basis of the HIP process, the room-temperature tensile properties of GrTMCs will be further improved by the thermal deformation process. For example, in the case of HIP and the forging process, the sample with 0.5 wt% GO exhibits tensile strength and yield strength of 1058 MPa and 1021 MPa, respectively, and the elongation was 9.3%. For the case of HIP and the rolling process, the tensile strength and yield strength reach 1061.7 MPa and 1026.3 MPa, respectively, and the elongation is 13.1%. Therefore, under hot deformation conditions such as forging and rolling, the reinforcing effect of a small amount of graphene on GrTMCs is more significant; that is, plasticity is slightly reduced, whereas the tensile strength and yield strength are both increased by nearly 20% compared with the matrix. Compared with other nanomaterials, graphene is an even more ideal nano-reinforcement, and has shown a more prominent role in the strengthening and toughening of conventional titanium alloys. The microscopic mechanism will be discussed later. For the cases of other sintering methods, graphene also plays a significant role in improving room-temperature mechanical properties. For example, the tensile strength and other properties of GrTMCs prepared by SPS have been greatly improved, the strength of the pure titanium matrix

102

Graphene Composite Materials

Figure 2.71. by HIP.

Comparison of room-temperature tensile properties of GrTMCs prepared

Figure 2.72. Comparison of tensile strength of GrTMCs fabricated by SPS under different process conditions.

with 0.5 wt% GO is increased by up to 94%, the tensile strength of hightemperature titanium alloy with 0.1 wt% GO is greatly improved, and the maximum increment is 15% (Figure 2.72). The elastic modulus and hardness of laser-sintered samples are greatly improved, and the hardness of GrTMCs is increased by more than 1.6 times when the addition amount of graphene is 2.5 wt%. (2) High-temperature tensile properties Table 2.16 shows the high-temperature tensile properties of GrTMCs prepared by HIP under the same process conditions. It can be evidenced

Graphene-Reinforced Metal Matrix Composites 103 Table 2.16. Material TN2

GrTMCs

High-temperature tensile properties of GrTMCs prepared by HIP. 400°C

Measured value

Rm (MPa)

Rp0.2 (MPa)

A (%)

Z (%)

E (GPa)

543

415

24.8

59.5

85.7

540

413

22.9

59.3

79.9

541

415

22.8

59.1

87.1

Average value

541.3

414.3

23.5

59.3

84.2

Measured value

606

475

23.3

59.7

87.6

606

478

19.6

47.5

89.2

605

476

22

59.2

89.1

605.7

476.3

21.6

55.5

88.6

Average value

Note: The specification of the billet is Φ50 mm.

Figure 2.73. by HIP.

Comparison of high-temperature tensile properties of GrTMCs prepared

from Table 2.16 that the value scattering of 400°C tensile properties of GrTMCs billet (Φ50 mm) with 0.15 wt% GO is small. Compared with the matrix without GO, the tensile strength and yield strength are increased by 64.4 MPa and 62.0 MPa, respectively, the plasticity is decreased slightly, and the elastic modulus is increased slightly. The average values of the measured results of the two samples are shown in Figure 2.73. When prepared by HIP and rolling, the 400°C tensile strength and yield strength of the samples reach 719.3 MPa and 611 MPa, respectively, and the elongation is 13.6%.56 Therefore, the addition of graphene can also significantly improve the high-temperature performance of the titanium alloy matrix regardless of HIP or thermal deformation process.

104

Graphene Composite Materials

Figure 2.74. Room-temperature friction and wear properties of GrTMCs prepared at different sintering temperatures.50

2.4.4.2 Tribological behavior (1) Graphene-reinforced high-temperature titanium alloys Figure 2.74 shows the room-temperature friction coefficient and wear rate under light load for the Ti60 titanium alloy matrix without and with GO prepared at different SPS temperatures. As shown in Figure 2.74, at the same sintering temperature, the friction coefficient of Ti60 titanium alloy with 0.3 wt% GO is reduced by about 15%, and the wear rate is reduced by 60%. In the case of the same particle size of titanium alloy powder and the same amount of GO loading, with the elevation of sintering temperature, the friction coefficient and wear amount of the sample exhibit a declined trend; in particular, the friction coefficient decreases significantly when the temperature increases from 1000°C to 1050°C. When the same amount of GO is incorporated, the friction coefficients of powder 1 (diameter 150–220 µm) and powder 2 (diameter 100–150 µm) present little difference when sintered at 1050°C, 1100°C, and 1150°C. However, when the sintering is performed at 1000°C, the powder with smaller particle size exhibits a lower friction coefficient. Figure 2.75 shows the experimental results of high-temperature friction and wear properties of GrTMCs prepared by SPS. One can find that the friction coefficient measured at high temperature of the titanium alloy matrix with 0.3 wt% GO has no significant difference compared with that without GO, which is much higher than that measured under light load at room temperature. Compared with the friction and wear properties of Ti60 titanium alloy without GO, the samples with 0.3% GO still show better wear resistance, and the wear rate is reduced by about 24.9%. It indicates

Graphene-Reinforced Metal Matrix Composites 105

Figure 2.75.

High-temperature tribological properties of GrTMCs sintered at 1150°C.50

that under the conditions of high temperature and heavy load, graphene has no obvious effect on the friction coefficient, but it significantly improves the wear rate. (2) Graphene-reinforced TiAl intermetallics In the family of TiAl intermetallic compounds, compared with Ti3Al and Ti2AlNb, TiAl intermetallic compounds have become the most promising materials for high-temperature structural applications in aeroengines due to advantages such as low density, high specific modulus, high creep resistance, and flame retardancy. At present, the studies on the performance of TiAl intermetallic compounds reinforced by multi-layer graphene are mainly focused on the tribological properties. Figure 2.76 shows the experimental results of the friction coefficient and wear rate of GrTMCs with graphene loadings of 0.25 wt%, 0.75 wt%, 1.25 wt%, 1.75 wt%, and 2.25 wt%.57 It is evidenced that the room-temperature friction coefficient and wear rate under different loads of TiAl intermetallic compounds are both decreased to a constant as the content of graphene rises, and the best tribological properties can be achieved for the GrTMCs with 1.75 wt% graphene loading. It demonstrates that the incorporation of more than 1.0% multi-layer graphene to the TiAl intermetallic compound can significantly improve the tribological properties of the material.

2.4.4.3 Thermal conductivity Compared with metal materials such as aluminum alloy, the thermal conductivity of titanium alloy and titanium-aluminum intermetallic

106

Graphene Composite Materials

(a)

(b)

Figure 2.76. Influence of graphene addition on the tribological properties of GrTMCs. (a) Friction coefficient; (b) wear rate.57

compound is relatively lower. For example, the thermal conductivity of Ti60 titanium alloy at 100°C is 6.1 W/(m·K), so improving the thermal conductivity of the titanium matrix by incorporating graphene is the focus of research on GrTMCs. The incorporation of a small amount of graphene (0.5 wt%) to the titanium alloy results in the increment of the roomtemperature thermal conductivity by more than 15%, whereas the thermal conductivity changes significantly when more than 1.0% graphene is incorporated. For instance, the thermal conductivity of GrTMCs is increased by 86%, 211%, and 337% when 1.0 vol%, 3.0 vol%, and 5.0 vol% graphene, respectively, is incorporated into the titanium matrix.57 It is believed that the amount of introduced graphene has a crucial influence on the improvement of the thermal conductivity of GrTMCs, and the thermal conductivity continues to increase with the rising content of graphene. A positive linear relationship between graphene volume fraction and thermal conductivity is derived from theoretical approximations, as shown in Equation (2.9), which is similar to the case of carbon nanotubes58,59: Vf λ f λe = 1+ 3λ m λm

(2.9)

In the equation, λ e is the thermal conductivity of composite materials, λ m is the thermal conductivity of the matrix, λ f is the thermal conductivity of graphene, and Vf is the volume fraction of graphene.

Graphene-Reinforced Metal Matrix Composites 107

2.4.5 Reinforcing mechanism of graphene At present, although GrTMCs have exhibited distinguished advantages in mechanical properties and physical properties, there is still a big gap when compared with the application potential of graphene. The effective strengthening effect of graphene on titanium alloys should be further explored, and the reinforcing mechanism needs to be deeply understood and recognized. In terms of the strengthening mechanism for mechanical properties, the well-recognized viewpoints are the comprehensive effects of dislocation strengthening and grain refinement strengthening. First, during the thermal processing, the TiC particles, which are produced via the interface reaction between graphene and its surrounding, become obstacles for dislocation motion and play a strengthening role. Second, the growth of the grains is effectively restrained since graphene nanosheets with large specific surface area are coated on the powder, and according to the Hall–Petch relationship, the grain refinement greatly reinforces the mechanical properties. Third, the interfacial interaction transfers the load to the graphene. Figure 2.77 shows the SEM morphology of the tensile fracture of GrTMCs, where graphene is presented at the fracture dimples. Fracture analysis of the composite samples indicates that graphene maintains the characteristics of 2D thin film and wrinkle structure, and it forms an excellent interfacial bond with the matrix, thereby increasing the strength and toughness of the titanium alloy.

Graphene

Figure 2.77.

SEM morphology of GrTMC tensile fracture.

108

Graphene Composite Materials

For the tribological behavior, under the friction condition of low speed and light load with the absence of lubricant, the abrasive wear and adhesive wear are the dominating wear mechanisms.60 Due to the higher hardness of the materials selected for the friction pair in the test process, the adhesive wear does not easily occur. From the microstructure analysis of Ti60 titanium alloy without and with graphene after tribological tests, it is concluded that the dominating wear mechanism of the matrix under room temperature and light load is abrasive wear. Many uneven laminated plows are produced in the friction process, which increases the effective area of friction and consequently deteriorates the condition of friction contact. After being incorporated with graphene, the friction surface of GrTMCs is much smoother, and no obvious exfoliation is found on it. The graphene in the matrix is transformed from an original lamellar structure into circular plates, around which the measured carbon content is higher than that in neat matrix, indicating that graphene is dispersed on the friction interface after tribological tests.50 With the occurrence of wear on the interface, the graphene gradually disperses and covers the interface around the site of the original graphene, and finally a local lubricant layer of graphene is formed on the surface, thereby changing the composition of the friction pair as well as improving the friction environment on the interface, as shown in Figure 2.78. Therefore, the deteriorated tribological behavior of GrTMCs can be attributed to this “wear-cover-lubrication” mechanism of graphene.

(a)

(b)

Figure 2.78. The reinforcing mechanism of graphene during the tribological test of GrTMCs. (a) Before test; (b) after test.50

Graphene-Reinforced Metal Matrix Composites 109

2.4.6 Potential applications and prospects Graphene has quickly become an ideal reinforcement for structuralfunctional-integrated materials due to its extraordinary properties, which provides a new idea for the development of titanium alloy material technology. The aforementioned studies indicate that graphene as reinforcement has shown application potential in the reinforced high-temperature titanium alloys and titanium-aluminum alloys. Compared with the matrix, GrTMCs have lower density, higher strength, and greatly improved tribological properties. In this cutting-edge technology field, XG Sciences and Oak Ridge National Laboratory launched a joint-development program in 2012 for manufacturing GrTMCs using advanced powder metallurgy processes, saying that this research “will advance the graphene-based material technologies and help maintain the competitive advantages of the United States in developing and manufacturing high-tech products”. Although it is impossible to know whether or not the goal of “manufacturing high-tech products” is aimed at high-performance aeroengines, the unique advantages and attractive application prospects of this material system have been well known, and have attracted great attention from the world’s aviation powers. Advanced aeroengines are now developing in the direction of high pre-turbine temperature, high thrust-to-weight ratio, long life, and low fuel consumption. In addition to the technology of advanced design, the improvement of aeroengine performance strongly depends on the development of advanced material technology. The key and important components of engine compressor fabricated by titanium alloys urgently require new materials with high temperature resistance, high specific strength, high specific modulus, oxidation resistance, and flame retardancy. With the elevation of the operating temperature, the hightemperature performance of the material becomes more and more important. The development history of high-temperature titanium alloys in China is shown in Figure 2.79. The application of titanium alloy materials in the low-temperature section below 400°C of the aeroengine is competed by resin-based composite materials with lower density, while the creep, durability, microstructure stability, anti-oxidation, and other properties of ordinary titanium alloy materials above 600°C no longer meet the requirements of the aeroengines. Compared with nickel-based superalloys, 600°C high-temperature titanium alloys, titanium-aluminum intermetallic compounds, SiC fiber-reinforced titanium matrix composites

110 Graphene Composite Materials T( )

Operating temperature

SiC/TiAl

Materials in service Materials under developing

800

850-TiAl TiAl

750 SiC/Ti 700

SiC/Ti2AlNb

Ti2AlNb

650

TD2(Ti3Al) TD3(Ti3Al)

650

GrTMNCs Alloy

600

TC11

500 450 400

TG6(600

TA12 Ti60

550

TC6

TA7

Alloy)

550 Fireproof Alloy TA15 TA19 TB12(Ti40)Fireproof Alloy TA11 TC17

TC4

350 1980

1985

1990

1995

2000

2005

2010

2015

Timeline Figure 2.79.

Development history of high-temperature titanium alloys in China.

(SiCf/Ti), and GrTMCs exhibit obvious advantages such as excellent specific strengths in the temperature range of 500–850°C. In the case of maintaining the same service performance, weight can be reduced by more than 40% when the nickel is replaced by titanium, which is of great significance for improving the thrust-to-weight ratio and performance of the aeroengine. The combination of these new materials and the blisk, bling, and other lightweight structures, is expected to be applied to highpressure compressors and low-pressure turbine components for nextgeneration aeroengine. With the continuous deepening of studies on the advanced hightemperature titanium alloy materials, typical components such as blisks, centrifugal impellers, and blades have been tested for strength and installed on advanced aeroengines, and the technical maturity has been improved. Based on these engineering productions and application experiences, the researched and developed GrTMCs have more excellent mechanical properties, and can overcome the deficiencies in thermal conductivity and wear resistance of titanium alloys. GrTMCs are expected to further develop the performance and application potential of high-temperature titanium alloys, promote the establishment and improvement of the

Graphene-Reinforced Metal Matrix Composites 111

titanium alloy material technology system for aeroengines in China, and take the road of independent innovation and development. For example, on the one hand, GrTMCs provide new candidate materials for the modification and upgrading of advanced engines in service and under research; on the other hand, GrTMCs accumulate the experience of the material technologies for the next generation of high-performance aeroengines and concept aeroengines. At present, the application potential of GrTMCs has emerged and has shown a bright application prospect, but the technological maturity is relatively low, and this idea will be turned into reality after the fundamental problems of related materials are solved.

2.5 Graphene-Reinforced Copper Matrix Nanocomposites Copper matrix composites have excellent electrical, thermal, and mechanical properties, and are widely used in electronic packaging materials, friction materials, and electrical and electrical contact materials.61 The reinforcements of traditional copper matrix composites mainly include ceramic particles and carbon materials, such as silicon carbide, alumina, graphite, and diamond. Ceramic particles have superior mechanical strength but they are not conductive, and using them as reinforcement will seriously weaken the electrical conductivity of the copper matrix. Ding used in-situ synthesis to prepare copper matrix composites with 3.0 vol% Al2O3, and its electrical conductivity is only 70% IACS.61 Graphite has good electrical and thermal conductivity, but its mechanical properties are poor. Liu used spark plasma sintering to prepare copper matrix composites with 60 vol% flake graphite; the thermal conductivity along the arrangement direction of flake graphite is as high as 668 W/(m·K), but its flexural strength is only 47.8 MPa.63 The development of modern industry has put forward higher requirements for the comprehensive properties of copper matrix composites. For high-speed trains with a speed higher than 250 km/h, the contact wire made by copper requires a tensile strength higher than 580 MPa, a conductivity higher than 78% IACS, a high softening temperature, and excellent wear resistance. Pure copper has good electrical conductivity, but its tensile strength is only about 350 MPa. With the miniaturization, light weight, and integration of electronic information products, its lead frame is also

112 Graphene Composite Materials

developing in the direction of being short, small, light, and thin, and the thickness of the lead frame material is gradually reduced from the original 0.25 mm to 0.1–0.15 mm, even 50 µm; thus, higher requirements are put forward for the electrical conductivity, thermal conductivity, and mechanical properties of lead frame materials. At the present stage, scientists from all over the world are committed to developing high-strength and highconductivity lead frame materials, which require mechanical properties to be higher than 550 MPa and electrical conductivity to reach 80–85% IACS.64 Compared with traditional reinforcements, graphene has both extraordinary mechanical properties and exceptional electrical and thermal conductivity, which is considered to be an ideal reinforcement for copper matrix. Given graphene as a reinforcement material to prepare copperbased composite materials, it can significantly improve the mechanical properties of copper matrix without weakening the electrical and thermal conductivity. The preparation of reinforced copper matrix composites with graphene as a reinforcement mainly has the following difficulties: (1) The specific surface area of graphene is large; for instance, the specific surface area of a single-layer graphene sheet is as high as 2630 m2/g, and the graphene easily agglomerates. (2) The density of graphene and copper matrix varies greatly; the density of pure copper is about 4 times that of graphene, and it is difficult for graphene to achieve uniform dispersion in the copper matrix. (3) There is neither wetting nor chemical reaction between graphene and the pure copper matrix, so it is difficult to achieve good interfacial bonding between graphene and copper matrix. At present, the preparation method of graphene-reinforced copper matrix composites is dominated by the powder metallurgy process, and the preparation process mainly includes the mixing of graphene and copper powder and the molding of graphene/copper composite powder. Graphene-reinforced copper matrix composites prepared by casting have not been reported yet; the main reason is that graphene is easy to agglomerate and floats in copper melts, and graphene is difficult to disperse uniformly in copper melts. Other reported processes include the template method, accumulative roll bonding process, and electrochemical deposition coating process. Xiong et al. used fir wood as a template to prepare porous copper sponge through chemical synthesis, calcination, and hightemperature hydrogen reduction.65 Then, the graphene was filled into the pores of porous copper sponge, and the final bulk of graphene-reinforced

Graphene-Reinforced Metal Matrix Composites 113 (a)

(b)

Figure 2.80.

(d)

(c)

Schematic representation of fabricating graphene-copper artificial nacre.65

copper matrix composite was fabricated by the hot pressing sintering process (Figure 2.80). The tensile strength of the bulk material is as high as 308 MPa, which is about 40% higher than that of pure copper, and its electrical conductivity is still as high as 56.6 × 106 S/m. Liu et al. used 1mm copper strips as raw material, between which the graphene was coated, and a bulk of graphene/copper matrix composite material with layered structure was fabricated by accumulative roll bonding, its tensile strength is higher than that of the pure copper matrix.66 The electrochemical deposition coating process involves directly producing a graphene/ copper composite coating with graphene and copper salt on the cathode sheet via the electrochemical effect, which significantly improves the hardness of the coating. The key of preparing graphene-copper matrix composites by powder metallurgy is to achieve uniform dispersion of graphene and copper powder, as well as good interfacial bonding between graphene and copper matrix. So far, the preparation methods of graphene/copper composite powder mainly include mechanical mixing, adsorption mixing, chemical/ electrochemical synthesis, and CVD in-situ synthesis. As for the powder molding, the processes used at the present stage mainly include cold pressing sintering, hot pressing sintering, spark plasma sintering, and powder hot rolling. In order to further improve the density and microstructure of the copper matrix composites, the material can also be subjected to deformation treatments such as hot extrusion and rolling. Since

114 Graphene Composite Materials

there is neither wetting nor chemical reaction between graphene and the pure copper matrix, the sintering process has little effect on the graphene/ copper matrix composites, and the preparation of graphene/copper composite powders is directly related to the distribution of graphene in the copper matrix and the interfacial bonding between graphene and copper matrix, which is the key process for the preparation of graphene/copper matrix composites. According to the relevant literature reported at the present stage, the mechanical properties of the copper matrix prepared by these four dispersion processes have been improved to a different extent. The following describes the research progress of graphene/copper matrix composites on the basis of the different dispersion processes of graphene.

2.5.1 Mechanical mixing The mechanical mixing is able to achieve uniform mixing of graphene and copper powder by the effect of an external mechanical force, which mainly includes mechanical stirring and mechanical ball milling. In mechanical stirring, rotors are employed to rotate the graphene and the copper powder to achieve mixing of graphene and the copper powder, where the mechanical force is small and the dispersion effect is poor. As to the mechanical ball milling, mixing of graphene and copper powder is achieved through the centrifugal force driven by rotating container as well as the collisions of the rigid balls, the mechanical force is large and the dispersion effect is excellent. The main parameters of the ball milling include rotational speed, ball-to-material ratio, and ball milling duration. The rotational speed of the rolling ball milling is about 60 rpm, the highspeed ball milling could reach 500 rpm, the ball-to-material ratio is usually set to 10:1–20:1, and stainless steel and ceramic beads are commonly used milled balls. The ball milling can be instigated in two states: dry ball milling and wet ball milling. The wet ball milling process first disperses graphene in alcohol or water, and mixed copper powder and graphene solution are then ball milled. In order to avoid oxidation of copper powder and improve the drying efficiency of composite powder, alcohol is mostly used as dispersant. The wet ball milling process generally adopts ultrasonic technology to uniformly disperse the graphene in the solution prior to the ball milling, and its dispersion effect is better than that of dry ball milling. Figure 2.81 shows the preparation of graphene/copper composite powder via mechanical ball milling by AECC Beijing Institute of

Graphene-Reinforced Metal Matrix Composites 115 (a)

(b)

Figure 2.81. SEM images of graphene/copper composite powder prepared by mechanical ball milling. (a) Low magnification; (b) high magnification.67

Aeronautical Materials.67 Graphene adheres to the copper surface under the collision of milled balls, which indicates that in the process of mechanical ball milling, the collision of the milled ball is able to create good mechanical bonding between graphene and copper powder. Then, pure copper and graphene/copper matrix composites are prepared by hot pressing sintering, and the mechanical test results show that the introduction of graphene can improve the hardness and tensile strength of copper matrix by about 8.6% and 28.0%. In the process of wet stirring and wet ball milling, due to the great difference between the density of graphene and copper powder, graphene is inclined to float on the upper layer of the mixed solution, resulting in the macro-segregation of graphene, and graphene is inclined to reagglomerate during the ball milling process. In addition, mechanical ball milling can only achieve physical mixing of graphene and copper powder, and it is difficult to achieve strong interfacial bonding between them via sintering. Therefore, the researchers have attempted to solve the above problems by pre-planting a metal layer on the surface of graphene before mechanical mixing. At the present stage, the coating metals reported in the literature are mainly copper, silver, and nickel, and three kinds of metal coatings are mainly prepared by the electroless plating process.68,69 By coating a metal layer on the surface of graphene, the density difference between graphene and copper powder is reduced, and the graphene sheets are not easy to agglomerate due to the presence of the metal layer; the interface strength between graphene and the copper matrix is significantly

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Figure 2.82. Graphene coated by silver by electroless plating. (a) XRD pattern; (b) SEM image; (c) TEM image.70

enhanced due to the presence of the metal coating, finally improving the reinforcing effect of graphene. Figure 2.82 shows the characterization results of silver-coated graphene prepared by a reduction method using graphene and silver nitrate as raw materials by Luo et al. from China University of Mining and Technology.70 The silver particles are grown and uniformly dispersed on the graphene nanosheets, indicating excellent interfacial bonding between graphene and silver particles. Subsequently, bulk graphene/copper matrix composite is prepared by ball milling and hot pressing sintering, and the microhardness is as high as 89.1, which is 27.3% higher than that of pure copper. Mechanical ball milling is a common powder mixing process in powder metallurgy; it is able to achieve the uniform dispersion of graphene and spontaneously refine the copper powder. Meanwhile, the mechanical ball milling has high production efficiency and it is easy to industrialize.

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It should be noted that impurities are easily introduced during mechanical ball milling, which affects the mechanical, electrical, and thermal behavior of the composites.

2.5.2 Adsorption mixing Adsorption mixing achieves the uniform dispersion of graphene in copper powder through electrostatic adsorption of copper and graphene in solution. Generally, GO is chosen for the adsorption mixing method because the surface of GO contains a large number of negatively charged functional groups, which makes the surface of graphene negatively charged. Studies have shown that the surface potential of copper powder in alcohol solution is +8.25 mV. By stirring the mixed solution of copper powder and negatively charged GO, followed by standing, graphene can be adsorbed on the surface of copper powder under the effect of charge interaction, and finally graphene and copper powder are deposited at the bottom of the solution. The graphene/copper composite powder is obtained after the supernatant is removed.71 Figure 2.83 depicts the schematic of the fabrication of graphene/ copper matrix composites via electrostatic adsorption by Shanghai Jiaotong University.72 The graphene is prepared by chemical unzipping of carbon nanotube, and the surface is negatively charged, which gives rise to a good dispersion with copper powder through electrostatic adsorption. In addition, some researchers have improved the dispersion of graphene in copper powder by modifying the surface of copper powder. Harbin University of Science and Technology used hexadecyl trimethyl ammonium bromide (HTAB), a cationic surfactant, to coat the copper powder, so that copper powder can infiltrate into water and get positively

Figure 2.83. Schematic illustration for the production of graphene/copper matrix composites by electrostatic adsorption process.72

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Figure 2.84. SEM images of composite powder. (a), (b) Graphene/copper composite powder; (c), (d) GO/copper composite powder; (e), (f) graphene agglomeration in graphene/copper composite powder.74

charged under the effect of cationic active hydrolysis, and finally excellent dispersion of graphene was achieved through electrostatic adsorption.73 Ningbo Institute of Materials Technology and Engineering first modified the surface of copper powder by coating copper powder with polymer polyvinyl alcohol, and then stirred and mixed the modified copper powder with GO or polyvinylpyrrolidone-modified graphene nanosheets.74 The dendritic effect of polymers achieves uniform mixing of graphene and copper powder. Figure 2.84 presents SEM images of the composite powders prepared by copper powder and modified graphene or GO. It is evidenced that either graphene or GO can be coated on the surface of the copper powder particles to form a good dispersion. Compared with GO, graphene powder is easier to agglomerate into blocks, which significantly influences its dispersion effect. In graphene/copper composite powder prepared by adsorption mixing, there is only simple physical mixing between graphene and copper. Ningbo Institute of Materials Technology and Engineering used the spark plasma sintering to prepare the graphene/copper matrix composite material, and the compressive and tensile mechanical properties were measured, respectively. The results are shown in Figure 2.85. It can be observed that the addition of graphene can significantly improve the compressive mechanical properties of the composite while deteriorating its

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Figure 2.85. Mechanical properties of graphene-reinforced copper matrix composites. (a) Compression curves; (b) tensile curves.74

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Figure 2.86. Tensile fractures of pure copper and graphene/copper composites. (a) Pure copper; (b), (c) GO/copper composites; (d), (e) graphene/copper composites.74

tensile mechanical properties, and can lead to a significant decrease of the elongation after tensile fracture, which is lower than 10%. Figure 2.86 is the SEM images of the tensile fracture of pure copper and composite materials. It can be evidenced that the copper particles in the composite material cannot be bonded together by sintering. The presence of graphene is observed on the surface of the copper particles at the fracture, and the cracks along the grain boundary are the evidence of typical intercrystalline fracture.

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Adsorption mixing realizes the self-assembly and dispersion of graphene and copper powder through the electrostatic effect; the dispersion effect is excellent and the efficiency is high. Compared with ball milling, the energy consumption of the adsorption mixing is greatly reduced, and it is an ideal industrialized method for dispersion of graphene. On the other hand, when the composite powder is prepared by this process, there is only simple physical mixing between the graphene and the copper powder, and the interfacial bonding strength between the graphene and the copper matrix is low; therefore, an appropriate powder molding process is necessary for preparing composites with excellent performance.

2.5.3 Chemical synthesis In the case of chemical synthesis, the graphene material is first dispersed into a copper salt solution, and then copper or copper oxide is directly grown on the surface of graphene by chemical means to obtain a composite powder of graphene/copper or copper oxide, and the final graphene/ copper composite powder can be obtained by reducing the graphene/ copper oxide with hydrogen at high temperature. Since the copper powder prepared by this method directly uses graphene as the nucleation site during the process of nucleation and growth, the graphene can be evenly dispersed into the copper powder particles, and good interfacial bonding can be formed between the graphene and the copper powder. The graphene/copper matrix composites prepared by this method generally have excellent mechanical properties. There are various processes for preparing graphene/copper composites by chemical synthesis, mainly including the electroless copper plating process and co-precipitation-calcination-reduction process. In the electroless plating process, the mixed solution of graphene and copper salt is directly reduced by a reducing agent, and the graphene/composite powder is obtained in one step. Prior to the electroless plating process, graphene can also be activated and sensitized by zinc stannate and palladium chloride in advance to promote the nucleation of copper on the surface of graphene, and to improve the dispersion effect of graphene as well as the strength of interfacial bonding between graphene and copper matrix. Zhao et al. used GO and copper sulfate as raw materials to obtain the graphene/ copper composite powder through activation, sensitization, and chemical reduction processes, and final graphene/copper composite materials were prepared by spark plasma sintering.75 Figure 2.87 shows the

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Figure 2.87. The preparation and characteristics of graphene/copper composites. (a) SEM image of graphene; (b) TEM image of graphene; (c) AFM image of graphene; (d) SEM image of graphene/copper composite powder; (e) TEM image of graphene/ copper composite powder; (f) particle size distribution of copper powder.75

characterization results of the graphene materials used and the graphene/ copper composite powder prepared. It can be observed that the copper powder is uniformly distributed on the graphene nanosheets. Then, the powders were prepared into graphene/copper matrix composite bulks by spark plasma sintering. The Young’s modulus and tensile strength of the composite material are 103 GPa and 485 MPa, which are 21% and 107% higher than that of pure copper, respectively. In the co-precipitation-calcination-reduction process, the intermediate products of graphene/copper oxide or graphene/copper hydroxide are obtained at first by evaporating the solvent or adding a strong base, and then the graphene/copper composite powder is obtained by hightemperature calcination and high-temperature hydrogen reduction. Chen et al. employed graphene and copper nitrate as raw materials to prepare graphene/copper composite powders and bulks by the co-precipitationcalcination-reduction process, and the graphene is homogeneously distributed in the graphene/copper oxide and graphene/copper composite powders, as shown in Figure 2.88.76 The results of Raman spectroscopy further confirmed that the carbon materials in the composite powders and bulk materials exist in the form of graphene.

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Figure 2.88. Preparation and characteristics of graphene/copper oxide composite. (a) SEM image of graphene/copper oxide composite powder; (b) SEM image of graphene/ copper composite powder; (c) digital image of composite powder and bulk; (d) XRD results of graphene/copper and graphene/copper oxide composite powders; (e) Raman spectroscopy of composite powder and bulk composites.76

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Figure 2.89. Mechanical properties tests of composites. (a) Tensile stress–strain curve; (b) yield strength and fracture elongation; (c) elastic modulus and hardness of the composite.76

Subsequently, a bulk graphene/copper composite was prepared by spark plasma sintering, and the relevant mechanical properties are shown in Figure 2.89. It can be observed that the addition of graphene can significantly improve the yield strength, tensile strength, microhardness, and elastic modulus of the composite, but its elongation decreases sharply. Moreover, the addition of graphene can greatly improve the wear resistance of the matrix. Furthermore, the tests of its electrical conductivity find

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that the electrical conductivity of the composites with low graphene content is above 85% IACS, which indicates that the addition of graphene would not significantly deteriorate its electrical conductivity. The chemical synthesis has obvious advantages in the preparation of graphene/copper matrix composites. By means of this method, the uniform dispersion of graphene can be obtained in the copper matrix, and good interface bonding between graphene and copper matrix can be achieved in the process of powder preparation. The prepared graphene/ copper composites have shown excellent mechanical properties and good electrical conductivity. On the other hand, the chemical synthesis method for preparing graphene/composite powder has a lengthy process, extremely low production efficiency, and it requires a large amount of chemical reagents, resulting in high preparation cost and unfriendly environment, which poses serious challenges for the industrialization of this method.

2.5.4 In-situ CVD synthesis Copper is an excellent catalyst for graphene preparation by the CVD method. In the in-situ CVD synthesis method, copper powder is used as raw material and catalyst, one can directly prepare graphene on the surface of copper powder, and graphene/copper composite powder is fabricated. Since graphene is directly deposited on the surface of the copper powder, the graphene is uniformly dispersed in the copper powder, and the interfacial bonding between them is excellent. At the present stage, the carbon sources used in the preparation of graphene/copper composite powders by in-situ CVD synthesis are mainly divided into gaseous carbon sources and solid carbon sources. The temperature for gaseous carbon sources is higher, which is about 1000°C according to the literature. The solid carbon source mostly used is polymethyl methacrylate (PMMA), and the temperature for graphene growth is about 700–900°C. Xiao et al. fabricated the graphene/copper composite powder using methane as the carbon source, and the characterization results are shown in Figure 2.90.77 The graphene growth temperature on the copper powder is 1000°C, the growth duration is about 45 min, and the growth pressure is constant. It can be evidenced from the SEM images and the Raman spectra of the composite powder that high-quality multi-layer graphene sheets are successfully grown on the surface of the copper powder by the

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Figure 2.90. Relevant characteristics of graphene/copper composite powders. (a), (b) Raman spectra of the composite powder; (c), (d) SEM images of the composite powder in low and high magnification.77

CVD process. On the other hand, due to its high growth temperature, the copper powder particles are bonded together by sintering. Subsequently, the graphene/copper matrix composite material was prepared by hot pressing sintering, and its mechanical properties, electrical conductivity, and thermal conductivity were tested; the results are shown in Figure 2.91. It can be observed that the addition of graphene significantly improves the microhardness of the material, but its electrical and thermal conductivity decline. In order to prevent the copper powder from sintering together during the CVD process of growing graphene, Wang et al. used magnesium oxide nanoparticles to separate the nanoscale copper powder, and then employed methane and carbon source to grow graphene at 1050°C.78 Then, magnesium oxide is eroded with dilute hydrochloric acid to obtain graphene/copper composite powder. Subsequently, graphene/copper matrix composites were fabricated by hot pressing sintering, and the

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Figure 2.91. Performance test of pure copper and composite materials (C0 is pure copper, C1 is 0.07 vol% Gr/Cu composite, and C2 is 0.115 vol% Gr/Cu composite).77

microhardness and tribological properties of pure copper were tested. The hardness of pure copper is 1.01 GPa and the wear rate is 379.4 × 10–5 mm3/(N·m). After incorporating graphene, the hardness of the composite is increased to 2.53 GPa, and the wear rate is only 2.8 × 10–5 mm3/(N·m), which is two orders of magnitude lower than that of pure copper. In addition, the resistivity of graphene/copper composites measured by the fourprobe method is 1.72 × 10–6 Ω·cm, which did not decrease significantly compared with that of pure copper (1.69 × 10–6 Ω·cm). Chen et al. used PMMA as solid carbon source to prepare graphene/ copper matrix composites with excellent performance.79 The detailed preparation process is as follows: First, the PMMA and copper powder are uniformly mixed by ball milling, and then the composite powders are calcined for 10 min at 800°C under Ar and H2 atmosphere to grow graphene on the surface of copper powder; the final graphene/copper matrix composites are prepared by hot pressing sintering (Figure 2.92). Figure 2.93 represents the SEM, TEM, and Raman results of the fabricated graphene/copper composite powders. It can be evidenced that the graphene grows uniformly on the surface of the copper powder. After etching the copper substrate with chemical reagents, it is observed by TEM that the graphene is presented in the shape of a transparent thin veil.

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Figure 2.92. Schematic diagram of fabricating process to prepare graphene/copper composites with PMMA as solid carbon source.79

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Figure 2.93. Characteristics of graphene/copper composite powder prepared with solid carbon source PMMA.79

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Figure 2.94 shows the stress–strain curve of these composite materials, which indicates that the addition of graphene can significantly improve the yield strength and tensile strength. It is particularly noteworthy that the addition of graphene does not reduce its elongation. In the meantime, the electrical conductivity of composites are superior to pure Cu, which is attributed to the “conductive films” effect of graphene within the matrix, guaranteeing the transferring of current in composites. In this light, electricity conductivities of graphene/Cu composites enhanced by in-situ grown graphene keep at a satisfactory level for the usage in electronics. may be attributed to that this method generates graphene in good quality, and a good conductive network is formed in the copper matrix. In addition, Cao et al. made corresponding improvements on the basis of the above preparation process.80 First, the PMMA was dissolved in anisole, and then the flaky copper powder was introduced into the anisole solution of PMMA, stirred, and dried to obtain the flaky copper powder coated with PMMA. The nano-laminated graphene/copper composites were prepared by CVD, hot pressing, and hot rolling, which had tensile strength of up to 378 MPa and excellent plasticity. At the same time, the conductivity is still as high as 93.8% IACS (Figure 2.95). It can be concluded that the use of solid carbon sources to prepare graphene/copper matrix composites has the following advantages over the

Figure 2.94. Mechanical properties of graphene/copper matrix composites prepared by in-situ synthesis.79

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Figure 2.95. Stress–strain curves for graphene/copper matrix composite with nanolaminated structure.80

above methods: (1) the temperature for graphene growth is low, which conserves energy; (2) the gas circuit for preparing graphene/copper matrix composites with solid carbon sources is simple, which is similar to that of general hydrogen reduction treatment. In general, the graphene/copper matrix composites prepared by in-situ CVD synthesis are able to achieve good dispersion of graphene in the copper matrix and good interfacial bonding with the matrix. It is worth noting that the composites prepared by this method have electrical conductivity comparable to that of pure copper, which is of great significance for the preparation of high-strength and high-conductivity copper matrix composites. In addition, since the in-situ CVD synthesis method does not require expensive graphene as raw material, this greatly saves the preparation cost of the graphene/copper matrix composite, which is beneficial to the popularization and application of the graphene/copper matrix composite. In general, the above four dispersion processes for preparing graphene/copper matrix composites can all achieve the dispersion of graphene in the metal matrix. Graphene/copper matrix composites prepared by chemical synthesis exhibit the best mechanical properties and excellent dispersion of graphene, but their low yield, high cost, and unfriendly environment all severely restrict their production and application. Both mechanical mixing and adsorption mixing are suitable for industrialized large-scale production and preparation, but the graphene dispersion of

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these methods is worse than other methods, and the performance of the prepared graphene/copper matrix composite material is worse than that of the other two methods. Therefore, it is necessary to conduct in-depth research and improvement on these methods, especially on their powder molding and subsequent processing technology. The in-situ CVD synthesis process has good graphene dispersion, excellent mechanical properties, and electrical conductivity, and it is a promising process for the preparation of graphene/copper matrix composites.

2.6 Graphene-Reinforced Magnesium Matrix Composites Magnesium alloys have become one of the most promising light alloys due to their low density, high specific strength, good electromagnetic shielding, and excellent machinability, and they show unique application advantages in electronics, automotive, military and aerospace, and other fields. The traditional magnesium alloys rely on the solid solution strengthening and grain refinement strengthening produced by the addition of alloying elements to strengthen the alloy, but the plastic loss of the alloy is serious. The magnesium matrix composites made by introducing reinforcements such as SiC, Al2O3, B4C, and carbon nanotubes have higher specific strength and specific stiffness, as well as better wear resistance and high temperature resistance, which make them one of the most promising composite materials in the high-tech field, leading to wide usage in the aerospace, military, and sporting goods fields.81–84 For example, magnesium matrix composites have been used to prepare guide vanes for aerospace, armored tiles for armored vehicles, brake pads and other friction materials, and sports equipment. However, the size of the particle reinforcements is often large, which also leads to the reduction of the plasticity of the magnesium matrix composites. The dispersibility of the carbon nanotubes, as well as the interface between the carbon nanotubes and the magnesium matrix, is still the issue to be solved to improve the performance of the carbon nanotube-reinforced magnesium matrix composites. As an allotrope of carbon nanotubes, graphene has more excellent mechanical properties and better dispersibility, so graphene has become an ideal reinforcement for the preparation of lightweight alloy composites. However, the specific surface area of graphene is too large that it

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tends to agglomerate seriously, and the interface reaction and bonding strength with the metal matrix are also problems that need to be solved in the preparation of high-performance graphene-reinforced magnesium matrix composites. In order to improve the dispersion of graphene in the magnesium matrix, Rashad et al. prepared relatively uniform composite powder composed of Mg-1Al-1Sn alloy powder with a particle size of less than 80 µm and 0.18 wt% multi-layer graphene through the liquid dispersion method (Figure 2.96).85 Graphene/magnesium matrix composites were prepared by hot pressing sintering. The results show that the yield strength of the composites is 208 MPa, which is 29% higher than that of magnesium alloys without graphene. However, the fracture morphology shown in Figure 2.97 indicates that some graphene is perpendicular to the extrusion direction, which makes it difficult to exert the high toughness of graphene.

Figure 2.96. Schematic diagram of the preparation of multi-layer graphene/magnesium matrix composites by liquid dispersion method.85

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Figure 2.97. 1Sn alloy.85

SEM fractured images of (a) Mg-1Al-1Sn alloy and (b) graphene/Mg-1Al-

Figure 2.98. of GO.86

SEM morphology of AZ91 alloy powder mixed with different contents

In addition, the strength of multi-layer graphene is low, which leads to decreased failure strain of graphene/Mg matrix composites by 34.7%. Yuan et al. used GO with better dispersibility to replace graphene as reinforcement, and then prepared rGO/AZ91 composites by powder metallurgy and hot extrusion.86,87 In the SEM morphology of the mixed powder, alloy powder particles are wrapped by GO, and a very good dispersion effect of GO in the magnesium alloy powder is evidenced (Figure 2.98). The mechanical test results show that when the GO content is 0.5 wt%, the tensile strength, yield strength, and elongation of rGO/AZ91 composites all reach the maximum values of 355 MPa, 312 MPa, and 11.3%, which are improved by 65.1%, 85.7%, and 61.4%, respectively, as

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Figure 2.99.

Stress–strain curves of AZ91 and rGO/AZ91 composites.87

compared with AZ91 alloys (168 MPa, 215 MPa, and 7.0%). The main reason is that strong interfacial bonding is established between rGO and magnesium matrix under the “bridging” effect of MgO, which makes the stress transfer strengthening effect extremely significant, and another reason is the grain refinement strengthening effect caused by the incorporation of GO (Figure 2.99). Since graphene shows better wettability with aluminum, the “bridging” effect of Al is expected to improve the interface strength between graphene and magnesium matrix. Rashad et al. first incorporated 1 wt% aluminum powder on the surface of graphene nanosheets, then uniformly mixed it with magnesium powder, and the final graphene/magnesium matrix composites were fabricated by cold pressing, sintering, and hot extrusion (Figure 2.100).88 The mechanical test results shown in Figure 2.101 suggest that when 0.3 wt% graphene is incorporated, the strength and ductility of the composites are significantly improved at the same time; the elastic modulus, yield strength, and elongation at break are increased by 131%, 49.5%, and 74.2%, respectively. The experimental results confirm that the idea of improving the interface strength between graphene and magnesium matrix by “bridging” is feasible. Yuan et al. fabricated graphene/AZ91 composites with GO by taking advantage of the “bridging” effect.86 During the sintering process under protective argon atmosphere, the oxygen-containing functional groups of GO react with magnesium, and the product is MgO. At the same time,

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Figure 2.100. TEM micrographs of 0.3 wt% graphene/magnesium matrix composite.88

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Figure 2.101. Room-temperature mechanical properties of pure magnesium and graphene/magnesium matrix composites. (a) Microhardness; (b) stress–strain curve.88

GO is reduced to rGO, and a strong interfacial bond is formed between them. On the other hand, along the direction [011]MgO or [2423]α-Mg at the MgO/α-Mg interface, the crystal planes (200)MgO and (1102)α-Mg form a semi-coherent interfacial bond, and the crystal atomic spacing misfit is about 8.6%. Therefore, MgO nanoparticles, the interfacial reaction product of GO and magnesium matrix, effectively improve the interfacial bonding strength of rGO and magnesium matrix, which ultimately helps to improve the strength and toughness of composite materials at the same time.

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2.7 Applications and Trends in Graphene-Reinforced Metal Matrix Nanocomposites Graphene has exceptional mechanical, electrical, and thermal properties, and it has attracted widespread attention from scientists all over the world since its discovery, and reports on graphene applications have emerged in an endless stream. In the field of metal matrix composites, compared with conventional reinforcements, graphene has extraordinary mechanical properties, excellent electrical and thermal conductivity, and temperature and corrosion resistance, which is considered to be an ideal reinforcement for metal matrix composites. Using graphene as reinforcement, researchers have successively carried out studies on its application in metal matrix such as aluminum, magnesium, copper, titanium, and nickel, and fruitful results have been achieved. Due to the short development history of graphene-reinforced metal matrix composites and the constraints of graphene raw materials and preparation processes, large-scale industrial production and application of graphene-reinforced metal matrix composites have not been realized yet. However, the excellent mechanical, thermal, electrical, and other properties exhibited by graphene-reinforced metal matrix composites in the laboratory stage still show their huge application potential. Graphene-reinforced metal matrix nanocomposites are expected to achieve a wide range of applications in the aerospace, automotive industry, and electrical and electronic fields in the future.

2.7.1 Applications of graphene-reinforced metal matrix nanocomposites Metal matrix composites are known as a relatively new material science developed in the 1960s and they are a branch of composite materials. With the rapid development of aerospace, aviation, electronics, automobiles, and advanced weapon systems, increasing performance requirements are placed on materials. In addition to requiring materials to have some special properties, they must also have excellent comprehensive properties, both of which effectively promote the rapid development of advanced composite materials. The rapid development of civil industries such as electronics and automobiles provides broad prospects for the applications of metal matrix composites. Compared with traditional metal matrix composites, graphene-reinforced metal matrix composites have better mechanical properties and

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electrical and thermal conductivity. For instance, lightweight and highstrength graphene-reinforced aluminum matrix and magnesium matrix composites are of great significance to reduce the weight of spacecraft, aircraft, and vehicles. High-strength and high-conductivity graphenereinforced copper matrix composites are expected to be widely used in electronic packaging and electrical contact materials. Graphenereinforced titanium matrix composites show excellent mechanical properties and thermal conductivity, and can be widely used in weapon systems for aviation.

2.7.1.1 Aviation, aerospace, and weapon systems With the development of aerospace technology, reducing the manufacturing cost, increasing the aircraft carrying capacity, and realizing the light weight of aircraft structural components are new requirements in this field. The low density and high toughness of aluminum matrix composite make it a new hot issue for application and research in the manufacturing process of aircraft. For example, Boeing and Airbus have realized the application of the landing gear and engine nacelle fabricated by aluminum matrix composites. The research results show that the graphene/aluminum matrix composites prepared by powder metallurgy not only greatly improve the tensile strength and yield strength but also significantly improve the ductility,10,25 which will be beneficial to expand the application range of aluminum matrix composites in the aerospace field. The high strength and excellent corrosion resistance and wear resistance of nickel matrix composites give them wide application prospects in the aviation field, while graphene as a second phase particle is expected to further improve their mechanical properties such as hardness and strength as well as machinability spontaneously. The graphene-reinforced nickel matrix composites are prepared by electrodeposition, and the thermal conductivity, electrical conductivity, and mechanical properties of graphenereinforced nickel matrix composites are improved when compared with pure nickel.89 Titanium alloys feature light weight and high strength, and have been intensively applied in various advanced weapon systems for aviation. However, the poor thermal conductivity of titanium alloys makes it difficult for processing and molding, which will also limit the application of titanium and titanium alloys in many fields. Graphenereinforced titanium matrix composites can significantly improve the thermal conductivity and processing properties of titanium alloys, which is of

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great significance for reducing the processing cost of titanium alloys and improving the yield of titanium matrix composites. In bulletproof armor and other weapons and equipment systems, reducing weight and thickness while improving elastic resistance has always been a difficult problem for current research. Graphene-reinforced metal matrix composite materials are expected to solve this problem. AECC Beijing Institute of Aeronautical Materials has carried out a large number of research studies since 2014, and has made many breakthroughs, the results of which have been patented and are expected to be industrialized in the near future.

2.7.1.2 Automobile industry In the automobile industry, new energy vehicles continue to put forward new requirements for the light weight of automobiles. Traditional lightweight metal materials, such as magnesium, aluminum, and their alloys, have gradually been unable to meet the increasing performance requirements. Therefore, metal matrix composites show a very broad application prospect in the automotive industry. Studies have shown that without reducing the rigidity and crash performance of the car, the weight can be reduced by 10% and the fuel consumption can be lowered by 6–8%. Due to its excellent mechanical properties and low density, graphene is an ideal reinforcing phase for improving the mechanical properties of magnesium, aluminum, and their alloys, and is widely used for the development of automotive composite materials. There have been reports of related studies in China since 2012. In 2012, Wang et al. used flaky aluminum powder and GO as raw materials to prepared 0.3 wt% graphene/ Al composite material by hot extrusion, and its tensile strength was 62% higher than that of pure aluminum matrix.10 Zhang et al. prepared GNPs/5083Al composites by hot pressing sintering and hot extrusion; at the loading of 1.0 wt%, the yield strength is increased by 52% and the tensile strength is increased by 56%.90 Yuan et al. used GO with better dispersion to replace graphene as a reinforcement, and then used powder metallurgy and hot extrusion to prepare rGO/AZ91 composites.86,87 The mechanical test results show that when the GO content is 0.5 wt%, the tensile strength, yield strength, and elongation of rGO/AZ91 composites all reach their maximum values, 355 MPa, 312 MPa, and 11.3%, which are improved by 65.1%, 85.7%, and 61.4%, respectively, when compared with AZ91 alloy (168 MPa, 215 MPa, and 7.0%). A large number of studies have shown that the addition of graphene has a significant

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strengthening effect on lightweight aluminum alloys and magnesium alloys, which is of great significance for expanding their application in the field of automobile manufacturing and promoting the light weight of the automobile industry.

2.7.1.3 Electronic and electrical field As a reinforcing material, the most significant advantage of graphene is that it does not significantly deteriorate the electrical conductivity on the basis of improving the mechanical properties of the metal matrix, which is of great significance for its applications in the electronic and electrical field. At present, pure aluminum and aluminum alloy wires are mainly used for power transmission. Pure aluminum has excellent electrical conductivity but poor mechanical strength, whereas the aluminum alloys have high strength but poor electrical conductivity. The graphene-reinforced aluminum wire developed by AECC Beijing Institute of Aeronautical Materials features high strength and high electrical conductivity. The Institute has applied for relevant national standards for the wire and has been approved, as it can meet the performance requirements of largespan transmission cables, namely, mechanical strength and electrical conductivity. Electrical contacts are one of the core components of electrical switches and instruments, and are mainly responsible for the important tasks of breaking, connecting circuits, and loading currents. The requirements of the contact material are multifaceted, i.e., it is required to have good electrical conductivity, thermal conductivity, low and stable contact resistance, high corrosion resistance, fusion weld resistance, and good mechanical strength. The contact materials made of copper matrix material reinforced by graphene show simultaneously improved electrical conductivity, thermal conductivity, and mechanical properties, which are able to improve the mechanical life of electrical contact switches, reduce contact resistance, and lower the rise of temperature. Such metal composites are of great significance for the development of contact materials. The Joint Graphene Laboratory established by Shanghai Institute of Microsystem and Information Technology and companies has achieved a homogeneous bulk graphene/copper composite after several years of joint research and development; the electrical conductivity of the composite material has been improved by 3%, and the thermal conductivity, the mechanical strength, and corrosion resistance can be increased

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Figure 2.102. (a) Uniform graphene-copper composite powder; (b) SEM image of graphene evenly coated by copper particles; (c) digital image of bulk alloy.91

simultaneously.91 It is expected to be widely used in the fields of heat dissipation components, contacts, and wires and cables. The alloy powder, microscopic morphology, and macroscopic bulk are shown in Figure 2.102. These are two products of graphene metal matrix composites that are about to be launched in China. There are also many graphene metal matrix composites that have made breakthroughs in laboratory, but still require more time for mass production and application.

2.7.2 Constraints for the applications of graphene-reinforced metal matrix nanocomposites 2.7.2.1 Cost factor The cost factors restricting the application of graphene metal matrix composites mainly include two aspects: graphene raw material cost and preparation process cost. At present, the methods for preparing highquality graphene in China are mainly the mechanical exfoliation method and vapor deposition method. The mechanical exfoliation method is the dominating method for preparing graphene powder. Although the cost is low, its controllability is poor and it is difficult to achieve mass production. At present, the price for single-layer graphene powder obtained by this method is between 800 CNY/g and 1000 CNY/g. CVD is the main method for obtaining graphene thin films, and it is the most promising method for mass production of large-scale graphene, but its complex process, high energy consumption, and expensive preparation equipment lead to an increase in cost. The price of single-layer graphene film prepared by the CVD method is between 600 CNY/g and 800 CNY/g. Such high

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material cost has restricted the applications of high-quality graphene materials, which can only be used by downstream enterprises with economic strength, and it is difficult for graphene to transform from earlystage application to large-scale application. Therefore, graphene nanoplatelets are mostly used in current graphene applications, and the price is about 20 CNY/g. Although the cost has obvious advantages compared with high-quality graphene, the performance of graphene nanoplatelets still cannot achieve the most ideal effect, so it can only meet the low- and mid-end applications of graphene. The high cost of preparation leads to a high price of high-quality graphene materials, which is one of the reasons for limiting its large-scale application. At the present stage, graphene metal matrix composites are intensively prepared by powder metallurgy, and the main processes include metal powder preparation, powder mixing, sintering, and post-processing. Compared with the melting and casting process, the powder preparation, sintering, and other process costs in the powder metallurgy are extremely high, resulting in a high overall preparation cost, and downstream application companies cannot afford such high price.

2.7.2.2 Preparation process There are still many problems that need to be solved in the preparation of metal matrix composites with graphene as a nano-reinforcement. First, graphene is very easy to agglomerate, and it is difficult to disperse uniformly in the metal matrix. How to ensure the uniform mixing of graphene in the matrix in the industrial production process is still very challenging. Second, excellent interfacial bonding between graphene and metal matrix is an important prerequisite for the excellent performance of graphene. Chemical reactions may take place between graphene and aluminum matrix to form Al4C3, and the degree of these reactions should be strictly controlled during the preparation of graphene–aluminum matrix composites so as to achieve optimal mechanical properties. There is neither wetting nor chemical reaction between graphene and copper matrix; how to optimize the interfacial bonding between them through the industrial preparation process to prepare graphene-copper matrix composites with excellent performance is still an important scientific issue to be solved. Titanium is a strong carbide-forming element, and the molding temperature of graphene-reinforced titanium matrix composites is generally very high. Controlling the interface reactions between graphene and

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titanium matrix has become a key issue in the preparation of excellent graphene-reinforced titanium matrix composites. Magnesium is an active metal, and the production process must be strictly controlled in the process of preparing magnesium-based composite materials, otherwise it is easy to explode. If the above problems are solved in the engineering production process, can graphene metal matrix composites be truly applied?

2.7.2.3 Disjointed industrial chain At present, thousands of organizations and enterprises are engaged in graphene production, research, and development in China. Although most enterprises have basic capabilities of design and production, the product differentiation is not large. In spite of increasing graphene production year by year, due to the lack of breakthroughs in downstream applications, the market of graphene has not been fully exploited. Currently, widespread news, such as a production line with an annual output of several tons of graphene being put into action by a certain company, is of common occurrence, but there are very few reports on the industrial application of graphene. If one just blindly increases the output of graphene instead of expanding the downstream applications of graphene on the basis of present products, the industry chain of graphene will not be established.

2.7.3 Development trend of graphene-reinforced metal matrix nanocomposites At the present stage, the research system of graphene-reinforced metal matrix nanocomposites is relatively simple. The research works that have been carried out so far mainly focus on the metal matrix and a few metal alloys, and generally in these cases the additive phase is no more than graphene. The increasing development of science and technology inevitably requires the development of metal matrix composites toward high performance and multifunctionality. Therefore, graphene-reinforced metal matrix nanocomposites will definitely develop in the direction of “structural complexification” and “integration of structure and function”.

2.7.3.1 Structural complexification The structural complexification mainly includes two aspects, one of which is the diversification of reinforcements. The current research shows

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that, compared with adding graphene alone, the incorporation of graphene and carbon nanotubes as reinforcements in the aluminum matrix can reinforce the strength of graphene more effectively. In addition, due to the high specific surface area of graphene, the loading of graphene in the metal matrix is often controlled at a low content level, resulting in a moderate enhancement of the performance of the metal matrix, which includes the reduction of the density, the improvement of electrical and thermal conductivity, and the reduction of the thermal expansion coefficient. However, electronic packaging functional materials often have strict requirements for the density, thermal conductivity, and thermal expansion coefficient of the material, and it is difficult to simply incorporate graphene to comply with the conditions for applications. Therefore, the future graphene/copper matrix composites will develop toward the diversification trend of reinforcements. Another aspect of structural complexity is the ordering of the microstructure of metal matrix composites. At the scale micrometer, inspired by a superior combination of strength and toughness achieved by the natural biological laminated structure, the research on microlaminated metal matrix composites with alternating ductile and brittle layers has attracted more and more attention, mainly including metal/metal, metal/ceramic, and metal/metal matrix composites microlaminates. The main purpose of microlaminated materials is to compensate for the shortage of inherent properties of single-layer materials through micro-lamination to meet a variety of special application requirements. Shanghai Jiaotong University has developed a flaky powder metallurgy process, and developed graphene-reinforced aluminum matrix and copper matrix composites with nano-laminated structure, which has achieved a good balance between the strength and plasticity of the metal matrix; that is, the plasticity of the metal matrix is guaranteed while the mechanical strength of the metal composite is greatly improved.92,93

2.7.3.2 Integration of structure and function With the development of science and technology, the requirements for the use of metal materials are no longer limited to mechanical properties, but extend to feature integration of structure and function as well as multifunctional response under multiple service conditions. The graphene introduced into the metal matrix can not only be used as a reinforcement to improve the mechanical properties of the metal material but can also be used as a functional body to impart physical and functional properties that

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the metal material does not have. Taking the graphene metal matrix composite used as electronic packaging for an example, it is required to bear a large load of mechanical stress, and it needs to have both appropriate electrical and thermal conductivity.

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Chapter 3

Graphene-Reinforced Resin Matrix Composites

Modern synthetic organic chemistry has laid the foundation for polymer resin materials. High-performance resin materials have many unparalleled advantages such as light weight, environmental resistance, and high strength. These performance characteristics of resin materials make them commonly utilized by means of composites, and their development can be traced back to the early 20th century. Meanwhile, they are the earliest and fastest-growing category of matrix materials employed in the field of composites. So far, resin matrix composites are versatile materials with a wide range of high performance in plenty of industrial fields; in particular, several performances such as specific strength and specific modulus have far surpassed those of all the ordinary materials. Graphene has shown exceptional physical and chemical properties, and the discovery of graphene provided an immense boost up and new dimension to materials research and nanotechnology. The combination of graphene and resin matrix offers further improvement of the material properties, which is an important subject for the development of novel resin matrix composites. The introduction of graphene as a reinforcing phase into the resin matrix can significantly improve the mechanical, electrical, thermal, and other properties of the resin matrix. In addition, graphene is capable of improving the interfacial interactions between the resin matrix and other reinforcements, which brings benefits to the optimization of the overall performance of the material.

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Graphene Composite Materials

Three-dimensional (3D) printing, also known as additive manufacturing, refers to the technology of manufacturing 3D solid objects by layer-by-layer superposition of materials based on digital model files. Compared with traditional “subtractive” manufacturing, 3D printing technology has outstanding advantages such as intelligence, rapidity, and efficiency, and it is a promising new material manufacturing technology. 3D printing technology also provides new ideas for the preparation of resin matrix composites, and is capable of realizing the rapid manufacturing of composites and products with complex structures. The incorporation of graphene makes 3D printed products have better mechanical and functional properties; in the meantime, it is more convenient to prepare functional gradient products. For the purpose of engineering applications, in addition to the performance improvement of the matrix and reinforcement of the composites, the sandwich structure and other designs are also intensively adopted to further improve the overall performance of the components and parts, and to endow them with a variety of functionalities. Due to the exceptional electrical and thermal properties as well as low-density advantage, graphene can be used as a functional layer in sandwich-structured composites for improving the overall performance of the composites, and the typical application for these structural-functional-integrated composites is for radar absorption. In this chapter, the influence of graphene on various properties of resin matrix composites is analyzed, the applications of graphene in modifying resin and improving resin–fiber interface are reviewed, the preparation and applications of graphene-reinforced resin matrix composites in the field of 3D printing are described, and finally the graphene sandwichstructured composites are introduced.

3.1 Introduction to Graphene-Reinforced Resin Matrix Composites Resin matrix composites are classified as a kind of polymer matrix composite, and they are the earliest and fastest-growing category of family of composite materials, which can be traced back to the phenolic resin matrix composites that emerged at the beginning of the 21st century. So far, resin matrix composites have formed a variety of categories and have found a very wide range of applications. Their development, application,

Graphene-Reinforced Resin Matrix Composites 151

and derived science and technology have generated a significant influence on modern materials science and industry. Resin matrix composites are composed of a colossal system; they are usually classified by the type of matrix and reinforcement, including hundreds of resin matrixes with diverse properties as well as composites which are several times as large as a resin matrix. Moreover, with the continuous development of disciplines such as synthetic polymers, the family of resin matrix composites is still getting even bigger. As to the practical applications of resin matrix composites, since the molding techniques are improving day by day and performance is gradually enhanced, the application scope of resin matrix composites is growing rapidly, and consequently the composites industry is nowadays playing a more important role in the national economy. Graphene-reinforced resin matrix composites are new materials produced by combining the graphene technology that has received widespread attention in recent years with the rapidly developing composite technology. At present, the research on graphene-reinforced resin matrix composites is still in its infancy. Nevertheless, this kind of new material has shown superior performance in many aspects and great application potential. In the meantime, it is also one of the most promising development directions in the field of composite materials.

3.1.1 Overall performance characteristics of resin matrix composites Resin matrix composites are made up of a big material family, which can be classified into various groups according to different standards. For example, based on the type of reinforcing fiber, they can be classified into glass fiber-reinforced resin matrix composites, carbon fiber-reinforced resin matrix composites, aramid fiber-reinforced resin matrix composites, etc. Meanwhile, another important criterion for classification is the type of resin matrix: thermosetting resin matrix composites and thermoplastic resin matrix composites. Thermosetting resin matrix mainly includes unsaturated polyester resin, epoxy resin, and phenolic resin. Their primary common feature is that under the effect of heat, pressure, or additives, resin can be converted into bulky macromolecules with a 3D network structure by means of certain conditions, thus giving rise to products with excellent mechanical

152

Graphene Composite Materials

properties. Thermoplastic resins are generally organic macromolecular compounds with a linear or branched structure. They are materials that soften to a liquid in high heat, and then harden again when cooled. Thus, they are recyclable; however, their mechanical properties are often worse than those of thermosetting resins. Thermosetting carbon fiber-reinforced resin matrix composites are popular in the modern aviation industry. Due to their high strength-toweight ratio, stiffness-to-weight ratio, fatigue resistance, corrosion resistance, and other excellent properties, they can be used as a structural material for aircraft to substitute metal materials, enabling lighter airplanes that burn less fuel. Carbon fiber-reinforced composites in aircraft have gradually developed from use in secondary load-bearing structures such as fairing and rudder surfaces to primary load-bearing structures such as wings, tails, and fuselage; for example, the usage of carbon fiber-reinforced composites in both Airbus A350 XWB and Boeing B787 is more than 50% by weight. Figure 3.1 shows the composite structure of A380.

3.1.2 Molding methods of resin matrix composites With the rapid development of the composites industry, the performance of resin matrix composites is improving and the application is

Figure 3.1.

Composite structure of A380.

Graphene-Reinforced Resin Matrix Composites 153

continuously expanded. As one of the foundations, the molding process of composites has also received more and more attention. The molding of resin matrix composite materials is the most crucial part of the entire application process. Since the material formation and product molding are generally completed at the same time, the molding process often directly determines the overall performance of the composite product, which is the unique feature of composite molding. At present, there are more than 20 molding methods for resin matrix composites. Commonly used molding processes are hand lay-up molding, injection molding, resin transfer molding, etc., but the basic process can be summarized as raw material mixing, upper mold, curing, demolding, and subsequent processing; diverted molding methods are based on the different operation methods of raw materials and molds.

3.1.2.1 Hand lay-up molding Hand lay-up molding is a molding method in which the matrix and reinforcements are manually positioned in a specific mold and then solidified, which is the oldest technique of composite manufacturing. It has low requirements on equipment and it is not limited by the shape and size of products. To this day, the hand lay-up technique is still a relatively common method for manufacturing multi-variety, small-scale, and large-sized composite components, as shown in Figure 3.2. Hand lay-up is classified into dry lay-up and wet lay-up according to the form of raw materials. During dry lay-up, the prepregs are cut into required shapes and placed over each other on the surface of the mold.

Figure 3.2.

Hand lay-up process.

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Graphene Composite Materials

As for wet lay-up, fiber or some other reinforcing mat or woven fabric or roving is positioned manually in the open mold, and resin is poured, brushed, or sprayed over and into the chopped fiber where the secondary spray-up layer imbeds the core between the laminates. Finally, the laminates are left to cure under standard atmospheric conditions. In the process of hand lay-up, skilled personnel are crucial to laminate the reinforcement and matrix, in processes such as resin mixing, laminating of resin contents, and checking the quality of the laminate. In addition, although hand lay-up does not rely on equipment, the lay-up process is slow and the efficiency is low. Therefore, this molding technique is now being gradually replaced by some more recent and automated methods with large batches and high stability requirements.

3.1.2.2 Injection molding Injection molding is an improved process of hand lay-up molding, whose automation is much improved, and it is also a relatively common molding technique in composites molding. Injection molding is to spray two resins mixed with initiator and accelerator from both sides of the spray gun, and the chopped fibers as reinforcement are sprayed from the center of the spray gun, so that the matrix and reinforcement can be composited during the spraying process. The ejected fibers are mixed with the resin and deposited directly on the mold; after reaching the predetermined thickness, it is pressed by hand and then solidified. Since injection molding offers automatic spraying, the work efficiency is high, which is generally 2 to 4 times higher than that of hand lay-up molding. The equipment of injection molding is relatively simple, and it is suitable for components with complex shapes; therefore, injection molding has become a welcome method for large-scale production, and it is usually employed for manufacturing cases and covers for ships, automobiles, and daily necessities. However, injection molding is unable to handle the situation where continuous reinforcing fibers are adopted; therefore, its applications are also quite limited.

3.1.2.3 Resin transfer molding Resin transfer molding (RTM) is a closed molding technology. The fiberreinforced preform is first assembled and placed in the mold cavity

Graphene-Reinforced Resin Matrix Composites 155

Figure 3.3.

Schematic of resin transfer molding process.

according to the design requirements, and the gas in the preform and the cavity is removed by vacuuming; then, with help of a special resin injection machine, the resin is pressed or injected into the closed cavity until the fiber-reinforced preform in the entire cavity is completely impregnated, and the composite formed after injection can be directly used for subsequent curing molding. This molding process is illustrated in Figure 3.3. Although the RTM technique requires a large budget in equipment, it has many advantages such as high efficiency, good product quality, high dimensional accuracy, and little environmental impact. Moreover, the weave of the reinforcing materials is not limited to laminates, and thus the fibers can be arranged according to the load-bearing direction of the component, and the comprehensive performance of the product is consequently improved. This technique is still in the stage of rapid development, and it has been widely used in many fields, including aerospace, automobiles, shipbuilding, electronics, and other cutting-edge industries. In the case of RTM, since the resin needs to transfer over a large number of fine pores among fibers, there are higher requirements for the quality of the resin, such as fluidity, wettability, and impurity content.

3.1.2.4 Filament winding Filament winding is a molding method in which continuous fibers are impregnated with resin and wound on a finished inner surface as well as a laminate surface on the outside diameter of the product under the condition of controlling the fiber tension and predetermined fiber type, after which the subsequent curing is performed. Since in the filament

156

Graphene Composite Materials

winding process, continuous winding is controlled by special equipment, the comprehensive mechanical properties of the product are excellent, the uniformity is great, and the production efficiency is high. Filament winding has certain requirements on the shape of the product, and it is generally used for convoluted bodies with circular crosssections, such as pipes and pressure storage tanks. Due to the highperformance characteristics of winding products, they have also received much attention in aerospace and other fields, and have been used in the manufacturing of aeroengine casings and other components.

3.1.2.5 Pultrusion molding Pultrusion is a continuous process for manufacturing composite parts with constant cross-sections by pulling fiber and resin through a die, where the length of the final products can be adjusted on demand. It allows continuous production of long parts such as rods or tubes with various profiles (square, I, U, C, etc.) that would be impractical to make using a mold. This technique has the advantages of high production efficiency, no waste, stable performance, and repeatability, and it is an important process for manufacturing lineal. The characteristics of each molding method are summarized in Table 3.1. The curing process of resin matrix composites is another important step that affects the final quality of the material. Curing is the process of thermosetting resins undergoing chemical reactions to produce composite parts with excellent mechanical properties while maintaining a certain shape, pressure, and temperature. The autoclave processing is the most important method used for curing thermosetting resins; that is, the product is wrapped in a soft vacuum bag, and high pressure and the temperature required for curing are applied in the autoclave to cure the product. Due to the unique applied pressure, the obtained product usually shows features of low porosity and excellent consistency in performance, and it has become the best processing method in the molding of high-performance composites. In addition, the process of autoclaving is also suitable for bonding of composites or co-curing of sandwich composites. Another commonly used curing process is pressure molding, in which the temperature and pressure required for curing are applied through a rigid mold to ensure the performance of the composite product (Figure 3.4).

Table 3.1. Molding method

Product quality and performance

Characteristics of resin matrix composite molding process. Characteristics of production

Variety of parts

Equipment requirements

Working environment

Depends generally on the operating process

Suitable for large-size, small-batch, and multi-variety production

Various

N/A

Harsh

Injection molding

Medium

Automated, suitable for mass production without high demands

Chopped fiber reinforced plastics (FRP) parts

Low

Harsh

RTM

Medium

Designable performance, suitable for complex product, and excellent scalability

Various

High

Good

Filament Winding

Good

High efficiency of continuous molding, geometric limitation of available tools

Continuous fiber parts

High

Medium

Pultrusion Molding

Longitudinally excellent

High efficiency of continuous molding, only suitable for straight parts

Normally fiber reinforced plastic

High

Good

Graphene-Reinforced Resin Matrix Composites 157

Hand lay-up molding

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Graphene Composite Materials

Figure 3.4.

Large autoclave equipment used for composite curing and molding.

3.1.3 Development of graphene-reinforced resin matrix composites Graphene has aroused the interest of researchers due to its extraordinary properties, and has set off a research upsurge with regard to its properties, preparation methods, and applications in various fields. The preparation of graphene-reinforced resin matrix composites is one of these important topics. Due to its unique structure, graphene has a Young’s modulus of 1 TPa and a breaking strength of 130 GPa,1,2 and its excellent mechanical and electrical properties would help to improve the mechanical and electrical properties of epoxy resins, which provide the possibility to manufacture lightweight, high-strength, and structural-functional-integrated composites.3–8 Carbon nanotubes (CNTs), carbon nanofibers (CNFs), and other reinforced polymer matrix composites have lots of research achievements, but they also suffer from complex preparation processes and high costs

Graphene-Reinforced Resin Matrix Composites 159

that restrict their practical applications.9–11 In contrast, the exceptional performance of graphene and the development of low-cost and large-scale graphene preparation have made graphene industrial-scale applications possible. By introducing graphene as reinforcement into the polymer matrix, the mechanical, electrical, thermal, and other properties of the polymer can be significantly improved, and the resulting composites have broad application prospects. Since graphene/polystyrene (PSt) conductive nanocomposites were first reported by Ruoff et al. in 2006, graphene has been introduced into a variety of polymer matrices, and a large number of high-performance graphene-reinforced polymer matrix nanocomposites have been prepared.12

3.2 Graphene-Reinforced Resin In the graphene-reinforced polymer matrix composites, uniform dispersion of graphene in the resin matrix, excellent compatibility, and interfacial interaction between graphene and matrix play a great role in giving exceptional properties to graphene and preparing composites to provide excellent performance. The preparation method of graphene-reinforced matrix composites has an important influence on the uniform dispersion of graphene in the matrix and the performance of composite materials. At present, there are mainly three methods, solution mixing, melt mixing, and in-situ polymerization. The superior performance of graphenereinforced resins gives rise to several outstanding advantages in many applications.

3.2.1 Principle of graphene-reinforced resin As mentioned above, graphene is an ultra-thin 2D layered material with a graphite-like structure. When graphene is mixed with a high polymer such as resin, due to the shape and chemical composition features of graphene, a strong van der Waals force is generated between the reinforcing graphene and the matrix, such that good interfaces are formed, which is helpful to exhibit the features of both the resin and the graphene. In terms of the load-bearing structure, the reinforcing principle of graphenereinforced resin and traditional fiber-reinforced resin is quite similar; however, there are also obvious differences between graphene and

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Graphene Composite Materials

common reinforcements: given the atomic thickness and small sheet of graphene, sliding would occur between graphene and resin when the graphene interacts with resin. Therefore, the performance characteristics of the composite consisting of graphene sheet and the resin are also different from those of the common composite materials such as fiber-reinforced resins. After the resin is reinforced by graphene, some properties are generally improved to a certain extent, including comprehensive mechanical properties, thermal conductivity, electrical conductivity, and temperature resistance. Among them, mechanical properties are generally the most important properties to be impacted when resin is used as a matrix. In general, the strength of the resin reinforced by graphene is increased by 10–50%. In some reinforcing methods, the incorporation of graphene also has a greater influence on the modulus, toughness, and wear resistance of the resin. Therefore, the research on graphene-reinforced resin has attracted great attentions in the composite material industry, and shows great application prospects. Moreover, after the resin material is reinforced by graphene, the thermal conductivity and electrical conductivity are often greatly improved. For example, incorporation of just 1 wt% graphene into epoxy resin could result in 3–4 orders of magnitude improvement of electrical conductivity, and even an improvement of seven orders of magnitude according to individual research reports. Therefore, graphene-reinforced resins have great potential for application in electrical conductivity. In addition, after incorporating graphene, the thermal conductivity of the resin also changes significantly, and for some matrix resins, the improvement can be several times as much. Therefore, in addition to their applications of conventional composites as matrix materials, graphene-reinforced resin also plays an important role in various functional composite materials. The excellent mechanical properties of the graphene-reinforced resin mean that graphene is much better than the traditional thermal and conductive fillers, and it occupies an important position in the advanced structuralfunctional-integrated composites. Meanwhile, graphene-reinforced resin is an important category of graphene-reinforced composite materials, and it is a research subject with a large number and wide range of research works at present. Many composite structures with special requirements need to be studied from the perspective of graphene-reinforced resins.

Graphene-Reinforced Resin Matrix Composites 161

3.2.2 Preparation method of graphene-reinforced resin 3.2.2.1 Solution mixing Solution mixing means to dissolve the polymer in a suitable solvent, while graphene oxide (GO) or graphene is also dissolved or dispersed in this solvent; mechanical stirring, ultrasonic mixing, etc., are performed to uniformly disperse them in the solvent, and finally the solvent is removed to obtain graphene-reinforced polymer matrix composites. The operation of this method is simple and special equipment is not required, and the obtained graphene dispersion is relatively uniform, and thus it is widely used. The dissolution and dispersion of graphene in the solvent are the key issues of this method. The oxygen-containing groups on the surface increase the polarity of GO, which can be uniformly dispersed in solvents with high polarity, such as water, N,N-dimethylformamide (DMF), and N-methyl pyrrolidone (NMP);13 therefore, polymers soluble in these solvents can be mixed with GO to prepare composites by solution mixing, such as polyvinyl alcohol (PVA), polyethylene oxide (PEO), and polyvinylidene fluoride (PVDF).14–16 In addition, GO can also be modified to improve its dispersibility in the organic solvents.17,18 Excellent dispersion and strong interfacial interaction greatly improve the mechanical and thermal properties of the prepared composites compared with pure polymers. In the process of solution mixing, chemical reducing agents such as hydrazine hydrate and hydroiodic acid can be added to the solvent, where reduced GO (rGO) is produced, and it can be utilized to prepare composites with more outstanding electrical and thermal conductivity; meanwhile, rGO is able to mix with directly polymer in solution.19,20 Although the solution mixing method is facile and feasible, it is difficult to remove and recycle the solvent, which may pollute the environment; and at the same time, due to the limited dispersibility of graphene and polymers in the solvent, it is generally not suitable for the preparation of graphenereinforced polymer matrix composites in large quantities.

3.2.2.2 Melt blending Melt blending involves melting of polymer to form viscous liquid followed by blending with graphene under shear mixing, thereby preparing the graphene-reinforced polymer matrix composite. Melt blending is also a common method for preparing polymer matrix composites, mainly for

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Graphene Composite Materials

thermoplastic polymers. This method has features of being solvent-free and emitting less environmental pollution, and conventional equipment such as a twin-screw extruder can be employed for shearing and blending, which is suitable for large-scale industrial production. Intrinsic graphene, GO, and rGO can all be composited with polymers by this method.21 During the melt blending process, GO can be thermally reduced under certain conditions, giving thermally reduced graphene oxide (TrGO). You et al. reported that GO could be thermally reduced in-situ during the melt blending of GO with styrene-ethylene/butylenestyrene (SEBS) block copolymer at 225°C for 25 min, and TrGO/SEBS composites can be easily prepared.22 There are also many studies that directly use rGO and polymer to prepare composite materials by the melt blending method. The resin matrix includes thermoplastic polyurethane (TPU),23 polymethyl methacrylate (PMMA),24 polycarbonate (PC),25 polyethylene terephthalate (PET),26 poly(vinyl chloride) (PVC),27 and polypropylene (PP).28 The disadvantage of melt blending is that the interaction between graphene and polymer is not strong, and graphene is hard to disperse uniformly. Under the effect of strong shear force, graphene sheets may be re-aggregated or curled, resulting in a decrease in aspect ratio. In addition, graphene containing unstable modified groups at high temperature cannot be used to prepare polymer matrix composites by melt blending.

3.2.2.3 In-situ polymerization In-situ polymerization deals with graphene or modified graphene material dispersion into the monomer or prepolymer matrix in the presence or absence of solvent which is followed by standard methods of polymerization. This method enables complete dispersion of graphene in polymeric matrix and strong interaction between graphene and matrix, which is beneficial to maximize the performance of graphene. A number of graphene-reinforced polymer matrix composites have been prepared by means of this method, and the polymeric matrix includes polyimide (PI),29 polymethyl methacrylate (PMMA),30 polyurethane (PU),31 polyaniline (PANi),32,33 epoxy resin,4 polyvinylidene fluoride (PVDF),16 and polystyrene (PS).34 The polymerization methods cover solution polymerization, emulsion polymerization, and bulk polymerization.35,36 The disadvantage of this method is that the addition of graphene increases the viscosity of the polymerization system, which has a significant impact on the

Graphene-Reinforced Resin Matrix Composites 163

polymerization process, i.e., more a complicated polymerization reaction and a more difficult operation.

3.2.3 Applications of graphene-reinforced resin matrix composites Owing to the advantages of high specific strength, high specific modulus, fatigue resistance, stable dimensional structure, and high degree of designability, fiber-reinforced composites made using epoxy resin formulations are intensively used in high-tech fields such as aerospace. The characterization and analyses of the performance of epoxy resins mainly include mechanical properties such as tensility, compression, and flexural and shear strength, as well as processing properties such as viscositytemperature characteristic, film-forming property, resin film quality, and curing condition.

3.2.3.1 Preparation of GO-modified epoxy resin Epoxy resin raw materials are blended according to a specific formulation, and appropriate amount of curing agent and GO is added and uniformly mixed by a kneader followed by treatments of a grinder to obtain an intermediate with incorporation of graphene. Then, more as-mixed epoxy resin raw materials are added into the container for continuous mixing; finally, the GO-modified epoxy resin (GH81) is obtained. Through manipulating the formulation, one can make the resin system meet the requirements of the prepreg hot-melt preparation process for the resin film, and this resin system without incorporation of graphene is designated as H81. In order to achieve successful preparation of resin film and its complete infiltration of carbon fiber, and to ensure appropriate operability of the prepreg at room temperature, the resin matrix is required to have excellent fluidity when heated, and a certain degree of viscosity and ease to lay at room temperature. The viscosity-temperature characteristic test via the rheometer and the gel time test via the flat plate method can determine the processing temperature of the resin and confirm that the resin can meet the processing requirements of hot-melt prepreg. The prepared H81 resin film is uniformly continuous, colorless, and transparent (Figure 3.5(a)), and not sticky to the touch or on paper, which meets the

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Graphene Composite Materials

requirements of the hot-melt processing of prepreg preparation for resin film. Similarly, the GH81 resin is not sticky to the touch or on paper and is uniform and continuous without impurities, and it has a brownish yellow color which originates from the intrinsic color of GO. In order to confirm the uniform dispersion of GO in the matrix resin, the GH81 is characterized using an optical microscope before and after grinding treatment. As shown in Figures 3.5(c) and 3.5(d), for GH81 without mechanical grinding, the agglomeration of GO is serious and the dispersion is uneven, while after mechanical grinding, there is no obvious GO agglomeration, and the dispersion is very uniform. This result indicates that the extrusion, shearing, and other applied forces during the mechanical grinding process can improve the dispersion of GO and achieve uniform dispersion of GO in epoxy resin. Subsequently, the resin film and the CCF300-3K carbon fiber are compounded by means of the hot-melt method to obtain the carbon fiber prepreg. The prepreg is cut and laminated, followed by a hot-press molding process via the compression molding method. The molding temperature of prepregs and their composites is determined by the characteristic

(a)

(b)

(c)

(d)

Figure 3.5. (a) H81 resin film; (b) GH81 resin film; optical microscopic images of GH81 (c) before and (d) after mechanical grinding.37

Graphene-Reinforced Resin Matrix Composites 165

(a)

(b)

(c)

Figure 3.6. (a) DSC curves of H81 and GH81, the heating rate is 5°C/min; (b) DSC curves of GH81 under different heating rates; (c) extrapolation of curing temperature of GH81.37

curing temperature of the matrix resin. According to the results of the curing exothermic curves of H81 and GH81, it can be evidenced from Figure 3.6(a) that when the heating rate is 5°C/min, the curing exothermic curves of H81 and GH81 are basically overlapped. The curing exothermic curves under different heating rates are shown in Figure 3.6(b), the characteristic curing temperatures are derived by means of extrapolation when β = 0°C/min, and the results are shown in Figure 3.6(c). The results also indicate that GO has a limited effect on the curing behavior of H81, which means that both resins can be cured under the same process conditions. GH81 is cured by the temperature-programmed method, and after experimental verification, a progress of 130°C/90 min + 180°C/120 min + 190°C/180 min is determined as the curing condition of GH81.

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Graphene Composite Materials

3.2.3.2 Mechanical properties of carbon fiber-reinforced composites The application scenarios strongly depend on the mechanical properties of carbon fiber-reinforced composites; thus, prepregs with high-temperature curing epoxy resin, before and after GO modification with matrix resin and carbon fiber (CCF300-3K) as reinforcement, are prepared by the hotmelt method, and the carbon fiber-reinforced composites are then prepared by the method of compression molding. The mechanical properties involving tensility, flexural strength, compression, interlaminar shear, and compression after impact are tested, and the results are listed in Table 3.2 and Figure 3.7. In the meantime, Table 3.2 also lists the mechanical properties of the composite 5228-300 with epoxy resin 5228 as the matrix resin and carbon fiber T300 as the reinforcement as reported in the literature [38] for comparison. It can be learned from the results that the longitudinal tensile, flexural, and compressive properties of GO-modified carbon fiber-reinforced composites GO 0.1-300, GO 0.5-300, GO 1.0-300 (GH81-300), and GO 1.2300 are all higher than those of the unmodified carbon fiber-reinforced composite GO 0-300 (H81-300). Compared with H81-300, the longitudinal tensile strength, flexural strength, and compressive strength of GH81300 are increased by 6.4%, 7.2%, and 7.1%, respectively, indicating that the uniformly dispersed GO is helpful to improve the strength of carbon fiber-reinforced composites along the fiber direction. This is mainly due to the fact that the GO dispersed in the matrix resin is capable of bearing part of the load when subjected to external force, thereby improving the strength of the matrix resin.39 On the other hand, during the infiltration of carbon fiber with GH81, GO with a wrinkled structure will partially wrap on the surface of carbon fiber, thereby improving the roughness of carbon fiber and increasing the surface area of carbon fiber, which is helpful for the carbon fiber and matrix resin to fully contact each other; consequently, the wetting effect and mechanical meshing effect are improved, and the interfacial bonding strength is increased.40 In order to better understand the reinforcement and modification effect of GO on carbon fiberreinforced composites, the GH81-300 specimens after longitudinal tensile test are analyzed by SEM. It can be evidenced from Figure 3.7(c) that the surface of the carbon fiber is still covered with a large amount of resin after tensile fracture, and the tearing pattern of the fracture cross-section is observed, indicating that the fiber is not easy to pull out from the resin,

Graphene-Reinforced Resin Matrix Composites 167 Table 3.2.

Mechanical properties of carbon fiber-reinforced composites.

Performance

GO GO GO GO GO 0-300 0.1-300 0.5-300 1.0-300 1.2-300 5228-30038

Longitudinal tensile modulus (GPa)

136

160

160

159

158

137

Longitudinal tensile strength (MPa)

2133

2268

2195

2270

2441

1744

Longitudinal flexural modulus (GPa)

160

140

134

162

127

130

Longitudinal flexural strength (MPa)

2089

1847

2060

2239

1804

1780

Longitudinal compression modulus (GPa)

142

145

136

142

142

110

Longitudinal compressive strength (MPa)

1427

1579

1445

1529

1536

1230

Longitudinal interlaminar shear strength (MPa)

91

56

75

65

54

106

Transverse tensile modulus (GPa)

9.8

11

10.6

10.7

10.2

8.8

Transverse tensile strength (MPa)

111

30

14.8

39

23

81

Transverse flexural modulus (GPa)

10.3

10

10.2

10.9

9.9

—

Transverse flexural strength (MPa)

100

52

47

43

72

—

Transverse compressive modulus (GPa)

11.1

10.5

10.6

10.8

11.6

9.3

Transverse compressive strength (MPa)

208

205

182

210

208

212

and there is a strong interfacial bonding between the carbon fiber and the matrix resin. In addition, compared with composite 5228-300 reported in the literature, the longitudinal flexural modulus and longitudinal compressive modulus of GH81-300 are increased by 24.6% and 29%, respectively, indicating that GH81-300 has higher flexural and compressive modulus. This means that GH81-300 has greater stiffness, and it is not easy to

168

Graphene Composite Materials (a)

(c)

(b)

(d)

(e)

Figure 3.7. (a) Strength, (b) modulus, and (c) tensile fracture morphology of H81-300; (d) thermogravimetric curve of GO; and (e) tensile fracture morphology of GH81-300.

deform when subjected to bending, compression, and other forces, suggesting the dimensional stability is improved. GH81-300 shows not only better modulus but also increased tensile strength, flexural strength, and compressive strength by 30.2%, 25.8%, and 24.3%, respectively, compared with composite 5228-300.38 It is worth noticing that GO is a high-energy, thermally unstable material prone to the disproportionation reaction even under mild heating conditions (Figure 3.7(d)). When the temperature is raised to ~100°C, its weight loss is as high as ~20%. Defects such as pinholes, microcracks,

Graphene-Reinforced Resin Matrix Composites 169

and delamination generated in the process of thermal decomposition would seriously affect the performance of composite materials. SEM images (Figure 3.7(e)) show that the existence of a large number of pinholes in the resin of the composite could lead to decrement of transverse tensile strength and transverse flexural strength of GH81-300.

3.3 Toughening of Epoxy Matrix Composites by Graphene Epoxy is a thermosetting resin containing more than one epoxy group per monomer. Epoxy resins are the polymer of choice in many fields such as aerospace, shipbuilding, and the electronic industry because of their simple molding, excellent mechanical properties, strong resistance to acid and alkali corrosion, and superior electrical properties. The applications cover the matrices for adhesives, coatings, electronic packaging materials, and carbon fiber matrix composites. Fiber-reinforced epoxy matrix composites are widely used in high-performance structural materials due to their high specific strength and high specific modulus. Carbon fiber-reinforced resin matrix composites are mainly composited of matrix resin and carbon fiber. Epoxy is brittle due to the formation of a cross-linked 3D network structure in the curing process. The performance difference between fiber and resin, as well as the laminated structure characteristics of composite materials, gives rise to an obvious interlaminar effect, which results in low interfacial bonding and interlaminar toughness, and the interlaminar and interface damage is prone to occur under various loads such as impact and bending. Therefore, the toughening of epoxy and its fiber-reinforced composite materials has become an important issue in the research of epoxy matrix composites. In fiber-reinforced epoxy matrix composites, the performance of the interface between epoxy and fibers, e.g., carbon fibers and glass fibers, is the key to determining the mechanical properties of composites. Therefore, the toughening of resin-based carbon fiber composites is mainly carried out from three aspects: toughening of matrix resin, surface modification of carbon fiber, and interlaminar toughening. Its flake-like structure and chemical feature of ease of functionalization provide graphene with excellent affinity for both the resin matrix and carbon fiber. Therefore, graphene can play a certain role in the abovementioned three main aspects. In addition, the electrical properties of epoxy resins are poor, and it is difficult to meet the requirements of the electrical properties of materials in various applications, while graphene

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Graphene Composite Materials

can also change the electrical properties of epoxy with its own excellent electrical conductivity.

3.3.1 Toughening of matrix resin Toughening and reinforcing the matrix resin is commonly done to improve the performance of carbon fiber-reinforced resin matrix composites. Epoxy is one kind of thermosetting resin; when it is used as the matrix of carbon fiber composites, modification is necessary because of its high cross-linking density, brittleness, and poor toughness. Yokozeki et al. uniformly dispersed cup-stacked carbon nanotubes into epoxy resin, and composites were prepared with the carbon fiber.41 The results show that the mode-I interlaminar fracture toughness GIC and the mode-II interlaminar fracture GIIC of the composite are 0.17 kJ/m2 and 0.79 kJ/m2, respectively, which are increased by 97.7% and 29.9% compared with unmodified composites. Graphene has also become one of the high-performance additives for toughening and modification of epoxy resins due to its exceptional mechanical properties and bonding characteristics. Shen studied the low-temperature tensile properties and impact resistance of graphene-modified epoxy resins when the graphene loading is 0.05–0.5%.5 When the loading of graphene is 0.1%, the low-temperature tensile strength, Young’s modulus, and impact strength of graphene-reinforced epoxy resin composites are significantly improved. A major scientific issue in the study of graphene-reinforced epoxy composites is the dispersion of graphene and interfacial bonding in composites. The interlayer van der Waals forces and π–π interactions of graphene or rGO obtained by redox methods allow graphene to aggregate in epoxy resins. In general, when the amount of graphene is increased to a certain extent, graphene cannot be uniformly dispersed in the epoxy resin due to the interlayer interactions, and it is difficult for graphene sheets with an inactive surface to interact with epoxy and generate an effective interfacial bonding; thus, a drop in the mechanical performance of graphene-reinforced epoxy composites is observed.42 Ball milling is used to improve the dispersion of thermal rGO in epoxy resin; the Tg and KIC of the composite with 0.2 wt% graphene are increased by 11°C and 52%, respectively, and meanwhile the modulus of the composite is also significantly increased due to the better dispersion of graphene. In contrast, as for the case with poor dispersion of graphene, the improvement of KIC is 24%, as shown in Figure 3.8.43 The surface functionalization of graphene is able to weaken the interlayer interactions, reduce the polarity difference between the sheet surface

Graphene-Reinforced Resin Matrix Composites 171

(a)

(b)

(c)

(d)

Figure 3.8. The impact on epoxy toughness with different dispersion degrees of graphene.43

and the epoxy resin, as well as increase the interaction between them, so as to achieve uniform dispersion and formation of effective interfacial bonding. As the precursor of graphene prepared by the redox method, GO contains plenty of chemical functional groups such as hydroxyl, carboxyl, and epoxy on the surface and peripheries of the sheet, which is distinct from the graphene sheet. As a result, the sheet surface polarity and the interactions between the sheets are alternated, which paves the way for improving conditions for improving the dispersion and interfacial bonding of nanofillers in graphene-reinforced epoxy composites, as well as enhancing the mechanical properties of the composites. First of all, the oxygen-containing functional groups on the surface and peripheries of GO sheets have a significant influence on the curing reaction of epoxy resin and the mechanical properties of graphene epoxy composites. Surface oxygen-containing functional groups such as hydroxyl and carboxyl groups are able to catalyze the curing reaction of amine-cured epoxy, and the catalytic effect is increased with the

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Graphene Composite Materials

augmentation of GO content.44 As a catalyst, GO is capable of reducing reaction temperature and time of the cross-linking reaction for aminecured epoxy resin by catalyzing the addition reaction of the primary amine-epoxy group, secondary amine-epoxy group, and main hydroxylepoxy group.45 After being washed by alkaline, surface oxygen-containing functional groups are removed from GO, and the enhancement effect on epoxy resin is worse than that of GO.46 These studies demonstrate the importance of functional groups on the graphene surface in promoting uniform dispersion in epoxy resins and forming excellent interfacial stress transfer. For example, Naebe et al. carried out carboxyl functionalization of TrGO, and prepared a graphene-reinforced epoxy resin composite with 1 wt% graphene by solution mixing.47 The dispersion of graphene is uniform, and the interfacial bonding is strong; meanwhile, the flexural strength of the composite is increased by 22%, as shown in Figure 3.9. Second, the oxygen-containing functional groups on the surface of GO will act as co-curing agents to catalyze the curing reaction of the

(a)

(b)

Figure 3.9. Surface carboxylation of graphene and the flexural performance of reinforced epoxy resin composites.47

Graphene-Reinforced Resin Matrix Composites 173 (a)

(b)

(c)

Figure 3.10. Amino functionalization of carboxyl functional groups on the peripheries of GO.51

corresponding epoxy curing system, e.g., amidation reaction between carboxyl groups and epoxy curing agents such as diamino diphenyl sulfone (DDS),48 isophorone diisocyanate (IPDA),49 and imidazole,50 which results in composites with increased cross-linking density and enhanced covalent bonding of interfaces. At the same time, the amine groups on the surface of functionalized GO promote the uniform dispersion of the sheets, which improves the tensile strength and modulus, flexural properties, and impact strength of the graphene epoxy composites. Amino-functionalized graphene is prepared through a reaction between carboxyl groups at the peripheries of GO and ammonia, as shown in Figure 3.10.51 With the addition of 0.1 wt% amino-functionalized graphene, the tensile modulus and flexural strength of E-51 epoxy resin are increased by 14.16% and 94.38%, reaching 2.81 GPa and 135.58 MPa, respectively. When the addition is 0.5 wt%, the tensile strength and flexural modulus are increased by 27.84% and 7.75%, reaching 67.28 MPa and 3.48 GPa, respectively. In addition,

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Graphene Composite Materials

n-Dodecyl-beta-D-maltoside (DDM)-functionalized GO is capable of increasing the viscosity of the toughened resin system by promoting the curing reaction of epoxy, which is believed to suppress the separation of curing phases and improve the modulus and toughness.52 Third, the reactive grafting of the end functional polymers onto the oxygen-containing functional groups on the GO surface would give rise to a polymer coating layer, which improves the dispersion and interfacial bonding of the graphene sheets in epoxy resin. The epoxy coating layer on the surface of the GO by the ring-opening reaction of the carboxyl and epoxy group would promote the uniform dispersion of GO in epoxy and the formation of covalent interfacial bonding, which increases the Tg of epoxy resin. When graphene loading is 0.25 wt%, the tensile strength and modulus are increased by about 13% and 75%, reaching 92.94 MPa and 3.56 GPa, respectively, and KIC is increased by about 41%.53 Flexible amine-terminated poly(butadiene-acrylonitrile) (ATBN) chains are grafted onto the surface of GO using 4,4′-methylene diphenyl diisocyanate (MDI) as the coupling agent; an impressive toughening effect is observed for the prepared composites, which shows a 1.5-fold improvement in KIC and a corresponding 2.4-fold improvement in GIC at 0.04 wt% of graphene loading.8 Polybenzimidazole is grafted onto dicarboxydiphenyl ether-functionalized thermally expandable graphene by in-situ polymerization to achieve polymer functionalization on the surface of graphene, which promotes more uniform dispersion of sheets in the resin and results in covalent bonding, thereby improving the modulus, strength, and fracture toughness of graphene epoxy resin composites, as shown in Figure 3.11.54 Finally, the dispersion and interfacial bonding of graphene in the epoxy resin are influenced by functionalization of surface functional groups on graphene and structural diversity of functionalized molecules; thereby, the performance of graphene epoxy composites can be manipulated. Studies have shown that the epoxy groups of the GO modified by coupling agent (3-glycidoxypropyl) trimethoxysilane (GPTS) contribute to excellent compatibility between GPTS-GO and epoxy; the dispersion is much improved and a large number of soft interphases are created, which this more effective in enhancing toughness of the matrix resin.46 Meanwhile, the strong covalent bonding formed between the GO sheets modified by (3-aminopropyl) trimethoxysilane (APTS) and the epoxy matrix optimizes the stress transfer, where the enhancement of tensile strength is more obvious as shown in Figure 3.12. In addition, amineterminated polyetheramines (PEA) with different chain lengths are grafted onto GO through amidation reaction; strong covalent interfacial bonding

Graphene-Reinforced Resin Matrix Composites 175

Figure 3.11. Mechanical properties of polybenzimidazole-grafted graphene epoxy resin composites.54

is formed between such modified GO and epoxy, and the strength and toughness of the epoxy composite are dependent on the length of the OEA chain.55 A more flexible and soft interphase is formed between epoxy resin and functionalized GO with a long PEA chain rather than that with a short chain, and greater deformation and load transfer occur upon stress. As shown in Figure 3.13, the toughness of two composites is increased by 119% and 90% at 0.50 wt% loading, whereas the tensile strength is increased by 51% and 63%. The above studies show that the dispersion and interphase problems of graphene sheets in epoxy resin can be solved through the functionalization of graphene surface as well as chemical structure control of functionalized groups and molecules, and the purpose of improving the mechanical properties of epoxy resin composites can be achieved.

3.3.2 Surface modification of carbon fiber Surface modification of carbon fiber is another common method in the modification of carbon fiber-reinforced resin matrix composites.

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Graphene Composite Materials

(a)

(c)

(b)

(d)

Figure 3.12. Dispersion, interphase, and mechanical properties of GO-reinforced epoxy resin composites treated with different silane coupling agents.46

By means of surface modification of carbon fiber, the interfacial bonding strength between carbon fiber and matrix resin is improved, and toughening and reinforcement of composite materials are achieved. Since the carbon fiber precursor has a low surface energy and low specific surface area and it is difficult to form bonds with resins and give rise to effective adhesion, the interface strength between the carbon fiber and matrix

Graphene-Reinforced Resin Matrix Composites 177

Figure 3.13. Schematic illustration of the interphase and deformation under load for PEA-grafted GO-reinforced epoxy resin composites.55

resin is weak, and the toughness and other mechanical properties of the composites are reduced.56 One of the methods for carbon fiber surface modification is the coating technique; that is, through vapor deposition, sizing treatment, and coupling agent treatment, a polymer transition layer is formed on the surface of carbon fiber, which improves the surface energy of carbon fiber, reduces the contact angle between carbon fiber and the matrix, enhances the wettability of the resin to the carbon fiber, increases the surface roughness of the carbon fiber, and enhances the interface mechanical interlocking effect between the carbon fiber and the resin. Another method of carbon fiber surface modification is the oxidation technique; that is, the carbon fiber is subjected to an oxidation treatment, through which the surface is oxidized and reactive functional groups are generated, such as carboxyl groups, hydroxyl groups, and epoxy groups, and the chemical bonding between the carbon fiber and the resin matrix is enhanced. Consequently, the bonding strength between resin and carbon fiber is enhanced, and thus the toughness of the composite material is improved.57 Carbon nanotubes were incorporated on the carbon fiber surface, and a composite material was prepared with methylphenylsilicone resin.58

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Graphene Composite Materials

The impact strength of this composite material is 77.85 kJ/m2, which is 33.17% higher than that of the unmodified carbon fiber composite material. The sizing agent was prepared by diazepine biphenyl polyaryl ether ketone, and then the sized carbon fiber and epoxy resin were used to prepare the composite material.59 The type-II interlaminar fracture toughness GIIC of the toughened composite is 1.04 kJ/m2, which is 51% higher than that of the unmodified composite, and the interlaminar shear strength is also increased by 12%. Graphene is considered to be a transition layer material with excellent performance. Due to its thin or ultra-thin nature and large specific surface area, it can interact strongly with carbon fibers and matrix materials at the same time. The introduction of graphene on the fiber surface can effectively improve the interfacial interaction of composite materials, and a toughening effect is achieved. In addition, it is easy for graphene to be functionalized, and the strength of such an interaction can be further enhanced by appropriate functionalization. The introduction of GO into carbon fiber-reinforced epoxy resin composites is able to improve the interface properties between the fiber and the matrix. It has been found that the addition of GO can simultaneously improve the interfacial shear strength, interlaminar shear strength, and tensile properties of carbon fiber composites.60 The existence of graphene sheets on the fiber surface improves the interface properties between fibers and resins, which provides a good pathway to improve the mechanical properties of composites. The aminofunctionalized graphene was directly grafted on the surface of the carbon fiber treated with strong acid oxidation by the amidation reaction, which significantly increases the surface energy and content of functional groups on the carbon fiber surface, thereby enhancing the interface strength between the carbon fiber and the resin matrix, and the interfacial shear strength is increased by 36.4%.61 Negatively charged GO sheets can be deposited on the surface of fiber by electrophoresis, and the interfacial shear strength between the fibers with deposited GO on the surface and epoxy resin can be manipulated by adjusting the magnitude of the electric field.62 The reduction of GO sheets deposited on the surface of carbon fibers is helpful to improve the AC conductivity of carbon fiber-reinforced epoxy composites.63 0.5 wt% silanized GO was deposited on the surface of carbon fiber, giving a rigid gradient interphase containing graphene between the carbon fiber and epoxy, and the interlaminar shear strength (ILSS), flexural strength, and flexural modulus of such multi-scale epoxy

Graphene-Reinforced Resin Matrix Composites 179

resin composite are increased by 19%, 15%, and 16%, respectively.64 T700 carbon fiber was sized by a graphene-containing sizing agent, the interfacial shear strength (IFSS) of multi-scale carbon fiber epoxy resin composite is increased by 36.3%, and the ILSS is raised by 12.7%.60 Two kinds of nanomaterials, polyarylimide nanofibers and aminated GO, are introduced onto the surface of glass fibers by means of the layer-by-layer self-assembly method; the large number of functional groups on the surface of glass fibers increases the interfacial bonding with epoxy resin. In addition, the GO sheet has excellent mechanical properties, which can realize the adjustment of the interfacial shear strength between fibers and epoxy resin, with a maximum increment of 39.2%.65 Graphene can also serve as multi-scale reinforcement by improving the in-plane and out-of-plane mechanical properties of composites. For example, graphene sheets were electrostatically sprayed on the surface of carbon fibers and simultaneously dispersed uniformly in epoxy resin; the fracture energy and flexural strength of this multi-scale composite are found to increase by 55% and 51%, respectively.66 The introduction of graphene nanoplatelet-modified epoxy resin laminates between the laminates of carbon fiber epoxy resin composites increases the interlaminar fracture toughness by 51% when the incorporation of graphene nanoplatelets is 0.5 wt% in the final composite.67 When the GO-reinforced epoxy interleaf is incorporated into the interface of CFRP laminates with 2 g/m2 addition of GO, the interlaminar fracture toughness and resistance of the specimen increase by 170.8% and 108.0%, respectively, compared to those of the plain specimens.68 Remarkable increases of 70% and 206% in mode-I and mode-II interlaminar fracture energies, respectively, and 36% enhancement in interlaminar shear strength are achieved in the glass fiberreinforced epoxy composite by incorporating 3D network graphenemodified epoxy resin with excellent mechanical properties as the interleaf.69 Another method of modifying resin matrix composites is interlaminar toughening, also known as ex-situ toughening; that is, by compounding pure toughening phase components on the surface of pristine prepreg, the evolution of the phase structure is controlled through diffusion and reaction at the interface as well as interaction of each phase, thereby toughening the interlaminar strength.70,71 By means of this method, the toughening effect can be localized at the interlaminar sites that contribute the most to the toughness of the composite, and the toughening of the composite is maximized while the manufacturability of the original resin-based

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Graphene Composite Materials

prepreg is retained. However, this technique is currently in the laboratory research stage and has not been widely used.

3.3.3 Graphene macrostructure-reinforced resin matrix composites Graphene is currently the hardest and strongest material ever known. Its Young’s modulus and tensile strength are 1 TPa and 130 GPa, respectively, and it has a high specific surface area, which makes graphene not only an excellent additive for modifying resin matrix or carbon fiber surface but also a suitable reinforcement directly used for resin matrix composites. Studies have shown that small amounts of graphene can significantly improve the properties of polymers. Rafiee et al. prepared graphene by thermal exfoliation of GO, and compared the modification effects of graphene, single-walled carbon nanotubes, and multi-walled carbon nanotubes on the mechanical properties of the composites.3 The results show that the tensile strength of graphene-modified epoxy resin is 40% higher than that of pure epoxy resin, and 14% higher than that of multi-wall carbon nanotube/epoxy composites. The Young’s modulus of graphenemodified epoxy resin is 31% higher than that of pure epoxy resin and 3% higher than that of single-walled carbon nanotube/epoxy composites. The multi-scale reinforcement consisting of carbon nanotube and graphene can be used for reinforcing epoxy resins. Carbon nanotubes were grown on graphene directionally, providing a good interfacial bonding between the nanofiller reinforcement and the matrix, thereby effectively improving the mechanical properties of the composite material.72 When the loading of multi-scale CNT-graphene reinforcement is 0.5 wt%, the tensile modulus shows 40% increment and the tensile strength is enhanced by 36% with respect to the neat epoxy. Compared with the composites modified by a single filler, the composites reinforced synergistically by graphene nanosheets and carbon fibers show greatly improved mechanical properties and thermal stability. For example, a synergistic enhancement effect is observed for graphene and carbon fiber on the polyethylene terephthalate matrix.73 Traditional graphene-modified resin materials are expected to improve the mechanical properties of the matrix material, and also to exhibit excellent electrical conductivity. However, the sheet resistance of common GO is 1012 Ω/sq (square) or even higher, and the existence of

Graphene-Reinforced Resin Matrix Composites 181

defects and functional groups on the sheet would deteriorate the longrange electron transport. Therefore, removal of functional groups on the GO surface and restoration of the complete layered structure by reduction methods are required for achieving excellent electrical performance.74 After the reduction of GO, the interaction between sheets would lead to problems of dispersion and interfacial bonding of rGO in the epoxy resin, giving rise to reduced content of the conductive graphene in the composite, due to which it is difficult to construct an effective electron transport structure and achieve simultaneous improvement of mechanical properties and electrical conductivity of the composite. For example, the AC conductivity of microwave-assisted exfoliated GO-reinforced epoxy composites can reach 10–5 S/m, but the enhancement of mechanical properties is weakened when the rGO content exceeds 0.25 wt%.75 The continuous structures architeched by graphene, such as graphene fibers and graphene films, lead to a continuous pathway for the electrons in the their composites due to the conductive structure given by the arrangement and stacking of graphene sheets, thereby improving the electrical conductivity and mechanical properties of the resin matrix composites.76 The large-sized GO sheets in the water-based epoxy are reduced by hydrazine and self-aligned, and a continuous conductive structure and enhanced mechanical performance are achieved in the final composite. At 1.5 wt% loading, the orientational tensile strength and Young’s modulus are increased by 500% and 70%, respectively, and the orientational conductivity could reach 10 S/m at 2 wt% addition.77 A hydrogel with a laminar structure is obtained as GO is thermally reduced in HI solution, and after cold drying, an aerogel with a continuous laminar structure can be obtained with electrical conductivity of 0.04–0.25 S/cm.78 Figure 3.14 shows the composite with anisotropic electrical conductivity prepared by vacuum-assisted infiltration of epoxy resin, where the electrical conductivity is up to 2 × 10−3 S/cm and the improvement of fracture toughness is elevated by 64%. Graphene grown on the template by the CVD method shows an intact layered structure and 3D network, and the template was etched after infiltration of epoxy resin and cured by hot pressing, following which a final 3D graphene/epoxy composite was obtained, as shown in Figure 3.15.79 A remarkable electrical conductivity of 3 S/cm is delivered at 0.2 wt% graphene, and there is a notable increment of Tg by 31°C, along with 53% (130 MPa) and 38% (4GPa) enhancements in flexural modulus and strength, respectively, and the fracture toughness of the composite is up to 1.78 MPa/m2 at 0.1 wt% graphene content.

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Figure 3.14. Properties and microstructure of 3D layered graphene aerogel-reinforced epoxy resin composites.78

Figure 3.15. Flowchart for the preparation of 3D graphene/epoxy composites prepared by template-directed CVD process and their performance.79

In addition, 3D graphene conductive structures can also be created by the reaction between GO and reactive compounds such as phenolic, polyamines, and cross-linked polymers. For example, the chemically crosslinked 3D graphene structure via the reaction of GO with glutaraldehyde and resorcinol gives an electrical conductivity of 3.4 × 10–2 S/cm after cool drying followed by reduction in a hydrazine hydrate atmosphere.80 The sol–gel polymerization of formaldehyde and resorcinol in GO aqueous solution results in chemically cross-linked GO gels, which are then pyrolyzed to obtain graphene aerogels with a conductivity of 1 S/cm.81 Upon taking ethylenediamine as a cross-linking agent to react with GO aqueous dispersion at 95°C for 6 h, and then lyophilizing and subjecting it to microwave-assisted reduction treatment for 1 min, an ultra-light

Graphene-Reinforced Resin Matrix Composites 183

conductive graphene aerogel can be obtained, which is infiltrated in epoxy resin and then cured to give the composite; the conductivity of this composite is up to 3.1 × 10–3 S/cm (Figure 3.16).82 3D graphene aerogel with good mechanical properties was prepared via the in-situ reduction assembly method by taking paraphenylene diamine (PPD) as a reducing and functionalizing agent followed by lyophilization; the composite was then fabricated by vacuum-assisted suction, whose electrical conductivity is greater than 10–3 S/cm with 0.95 vol% loading of aerogel (Figure 3.16).83 Epoxy-based composites reinforced by 3D graphene skeleton (3DGS) were fabricated by the resin transfer molding (RTM) method, in which the graphene sheets is in good dispersion and arrangement.84 3DGS was synthesized in the process of self-assembly and reduction with poly(amidoamine) dendrimers, and its tensile and compressive strength significantly increased by 120.9% and 148.3%, respectively, as well as an increase in the glass transition temperature by a notable 19°C, compared with the thermally exfoliated graphene/epoxy composites via the direct mixing route at only 0.20 wt% content of fillers. The π–π interaction and hydrogen bonding between 1D nanomaterials, e.g., carbon nanotubes and nanofibers, and graphene or GO are utilized to construct a 3D conductive structure with good electrical conductivity. For example, cellulose nanofibers (CNF), GO, and hydrazine hydrate were

Figure 3.16. 3D graphene structures chemically cross-linked by polyamine and their epoxy resin composites.82,83

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Graphene Composite Materials

mixed by ball milling to in-situ reduce GO, and the CNF/rGO hydrogel was obtained with hydrogen bonding between them; then, fiber hybrid graphene conductive aerogel with 3D porous structure was fabricated with a conductivity of 15.28 × 10–2 S/cm.85 Microfibers with conductivity of 649 ± 60 S/cm were fabricated through a carbonization of well-aligned GO–nanofibrillated cellulose (NFC) hybrid fibers.86 GO acts as a template for NFC carbonization, which changes the morphology of carbonized NFC from microspheres to sheets while improving the carbonization of NFC. Meanwhile, the carbonized NFC repairs the defects of rGO and links rGO sheets together. The GO-templated carbonization of NFC, as well as the alignment of the building blocks along the fiber direction, leads to excellent conductivity. Conductive carbon nanofiber/rGO nanoporous aerogels were fabricated by the carbonization of 3D structure consisting of GO and polyacrylonitrile nanofibers, which is also known as the precursor of the carbon nanotube.87 The electrical conductivity of this composite is measured to be 9.26 × 10–2 S/cm, and it shows excellent compressive behavior, as shown in Figure 3.17. The conductive

Figure 3.17. aerogel.87

Compressive performance and microstructure of carbon nanofiber/rGO

Graphene-Reinforced Resin Matrix Composites 185 (a)

(b)

(c)

(d)

Figure 3.18. Micromorphology of MWCNT/GO conductive aerogel and the electrical conductivity of its composites.88

multi-walled carbon nanotubes (MWCNTs) and GO are uniformly mixed in water, followed by freeze drying, and then heat treating to obtain conductive aerogels.88 The microscopic morphology is shown in Figure 3.18, and the best conductivity of this epoxy resin composite is 5.2 × 10–2 S/cm. The research on the graphene epoxy composite material with continuous structure shows that the graphene/epoxy 3D composite prepared by constructing a 3D graphene conductive structure has the characteristics of a continuous and uniform graphene phase, excellent mechanical properties, and electrical conductivity. In the 3D continuous conductive graphene structures prepared by the template method, chemical cross-linking method and nanofiber hybridization, the sheet-sheet interaction and sheetfiber interaction is able to improve the mechanical properties and electrical conductivity of 3D graphene structures. The well-aligned microstructure of 3D graphene determines that the continuous conductive structure of graphene sheets is created in epoxy resin, and its porosity facilitates the molding of epoxy resin composites, which provides a favorable way to research the structural-functional-integrated graphene-reinforced epoxy resin composites.

3.3.4 Applications of graphene-toughened resin matrix composites As we all know, one of the bottlenecks in performance of carbon fiber-reinforced resin composites in practical applications is the brittleness caused by the low interlaminar bonding strength; the compressive strength after impact (CAI) is also an important and unique specification

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for evaluating the performance of resin matrix composites. The CAI of graphene-toughened resin composites can be increased by nearly 30%, which will greatly improve the comprehensive performance of resin matrix composites, especially in high-end applications such as aerospace. The high requirements for performance in extreme environments are the best field for the application of graphene resin matrix composites. Its typical applications include parts with high requirements of performance that bear large loads and call for a high degree of safety, such as wing leading edge of aircraft and fan blades of aeroengines. In addition, due to excellent electrical and thermal properties of graphene resin matrix composites, they can also be used as functional composites, which are expected to show their potential in mechanical bearing, and electrical and thermal conductivity simultaneously. At present, such applications are still limited by design techniques, and the excellent properties of graphene resin matrix composites cannot be fully exerted, but it is certain that graphene resin matrix composites will develop rapidly in this field hereafter.

3.4 3D Printing of Graphene Resin Matrix Composites 3.4.1 Introduction to 3D printing technology 3D printing, also known as additive manufacturing, refers to the technology of manufacturing 3D entities via layer-by-layer superposition of materials based on digital model files. Compared with traditional “subtractive” manufacturing, the outstanding advantages of 3D printing technology include the following: (1) converting the structural information of products into data files through a computer, which can realize digital and intelligent manufacturing; (2) simplifying the production process and shortening the manufacturing cycle to achieve rapid prototyping; (3) toolfree or mold-free components with very complex structures can be manufactured; (4) “near-net shape manufacturing” can be achieved, alleviating the waste of raw materials and environmental pollution; (5) layer-by-layer processing method of 3D printing technology favors the preparation of inhomogeneous functionally graded composite materials; and (6) customized products and quick response to market demands. In recent years, 3D printing technology has developed very rapidly and has been widely used

Graphene-Reinforced Resin Matrix Composites 187

in medical, aerospace, architecture, art, food, and other fields, and it is a promising new manufacturing technology. 3D printing technology also provides new ideas for the preparation of polymer matrix composites. The combination of 3D printing technology and the preparation of graphene/polymer matrix composite can realize rapid manufacturing of composites with complex structures. The addition of graphene will improve the mechanical and functional properties of 3D printed products; in the meantime, it is more convenient to prepare functional graded products. In addition, the layer-by-layer manufacturing method of 3D printing inhibits the large-scale agglomeration of graphene in the polymer matrix, which is beneficial for achieving uniform dispersion.

3.4.2 3D printing process of graphene resin matrix composites With the continuous development and improvement of 3D printing technology, various types of new 3D printing processes have emerged in an endless stream. At present, the 3D printing processes suitable for graphene/polymer matrix composites mainly include inkjet printing, fused deposition modeling (FDM), stereolithography (SLA), and selective laser sintering (SLS), and the features of these printing processes are shown in Table 3.3. Each printing process has corresponding advantages and disadvantages, and decision should be made comprehensively according to the feature of printing materials, process characteristics, and the usage of products.

3.4.2.1 Inkjet printing Inkjet printing has developed from a technology originally used for text and picture printing to a rapid processing and prototyping method. As an additive manufacturing technology, inkjet printing has been widely used in electronic circuits, flexible devices, etc.109 As shown in Figure 3.19(a), in the common piezoelectric inkjet printing process, the printing material is first dissolved or dispersed in a solvent to form “ink”, and then a voltage is applied through the piezoelectric element according to the printing needs; it deforms, compressing the ink out of the capillary and ejecting a droplet. The process then repeats to produce a series of droplets accumulated on the substrate, giving rise to the pattern that needs to be printed.

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Table 3.3.

Process Inkjet

Features of 3D printing processes for graphene/polymer matrix composites.

Materials Electrochemically exfoliated graphene/EC Graphene/PVP

Wide range, low cost, simple equipment operation, compatible with soft materials

Lower mechanical strength, slower speed, poor interaction with the substrate

Refs. 89

3.44 —

0.008

—

100–500

4–10

10–90

0.2

94

0.4

—

95

Graphene/PLGA

100–1000

—

96

RGO/ABS or PLA

200–400

20

400

20

Modified graphene/ P(MAA-co-EG) Graphene/EC RGO/PANI

GO/TPU/PLA Graphene/PA12

200

60

RGO/PLA

200–400

—

Graphene/PCL

100–500

—

GO/GelMA/PEGDA

25

10

GO/epoxy-based photopolymer

—

—

GO/acrylate-based photopolymer

50

—

Graphene/GelMA SLS

—

Disadvantage

22

GO/PEDOT

SLA

0.025

Advantage

10–200

RGO/PVA

FDM

Z-axis Printing resolution speed (µm) (mm/s)

—

—

GO/PVA

100–200

6.67

Graphene/PA11

75–150

—

—

—

Graphene/PA2200

Low cost, fast, high strength, suitable for a variety of materials

High resolution, high surface quality

Anisotropic printed materials, easily damaged nozzle, easy to warp, and support part needs to be designed and separated High cost, limited materials, residual toxicity of reagents

90 91 92 93

97 98 99 100 101

102 103 104 105

High strength, High cost, serious easy separation surface of support granularity

106 107 108

Consequently, post-processing methods such as heat treatment and freeze drying are adopted to remove the solvent, and final product is obtained.110 The high carrier mobility of graphene makes it very suitable for the preparation of nanoelectronics, and inkjet printing is a commonly used

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(b)

(c)

(d)

Figure 3.19. Schematic diagram of typical 3D printing process for graphene/polymer matrix composites: (a) inkjet printing; (b) fused deposition modeling; (c) stereolithography; (d) selective laser sintering.111

convenient and efficient preparation method. The addition of polymers is able to stabilize the ink and prevent graphene from precipitation and stratification, and the viscosity of the ink can be adjusted to print with ease. Ethyl cellulose (EC)89 and polyvinylpyrrolidone (PVP)90 are usually added to graphene inks as stabilizers and viscosity modifiers. GO and polyvinyl alcohol (PVA) were dissolved and mixed in water, then reduced with hydrazine hydrate, and finally dispersed in a mixed solvent of N,Ndimethylformamide (DMF) and water to prepare rGO/PVA ink; the electrodes of organic field effect transistors were prepared by inkjet printing.91 In contrast to the traditional Au and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrodes, the field effect mobility of rGO/PVA electrodes printed using inkjet has been greatly improved. GO and conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) were dispersed in a mixed solution of water, ethanol, isopropanol, and hexanol to make inks for inkjet printing, and then the conductivity was restored by heat treatment; that is, flexible films with excellent conductive

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and dielectric properties were prepared.92 A pH-responsive surfactant can be prepared by grafting the polymer on the GO sheet, and the viscosity of the obtained ink can be manipulated by adjusting the pH; then, a 3D entity can be continuously printed by a 100-µm nozzle. The 3D inkjet printer is simple, low cost, and easy to operate, which is very suitable for the preparation of micro- and nanodevices and electronic circuits.93 The disadvantage of this method is that the strength of the prepared device is not very good, so defects are prone to occur after the solvent is removed after postprocessing, and the device easily falls off the substrate.

3.4.2.2 Fused deposition modeling (FDM) Fused deposition modeling is mainly suitable for 3D printing of thermoplastic polymers and currently is the most widely used 3D printing process. In this method, the polymer is prepared into a filament wire with standard diameter, and then the wire is transported to the nozzle by a stepping motor, heated to a molten state, and then extruded through the nozzle. The material is deposited on the build plate layer by layer until the required product shape and size are manufactured. The principle of the FDM process is illustrated in a schematic diagram in Figure 3.19(b),112 and a typical printer configuration is shown in Figure 3.20. 3D printing filament is prepared by graphene polymer matrix composite fabricated by melt blending, solution mixing, etc.; thus, the fabrication of the graphene polymer matrix composite can be performed by FDM. The introduction of graphene not only enhances the mechanical properties of 3D printed products but also results in excellent electrical, thermal, and frictional and wear properties. Figure 3.21 presents 3D printing filaments and products of graphene polymer matrix composite prepared by AECC Beijing Institute of Aeronautical Materials. Acrylonitrile-butadiene-styrene (ABS) copolymer and polylactic acid (PLA) are the most widely used polymers for FDM. A mixture of these polymers was used with GO by solution mixing followed by hydrazine hydrate reduction for preparing rGO/ABS and rGO/PLA composite, and their filaments were fabricated for FDM.97 The maximum loading of GO can reach 5.6 wt%, and the conductivity can reach 1.05 × 10−3 S/m. The addition of graphene increases the glass transition temperature (Tg) of the polymer; thus, the printing temperature needs to be appropriately increased with respect to the neat resin. Composites were fabricated by thermoplastic polyurethane (TPU) as well as PLA and GO via solution mixing for

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Figure 3.20. Typical FDM 3D printer.

FDM.98 TPU and PLA are mixed to complement each other, making the composite tough and rigid. The addition of GO not only improves the mechanical and thermal properties but also results in excellent antibacterial properties and biocompatibility. This composite material can be used for bioscaffolds and tissue engineering by means of FDM. The graphene nanoplatelets (GNPs) were melt blended with nylon 12 (PA12) and used for FDM.99 It is found that the GNPs are oriented during the extrusion process from the nozzle, and the thermal conductivity and elastic modulus of the 3D printed parts along the orientation direction are improved by 51.4% and 7%, respectively, with respect to the compression-molded products. FDM is able to print a wide range of filaments, and the features of low equipment cost, simple operation, fast printing speed, and multiple nozzles are available to print different kinds of filaments at the same time, making it one of the most promising printing methods for industrial applications. FDM technique faces several obstacles, such as printing accuracy being not high enough, nozzle blockage by a large amount of graphene addition, deterioration of printing effect by holes easily generated in the process of preparation of the composite filaments, easy warping of printed

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Figure 3.21. Graphene 3D printing filaments and products fabricated by AECC Beijing Institute of Aeronautical Materials.

product when the thermal stress is uneven, and anisotropic mechanical properties and low interlayer strength of printed product.

3.4.2.3 Stereolithography (SLA) Stereolithography, also known as optical fabrication or photo-solidification, is a form of 3D printing technology using photosensitive resin as the printing material. The laser beam sweeps the surface of the liquid photosensitive resin according to the designated route; thus, the specific area of the photosensitive resin is cured, resulting at a layer of cross-section of the model. Then, the platform descends a level, and the next section is then solidified. There are as many printing cycles as there are layers necessary to obtain the complete volume of the piece, as shown in Figure 3.19(c).113 Photosensitive resin is generally composed of polymer monomer or prepolymer, to which a light (ultraviolet) initiator or photosensitizer is added, and the widely used photosensitive resins are epoxy acrylates, unsaturated polyesters, urethane acrylates, etc.114 When the graphene/polymer matrix composite material is fabricated by stereolithography, in general, the graphene is dissolved in a solvent followed by the addition of photosensitive resin, or the graphene is directly added into the resin and mixed, and then photocuring is performed. GO is added to polyethylene glycol diacrylate (PEGDA) and gelatin methacrylate (GelMA) in phosphate-buffered solution (PBS) followed by introduction of photoinitiator to give photosensitive resin.102 Among them, GelMA and PEGDA are two commonly used photocurable biomaterials, and the addition of GO can promote the adhesion, growth, and differentiation of biological stem cells.115

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This photosensitive resin can be used for fabricating biological scaffolds via stereolithography, which promotes chondrogenic differentiation of human bone marrow mesenchymal stem cells. The GO is ultrasonically dispersed in anhydrous ethanol, and the silane compound is added to modify the surface of GO. Then, the epoxy-based photosensitive resin is dispersed in the mixture followed by drying in vacuum to remove excess ethanol. This as-prepared photosensitive resin is appropriate for stereolithography, and it is suitable for intraoral model fabricating in the dental industry.103 It is believed that GO can improve the tensile strength and absorbance at given wavelengths of the photosensitive resin. Stable dispersions of graphene in vinylpyrrolidone (VP) are prepared by applying ultrasound treatment to powdered graphite, followed by centrifugation to remove heavy aggregates. A polyvinylpyrrolidone (PVP) precursor is synthesized in a radical photopolymerization with addition of initiator, which could be used in the round of subsequent stereolithography. This method combines the process of the mechanical exfoliation of graphene and the process of compounding process between graphene and photosensitive resin, which effectively eliminates the troublesome steps of removing the excess solvent.104 In addition, it is reported that GO is directly added to commercial photosensitive resin for printing, which is used to improve the mechanical properties of products; or, high-temperature post-treatment of the printed products is performed to remove the polymer, and the GO is thermally reduced simultaneously, and the 3D rGO structures are thus prepared.116–118 Stereolithography 3D printing technology is particularly precise and accurate, which allows excellent surface quality, making it the ideal method for the production of complex prototypes and display models. The stereolithography has become a dominant technology taking over the high-end 3D printing market. The bottleneck of this technology is its high cost, and the residual photoinitiator and uncured photosensitive resin can be toxic. In addition, it is necessary to prevent graphene from separating out from the photosensitive resin during the printing process, resulting in uneven distribution of graphene in the products.

3.4.2.4 Selective laser sintering (SLS) Selective laser sintering is a 3D printing technology suitable for powder materials, which are mainly metal and ceramic powders, but also thermoplastic polymer powders. As shown in Figure 3.19(d), during the printing process of SLS, the material barrel first rises a certain distance. A thin

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powder layer is spread over a platform using a leveling roller and is selectively sintered by a PC-controlled scanning laser beam, according to the sliced pattern, and the process repeats for each layer until parts are complete.119 The graphene nanosheets and nylon 11 (PA11) can be melt-mixed and granulated with a twin-screw extruder, and then pulverized at low temperature to give powder for SLS. The addition of graphene improves the tensile modulus, flexural modulus, and thermal stability of PA11, and makes PA11 conductive, which can be used for static dissipation.107 Compared with other 3D printing technology, the composite obtained by SLS shows better conductivity, and the amount of graphene added for dissipation of electrostatic charge is low. In addition, graphene can enhance thermal conductivity of the composite, making the laser fusion sintering process easier. Kim et al. used a rotary mixer to mix graphene with nylon 2200 (a nylon material suitable for SLS developed by EOS, Germany) for SLS.120 After mixing for 8 h, the plasticity of nylon 2200 is improved, and lots of broken graphene sheets are embedded into the interior from the surface of nylon powder under the mechanical effect, and interconnection of a neck geometry is formed between the adjacent powders, which contributes to the interdiffusion of atoms during laser sintering. In addition, GO/PVA composite powder can be prepared by solution mixing, which is employed to fabricate bioscaffolds via SLS.106 Due to the strong hydrogen bond interaction, GO and PVA are tightly bonded, and the compressive strength, Young’s modulus, and tensile strength of the GO/PVA scaffold with 2.5 wt% GO are increased by 60%, 152%, and 69%, respectively, with respect to the neat resin. The advantage of SLS is that there are a variety of powder materials available, which can be mixed and sintered to form composite. In this 3D printing technology, supports are not required, and the material utilization rate is high. However, in the meantime, the powder materials used for SLS must follow several rules. The powder should have considerable thermal conductivity to avoid shrinkage and warpage due to thermal distortion in the fabricated parts. The powder should have considerable mechanical strength after printing, and the particle size should be uniform — 10–100 µm is preferred.108 In addition, the powder should exhibit good thermoplastic and processing properties. The introduction of graphene into the polymer powder can improve the thermal conductivity of the powder and significantly alleviate the thermal warpage. At the same time, graphene is also expected to improve the mechanical properties of printed parts. At present, there are relatively few reports on the use of SLS to fabricate graphene/polymer matrix

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composites, and they are mainly focused on nylon-based materials. It is expected that research in the future could be extended to more types of composite materials.

3.4.3 Applications of 3D-printed graphene resin matrix composites 3.4.3.1 Electronics Graphene has a large specific surface area and high carrier mobility, showing great application potential in the field of electronics. After compounding with an appropriate polymer matrix, graphene can be used to prepare flexible electronic devices; the application of 3D printing can easily and quickly manufacture complex and delicate electronic devices, and it can swiftly integrate electronic components. At present, one of the popular topics in graphene electronics is the use of graphene in field effect transistors (FET). Due to its high carrier mobility, graphene can be manufactured to transistors with a quicker response, which is able to greatly increase the cutoff frequency of the transistors. In addition, due to the extreme thinness of graphene, the feature size of transistors can be reduced, and Moore’s Law can be further extended, which is an important research theme in the field of integrated circuits in the future.121,122 The 3D printing technology used to prepare graphene FET is predominately inkjet printing. For example, by taking ionic liquid/copolymer gel as the gate dielectric layer, Xiang et al. deposited graphene on flexible Kapton substrates by inkjet printing, and FET was fabricated.123 Light-emitting diodes (LED) are optoelectronic devices that play an important role in communication, display, illumination, and other fields, and the excellent transparency and conductivity of graphene means that it can be used as the electrode material for LEDs.124,125 Specifically, graphene is prepared to ink with a hydrogel state for inkjet printing. In addition, electronic circuits prepared by 3D printing technology such as inkjet printing and FDM can be used to connect a variety of electronic devices.94,126

3.4.3.2 Energy The large specific surface area and excellent electrical conductivity of graphene have attracted great attention in energy applications, including supercapacitors and lithium-ion batteries for energy storage, as well as

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fuel cells and solar cells for energy conversion. The electrode materials of supercapacitor require high specific surface area, appropriate pore size distribution, and excellent conductivity; thus, graphene is considered an ideal electrode material for supercapacitors.127,128 There are many reports on the preparation of supercapacitor electrodes composited by graphene and conductive polymers. The printing of graphene electrode materials for supercapacitors is generally also done by inkjet printing. For example, Chi et al. prepared hydrothermally rGO/polyaniline (PANI) composites by insitu polymerization, which is dispersed in a solvent to give ink, and the supercapacitor electrodes are fabricated via inkjet printing.95 Graphene composites are mainly used as negative electrode materials for lithiumion batteries; the introduction of graphene can effectively alleviate the serious volume expansion of battery negative electrode materials during lithium deintercalation, and prolong the service life of electrodes. Meanwhile, the graphene conductive network also provides a channel for rapid charge conduction.129,130 The positive and negative electrodes are printed by inks mixed with GO and positive/negative active materials of lithium battery and followed by a heat treatment to reduce GO; the solid polymer electrolyte is then printed between the positive and negative electrodes, and the lithium-ion batteries are fabricated.131 In solar cells, graphene is mainly used as active material to promote the formation of photocurrent, and as a transparent electrode or electrode component. Researchers have reported the preparation of graphene-containing dyesensitized solar cell electrodes by inkjet printing.90 In fuel cells, graphene is mainly used as a catalyst carrier for electrode reactions or directly used as a catalyst after doping.132,133

3.4.3.3 Biomedical The applications of graphene in the biomedical field are also of great interests for researchers. Graphene has excellent biocompatibility and antibacterial properties,134 and the surface of GO is rich in oxygencontaining functional groups, which is convenient for modification and immobilization of drugs, and it can be used as drug carriers.135 Graphene has excellent photothermal conversion ability in the near-infrared (NIR) region and is used for photothermal therapy of tumors.136 Composite materials can be fabricated with graphene to enhance the wear resistance of artificial bone tissue and joints.137 GO films can promote cellular adhesion to improve proliferation and differentiation of cells, and they can be

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used for bioscaffolds.138 3D printed graphene/polymer composites have been studied as well in the field of biomedicine, mainly for the preparation of biological scaffolds, and the commonly used printing methods include inkjet printing, FMD, and stereolithography. Jakus et al. prepared an ink with a maximum graphene content of ~75 wt% by solution mixing, taking polylactide-co-glycolide (PLGA) as the adhesive.96 The bioscaffolds with filaments ranging in diameter from 100 µm to 1000 µm were printed with this ink, which is mechanically flexible, self-supporting, biocompatible, biodegradable, surgically friendly, and inducing differentiation of stem cells into neuron-like cells. Sayyar et al. used covalently linked graphene polycaprolactone (PCL) composites to fabricate bioscaffolds by FDM.101 The introduction of graphene enhances the tensile strength and Young’s modulus of PCL, and the differentiation of rat PC12 cells on this scaffold is demonstrated. Zhu et al. mixed gelatin methacrylamide (GelMA), graphene nanoplatelets, and neural stem cells to target the bioscaffold while using the stereolithography technique after a photoinitiator is added.105 The porous GelMA hydrogel in the scaffold provides a biocompatible microenvironment for the survival and growth of neural stem cells, and the stem cells present very good cell viability, and the differentiation into neurons and synapses is successfully observed. When these three printing methods are used in the biomedical field, the polymers are all required to have biocompatibility and low cytotoxicity. As for inkjet printing, the loading of graphene can reach a high level, and the bioscaffold prepared by FDM has good mechanical properties and high reliability. In contrast, high precision and accuracy can be achieved by stereolithography, and on some occasions, the liquid photosensitive material before curing can be injected into the cavity of the tissues and organs, and then laser irradiation is applied to solidify in vivo to prepare highly fitted biological parts.

3.4.3.4 Aerospace In the field of aerospace, graphene/polymer matrix composites also show considerable application potential.139,140 Due to its excellent mechanical properties, the introduction of graphene into polymer matrix may significantly improve the mechanical properties such as tensile strength and elastic modulus. When a small amount of graphene, modified graphene, or GO is incorporated into the commonly used aerospace resin matrix such as epoxy resin, bismaleimide, and phenolic resin, many mechanical

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properties are improved.141 Through the development of a graphene sizing agent, graphene is able to be introduced into the interface layer of carbon fiber composite materials, and thus the generation and expansion of cracks in the interface layer can be suppressed, which will subsequently improve the strength and toughness of carbon fiber composite materials, and the scope of their applications is expanded.142 In addition to improving the mechanical properties, graphene can also be used as functional enhancement. Graphene is capable of forming a conductive network in a polymer matrix to improve the conductivity of composite materials, which can be used for static dissipative materials and lightning strike protection of aircraft.143,144 The addition of graphene to the polymer matrix can enhance the thermal stability of the composites as well, and increase the carbon residue, which can be used as ablative thermal protection materials.145 In addition, graphene/polymer matrix composites can be used for microwave absorption and electromagnetic shielding, which are suitable for stealth aircraft.146 Due to the excellent mechanical properties and functionality of graphene, graphene/polymer matrix composites can also be used as structural-functional-integrated materials in future aircraft. The 3D printing technology of graphene polymer matrix composites presents features of rapidly and precisely manufacture of parts with both complex structures and excellent functionalities, which shows great application potential in the field of aviation as parts without resistance to stress loading.

3.4.4 Graphene polyetheretherketone matrix composites for 3D printing 3.4.4.1 Introduction Polyetheretherketone (PEEK) is an important thermoplastic engineering plastic with extraordinary properties.147–149 Since it was developed, it has been regarded as an important national defense strategic material, and its structure is shown in Figure 3.22. PEEK shows both stiffness and toughness, and its fatigue resistance under alternating stress is among the highest of all plastics. Its melting point is as high as 343°C, as is one of the plastics with best resistance to high temperature. The chemical resistance of PEEK is outstanding, even at elevated temperatures, resisting most common organic compounds, acid (except for sulfuric acid), and base hydrolysis; it is also radiation resistant and difficult to burn. This material shows self-lubricating property and is able to withstand a harsh friction

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Figure 3.22.

Chemical structure of polyetheretherketone.

environment. In addition, it is insulating and easy to handle. These superb performances make it have great application potential in many high-end fields such as aerospace, automobile, electrical and electronic, and medical. At the same time, due to the characteristics of these application fields and the limitation of high melting point on traditional molding processes, the 3D printing technology that allows rapid manufacture of products with complex structures has also become a hot issue in the field of PEEK research. Could this high-performance plastic be smoothly combined with graphene, which is also an emerging material? The answer is yes. Tewatia et al. mixed graphene with PEEK by melt blending and found that the addition of graphene could induce the surficial crystallization of PEEK, which enhances the mechanical properties and significantly improves the thermal stability of the composite.150 Yang et al. coated a layer of poly(ether sulfone) resin on the surface of thermally rGO, and graphene/ PEEK composite was fabricated by hot pressing, which has demonstrated excellent electrical conductivity.151 GO/carbon nanotube hybrid fillers were fabricated by Hwang et al., with chemical bonding between GO and the carbon nanotube; a hot-press melting method was used to prepare hybrid fillers/PEEK composites with different filler contents. It is found that the thermal conductivity is significantly increased, and the mechanical properties and thermal stability are also improved to a certain extent.152 Therefore, the combination with graphene can further expand the boundaries of PEEK applications, which is of great significance.

3.4.4.2 Preparation and processing of 3D printed graphene/PEEK composites (a) Melt blending of GO and PEEK After drying and surface treatment, the PEEK pellets are mixed with GO powder evenly, and a twin-screw extruder is employed to perform the melt blending in a temperature range from 360°C to 375°C.

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Figure 3.23. GO/PEEK composite pellets via melt blending, the GO loading from left to right are 0, 0.1 wt%, 0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt%. (a)

(b)

(c)

(d)

Figure 3.24. TEM images of GO in the ultra-thin section of melt-blended GO/PEEK composites; the GO loadings are (a) 0.5 wt%, (b) 1.0 wt%, (c) 1.5 wt%, and (d) 2.0 wt%.

The fabricated filaments undergo water cooling, drying, and granulation, then the GO/PEEK composite pellets are obtained (Figure 3.23). Figure 3.24 presents the TEM morphology of GO in the ultra-thin sections of melt-blended GO/PEEK composite. In all four kinds of

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GO/PEEK composites, the presence of wrinkles is obvious on GO, which indicated the thin nature of GO sheets, and serious agglomeration in the PEEK matrix is not observed. In addition, the interfacial bonding between GO and PEEK in these four composites is still very tight after the preparation process of ultra-thin sections, and no delamination scenario is observed, which indicates that a strong interfacial bonding is formed between GO and the PEEK matrix. This is of great significance for GO to fully realize its potential in enhancing the composites. Figure 3.25 shows the TGA curves of melt-blended GO/PEEK composites with different GO loadings. It can be evidenced from the figure that after the incorporation of GO, the thermal decomposition temperature of the composite, either the decomposition temperature at 5% weight loss (T5) or the decomposition temperature at 10% weight loss (T10), is higher than that of neat PEEK. The thermal decomposition temperature of the composite with 0.1 wt% GO shows a lower increment than those of others, and the thermal decomposition temperature of the composites with 0.3 wt% GO or more is almost identical. The increment of T5 for the composite is up to 6.1°C than that of neat PEEK, and the increment of T10 is up to 6°C, which indicates that the addition of GO can improve the thermal stability of the composites. During the processing of the composite,

Figure 3.25. TGA curves of melt-blended GO/PEEK composites with different GO loadings.

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the highest temperature is 375°C, at which most of the oxygen-containing functional groups on GO have been decomposed, and the thermal decomposition rate has become moderate. After GO is added, it acts as a physical barrier and hinders the thermal motion of molecular segments. In the meantime, a certain interface synergy effect may be formed between GO and the PEEK matrix, thus improving the thermal stability of the composite material.153 (b) Preparation of composite filaments and 3D printing Before 3D printing, resin pellets are required to be manufactured into filaments, which can be processed by the 3D printer; here, a high-temperature extruder for 3D printing consumables was employed. First, the PEEK pellets were dried and then extruded with a single-screw extruder, and the temperature is also controlled to be in the range of 360–375°C. After extrusion, the filaments underwent two-stage air cooling, and then the winder system is hired to carry out the filament winding. Figure 3.26 shows the neat PEEK and the composite filaments with different GO loadings prepared by the high-temperature filament extruder. Figure 3.27 shows the monitoring data of the filament diameter during the winding process of the composite with 1.5 wt% GO. It indicates that the diameter of the filament is well controlled to be within the range of 1.75 ± 0.05 mm, which is in accordance with the requirements for 3D printing. The 3D printing of graphene/PEEK composites was carried out using an FDM process. PEEK is a semi-crystalline polymer, so during the

Figure 3.26. The digital images of 3D printing consumables prepared from neat PEEK and GO/PEEK with different GO loadings. From left to right, the loading of GO is 0, 0.1 wt%, 0.3 wt%, 0.5 wt%, 0.7 wt%, 1.0 wt%, and 1.5 wt%.

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Figure 3.27.

Diameter distribution of 3D printing GO/PEEK filament with 1.5 wt% GO.

printing process, the material will undergo processes such as crystalline phase transition accompanied by a change of temperature, which increases the complexity of the printing process. According to the characteristics of PEEK, the main printing conditions need to be considered during the printing process, such as nozzle temperature, substrate temperature, building chamber temperature, printing speed, and layer thickness. Among them, the temperature of the building chamber and the printing speed are the primary factors affecting the temperature field of the 3D printing parts. During printing, rapid changes in the temperature field can cause uneven thermal stress distribution. Therefore, increasing the temperature of the building chamber and accelerating the printing without damage to the process of 3D printing will make the distribution of temperature field more uniform; thereby, the adhesion quality is improved, and the temperature gradient and cooling rate are reduced, which is beneficial for uniform crystallization of parts as well as the elimination of warpage caused by uneven shrinkage.154,155 Experiment results show that the overall warpage of the parts decreases with the increment of the building chamber temperature and the decrement of the layer thickness. With the increase of the nozzle temperature and the printing speed, the overall warpage of the parts first declines and then increases, and there is an optimal value. The temperature of the nozzle needs to be kept within an appropriate range, because if it is too low, the material will not be completely melted, and it

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cannot be smoothly extruded from the nozzle. If the temperature is too high, the material will be partially carbonized, which will easily block the nozzle. For general PEEK and graphene/PEEK composite materials, the optimal printing conditions are the nozzle temperature is 400°C, the temperature of the printing substrate is 160°C, the temperature of the building chamber is 70°C, the layer thickness is 0.2 mm, and the printing speed is set to between 20 mm/s and 40 mm/s. Figure 3.28 shows the digital images of the parts printed by neat PEEK and 1.5 wt% GO/PEEK composite filaments. A major challenge for practical applications of 3D printing is the poor mechanical properties of 3D printed parts, which are much worse than those fabricated by conventional processing methods such as injection molding, and they cannot meet the requirements of practical applications. It is found experimentally that the 3D printing technology would compromise the intrinsic excellent tensile properties of neat PEEK, but a such problem can be effectively solved by the incorporation of a small amount of GO. The mechanical test specimens fabricated by 3D printing of the composites with different GO loadings were measured. During printing, the infill density of the specimens was 100%, and the printing direction was 45°/–45°. After the specimens were printed, they were annealed in an oven to increase the crystallinity and reduce the internal thermal stress. The annealing conditions of the specimens were as follows: dried at 150°C for 2 h, then heated from 150°C to 200°C at a rate of 10°C/h, holding at 200°C for 4 h, followed by cooling to 140°C at a rate of 10°C/h, and finally cooling naturally to room temperature. After annealing, the tensile, flexural, and impact properties of the specimens were tested, which are compared with the standard ones fabricated by injection molding. The injection molding was carried out at 365–380°C, the injection

Figure 3.28.

Digital images of 3D printed parts by PEEK and GO/PEEK composite.

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pressure of each section was maintained at 90~140 MPa, and the holding pressure was kept between 60 MPa and 120 MPa. The tensile strength of composite specimens with different GO loadings prepared by injection molding (IM) and 3D printing (AM) is compared in Figure 3.29. It can be concluded that for 0.1 wt%, 0.3 wt%, and 0.5 wt% GO loading, the tensile strengths of the 3D printed specimens of melt-blended GO/PEEK composites are equivalent to those of neat PEEK specimens by injection molding. As for the 3D printed specimen of meltblended GO/PEEK composites with 0.1 wt% GO, its tensile strength is higher than that of neat PEEK specimens by injection molding. Compared with the neat PEEK 3D-printed specimens, the 3D-printed specimens of melt-blended GO/PEEK composites with 0.1 wt%, 0.3 wt%, and 0.5 wt% GO loading show an increase in tensile strength of 11.5%, 10.0%, and 10.3%, respectively. However, as the loading amount of GO rises, such as 1 wt% and 1.5 wt%, the tensile strength of the 3D-printed specimens of the composites is lower than that of neat PEEK prepared by both methods. As shown in Figure 3.30, the flexural strength also presents an overall trend of first increasing and then decreasing with the elevation of GO

Figure 3.29. Tensile strength of GO/PEEK composites with different GO loadings fabricated by injection molding and 3D printing.

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Figure 3.30. Flexural strength of GO/PEEK composites with different GO loadings fabricated by injection molding and 3D printing.

loading. Compared with neat PEEK 3D-printed specimen, the flexural strength of 3D-printed specimens with 0.1 wt% GO loading is increased by 9.6%, whereas those of 3D-printed specimens with 0.5 wt% and 1.0 wt% GO loading are lower than that of the neat PEEK 3D-printed specimen. The flexural strength of the 3D-printed specimen with 0.1 wt% GO loading is slightly higher than that of the injection-molded specimen, which is indicative that the flexural strength is not compromised at all for 3D-printed composites with low GO contents. The main reason for this scenario is that the printed layers of traditional 3D printing technology cannot be completely fused together, leaving gaps between layers, as shown in Figure 3.31(a). Then, stress concentrations occur around these gaps, and failures start at these localized spots. The existence of GO in the 3D-printed composites is able to improve the strength of the composites just like the injection-molded ones do, but also part of GO can bridge the layers of the 3D-printed parts, as shown in Figures 3.31(b) and 3.31(c). Such an effect controlled by GO leads to absorption of energy in the process of material destruction, so the strength of the composite material is increased to a greater extent, and the toughness of the composite can also be improved.

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(b)

(c)

Figure 3.31. SEM images of the cross-section of the impact test specimens fabricated by 3D printing: (a) neat PEEK; (b) GO/PEEK composite; (c) enlarged view of the white box in (b).

3.4.4.3 Preparation and molding of chemically modified graphene/PEEK composites GO was chemically modified with toluene-2,4-diisocyanate (TDI), and reactive isocyanate groups are attached to GO sheets, giving GO-TDI. The poly(aryl ether ketone) (PEK-L) containing pendant carboxyl groups was prepared by solution polymerization, and was prepared by solution polymerization, and the carboxyl groups on the side chain of PEK-L were able to react with the isocyanate groups of GO-TDI covalently, and PEK-L was grafted onto the surface of GO, namely, PEK-L-functionalized GO (GTPEKL) was prepared. The prepared GTPEKL was uniformly dispersed in a solvent, and then mixed with PEEK powder; after that, the mixed powder was added to a twin-screw extruder to prepare GTPEKL/PEEK composite pellets. By using the same aforementioned tests and research methods, it is found that the addition of GTPEKL to the PEEK matrix can also improve both the tensile strength and toughness, and enhance the tribological performance.

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Compared with the GO/PEEK composites, interfacial bonding and compatibility between GTPEKL and PEEK are much improved; thus, GTPEKL/PEEK composites show more outstanding mechanical and tribological properties than those of the GO/PEEK composites. For example, the strength and toughness of composites are increased, and in the meantime the improvement of friction and wear performance is also greater. Therefore, it is of great practical significance to combine the excellent performance of composites with advanced intelligent manufacturing technology to prepare GTPEKL/PEEK composite filaments for fabricating 3D-printed composite parts. According to the method mentioned above, the filaments were manufactured by as-prepared composite pellets with the same parameters adopted, and the printed parts are shown in Figure 3.32.

3.4.4.4 Application prospect The excellent performance of PEEK makes it a potential candidate for replacing traditional materials in many fields. For example, in the aerospace field, it can be used for radomes, fittings on the interior of an airplane, composite fasteners, cabin interior parts, cold-end parts of aeroengines, etc. In the field of automobile manufacturing, due to its excellent wear resistance, it can be used in manufacturing bearings, gears, gaskets, etc., which are intensively applied in motor control systems, braking systems, and so on. As for the electrical and electronic field, the insulation nature of PEEK causes it to be widely used in insulation and protective layers of motors, transformers, wires, etc., as well as heat- and

Figure 3.32.

3D-printed parts fabricated by GTPEKL/PEEK composite filaments.

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corrosion-resistant parts. In the medical field, PEEK is now replacing titanium and other metal materials to manufacture human bones, and medical devices that are resistant to high temperature and steam can be fabricated, such as dental equipment and endoscopes. A major challenge for the practical application of 3D printing is the poor mechanical properties of 3D-printed parts and they are not ready for load bearing. However, 3D-printed graphene/PEEK composites have achieved equivalent mechanical properties to those of neat matrix fabricated by injection molding, suggesting the availability toward broadened applications, such as parts with load bearing, friction and wear resistance, complex structures and hardness in preparation with traditional methods; therefore, they show great application potential in aerospace, automotive machinery, biomedicine, and other fields.

3.5 Graphene Sandwich Composites 3.5.1 Overview of sandwich composites In general, composites are composed of matrix and reinforcement; however, in practical applications, especially in the field of aerospace, there are many strict requirements on the dimensions, weight, and performance of the components. As a result, a conception of low-density core structure such as honeycomb and foam is promoted to optimize the performance of the entire component, and materials with such structure are usually considered as a specific type of composite, i.e., a sandwich composite. The history of this sandwich structure can be traced back to the architectural structure of ancient Rome, while the modern high-performance sandwich structure is mainly driven by the development of the modern aviation industry in the 20th century. As shown in Figure 3.33, a sandwich composite consists of two thin but stiff facesheets and a lightweight but thick core. The use of the core to separate the two stiff and strong composite facesheets produces composite sandwich structures that are especially well suited for flexural loading. Under such loading, the facesheets are subjected to tensile and compressive stresses associated with bending, whereas the central core is subjected primarily to shear stresses. Therefore, the efficiency of the load-bearing capacity of the material is effectively promoted, which makes it lightweight and show extremely high flexural stiffness; meanwhile it exhibits advantages of anti-buckling, fatigue resistance, sound

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Figure 3.33.

Configuration of a sandwich composite.

insulation, thermal shield, and so on. Therefore, sandwich composites have been widely used in aerospace and other fields that require highperformance material and that are sensitive to weight, and they are also applied in other industries such as construction and automobiles. Under most circumstances, the common choice of facesheet used for a sandwich composite is a resin matrix composite, which allows both the facesheet and core to cure at the same time, giving rise to easy formation of high-strength composites. However, in other cases, adhesive film is used for bonding the facesheets and core together. In very rare cases, a metal sheet can also be used as a facesheet, which can be bonded to the core layer by brazing them together. Open- and closed-cell-structured foams, such as polystyrene (PS), polyurethane (PU), polyvinylchloride (PVC) foams, balsa wood, and honeycombs, are commonly used core materials. Sometimes, an open- and closed-cell metal foam or honeycomb can also be employed as the core material. Under certain circumstances, the honeycomb structure is filled with other kinds of foams for higher strength. Recently, 3D graphene foam structures have also been investigated as cores, which are reviewed and discussed by Khurram et al.156 The graphene core has an ultra-low density; at the same time, it can be combined with a variety of traditional core materials, and the thermal and

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electrical properties of these cores can be greatly manipulated to achieve a variety of functions.

3.5.2 Graphene sandwich composites and applications The particularity of sandwich composites lies in the design and selection of the core materials. In practical applications, since the core is placed in the space separated by facesheets, the density and mechanical properties of the core are usually the biggest concerns. Moreover, due to the constraints of cost and weight, the selection of core materials is usually less crucial than that of the facesheets. However, due to the large volume ratio of the core material, requirements of some special applications can be easily satisfied by the core material. Graphene macrostructure is a general term for a kind of 3D graphene foam or network structure, which usually presents an ultra-low density and excellent physical properties such as high electrical conductivity and thermal conductivity, and it can easily modify the core material. The core

Figure 3.34.

Electromagnetic parameters of graphene-modified honeycomb core.

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materials modified by graphene show some special functions, one of the most important of which is the behavior of the material responding to electromagnetic waves. The commonly used honeycomb core or foam core is a low-dielectric material, which is completely transparent to electromagnetic waves, while graphene has controllable electrical conductivity and large specific surface area, and the graphene-modified core material will generate significant dielectric loss, as shown in Figure 3.34. Since the thickness of the core material is usually several centimeters or even more, the core material modified with a small amount of graphene is able to greatly influence the transmission and reflection properties of the microwave, which is an important electromagnetic frequency band. The specially designed sandwich composite prepared by the graphenemodified honeycomb core material exhibits special electromagnetic properties such as strong electromagnetic wave absorption, and this composite has a promising application prospect in electromagnetic shielding and other fields.

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116. Lin, D., Jin, S., Zhang, F., Wang, C., Wang, Y., Zhou, C. and Cheng, G. J. (2015). 3D stereolithography printing of graphene oxide reinforced complex architectures, Nanotechnology, 26 (43), p. 434003. 117. Korhonen, H., Sinh, L. H., Luong, N. D., Lehtinen, P., Verho, T., Partanen, J. and Seppälä, J. (2016). Fabrication of graphene-based 3D structures by stereolithography, Phys. Status Solidi, 213 (4), pp. 982–985. 118. Manapat, J. Z., Chen, Q., Ye, P. and Advincula, R. C. (2017). 3D printing of polymer nanocomposites via stereolithography, Macromol. Mater. Eng., 302 (9), p. 1600553. 119. Gu, D. D., Meiners, W., Wissenbach, K. and Poprawe, R. (2012). Laser additive manufacturing of metallic components: Materials, processes and mechanisms, Int. Mater. Rev., 57 (3), pp. 133–164. 120. Kim, H. C., Hahn, H. T. and Yang, Y. S. (2013). Synthesis of PA12/functionalized GNP nanocomposite powders for the selective laser sintering process, J. Compos. Mater., 47 (4), pp. 501–509. 121. Wang, H., Hsu, A., Lee, D., Kim, K., Kong, J. and Palacios, T. (2012). Delay analysis of graphene field-effect transistors, IEEE Electron Device Letters, 33 (3), pp. 324–326. 122. Schwierz, F. (2010). Graphene transistors, Nat. Nanotechnol., 5 (7), pp. 487–496. 123. Xiang, L., Wang, Z., Liu, Z., Weigum, S. E., Yu, Q. and Chen, M. Y. (2016). Inkjet-printed flexible biosensor based on graphene field effect transistor, IEEE Sens. J., 16 (23), pp. 8359–8364. 124. Wu, J., Agrawal, M., Becerril, H. A., Bao, Z., Liu, Z., Chen, Y. and Peumans, P. (2010). Organic light-emitting diodes on solution-processed graphene transparent electrodes, ACS Nano, 4 (1), pp. 43–48. 125. Sun, T., Wang, Z. L., Shi, Z. J., Ran, G. Z., Xu, W. J., Wang, Z. Y., Li, Y. Z., Dai, L. and Qin, G. G. (2010). Multilayered graphene used as anode of organic light emitting devices, Appl. Phys. Lett., 96 (13), p. 133301. 126. Paddubskaya, A., Valynets, N., Kuzhir, P., Batrakov, K., Maksimenko, S., Kotsilkova, R., Velichkova, H., Petrova, I., Biró, I., Kertész, K., Márk, G. I., Horváth, Z. E. and Biró, I. (2016). Electromagnetic and thermal properties of three-dimensional printed multilayered nano-carbon/poly(lactic) acid structures, J. Appl. Phys., 119 (13), p. 135102. 127. Zhu, Y., Murali, S., Stoller, M. D., Ganesh, K. J., Cai, W., Ferreira, P. J., Pirkle, A., Wallace, R. M., Cychosz, K. A., Thommes, M., Su, D., Stach, E. A. and Ruoff, R. S. (2011). Carbon-based supercapacitors produced by activation of graphene, Science, 332 (6037), pp. 1537–1541. 128. Du, Q., Zheng, M., Zhang, L., Wang, Y., Chen, J., Xue, L., Dai, W., Ji, G. and Cao, J. (2010). Preparation of functionalized graphene sheets by a lowtemperature thermal exfoliation approach and their electrochemical supercapacitive behaviors, Electrochim. Acta, 55 (12), pp. 3897–3903.

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Chapter 4

Graphene Rubber Matrix Composites

Rubber materials are used in various fields of the national economy. They are one indispensable and irreplaceable key material in the high-tech field, and widely applied in aviation equipment. The development and utilization of natural rubber has a history of more than 100 years, and the subsequent development of the chemical industry has led to a variety of new rubbers, which have greatly improved the performance of the materials. With continuous development of high-power and high-speed aircraft, the dual requirements of high performance and high functionality are proposed for rubber materials, which are strikingly marked by high loadbearing, high damping, and reliable sealing properties. The strength and elasticity of raw rubber are not good enough for practical applications; thus, reinforcing fillers, antioxidants, and other compounding agents must be used to tailor the material performance for each application. Carbon black (CB) and silica (SiO2) are the general reinforcements for rubber materials. As a novel material with exceptional performance, graphene has attracted the attention of the entire material community after its discovery. At present, as a kind of nanofiller with excellent performance for rubber, graphene and its derivatives have been widely used in the research of various graphene rubber composites. The widespread research studies on graphene rubber composites have been very fruitful, and new light is shined on the material, its processing, and the characterization method of the composites. In this chapter, rubber materials used for graphene rubber composites and their properties are introduced first, and the research progress of the preparation method,

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structure, properties, testing methods as well as applications for graphene rubber composites are then reviewed.

4.1 Overview of Graphene Rubber Matrix Composites Rubber materials are widely used in various fields of national economy, which makes them indispensable and irreplaceable key materials in the high-tech field. Among them, the development and utilization of natural rubber has a history of more than 100 years. In the 1930s, diene monomers were used to synthesize sodium-butadiene rubber and lithiumbutadiene rubber, and chlorine atoms were introduced to synthesize chloroprene rubber with the functions of flame retardancy and sunlight aging resistance. In addition, nitrile rubber with cyano groups was synthesized showing improved oil resistance, and the obtained fluorine rubber, in which fluorine atoms with high bonding energy are incorporated in the molecular side chain, presents greatly improved heat resistance and aging resistance. With continuous development of the chemical industry, rubber materials such as silicone rubber, ethylene propylene diene methylene, acrylic rubber, and styrene-butadiene rubber have been developed. With the development of application requirements and the diversification of rubber products, the rubber products are expected to exhibit excellent comprehensive properties, and higher requirements of functionalities are also raised for rubber products, especially for the materials used in aviation.1 The strength and elasticity of raw rubber are not good enough for practical applications; thus, reinforcing fillers, antioxidants, and other compounding agents must be used to tailor the material performance for each application. Carbon black (CB), as a general carbon-based reinforcement, is widely used in the reinforcement of various rubber composites together with silica (SiO2). Graphene is a recently developed carbonbased material with extraordinary physical properties, which has attracted great attention in both academia and industry.2,3 The properties of graphene, carbon nanotube, steel, plastic, fiber, and rubber are listed for comparison in Table 4.1.4 Graphene and its derivatives, as excellent rubber nanofillers, have been widely used in the research of various graphene/rubber composites. On the basis of meeting the requirements of functionalities, the related studies are mainly focused on improving the

Graphene Rubber Matrix Composites 229 Table 4.1.

Properties of graphene, carbon nanotubes, nanostructured steel, and polymers.4 Thermal conductivity (W/ (m∙K), room temperature)

Conductivity (S/m)

130 ± 10 GPa

(4.84 ± 0.44) × 103 – (5.30 ± 0.48) × 103

7200

60–150 GPa

3500

3000–4000

Nanostructured stainless steel

1769 MPa

5–6

1.35 × 106

Plastic (HDPE)

18–20 MPa

0.46–0.52

Insulator

Material

Tensile strength

Graphene CNTs

Organic fiber (Kevlar)

3620 MPa

0.04

Insulator

Rubber (natural rubber)

20–30 MPa

0.13–0.142

Insulator

performance of graphene/rubber composites in the following two aspects: (1) improving the dispersion of graphene and its derivatives in the rubber matrix, (2) enhancing the interfacial interaction between graphene as well as its derivatives and the rubber matrix. After recent years of efforts, the widespread studies on graphene rubber composites have been very fruitful, and new light is being shined on the material, its processing, and the characterization method of these composites. The development process, historical orientation, and development basis have been recorded by numerous reviews.5 This chapter will start from the rubber matrix and typical applications of graphene rubber composites, and then review the research progress of their preparation, functionalities, modification, structure, properties, testing methods, and applications.

4.2 Preparation Method of Graphene Rubber Matrix Composites At present, the preparation methods of graphene/rubber composites mainly include solution mixing, latex blending, and mechanical mixing.

4.2.1 Solution mixing method Solution mixing is a frequently used method for laboratory preparation of polymer matrix nanocomposites. The detailed process is mixing the

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colloidal suspension of the graphene sheet or graphene derivatives with the target polymer matrix. The polymer can be dissolved in the solvent alone or dissolved in the suspension of the graphene sheet. Then, the poor solvent of the target polymer is added to the suspension mixture, and as a result, the molecular chains of the polymer wrapped with the filler will precipitate, and then the precipitated mixture undertakes purification, drying, and further treatment for relevant experiments or applications. In addition, the solvent in the graphene/polymer composite solution can also be directly volatilized, but studies have shown that in this method, due to the slow volatilization rate of the solvent, graphene aggregation may occur, which ultimately deteriorates the performance of the final composite material. Singh et al. reported the preparation of graphene/rubber nanocomposites by solution coating, in which thermal rGO and nitrile butadiene rubber (NBR) were dispersed into solvent xylene to make homogeneous slurry, and then the resulting slurry was used to prepare a 2–3-mm-thick coating onto an aluminum panel, and the final graphene/rubber composite (rGO/NBR) was obtained after curing in air for 24 h (Figure 4.1).6 When the graphene/rubber composite is prepared by solution mixing, the graphene can be ideally exfoliated and uniformly dispersed in the rubber matrix, but this method also has several shortcomings. For example, graphene and its derivatives are generally difficult to disperse in a common solvent for the rubber matrix at the same time, such as chloroform

Figure 4.1.

SEM image of rGO/NBR composite.6

Graphene Rubber Matrix Composites 231

and toluene. Therefore, the modification of graphene is capable of further improving its dispersion, but chemical modification will generally affect the electrical conductivity of graphene. The usage of quantities of organic solvents will cause environmental pollution and will increase the production cost, which is not in agreement with the current environmental protection policy. The rubber compounding vulcanizing agent is also difficult to add through solution mixing.5 In addition, some studies have shown that small molecules of solvent are very likely to enter and attach onto the sheets of graphene, which is hard to completely detach; thus, it brings difficulty to the preparation of high-performance composites by solution mixing. In contrast, latex mixing is able to avoid these drawbacks.7

4.2.2 Latex mixing method The general process of the latex blending method is mixing the aqueous dispersion of graphene or GO with rubber latex to get a uniform dispersion, then the mixture is subjected to coagulation, drying, and vulcanization to obtain the graphene/rubber composites. This method is solvent-free, which means less pollution and relatively simple process. Li et al. prepared graphene (GR)-filled natural rubber (NR) by in-situ reduction of GO in natural rubber latex, and the procedure is schematically represented in Figure 4.2.8 The tube model analysis shows that the network molecular parameters of NR are shifted after the addition of GR and GO, and the effect of GR is more significant. Compared with neat NR, a noticeable enhancement in tensile strength is achieved for both GO and GR-filled NR systems, but GR has a better reinforcing effect than GO. Wide-angle X-ray diffraction (WAXD) results show that compared with NR/GO composites, increased crystallinity and special strain amplification effects are observed for NR/GR composites.

4.2.3 Mechanical mixing For the mechanical mixing method, graphene is directly mixed with rubber matrix either in a twin-roll mill or alternatively in a batch mixer; then a certain temperature and pressure are applied for vulcanizing the rubber to obtain a final graphene/rubber nanocomposite. The mechanical mixing method has several advantages such as low cost, simple process, solvent-free result, environment-friendly, and suitable for both polar and

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Figure 4.2.

Schematic illustration of the preparation of NR/GO and NR/GR composites.8

non-polar rubbers; thus, it is widely used in industrial production. However, due to the high surface energy of graphene and the high viscosity of rubber, the challenge in the preparation of graphene/rubber composites through mechanical mixing is the uniform dispersion of graphene in the rubber matrix. A1-solamy et al. first prepared NBR compounds filled with different contents of graphene nanoplatelets (10 nm in thickness) using a two-roll mixing mill, and then the rubber composites were compression molded into cylinders of 1 cm2 area and 0.01 m height for investigation of the electrical conductivity and piezoresistive effects.9 The percolation concentration of the investigated nanocomposites is found to be 0.5 phr. Of all the composites examined, the sample, in the region of the percolation transition, is found most sensitive to compressive strain. The electrical conductivity of this sample is changed by more than five orders upon 60% compression and more than two orders for 6 MPa pressure. The increase

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of electrical resistance with uniaxial pressure may be explained as a result of destruction of the structure of the graphene electro-conductive nanoplatelet network. The preparation process has a great influence on the properties of graphene/rubber nanocomposites. Zhan et al. compared the influence of different processing methods on dispersion, structure, and performance of graphene in GR/NR composites.10 For the cross-linked graphene/NR composites with the segregated network prepared by self-assembly in latex and direct hot pressing (NRLGRS), the percolation threshold is only 0.62 vol%, and the conductivity is 0.03 S/m when the graphene content is 1.78 vol%. The percolation threshold of the cross-linked GR/NR composites without the segregated network prepared by latex mixing and twinroll mixing (NRLGRS-TR) is 4.62 vol%, and the electrical conductivity is about 5.7 × 10–7 S/m with a graphene content of 1.78 vol%. As for those composites prepared by direct Haake mixing of graphene and rubber (NRGR-HM) or by direct twin-roll mixing of graphene and rubber (NRGR-TR), the electrical conductivity is only ~10–7 S/m even when the graphene content is as high as 9 vol%. These experimental data combined with the SEM in Figure 4.3 indicate that the “emulsion polymerization + hot pressing vulcanization” process helps to create a 3D segregated network of graphene in the polymer matrix, significantly enhancing the electrical conductivity. As for the case of the “emulsion polymerization + twin-roll mixing + hot pressing vulcanization” process, the segregated networks are destroyed under the dynamic shear force, and the graphene is uniformly distributed in the rubber matrix, giving rise to reduced electrical conductivity. In contrast, the graphene is not effectively exfoliated by direct twin-roll or Haake mixing, and the graphene still exists in the polymer matrix as agglomerates; thus, the fabricated composite shows a higher percolation threshold and lower conductivity.

4.2.4 In-situ polymerization The general principle of in-situ polymerization involves mixing of GO and monomers in a solvent, followed by in-situ polymerization by adding appropriate initiators. Chen et al. synthesized graphene/waterborne polyurethane composites by in-situ polymerization, where GO modified by 1-pyrenemethanol was employed to react with 4,4′-diphenylmethane diisocyanate (MDI) and poly(ethylene glycol) (PEG-200) under certain

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(b)

(c)

(c′)

(d)

(d′)

Figure 4.3. TEM image (a) GR; (b) the NR latex particles containing GR shell after 15 times dilution; (c) NRLGRS; (d) NRLGRS-TR; (c′) and (d′) are the magnified images of (c) and (d), respectively.10

conditions.11 When 2 wt%-modified graphene is added, the fracture strength, elastic modulus, and toughness of the composites are enhanced by 50.7%, 104.8%, and 47.3%, respectively. The filler can be uniformly dispersed in the matrix by means of in-situ polymerization, but the disadvantage of this method is that the introduction of fillers will result in an increased viscosity of the polymer, which makes the polymerization reaction complicated. The preparation of vulcanized GO/rubber by in-situ polymerization has not been reported so far, and this method needs to be further studied.

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4.2.5 Other methods In addition to the predominated preparation methods of graphene/rubber composites as mentioned above, many tricky methods have also been developed to construct delicate microstructures, such as the layer-by-layer self-assembly technique which assembles the graphene sheets into a layered structure. Wang et al. fabricated rubber composites by layer–layer electrostatic self-assembly, where desired multi-layered films of (PEI/ XNBR/PEI/GO)n (n represents the number of deposition cycles) were obtained by repeated deposition cycles of alternating GO, carboxylic acrylonitrile butadiene rubber (XNBR) latex, and polyethyleneimine (PEI).12 During the self-assembly process, negatively charged carboxyl groups on XNBR latex and GO sheets can be electrostatically bound by positively charged amine groups from PEI molecules. After thermal treatment, XNBR latex particles in each layer are gradually mixed together and become a continuous rubber film layer, and partial ionic bonds among XNBR latex, PEI, and GO sheets are changed into covalent amide bonds. The formation of the multi-layer XNBR/graphene film with ordered arrangement of GO sheets and XNBR latex layers was demonstrated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The obtained GO/XNBR multi-layer film exhibits a significant improvement in mechanical properties, i.e., 192% improvement of tensile strength and 215% increase of elastic modulus, when compared with pure XNBR film. The electrical conductivity of the multi-layer films is increased from 6.5 × 10–4 S/cm to 8.2 × 10–3 S/cm as their deposition cycles increase, which is attributed to the increment of the GO layers in the multi-layer films. Such multi-layered film shows great promise for their potential use as elastomeric conductive material.

4.3 Graphene-Reinforced General-Purpose Rubber Composites General-purpose rubbers, or GP rubbers for short, are a group of allpurpose, general-grade elastic materials which were used in the national economy. GP rubber has a long production history and dominates the rubber industry, and is widely used and produced in large volumes. The GP rubber matrix materials involved in this section mainly consist of natural rubber (including epoxidized natural rubber), styrene-butadiene rubber,

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butyl rubber, ethylene-propylene rubber, nitrile rubber, and carboxylated nitrile rubber.

4.3.1 Graphene/natural rubber composites (GNR) As a renewable natural resource with excellent comprehensive properties, natural rubber (NR) has the characteristics of high elasticity, high strength, high elongation, and wear resistance, and is widely used in aerospace, national defense and military industry, aircraft tires, medical elastomers, and other fields. At present, NR occupies a very important place in national economic construction. The interfacial properties of composites determine the performance of polymer/inorganic filler nanocomposites. She et al. introduced epoxy and hydroxyl groups into the NR molecular chain to anchor hydrogen bonding with the oxygen functional groups on the surface of GO sheets, thereby enhancing the interfacial interaction between GO and rubber.13 The SEM results show that epoxidized natural rubber (ENR) latex particles are assembled onto the surface of GO sheets by employing hydrogen bonding interaction as the driving force. This self-assembly depresses restacking and agglomeration of GO sheets and leads to homogeneous dispersion of GO within the ENR matrix. Compared with those of pure ENR, the nanocomposite with 0.7 wt% GO receives 87% increase in tensile strength and 8.7-fold increase in modulus at 200% elongation. The effects of functionalized graphene sheets (FGSs) on the mechanical properties and strain-induced crystallization of NR were investigated by Ozbas et al.14 FGSs are predominantly single sheets of graphene with a lateral size of several hundreds of nanometers and a thickness of 1.5 nm. The onset of crystallization occurs at significantly lower strains for FGSfilled NR samples compared with carbon black (CB)-filled NR, even at low loadings. Neat NR exhibits strain-induced crystallization around a strain of 2.25, while incorporation of 1 and 4 wt% FGS shifts the crystallization to strains of 1.25 and 0.75, respectively. In contrast, loadings of 16 wt% CB do not significantly shift the critical strain for crystallization. Small-angle X-ray scattering shows that FGS is aligned in the stretching direction, whereas CB does not show alignment or anisotropy. Moreover, Yan et al. also examined the effect of strain-induced crystallization on fatigue crack propagation in reduced graphene-reinforced NR nanocomposites.15 It is observed that graphene has an opposite effect on crack growth resistance of NR; that is, at lower fatigue strains, the

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inclusion of graphene accelerates the crack growth, whereas at higher strains, the crack growth is retarded. It is suggested that the reason for this behavior is a competition between strain-induced crystallization and cavitation at the crack tip. The fatigue crack growth resistance directly depends on the strain-induced crystallization at the crack tip. It is found that the graphene/NR composite starts to crystallize near the crack tip at a strain of ~30%, while unfilled NR does not. Under the same strain, the addition of graphene increases the crystallinity and crystallization zone of the NR crack tip, and the crystallization of the material crack tip hinders the crack growth, which promotes crack branching and increases the energy dissipation of crack growth, thereby resulting in a better crack growth resistance. Wu et al. reported in detail for the first time the influence of graphene on the vulcanization kinetics of NR with a sulfur curing system.16 It is found that on adding graphene, the induction period of the vulcanization process is remarkably depressed, whereas the vulcanization rate is enhanced at low graphene loading and then suppressed. As a result, the optimum cure time decreases dramatically at first and subsequently shows a slight increase with increasing graphene loading. At the same time, the cross-linking density of NR increases monotonically, because graphene takes part in the vulcanization process. The exothermal peak of the vulcanization reactions is split into two peaks on adding ≥0.5 phr graphene. It is interpreted in terms of two reaction stages, i.e., chemical-reaction-controlling stage and diffusion-controlling stage. The chemical reaction controlling stage has lower activation energy than the diffusion controlling stage. Addition of graphene decreases the activation energy of the former stage, while has an opposite effect on the activation energy of the latter stage. In addition to modifying NR with graphene directly, Lin et al. also synthesized zinc dimethacrylate-functionalized graphene (Z-GR) as reinforcing nanofiller for NR, and the results confirm the exfoliation and functionalization of graphene.17 It is also noticed that the tensile strength, tear strength, and modulus at 300% elongation of NR/Z-GR-20 nanocomposites with 1.4 phr Z-GR have been improved by 143%, 76%, and 231%, respectively. The thermal conductivity of NR/Z-GR-30 nanocomposites is enhanced 1.39 times that of the pure NR. This remarkable improvement is attributed to the formation of covalent cross-linked network and ionic cross-linked network. This work presents a new approach to synthesize functionalized graphene and expedites the potential applications of functionalized graphene in the rubber matrix.

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4.3.2 Graphene/styrene-butadiene rubber composite (GSBR) Styrene-butadiene rubber (SBR) is a random copolymer comprising butadiene and styrene, which is one of the general-purpose synthetic rubbers with the highest volume and the earliest industrialization. The processing performance of SBR as well as the wear resistance, heat resistance, and aging resistance of vulcanized rubber are close to those of NR, and it can be used in combination with NR and various synthetic rubbers. SBR is widely used in the fields of tires, shoes, tapes, hoses, wires, cables, and medical devices. Tang et al. studied the influence of graphene on the vulcanization kinetics of SBR.18 It turns out that with increasing graphene loading, the induction period of the vulcanization process of SBR is remarkably reduced at low graphene loading and then levels off; on the other hand, the optimum cure time shows a monotonous decrease. As a result, the vulcanization rate is suppressed at first and then accelerated, and the corresponding activation energy increases slightly at first and then decreases. Upon adding graphene, the cross-linking density of the nanocomposites increases, which may be attributed to the participation of graphene in the vulcanization process. Das et al. compared the influence of different carbon nanomaterials (graphene nanoplatelets (GNP), expanded graphite (EG), CNTs, and EG/ MWCNTs hybrid fillers) on the properties of solution styrene butadiene rubber (S-SBR) composites.19 The electrical percolation behavior is found to start at 15 phr for S-SBR/GNPs and 20 phr for S-SBR/EG, whereas a sharp drop is found at 5 phr for S-SBR/MWCNT-based composites. At a particular filler loading, the tensile properties and storage modulus of the nanocomposites are significantly improved, and such enhancement is in the order of EG/CNTs > MWCNT > GNP > EG. In addition, octadecylamine (ODA) is used by Wang et al. to modify graphene (GO-ODA), and GO-ODA/SBR nanocomposites are consequently prepared.20 GO-ODA sheets are uniformly dispersed in SBR as confirmed by SEM, TEM, and XRD. The interfacial interaction between GO-ODA and SBR is weaker than that between GO and SBR, which is proved by an equilibrium swelling test and dynamic mechanical analysis. GO-ODA/SBR composites show a more pronounced “Payne effect” than GO/SBR composites, indicating enhanced filler networks resulting from the modification of GO with ODA. GO-ODA/SBR composites exhibit higher tensile strength and elongation at break than those of SBR and

Graphene Rubber Matrix Composites 239

GO/SBR composites. The tensile strength and elongation at break for the composite with 5 phr GO-ODA increase by 208% and 172% versus neat SBR, respectively. Xing et al. prepared rGO/SBR nanocomposites using a modified emulsion method.21 It is found that molecular-level dispersion of rGO in the rubber matrix is achieved, and a strong interfacial interaction occurs between rGO and SBR. The conductivity of rGO/SBR rises remarkably with the increment of rGO filler. When the rGO is 3 phr, the conductivity of the nanocomposite has reached the antistatic level (10–6 S/m). When the filling amount reaches 7 phr, the rGO/SBR composite has become a conductor, and the electrical conductivity is increased by 7 orders of magnitude compared with the unfilled SBR. This increase in electrical conductivity is attributed to the high degree of reduction of rGO as well as the formation of relatively well-established conductive network structures in contact with each other in the matrix. In addition, the rGO/SBR nanocomposites also exhibit low heat buildup, excellent gas permeability, wear resistance, and thermal stability. Araby et al. compared the influence of solution mixing and mechanical mixing on the properties of SBR/GNPs nanocomposites.22 A percolation threshold of electrical conductivity is observed at 5.3 vol% of GNPs for SBR/GNPs nanocomposites prepared by solution mixing, while 16.5 vol% for nanocomposites are prepared by mechanical mixing. Compared with the mechanical mixing method, the mechanical properties of the SBR/GNPs prepared by the solution method are better. When the GNP loading is 16.7 vol%, the tensile strength, Young’s modulus, and tear strength of the nanocomposites prepared by solution mixing are improved by 413%, 782%, and 709%, respectively, confirming the formation of good interfacial bonding which efficiently transfers loads from the matrix to graphene. The thermal conductivity of SBR prepared by solution mixing shows a three-time increment at 24 vol% of GNPs.

4.3.3 Graphene/isobutylene-isoprene rubber composites (GIIR) Isobutylene-isoprene rubber (IIR), also known as butyl rubber, is the fourth largest synthetic elastomer in the world. It shows excellent gas impermeability and remarkable resistance to heat, aging, acids and bases, ozone, and solvent, as well as properties of electrical insulation, shock absorption, and low water absorption, which make it widely used in inner

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tubes, water tires, vulcanized bladders, gas-tight layers, tire sidewalls, wires and cables, waterproof building materials, shock absorption materials, and medicinal bottle stoppers. Sadasivuni et al. prepared graphene-modified IIR nanocomposites by solution mixing, and compared it with organic montmorillonite (cloisite 10A)/IIR composites.23 Blends with different loadings of maleic anhydride-grafted poly(isobutylene-co-isoprene) (MA-g-IIR) and IIR are made to maintain a balance between the beneficial polarity induced by MA grafting and the inevitable decrease in molecular weight (due to chain scission) induced by the free radical grafting process. The research shows that the interface properties of graphene and organic montmorillonite with IIR are greatly improved. Compared with unfilled IIR, the Young’s modulus of IIR nanocomposite with 5 wt% graphene loading increases from 0.9 MPa to 1.91 MPa, and the tensile strength increases from 0.8 MPa to 2.7 MPa, the elongation at break increases from 160% to 220%, and the oxygen transmission rate (OTR) decreases from 38.4 mL/(m2/24 h) to 28.4 mL/(m2/24 h). IIR/graphene nanocomposites exhibit higher reinforcement and lower gas permeability compared to the optimized clay nanocomposites with the same weight percentage. The aspect ratios of the two fillers clay and graphene are estimated to be 108 and 130, respectively; therefore, the relatively higher aspect ratio of graphene in IIR matrix compared to clay platelets is one of the main reasons for the higher OTR rate in the clay sample. In addition, graphene has a higher surface area (~2630 m2/g) than clay (~750 m2/g), and graphene shows a better exfoliation and dispersion in the rubber matrix, which contributes to a lower OTR of graphene/IIR nanocomposites. Frasca et al. studied the influence of preparation methods on the properties of multi-layer graphene (MLG) reinforced chlorine-isobuteneisoprene rubber (CIIR) nanocomposites.24 In this study, direct incorporation of 3 phr MLG into CIIR on a two-roll mill does not result in substantial enhancement of the material properties. In contrast, by pre-mixing 3 phr MLG with CIIR using an ultrasonically assisted solution mixing procedure followed by two-roll milling, the rheological, curing, and mechanical properties are improved substantially compared with the MLG/CIIR nanocomposites mixed only on the mill, and the Young’s moduli of the nanocomposites mixed in solution increase by 38%. In addition, the MLG/CIIR nanocomposites produced via solution show superior durability against weathering exposure.

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4.3.4 Graphene/butadiene rubber composite (GBR) Butadiene rubber (BR) is the second-largest global general-purpose synthetic rubber. Compared with NR and SBR, vulcanized BR presents excellent low-temperature resistance, abrasion resistance, and elastic properties, and it also exhibits low heat buildup, good aging resistance, and is available to be used in combination with NR, neoprene, or nitrile rubber. BR is widely used in the production of rubber products such as tires, impact resistance modification, tapes, hoses, and rubber footwear. Malas et al. developed expanded graphite and isocyanate modified graphite nanoplatelets-filled BR, SBR and SBR/BR blends in the presence of carbon black (CB), and found that isocyanate modification could make graphene better dispersed in the rubber matrix.25 Isocyanate-modified graphite nanoplatelets containing rubber compounds in the presence of CB showed an increase in the mechanical, dynamic mechanical, hardness, abrasion resistance, and thermal properties compared to the CB-filled rubber vulcanizates alone. The thermal decomposition temperature of expanded graphite/CB and isocyanate-modified graphite nanoplatelet/ CB-filled rubber composites shows considerable enhancement. It is believed that the expanded graphite and isocyanate-modified graphite nanoplatelets are dispersed properly in the rubber matrix and act as efficient heat shields by dissipating more heat and do not permit the heat to rise up within the matrix, thereby preventing oxidation at the early stage of degradation.

4.3.5 Graphene/ethylene propylene diene monomer rubber composite (GEPDM) Ethylene propylene diene monomer rubber (EPDM) is a terpolymer comprising ethylene, propylene, and non-conjugated diene monomers. The backbone of the molecule is saturated, but the side chain contains a small amount of unsaturated double bonds, which is caused by the introduction of the third monomer. Therefore, compared with other general-purpose rubbers, EPDM has excellent resistance to heat, oxidation, ozone, weather aging, and chemicals; meanwhile, it also has good electrical resistivity and low-temperature properties. In addition, EPDM has low relative density and large filling amount, and it exhibits good adaptability to various conventional rubber processing methods.

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Valentini et al. prepared GNP-reinforced EPDM (GEPDM) by mechanical blending and conventional vulcanization.26 The acceleration time history measured in the impact excitation test and a simplified analysis model based on the frequency response analysis are established for evaluating the shock response of the nanocomposites. Experimental results indicate that the interfacial interaction between GNPs and EPDM matrix results in enhancements in damping property. This experimental approach can be used to test the potential utilization of such polymer nanocomposites to be used for space or military components subjected to high-frequency, high-magnitude shock pulses. Castro et al. prepared a novel conductive material by depositing graphene in polyaniline (PANI)/EPDM composite conductive rubber (GEPDM-PANI) by the chemical vapor deposition (CVD) method.27 After the transfer process, the GEPDM-PANI electrode is ready to be used in organic electronic devices. Organic photovoltaic devices (OPVs) using the graphene/EPDM-PANI as transparent electrode were fabricated and characterized by various spectroscopic techniques. The OPVs presented the same open circuit voltage (Voc) of the similar ITO/glass based devices with lower short circuit current density (Jsc) due to the higher electrode resistance. The approach of non-covalent surface treatment utilizing poly(ethyleneimine) (PEI) polymer was applied to modify GNPs by Razak et al., and the effects of various GNP–PEI loadings on the performance of NR/EPDM blend nanocomposites were studied.28 It is found that the addition of GNPs significantly improves the blend’s processability, offering approximately a 104.30% increase in tensile strength obtained with the addition of 5.00 wt% GNP–PEI. A reduced swelling index of Qf/Qg in parallel with an increase in modified GNP–PEI content revealed enhancements in terms of rubber–filler interactions between the NR/EPDM matrix and GNP–PEI.

4.3.6 Graphene/nitrile butadiene rubber composite (GNBR) Nitrile butadiene rubber (NBR) is a polymer obtained by copolymerizing butadiene and acrylonitrile. Because of the polar cyano group in its molecule, NBR has excellent oil resistance, solvent resistance, and mechanical properties. Generally, this type of synthetic rubber is widely used in various oil-resistant products, and it has become the specialized synthetic rubber with the largest market share so far.

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Mensah et al. showed that the thermal stability of NBR composites was greatly improved after adding GO.29 The weight loss rate of the NBR is 92% at a maximum degradation temperature of 447°C, whereas that of GO/NBR composite with 4 phr GO loading is 88.4% at a maximum degradation temperature of 452°C. The reason for the thermal enhancement could be attributed to restricted chain mobility of NBR molecules due to the presence of GO; and the inflammable GO nanoparticles can form a jammed network of char layers that retard transport of the decomposition products. These results again suggest the good dispersion of GO in rubber matrix, and strong interfacial interaction is achieved between GO and NBR. Mensah et al. also prepared rGO by treating GO with hydrazine hydrate, and solution mixing was adopted to fabricate NBR/GO and NBR/ rGO composites.30 The results show that the glass transition temperature of the NBR/GO and NBR/rGO composites is significantly increased, which is indicative of strong interfacial interaction between graphene and the NBR matrix. The NBR/rGO nanocomposites demonstrate a higher curing efficiency and a change in torque compared to the gum and NBR/ GO compounds. The results manifest in the high hardness and high tensile modulus of the NBR/rGO compounds. For instance, the tensile modulus of NBR/rGO nanocomposite at a 0.1 phr rGO loading shows a great increment of 83%, 114%, and 116% at strain levels of 50%, 100%, and 200%, respectively, compared to that of NBR/GO nanocomposite with a 0.1 phr GO-loaded sample. Such evidenced enhancement is highly attributed to a homogeneous dispersion of rGO within the NBR matrix, which is confirmed by TEM and SEM. However, in view of the high ultimate tensile strength, the NBR/GO compounds exhibited an advantage; this was presumably due to strong hydrogen bonding or polar–polar interactions between the NBR and GO sheets. Varghese et al. prepared NBR/few-layer graphene nanoplatelet (FLG) nanocomposites by melt-mixing FLG with NBR in Brabender Plasticorder mixer, and the properties were compared with composites containing CB alone and a 1:1 mixture of FLG and CB (FLG-CB).31 Incorporation of a very small amount of FLG significantly improves the cure characteristics of NBR. Compared to graphene, similar loading of CB gives higher rheometer torque at the expense of scorch safety, whereas hybrid filler of FLG-CB gives optimum cure characteristics. There is more than 190% improvement in tensile strength when 5 phr FLG is

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added to the matrix. On the other hand, to achieve the same strength, five times more CB is needed when compared to FLG. In contrast to CB, FLG increases the compression set of the rubber. The compositions containing 1:1 mixture of the fillers display compression set values in between those containing FLG or CB alone. FLG-incorporated composites show significant reduction in the creep compliance. The effects of pressure and temperature on the electrical properties of GNBR nanocomposites were also investigated. NBR compounds filled with different concentrations of GNPs were successfully fabricated by Mahmoud et al. via a two-roll mechanical mixing method.32 The percolation threshold of the nanocomposite is 0.5 phr, and the I–V characteristic curves show that the nanocomposites exhibit ohmic behavior at a certain voltage and a later nonlinear behavior. The electrical conductivity increases as the temperature and the graphene nanosheet content rise; this may be due to the decreased hopping distance between graphene nanosheets, which enhances the conductivity of the nanocomposite at low concentrations. The best hopping distance is presented when the graphene loading in the composite is 0.5 phr.

4.3.7 Graphene/carboxylated nitrile rubber composite (GXNBR) Carboxylated nitrile rubber (XNBR) is a kind of synthetic rubber obtained by adding a small amount of acrylic acid or methacrylic acid monomer to the molecular chain of ordinary NBR. Liu et al. used sodium humate (SH), a cost-effective and environmentally friendly humic substance, to non-covalently functionalize graphene, and the prepared functionalized graphene (SHG) was incorporated into XNBR through latex co-coagulation to form XNBR/SHG composites.33 The results show that SHG could be steadily and individually dispersed in water at a very high concentration (up to 30 mg/mL), which is contributed to π–π interaction and hydrogen bonding between SH and the graphene layer. With the incorporation of 1 phr SHG, the fracture energy of the XNBR/SHG composite is doubled and the elongation is improved, while the modulus is nearly identical. This scenario may be correlated with the interactions between SHG and MgO, which pronouncedly affected the cross-linking of the rubber. Bai et al. dispersed GO into dimethylformamide (DMF) by ultrasound, and the GO/DMF solution was added to the dissolved

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hydrogenated XNBR (HXNBR)/tetrahydrofuran solution, followed by ultrasonic dispersion, co-deposition, drying, two-roll mechanical mixing, and hot pressing curing, and the final GO/HXNBR nanocomposites were obtained.34 The results show that GO is well dispersed in the rubber matrix, and the addition of 0.44 vol% of GO nanosheets enhances the tensile strength and modulus at 200% elongation of HXNBR by more than 50% and 100%, respectively. This is believed to be due to strong interfacial interactions between the oxygen-containing functional groups on the surfaces of GO nanosheets and the carboxyl groups in HXNBR.

4.4 Graphene-Reinforced Special Rubber Composites Special rubber material, also known as special synthetic rubber material, refers to synthetic rubber with special properties and special usage under harsh conditions. For example, fluoroelastomer displays excellent hightemperature resistance at 300°C, and superb resistance of strong corrosion, ozone, light, weather, radiation, and oil; silicone rubber presents low-temperature resistance at –100°C, high-temperature resistance at 260°C, low-temperature dependence, low viscous flow activation energy, and physiological inertness; acrylate rubber shows heat-resistant, solventresistant, and oil-resistant properties and good electrical insulation. These special materials are mainly used for fabricating components and call for higher performance in new aeronautical and aerospace equipment, deep mining equipment, and heavy equipment. The special rubber matrix materials involved in this section mainly include silicone rubber, fluoroelastomer, acrylate rubber, thermoplastic styrene-butadiene rubber, and hydrogenated nitrile rubber.

4.4.1 Graphene/silicone rubber composite (GSR) Silicone rubber (SR) is a kind of liner polysiloxane comprising a siloxane backbone with silicon-oxygen chain (Si-O) and an organic moiety (such as methyl, ethyl, vinyl, phenyl, and trifluoropropyl) bound to the silicon. SR shows outstanding high- and low-temperature resistance, excellent resistance to oil, solvents, UV rays, radiation, and aging, good electrical insulation, chemical stability, and physiological inertness. It has been widely used in the fields of aviation, aerospace, electrical, electronic,

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chemical, instrument, automobile, machinery, medical, and health care as well as daily life. GO was functionalized with triethoxyvinylsilane (TEVS) by dehydration reaction to improve the dispersion and compatibility in the liquid silicone rubber (LSR) matrix by Zhao et al., and the TEVS-GO/LSR composites were then prepared via in-situ polymerization.35 It is found that TEVS is successfully grafted on the surface of GO, and TEVS-GO is exfoliated and uniformly dispersed in the LSR matrix. The results show that the 10% weight loss temperature (T10) increases 16.0°C with only 0.3 wt% addition of TEVS-GO and the thermal conductivity possessed a two-fold increase, compared to the pure LSR. The TEVS-GO/LSR composites with 0.3 wt% TEVS-GO displayed a 2.3-fold increase in tensile strength and a 1.97-fold reinforcement in shear strength compared with the neat LSR. Cai et al. synthesized SR nanocomposites filled with CB, GNPs, and CB/GNPs hybrid fillers via a liquid mixing method, and the effects of these fillers on the electrical properties and piezoresistive properties (near the region of the percolation) of the conductive nanocomposites are studied.36 The results show that the dispersion of CB alone in the matrix is not enough, and even a slightly continuous structure is formed. The dispersion of only GNPs in the matrix is not uniform due to agglomeration. As for CB/GNPs, the granular CB enters the interlayers of GNPs and is uniformly dispersed on the surface of GNPs; that is, the space between the sheets of GNPs is occupied, resulting in both improved dispersion of CB and exfoliation of GNPs. It is found that the conductivity of the composite filled with CB/GNPs hybrid fillers in the mass ratio of 2:4 (total filler loading is 6 wt%) is much higher than that in another ratio. The percolation threshold for CB/GNPs/SR is found to be 0.18 vol%, which is far lower than that for CB/SR, 25.5 vol%. The GNPs/CB/SR composite with 19 vol% of GNPs/CB displays a better piezoresistivity property compared with CB/SR. Hence, GNPs/CB/SR composites are expected to be a potential applicant as high-performance piezoresistive sensors. Moreover, Hu et al. prepared graphene/CNTs/SR nanocomposites via solution mixing.37 The different dispersion behaviors of graphene and CNTs could be attributed to the difference in their interaction with polymer matrix and their geometry. Graphene acts as a compatibilizer since it shows strong interaction with both polymer matrix and CNTs; thus, the dispersion of CNTs in silicone rubber is dramatically improved by the addition of graphene. The synergic effect of the graphene/CNT hybrid

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material is in charge of the improved electrical property of the polymer composites. Chen et al. successfully prepared GNPs/SR nanocomposites with different GNP loadings by the solution method, and their properties are compared with that of SR composites filled with conventional graphite.38 The GNPs/SR nanocomposite steady-state volume resistivity as a function of the volume fraction of GNPs is displayed in Figure 4.4, compared with that of SR composite containing 800 mesh and 2000 mesh graphite. The results show that at a very low volume fraction of conducting filler content, the resistivity of GNPs/SR and graphite/SR composites is very close to that of an insulating SR matrix. As the fraction of conductive filler content rises, both nanocomposites show the percolation behavior of “insulating–conducting” transition, and the percolation zone of GNPs/SR nanocomposite is narrower than that of composites filled with conventional graphite. The percolation threshold values at which the critical resistivity transition occurs are estimated to be about 0.9 vol%, 5.3 vol%, and 7 vol% for SR composites filled with GNPs, 8000, and 2000 mesh graphite, respectively. Obviously, the percolation threshold value of GNPS/SR nanocomposite is far lower than that of composites with

Figure 4.4. Volume resistivity of composites as a function of GNPs and graphite content.38

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conventional graphite. The particular geometry of GN (30–80 nm in thickness), with a higher aspect ratio of about 100–500-fold than conventional graphite, is more benefited to form a conducting network at the low loading level. The much lower percolation threshold is obviously the notable advantage of these particular GNP-modified nanocomposites as a conducting filler and efficiently avoids the material redundancy and detrimental mechanical properties led by high loading for the conventional fillers.

4.4.2 Graphene/fluoroelastomer composite (GFKM) 2D one-atom-thick graphene inspires intensive studies due to its great potential in enhancing polymer’s mechanical, gas/liquid barrier, and thermal properties. The GO-reinforced fluoroelastomer (FKM) was solution processed to enhance the mechanical and liquid barrier properties of FKM for the first time by Wei et al.39 It is found that the GO/FKM has 1.5-fold increment in tensile strength compared with pure FKM and 1.2-fold increment in tensile strength compared with rGO/FKM at 150°C. The reduced permeability of GO/FKM to an organic solvent (such as acetone) indicates the improved liquid barrier properties. This research provides a costeffective solution process to efficiently enhance the thermal mechanical and liquid barrier properties of FKM with the addition of GO sheets. Since GO-filled GFKM begins to decompose at 200°C, and rGO-filled GFKM shows a higher decomposition temperature, but rGO is not involved in the matrix cross-linking process, resulting in poor improvement of mechanical properties. To solve this paradox, the allyl alcohol groups on the GO react with peroxide to provide the initial free radicals and to further assist GO to participate in the cross-linking reaction, resulting in improvement of the mechanical properties and addressing the heat resistance problem. This research result is expected to be extended to applications such as mechanical gaskets used in high-temperature environments.

4.4.3 Graphene/acrylic rubber composite (GACM) Acrylic rubber, known by the chemical name alkyl acrylate copolymer (ACM), is a type of special synthetic rubber that is widely used in harsh environments such as high-temperature oil seals, crankshaft valve stems,

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cylinder gaskets, and hydraulic oil pipes. Dao et al. synthesized thin alumina-coated graphene by first coating graphene with aluminum trisec-butoxide in anhydrous dimethylformamide, followed by rapid calcination in an inert atmosphere after the hydrolysis of the alkoxide.40 Then, this coated graphene was dispersed into acrylic rubber to prepare rubber composites with high thermal conductivity and low electrical conductivity. Thermogravimetry demonstrates that the uniformly coated alumina protective layer substantially improves the thermal stability of the graphene and that the electrically insulative alumina layer effectively reduces the electrical conductivity of the graphene. The enhanced polar nature of surface as well as the increased surface roughness due to the coated alumina improved the dispersion of the graphene in the polar acrylic rubber matrix and the interaction at the interface. This led to an effective improvement of the thermal conductivity and tensile modulus but marginal increase in electrical conductivity by the filler. Tensile modulus increases drastically to as high as 470% for the composite reinforced with the 5 phr (about 2.5 vol%) loading of the alumina-coated graphene.

4.4.4 Graphene/styrene-butadiene-styrene thermoplastic rubber composite (GSBS) Elastomeric and conductive graphene/styrene-butadiene-styrene thermoplastic rubber composites (GSBS) were prepared via solution mixing by employing rGO and hydroxylated styrene-butadiene-styrene thermoplastic rubber (HO-SBS) as raw materials. Xiong et al. improved the interfacial interaction between rGO and SBS matrix by introducing hydroxyl and carbonyl groups into the SBS matrix.41 The addition of rGO improves the thermal stability of GSBS at a slight sacrifice of mechanical properties; meanwhile, the conductivity reaches 1.3 S/m, demonstrating its perspective value for conductive materials.

4.4.5 Graphene/hydrogenated nitrile rubber composite (GHNBR) Compared with nitrile rubber (NBR), hydrogenated nitrile rubber (HNBR) exhibits enhanced oxidation and ozone aging resistance while maintaining oil resistance in harsh chemical environments, and the operating temperature and mechanical properties are also obviously improved. Therefore, in

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Figure 4.5.

Dielectric constant of GHNBR composites at different rGO contents.42

the medium of hydrogen sulfide, carbon dioxide, water vapor, and liquefied petroleum gas under high temperature and high pressure, the comprehensive performance of HNBR is better than that of NBR, ACM, and FKM. However, HNBR is prepared by catalytic hydrogenation of NBR, in which the catalytic process is complicated, and it is difficult to incorporate graphene into the emulsion. For the first time, Cao et al. achieved a one-step fabrication of rGO/HNBR composites (GHNBR) by catalyzing NBR rubber with graphene-supported Rh catalysts, thereby improving the electrical and mechanical properties of HNBR rubber; for example, with the increment of rGO content, the volume resistivity of the GHNBR composite decreases and the dielectric constant increases, as shown in Figure 4.5.42

4.5 Properties of Graphene-Reinforced Rubber Composites 4.5.1 Mechanical properties The tensile strength of graphene is as high as 130 GPa and the Young’s modulus is about 1.01 TPa, making it currently the hardest and strongest material. In addition, it also has an ultra-high specific surface area

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(about 2630 m2/g), which is 100–500% higher than that of conventional graphite. The aspect ratio of graphene is about 400, which is 40–80 times larger than that of carbon black. Incorporation of a small amount of graphene can significantly improve the performance of rubber composites, which is a remarkable benefit for the applications of graphene-reinforced nanocomposites. Araby et al. synthesized the nanocomposites by adopting 3.56-nmthick GNPs of high structural integrity to melt compounds with EPDM by mechanical blending.43 The Young’s modulus, tensile strength, and tear strength of the nanocomposites are improved by 710%, 404%, and 270%, respectively, at 26.7 vol% GNPs. GO/SR nanocomposites were prepared by Gan et al. via solution mix44 ing. It is found that the GO sheets can be uniformly dispersed within the SR matrix; meanwhile, both the thermal and mechanical properties of the SR composites are improved. It is also evidenced that when the SR composites with different vinyl concentrations are mixed together, the mechanical properties of the prepared rubber hybrids and their GO-filled composites improve more obviously than those of the SR composites with a single-vinyl concentration.

4.5.2 Fatigue endurance When rubber products are used in tires, high-speed trains, aviation, and aerospace, they are often in a cyclic and dynamic loading state, and the fatigue life of products is strongly determined by the fatigue and fracture performance of rubber materials. Therefore, in order to provide rubber products with safety, reliability, and durability, it is of great significance to improve the dynamic fatigue properties of rubber materials. Mahmoud et al. studied the effect of GNPs on the cyclic fatigue and hysteresis of NBR rubber.32 The loading path dissipated energy (LDE) gives an indication about the damage accumulation, and the LDE as a function of the number of cyclic stress–strain is represented in Figure 4.6. It shows that by raising the GNP concentrations, the specific surface area of these nanoplatelets will be increased; so, more friction between rubber and GNPs will occur, subsequently resulting in more hysteresis. As a result, the dissipation energy of the composite is increased with the number of cycles, the hysteresis effect is more obvious, and the rate of damage is accelerated. It is found that after a large amount of damage takes place over the first ten cycles, the rate of LDE becomes marginal with

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Figure 4.6. The damage accumulation (LDE) versus the number of cyclic stress–strain.32

subsequent cycling, and the damage dissipated energy of the nanocomposite is reduced.

4.5.3 Damping performance Damping rubber is a kind of functional material in which the inherent viscoelastic properties of rubber are utilized to convert vibrational mechanical energy into thermal energy and dissipate it, which is an important field of functional applications for rubber. The damping of rubber originates from the internal friction of macromolecular motion, which is the manifestation of the relaxation phenomenon of polymer mechanics and one of the major indications of the dynamic mechanical properties of rubber materials. Xing et al. prepared GR/NR nanocomposites by the modified emulsion mixing method and in-situ reduction with hydrazine hydrate, and their damping properties were investigated.45 It is found that the GR nanosheets are well dispersed in NR, and a strong interfacial interaction is established between GR and NR. The dynamic mechanical analysis (DMA) demonstrates that the storage modulus of the nanocomposites is greatly improved after the incorporation of GR, while the loss peak height is suppressed, which may be attributed to the weakened mobility of the rubber molecules. Stanier et al. also investigated the damping behavior of GO nanosheetfilled NR at different strain rates.46 The study shows that the Young’s

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moduli of the nanocomposites are improved as the strain rate rises, and the dissipative properties are enhanced when GO content is increased, which may be attributed to the increased friction caused by the presence of high-aspect-ratio GO platelets.

4.5.4 Thermal behavior The ultra-high thermal conductivity of graphene (3000–5000 W/(m∙K)) is superior to that of carbon nanotubes (3000 W/(m∙K)), which shows great potential for improving the thermal properties of rubber composites in practical applications. Wang et al. found that after the incorporation of GO nanosheets, the thermal conductivity and thermal diffusivity of carboxylated acrylonitrile butadiene rubber (XNBR) increase remarkably.47 The thermal conductivity and diffusivity of unfilled XNBR are 0.160 W/(m∙K) and 0.084 mm2/s, respectively. The thermal conductivity and diffusivity of the GO/XNBR vulcanizates with 1.6 vol% of GO have 1.4-fold and 1.2-fold improvements, respectively, compared to those of unfilled XNBR vulcanizate. It is well known that phonon works as heat carrier in polymer, which is a definite discrete unit or quantum of vibrational mechanical energy. In order to improve the thermal conduction of thermally conductive polymers, it is necessary to reduce the scattering of phonons or reduce the mismatch of acoustic impedance at the interface of filler particles and the polymer matrix. Therefore, the thermal conductivity of GO/XNBR is improved due to the reduced acoustic impedance mismatch, which is the result of strong interaction between the oxygen-containing functional groups on the GO surface and polar XNBR. Thermal stability is one of the important properties of rubber materials, and intensive investigations have been carried out. The effect of GO as well as thermally reduced graphene on the thermal stability of SR has been reported by Ma et al.48 It is found that the decomposition temperature of SR nanocomposites rises with the increasing concentration of graphene. In particular, the decomposition temperature increases from 356°C to 417°C when 1 wt% GO is incorporated, and the decomposition temperature can be further increased to 489°C by taking thermally reduced graphene as the filler. Since the thermal stability of thermally reduced graphene is better than that of GO, the improvement of the thermal stability brought by thermally reduced graphene is superior to that by GO.

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Bai et al. performed DMA on GO/HXNBR nanocomposites.34 The results show an increase of the glass transition temperature Tg of HXNBR from −23.2°C to −21.6°C at a GO content of 1.3 vol%. This is due to the fact that the GO sheets have abundant oxygen-containing groups (hydroxyls, carboxyls, epoxides) on the surfaces and high aspect ratio of over 1000, which could form hydrogen bonding with the carboxyl groups in the HXNBR chains, resulting in an elevated Tg.

4.5.5 Electrical properties As a 2D planar material, three electrons of each carbon atom in graphene are sp2 hybridized, and the one remaining electron in the outer shell occupies a π orbital. This π orbital merges with adjacent π orbitals creating a huge orbital which allows easy movement of electrons across the plane of the molecule, which gives graphene remarkable electrical conductivity (106 S/m).49 Therefore, graphene has broad application prospects in conductive rubber composite. After filling and modifying rubber as a new carbon nanomaterial, graphene can greatly improve the electrical conductivity of rubber, and it shows the advantages of features such as low levels of loading, being lightweight, high strength, easy processing and molding, as well as a large range of adjustable resistivity values. However, due to the presence of a large number of oxygen-containing groups on the surface of GO, the conjugated structure of graphene is destroyed, resulting in a decrease in its electrical conductivity. The research of Mensah et al. reveals that the dielectric constant of GO/NBR nanocomposites gives a 1.2-fold increment compared with pure NBR vulcanizates, and the conductivity is close to that of an insulator.29 Zhou et al. synthesized polyaniline non-covalently-modified rGO (PANI@rGO) nanohybrids by in-situ reduction of GO using aniline as both reducing and stabilizing agents, and then PANI@rGO/NR nanocomposites were prepared by adding nanohybrids into NR via emulsion mixing.50 In addition, the physical mixture of PANI and rGO (rGO/ PANI)-reinforced NR nanocomposites (rGO/PANI/NR) was also prepared for comparative analysis. The results show that the electrical conductivity of the nanocomposites is enhanced with the increase of rGO content. The as-prepared PANI@rGO/NR nanocomposites with this 3D conductive network exhibit a very low percolation threshold (3-fold lower than that of the rGO/PANI/NR blends) and enhanced electrical conductivity (up to six orders of magnitude improvement). SEM results in Figure 4.7(a)

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(b)

Figure 4.7. SEM images of the cross-section of cryo-fractured nanocomposite (a) PANI@rGO/NR and (b) rGO/PANI/NR.50

present that PANI@rGO nanohybrids are located in the interstitial space between the NR latex microspheres and organized into a continuous 3D hierarchical network. This indicates that the construction of a superior conductive network is an effective way to improve the electrical conductivity of nanocomposites. Furthermore, the PANI@rGO/NR composites also exhibit excellent mechanical properties, which are promising as chemical sensing materials. Studies have shown that the vulcanization process, temperature, and pressure all have influence on the electrical properties of graphene/rubber materials. Zhan et al. studied the effect of hot pressing vulcanization on the dielectric properties of graphene/rubber composite.51 The results show that the dielectric constant of NBR nanocomposites is greatly improved after adding graphene, and dielectric percolation is achieved only at 0.2 wt%. When the content of graphene is 1.5 wt%, the dielectric constant is 1700, which is again increased to 11,000 after the hot press vulcanization process. It is concluded that the hot press vulcanization has a good reduction effect on graphene. Mahmoud et al. studied the influence of temperature on the electrical conductivity of NBR nanocomposites filled with different GNP contents, and the results show that the electrical conductivity is increased as the temperature rises, and the negative temperature coefficient (NTC) effect is observed.52 In addition, the relationship between the electrical conductivity and temperature of the nanocomposites follows the Mott equation.

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Al-solamy et al. studied the piezoresistive effect of NBR nanocomposites filled with different concentrations of GNPs.9 It is found that the resistivity of the nanocomposites is raised as applied pressure increases, which may be due to the destruction of the graphene conductive network, resulting in a decrease in the number of conductive paths. Of all the composites examined, the sample, in the region of the percolation transition (0.5 phr), is found most sensitive to compressive strain. When the conductive filler content is around the percolation threshold (0.5 phr), the resistance of the material is most sensitive to uniaxial compressive strain. The electrical conductivity of this sample is increased by more than five orders upon a 60% compression and more than two orders for 6 MPa pressure.

4.5.6 Electromagnetic shielding performance The development of modern society has brought about more and more serious problems of electromagnetic radiation and electromagnetic pollution. Graphene has excellent electrical conductivity and dielectric properties, and the fabricated composite at a low filling amount is able to obtain a high attenuation coefficient of electromagnetic wave. The application of graphene in the field of rubber matrix-absorbing materials has received more and more attention. Singh et al. have studied the effect of different loadings of rGO on the microwave absorption properties of rGO/NBR composites.6 Simulation studies show that 10 wt% of GO in NBR matrix exhibits high values of reflection loss (>10 dB) over a wide frequency range 7.5– 12 GHz, and the maximum loss is 57 dB at 9.6 GHz at a thickness of 3 mm. The microwave absorption mechanism is schematically depicted in Figure 4.8. In rGO, the dihedral angle could be easily formed within the stacks of flakes of rGO sheets; the microwaves suffer multiple reflections from the dihedral angles, thereby increasing their propagation path in the absorber. These multiple reflections of microwave leading to the higher losses of electromagnetic energy may be due to the fact that the interaction of microwaves with dielectric materials causes molecular motions such as ionic conduction and dipolar relaxation, and resistive force resists these molecular motions, and in result energy is dissipated in the form of heat.

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Figure 4.8. A possible microwave absorption mechanism in rGO/NBR composite.6

Al-Ghamdi et al. studied the electromagnetic interference shielding effectiveness (EMI SE) of GNPs/NBR nanocomposites in the frequency range from 1 GHz to 12 GHz.53 The results reveal that the SE of GNPs/ NBR nanocomposites is dependent on the GNP content and frequency. The pure NBR vulcanizates exhibit hardly any SE ability, which may be due to the low electrical conductivity, dielectric constant, and loss tangent value of pure NBR vulcanizates. The linear nature of SE as a function of frequency suggests the homogeneous nature of conductive networks formed in the nanocomposite. At fixed frequency, the SE of GNPs/NBR nanocomposites is enhanced with the increase of GNP loading, which may be attributed to more continuous conductive paths of GNPs that are formed since the nanosheet aggregates in the network. Additionally, upon increasing GNP loading, the larger density of mobile charge carriers (electrons or holes) in the nanocomposites interacts with incident electromagnetic field which in turn improves the SE of nanocomposites. When the GNP content is 4.0 wt%, the SE value is found to be close to 77 dB. This implies that such GNPs/NBR nanocomposites can be used commercially as a shielding material against electromagnetic radiation such as radar, broadcasting satellites, vehicular detection, defense tracking, weather satellite, and household appliances.

4.5.7 Media resistance performance Graphene has an ultra-high specific surface area, which is able to hinder the penetration of the media, and the strong interaction between graphene and rubber molecules reduces the free volume of the rubber matrix, so

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the graphene/rubber nanocomposite shows excellent media resistance performance. Polyvinyl pyrrolidone (PVP)-modified rGO (PrGO) was prepared through chemical reduction of GO in the presence of PVP by Zhang et al.54 The PrGO/natural rubber (NR) nanocomposites were fabricated by mixing a PrGO aqueous dispersion with NR latex, followed by coagulation and vulcanization. The results reveal that GO is efficiently reduced by PVP molecules, and the adsorption of PVP molecules on the basal plane of rGO is done through non-covalent interactions. With the increment of the PrGO, the solvent uptake of PrGO/NR is decreased, and compared with unfilled NR, the PrGO/NR nanocomposite with 5 phr PrGO has a 37% decrease in solvent uptake. Yin et al. for the first time used ionic liquid 1-allyl-3-methylimidazolium chloride (AMICl) to fine-tune the surface properties of GO for preparing ionic liquid-functionalized GO (GO-IL), as shown in Figure 4.9, then the GO-IL was mixed into SBR for fabricating GO-IL/ SBR nanocomposites, and their solvent resistance was investigated.55 It is found that AMICl molecules can interact with GO via the combination of

Figure 4.9.

Schematic diagram of the interaction between AMICl and GO.55

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hydrogen bond and cation–π interaction. GO-IL can be well dispersed in the SBR matrix. The solvent resistance of GO-IL/SBR nanocomposite with 5 phr rubber GO-IL is increased by 31% and 17%, respectively, compared with those of neat SBR and GO/SBR nanocomposites.

4.5.8 Gas barrier properties The barrier properties of rubber matrices can be significantly improved by incorporation of impermeable lamellar graphene materials with high aspect ratio to produce a tortuous diffusion pathway for the gas molecules.56 In addition to that, studies have shown that the water vapor barrier properties of graphene/rubber composites are better than clay-based rubber materials.10 Schopp et al. synthesized TrGO and CrGO by reducing GO thermally and chemically, respectively, with sodium dithionite as the reducing agent; then, graphene-filled SBR composites were prepared by latex blending, and the reinforcing effect of graphene was compared with other carbon-based fillers (CB, expended graphite (EG), CNTs, multi-layer graphene (MLG)).57 It was revealed that filler type, content, and dispersion process all affect the properties of composites. SEM and TEM studies show that under identical processing conditions, MLG, CrGO, and TrGO are all composed of wrinkled nanosheets which are incorporated as single graphene layers and uniformly dispersed in the matrix with very few agglomerates, whereas CB, CNT, and EG fail to disperse homogeneously in the SBR matrix and large agglomerates are detected. The following is the filler ranking regarding the higher oxygen permeability: TrGO < MLG < CNTs < CB. Unlike the other carbon fillers, the good dispersion and ultra-high aspect ratio of TrGO nanosheets are likely to form labyrinth structures accounting for drastically larger diffusion pathways. Varghese et al. studied the gas barrier properties of multi-layer graphene (FLG)-modified NBR nanocomposites and compared those with the composites containing CB alone.31 The results showed that the presence of FLG causes 40–50% increase in gas barrier properties of the NBR matrix. CB alone or CB/FLG mixture cannot contribute significantly to the gas barrier properties since CB particles are spherical in shape. This improvement in gas barrier properties lies not only on the platelet structure of FLG but also on the processing conditions of nanocomposites. In this study, rubber compound is mixed in a two-roll mill, which induces preferential orientation of the graphene platelets toward the milling

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direction, giving rise to improved gas barrier properties. However, extending the mixing time does not contribute much to the gas barrier properties.

4.5.9 Tribological property The tribological property is a very important key indicator of rubber products, such as the wear resistance of rubber tires, braking performance, and driving efficiency, as well as wear resistance of seals.58 As the basic structural unit of carbonaceous solid lubricating materials (0D fullerene C60, 1D carbon nanotubes, 3D graphite), the research on friction and wear performance of graphene-filled or modified rubber composites is also one of hotspots in the field of tribology. The tribological properties of the GO/NBR nanocomposites were evaluated under dry sliding and water-lubricated conditions by Li et al.59 The results show that in the case of the dry sliding condition, the GO could easily transfer from the matrix to the counterpart surface, forming a continuous and compact transfer film. Thereby, the composites display excellent friction reduction and wear resistance properties at a low GO loading. When GO content is increased, the friction coefficient (COF) and specific wear rate of the composites increase with increasing GO content. SEM images of transfer film on counter-surfaces are shown in Figure 4.10. Under the water-lubricating condition, both the COF and the specific wear rate of nanocomposites are decreased with increasing GO contents. It could be attributed to (1) the hydrophilic groups of GO, which could form strong hydrogen bonding with the water molecules, leading to the formation of a thicker water film between the rubber block and counter-surface; (2) the debris is taken away by water so that it could effectively reduce the abrasive wear; (3) the friction heat is effectively reduced under the water-lubricating condition, and this lightened the adhesive wear. SEM images of worn surface of the specimens are shown in Figure 4.11.

4.5.10 Other properties In addition to the abovementioned properties, the researchers also investigated several other properties of graphene/rubber nanocomposites, e.g., antibacterial properties.

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(b)

(c)

(d)

Figure 4.10. SEM images of worn surface of GO/NBR nanocomposites under dry sliding condition: (a) NBR; (b) 0.5 wt%; (c) 1.5 wt%; (d) 3 wt%.59

(a)

(b)

(c)

(d)

Figure 4.11. SEM of the worn surface of GO/NBR nanocomposites under waterlubricating conditions: (a) NBR; (b) 0.5 wt%; (c) 1.5 wt%; (d) 3 wt%.59

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Matos et al. first prepared aqueous dispersions of different concentrations of GO and rGO with the surfactant cetyltrimethylammonium bromide (CTAB), which was then mixed with natural rubber latex under magnetic stirring followed by sonication, and the biodegradability properties of these as-prepared GO/NR and rGO/NR nanocomposites were investigated.60 The results show that after 12 months in soil, neat NR shows a significant degradation of ~86%, whereas the NR/rGO and NR/ GO nanocomposites containing 2 wt% GO or rGO show degradation of 67% and 57%, respectively. The lower degradability versus time of the graphene-containing species may be related to their known antibacterial properties. This behavior may be interesting in biomedical applications and different applications of short duration, such as agriculture devices, packaging, or other hygiene products.

4.6 Conclusion Due to its unique structure, graphene has exceptional physical and electronic properties. As a typical 2D nanofiller, the structure of graphene can be easily tailored, and it can be modified or dispersed with regard to various requirements of reinforcement and functionalization. Graphene is a multifunctional nanofiller which can play an important role in enhancing the electrical, thermal, piezoresistive, gas barrier, impact resistance, lightweight, and dielectric loss properties of different rubber vulcanizates at very low levels of loading, which is of great significance to improve the performance of rubber products, in particular for those utilized in the field of aviation. It can be concluded from the above that the current study shows the following trends: (1) More and more self-assembly design concepts are introduced into the preparation of graphene/rubber composites, aiming to solve the problems of harsh mixing conditions and economic latex production; (2) the studies are mainly focused on enhancing the internal interfacial interactions between graphene and the matrix through graphene modification and the preparation of graphene fillers; (3) the studies are focused on general-purpose rubber matrix materials such as natural rubber, styrene-butadiene rubber, and nitrile rubber that can be mixed via latex mixing; and (4) as the application studies of graphene/rubber composites are developing, some of them have reached and are close to the level of technological maturity in line with the industrial field.

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In view of the above development trends, the authors believe that the following issues should be paid attention to: (1) Proposing innovative mixing methods for improving the economic efficiency and environmental protection of composite materials preparation; (2) proposing innovative reinforcement methods for comprehensively introducing and utilizing various forms of interactions; (3) increasing the efforts on the fundamental and basic applied research of special rubber composites to meet the development needs of cutting-edge industries for a new generation of rubber matrix composites; and (4) promoting the integration of production, education, and research in a variety of approaches, accelerating the transformation of academic achievements, and striving to occupy the commanding heights of the development of new-generation materials and application technologies.

References 1. Liu, J., Su, Z. and Li, F. (2011) Aeronautical Rubber and Sealing Materials (National Defense Industry Press, Beijing), in Chinese. 2. Jiang, H. and Xu, L. (2014). Recent domestic and foreign progress of graphene researches, New Mater. Ind., 11, pp. 6–10, in Chinese. 3. Wan, Y., Ma, T., Feng, R., Jiang, S. and Huang, J. (2014). An overview of domestic and foreign policies for graphene researches, New Mater. Ind., 11, pp. 2–5, in Chinese. 4. Kuilla, T., Bhadra, S., Yao, D., Kim, N. H., Bose, S. and Lee, J. H. (2010). Recent advances in graphene based polymer composites, Prog. Polym. Sci., 35 (11), pp. 1350–1375. 5. Tang, Z.-h., Guo, B.-c., Zhang, L.-q. and Jia, D.-m. (2014). Graphene/rubber nanocomposites, Acta Polym. Sin., 7, pp. 865–877, in Chinese. 6. Singh, V. K., Shukla, A., Patra, M. K., Saini, L., Jani, R. K., Vadera, S. R. and Kumar, N. (2012). Microwave absorbing properties of a thermally reduced graphene oxide/nitrile butadiene rubber composite, Carbon, 50 (6), pp. 2202–2208. 7. Barroso-Bujans, F., Cerveny, S., Alegría, A. and Colmenero, J. (2010). Sorption and desorption behavior of water and organic solvents from graphite oxide, Carbon, 48 (11), pp. 3277–3286. 8. Li, F., Yan, N., Zhan, Y., Fei, G. and Xia, H. (2013). Probing the reinforcing mechanism of graphene and graphene oxide in natural rubber, J. Appl. Polym. Sci., 129 (4), pp. 2342–2351. 9. Al-solamy, F. R., Al-Ghamdi, A. A. and Mahmoud, W. E. (2012). Piezoresistive behavior of graphite nanoplatelets based rubber nanocomposites, Polym. Adv. Technol., 23 (3), pp. 478–482.

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10. Zhan, Y., Lavorgna, M., Buonocore, G. and Xia, H. (2012). Enhancing electrical conductivity of rubber composites by constructing interconnected network of self-assembled graphene with latex mixing, J. Mater. Chem., 22 (21), pp. 10464–10468. 11. Chen, Z. and Lu, H. (2012). Constructing sacrificial bonds and hidden lengths for ductile graphene/polyurethane elastomers with improved strength and toughness, J. Mater. Chem., 22 (25), pp. 12479–12490. 12. Wang, L., Wang, W., Fu, Y., Wang, J., Lvov, Y., Liu, J., Lu, Y. and Zhang, L. (2016). Enhanced electrical and mechanical properties of rubber/graphene film through layer-by-layer electrostatic assembly, Compos. B: Eng., 90, pp. 457–464. 13. She, X., He, C., Peng, Z. and Kong, L. (2014). Molecular-level dispersion of graphene into epoxidized natural rubber: Morphology, interfacial interaction and mechanical reinforcement, Polymer, 55 (26), pp. 6803–6810. 14. Ozbas, B., Toki, S., Hsiao, B. S., Chu, B., Register, R. A., Aksay, I. A., Prud’homme, R. K. and Adamson, D. H. (2012). Strain-induced crystallization and mechanical properties of functionalized graphene sheet-filled natural rubber, J. Polym. Sci., Part B: Polym. Phys., 50 (10), pp. 718–723. 15. Yan, N., Xia, H., Zhan, Y. and Fei, G. (2013). New insights into fatigue crack growth in graphene-filled natural rubber composites by microfocus hard-Xray beamline radiation, Macromol. Mater. Eng., 298 (1), pp. 38–44. 16. Wu, J., Xing, W., Huang, G., Li, H., Tang, M., Wu, S. and Liu, Y. (2013). Vulcanization kinetics of graphene/natural rubber nanocomposites, Polymer, 54 (13), pp. 3314–3323. 17. Lin, Y., Liu, K., Chen, Y. and Liu, L. (2015). Influence of graphene functionalized with zinc dimethacrylate on the mechanical and thermal properties of natural rubber nanocomposites, Polym. Compos., 36 (10), pp. 1775–1785. 18. Tang, M.-Z., Xing, W., Wu, J.-R., Huang, G.-S., Li, H. and Wu, S.-D. (2014). Vulcanization kinetics of graphene/styrene butadiene rubber nanocomposites, Chin. J. Polym. Sci., 32 (5), pp. 658–666. 19. Das, A., Kasaliwal, G. R., Jurk, R., Boldt, R., Fischer, D., Stöckelhuber, K. W. and Heinrich, G. (2012). Rubber composites based on graphene nanoplatelets, expanded graphite, carbon nanotubes and their combination: A comparative study, Compos. Sci. Technol., 72 (16), pp. 1961–1967. 20. Wang, C., Liu, Z., Wang, S. and Zhang, Y. (2016). Preparation and properties of octadecylamine modified graphene oxide/styrene-butadiene rubber composites through an improved melt compounding method, J. Appl. Polym. Sci., 133 (4), p. 42907. 21. Xing, W., Tang, M., Wu, J., Huang, G., Li, H., Lei, Z., Fu, X. and Li, H. (2014). Multifunctional properties of graphene/rubber nanocomposites fabricated by a modified latex compounding method, Compos. Sci. Technol., 99, pp. 67–74.

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22. Araby, S., Meng, Q., Zhang, L., Kang, H., Majewski, P., Tang, Y. and Ma, J. (2014). Electrically and thermally conductive elastomer/graphene nanocomposites by solution mixing, Polymer, 55 (1), pp. 201–210. 23. Sadasivuni, K. K., Saiter, A., Gautier, N., Thomas, S. and Grohens, Y. (2013). Effect of molecular interactions on the performance of poly(isobutylene-co-isoprene)/graphene and clay nanocomposites, Colloid. Polym. Sci., 291 (7), pp. 1729–1740. 24. Frasca, D., Schulze, D., Wachtendorf, V., Morys, M. and Schartel, B. (2016). Multilayer graphene/chlorine-isobutene-isoprene rubber nanocomposites: The effect of dispersion, Polym. Adv. Technol., 27 (7), pp. 872–881. 25. Malas, A., Pal, P. and Das, C. K. (2014). Effect of expanded graphite and modified graphite flakes on the physical and thermo-mechanical properties of styrene butadiene rubber/polybutadiene rubber (SBR/BR) blends, Mater. Des., 55, pp. 664–673. 26. Valentini, L., Bolognini, A., Alvino, A., Bittolo Bon, S., Martin-Gallego, M. and Lopez-Manchado, M. A. (2014). Pyroshock testing on graphene based EPDM nanocomposites, Compos. B: Eng., 60, pp. 479–484. 27. de Castro, R. K., Araujo, J. R., Valaski, R., Costa, L. O. O., Archanjo, B. S., Fragneaud, B., Cremona, M. and Achete, C. A. (2015). New transfer method of CVD-grown graphene using a flexible, transparent and conductive polyaniline-rubber thin film for organic electronic applications, Chem. Eng. J., 273, pp. 509–518. 28. Abd Razak, J., Haji Ahmad, S., Ratnam, C. T., Mahamood, M. A. and Mohamad, N. (2015). Effects of poly(ethyleneimine) adsorption on graphene nanoplatelets to the properties of NR/EPDM rubber blend nanocomposites, J. Mater. Sci., 50 (19), pp. 6365–6381. 29. Mensah, B., Kim, S., Arepalli, S. and Nah, C. (2014). A study of graphene oxidereinforced rubber nanocomposite, J. Appl. Polym. Sci., 131 (16), p. 40640. 30. Mensah, B., Kumar, D., Lim, D.-K., Kim, S. G., Jeong, B.-H. and Nah, C. (2015). Preparation and properties of acrylonitrile–butadiene rubber–graphene nanocomposites, J. Appl. Polym. Sci., 132 (36), p. 42457. 31. Varghese, T. V., Ajith Kumar, H., Anitha, S., Ratheesh, S., Rajeev, R. S. and Lakshmana Rao, V. (2013). Reinforcement of acrylonitrile butadiene rubber using pristine few layer graphene and its hybrid fillers, Carbon, 61, pp. 476–486. 32. Mahmoud, W. E., Al-Ghamdi, A. A. and Al-Solamy, F. R. (2012). Evaluation and modeling of the mechanical properties of graphite nanoplatelets based rubber nanocomposites for pressure sensing applications, Polym. Adv. Technol., 23 (2), pp. 161–165. 33. Liu, X. Y., Sun, D., Wang, L.-J. and Guo, B. (2013). Sodium humate functionalized graphene and its unique reinforcement effects for rubber, Ind. Eng. Chem. Res., 52 (41), pp. 14592–14600.

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34. Bai, X., Wan, C., Zhang, Y. and Zhai, Y. (2011). Reinforcement of hydrogenated carboxylated nitrile–butadiene rubber with exfoliated graphene oxide, Carbon, 49 (5), pp. 1608–1613. 35. Zhao, X.-W., Zang, C.-G., Wen, Y.-Q. and Jiao, Q.-J. (2015). Thermal and mechanical properties of liquid silicone rubber composites filled with functionalized graphene oxide, J. Appl. Polym. Sci., 132 (38), p. 42582. 36. Cai, W., Huang, Y., Wang, D., Liu, C. and Zhang, Y. (2014). Piezoresistive behavior of graphene nanoplatelets/carbon black/silicone rubber nanocomposite, J. Appl. Polym. Sci., 131 (3), p. 39778. 37. Hu, H., Zhao, L., Liu, J., Liu, Y., Cheng, J., Luo, J., Liang, Y., Tao, Y., Wang, X. and Zhao, J. (2012). Enhanced dispersion of carbon nanotube in silicone rubber assisted by graphene, Polymer, 53 (15), pp. 3378–3385. 38. Chen, L., Lu, L., Wu, D. and Chen, G. (2007). Silicone rubber/graphite nanosheet electrically conducting nanocomposite with a low percolation threshold, Polym. Compos., 28 (4), pp. 493–498. 39. Wei, J., Jacob, S. and Qiu, J. (2014). Graphene oxide-integrated hightemperature durable fluoroelastomer for petroleum oil sealing, Compos. Sci. Technol., 92, pp. 126–133. 40. Dao, T. D., Lee, H.-i. and Jeong, H. M. (2014). Alumina-coated graphene nanosheet and its composite of acrylic rubber, J. Colloid Interface Sci., 416, pp. 38–43. 41. Xiong, Y., Xie, Y., Zhang, F., Ou, E., Jiang, Z., Ke, L., Hu, D. and Xu, W. (2012). Reduced graphene oxide/hydroxylated styrene–butadiene–styrene tri-block copolymer electroconductive nanocomposites: Preparation and properties, Mater. Sci. Eng.: B, 177 (14), pp. 1163–1169. 42. Cao, P., Huang, C., Zhang, L. and Yue, D. (2015). One-step fabrication of RGO/HNBR composites via selective hydrogenation of NBR with graphenebased catalyst, RSC Adv., 5 (51), pp. 41098–41102. 43. Araby, S., Zaman, I., Meng, Q., Kawashima, N., Michelmore, A., Kuan, H.-C., Majewski, P., Ma, J. and Zhang, L. (2013). Melt compounding with graphene to develop functional, high-performance elastomers, Nanotechnology, 24 (16), p. 165601. 44. Gan, L., Shang, S., Yuen, C. W. M., Jiang, S.-X. and Luo, N. M. (2015). Facile preparation of graphene nanoribbon filled silicone rubber nanocomposite with improved thermal and mechanical properties, Compos. B: Eng., 69, pp. 237–242. 45. Xing, W., Wu, J., Huang, G., Li, H., Tang, M. and Fu, X. (2014). Enhanced mechanical properties of graphene/natural rubber nanocomposites at low content, Polym. Int., 63 (9), pp. 1674–1681. 46. Stanier, D. C., Patil, A. J., Sriwong, C., Rahatekar, S. S. and Ciambella, J. (2014). The reinforcement effect of exfoliated graphene oxide nanoplatelets

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47.

48.

49. 50.

51.

52.

53.

54.

55.

56. 57.

58.

on the mechanical and viscoelastic properties of natural rubber, Compos. Sci. Technol., 95, pp. 59–66. Wang, J., Jia, H., Tang, Y., Ji, D., Sun, Y., Gong, X. and Ding, L. (2013). Enhancements of the mechanical properties and thermal conductivity of carboxylated acrylonitrile butadiene rubber with the addition of graphene oxide, J. Mater. Sci., 48 (4), pp. 1571–1577. Ma, W., Li, J., Deng, B., Lin, X. and Zhao, X. (2013). Properties of functionalized graphene/room temperature vulcanized silicone rubber composites prepared by an in-situ reduction method, J. Wuhan Univ. Technol.-Mater. Sci. Ed., 28 (1), pp. 127–131. Bu, Q., He, F. and Xia, H. (2014). Progress in graphene/rubber nanocomposites, Acta Polym. Sin., 6, pp. 715–723, in Chinese. Zhou, Z., Zhang, X., Wu, X. and Lu, C. (2016). Self-stabilized polyaniline@ graphene aqueous colloids for the construction of assembled conductive network in rubber matrix and its chemical sensing application, Compos. Sci. Technol., 125, pp. 1–8. Zhan, X. (2013) Preparation Methods/Micro-structure/Mechanical and Dielectric Properties of Graphene/Elastomer Nanocomposites, Master thesis, Beijing University of Chemical Technology, in Chinese. Mahmoud, W. E. and Al-Ghamdi, A. A. (2012). Charge transport mechanism of graphite-nanosheet-loaded rubber nanocomposites, Polym. Int., 61 (1), pp. 51–54. Al-Ghamdi, A. A., Al-Ghamdi, A. A., Al-Turki, Y., Yakuphanoglu, F. and El-Tantawy, F. (2016). Electromagnetic shielding properties of graphene/ acrylonitrile butadiene rubber nanocomposites for portable and flexible electronic devices, Compos. B: Eng., 88, pp. 212–219. Zhang, X., Wang, J., Jia, H., You, S., Xiong, X., Ding, L. and Xu, Z. (2016). Multifunctional nanocomposites between natural rubber and polyvinyl pyrrolidone modified graphene, Compos. B: Eng., 84, pp. 121–129. Yin, B., Zhang, X., Zhang, X., Wang, J., Wen, Y., Jia, H., Ji, Q. and Ding, L. (2017). Ionic liquid functionalized graphene oxide for enhancement of styrenebutadiene rubber nanocomposites, Polym. Adv. Technol., 28 (3), pp. 293–302. Bunch, J. S., Verbridge, S. S., Alden, J. S., van der Zande, A. M., Parpia, J., Craighead, H. G. and McEuen, P. L. (2008). Impermeable atomic membranes from graphene sheets, Nano Lett., 8 (8), pp. 2458–2462. Schopp, S., Thomann, R., Ratzsch, K.-F., Kerling, S., Altstädt, V. and Mülhaupt, R. (2014). Functionalized graphene and carbon materials as components of styrene-butadiene rubber nanocomposites prepared by aqueous dispersion blending, Macromol. Mater. Eng., 299 (3), pp. 319–329. Zhang, S. W. (1998). State-of-the-art of polymer tribology, Tribol. Int., 31 (1), pp. 49–60.

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59. Li, Y., Wang, Q., Wang, T. and Pan, G. (2012). Preparation and tribological properties of graphene oxide/nitrile rubber nanocomposites, J. Mater. Sci., 47 (2), pp. 730–738. 60. Matos, C. F., Galembeck, F. and Zarbin, A. J. G. (2014). Multifunctional and environmentally friendly nanocomposites between natural rubber and graphene or graphene oxide, Carbon, 78, pp. 469–479.

Chapter 5

Graphene Composite Coating

Coating materials are the first barrier for various structural materials to contact the external environment. The painting of coating materials is also the most common and effective technical means to realize the functionality of structural materials. The large specific surface area and excellent thermal and electrical conductivity of graphene make it a kind of functional filler that can be used in coating materials to effectively enhance or modify the performance of coatings. The interphase problem of graphene in the composite coating system and the improvement of the dispersion and compatibility of graphene, as well as the increment of the incorporated amount, are the technical bottlenecks of graphene applications in coating materials. In this chapter, several effective surface modification methods of graphene will be introduced. In addition, the isolation of graphene in 2004 inspired worldwide efforts to develop a whole new family of materials, known as 2D materials, such as hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS2). The assemblies and applications of mixed-dimensional homostructures, mixed-dimensional heterostructures as well as homodimensional heterostructure nanomaterials with graphene and other 2D materials provide a new direction for graphene application technology, which is of significance for fostering strength and circumventing weakness, and provide a new solution for creating coating materials with a functional design and a multifunctional synergetic effect.

269

270

Graphene Composite Materials

5.1 Introduction to Graphene Composite Coating A coating is a covering that is applied to the surface of an object. The purpose of applying the coating may be decorative, functional, or both. Functional coatings are applied to change the surface properties of the substrate, such as corrosion resistance, electrical conductivity, wear resistance, and wettability. Graphene has many outstanding characteristics such as a large specific surface area, excellent thermal and electrical conductivity, and good chemical stability, so it can be used as a functional additive to enhance or modify the performance of the coating, giving the coating excellent electrical conductivity, thermal conductivity, corrosion resistance, and wear resistance. In addition, the properties of graphene can be manipulated and tuned by preparation methods, thus causing it to be well adapted to a variety of coating substrates, further broadening the applications of graphene in the field of coating materials.

5.2 Graphene-Based Polymer Coating 5.2.1 Surface modification of graphene Graphene is considered an inert material, but the commonly used graphene-based material is GO, prepared by the modified Hummers redox method, bearing hydroxyl, epoxy, carboxyl, carbonyl, diol, and ketone functional groups distributed on the surface. The existence of these functional groups can alter the van der Waals interactions and lead to a range of solubility in water and organic solvents as shown in Figure 5.1.1 The presence of additional carbonyl and carboxyl groups located at the edge of the sheets makes GO sheets strongly hydrophilic, allowing them to readily swell and disperse in water. In order to retain the intrinsic performance of graphene to the greatest extent, the reduction of GO must be carried out chemically, thermally, or photochemically. However, the reduction of GO dispersion without stabilizer leads to precipitation of graphene particles due to the rapid, irreversible aggregation of graphene sheets. Therefore, prior to reduction, surface modification of GO sheets is usually carried out by covalent modifications or non-covalent functionalizations, followed by reduction. Table 5.1 shows different kinds of covalent modifications of GO using different

Graphene Composite Coating 271

Figure 5.1. Digital pictures of as-prepared GO dispersed in water and 13 organic solvents through bath ultrasonication (1 h). Top: Dispersions immediately after sonication. Bottom: Dispersions three weeks after sonication.1

Table 5.1. Different kinds of covalent modifications of GO using different modifying agents, their dispersion stability in various solvents, dispersibility, and electrical conductivity.2 Modification techniques

Modifying agent

Nucleophilic Alkyl amines/amino substitution acids 4-Aminobenzene sulfonic acid

Dispersing medium CHCl3, THF, toluene, DCM Water

4,4′-Diaminodiphenyl Xylene, methanol ether

Electrical Dispersibility conductivity (mg/ml) (S/m)

Refs.

/

/

3

0.2

/

4

0.1

/

4

POA

THF

0.2

/

5

Allylamine

Water, DMF

1.55

/

6

APTS

Water, ethanol, DMF, DMSO

0.5

/

7

IL-NH2

Water, DMF, DMSO

0.5

/

8

PLL

Water

0.5

/

9

Dopamine

Water

0.05

/

10

Polyglycerol

Water

3

/

11 (Continued )

272

Graphene Composite Materials Table 5.1.

Modification techniques

Modifying agent

(Continued )

Dispersing medium

Poly(norepinephrine) Water, methanol, acetone, DMF, NMP, THF, toluene Electrophilic ANS substitution 4-Bromo aniline

Condensation reaction

Refs.

0.1

/

12

Water

3

145

13

DMF

0.02

/

14

Sulfanilic acid

Water

2

1250

15

NMP

Ethanol, DMF, NMP, PC, THF

0.2~1.4

21,600

16

Organic isocyanate

DMF, NMP, DMSO, HMPA

1 (DMF)

/

17,18

/

1.9 × 10–4

19

0.5 (THF)

/

20

0.2

/

21

Organic diisocyanates DMF ODA

THF, CCl4, 1,2-dichloroethane

TMEDA PEG-NH

Addition reaction

Electrical Dispersibility conductivity (mg/ml) (S/m)

THF Water

1

/

22

CS

Water

2

/

23

TPAPAM

THF

/

/

24

β-CD

Water, acetone, DMF

1 (DMF)

/

25

α-CD, β-CD, γ-CD

Water, ethanol, DMF, DMSO

>2.5

/

26

2

PVA

Water, DMSO

/

/

27

TPP-NH2

DMF

/

/

28

Adenine, cystine, Water nicotinamide, OVA

0.1

/

29

POA

THF

0.2

/

5

Polyacetylene

Ortho dichlorobenzene (O-DCB)

0.1

/

30

Aryne

DMF, O-DCB

0.4

/

31

Cyclopropanated malonate

Toluene, O-DCB, DMF, DCM

0.5

/

32

Graphene Composite Coating 273

modifying agents, their dispersion stability in various solvents, dispersibility, and electrical conductivity as reported in the literature.2 In covalent modification, nucleophilic substitution of an amine-terminated organic modifier is the simplest way to produce functionalized graphene in large quantities; however, the electrical conductivity of the resulting functionalized graphene is greatly reduced. In order to obtain highly conductive graphene, electrophilic substitution of diazonium salt onto the surface of partially reduced graphene is highly recommended. Non-covalent modification of a small organic polymer on a pre-reduced graphene surface can also yield excellent electrical conductivity. Table 5.2 shows the research results on different non-covalent modifications of GO.2 However, in all these cases, the electrical conductivity of the functionalized graphene is significantly decreased. Therefore, research efforts should focus on seeking modification technologies that can effectively improve the dispersibility and spontaneously retain the electrical conductivity of graphene. Moreover, the removal of excess organic modifier from the functionalized graphene is a major concern. Based on this situation, much effort has been directed toward the synthesis of functionalized graphene directly from graphite. Different kinds of organic modifier used for the preparation of functionalized graphene, their dispersion stability, and electrical conductivity are listed in Table 5.3.2

5.2.2 Anti-corrosive coating Elcora Advanced Materials Corp., a Canadian company, has been actively conducting research on graphene coatings, expecting to develop graphene hydrophobic coatings that can be utilized for boat hulls, non-stick pan liners, mirrors, windows, and windshields. Meanwhile, conductive coatings are another kind of product which can be used for the displays of cell phones, tablet PCs, computers, and TVs. In addition, graphene can also be used for anti-corrosive coatings, exhibiting excellent resistance to chemicals, water, corrosion, UV radiation, and fire. In the field of medical devices, graphene anti-corrosive coatings are biocompatible and prevent the coating from degrading in the presence of organisms. Elcora predicts that in the next decade, various types of graphene coatings will be gradually popularized, with an annual output value of 12 billion US dollars. There are mainly two types of graphene anti-corrosive coatings, one is pure graphene coating and the other is graphene-based polymer composite coating.

274

Graphene Composite Materials

Table 5.2. Non-covalent modification of GO using different modifying agents, their dispersion stability in various solvents, dispersibility, and electrical conductivity.2

Modifying agent

Dispersing medium

Dispersibility (mg/ml)

Electrical conductivity (S/m)

Refs.

PSS

Water

1

/

33

SPANI

Water

>1

30

34

PBA

Water

0.1

200

35

Amine terminated polymer

1,3-Dimethyl-2imidazolidinone, γ-butyrolactone 1-propanol, ethanol, ethylene glycol, DMF

0.4

1500

36

PNIPAAM

Water

/

/

37,38

PSS-g-PPY

Water

3

/

39

Poly(propyleneimine) dendrimers

Water

/

/

40

Coronene derivative

Water

0.15

/

41

PPES O3−

Water

0.25

30 kΩ (resistance)

42

SDBS

Water

1

80 Ω (resistance)

43,44

MG

Water

0.1

/

45

SLS, SCMC, HPC-Py

Water

0.6–2

/

46

PYR-NHS

Water

/

/

47

Porphyrin

Water

0.02

370 Ω cm

48

PIL

Water

1.5

3600

49

Krishnamoorthy et al. reported the use of GO nanosheets for inhibition of Cu metal corrosion, in which GO nanosheets were synthesized according to the modified Hummers method and thin films of GO nanosheets were deposited onto copper substrates by the drop casting method.50 The experimental details were as follows: An appropriate amount of GO in 80:20 (v/v) of water and ethanol was sonicated for 30 min until a uniform suspension was attained. Then, the GO dispersion was

Graphene Composite Coating 275 Table 5.3. Functionalized graphene directly from graphite using different modifying agents, their dispersion stability in various solvents, dispersibility, and electrical conductivity.2 Modifying agent

Sono-chemical

TCNQ

DMF, DMSO

/

/

51

SDBS

Water

0.05

35~1500

52,53

NaCl

Water

0.3

7000~17,500

PCA

Water

/

/

54

Benzoyl peroxide

Water

2.1

212 Ω (resistance)

55

Styrene

Toluene, DMF, THF, CHCl3

2

/

56

Ionic liquid

DMF

1

/

57

Electrochemical

Dispersing medium

Dispersibility (mg/ml)

Electrical conductivity (S/m)

Modification techniques

Refs.

drop-casted onto the cooper foil and allowed to dry at 80°C for 30 min. After the evaporation of solvent, uniform thin films of GO were formed on the Cu foil. Potentiodynamic polarization and AC impedance measurements were performed on the obtained GO coatings in 3.5% NaCl solution to characterize their anti-corrosive properties. In general, the corrosion of copper in NaCl solution can be accompanied by two processes: (i) anodic oxidation: Cu → Cu + + e − (fast ) Cu + → Cu 2+ + 2e − (slow )

(5.1) (5.2)

(ii) cathodic reduction: O 2 + H 2 O + 4e − → + OH −

(5.3)

These two reactions complement each other so that impeding one of them slows the overall corrosion process. The potentiodynamic polarization curves, obtained for the bare Cu substrate and GO-coated copper substrate, are shown in Figure 5.2. Compared with the bare copper substrate, the corrosion potential of the GO-coated copper substrate exhibit is

276

Graphene Composite Materials

Figure 5.2. Tafel plots of bare copper and GO-coated copper substrates.58

shifted to larger potentials, that is, from −269.89 mV to −131.73 mV. The corrosion current density (Icorr) can be obtained by fitting the polarization curves with the Tafel equation. By using Equation (5.4), the protective efficiency of the GO-coated copper substrate approaches 70% when compared to that of the bare copper foil: Pi ( % ) = 1 − ( I corr /I corr ′ ) × 100

(5.4)

Pi is the protective efficiency, and Icorr and I′corr indicate the corrosion current density of GO-coated Cu substrate and bare copper substrate, respectively. Singh et al. fabricated GO/polymer composite coatings on the surface of copper substrates by electrophoretic deposition (EPD).59 An aqueous dispersion of GO was prepared at a concentration of 0.01–0.1 g/l with optimum dosages of polymeric isocyanate cross-linked with hydroxy functional acrylic adhesive (PIHA). A homogeneous aqueous dispersion of GO and polymer was obtained by magnetically stirring the suspensions at moderate speed for 10 min followed by ultrasonication for 20 min. Electrophoretic deposition of a well-dispersed PIHA/GO system on copper was performed using the EPD setup as depicted in Figure 5.3. The well-dispersed PIHA/GO system was used as the EPD solution; two parallel copper plates (9 mm × 30 mm) separated by a 10-mm gap were used as the electrodes, with one of them working as the depositing substrate (cathode) and the other as the counter electrode. EPD was performed at

Graphene Composite Coating 277

Figure 5.3. A schematic of the electrophoretic deposition setup for fabricating PIHA/GO coatings.59

constant DC voltage mode 10–30 V with deposition time of 5–50 s. Finally, the PIHA/GO coating with a thickness of 40 nm was obtained. The deposited samples were then taken out from the suspension and allowed to dry overnight at room temperature, followed by treating with silicone fluid (KF-99) to increase the hydrophobicity of the resultant coatings. Figure 5.4 shows the electrochemical impedance spectroscopy (EIS) data in a complex plane diagram (Nyquist plot) of the bare copper, KF99-coated copper substrate, and PIHA/GO-coated copper substrate. The corrosion resistance of the PIHA/GO-coated sample is found to be almost three times higher than the bare copper substrate. Krishnamurthy et al. carried out research on the inhibition effect of graphene coating on microbially induced corrosion (MIC), which is a ubiquitous problem in the natural environment as indigenous microbes are adept at corroding metallic structures under ambient temperatures and neutral pH conditions.60 Especially in the environment where the microbial, chemical, and electrochemical corrosion media coexist, MIC often accelerates the chemical and electrochemical corrosion process; thus, the prevention of MIC is also an important approach to control corrosion in complex corrosive environments. The MIC resistance of Ni was

278

Graphene Composite Materials

Figure 5.4. EIS spectra of (a) bare copper, (b) copper coated with only KF99, and (c) PIHA/GO-coated copper after KF-99 treatment.59

compared in the presence of three different coatings: (1) parylene-C coating on Ni (PA/Ni), (2) polyurethane coating on Ni (PU/Ni), and (3) graphene coating on Ni (Gr/Ni). An Ni foam was used as the substrate for all three coatings. Chemical vapor deposition (CVD) was used to obtain a uniform coating of PA-C on the Ni surface with the average thickness of ~46 nm. The PU films were deposited using standard spray coating, and the thickness of PU/Ni is determined to be in the range of 20~80 µm. The graphene films were deposited on the Ni foam using CVD, and the number of layers is 3–4. The microstructures of the nickel foam substrates with the three coatings are shown in Figure 5.5. The biofilms were grown on the surfaces of three coatings by means of the fed-batch mode, and microbial corrosion on the surface of coatings was observed after continuous exposure to the MIC environment for 30 days. Visual examination shown in Figure 5.6 indicates that the graphene coating is able to preserve the underlying Ni material, while the presence of green corrosion by-products is clearly observed in the case of the PA and PU coating, and the edges of the PA/Ni are found to be severely corroded, which is metal corrosion induced by microbes. The

Graphene Composite Coating 279

Figure 5.5. Dimensional characteristics of three coatings on Nickel foam surfaces: (a) PA-coated nickel, (b) PU-coated nickel, and (c) conformal coating of graphene film on an Ni foam.60

280

Graphene Composite Materials

(a)

(b)

(c)

(d)

(e)

(f)

Figure 5.6. Microbial corrosion of three coating surfaces. (a) SEM image of biofilm on Gr/Ni, (b) MIC-resistant Gr/Ni anode after 30 days of MIC testing, (c) SEM image of biofilm on PA/Ni, (d) corroded Ni/PA anode after 30 days of MIC experiment, (e) SEM image of biofilm on PU/Ni, and (f) corroded Ni/PU anode after 30 days of MIC experiment.60

study also suggests that the graphene coating grown in-situ by CVD is both resistant to microbial attack and extremely conformal and defectfree when compared to transferred graphene films for anti-MIC applications.

Graphene Composite Coating 281

5.2.3 Thermally conductive coating The in-plane thermal conductivity of graphene at room temperature is among the highest of any known materials, about 2000–4000 W/(m·K) for freely suspended samples. Based on this, graphene is expected to fabricate thermally conductive organic coatings, but there are still some technical problems in practical applications. Because of its 2D nature, graphene has very high anisotropy of its thermal properties between the in-plane and out-of-plane directions. Whereas the in-plane thermal conductivity is excellent, the out-of-plane thermal coupling is limited by weak van der Waals interactions and could become a thermal dissipation bottleneck. To overcome this effect in practice, 3D architectures could incorporate CNT-pillared graphene network structures.61 The lamellar structure of graphene and the pillar structure of carbon nanotubes can be fully leveraged to construct a 3D nanoarchitecture as shown in Figure 5.7, which overcomes the shortcomings of the anisotropic thermal properties of graphene.62 From the perspective of thermal transport, this modeling study suggests that the lateral CNT separation, called the interjunction distance (IJD), and the interlayer distance (ILD) between graphene sheets play a critical role in determining the thermal transport properties in these

Figure 5.7. sheets.62

Schematic of a 3D nanoarchitecture that combines CNT pillars and graphene

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3D architectures. These junctions, in turn, will govern the thermal conductivity of graphene in the out-of-plane direction. For instance, the outof-plane thermal conductivity of graphene could be raised by reducing IJD and increasing ILD, while the out-of-plane thermal conductivity of graphene could be declined by increasing IJD and reducing ILD. As mentioned above, graphene has remarkable anisotropy of its thermal properties between the in-plane and out-of-plane directions, which limits its wide applications. However, the fact is that graphene’s extraordinary in-plane heat conductivity properties still show its potential for applications. Han et al. show experimentally that the thermal management of a micro heater is substantially improved by introducing alternative heat-escaping channels into a graphene-based film bonded to functionalized GO through amino-silane molecules.63 As shown in Figure 5.8(e), graphene-based film/functionalized GO (GBF/FGO) has the best heatspreading capability; thus, the resulting hotspot temperature is lowest (Figure 5.9). The results of this study indicate that the combination of graphene and GO can effectively improve the in-plane thermal conductivity of graphene coatings, which can be used for the thermal management of hotspots in high-power electronics at the micro- and nanometer length scale.

5.2.4 Electrically conductive coating A recent work by Wang et al. from Rice University shows that a perfluorododecylated graphene nanoribbon (FDO-GNR) coating exhibits (a)

(c)

(b)

(d)

Figure 5.8.

(e)

Graphene-based film on FGO as heat spreader for hotspot.63

Graphene Composite Coating 283 (a)

(b)

(c)

Figure 5.9. Heat-spreading performance of coating comprising graphene and FGO with different functional agents (Eph: phonon transmission, Gth: phonon thermal conductance).63

excellent de-icing and anti-icing effects down to −14°C.64 The anti-icing and de-icing of cables, wires, and aircraft wings have always been a problem across the world. Anti-icing and de-icing are two major pathways for solving the icing problem of critical industrial components and facilities such as cables and aircraft wings. The coating prepared by the Rice University researchers theoretically and experimentally proved to show both anti-icing and de-icing capabilities. The basic mechanism is that the FDO-GNR coatings are superhydrophobic and then exhibit an anti-icing property that prevents freezing of incoming ice-cold water. Meanwhile, the excellent electrical conductivity of the coatings with a sheet resistance below 8 kΩ/sq is helpful for thermal de-icing. The water contact angle and sheet resistance of the coatings can be controlled as needed. As shown in Figure 5.10, when the water contact angle of the coating surface is 161°, the water droplets cannot stick to the coating surface in a −14°C environment. When the water contact angle is 131°, ice water droplets fall on the surface of the coating without freezing under the same temperature, but the water drops stay on the surface. As the temperature is raised to 4°C,

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(b) (a)

(c)

(e) (d)

(f)

Figure 5.10. Anti-icing effect of FDO-GNR coating. (a) Contact angle (161°) image of a water drop on a coating surface. Photographs of the coating maintained in a −14°C environment (b) before and (c) after ice-cold water is dropped onto the coating surface. (d) Contact angle (131°) image of a water drop on a coating surface. Photographs of the coating maintained at (e) −14°C and (f) 4°C environment after ice-cold water is applied onto the coating surface.64

the water droplets do not freeze and the phenomenon of attaching to the coating surface is alleviated. This suggests that superhydrophobicity is a key factor in preventing freezing of incoming water. This study shows that the lowest temperature for the anti-icing nature of graphene coating is −14°C. As shown in Figure 5.11, when the ambient temperature is −32°C, water droplets are frozen on the surface of the coating without applying any other heat sources. However, since graphene coating has good electrical conductivity, the ice on the surface of the coating can be melted by applying current, as shown in Figure 5.11(c). In order to obtain a better de-icing effect, heptacosafluorotributylamine is added as a lubricant to wet the coating, which results in a slippery surface, as shown in Figure 5.12. The entire piece of ice starts to fall off when the frozen coating is electrically heated (Figure 5.12(d)). The coating becomes less sticky and more cleanable after de-icing when compared with that without addition of the lubricant (Figure 5.12(b)). In this study, graphene nanosheets are not used in these coatings but graphene nanoribbons are adopted,

Graphene Composite Coating 285

Figure 5.11.

De-icing effect of FDO-GNR coating.64

(a)

(c)

(b)

(d)

Figure 5.12.

De-icing effect of FDO-GNR coating after adding lubricant.64

which easily contact with each other, giving rise to coatings with better electrical conductivity and better de-icing effect.

5.2.5 Other coatings Secondary electron emission is a limiting factor for many vacuum-related industries ranging from small gauges and detectors to waveguides and charged particle accelerators. Several methods have been proposed to suppress such effect, e.g., introduction of a layer of material with low secondary electron yield (SEY). The SEY of a material depends upon the atomic number of the materials, surface chemistry, the surface topology, and to a

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lesser effect the work function of the material. Carbon material is known to have a low SEY in its graphite allotropic form mainly ascribing to the sp2 hybrids in graphite, and is already widely used in a wide range of industries; for example, coating copper with amorphous carbon by DC magnetron sputtering has been found to reduce the maximum SEY from 2.4 to 1.1 for “as-received” samples and coating with highly oriented pyrolytic graphite (HOPG) reduces the maximum SEY to 1.26. Graphene has unusual electrical, mechanical, and thermal properties and since it is a form of carbon, it should result in a low secondary electron yield. Sian et al. used electrophoretic deposition (EPD) to deposit a layer of graphene coating on the surface of stainless steel substrate.65 During the deposition process, the pH value of the graphene aqueous solution was adjusted to 3 with hydrochloric acid, which caused the graphene to become positively charged, and the coating was deposited onto the surface of the negative electrode. Figure 5.13 shows the SEY data as a function of primary electron energy for all the samples using various EPD biases and deposition times. It can be evidenced that a longer deposition time at the same bias leads to a lower SEY, and a higher bias for the same deposition time also leads to a lower SEY.

Figure 5.13. The SEY data for stainless steel coated with graphene using various EPD biases and deposition times.65

Graphene Composite Coating 287

5.3 Graphene-Based Inorganic Coating 5.3.1 Metal composite coating The coatings or films comprising graphene and metal often exhibit multifunctionality, giving rise to many potential applications. The hybrid coating based on graphene and silver nanowire (AgNW) exhibits efficient antimicrobial activity and shows great application prospects in medical devices and implantable human organs. Zhao et al. from Peking University fabricated an antimicrobial graphene/AgNW coating by the CVD method, which can be applied on ethylene vinyl acetate/polyethylene terephthalate (EVA/PET) plastic films by a full roll-to-roll process.66 Studies indicate that the graphene/AgNW hybrid coating shows broadspectrum antimicrobial activity against Gram-negative (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus), and fungi (Candida albicans), as shown in Figure 5.14. This effect is attributed to a weaker microbial attachment to the ultra-smooth hybrid coating and the sterilization capacity of Ag+ (Figure 5.15). Owing to the good electrical conductivity of the graphene/AgNW hybrid coating, the antimicrobial activity is investigated by applying electrical input. As shown in Figure 5.16(a), a graphene/AgNW/EVA/PET hybrid coating with sheet resistance of

Figure 5.14. Scheme and mechanism of the antimicrobial activity of graphene/AgNW hybrid coating.66

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Figure 5.15.

Images of graphene/AgNW/EVA/PET hybrid coating.66

~20 Ω/sq was used as a cathode, a chemically inert electrode like platinum (Pt) wire as the anode, and C. albicans SC5314 culture suspension as the electrolyte. A small voltage of ~5 V was applied with a current flow of ~3 mA. Note that such low voltage and small current flow do not harm human health. Water electrolysis takes place in the electrolyte when the hybrid coating as the cathode is subjected to a current flow. The generation of large amounts of hydroxyl ions at the surface of the hybrid coating allows the neutral microbial suspension (pH value at 7) to become strongly basic (pH value at 10), which is an unsuitable environment for many microorganisms, thereby enhancing the inhibition of microbial growth. Silver/reduced GO (Ag/rGO)-coated polyester (PET) fabrics with dopamine as the adhesive agent were prepared by Wang et al., and the electromagnetic interference (EMI) shielding effectiveness (SE) was investigated.67 The Ag/rGO-coated fabric possesses high electrical conductivity with low surface resistance of 0.678 Ω/sq, and the EMI SE of Ag/rGO-coated fabric ranges from 58 dB to 65 dB in the range of 1–18 GHz X-band. The mechanism by which Ag/rGO is coated on PET fabric modified with dopamine under microwave irradiation is illustrated in Figure 5.17. Figure 5.18 presents surface morphologies of Ag/rGO-coated PET fabric with and without treatment of dopamine. It is obviously evidenced that Ag/rGO particles are still densely coated on the PET fibers modified with dopamine.

Graphene Composite Coating 289 (a)

(c)

(f)

(b)

(d)

(g)

(e)

(h)

Figure 5.16. Enhanced antimicrobial activity of coating by water electrolysis using a graphene/AgNW electrode.66 (a) Scheme diagram of the electrolysis cell. (b) Death rate versus electrolysis time; (c)–(e) Representative photograph of colony-forming units (CFU) showing various electrolysis time points, i.e., (c) 0 s, (d) 30 s, and (e) 4 min. (f) Image of hyphae on the surface of a denture plate without graphene/AgNW coating. (g) Image of the surface of a denture plate with graphene/AgNW coating with 4-min electrolysis time. (h) The photograph of a usable denture coated with graphene/AgNW. The dashed boxes show the graphene/AgNW-coated areas.

5.3.2 Non-metallic composite coatings In addition to the hybridization with metals, graphene can also be hybridized with various oxides to form composite coatings, including TiO2,

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Figure 5.17.

Mechanism of Ag/rGO coating on PET fabric modified with dopamine.67

ZnO, SnO2, MnO2, Co3O4, Fe3O4, Fe2O3, NiO, and Cu2O. The prepared coatings may result in some particular properties when different metal oxides are composited. A variety of techniques have been developed for fabricating the graphene/metal oxide composites. (1) Solution mixing method: The solution mixing technique is an efficient and direct method. The graphene/SnO2 composites can be prepared by solution mixing: SnO2 sol is first synthesized by hydrolysis of SnCl4 with NaOH, and then graphene dispersion is adopted and mixed with the sol in ethylene glycol to form the graphene/SnO2 composites.68 TiO2 nanoparticles are mixed with Nafion-coated graphene to fabricate graphene/TiO2 composites, which can be used for dye-sensitized solar cells.69 TiO2 and GO colloids have been mixed ultrasonically, followed by

Graphene Composite Coating 291

(a)

(b)

(c)

(d)

Figure 5.18. Morphology of hybrid coating on PET fabric. (a), (b) Ag/rGO-coated PET fabric without pretreatment with dopamine. (c), (d) Ag/rGO-coated PET fabric with pretreatment with dopamine.67

ultraviolet-assisted photocatalytic reduction of GO to yield graphene/TiO2 composites.70 (2) Sol–gel method: The sol–gel process is a popular approach for the preparation of metal oxide structures and film coatings, with the metal alkoxides or chlorides as precursors that undergo a series of hydrolysis and polycondensation reactions. Taking TiO2 as an example, the typical precursors used are TiCl3,71 titanium isopropoxide,72 and titanium butoxide,73 which result in nanorods, nanoparticles, or a macro-mesoporous framework of TiO2 depending on the different experimental conditions applied. The direct growth of TiO2 nanocrystals on GO sheets was achieved by a two-step method, in which amorphous TiO2 was first coated on GO sheets by hydrolysis and then crystallized into anatase nanocrystals by the hydrothermal treatment in the second step.74 This method offers easy access to the GO/TiO2 nanocrystal hybrids with a uniform coating

292 (a)

Graphene Composite Materials (b)

(c)

Figure 5.19. Synthesis of graphene/TiO2 composite coatings through sol–gel method. (a) AFM image of a starting graphene sheet; (b) SEM image of TiO2 particles grown on a GO sheet after the first hydrolysis reaction step; (c) SEM image of TiO2 nanocrystals on GO after hydrothermal treatment in the second step. The scale bars are 100 nm.74

and strong interactions between TiO2 and the underlying GO sheets, as shown in Figure 5.19. (3) Hydrothermal/solvothermal method: Hydrothermal/solvothermal is a powerful tool for the synthesis of inorganic nanocrystals, which operates at an elevated temperature in a confined volume to generate high pressure. The one-pot hydrothermal/solvothermal process can give rise to nanostructures with high crystallinity without post-synthetic annealing or calcination. A series of graphene/TiO2 composites were synthesized using a hydrothermal or solvothermal method.75–79

5.4 Composite Coatings of Graphene and Other Nanomaterials 5.4.1 Composite coating of graphene and 0D nanomaterials Hybridization and compounding of graphene and other nanomaterials in different dimensions have become important widespread technical approaches to improve the performance and application potential of graphene. With the development of various 0D nanomaterial preparation technologies, scientists have attempted to hybridize graphene with 0D nanomaterials for better performance. Xiao et al. developed a layer-bylayer (LbL) self-assembly approach for construction of new hybrid nanomaterials with 0D CdS quantum dots (QDs) with graphene nanosheets (GNs).80 First, the 0D CdS QDs were synthesized,81 then

Graphene Composite Coating 293

Figure 5.20. Schematic illustration for LbL self-assembly of CdS QDs/GNs, pure GNs, and pure CdS QDs.80

GNs-poly(allylamine hydrochloride) (PAH) was prepared,82 and the final 0D CdS QD/GN self-assembled hybrids were prepared by the detailed process illustrated in Figure 5.20. These resultant self-assembled hybrids are composed of tailor-made negatively charged CdS QDs and positively charged GNs-PAH, which are judiciously stacked in an alternating manner based on pronounced electrostatic interaction; by this means, largearea, smooth, and uniform hybrid films are thus fabricated. It is found that the as-assembled materials demonstrate promising photoelectrochemical and photocatalytic performances under visible light irradiation, as Figures 5.21–5.23 show.

5.4.2 Composite coating of graphene and 1D nanomaterials The combination of graphene and carbon nanotubes is effective to extend the properties into three dimensions. Zhu et al. synthesized a new 3D carbon material by directly growing aligned single-walled carbon nanotube (SWCNTs) arrays on the graphene, and this process is shown in

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(a)

(c)

(b)

(d)

(e)

(f)

Figure 5.21. UV-vis absorption spectra of (a) pure CdS QDs and GNs assembled after five deposition cycles, and (b) (CdS QDs/GNs)n (n = 1, 5, 10, 15, 20) multilayered films with varied deposition cycles. (c) and (d) Plot of transformed Kubelka−Munk function versus energy of light for (a) and (b), respectively. (e) Stacking model between CdS QDs and GNs-PAH. (f) Absorption of (CdS QDs/GNs)n multilayered films at wavelength of 475 nm versus the number of deposition cycles.79 (a)

(c)

(e)

(b)

(d)

(f)

Figure 5.22. TEM images of (a) GO, (b) GNs-PAH, and CdS QD/GN composite film with (c) one and (d), (e) five deposition cycles, and (f) high-resolution TEM image of CdS QDs on the GNs-PAH with selected area electron diffraction (SAED) pattern in the inset.79

Graphene Composite Coating 295 (a)

(b)

(c)

(d)

(e)

(f)

Figure 5.23. (a) and (d) Transient photocurrent responses of CdS QD and GN films with the same number of deposition cycles, and (CdS QDs/GNs)n multilayered films with different number of deposition cycles. (b) and (e) Photocurrent–voltage curves and (c) and (f) electrochemical impedance spectroscopy (EIS) Nyquist plots of CdS QD film and CdS QD/GN multilayered film with the same number of deposition cycles in 0.1 M Na2S aqueous solution under visible light irradiation (λ > 420 nm).79

(a)

Figure 5.24.

(b)

(c)

(d)

Scheme for the synthesis of CNT carpets directly from graphene.83

Figure 5.24.83 The material with such structure exhibits a specific surface area of >2000 m2/g with ohmic contact from the vertically aligned SWCNTs to the graphene. The coating material with such a structure is useful in energy storage and nanoelectronic technologies. The growth process is shown in Figure 5.24. First, the graphene is grown on a copper foil (Figure 5.24(a)), then the iron catalyst and alumina buffer layer are deposited on the graphene in series by electron beam (e-beam) evaporation (Figure 5.24(c)). During the growth, the catalyst and alumina are lifted up and the CNT carpet is grown directly out of the graphene (Figure 5.24(d)). This growth strategy results in seamless covalent bonds between the graphene and CNT carpet. SEM images of a nanotube carpet grown from a graphene-covered copper foil are shown in Figure 5.25. Figures 5.25(a) and 5.25(b) present the samples grown by

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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

Figure 5.25.

(m)

(n)

Characterization of CNT coating grown from graphene.83

using 0.3 nm of iron and 3 nm of alumina. The dark regions in Figure 5.25(a) are gaps in the broken alumina layer and the brighter regions are aluminacovered areas, and the scale bar is 50 µm. Figure 5.25(b) displays an enlarged image for one of the lifted areas where the CNTs are grown, and the scale bar is 10 µm. Figures 5.25(c) and 5.25(d) show the samples grown by using 0.5 nm of iron and 3 nm of alumina, and the scale bars are both 10 µm. It is evidenced that the top alumina layer is fully removed or

Graphene Composite Coating 297

partially broken during growth. Figures 5.25(e)–5.25(h) present the samples grown by using 1 nm of iron and 3 nm of alumina, and the scale bars in Figures 5.25(e) and 5.25(f) are 50 µm and 10 µm, respectively. Figure 5.25(g) shows the side view of the CNT carpet grown on the graphene; the height of the CNT is ~120 µm, and the scale bar is 10 µm. The close-up view on the upper portion of the CNT carpet is shown in Figure 5.25(h), and the scale bar is 1 µm. Figures 5.25(i) and 5.25(j) give the Raman spectra of the CNT carpet and graphene, respectively, and Figure 5.25(k) shows the diffraction pattern of the graphene after CNT growth. Figures 5.25(l)–5.25(n) show TEM images of a single-walled, double-walled, and triple-walled nanotube grown in this work, and the scale bars are all 5 nm. Figure 5.26 shows the electrical properties of graphene and CNT carpet hybrid materials and the supercapacitor device’s characterization. The I–V curves of three experiments (Figure 5.26(a)) are described in Figure 5.26(b): I, one probe is in contact with the platinum electrode and

(a)

(c)

(b)

(d)

(e)

Figure 5.26. Electrical properties of graphene and CNT carpet hybrid materials and the supercapacitor device’s characterization.83

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the other probe is on the graphene electrode; II, one probe is on the platinum electrode and the other probe is suspended above the surface; III, one probe is on the platinum electrode and the other probe is in contact with the CNT carpet side walls. The inset of Figure 5.26 is the SEM image of the device; graphene (the dark area) is patterned into a Hall-bar shape. “Pt” is the platinum electrode deposited on graphene, “CNT” is the CNT carpet electrode grown on graphene, and “G” is the bare graphene electrode. Figure 5.26(c) shows the cyclic voltammetry of the supercapacitor device under different scan rates. Figure 5.26(d) shows the Galvonostatic discharge curves under different discharge currents. Figure 5.26(e) shows the Ragone chart of energy density versus power density, with the voltage window being 4 V.

5.4.3 Composite coating of graphene and 2D nanomaterials The isolation of graphene has inspired worldwide efforts to develop a whole new family of materials, known as 2D materials, such as h-BN, black phosphorus (BP), and MoS2. The assemblies of graphene and these 2D materials have gradually become a new research and application direction in the quest to achieve new or better properties. BP materials have attracted considerable attention owing to their ultra-sensitive humidity-sensing characteristic because of the natural adsorption of water molecules on the BP surface caused by the specific 2D-layer crystalline structure. On the other hand, the BP-based humidity sensor is less repeatable due to the instability of BP with water molecules due to which the stability of the sensor is reduced. The latest research shows that this limitation of the BP-based humidity sensor can be overcome by preparing a BP/graphene hybrid as a novel humidity-sensing nanostructure.84 The BP/graphene interface improves the stability of the humidity sensor with a linear response within the relative humidity (RH) range of 15–70%. The sensor’s response/recovery speed of the humidity sensor is extremely fast within a few seconds. The response of the humidity sensor based on the BP/graphene hybrid is 43.4% at RH = 70%, and the estimated response and recovery times of the sensor are only 9 and 30 s, respectively, at room temperature. The experimental investigation reveals that the BP/graphene hybrid not only improves the reversibility and hysteresis factors but also enhances the stability of the humidity sensor, as Figures 5.27–5.29 show.

Graphene Composite Coating 299 (a)

(b)

(c)

(e)

(d)

Figure 5.27. Sequence of sensor fabrication process of the humidity sensor using BP/graphene heterojunction. (a) Schematic diagram of the sensor, (b) image of the full wafer of fabricated sensor, (c) optical image of single graphene chip, (d) electrospray system to deposit BP on graphene, and (e) schematic diagram of the sensor based on the BP/graphene heterojunction.84

(a)

(b)

(c)

(d)

Figure 5.28. Morphology and structure of graphene/BP heterohybrid coating. (a), (b) SEM images of humidity sensor based on graphene/BP hybrid, (c) TEM images, and (d) SAED of BP flakes.84

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(a)

(b)

Figure 5.29. Transient response and estimated stability of the humidity sensor after 1 h based on (a) BP only and (b) BP/graphene heterojunction.84

Figure 5.30. Typical Raman spectrum of monolayer graphene, monolayer MoS2, and graphene/MoS2 heterostructure.85

MoS2 is a traditional lubricant that is often used as a filler to improve the lubricating properties of coatings. Recent studies have shown that better lubricating properties can be obtained through the hybridization of 2D nanosheets of MoS2 and graphene.85 Graphene/MoS2 heterostructure samples were fabricated by transferring MoS2 monolayers onto graphene monolayers lying on SiO2/Si substrates with a wet transfer strategy. The Raman shift in Figure 5.30 confirms that there is an interlayer coupling between the monolayer MoS2 and the monolayer graphene in the hybrid film. It is found by calculation that after the hybridization of the two heterogeneous 2D materials, although its out-of-plane force constant is

Graphene Composite Coating 301 Table 5.4. Comparison of the interlayer force constant per unit area in graphene/MoS2 heterostructure, graphene, and MoS2.85 Out-ofplane interlayer force constant

In-plane interlayer force constant

S-Mo sublayer out-ofplane force constant

S-Mo sublayer in-plane force constant

Graphene/MoS2 heterostructure

2.88 × 1019

5.45 × 1017

3.47 × 1021

1.90 × 1021

MoS2

8.90 × 1019

2.82 × 1019

3.46 × 1021

1.88 × 1021

/

/

System

Graphene

11.56 × 10

19

1.82 × 10

19

Figure 5.31. Schematic of the wet transfer process to fabricate 2D graphene/h-BN heterostructure.86

basically the same as that of bilayer graphene and bilayer MoS2, the lateral force constant of bilayer graphene is two orders of magnitude lower than that of bilayer MoS2 in the heterostructure (see Table 5.4). In Table 5.4, the first column lists the out-of-plane (vertical) interlayer force constant of the bilayer heterostructure, bilayer graphene, and bilayer MoS2. The second column is the in-plane (horizontal) interlayer force constant. The third and fourth columns are the out-of-plane and in-plane interlayer force

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(a)

(b)

(c)

(e)

(f)

(g)

(d)

(h)

Figure 5.32. Schematic of the liquid phase exfoliating method to fabricate 2D graphene/ h-BN heterostructure.87

constants between S and Mo, respectively. This result shows that in the graphene/MoS2 heterostructure, extremely low interlayer friction is presented, which means superlubricity is achieved by the architecture of such a 2D heterostructure, and it can be used as a lubricating coating or filler. h-BN is also a typical 2D material, and the 2D heterostructure with hybridization of graphene and h-BN has exhibited outstanding optical, electric, and magnetic properties.88 The preparation methods include the wet transfer method (as shown in Figure 5.31), liquid phase exfoliating method (as shown in Figure 5.32), dry transfer method, CVD method, transition metal catalysis, physical transfer method, gas phase epitaxy technology method, and co-segregation method.

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27.

28.

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67.

68.

69.

70.

71.

72.

73.

74.

75.

76.

77.

Graphene Composite Materials

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Index

A 3D architectures, 281–282 ablative thermal protection materials, 198 abrasion resistance, 241 abrasive wear, 108 absolute temperature, 55 accumulative roll bonding, 112–113 acoustic impedance mismatch, 253 activation energy, 238, 245 additive manufacturing, 150, 186 adhesives, 169 adhesive wear, 108 adsorption effect, 59 adsorption mixing, 113 adsorption theory, 59 AECC Beijing Institute of Aeronautical Materials, 23, 66, 114, 136–137 aeroengine, 14, 18, 24, 80, 109, 156 aerospace, 9, 13, 16, 18, 53, 80, 129, 134, 155, 163, 169, 186–187, 199, 208–209, 236, 245, 251 agglomerates, 259 agglomeration, 17, 25–26, 54, 58, 66, 83, 87, 97, 164, 187, 201, 236, 246

aggregates, 193 aggregation, 230, 270 aging resistance, 228, 241 agriculture devices, 262 air cooling, 202 aircraft, 18, 135, 236 allotropic transformation, 18 alloy, 10, 21 alloying ability, 18 alloying element 18, 26, 53 alloying methods, 81 aluminum alloy, 14 amidation reaction, 173, 178 amino-functionalized graphene, 173 amorphous alloys, 57 anisotropic, 181 anisotropy, 281 anodic oxidation, 275 antibacterial, 191, 196, 262 anti-buckling, 209 anti-corrosive, 273 anti-icing, 283 antimicrobial activity, 287 anti-oxidation, 109 aramid fiber-reinforced resin matrix composites, 151

311

312

Graphene Composite Materials

architecture, 187 argon atomization, 19, 21 armored tiles, 129 aspect ratio, 24, 162, 240, 254, 259 ASTM grain size, 37, 40–41 astroloy, 19 atomic diffusion, 61, 99 atomic force microscopy (AFM), 235 attenuation coefficient, 256 automatic, 154 automobile, 9, 16, 134, 154–155, 199, 208, 210, 246 automotive, 53 average section shrinkage rate, 32 aviation, 13, 53, 134, 152, 245, 251 B B777, 20 baffles, 22 ball milling, 56, 75–76, 170 ball milling duration, 114 ball-to-material ratio, 57, 69, 86, 114 basal plane, 24 batch mixer, 231 bilayer graphene, 301 bilayer MoS2, 301 biocompatibility, 191, 196 biocompatible, 197, 273 biodegradability, 262 biodegradable, 197 biomedicine, 197 bioscaffolds, 191 Boeing, 80 bonding interface, 50 bonding states, 79 bonding strength, 130 brake pads, 129 braking performance, 260 breaking strength, 3 “bridging” effect, 132 brittle, 169 brittleness, 170, 185

broadcasting satellites, 257 building chamber, 203 bulk material, 121 bulk polymerization, 162 bulletproof armor, 136 Burger vector, 76 C 3D conductive structure, 183 carbon black, 227–228, 236, 251 carbon fiber, 23, 53, 81, 169, 198 carbon fiber-reinforced resin matrix composite, 151 carbonization, 184 carbon nanofibers, 158 carbon nanotube, 23, 53, 73, 77, 79, 81, 106, 117, 158, 199, 228 carboxyl, 5 carrier mobility, 195 casting, 17 catalytic hydrogenation, 250 cathodic reduction, 275 cationic active hydrolysis, 118 cellular adhesion, 196 cellulose nanofibers, 183 centimeter-scale, 7 Central Iron and Steel Research Institute, 23 centrifugal casting, 17 centrifugation, 193 ceramic matrix composites, 9 chain mobility, 243 chemical, 246 chemical/electrochemical synthesis, 113 chemical equation, 55 chemical industry, 227 chemical intercalation exfoliation techniques, 16 chemical reducing agents, 161 chemical resistance, 198 chemical sensing materials, 255

Index 313

chemical sensors, 3 chemical stability, 245, 270 chemical state, 47 chemical synthesis, 120, 123 chemical unzipping, 117 chemical vapor deposition (CVD), 3, 6, 16, 24, 78, 113, 123, 128–129, 278 CNT carpet, 297 coagulation, 231 coarse-grained structure, 36 coatings, 169 coating technique, 177 coefficients of thermal expansion, 74 cold isostatic pressing, 56 cold pressing sintering, 113 cold welding, 57–58 colloidal suspension, 58, 230 compatibility, 208, 269 compatibilizer, 246 complexity, 203 composite coatings, 9 comprehensive performance, 1 comprehensive properties, 18 compression, 166 compression molding, 56 compressive behavior, 184 compressive strain, 232 compressive strength, 166, 183 compressive strength after impact, 185 compressor disk, 19 computational simulation, 78 computer simulation, 74 conduction band, 2 conductive, 194 conductivity, 111 consolidation, 56 construction, 9, 210 contact angle, 177 contact resistance, 137 contact wire, 111

continuous conductive paths, 257 continuous metallic matrix, 15 continuous phase, 8 cooling rate, 203 copolymer gel, 195 copper foil, 6 copper sponge, 112 co-precipitation-calcination-reduction process, 120–121 core materials, 211 corrosion current density (Icorr), 276 corrosion potential, 275 corrosion resistance, 14, 18, 79–80, 137, 152, 270 co-segregation method, 302 cost-effective, 244 coupling agent, 174, 177 covalent bonding, 173 covalent modifications, 270 crack growth, 19, 22, 53, 237 crack growth resistance, 36, 236 crack propagation, 24 crash performance, 136 creep, 14, 45, 80 creep resistance, 14, 46, 105 creep strength, 19 critical strain for crystallization, 236 cross-linked 3D network structure, 169 cross-linked polymers, 182 cross-linking reaction, 172, 248 cross-section, 192 cross-sectional area, 77 crystal atomic spacing misfit, 133 crystal defects, 59 crystal dislocation, 32 crystalline phase transition, 203 cup-stacked carbon nanotubes, 170 curing, 163 curing efficiency, 243 curing reaction, 171 cutoff frequency, 195

314

Graphene Composite Materials

cutting-edge, 13, 155 cyclic fatigue, 251 cyclic stress–strain, 251 cyclic voltammetry, 298 cytotoxicity, 197 D daily life, 246 daily necessities, 154 damage accumulation, 251 damage tolerance, 14, 24 damage-tolerant, 20, 37 damping, 13 damping property, 242 decomposition temperature at 5% weight loss (T5), 201 decomposition temperature at 10% weight loss (T10), 201 defect-free, 280 defense industries, 9 defense tracking, 257 deformation, 57, 75 deformation process, 53 deformation resistance, 46 degassing reduction, 29 degradation temperature, 243 de-icing, 283 delamination scenario, 201 delocalized π bonds, 2 dendritic effect, 118 densification, 17, 63, 88, 95 densification effect, 88 densification rate, 61 designability, 56, 163 development trend, 18 dielectric constant, 254–255, 257 dielectric loss properties, 262 dielectric percolation, 255 diffraction pattern, 297 diffusion, 17, 55, 259 dimensional, 155, 163, 168 dimensions, 209

dipolar relaxation, 256 dirac cones, 2 dislocation, 53, 74–76, 107 dispersibility, 58, 84, 87, 129, 131, 273 dispersion, 114, 273 disproportionation reaction, 168 driving efficiency, 260 drug carriers, 196 dry ball milling, 114 dry lay-up, 153 dry milling, 57 dry sliding, 260 dry transfer method, 302 dual-property, 21 dual-structure, 20 ductility, 53, 69, 132, 135 durability, 80, 109, 240, 251 dynamic mechanical analysis (DMA), 238, 252 dynamic mechanical properties, 252 E EP741NP, 19 economic efficiency, 263 elasticity, 227, 236 elastic material, 9 elastic modulus, 73, 102–103, 122, 132, 191, 197, 234–235 elastic resistance, 136 electrical, 54, 199, 245, 262 electrical conductivity, 44, 111–112, 123, 137, 160, 170, 181, 183, 199, 231–232, 235, 239, 249, 254, 256, 270, 273, 285 electrical insulation, 239, 245 electrical resistance, 43, 233 electrical switches, 137 electrochemical deposition coating process, 112 electrochemical effect, 113

Index 315

electrochemical impedance spectroscopy (EIS), 277 electrodeposition, 135 electroless copper plating process, 120 electroless plating, 78 electroless plating process, 115 electromagnetic interference shielding effectiveness (EMI SE), 257, 288 electromagnetic pollution, 256 electromagnetic radiation, 256 electromagnetic shielding, 129, 198, 212 electromagnetic wave absorption, 212 electron beam (e-beam) evaporation, 295 electronic circuits, 187, 190 electronic industry, 169 electronic packaging materials, 111, 169 electronics, 9, 53, 134, 155, 199, 245 electron probe microanalysis (EPMA), 25 electrophilic substitution, 273 electrophoresis, 178 electrophoretic deposition (EPD), 276, 286 electrostatic adsorption, 117 electrostatic attraction, 60 electrostatic charge, 194 electrostatic effect, 120 electrostatic interaction, 58–59 elemental analysis, 92 elongation, 16, 32, 53, 70, 75, 78, 101, 119, 122, 127, 131–132, 136, 236, 238, 240, 244–245 emulsion mixing, 254 emulsion polymerization, 162, 233 energy consumption, 138 energy conversion, 196

energy dispersive spectroscopy (EDS), 28 energy dissipation, 237 energy spectrum, 41, 49, 99 energy storage, 195, 295 engine nacelle, 135 enthalpy, 44 environmental protection, 231, 263 environmental resistance, 149 environment-friendly, 231, 244 epitaxial, 3 exfoliation, 246 ex-situ toughening, 179 extrusion ratio, 40, 66–67, 72 F face-centered cubic structure, 76 face-centered cubic unit cell, 18 facesheets, 209 failure strain, 131 fatigue, 14, 18, 44–45 fatigue crack propagation, 236 fatigue life, 251 fatigue resistance, 13, 81, 152, 163, 198, 209 fatigue strains, 236 feather-like, 41 Fermi level, 2 FGH95, 21 FGH97, 21 fiber, 13, 16, 54 field effect transistors, 189, 195 filaments, 197 filament winding, 155, 202 filament wire, 190 film-forming property, 163 fine-grained phase, 57 fine-grained structure, 19, 36 fine-grain strengthening, 53 fire control systems, 18 first Brillouin zone, 2 flake powder metallurgy, 71

316

Graphene Composite Materials

flame retardancy, 80, 105, 109, 228 flat plate method, 163 flexible devices, 187 flexible electronic devices, 3 flexural loading, 209 flexural modulus, 173, 178, 181, 194 flexural strength, 73, 111, 166, 172–173, 178–179, 206 flow stress, 76 fluidity, 155, 163 foam, 209 focused ion beam scanning electron microscope (FIB-SEM), 49 Fourier Transform Infrared Spectroscopy (FTIR), 72 fracture, 16, 57 fracture analysis, 107 fracture energy, 179, 244 fracture morphology, 50, 130 fracture strength, 234 fracture toughness, 170, 181 freeze drying, 185 frequency response analysis, 242 friction coefficient (COF), 104, 260 friction heat, 260 friction interface, 108 friction materials, 111 friction pair, 108 friction stir process, 63 fuel cells, 196 functional groups, 84, 87, 94, 132, 171, 196, 202, 236, 245, 253 fused deposition modeling (FDM), 187 fusion weld resistance, 137 G gas barrier, 259, 262 gas constant, 55 gasification, 96 gas permeability, 239 gas phase epitaxy technology method, 302

gas pressure infiltration, 17 GE9X, 21 GE90, 20 GE Corporation, 19 geometry, 246 Gibbs free energy, 55, 94 glass fiber-reinforced resin matrix composites, 151 glass fibers, 169 glass transition temperature, 183, 190, 243, 254 grafting, 174 grain boundary, 26, 53, 75, 91, 96, 119 grain coarsening, 61–62 grain growth, 61–62, 75 grain refinement, 74–75, 107 grain refinement strengthening, 75, 107, 129, 132 grain size, 32, 56, 75, 90 graphene derivatives, 230 graphene nanoribbons, 284 graphene oxide, 161 graphite oxide, 3 guide vanes, 129 H Haake mixing, 233 Hall–Petch equation, 75 Hall–Petch relation, 61, 107 hand lay-up molding, 153 hardness, 15, 73, 82, 102, 108, 113, 115, 241 harsh conditions, 16 harsh environments, 248 health care, 246 heat buildup, 239, 241 heat dissipation, 138 heat-escaping channels, 282 heat resistance, 228, 238 heat shields, 241 heat-spreading capability, 282

Index 317

heat treatment, 44, 53, 75 heterostructure, 300 high-aspect-ratio, 253 high-end applications, 186 high-end fields, 199 high-energy ball milling, 57, 70 highly oriented pyrolytic graphite (HOPG), 4, 286 high-quality, 6 high specific modulus, 163, 169 high specific strength, 163, 169 high-speed trains, 111, 251 high-tech fields, 163 high temperature, 16 high-temperature creep residual strain, 32 high temperature resistance, 109, 129, 245 high-temperature tensile properties, 102 high vacuum, 16 homogenization, 63 honeycomb, 2, 209 hopping distance, 244 hot compaction, 19 hot deformation, 101 hot-end components, 18 hot extrusion, 19, 21–22, 30, 44, 57, 62, 66, 75, 136 hot forging, 57, 62 hot isostatic pressing, 21, 25, 29–30, 32, 44, 57, 61–62, 87 hot-melt method, 164 hot pressing, 17, 57, 199, 233 hot pressing sintering, 61, 113, 115, 124–125, 130, 136 hot rolling, 57, 62, 75 human bones, 209 humidity, 298 Hummers method, 270, 274

hybrid fillers, 199, 246 hybridization, 289, 292, 300 hydrazine, 5 hydrogen bonding, 183, 194, 236, 243–244, 260 hydrogen–oxygen bonding, 59 hydrolysis, 198, 291 hydrophilic, 5 hydrophobic, 5 hydroxy functional acrylic adhesive, 276 hydroxyl, 5 hygiene products, 262 hysteresis, 251, 298 I π–π interaction, 170, 183, 244 impact resistance, 262 impact strength, 170, 178 impermeability, 239 impurities, 58 impurity, 155 IN100, 19 industrial-scale, 3, 159 inert atmosphere, 249 infiltration, 163 inhibition effect, 277 inhomogeneous, 18 injection molding, 153 inkjet printing, 187 in-plane, 179, 281, 301 in-situ, 78, 113, 123, 128–129, 159, 162, 174, 183, 252 instrument, 246 insulating–conducting transition, 247 intelligent manufacturing technology, 208 interface mechanical interlocking effect, 177 interface reactions, 61, 79 interface strength, 115, 176

318

Graphene Composite Materials

interfacial bonding, 15, 60, 64, 68, 78–79, 85, 87, 97, 99, 112–113, 115, 120, 123, 132, 169–170, 174, 180–181, 201, 208, 239 interfacial diffusion, 51 interfacial interaction, 107, 149, 161, 236, 238, 242–243, 249, 245, 252, 262 interfacial shear strength, 178–179 interfacial strength, 64 interfacial wetting, 16 interjunction distance (IJD), 281 interlaminar bonding strength, 185 interlaminar effect, 169 interlaminar fracture toughness, 178 interlaminar shear, 166, 178 interlaminar toughening, 169 interlayer coupling, 300 interlayer distance (ILD), 281 interlayer force, 4 interlayer force constant, 301 interlayer friction, 302 interlayer spacing, 99 interleaf, 179 intermetallic compound, 91, 105 internal thermal stress, 204 interphase problem, 269 ionic bonds, 235 ionic conduction, 256 ionic liquid, 195, 258 isolation, 269 isostatic pressing, 22 isothermal forging, 21–22, 25, 32, 99 I–V curves, 244, 297 L 2D-layer crystalline structure, 298 labyrinth structures, 259 lamellar structure, 93, 108 laminated structure, 169 landing gear, 135

large-area, 6 large-scale industrial production, 162 large-scale production, 128, 154 large-span transmission cables, 137 laser cladding, 81 laser sintering, 87, 96 lattice distortions, 59 lattice friction, 75–76 lattice potential field, 2 layer-by-layer, 150, 179, 186, 235, 292 light-emitting diodes (LED), 195 lightning strike protection, 198 light (ultraviolet) initiator, 192 light weight, 54, 81, 111, 129, 135, 137, 149, 158, 209, 254, 262 linear energy–momentum relation, 2 linear response, 298 liquid barrier properties, 248 liquid dispersion method, 130 liquid phase exfoliating method, 302 liquid-state metal, 17 lithium deintercalation, 196 lithium-ion batteries, 195 load-bearing, 18 load-bearing capacity, 209 load-bearing structure, 159 loading path dissipated energy (LDE), 251 longitudinal compressive modulus, 167 longitudinal flexural modulus, 167 long-range electron transport, 181 long-term aging, 51 loss tangent value, 257 low-cycle fatigue, 47 low-temperature ball milling, 28 low-temperature resistance, 241 low-temperature solvents, 57 low water absorption, 239 lubricant, 284, 300 lyophilization, 183

Index 319

M M88, 20 machinability, 53, 129, 135 machinery, 246 macrosegregation, 18, 115 magnetic stirring, 262 manufacturability, 179 mass fraction, 51, 94 massless Dirac fermion, 3 mass production, 5, 138 mass ratio, 246 mathematical model, 78 mathematical modeling, 74 mechanical ball milling, 114 mechanical blending, 251 mechanical bonding, 115 mechanical equipment, 9 mechanical exfoliation, 3, 16, 58, 138 mechanical force, 114 mechanical grinding, 164 mechanical meshing effect, 166 mechanical mixing, 113, 239 mechanical properties, 24 mechanical stirring, 114, 161 medical, 187, 199, 246 medical elastomers, 236 melt blending, 161, 190, 199 melting point, 54, 199 melt mixing, 159 MERL76, 19 metal-forming process, 60 metal matrix composites, 9, 13 metastable crystalline phase, 57 micro-arc oxidation, 81 microbial corrosion, 278 microbially induced corrosion (MIC), 277 microdevice, 190 microfibers, 184 microhardness, 122, 124 micromechanical exfoliation, 16

microscopic bonding mechanism, 78 microscopic morphology, 8 microstructure, 17, 79–80, 87, 92, 108–109 microwave absorption, 198, 256 microwave plasma, 78 military, 13, 129, 236 milling, 57 mixing law, 79 mixing method, 30 mobile charge carriers, 257 modifying agents, 273 modulus, 53, 160, 236, 244–245 molecular-level dispersion, 239 monolayers, 300 Moore’s Law, 195 morphology, 47 Mott equation, 255 multifunctional, 262 multi-layer graphene (FLG), 2, 47, 259 multi-phase composition, 8 multi-scale reinforcement, 179 multi-walled carbon nanotubes, 180 N 0D nanomaterial, 292 2D nanosheet, 300 N18, 20 nanocrystalline materials, 16 nanodevices, 190 nanoelectronic, 295 nano-enhancing phase, 97 nanofiller, 13, 180 nano-quantum effects, 54 nano-reinforcement, 74, 139 nano-reinforcing phase, 54 nanotechnology, 16 national defense, 23, 236 national economy, 9, 151, 227–228, 235 near-infrared (NIR), 196

320

Graphene Composite Materials

negative temperature coefficient (NTC), 255 neural stem cells, 197 new energy vehicle, 136 new-generation materials, 263 nickel-based alloys, 23 Nimonic®90, 19 Nimonic®100, 19 non-covalent modification, 273 non-covalent surface treatment, 242 nonlinear behavior, 244 non-moisture absorption, 13 non-outgassing, 13 nozzle, 190 nucleation, 75, 120 nucleophilic substitution, 273 Nyquist plot, 277 O π orbital, 254 ohmic contact, 295 oil resistance, 228, 242, 249 one-atom-thick, 2 one-pot, 292 one-step fabrication, 250 operating temperature, 14, 22, 80, 109, 249 optical, 54 optical fabrication, 192 optical microscope, 164 optoelectronic devices, 3 orientational conductivity, 181 orientational tensile strength, 181 out-of-plane, 179, 281, 301 oxidation resistance, 18, 80, 109 oxygen-containing functional groups, 5 oxygen transmission rate (OTR), 240 P 3D porous structure, 184 packaging, 262 particles, 15, 54

particle-to-particle interfaces, 96 Payne effect, 238 penetration, 257 percolation, 232–233, 239, 244, 246–247, 254, 256 perfluorododecylated graphene nanoribbon (FDO-GNR), 282 permeability, 248 peroxidation, 5 phase transformation theory, 92 phase transition point, 92 photocatalytic, 293 photoinitiator, 192 photosensitive resin, 192 photosensitizer, 192 photo-solidification, 192 photothermal conversion, 196 photothermal therapy, 196 physical mixture, 254 physical transfer method, 302 physiological inertness, 245 piezoresistive, 262 piezoresistive effects, 232 planetary ball milling, 25, 57, 69 plasma discharge sintering, 57 plasma rotary electrode process, 21 plasticity, 14–16, 44, 54, 68, 75, 101, 103, 127, 129, 141, 194 polarity, 161, 170 polar–polar interactions, 243 polycondensation reaction, 291 porous copper sponge, 112 potentiodynamic polarization curve, 275 powder disk, 22 powder hot rolling, 113 powder metallurgy, 64, 81–82, 112, 116, 136 powder metallury, 17, 19 powder mixing, 17 power industries, 18

Index 321

power transmission, 137 Pratt & Whitney, 19 prealloyed powder, 19 precipitated mixture, 230 precipitation hardenable, 22 precursor, 184, 291 pressure-assisted sintering, 61 pressureless sintering, 17, 57, 60 pressure molding, 156 primary electron energy, 286 primary load-bearing structures, 152 primary particle boundaries, 19 primary γ′ phase, 37 printing speed, 203 prior particle boundary, 40 processability, 54 process-interface-performance, 97 process parameters, 79 protective atmosphere, 16 protective efficiency, 276 purification, 230 PW4084, 20 Q quantum dots (QDs), 292 quantum Hall effect, 3 R radar, 257 radiation resistant, 198 Ragone chart, 298 Raman, 58–59, 93, 121, 123, 297, 300 rapid solidification/powder metallurgy (RS/PM), 56 reaction, 16 reaction activation energy, 55 recrystallization, 75 redox method, 5 reducing, 254 refinement, 63 reflection loss, 256 reinforcing effect, 101, 116, 231, 259

reinforcing mechanism, 74, 107 reinforcing phase, 72, 79, 136, 149 reinforcing principle, 159 relative humidity (RH), 298 relaxation phenomenon, 252 reliability, 251 René88DT, 20 René95, 19 René104, 21 René130, 21 repulsive force, 58 residence time, 55 residual strain, 46 resin matrix composites, 9 resin transfer molding, 153 resistance, 44 restacking, 236 reversibility, 298 rigidity, 136 ring-opening reaction, 174 roll-to-roll, 287 rotational speed, 114 RR1000, 21 rubber composites, 9 S safety, 251 sandwich composite, 150, 209 scanning electron microscopy (SEM), 235 scientific and engineering issues, 17 sealing disks, 22 secondary electron emission, 285 secondary electron yield (SEY), 285 secondary load-bearing structures, 152 secondary γ′ phase, 37 section shrinkage, 75 segregated network, 233 selective laser sintering (SLS), 187 self-assembly, 183, 233, 235, 236, 262, 292

322

Graphene Composite Materials

self-lubricating, 198 self-supporting, 197 semi-coherent interfacial bond, 133 semi-metallic, 3 sensor, 298 shear force, 233 shearing effect, 83 shear mixing, 161 shear strength, 77, 246 shielding, 129 shipbuilding, 155, 169 ships, 9, 154 shock absorption, 239 shock response, 242 short-fiber model, 77 shrinkage, 194 silicon carbide, 6 single-crystal silicon, 3 single-layer graphene, 2, 138 single-walled carbon nanotubes, 180 sintering, 17, 56, 79 sizing agent, 178, 198 sizing treatment, 177 small-angle X-ray scattering, 236 softening temperature, 111 solar cells, 196 sol–gel polymerization, 182 solid polymer electrolyte, 196 solid solution strengthening, 129 solid solution temperature, 20 solid-state metal, 17 solution-aging treatment, 25 solution coating, 230 solution mixing, 159, 172, 190, 239–240, 246, 290 solution polymerization, 162 solvent-free, 162, 231 solvent resistance, 242 solvent uptake, 258 sound insulation, 209 sp2-hybridized, 2, 286 spacecraft, 135

spark plasma sintering, 17, 61, 87, 90, 111, 113, 118, 120, 122 specific modulus, 105, 109, 149 specific stiffness, 129 specific strength, 80, 109, 110 129, 149 specific surface area, 3, 14, 24, 53, 71, 73, 81, 107, 112, 129, 141, 176, 178, 180, 195, 250, 269, 295 sporting goods, 9, 129 squeeze casting, 17 stability, 298 stabilizer, 270 stabilizing agent, 254 static dissipation, 194 static dissipative materials, 198 steady-state volume resistivity, 247 stealth, 198 stereolithography (SLA), 187 steric hindrance, 58 sterilization, 287 sticky to the touch, 163 stiffness, 15, 198 stiffness-to-weight ratio, 152 stirring casting, 17 storage modulus, 238, 252 strain amplification effect, 231 strain-induced crystallization, 236 strain rate, 253 strength, 16, 53, 127, 132, 149, 227, 236 strengthening effect, 75, 107 strengthening efficiency, 78 strengthening mechanism, 74 strengthening phases, 57 strength-to-weight ratio, 152 stress–strain curve, 127 stress transfer, 51, 74, 174 stress transfer strengthening, 77, 132 structural defects, 58 structural design, 53 structural-functional-integrated, 1, 54, 109, 150, 158, 160, 185, 198

Index 323

structural integrity, 251 structural material, 3, 152 sunlight aging resistance, 228 superalloy, 14, 18 supercapacitors, 195 superhydrophobic, 283 superlubricity, 302 superplastic, 63 superplastic forging, 19 supersaturated solid solution, 57 surface chemistry, 285 surface energy, 176, 232 surface ionization, 91 surface modification, 58–59, 81, 169, 177, 269 surface morphologies, 288 surface roughness, 177 surface topology, 285 surfactants, 58 surficial crystallization, 199 surgically friendly, 197 suspension adsorption, 79 swelling index, 242 synergistic effect, 1, 8, 180, 246 synthetic elastomer, 239 T Taylor factor, 76 tearing ridge, 50 tear strength, 237, 239, 251 temperature-bearing capacity, 18 temperature gradient, 203 temperature resistance, 80 template method, 112 tensile, 44, 127 tensile fracture, 69, 119, 166 tensile modulus, 173, 180, 194, 243, 249 tensile plasticity, 35 tensile strength, 16, 22, 32, 44, 54, 64, 68, 72–73, 100–101, 111, 115, 121–122, 127, 135–136, 166, 170,

175, 180, 193–194, 197, 205, 207, 235–240, 242–243, 245–246, 248, 250–251 tensility, 166 tertiary γ′ phase, 37 TGA curves, 201 theoretical model, 75 theoretical research, 79 thermal, 54, 262 thermal barrier, 80 thermal conductivity, 3, 15, 43, 81, 105–106, 110, 112, 135, 160, 191, 194, 199, 239, 246, 249, 253, 281 thermal decomposition, 201 thermal deformation, 17, 99, 103 thermal diffusivity, 43–44, 253 thermal dissipation, 281 thermal distortion, 194 thermal expansion, 13 thermal expansion coefficient, 141 thermal management, 282 thermal mismatch strengthening, 74 thermal processing, 24, 99, 107 thermal residual stress, 74 thermal shield, 210 thermal stability, 73, 180, 194, 198–199, 239, 249, 253 thermal strength, 80 thermal stress distribution, 203 thermal warpage, 194 thermogravimetry, 249 thermoplastic resin matrix composites, 151 thermosetting resin matrix composites, 151 three-dimensional (3D) printing, 150, 186 thrust-to-weight ratio, 14, 18, 80, 109 TiAl intermetallic compounds, 105 tires, 251 tissue engineering, 191

324

Graphene Composite Materials

titanium alloys, 14 titanium-aluminum intermetallic compounds, 14 toughening, 54, 174 toughness, 15, 18, 160, 170, 198, 207, 234 transformative technologies, 15 transition metal catalysis, 302 transmission electron microscope (TEM), 26 transparency, 3 transparent conductive films, 3 transparent electrodes, 3 transportation, 16 transverse flexural strength, 169 transverse tensile strength, 169 tribological behavior, 108 turbine disk, 14, 18–19, 22, 24 turbine inlet, 14, 18 turboshaft engine, 22 twin-roll mill, 231 twin-screw extruder, 162, 199, 207 two-dimensional (2D), 2 two-roll mill, 240 two-roll mixing mill, 232 U U720, 36 Udimet720, 19 ultrafine-grained, 75 ultra-sensitive, 298 ultrasonic mixing, 161 ultrasonic treatment, 58 uniaxial pressure, 233 uniaxial thermal compression, 46 V vacuum-assisted, 181, 183 valence band, 2 van der Waals force, 84, 57–58, 159, 170

van der Waals interactions, 2, 270, 281 vapor deposition, 138, 177 vehicles, 135 vehicular detection, 257 versatile materials, 149 viscosity, 162–163, 190, 232, 234 viscosity-temperature characteristic, 163 visible light irradiation, 293 volatilization, 230 volume expansion, 196 volume fraction, 88, 106, 247 vulcanization, 233 vulcanization kinetics, 237 vulcanizing agent, 231 W 10% weight loss temperature (T10), 246 water-based epoxy, 181 water contact angle, 283 water electrolysis, 288 water-lubricated, 260 weaponry, 80 wear-cover-lubrication, 108 wear mechanisms, 108 wear rate, 104, 125, 260 wear resistance, 13, 15, 81–82, 110–111, 122, 129, 160, 196, 208, 236, 238–239, 260, 270 weather satellite, 257 weight loss rate, 243 weight reduction, 80 wet, 29 wet lay-up, 153 wet mechanical stirring, 84 wet milling, 57, 114 wettability, 24, 78, 97, 132, 155, 177, 270 wetting effect, 166 wet transfer, 300

Index 325

whiskers, 13, 15, 54 wide-angle X-ray diffraction (WAXD), 231 work function, 286 wrinkled morphology, 53 wrinkled structure, 68, 166 wrinkles, 201

Y yield strength, 22, 44, 54, 68, 70, 73, 75–77, 82, 100–101, 122, 127, 130, 132, 135–136 Young’s modulus, 3, 121, 158, 170, 180–181, 194, 197, 239–240, 250–251

X X-ray diffraction (XRD), 66

Z zero-bandgap, 3