Chemistry of Engineering Materials [First Edition] 9789354663857

Table of contents :
Cover
Half Title Page
Title Page
Copyright
Foreword
Preface
Acknowledgements
Contents
1. Functional Materials
1.1 Introduction to Ceramic Materials
1.2 Processing of Ceramics
1.2.1 Powder Processing
1.2.2 Shaping
1.3 Classification of Ceramic Materials
1.3.1 Classification Based on Composition
1.3.2 Classification Based on Applications
1.3.3 Classification Based on Functionality
1.4 Properties of Ceramic Materials
1.4.1 Mechanical Properties
1.4.2 Electrical Properties
1.4.3 Magnetic Properties
1.4.4 Chemical Properties
1.4.5 Thermal Properties
1.4.6 Optical Properties
1.4.7 Nuclear Properties
1.4.8 Biological Properties
1.4.9 Piezoelectric Property
1.5 Advanced Ceramic Materials
1.5.1 Alumina (Al2O3)
1.5.2 Carborundum or Silicon Carbide (SiC)
1.6 Superhard Ceramics
1.6.1. Tungsten Carbide
1.6.2 Boron Nitride
1.6.3 Transparent Ceramics
1.7 Composites
1.7.1 Matrix
1.7.2 Reinforcement
1.7.3 Classification
1.7.4 Properties of Composites
1.7.5 Applications
1.8 Advanced Composites
1.8.1 Polymer-nanoclay Composites
1.8.2 Synthesis of Polymer-nanoclay Composite
1.8.3 Advantages
1.8.4 Applications
1.8.5 Polymer-carbon Nanotube Composites
1.9 Alloys
1.9.1 Phase Mixture of Solid Solutions
1.9.2 Factors Affecting Alloying or Solubility of Metal with Each Other
1.9.3 Ferro Alloys
1.9.4 Copper Alloys
1.9.5 Aluminium Alloys
1.10 Glass
1.10.1 Introduction
1.10.2 Chemical Constitution of Ordinary Glass
1.10.3 Classification Based on Application
1.10.4 Manufacture of Glass Using Tank Furnace
1.10.5 Manufacture of Glass Using Pot Furnace
1.11 Lubricants
1.11.1 Some Properties of a Good Lubricant
1.11.2 Some Important Functions of a Good Lubricants
1.11.3 Classification
1.11.4 Solid Lubricants
1.11.5 Semi-solid Lubricants
1.11.6 Liquid Lubricants
1.11.7 Mechanism of Lubrication
1.11.8 Applications of Lubricants
1.12 Refractories
1.12.1 Properties
1.12.2 Measuring Pyrometric Cone Equivalent (PCE) Value
1.12.3 Classification of Refractories
1.12.4 Manufacture of Refractories
Conceptual Questions
Descriptive Questions
Multiple Choice Questions
2. Commodity and Engineering Polymers
2.1 Introduction to Liquid Crystals
2.2 Structural Requirement for a Liquid Crystal
2.3 Examples of Polymeric Fragments Exhibiting Liquid Crystal Behaviour
2.4 Liquid Crystal Polymers
2.5 Structure–Activity Relationship
2.5.1 Rigid Main Chain
2.5.2 Semiflexible Main Chain
2.5.3 Role of Substituents
2.6 Phase Behavior
2.7 Classification of Liquid Crystals Based on Their Method of Synthesis
2.8 Applications of Liquid Crystals
2.9 Conducting Polymers
2.9.1 Mechanism of Conduction
2.9.2 The Role of Dopants
2.9.3 Applications of Polyaniline
2.10 High Performance Fibres
2.10.1 Fibres
2.11 Photonic Polymers
2.11.1 Polymethyl Methacrylate (PMMA)
2.12 Elastomers
2.12.1 Structural Activity
2.12.2 Natural Rubber or Raw Rubber
2.12.3. Synthetic Rubber
2.13 Inorganic Polymers
2.13.1 Polyphosphazines
2.14 Bioderadable Polymers
Conceptual Questions
Descriptive Questions
Multiple Choice Questions
3. Electronic Materials
3.1 Introduction
3.1.1 Energy Levels and Bands
3.2 Conduction in Semiconductors
3.2.1 Doping
3.2.2 Intrinsic Semiconductors
3.2.3 Extrinsic Semiconductor
3.3 Fermi Level in Intrinsic Semiconductor
3.3.1 Fermi Level in Extrinsic Semiconductor
3.4 E-K Diagrams
3.5 Band Gap Engineering
3.6 Applications of Semiconductors in Optoelectronic Devices
3.6.1 Light Emitting Diode (LED)
3.6.2 Solar Cell
3.6.3 Phototransistors
3.6.4 Photoconductors
3.6.5 Photodiodes
3.7 Safe Disposal of Electronic Materials
3.8 Bioimplants
3.8.1 Metal Bioimplants
3.8.2 Ceramic Bioimplants
3.8.3 Polymeric Bioimplants
3.8.4 Tissue Engineering
3.8.5. Biocompatibility
3.9 Biosensors
3.10 Classification of Biosensors
3.10.1 Optical Biosensors
3.10.2 Resonant Biosensor
3.10.3 Electrochemical Biosensors
3.10.4. Mass-based Biosensors
3.10.5 Enzymatic Biosensors
3.10.6 Thermal-detection Biosensors
3.10.7 Ion-sensitive Biosensors
3.11 Glucose Biosensors
3.11.1 Enzymatic Glucose Biosensors
3.12 Cholesterol Biosensors
3.13 DNA Biosensors
3.13.1 Electrochemical DNA Biosensors
Conceptual Questions
Descriptive Questions
Multiple Choice Questions
4. Characterization of Materials
4.1 Introduction
4.2 Electron Microscope
4.3 Transmission Electron Microscope (TEM)
4.3.1 Principle
4.3.2 Construction
4.3.3 Advantages
4.3.4 Disadvantages
4.3.5 Applications
4.4 Scanning Electron Microscope (SEM)
4.4.1 Principle
4.4.2 Construction
4.4.3 Advantages
4.4.4 Disadvantages
4.4.5 Applications
4.5 Scanning Probe Microscopy
4.6 Scanning Tunneling Microscope (STM)
4.6.1 Principle
4.6.2 Construction
4.6.3 Advantages
4.6.4 Disadvantages
4.6.5 Applications
4.7 Atomic Force Microscopy (AFM)
4.7.1 Principle
4.7.2 Construction
4.7.3 Applications
4.8 X-ray Methods
4.8.1 X-ray Photoemission Spectroscopy (XPS)
4.8.2 Analysis of the Spectrum
4.9 X-ray Powder Diffraction (XRD)
4.9.1 Principle
4.9.2 Lattice Planes
4.9.3 Analysis of Diffractogram
4.9.4 Calculation of Unit Cell Parameters from Diffraction Peak Positions
4.9.5 Debye-Scherrer Equation to Calculate the Particle Size
4.10 Thermal Analysis
4.11 Thermogravimetric Analysis (TGA)
4.11.1 Instrumentation
4.11.2 Factors Affecting TGA
4.11.3 Application of TGA
4.12 Differential Thermal Analysis (DTA)
4.12.1 Principle
4.12.2 Procedure
4.12.3 Factors Affecting DTA
Conceptual Questions
Descriptive Questions
Multiple Choice Questions
5. Nanotechnology
5.1 Introduction
5.2 Synthesis
5.2.1 Top-down
5.2.2 Bottom-up
5.3 Biosynthesis of Nanoparticles
5.4 Quantum Dots
5.4.1 Properties
5.4.2 Synthesis of Quantum Dots
5.4.3 Toxicity of Quantum Dots
5.4.4 Eco-friendly Quantum Dots
5.4.5 Nanoscale Materials
5.5 Applications of Nanomaterials in Drug Delivery Mechanism
5.6 Applications of Nanomaterials in Photovolaics
5.7 Applications of Nanomaterials in Sensors
Conceptual Questions
Descriptive Questions
Multiple Choice Questions
Index
Back Cover

Citation preview

Chemistry of Engineering Materials

Chemistry of Engineering Materials Malini S MSc MPhil PhD Assistant Professor Department of Chemistry BMS College of Engineering Bengaluru, India

KS Anantha Raju MSc MPhil PhD Professor and Head Department of Chemistry Dayananda Sagar College of Engineering Bengaluru, India

CBS Publishers & Distributors Pvt Ltd New Delhi • Bengaluru • Chennai • Kochi • Kolkata • Lucknow • Mumbai Hyderabad • Jharkhand • Nagpur • Patna • Pune • Uttarakhand

Disclaimer Science and technology are constantly changing fields. New research and experience broaden the scope of information and knowledge. The authors have tried their best in giving information available to them while preparing the material for this book. Although, all efforts have been made to ensure optimum accuracy of the material, yet it is quite possible some errors might have been left uncorrected. The publisher, the printer and the authors will not be held responsible for any inadvertent errors, omissions or inaccuracies. eISBN: xxxx Copyright © Authors and Publisher First eBook Edition: 2022 All rights reserved. No part of this eBook may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system without permission, in writing, from the authors and the publisher. Published by Satish Kumar Jain and produced by Varun Jain for CBS Publishers & Distributors Pvt. Ltd. Corporate Office: 204 FIE, Industrial Area, Patparganj, New Delhi-110092 Ph: +91-11-49344934; Fax: +91-11-49344935; Website: www.cbspd.com; www.eduport-global.com; E-mail: [email protected]; [email protected] Head Office: CBS PLAZA, 4819/XI Prahlad Street, 24 Ansari Road, Daryaganj, New Delhi-110002, India. Ph: +91-11-23289259, 23266861, 23266867; Fax: 011-23243014; Website: www.cbspd.com; E-mail: [email protected]; [email protected].

Branches Bengaluru: Seema House 2975, 17th Cross, K.R. Road, Banasankari 2nd Stage, Bengaluru - 560070, Kamataka Ph: +91-80-26771678/79; Fax: +91-80-26771680; E-mail: [email protected] Chennai: No.7, Subbaraya Street Shenoy Nagar Chennai - 600030, Tamil Nadu Ph: +91-44-26680620, 26681266; E-mail: [email protected] Kochi: 36/14 Kalluvilakam, Lissie Hospital Road, Kochi - 682018, Kerala Ph: +91-484-4059061-65; Fax: +91-484-4059065; E-mail: [email protected] Mumbai: 83-C, 1st floor, Dr. E. Moses Road, Worli, Mumbai - 400018, Maharashtra Ph: +91-22-24902340 - 41; Fax: +91-22-24902342; E-mail: [email protected] Kolkata: No. 6/B, Ground Floor, Rameswar Shaw Road, Kolkata - 700014 Ph: +91-33-22891126 - 28; E-mail: [email protected]

Representatives Hyderabad Pune Nagpur Manipal Vijayawada Patna

Foreword

T

he words ‘materials science’ encompass a wide variety of subjects and perhaps connect all branches of science and engineering. It is therefore possible to look at it from different angles and interests. Ever since this branch has been introduced, a number of books and monographs have appeared under different titles such as Materials Science, Materials Engineering, Electronic Materials, Optical Materials, Magnetic Materials and so on. The emphasis in each one of them would be to serve different groups of researchers and knowledge seekers. In view of its all-encompassing nature, many colleges and universities made it compulsory for students to credit this subject as a part of engineering curriculum. The exact coverage would depend on the main subject being pursued. In such a scenario, it would be difficult for any student to find a single book that will cater to all aspects of engineering materials. The authors, in a way, seem to address this issue by putting in their years of teaching experience to evolve a book aimed at the engineering student and the result is Chemistry of Engineering Materials. It is obvious that in such an attempt, one cannot be comprehensive in including all that conforms to the description of materials engineering. That perhaps is the reason why word ‘Chemistry of’ is included in the title and appears appropriate. The book covers the subject in 5 chapters: Functional materials, commodity and engineering polymers, electronic materials, characterization of materials and nanotechnology. The book is obviously intended for students of engineering who undergo a basic course in materials engineering. The authors have carefully chosen to limit the descriptions to introductory level in all these chapters. Initially I was wondering as to why only ceramic, electronic, polymer and nanomaterials. Then I realised that there are so many books that cover metallic materials and composites. A simple treatment of these four classes of materials has therefore been summarised for the benefit of the engineering student community. Having been in the field of engineering materials for over 5 decades, my mind automatically thought of asking the authors to revise several portions of the book to include more details. However, I realized that the present day student require to earn only 3 credits in the subject and hence the contents of the book seem to be more than adequate. The intention of the authors is not to make the reader an expert in materials engineering, rather it is to provide an insight into the fundamental principles, material synthesis and characterization and some industrial applications. I surmise that the book will be a welcome addition to textbooks on the subject and would be used by both the students and the teachers in materials engineering. I wish the book all success and congratulate the authors for this fine effort.

E S Dwarakadasa Former Professor Department of Metallurgy Indian Institute of Science, Bengaluru Chairman Karnataka Hybrid Micro Devices Ltd and Cosmic Industrial Laboratories, Bengaluru

Preface

M

any universities have courses related to materials science, as it is a component of biochemistry, nanotechnology, chemical engineering, biotechnology, geology, and metallurgy. The concepts used in the study of engineering materials have their origin in the principles of chemistry and without the understanding of it, the student is likely to miss much of the subtlety of the subject. In this book, we have attempted to provide students with an initial source of fundamental ideas to learn chemistry of materials, relate to its macro- and micro- molecular chemical properties in a simple language, lucidly and clearly rather than to attempt a comprehensive treatment. Indeed, when dealing with such a complex area of study, much has to be omitted from a text if it is to be kept to a sensible size. However, an honest attempt is made to cover varied materials and to remain contemporary. Wherever required, the physical significance of mathematical formulations is explained in a simplified form and a very little mathematical background is expected from the reader. The subject matter is arranged in the traditional way with questions at the end of every section to assist self-study and the text attracts the reader's attention by stating some amazing facts at the corner of every concept. We earnestly hope that this book will serve the best of interests of the students and as a reference to the teachers. Suggestions and constructive criticisms for the improvement of this book are welcome.

Malini S KS Anantha Raju

Acknowledgements

W

e have consulted several similar works in the preparation of this book and are grateful to all the authors for their endeavour. We are thankful to the staff of BMS College of Engineering and Dayananda Sagar College of Engineering, for their support and encouragement. Thanks are due to colleagues, friends and many individuals in both the institutions who have contributed in countless ways. We with gratitude, thank Dr E S Dwarakadasa, Former Professor, Department of Metallurgy, Indian Institute of Science; Chairman, Karnataka Hybrid Micro Devices Ltd and Cosmic Industrial Laboratories, Bengaluru, who has presented the foreword, and helped us to refine the text with his advice and suggestions. We wish to express our thanks to CBS Publishers and Distributors, New Delhi, for the publication of this book. Malini S KS Anantha Raju

Contents Foreword Preface

v vii

1. Functional Materials 1.1 1.2

1.3

1.4

1.5

1.6

1.7

1.8

Introduction to Ceramic Materials 1 Processing of Ceramics 2 1.2.1 Powder Processing 2 1.2.2 Shaping 2 Classification of Ceramic Materials 2 1.3.1 Classification Based on Composition 3 1.3.2 Classification Based on Applications 3 1.3.3 Classification Based on Functionality 4 Properties of Ceramic Materials 6 1.4.1 Mechanical Properties 6 1.4.2 Electrical Properties 6 1.4.3 Magnetic Properties 6 1.4.4 Chemical Properties 6 1.4.5 Thermal Properties 6 1.4.6 Optical Properties 6 1.4.7 Nuclear Properties 7 1.4.8 Biological Properties 7 1.4.9 Piezoelectric Property 7 Advanced Ceramic Materials 7 1.5.1 Alumina (Al2O3) 7 1.5.2 Carborundum or Silicon Carbide (SiC) 8 Superhard Ceramics 10 1.6.1. Tungsten Carbide 10 1.6.2 Boron Nitride 11 1.6.3 Transparent Ceramics 12 Composites 13 1.7.1 Matrix 13 1.7.2 Reinforcement 13 1.7.3 Classification 14 1.7.4 Properties of Composites 15 1.7.5 Applications 15 Advanced Composites 15 1.8.1 Polymer-nanoclay Composites 15 1.8.2 Synthesis of Polymer-nanoclay Composite 16

1

xii Chemistry of Engineering Materials

1.8.3 Advantages 16 1.8.4 Applications 16 1.8.5 Polymer-carbon Nanotube Composites 17 1.9 Alloys 18 1.9.1 Phase Mixture of Solid Solutions 18 1.9.2 Factors Affecting Alloying or Solubility of Metal with Each Other 19 1.9.3 Ferro Alloys 19 1.9.4 Copper Alloys 19 1.9.5 Aluminium Alloys 21 1.10 Glass 23 1.10.1 Introduction 23 1.10.2 Chemical Constitution of Ordinary Glass 23 1.10.3 Classification Based on Application 23 1.10.4 Manufacture of Glass Using Tank Furnace 24 1.10.5 Manufacture of Glass Using Pot Furnace 24 1.11 Lubricants 24 1.11.1 Some Properties of a Good Lubricant 25 1.11.2 Some Important Functions of a Good Lubricants 25 1.11.3 Classification 25 1.11.4 Solid Lubricants 26 1.11.5 Semi-solid Lubricants 26 1.11.6 Liquid Lubricants 27 1.11.7 Mechanism of Lubrication 28 1.11.8 Applications of Lubricants 28 1.12 Refractories 29 1.12.1 Properties 29 1.12.2 Measuring Pyrometric Cone Equivalent (PCE) Value 29 1.12.3 Classification of Refractories 29 1.12.4 Manufacture of Refractories 30 Conceptual Questions 30 Descriptive Questions 31 Multiple Choice Questions 32

2. Commodity and Engineering Polymers 2.1 2.2 2.3 2.4 2.5

2.6 2.7

34

Introduction to Liquid Crystals 34 Structural Requirement for a Liquid Crystal 35 Examples of Polymeric Fragments Exhibiting Liquid Crystal Behaviour 35 Liquid Crystal Polymers 35 Structure–Activity Relationship 35 2.5.1 Rigid Main Chain 36 2.5.2 Semiflexible Main Chain 36 2.5.3 Role of Substituents 36 Phase Behavior 36 Classification of Liquid Crystals Based on Their Method of Synthesis 37

Contents xiii

2.8 2.9

2.10 2.11 2.12

2.13 2.14

Applications of Liquid Crystals 40 Conducting Polymers 41 2.9.1 Mechanism of Conduction 41 2.9.2 The Role of Dopants 41 2.9.3 Applications of Polyaniline 43 High Performance Fibres 43 2.10.1 Fibres 44 Photonic Polymers 46 2.11.1 Polymethyl Methacrylate (PMMA) 46 Elastomers 48 2.12.1 Structural Activity 48 2.12.2 Natural Rubber or Raw Rubber 48 2.12.3. Synthetic Rubber 49 Inorganic Polymers 51 2.13.1 Polyphosphazines 51 Bioderadable Polymers 52 Conceptual Questions 55 Descriptive Questions 56 Multiple Choice Questions 57

3. Electronic Materials 3.1 3.2

3.3 3.4 3.5 3.6

3.7 3.8

Introduction 59 3.1.1 Energy Levels and Bands 61 Conduction in Semiconductors 61 3.2.1 Doping 62 3.2.2 Intrinsic Semiconductors 3.2.3 Extrinsic Semiconductor Fermi Level in Intrinsic Semiconductor 63 3.3.1 Fermi Level in Extrinsic Semiconductor 63 E-K Diagrams 64 Band Gap Engineering 65 Applications of Semiconductors in Optoelectronic Devices 65 3.6.1 Light Emitting Diode (LED) 66 3.6.2 Solar Cell 67 3.6.3 Phototransistors 68 3.6.4 Photoconductors 69 3.6.5 Photodiodes 70 Safe Disposal of Electronic Materials 71 Bioimplants 72 3.8.1 Metal Bioimplants 73 3.8.2 Ceramic Bioimplants 77 3.8.3 Polymeric Bioimplants 78 3.8.4 Tissue Engineering 78 3.8.5. Biocompatibility 78

59

62 62

xiv Chemistry of Engineering Materials

3.9 Biosensors 79 3.10 Classification of Biosensors 80 3.10.1 Optical Biosensors 80 3.10.2 Resonant Biosensor 80 3.10.3 Electrochemical Biosensors 80 3.10.4. Mass-based Biosensors 81 3.10.5 Enzymatic Biosensors 81 3.10.6 Thermal-detection Biosensors 81 3.10.7 Ion-sensitive Biosensors 81 3.11 Glucose Biosensors 81 3.11.1 Enzymatic Glucose Biosensors 81 3.12 Cholesterol Biosensors 82 3.13 DNA Biosensors 84 3.13.1 Electrochemical DNA Biosensors 85 Conceptual Questions 86 Descriptive Questions 87 Multiple Choice Questions 87

4. Characterization of Materials 4.1

Introduction 90

4.2 4.3

Electron Microscope 90 Transmission Electron Microscope (TEM) 92 4.3.1 Principle 92 4.3.2 Construction 92 4.3.3 Advantages 93 4.3.4 Disadvantages 93 4.3.5 Applications 93 Scanning Electron Microscope (SEM) 93 4.4.1 Principle 93 4.4.2 Construction 93 4.4.3 Advantages 93 4.4.4 Disadvantages 94 4.4.5 Applications 94 Scanning Probe Microscopy 94 Scanning Tunneling Microscope (STM) 95 4.6.1 Principle 95 4.6.2 Construction 95 4.6.3 Advantages 95 4.6.4 Disadvantages 95 4.6.5 Applications 96 Atomic Force Microscopy (AFM) 96 4.7.1 Principle 96 4.7.2 Construction 96 4.7.3 Applications 97

4.4

4.5 4.6

4.7

90–115

Contents xv

4.8

X-ray Methods 97 4.8.1 X-ray Photoemission Spectroscopy (XPS) 97 4.8.2 Analysis of the Spectrum 99 4.9 X-ray Powder Diffraction (XRD) 100 4.9.1 Principle 101 4.9.2 Lattice Planes 101 4.9.3 Analysis of Diffractogram 104 4.9.4 Calculation of Unit Cell Parameters from Diffraction Peak Positions 107 4.9.5 Debye-Scherrer Equation to Calculate the Particle Size 107 4.10 Thermal Analysis 108 4.11 Thermogravimetric Analysis (TGA) 108 4.11.1 Instrumentation 108 4.11.2 Factors Affecting TGA 109 4.11.3 Application of TGA 110 4.12 Differential Thermal Analysis (DTA) 110 4.12.1 Principle 110 4.12.2 Procedure 110 4.12.3 Factors Affecting DTA 110 Conceptual Questions 112 Descriptive Questions 113 Multiple Choice Questions 113

5. Nanotechnology 5.1 5.2

5.3 5.4

5.5 5.6 5.7

Introduction 116 Synthesis 118 5.2.1 Top-down 118 5.2.2 Bottom-up 121 Biosynthesis of Nanoparticles 127 Quantum Dots 128 5.4.1 Properties 129 5.4.2 Synthesis of Quantum Dots 129 5.4.3 Toxicity of Quantum Dots 131 5.4.4 Eco-friendly Quantum Dots 132 5.4.5 Nanoscale Materials 132 Applications of Nanomaterials in Drug Delivery Mechanism 138 Applications of Nanomaterials in Photovolaics 140 Applications of Nanomaterials in Sensors 140 Conceptual Questions 141 Descriptive Questions 142 Multiple Choice Questions 142

116

1 Functional Materials

1.1 INTRODUCTION The term ‘ceramic’ originates from the Greek word ‘Keramikos’ meaning ‘burnt stuff’ which implies that the properties are obtained through processes at high temperature. Traditional ceramics such as porcelain, bricks and tiles have clay as one of the important constituents while on the contrary, modern ceramics are made of oxides and nonoxides of metals and nonmetals. These materials are considered to be one The word ‘ceramic’ comes from the Greek of the popular industrial materials as they word ‘Keramikos’ meaning ‘pottery,’ or are hard, inert and thermally stable. These ‘potter’s clay’ is also related to an old Sanskrit word meaning ‘to burn’ materials are neither composed of only organic compounds nor metals. Since ceramics are composed of more than one element, the crystal structure is complex with bonds ranging from being completely ionic to fully covalent. However, when the ionic character predominates, crystal structure may be considered as being composed of electrically charged ions with equal number of cations and anions rather than atoms. Also a stable structural framework is achieved when anions surrounding the materials are a part of the The oldest human-made ceramics diversified group of industry products composed of date back to about 24000 BC and silicates, including refractories, cements, abrasive clay were found in Czechoslovakia. products, lime, plaster, glass, high and low tension electrical insulators. Ceramics are versatile in nature which are crystalline, glassy, insulating or semiconducting and used as spark plugs, fiber optics (Al2O3 and ZrO2 are the ceramics popularly used), artificial joints, space shuttle tiles, cooktops, race car brakes, micro positioners, chemical sensors, self-lubricating bearings, body armor, skis, etc. Raw Materials of Ceramics The important raw material required in the manufacturing of ceramic materials are: i. Clay: It is made of aluminum silicate formed during weathering of igneous rocks which contains feldspar. In presence of water, clay gains plasticity and can be easily moulded.  K2CO3 + 4SiO2 + Al2O3 2SiO22H2O K2O Al2O3 6SiO2 + 2H2O + CO2  Potash Feldspar

Kaolinite Clay

1

2 Chemistry of Engineering Materials

On heating, water is eliminated to result a glassy, vitreous hard material. 873K Al 2 O 3 2SiO 2  2H 2 O   Al 2 O 3 2SiO 2 + 2H 2 O 1200K Al 2 O 3 2SiO 2  2H 2 O   3Al 2 O 3 2SiO 2 + 6H 2 O + 4SiO 2

China Clay

Mullite

ii. Silica: Added in the form of quartz or flint, it provides a firm skeletal structure to the ceramic materials and is highly resistant to heat. iii. Feldspar: These are aluminosilicates of sodium, potassium or calcium having a low fusion temperature acting as binding material. The three common type of feldspar are: • Potash feldspar: K2O Al2O3 6SiO2 • Soda feldspar: Na2O Al2O3 6SiO2 • Lime feldspar: Ca2O Al2O3 6SiO2 iv. Fluxing agents: These are alkaline oxides that promote liquification such as borax, soda ash and lead oxide. v. Refractory agents: Imparts thermal stability often in the form of ThO2 or ZrO2 vi. Colouring agents: Oxides of cobalt, chromium, nickel and manganese impart colour. 1.2 PROCESSING OF CERAMICS A great deal of research has gone into developing fabrication methods of ceramic, as it decides the cost, durability and cost of the ceramic end products as well. Powder processing and shaping technique are the two important fabrication steps often employed in the processing of ceramics. 1.2.1 Powder Processing In solid phase, the raw materials in the form of hydroxides, oxalates or carbonates are blended and allowed to solidify. Synthesis in liquid phase is preferred to prepare ceramic fibers and thin films which can be achieved through sol-gel or co-precipitation methods. High purity ceramics are synthesized in the vapor phase where the raw materials are vaporized and homogeneously mixed in the form of fine nonaggregated particles using the technique of chemical vapor or physical vapor deposition. 1.2.2 Shaping Popularly referred to as forming, the ceramic in the powdered form is given a formation under an ambient temperature, and can be subjected to dye pressing, slip casting or extrusion at room temperature called as cold forming or hot isostatic pressing and injection moulding called as hot forming. 1.3 CLASSIFICATION OF CERAMIC MATERIALS As ceramics have achieved broader applications, classification is necessary to gain a systematic view of these materials.

Functional Materials 3

1.3.1 Classification Based on Composition Class 1: Silicate ceramics These constitute Si and O as the prime components and are generally produced by glass ceramic techniques. They have a low thermal expansion and are available in coarse and fine textures with high mechanical strength. Example: Porcelain, steatite.

One type of advanced ceramic, called ‘shuttle ceramic tiles’, has been created to withstand temperatures upto 1280°C (2336 F). Space shuttles are covered in 30000 of these lightweight tiles. They protect the shuttle from heat when it re-enters Earth’s atmosphere from space.

Class 2: Oxide ceramics This category includes oxides of Si, Al, Zr, Ti, Mg and other metals. They possess high melting point with exclusive electrical properties and are manufactured by modeling process. Example: Magnesia, silica. Class 3: Nonoxide ceramics Generally produced by conventional dry-pressing processes, they include carbides, nitrides, borides and silicide possessing high thermal conductivity and abrasive properties. Example: B4C, WC Class 4: Glass ceramics These are materials produced by controlled crystallisation constituting a structure with dimension of 1 μm, existing between amorphous and highly crystalline state possessing high impact resistance and translucency. Example: Li2O Al2O3 SiO2 1.3.2 Classification Based on Applications Class 1: Classic ceramics These are the traditional ceramics which are highly porous, made for everyday use with basic components such as clay, feldspar and silica are made in large quantities by inexpensive manufacturing methods. Calcium carbonate (CaCO3) is used as a polishing agent Example: Abrasives, refractories, clay and cement or an abrasive in tooth paste.

Class 2: Electroceramics Ceramic materials which are used to execute electronic functions such as insulators and conductors are termed as electroceramics. These materials have specifically designed formulation with highly controlled processing. They can be subdivided into two classes as follows: a. Dielectrics or insulator ceramics: Popularly used in capacitors, as these materials resist the flow of electrons. Example: Al2O3, BaTiO3 b. Conducting ceramics: Popularly used in spark igniters, accelerometers and sonar imaging, as these materials contain electrically polarized domains in their structure which respond to any electric field applied. Example: Piezoceramics, magnetoceramics

4 Chemistry of Engineering Materials

Class 3: Modern ceramics These are the recently developed inorganic ceramics with high commercial value include a broad spectrum of materials which can withstand high temperature, high strength and hardness, wear and corrosion resistance, biocompatibility, with unique optical and electronic properties. Example: Nuclear ceramics, bioceramics The blade of a ceramic knife will 1.3.3 Classification Based on Functionality Based on the performance and purpose of use, ceramics can be classified as traditional and advanced ceramics.

stay sharp for much longer than that of a steel knife, although it is more brittle and can be snapped if dropped on a hard surface

Traditional Ceramics Class 1: Clay products Ceramic cement is successful in These are made of natural materials with 45% quartz Portland cement replacement acting as filler and 55% feldspar providing the shape. range upto 40% by mass and thus As they exhibit load bearing ability, they are pressed considered to be ecofriendly or extruded into shape while in a wet plastic state and then dried and fired. Example: Bricks and tiles Class 2: Bricks and tiles A homogeneous mixture of alumina (aluminum oxide), silica (silicon oxide), and dolomite (calcium magnesium carbonate) constitutes bricks or tiles with clay being fired in a kiln to increase hardness. Example: Construction materials Class 3: Abrasives Hard materials used to provide shape or finishing to an object through rubbing, often referred to as polishing or buffing. Example: Diamond, boron nitride Class 4: Refractories These are oxides of Al, Si, Mg and Ca, are heat-resistant materials used in making furnaces, crucibles, kilns, boilers, stills for cracking petroleum, ladles for pouring molten metal into moulds, electrolytic cells and incinerators which operate upto 2150°C. Example: i. Acidic refractories (Al2O3 and SiO2)—resistant towards acids, but attacked by base. ii. Basic refractories (MgO)—resistant towards bases, but attacked by acids. iii. Neutral refractories (Cr2O3)—resistant towards both acid and base. Special refractories are high purity oxides, relatively expensive, with very little porosity which includes beryllia (BeO), zirconia (ZrO2), mullite (3Al2O3.2SiO2) and silicon carbide (SiC). Carbon and graphite can act as refractories but rarely used due to its susceptibility to oxidation at high temperatures of about 800°C. Class 5: Cements Ceramic dust which is a nonrecyclable byproduct in ceramic industry is used as a supplementary binder in Portland cement that hardens the concrete. The lack of Ca2+ ions, decreases the porosity, and thus makes it denser and leads to the formation of

Functional Materials 5

fine aggregate. This cost effective cementitious material popularly used in making hollow bricks, has decreased the energy consumption and CO2 production associated with the production of portland cement. Example: MgO SiO2 Al2O3 dust

Silicate Structure in Traditional Ceramics Silicates are occasionally considered as discrete molecules but exist as arrays where ceramic materials can exist in any of the following forms with a combination of covalent, ionic and metallic bonding. Silicon oxygen tetrahedron [SiO4]4– : Here one silicon atom is bound interstitially to 4 oxygen atoms. Each oxygen atom has only 7 electrons in its outermost shell. Thus to reach the octet structure, it takes the electron from a metal or shares an electron pair with the 2nd silicon and becomes double tetrahedron silicate [Si2O7]6–. Polytetrahedral structure: When three or more tetrahedral units are linked together, a ring structure evolves, where one oxygen atom is a member of two units. The composition is Si3O9 producing (Si3O9)6– ions. Chain structure: Two corners of each tetrahedra when linked, form a single chain structure where one oxygen is common to two adjacent tetrahedra. When two such structure in parallel chain structures are polymerized through a common oxygen atom, results in a double chain structure. Sheet structure: When a double chain structure extends infinitely, sheets results which are found in clay, mica and talc. The sheets impart plasticity in clay and lubricating characteristics in talc. Framework structure: The sheet structures when extended to 3D give rise to framework structure results in hard but low density lightweight ceramics. Example: Roof tiles. Vitreous structure: A vitreous silicate is a 3D framework with covalent bonds, which provides rigidity. Example: Glass. Polymorphism in Ceramics It is the property of a material to exist in more than one type of crystal structure or space lattice. Silica can exist in three crystalline forms and each differs in their physicochemical properties such as thermal and chemical stability. (i) Quartz (ii) Tridymite (iii) Cristobalite Since the change from one form to the other is reversible, they are known as allotropic forms. The change involves breaking of Si-O-Si bonds and the rearrangement of silicon tetrahedral. 870 ºC 

Quartz

Tridymite 2270 kg/m3

2655 kg/m3 573 ºC

 Quartz   Quartz 120ºC

 Tridymite   Prime tridymite 160ºC  Tridymite  Prime tridymite  200 ºC  Cristobalite   Cristobalite

1470 ºC   Cristobalite

2300 kg/m3

6 Chemistry of Engineering Materials

1.4 PROPERTIES OF CERAMIC MATERIALS Due to structural diversity, a strong emphasis is laid on properties of ceramics for its technological functions. The following properties make ceramic materials ideal for engineering applications. 1.4.1 Mechanical Properties i. Extremely hard and has resistance to wear Example: Carborundum 2480 knoop and boron nitride 7000 knoop ii. Very low tensile strength of 1900 kg/cm2 Example: Al2O3 iii. High compressive strength 19500–35000 kg/cm2 Example: Al2O3 iv. Low fracture strength and value of modulus of elasticity for most ceramic range of 7 × 1010 N/m2 to 40 × 1010 N/m2 1.4.2 Electrical Properties i. Most ceramics like procelain, steatite, forsterite and alumina are used as insulators with resistivity of order of 1012 m. ii. A small number of ceramics can conduct electricity and are used as semiconductors. Example: Iron oxide, nickel oxide and cobalt oxide iii. Most ceramics can be made piezoelectric by application of high voltage where the material changes the property of transforming mechanical deformation into voltage changes. Example: Barium titanate used in gramophone pickups. 1.4.3 Magnetic Properties Ceramics consisting of iron oxides have magnetic properties Example: Ferrites like ferroxcube form soft magnetic materials (used in making inductors and transformers) and ferroxdure form hard magnetic materials (used in making household materials). 1.4.4 Chemical Properties i. Ceramics show high resistance organic solvent, acids and caustic solutions ii. Can resist oxidation at high temperatures iii. Ceramics resist molten metals and hence used as furnace lining Example: Magnesia, Al2O3 1.4.5 Thermal Properties Ceramics possess excellent specific heat capacity Example: Silica and zirconia have specific heat of about 1.1 and 0.6 × 103 W/mºC and thermal conductivity of 1.7 and 2.3 W/mºC respectively. 1.4.6 Optical Properties i. Glass which is a major class of ceramics is a polycrystalline material in production of windows, bulbs and optical lenses. ii. Glass with selective absorption and transmission with refraction index varying from 1.46 to 2.0 can be designed.

Functional Materials 7

1.4.7 Nuclear Properties Ceramics offer a wide range of properties to capture and scatter neutrons thus are used as shields, moderators and chambers used to immobilize and store nuclear wastes. 1.4.8 Biological Properties Silica ceramic is inert to the tissues and body fluids making it corrosion resistant and biocompatible. It finds applications in making artificial teeth, bones implant and a major part in surgical glue. 1.4.9 Piezoelectric Property It is the property of a material to transform mechanical deformations into voltage changes and vice versa. Piezoelectric ceramic can be prepared by suitably treating ceramic at a high voltage known as “polarization”. Example: Barium titanate and lead zirconate titanate. 1.5 ADVANCED CERAMIC MATERIALS Ceramics with extreme hardness, physical and chemical stability, good conductivity and biocompatibility are generally termed as advanced ceramic materials making them one of the most important industrial elements. Example: Silicon nitride in turbine blades, silicon carbide in bearings, boron nitride in high temperature equipment, titanium diboride used as wear-resistant parts, tungsten carbide used in cutting tools, hot pressed zirconia used as a refractory material, alumina used as abrasion resistant tiles, aluminum titanate used as port liners in automobiles, glass ceramics in vacuum tube components are a few examples. 1.5.1 Alumina (Al2O3) It is a well-known oxide ceramic material with aluminium and oxygen sharing ionic bond in between them, available in both coarse and fine grain sizes. It exists in most stable hexagonal -phase at high temperatures. It can be used in both oxidizing and reducing atmospheres upto 1925°C. When alumina is combined with SiO2, it becomes a part of silicon matrix by dissolving and yields glaze melts. Alumina controls the flow of the glaze, prevents crystallization, imparts chemical stability, increases melting point, improves tensile strength, lowers expansion, reduces phase separation and improves hardness. Example: Kaolin, pyrophyllite or feldspar are good sources of alumina.

Synthesis Extraction of Al2O3 from Bauxite by Bayer’s process (Fig. 1.1): Bauxite is a mixture of Al2O3, SiO2, Fe2O3 and other materials. Step 1: A fine powder of bauxite is made to react with NaOH. Only Al2O3 dissolves.  NaAl (OH)4 Al2O3 + H2O + NaOH  NaAl(OH)4 on heating decomposes to pure Al(OH)3 and NaOH Step 2: Calcination 1100ºC   Al2O3 + 3H2O 2Al2 (OH)3 

8 Chemistry of Engineering Materials

Step 3: Filtration: SiO2 and Fe2O3 do not dissolve in NaOH and are removed by filtration.

Fig. 1.1: Extraction of Al2O3 from bauxite by Bayer’s process

Properties i. Strongest, hardest of all oxide ceramics ii. Good electrical insulation upto 1 × 1014–1015 cm–1 iii. Good thermal conductivity upto 20–30 W/mK iv. Highly resistant to flammability, acids, alkali and oxidation. v. High melting point of about 2369 K and specific heat of 955 J/kgK vi. Excellent dielectric constant of 11. 1. Applications General applications i. Used in electronics, high voltage insulators and thermometry sensors ii. Used as thermometry sensors and furnace lining iii. Used in water purification iv. Used as abrasives v. Used as a filler material in pharmaceuticals and cosmetics Specific applications i. As it is chemically inert, it is used as filler in cosmetics ii. As it is highly porous, it is used to remove water from vapours iii. It is used as a catalyst support in Ziegler-Natta polymerisation and also used as a catalyst in dehydration of alcohols. iv. Electronic industry material: Used as an insulating barrier in capacitors and spark plugs v. Fibres of Al2O3 such as fibre FP, Nextel 610 and 720 are used in composites vi. Abrasive: Sandpaper with Al2O3 crystals are popularly used as abrasive. Fine powder of Al2O3 is used in scratch-repair kits. 1.5.2 Carborundum or Silicon Carbide (SiC) Composed of carbon and silicon in 1:1 ratio, SiC occurs in nature as a part of rare mineral called moissanite in various form as 2H-SiC(-cubic form), 4H-SiC (hexagonal form) and 6H-SiC (-hexagonal form) (Figs 1.2 and 1.3). The hexagonal crystal structure being the most common forms, has a predominantly covalent character with the molecular structure Si-C. It is a popular semiconductor with high thermal conductivity which is highly stable, chemically inert, and has Mohs hardness rating of 9, close to that of the diamond.

Functional Materials 9

Synthesis • Synthesis by Acheson process: It is a process used in the large scale production of carborundum in an Acheson graphite electric resistance furnace. Silica and quartz is mixed with coke and heated to about 2000°C in furnace, where the following chemical reactions take place.  SiC + 2CO SiO2 + 3C 

SiO2 + CO   SiO + CO2

Fig. 1.2: Cubic form

SiO + 2C   SiC + CO(g) C + CO2   2CO 2C + SiO   SiC + CO Note: Addition of NaCl to furnace prevents corrosion of steel furnace. Addition of sawdust to furnace stops emissions. The product formed is a mixture of different grades of Si which requires further separation. • Synthesis by physical vapour transport: Popularly known as seeded sublimation growth or Lely method, it is a simple Fig. 1.3: Hexagonal form process to obtain large sized SiC single crystal. At low Argon pressure, a source is placed to close proximity of seed of SiC, maintaining a temperature gradient which facilitates the movement of material vapour. However, this method has a disadvantage that an uncontrollable nucleation and dendrite-like growth may be observed. • Synthesis by chemical vapour deposition: This method is most suitable for generating SiC thin films, powders, whiskers and nanorods. The process implements hermolysis, hydrolysis, oxidation, reduction, nitration and carboration, depending on the precursor species used. The gaseous species and the substrate are kept in close proximity which allows a slow diffusion controlled absorption of species.  2SiC(g) + 5H2(g) 2 SiH4(g) + C2H2(g) 

• Synthesis by sol-gel process: This method is most suitable for generating SiC in highly pure form with extremely uniform and dispersed nanostructures. Step 1: Hydrolysis: An alkoxide is mixed with a suitable solvent to produce siloxane (SiOSi). Tungsten carbide begins as a fine powder Step 2: Polycondensation: Formation of gel by during the jewelry-making process, but shrinkage and densification is achieved by after heating, it’s approximately 10 times heating and removing the solvent. The product harder than 18K gold and two times is a network of nanostructures of SiC in the pure denser than steel! form.

Properties i. Extremely hard due to tetrahedral crystal lattice of covalently bonded silicon and oxygen ii. Good thermal conductivity but low thermal expansion and hence can resist thermal shock

10 Chemistry of Engineering Materials

iii. Has semiconductor characteristics iv. Remain undamaged up to 1800°C

Carbide cutting tools remain sharp, not only at ordinary cutting temperatures of 1700 to 2000 F, but even at temperatures of upto 3000 F, which can exist at the interface between the cutting tip and the metal being cut.

Applications i. Used as a structural material in composite armor, brake disc and plastic alloys for automobiles and microelectronics ii. SiC fibers are used in gas temperature iii. Used as a diamond substitute in jewellery iv. Used in heat treatment of metals, melting of glass and production of electronic components v. Used in making pallets that connects power lines and earth. 1.6 SUPERHARD CERAMICS Ceramic materials having extreme hardness to withstand more than 40 GPA with high thermal and chemical stability are generally termed superhard ceramics. Precision ceramics are made using superhard ceramics which play a major role among technical ceramics over-ridden only by diamond in terms of toughness. Example: B4O known for abrasion resistance are used in pumping nozzles, SiC known for chemical resistance and low thermal expansion. Two of the superhard ceramics popularly used in industries are discussed below. 1.6.1. Tungsten Carbide Its molecular formula is WC, an important compound of carbon with some other equivalents being titanium carbide, molybdenum carbide, tantalum carbide and chromium carbide. With a high density and Young’s modulus of about 530–700 GPa, it overrides the stiffness of steel and is considered to be one of the hardest material. Its unique thermal stability makes it an excellent material for a wide range of industrial applications which includes metal cutting, mining, dental drills, bearings, dyes and seal rings. Available as grey fine powder, it exists in two forms: i. Hexagonal form (-WC): Distance between tungsten atoms is 291 pm and between layers is 284 pm. Tungsten-carbon bond length is 220 pm. The hexagonal state is stable from a range between 2670–2720 K to 3000–3050 K. ii. Cubic form (-WC): Generally formed at lower temperatures, exists as thin films made of grains with granular size 400–500 Å.

Properties 1. Twice as hard as to deal with Young's modulus of 530–700 GPA and twice as dense as steel. 2. Refractory grade WC has hardness of 2000–2700 HV with a score of 9 in mohs scale 3. Highly resistant to strong acids 4. Has melting point 2870°C and boiling point 6000°C with thermal conductivity 110 Wm–1  K–1 and coefficient of thermal expansion 5.5 m–1 K–1 5. Low electrical resistivity of 0. 2 m 6. High resistance to galling and welding

Functional Materials 11

Applications i. Cutting tools for machining ii. Used in armor piercing ammunition and rock drill bits. iii. Used as an effective neutron reflector. iv. Tyres with WC studs provide a good traction on ice. v. Used in manufacturing of guage blocks. Synthesis Method 1: Dry tungsten powder blend with carbon black and fired in hydrogen atmosphere at 1400 to 2650°C. 1400  2650ºC  WC W + C  Method 2: Tungsten hexachloride is reduced with H2 and made to react with CH4 which acts as a source of carbon.

670ºC  WC + 6HCl WCl6 + H2 + CH4 

Method 3: To achieve higher reaction rates tungsten hexafluoride is reduced with H2 and made to react with CH3OH acting as a source of carbon.  WC + 6HF + H2O WF6 + 2H2 + CH3OH 

1.6.2 Boron Nitride The structural variety in BN is similar to carbon solids as the pairs of boron and nitrogen atoms is isostructural and isoelectronic. It is a popularly used broadband semiconductor compound with extreme chemical and thermal stability having strong covalent bonds and equal number of boron and nitrogen atoms. Due to dipolar nature of B-N bond, it is also considered as a good material for adsorption of hydrogen and hydrogen storage applications. BN can exist in amorphous and crystalline forms varying in the arrangement of atoms. Amorphous form: No regularity in the structure and resembles amorphous carbon. Crystalline form: Atoms have fixed position and Fig. 1.4: Hexagonal BN regularity in structure. Hexagonal form (h-BN or -BN): It is the most stable form resembling graphite, has ABAB… stacking structure. The hexagonal rings of the basal planes directed above each other are rotated by 180° angle between alternative layers (Fig. 1.4). Cubic form (c-BN or -BN): It is a less stable form resembling diamond has ABCABC… stacking structure. The cubic structure has tetrahedrally coordinated boron and nitrogen atoms with {111} planes arranged in a three layer stacking (Fig. 1.5). Wurtzite form (-BN): These are six membered rings Fig. 1.5: Cubic BN constituting boron and nitrogen and have chair configuration with two layers of stacking planes ABAB… (Fig. 1.6).

12 Chemistry of Engineering Materials

Synthesis i. Pure ammonia is passed onto borax at elevated temperature. 900ºC B2O3 + 2NH3   2BN +3H2O ii. Boric acid may be used to obtain BN at in-situ (lab) temperature

 BN + 3H2O B(OH)3 +NH3  iii. Boron oxide reacted with urea

Fig. 1.6: Wurtzite BN

B2O3 +Co (NH3)2   2BN + 2H2O + CO2

Properties i. High thermal stability: Stable upto 1000°C in air, 2800°C in inert atmosphere and 1400°C in vacuum. ii. Electrical conductivity: Band gap is wide with 5.9 eV and hence used as an electrically insulating filler material. iii. Chemical stability: Insoluble in acids and it resists oxidation upto 1300°C, if it undergoes oxidation at about 1300°C to B2O3, further oxidation is prevented by a thin layer of oxidized product. Application i. Hexagonal BN is an excellent lubricant and is a good alternative for graphite ii. It imparts self-lubricating properties used as a constituent of plastics, resins, ceramics and alloys iii. Used as a semiconductor base because of its excellent dielectric property. iv. Used as a charge leakage barrier layer of photo drum in laser print v. Nanosheets of BN serve as proton conductors and hence used in water electrolysis process 1.6.3 Transparent Ceramics Glass and polymers, considered being optically transparent or "see through" materials are of high importance in industry, but generally lacks mechanical and chemical stability. Hence, transparent ceramics are excellent candidates to replace conventional transparent materials in optoelectric devices, biomedical materials, scintillation processes and solid-state lighting owing to their chemical durability. Transparency is imparted by crystallographic orientation through different fabrication methods. As grains are the main phase of ceramics, the transparency of ceramics is governed by grain boundaries where scattering of light occurs and impurities reside. Ceramics materials with high degree of crystallinity with cubic, tetragonal and hexagonal structures can lead to isotropic scattering and are preferable, despite their laborious processing routes. Among the three steps involved in ceramic making namely, preparation of powder, body forming and sinctering, the capacity of light transmittance of the ceramics depends on temperature of sintering which decides the grain size. Sintering is a process of compacting the ceramic material with homogeneous microstructure by application of pressure and heat below its melting point, to reduce the water content and porosity.

Functional Materials 13

To achieve transparency in ceramics, sintering is generally done to optimize the grain size to about 200 m through spark plasma sintering, hot isostatic pressing and cold isostatic pressing. The best materials for the preparation of transparent ceramics are mechanically and chemically stable oxides such as Al2O3, MgO, ZrO2, Y2O3, Sc2O3, Lu2O3. Further, pre-sintering, calcination and doping can enhance the controllability of grain size. Some of the fabrication methods adopted are • Nonaqueous tape casting: Thin ceramic flat tapes are produced using organic solvents. • Vacuum tape casting: Long thin ceramic tapes are produced using a vacuum pump. • Slip casting: A liquid consisting of ceramic solid particles is molded into a shape without application of heat. 1.7 COMPOSITES These are materials comprising two or more constituent material with different physical or chemical properties, which on combination, lead to new properties that were absent in the individual component. However, they remain separate and distinct from finished structure. These materials are a combination of matrix acting as a binder and reinforcement. 1.7.1 Matrix It is a continuous phase which surrounds the fiber dispersed phase. Matrix helps in retaining the composite mass, shape, configuration and binds the fiber together retaining its entity. Matrix protects the surface of composites from corrosion, mechanical damage, temperature changes and decides the distribution of structural and environmental load transfer which affects the efficiency of the composite.

Matrix Used in Composites Include Thermosetting resins: These materials which sets at a high temperature are irreversibly hardened and imparts specific tensile strength and stiffness properties. They can be further processed only by chemical dissociation. Example: Epoxides, furan resins, polyimides, silicones, polyamides, vinyl esters, polyesters, etc. Thermoplastic resins: These are moldable at a specific temperature when intermolecular forces weaken and impart flexibility but solidify on cooling. Example: Polymethylmethacrylate, acrylonitrile butadiene styrene, polylactic acid, polyimide sulphone, ether imide, poly benzimidazole, etc. 1.7.2 Reinforcement It is a component which can be particulates, fibre particulates or whiskers. The most commonly used reinforcement is fiber. Fibers are molecules with long chain polymers of varying length and diameter of a few microns. The strength and stiffness of fibers reflects the strength and stiffness of the composites. Fibres used in a composite include the following. i. Glass fibers: Imparts chemical resistance to composites Example: Sodium silicate glass ii. Carbon fibers: Imparts high modulus and strength to composites. Example: Rayon carbon filament, polyacrylonitrile fibers

14 Chemistry of Engineering Materials

iii. Boron fibers: Imparts elasticity and strength to composites Example: Boron filaments iv. Polyolefin fibers: Imparts low dielectric properties and good microwave transparency Example: Polybutene-1 v. Silicon carbide fibers: Imparts high thermal stability to the composite. Example: Sintered SiC fibres 1.7.3 Classification

Classification of Composites Based on Occurrence i. Natural composites: These are compounds derived from nature and are renewable. Example: 1. Wood containing lignin acts as matrix and binds the cellulose fibre reinforcement materials. 2. Bone containing calcium phosphate matrix, binding the collagen protein reinforcement material. ii. Synthetic composites: These are manmade artificial composites with better properties than natural composites. Example: 1. Reinforced cement concrete containing cement as matrix and metal rod as reinforcement material 2. Fiber glass containing polymethylmethacylate matrix and fine threads of glass as fiber. 3. Carbon fiber containing polystyrene matrix and carbon fibre as reinforcement. Classification of Composites Based on Matrix Composites are generally classified based on the matrix used. Thermoplastics when used as a matrix tend to lose strength at elevated temperature. However, qualities like rigidity and toughness it imparts make it industrially important. i. Metal matrix composites: Most metal and metal alloys can be used as matrix. Ti, Al and Mg are used as are lightweight elements with high modulus. Metal matrix composites are popularly used as conducting materials and the strength to weight ratio is higher than traditional alloys. Example: Copper reinforced with tungsten particles ii. Ceramic matrix composites: These materials when used as matrix imparts high melting point and load bearing capacity with high compressive strength making them good materials for high temperature application. Example: SiC reinforced with carbon fibre iii. Polymer matrix composites: Polymers qualify as excellent matrix materials owing to light weight and desirable mechanical properties. Thermosets and thermoplastics are widely used as matrix materials. Thermosetting polymers like epoxy polyesters, phenolic and polyimides which do not melt but decompose on hardening and hence used as a premixed and moulding component. Example: Glass strand reinforced polyurethane

Functional Materials 15

iv. Carbon matrix composites: Carbon and graphite are highly superior elements as they have high strength rigidity and temperature resistance upto 2300°C. Example: Graphite with Cu particles

Classification of Composites Based on Reinforcement i. Fibre reinforced composite: Strength and stiffness of fiber used as reinforcement decides the properties of composite. ii. Particulate reinforced composite: These are small compact molecules which when used as reinforcement can impart special properties such as colour and flexibility. iii. Whisker reinforced composite: These are short fibers of length ranging from 3–5. 5 mm and popularly used for its zero defects iv. Flakes reinforced composite: Light weight, inexpensive flakes are used in place of fibers which are densely packed. When metal flakes are used, conducting properties are induced. 1.7.4 Properties of composites i. High strength to weight ratio ii. High flexibility and toughness iii. Fire retarding and fire resisting properties iv. Good insulators with high chemical resistance v. Low thermal conductivity vi. Highly ductile and malleable 1.7.5 Applications i. Used in making laminates and electronic gadgets ii. Used in making chimney ducts, chemical storage tanks and boat hulls iii. Manufacturing of containers used for microwaves and form insert iv. Used in making fiberglass v. Used in acoustical insulation 1.8 ADVANCED COMPOSITES Traditional polymer composites have a bulk polymer and reinforcement. Often, there is a poor interfacial bonding between them resulting in disproportion of properties. To improve the interfacial interaction, a third minor reinforcement component in the nanometre scale is added to the composite as an additive which can disperse homogeneously in the polymer matrix and enhances the properties of the composite. 1.8.1 Polymer-nanoclay Composites Materials with polymers as continuous matrix with dispersed nanomaterial in the form of fibres, spheroids, pallets of at least one dimension less than 100 nm mixed with clay are termed as polymer-nanoclay composites. The nanoclay dispersion aggregates within the polymer matrix and the polymer chains are separated from the clay layers, formimg an intercalated structure between the clay layers by modifying the geometry of the clay layer. Example: 1. Montmorillonite nanoclay composed of 1 nm thick aluminio silicate layers gets dispersed in a polymer matrix with interlayer distance of 10 m enhancing the mechanical strength and flame resistance.

16 Chemistry of Engineering Materials

2. Montmorillonite nanoclay coated with siloxane of 50 m disperses into polymer matrix imparting high cation exchange capacity

Surfboards are made from fibreglass, a cheap composite material of glass fibre and a plastic polymer

1.8.2 Synthesis of Polymer-nanoclay Composite There are three synthetic methods to prepare polymer nanoclay composites which are popularly in use. Method 1. Melt blending method A desired amount of nanomaterial intercalated in the clay is blended with the polymer at a temperature above the polymer’s softening point in presence of an inert gas. This method provides a better mixing of the polymer and nanoclay which is compatible with the current industrial process such as extrusion and injection, moulding in thermoplastic and elastomeric industries. Advantage: No organic solvents are required and Gore-Tex is a composite which is used to make clothing. It contains hence environmental friendly. layers of different materials which Disadvantage: High temperature is required and work together to create a fabric sometimes results in aggregation of nanomaterials. which is waterproof and breathable Method 2. Solution blending method The polymer and the nanomaterials are dissolved homogeneously in a suitable solvent and added to clay. This causes the layers of the clay to swell. The solvent is later removed by vaporization or precipitation to obtain pure nanoclay composite. Advantage: Gives a uniform nanoclay dispersion. Disadvantages: • Applicable only to a few polymers • A large quantity of organic solvents are required and hence not eco-friendly Method 3. In-situ polymerization technique Layered silicates containing nanomaterials is incorporated into the monomers before the polymerization to form monomer-clay sheet. During this polymerization step, clay layers are exfoliated and easily get dispersed. Advantage: Permits versatile molecular designs and is currently in use. Disadvantage: Applicable only to slow reactions. 1.8.3 Advantages i. Introducing nanoscale phase increases the strength and imparts corrosion resistance ii. Nanoparticle enhances dielectric and mechanical properties iii. Polymer nanoclay composites have good flexural strength barrier properties and excellent heat distortion temperature iv. Has high sintering property 1.8.4 Applications i. A base for catalyst in biochemical reaction ii. A structural material in making chemical sensor capacitors iii. Used as abrasive and wear resistant coating material iv. Used in making food grade containers and heat resisting food wraps

Functional Materials 17

1.8.5 Polymer-carbon Nanotube Composites Materials with polymer as continuous matrix and carbon nanotube dispersed as fibers are termed polymer carbon nanotube composites. Carbon nanotubes are the cylindrical nanostructures which are allotropes of carbon possessing unusual electrical, optical and mechanical properties. These carbon nanotube fibres are fabricated and dispersed into a continuous polymer matrix like polypropylene, polyimide, polyaniline and polyacrylamide where the nanotubes bond to the polymers by strong - stacking which brings dispersion stabilility. This incorporates many useful properties to the polymers. However, the interfacial bonding between the polymer and the carbon nanotube is still a concern, because of the chemical inertness of carbon nanotubes. Example: 1. Polyethylene carbon nanotube composite: Incorporated carbon nanotubes, increase the elastic melt properties, strain energy density, viscosity, ductility, toughness and crystallization rate. Hence it is used in fatigue resistant materials. 2. Polyimide carbon nanotube composite: Incorporated carbon nanotubes impart antiwear capacity, tensile strength, friction resistance and load capacity. 3. Polyaniline carbon nanotube composite: Carbon nanotubes incorporated enhance the conductivity in the composites. 4. Polystyrene carbon nanotube composite: Carbon nanotubes form a network of fibers inside the composite which increases the viscoelastic behavior and loading capacity of the composites.

Synthesis of Carbon Nanotube Composites i. In-situ polymerization: An efficient way of synthesis where a catalyst like methylaluminoxane covalently bonded to both carbon nanotube and the polymer melt is added on to the surface of nanotubes. This method provides better crystallization and gives rise to thermal and mechanically stable composites. ii. Blending of functionalized carbon nanotube and polymer: Carbon nanotubes are susceptible to substitution reactions. Hence, they are functionalized with functional groups capable of bridging themselves with polymers. Advantages i. Excellent thermal conductivity ii. Exceptionally high mechanical strength and Young's modulus iii. Exhibits nonlinear optical properties iv. Highly versatile and can be fabricated v. Corrosion resistant and lightweight Applications i. Used as components of LED devices ii. Used as reinforcing and conductive filler iii. As a structural material in molecular pressure sensor and high temperature gas sensors iv. Good mechanical strength to replace steel in military equipment v. Used in making anti-static devices and frequency selective coatings

18 Chemistry of Engineering Materials

1.9 ALLOYS An alloy is a metallic solid obtained from a combination of two or more metals. The metal in larger concentration is the parent metal which is melted in a crucible then solid pieces of alloying metals are added and mixed. Example: Cu-Ni, Ag-Au, Au-Pt, K-Cs, Ti-Zr, As-Sb If both the metal atoms are electrochemically same, an ordered solid solution or “superlattice” is formed, called “primary solution”. Example: Cu-Zn, Cu-Au, Cu3-Au If both the metal atoms are electrochemically different, the bond among the atoms becomes partially ionic leading to “intermetallic compound”, called secondary solutions or “intermediate phase”. Example: MgCu2, KNa2, MgNi2 Unlike atoms coming together, this is a microscopically heterogeneous mixture often termed phase mixtures where each phase is rich in atoms of one type and are separated by grain boundaries. 1.9.1 Phase Mixture of Solid Solutions Phase mixtures exists in heterogeneous equilibrium where homogeneous and distinct crystal grains in different phases are mixed together and joined to one another along well defined narrow interfaces. Experimental studies reveal that alloy systems are generally represented as phase diagrams which specifies the range of composition and temperature in which various phases are stable. The grains in various constituents of a phase mixture in a metallographically prepared section of alloy can be examined under an optical microscope. The quantity of each element present in each phase is defined by a set of equations called lever rule. Consider a binary alloy of components A and B in concentrations c and (1 – c) respectively. Let the alloy consist of two different phases, 1 and 2 in which the concentration of A are c1 and c2 respectively. Let the proportion of phase 1 in the alloy be x then phase 2 will be (1 – x). If there are N atoms, we have for the A atoms, Nc = Nxc1 + N (1 – x) c2 x = (c – c2)/(c1 – c2) = m/l (1 – x) = (c1 – c)/(c1 – c2) = n/l x/(1 – x) = (c – c2)/(c1 – c) = m/n where l, m and n are the lengths defined in the diagram below.

Fig. 1.7: Representation of parameters in lever rule

Functional Materials 19

These relations are the lever rule and necessary for determining the constitution of phase mixtures. In a metallurgical study, it is often important to determine the proportion of phases present in the alloy which can be determined in several ways (Fig. 1.7). Method 1: Area method—the ratio of area of the phases (as view) measured on a random section of the material. The ratio of volumes of the phases considering the analytic section as a wafer slice of material is measured. Method 2: Line intercepts—straight lines are scribed at random across a metallographic section and the proportion by length of the phases intercepted by them are measured. Method 3: Point counting—a grid is laid randomly on the section and proportion by volume is measured from the proportion of grid points which fall on various phases. 1.9.2 Factors Affecting Alloying or Solubility of Metal with Each Other i. Atomic size: Solubility of metals decreases with increase and difference in atomic size and if the Roofs made of copper alloys diameter is more than 14% then the size is undergo a slight atmospheric corrosion and forms a popular unfavorable. ‘green platina’ with enhanced ii. Electrochemical nature: If the metals have a large attractive appearance. difference in electronegativity, they form compounds and not alloys and thus have low solubility. iii. Relative valence: A metal of low valence dissolves one of higher valency. iv. Temperature: As disordered solution has higher entropy, solubility of metals with each other increases with increasing temperature. G = H – TS as TS increases, G decreases and the system becomes highly stable. Alloys can be broadly classified as ferro and nonferro alloys. Due to the abundance and versatile physicochemical properties of iron, ferro alloys constitutes a major class of industrial alloys. 1.9.3 Ferro Alloys The main ferro alloys produced are steel, ferrosilicon, ferromanganese, ferrochromium, ferrotungsten, ferromolybdenum, ferrovanadium and ferrotitanium. Among all, the ferro alloy with largest production is steel with a high industrial importance. Steel: Carbon atoms diffused into the surface of the solid iron producing a hard surface layer is called steel. The essence of steel making is tailoring the carbon content to obtain varied properties. The most popular methods used in the industrial production of steel are crucible steel process, Bessemer process, open-hearth process and electric steel making. The elements like Ni, Cr, Mn, V, Mo, W, Nb and Ti are often added to steel to incorporate desired properties. 1.9.4 Copper Alloys Copper is a very popular alloying metal, it is chemically inert, highly conductive, easy to solder or braze, with high thermal conductivity and good fabricating properties. However, it cannot compete with steel or aluminum in applications.

‘Admiralty brass’ a popular alloy is used for marine applications (29% Zn, 1% Sn and 0.05% As) has corrosion resistance to sea water.

20 Chemistry of Engineering Materials

It is an excellent solvent for many metals like Al, As, Au, Ga, Ge, Mn, Ni, Pd, Pt, Sn, and Zn forming extensive primary solid solutions. Copper alloys are mostly strengthened by solid solution hardening and work hardening. The most common alloying elements added to copper are: i. Zinc: It is the most common alloying metal with copper and a combination of Cu and Zn is termed brass. Addition of Zn to Cu increases the strength of Cu alloy by 50%, ductility by 30%, and also reduces the cost and lowers the melting point. The properties varied drastically with percentage of Zn. 36% Zn in primary solid solution is called alpha brass with high ductility, 5% Zn in primary solid solution is called gilding brass with high corrosion resistance. 15% Zn in primary solid solution is Pb forms dispersed insoluble called red brass with high corrosion resistance. soft particles in the matrix of 30% Zn in primary solid solution is called cartridge copper alloys to give freemachining properties. It acts as brass with high ductility. 34% Zn in primary solid a lubricant for alloys used as solution is called yellow brass which is highly sliding bearing. economical. Beyond 36% of Zn, the BCC -phase appears along with -phase, making cold working difficult. 40% Zn in primary solid solution is called muntz metal is drawn into tubes and other irregular shapes like in hard solder by hot extrusion as it cannot be cold worked. ii. Tin: Popularly called as bronze, increases the elastic limit of Cu alloy by 10%. iii. Aluminum: Increases the corrosion resistance of Cu alloy by 50%. iv. Silicon: Increases the welding properties of Cu alloy. iv. Beryllium: Increases the mechanical strength of Cu alloy. v. Nickel: Increases the corrosion resistance of Cu alloy.

Copper Aluminum or Aluminum Brass (Cu-Al) The combination of copper and aluminum is referred to as aluminum brass. Cu-Al alloy is homogenous upto 79% Cu. The addition of Al to Cu improves the hardness, strength and corrosion resistance but decreases ductility. The increasing content of Al makes the system heterogeneous and resistant to The name ‘Brazing’ is derived corrosion and hence used in marine condenser tubes and from the original use of name power stations. ‘brass’. Aluminum content in the range 7.3–9.4% solidifies at eutectic point. Properties i. Tough and suitable for cold and hot forming ii. Retains its mechanical strength even at high temperature iii. Can with stand stress and strain iv. Highest tensile strength 380 MPa and hardness 70-110 HB Application i. Used in shipbuilding, municipal water system and marine hardware ii. CuAl5 formed by cold forming is used in making stressed gear wheels

About 6000 years ago, early people made the alloy bronze by roasting together copper and tin ores. Bronze is stronger and longer-lasting than pure copper.

Functional Materials 21

Copper Silicon Copper has an atomic mass twice as that of silicon and has higher density than silicon. Copper has a valency of 1, whereas silicon has a valency of 4 and hence the lattice bonding of silicon is incomplete thereby increasing the internal energy of the system. Alloying of Cu-Si Copper-silicon melt is mixed at 1450°C for an hour in an environment of Ar gas to prevent oxidation. After reaching room temperature, the solid is removed from furnace. As the density difference between copper and silicon is high, a complex system is formed during solidification (Tables 1.1 and 1.2). Generally for impurity removal, an understanding Electrum is a naturally occurring of phase diagram is necessary. alloy of gold and silver with small amounts of copper and other Properties of Cu-Si metals i. Highly wieldable ii. Good semiconducting properties iii. Good acid resisting, nonrusting and nonmagnetic Applications of Cu-Si i. Aquaculture enclosures are made due to their lightweight. ii. Used in making chemical furnaces. iii. Used in making semiconducting materials. Table 1.1: Atomic structure of silicon-copper Atomic radius Atomic volume Covalent radius Crystal structure Electron configuration

Copper

Silicon

1.57 Å 7.1 cm3 mol–1 1.17 Å Cubic face centered 1s22s22p63s23p63d104s1

1.46 Å 12.1 cm3 mol–1 n/a Cubic face centered 1s22s22p63s23p6

Table 1.2: Comparison of physical properties of Cu and Si Atomic mass Density Melting point Boiling point Coefficient of linear thermal exapansion Electrical conductivity Thermal conductivity

Physical properties of copper

Physical properties of silicon

63.546 8.96 g cm–3 (300 K) 1358 K 2833 K

28.0855 2.329 g cm–3 (293 K) 1687 K

16.5 × 10–6 K–1

3.0 × 10–6 K–1

60.7 × 106 Scm–1 401

1.2 × 10–5 Scm–1 149

1.9.5 Aluminium Alloys In contrast to traditional metals such as iron and copper, the tonnage use of which has remained fairly steady in recent years, aluminum has increased steadily in amount used by about 7% a year and is now the second only to steel in amount produced. Amongst the common materials, only plastics have a faster rate of growth of production and use than Al.

22 Chemistry of Engineering Materials

Aluminum is an excellent alloying parental metal as it is corrosion resistant, light, and economical with good fabricating properties. Its strong alloys have a higher strength/weight ratio than steel and hence are a major structural material for aircraft, transport and other engineering uses. The main alloying elements used in aluminium alloys are Si, Mg, Cu and Zn. Other alloying elements like Cu cannot be used frequently as it forms oxide (weak spots) and comes with less corrosion resistance. Various combinations are: • Addition of Zn to Al enhances ductility of Al • Addition of Mg to Al enhances welding properties of Al • Addition of Ag to Al enhances grain boundary properties of Al

Silumin The simple eutectic which Al forms with Si at 12% silicon is the basis of important casting alloys that contain 11–13% silicon. This alloy does not have the problem of formation of weak spots and oxide film at the sites of phase boundary and thus have a good corrosion resistance. These have excellent casting fluidity and in making intricate shapes for engine cylinder blocks gear and crank cases, etc. Alloying Al with Si between 5–20% enhances the casting characteristics. Based on silicone concentration, alloys are of differentiated into three categories: • Hypoeutectic: Solidify with Al in primary phase • Eutectic: Both Si and Al to 50% in primary phase • Hypereutectic: Solidify with silicon in primary phase Properties of Al-Si i. Both components are inexpensive ii. Resistant to humidity iii. Highly ductile and has low density iv. High fluidity and low shrinkage v. Low thermal expansion coefficient vi. Excellent corrosion resistance, low density, good mechanical properties and excellent weldability. Applications of Al-Si i. Used in highwear resistant materials like piston, engine blocks and bearing brushes ii. Widely used in making castings with fatigue resistance iii. Can be heat treated to obtain necessary properties iv. Laser hardening can be achieved to produce chemically resistant surface

Aluminium-Lithium Li being a light element makes light weight low-density alloy. Every crystal lattice of Al has one atom of Al replaced by Li. The alloy exists in two forms: i. Metastable Al3-Li (-phase): Has a coherent crystal structure that strengthens the metal by hampering the dislocation motion at the time of deformation. ii. Stable Al-Li (-beta phase): Has precipitate free zones at grain boundaries and hence has poor resistance to corrosion.

Functional Materials 23

Properties i. Poor fatigue strength when compressed ii. Low density with high castability iii. Low ductility iv. Cold work is required for optimum properties Applications i. Used in jet linear air frames, fuel and fuel and oxidant tanks ii. Used in composites to reduce weight iii. Better alternative for Ti-composite 1.10 GLASS 1.10.1 Introduction Glass is a transformative material with incredible versatility and is considered to be one of the world’s most valuable engineering material. It is considered as an important engineering component due to its high thermal and mechanical stability gained by continual silicon-oxygen bonds, unparalleled transparency, extreme impermeability, no sharp melting point, solidifying to a transparent solid from a liquid state without crystallization. It constitutes a homogeneous mixture of two or more silicates, represented as XR2O YMO 6SiO2, where R = alkali metal and M = bivalent metal. Example: Ordinary soda glass consists of Na2O CaO 6SiO2 1.10.2 Chemical Constitution of Ordinary Glass The production of glass involves addition of various chemical compounds, depending upon the properties desired. Generally, the raw materials used are as follows: i. Quartz sand: SiO2 forms a network of oxides ii. Soda: Na2O enables thermal expansion and is generally obtained by Na2CO3 or Na2SO4. iii. Potash: K2O which imparts hardness to glass. iv. Feldspar: Aluminosilicate of sodium or potassium which decreases the melting point. v. Calcium oxide: CaO, generally obtained by CaCO3, imparts thermal stability. vi. Colouring agents: Transition metal oxides of cadmium, chromium, cobalt, copper, and manganese impart yellow, green, blue, red and violet colour, respectively ranging from 0.1% to 2%. 1.10.3 Classification Based on Application The properties of glass varies depending upon the manufacturing process and chemical compositon.The popularly used type of glass are: i. Sodalime glass: Most commonly used glass, consists of 74% SiO2, 14–17% N2O and 9% CaO in the form of Na2O CaO 6SiO2. The glass has fine pores and light permeable. It can be further hardened by replacing Na with K to obtain K2O CaO 6SiO2 generally referred to as “hard glass”. Application: Window panes, bottles and inexpensive glassware.

24 Chemistry of Engineering Materials

ii. Borosilicate glass: Used as heat proof and chemical resistant glass consisting of 81% SiO2, 12%B2O3, 2% Al2O3. Application: Thermometers, lab apparatus and electronic device parts iii. Optical glass: It has a homogeneous chemical composition of 73% SiO2, 7 to 15% PbO, 5 to 7% K2O and is transparent with an optimum refractive index. It is devoid of air bubbles, has a low melting point and is also highly resistant towards moisture and atmospheric gases. Application: Lenses and prisms for telescopes, microscopes and spectrophotometers. Optical glass is available under following categories Flint glass: High density glass used to make cathode ray tubes and x-ray shields. Crown glass: Highly homogeneous glass used in making prisms, it contains adducts such as BaO, K2O and ZnO. Crooke glass: High absorptivity towards UV radition due to addition of CeO2, it is popularly used in making spectacles. 1.10.4 Manufacture of Glass Using Tank Furnace Soda glass is prepared in a large scale using tank furnace with a capacity of about 1400–1500 tons, coated inside with refractory bricks. A homogeneous mixture of powdered raw material often referred to as “Batch material” consisting of sand, limestone, soda ash, salt cake and charcoal is fed into a tank furnace heated to 1700 K progressively. The following reactions occur in the tank furnace (Fig. 1.8).

Na2 CO 3 + SiO 2   Na2 SiO 3 + CO 2 CaCO 3   CaO + CO 2 CaO + SiO 2   CaSiO 3 The molten glass is uniformly mixed, allowed stand for the gas bubbles to escape and the impurities floating on the surface is skimmed off. The viscous molten glass is moulded to a desired shape.

Fig. 1.8: Schematic representation of tank furnace

1.10.5 Manufacture of Glass Using Pot Furnace High performance special type glass is prepared in a large scale using pot furnace made of alumina (Fig. 1.9) with a capacity of 2 tonnes heated using producer gas. The chemical reactions are the same as in tank furnace except for the fact that the molten glass product does not come in direct contact with products of combustion. Hence a glass of uniform concentration with high quality borosilicate and optical glass can Fig. 1.9: Schematic representation of pot furnace be obtained.

Functional Materials 25

1.11 LUBRICANTS Lubricants are substances which decrease the coefficient of friction between two contacting surfaces in mechanical machinery, increasing the efficiency and wear resistance. Economic and ecological factors must be considered during the selection of lubricant to efficiently contribute towards minimizing waste, reducing emission, saving and conserving the natural resources. 1.11.1 Some Properties of Good Lubricants • High temperature of evaporation and fluidity • High boiling point • Low freezing point • High viscosity index • High thermal stability • High hydraulic stability • High resistance to oxidation and degradation 1.11.2 Some Important Functions of Good Lubricants • Reduces frictional resistance • Prevents direct contact and interlocking • Reduces surface deformation, wear and tear • Acts as coolant • Increases the efficiency of the machine and machine life • Reduces maintenance cost and corrosion • Acts as sealants for gases 1.11.3 Classification Lubricants can be classified in different ways 1. Classification based on the use • Automotive lubricants Example: Grease • Industrial lubricants Example: Mineral oil • Special lubricants Example: Alkyl benzene 2. Classification based on the raw material • Petroleum oil Example: Mineral oil • Vegetable oil Example: Olive oil • Animal oil Example: Whale oil • Synthetic oil Example: Phosphoric acid ester

26 Chemistry of Engineering Materials

3. Classification based on the origin • Renewable: Vegetable oil • Nonrenewable: Mineral oil based 4. Classification based on the physical state • Solid Example: MoS2, PbO, graphite, • Semi solid Example: Grease • Liquid Example: Palm oil • Gases Example: Compressed air 1.11.4 Solid Lubricants These lubricants are used when the action of lubrication has to be exerted withstanding high pressure and temperature at a fixed area. Example: Graphite consists of carbon sheets separated by a distance of 3.4 Å and stacked by weak van der Waals forces. Hence graphite is soapy, slippery, non-inflammable and resistant to oxidation below 375ºC. Suspension of graphite with water is called ‘aquadag’, an oil free lubricant prepared using tannin as an emulsifying agent. Application: Commonly used in air compressors, food industry, ball bearings and railway tracks joints etc. Example: Molybdenum disulphide (MoS2) A sandwich like structure with molybdenum atoms between two layers of sulphur atoms. MoS2 has a poor interlaminar attraction and hence a low coefficient of friction but has excellent adhesive property, high load carrying capacity and stable in air up to 400ºC. Application: Commonly used in space vehicles due to its stability towards extreme temperature, low pressure and nuclear radiations. 1.11.5 Semi-solid Lubricants Liquid lubricants converted to gel by dispersing thickening agents such as soaps of lithium, sodium and calcium, etc. Non-soap thickeners like carbon black, clay, aspaltenes and siloxane are called semi solid lubricants. Example: Grease

Preparation Greases are prepared by the saponification of fat with alkali such as NaOH or Ca(OH)2 followed by the addition of hot lubricating oil with constant stirring.

Functional Materials 27

Greases are classified on the basis of the soap used in their manufacture Calcium based greases: Emulsions of petroleum oils with calcium soaps—it is insoluble in water Soda-based greases: Petroleum oils thickened by mixing sodium soap—not water resistant because sodium content of soap soluble in water. Lithium based Greases: Petroleum oil thickened by mixing lithium soap—water resistant but can be used up to 15ºC only. Non-soap greases: Mineral oil with solids like graphite, carbon black and soap stone 1.11.6 Liquid Lubricants These are the most popularly used lubricants owing to its capability to form a continuous interface across two surfaces, long life and low cost. Liquid lubricants can be further classified as: 1. Vegetable and animal oils 2. Mineral oils or petroleum oils 3. Blended oil or compound oils 1. Vegetable and animal oils: These oils contain glycerides of higher fatty acids which catches up moisture, undergo oxidation or hydrolysis and increases in viscosity. However, they are very expensive due to the associated tedious procedure of extraction. As they have the property of “oiliness”, they are blended with mineral oils to make it thinner. Example: Olive oil, mustard oil, castor oil, palm oil, whale oil etc. 2. Mineral oil or petroleum oils: The residual hydrocarbon fraction constituting a chain of 12 to 50 carbon atoms obtained after fractional distillation of crude petroleum followed by vacuum distillation is referred to as mineral oil or petroleum oil. They are inexpensive, less viscous than vegetable oils and highly resistant towards hydrolysis and oxidation and hence are popularly used as lubricants. 3. Blended or doped or compound oils: Introducing additives or other oils to obtain a mixture with desired properties are called the blended oil. The following are the common additives i. Antioxidant or inhibitors: These substances preferentially get oxidized and protect other constituents of lubricant Example: Amino compounds and aromatic phenolic compounds. ii. Viscosity index improvers: These substances increase the viscosity index by altering the intermolecular forces. Example: Hexanol. iii. Pour point depressants: These substances reduce the “pour point” of the oil, the temperature below which the liquid loses its flow characteristics. Example: Alkylated naphthalene iv. Metal deactivators: These substances retard the catalytic effect of metals on the oxidation states of constituents undergoing degradation. Example: Sulphides or phosphides v. Antifoaming agents: These substances prevent the formation of air bubbles and thereby retard oxidation process. Example: Glycol, glycerol etc.

28 Chemistry of Engineering Materials

vi. Corrosion and abrasion iInhibitors: These substances increase the inertness of the lubricant and prevent its erosion. Example: Organometallic compounds Example: Tricresyl phosphate vii. Emulsifiers: These substances help in the formation of emulsions of lubricating oil with water Example: Sodium salt of sulphonic acids 1.11.7 Mechanism of Lubrication Two mechanisms are proposed to explain the working mechanism of lubrication 1. Boundary lubrication or fluid film lubrication Definition: It is a process where a thin molecular layer of lubricant separates the sliding surfaces such that physical and chemical nature of surfaces are retained over a wide range of temperature, pH and other environmental factors. A film of lubricant generally of 20–30 Å thick is adsorbed through weak Vander Waals forces across two surfaces, preventing surface to surface contact and reducing the friction. This is ideal for lubricants with low viscosity used for low speed and high pressure heavy loading applications. Example: Zinc dialkyldithophosphate used as engine lubricants Applications of boundary lubrication • Bearing of diesel engines and two stroke engines • Used as hydrodynamic lubrication in Hydraulic pumps • Used in rotating shaft and rolling components Properties of boundary lubricants • High viscocity and adhesiveness • High thermal stability due to lateral attraction across the long hydrocarbon chains • High resistance to oxidation • Low cloud and pour point. 2. Extreme pressure lubrication Defintion: Lubricants used to reduce or prevent the wear of surfaces exposed to high pressures are referred to as extreme pressure lubricants. Example: Methylenebis(dibutyldithiocarbamate) as an additive to lubricant minimizes the oxidation of metal surface. Organic compounds introduced as additives, contain Chlorine, Sulphur, and Phosphorus reacts with metal surface and increase the thermal stability and prevent generation of high temperature, surface deformation, vaporization and deformation. 1.11.8 Applications of Lubricants • As antiwear, antioxidants, demulsifying and antifoaming agents. • As corrosion inhibitors. • As engine oils, compressor oils, gear oils, piston oils and gear box fluids • As cutting fluids in cutting and grinding,

Functional Materials 29

1.12 REFRACTORIES Compounds which resist extremely high temperatures without fusion or deformation are called refractories. 1.12.1 Properties A material can be called as refractory if the following conditions are satisfied. i. Thermal resistance: Ability to withstand high temperatures without undergoing deformation. It is represented in terms of pyrometric cone equivalent (PCE) whose value is generally above 26. ii. Melting point: Metal oxides with high melting point acts as a good refractory material such as SiO2 (mp = 1986), Al2O3 (mp = 2290), CaO (mp = 2770), ZrO2 (mp = 2970) and ThO2 (mp = 3770) iii. Spalling resistance: Ability to withstand high temperatures without undergoing expansion or contraction. iv. Thermal conductivity: Materials with low prevent heat loss from the walls. Materials such as alumina, silica and dolomite in the form of crucibles and muffle furnace act as refractory materials. v. Porosity: Materials with low porosity prevent diffusion of molten charge, flux, slag and gases and are useful refractories as they resist abrasion and spalling. vi. Chemical inertness: Refractory material must resist the attack of slag, furnace gases, fuel, and ashes inside the furnace. It is preferable to have an acidic refractory when products are alkaline and vice versa. vii. Erosion and corrosion resistant: A good refractory material should have erosion and corrosion resistance. 1.12.2 Measuring Pyrometric Cone Equivalent (PCE) value Refractory materials are mixtures of metal oxides and hence its thermal stability is conventionally indicated by softening temperature in terms of pyrometric cone equivalent (PCE) value. To calculate the PCE value, the following procedure is adopted Step 1: Standard pyramidal pyrometric cones or seger cones of known composition with height 38 mm and triangular base of 19 mm are prepared. Step 2: Seger cones of material whose PCE value is to be determined are prepared. Step 3: Temperature of the analyte and standard cones are raised at a rate of 10 degrees per minute. Step 4: The PCE value of given refractory is noted as temperature at which the standard one softens and bends over the base. 1.12.3 Classification of Refractories The efficiency of the furnace is affected by the chemical composition and properties of refractory. Refractories can be classified into three types: Acidic refractories: These are light highly rigid and mechanically strong acidic oxides with low porosity withstanding a load of about 3.5 kg/cm2 up to 1500ºC– 1600ºC which are resistant towards acidic slags and gases. They are widely

30 Chemistry of Engineering Materials

used in iron and steel industries for making open hearth furnace, reverberatory furnaces, gas retorts and wall of coke ovens. Example: SiO2 and Al2O3 Basic refractories: These are basic oxides which are resistant towards basic slags and gases. Example: CaO, MgO Neutral or inert refractories: These are neutral refractories which are resistant towards both acidic and basic slags and gases. Example: SiC, Cr2O3 FeO and ZrO2 1.12.4 Manufacture of Refractories Manufacture of a refractory consists of the following steps: 1. Crushing: Lumps of raw materials are broken into small particles of about 25 mm. 2. Grinding: Small pieces of raw material is finely ground to 200 mesh size. 3. Screening: The unwanted materials and impurities are removed by screening, settling, magnetic separation and froth flotation. 4. Blending and mixing: The raw materials are then mixed to a homogeneous mixture with organic or inorganic binders like molasses starch, shellac, gum and sodium silicates, lime, calcined gypsum. 5. Moulding: High pressure is applied either by hand moulding or by mechanical moulding 6. Drying: Under an optimum humidity and temperature, applied slowly, the product is dried which removes moisture. 7. Firing or burning: This is the last step of the manufacture carried on inside tunnel kilns, shaft kilns or rotary kilns where the refractory is heated to a high temperature to stabilize the refractory through dehydration, calcination, oxidation, shrinkage and crystal structure transformation (verification). Example: 1480ºC for high fired super duty bricks; 1700ºC for kaolin bricks and 1870ºC for basic bricks. CONCEPTUAL QUESTIONS

Justify the followings. 1. Ceramics are bad conductors of heat and electricity. Ans. Absence of free electrons is responsible for making most ceramics poor conductors of electricity and heat. 2. Metals bend on hammering but ceramics shatter. Ans. Metals can bend, stretch, and mould into wires because of their rows of regularly packed atoms which will slide past one another. But in ceramics, there are no rows of atoms and atoms are locked in a regularly repeating 3D crystal. 3. Silicon carbide can resist thermal shock. Ans. High thermal conductivity coupled with low thermal expansion makes them resistant to thermal shock.

Functional Materials 31

4. Surface interaction between matrix and fiber decides the quality of a composite. Ans. Surface interactions between matrix and fiber enable good binding and load transfer across the composite and hence its properties. 5. Copper is the most popular alloying metal. Ans. Cu has the best electrical and thermal conductivity. 6. Alloys are preferred over pure metals. Ans. These are specifically crafted to enhance the properties of the metals and elements they contain and hence preferred over pure metals. 7. Some alloys possess shape memory Ans. Within recoverable ranges, over a range of temperatures, the alloy undergoes a reversible solid state transformation. 8. Silumin has low plasticity. Ans. Silicon particles in silumin are in lamellar form and a small force deforms the -matrix. Also a large volume of silicon particles initiates cracking. 9. Chromium gives the steel resistance to 'stain' in stainless steel. Ans. Because Cr2O3 is a stable corrosion product which resists further corrosion. 10. Composites are promising ecofriendly products. Ans. Large variety of fibers incorporated in composites is naturally available. 11. Ceramic bio-implants have excellent biocompatibility. Ans. These do not release ions during degradation and hence are biocompatible. DESCRIPTIVE QUESTIONS

1. Define (a) superhard ceramics (b) composites (c) matrix (d) fibre. 2. Briefly outline the classification of ceramics with suitable examples. 3. Enumerate the classification of composites based on matrix. 4. Briefly outline the classification of composites based on reinforcement. 5. What are the advantages of composite materials? 6. What is the role of matrix is a composite material? 7. What is the role of reinforcement in composite materials? 8. Explain the various methods to increase surface interaction between matrix and fiber. 9. List the characteristics of matrix and fiber material. 10. List the properties of Al-Si and Al-Li. 11. Define ‘advanced ceramics’. Elaborate on the synthesis of silicon-carbide (SiC) by Acheson process. 12. Differentiate between melt and solution blending methods of synthesis of polymer nanoclay composites. 13. List the factors affecting alloying. 14. Appraise the industrial importance of silumin. 15. List the differences between ferro and nonferro alloys. 16. Describe the synthesis properties and application of (i) BN and (ii) WC 17. Outline the synthesis, properties and applications of (i) Al2O3 and (ii) SiC 18. Define an alloy. Elaborate on the composition, properties and applications of Cu-Al and (ii) Li-Al alloys

32 Chemistry of Engineering Materials

MULTIPLE CHOICE QUESTIONS

1. Identify which of the following kind of stress ceramics cannot withstand? (a) Shear (b) Compressive (c) Tensile (d) Normal 2. Which of the following leads to low tensile strength of ceramics? (a) Dislocations (b) Structural defects (c) Grain boundaries (d) Vacancy 3. Which of the following processes is important in fabrication of ceramics? (a) Sintering (b) Rolling (c) Forging (d) Casting 4. The raw materials used in the synthesis of BN are (a) Quartz and coke (b) Quartz (c) Coke (d) Borax and ammonia 5. The main constituents of composites are (a) Fibre (b) Matrix (c) Both a and b (d) None 6. The composition of whiteware ceramics is (a) Clay (b) Silica (c) Feldspar (d) All of these 7. Specify the example for natural composite. (a) Kevlar (b) Bone (c) Carbon (d) None 8. Ceramic materials can conduct (a) Heat (b) Electricity (c) Light (d) None of these 9. Ceramics have properties of (a) High melting and boiling points (b) Hard (c) Chemically inert (d) All of these 10. Which of the following carbides are used in cutting tools? (a) Silicon-carbide (b) Tungsten-carbide (c) Vanadium-carbide (d) Chromium-carbide 11. Identify the composition of aluminum bronze. (a) Cu-Sn (b) Cu-Al (c) Cu-Zn (d) Al-Si 12. Identify the example of high performance fiber. (a) Carbon fiber (b) Polyester fiber (c) Kevlar fiber (d) Liquid crystal fiber 13. Identify an example for nonoxide ceramic material. (a) ZrO2 (b) Al2O3 (c) SiC (d) SiO2 14. Identify a good ceramic substrate for catalysis (a) ZrO2 (b) Al2O3 (c) SiC (d) SiO2

Functional Materials 33

15. Which of the following is a property of porcelain? (a) Soft (b) Absorbent (c) Vitreous (d) Expensive 16. Which of the following is not an alloy? (a) Steel (b) Copper (c) Brass (d) Bronze 17. Composite materials are classified based on (a) Type of matrix (b) Size and shape of reinforcement (c) Both (d) None of these 18. Which of these is the major load carrier in composites? (a) Matrix (b) Fiber (c) Both (d) Can’t be defined 19. Mechanical properties of fiber-reinforced composites depend on (a) Constituents (b) Interface strength (c) Fiber length and orientation (d) All the above 20. The stronger constituent of a composite is (a) Matrix (b) Reinforcement (c) Both are of equal strength (d) Can’t be defined 21. Which one of the following ceramics can be used as a pigment in paints? (a) TiO2 (b) SiC (c) SiO2 (d) none 22. Ceramics can conduct (a) Heat (b) Electricity (c) Light (d) None 23. Ceramics are well known for the properties of (a) Hardness (b) Chemical inertness (c) Corrosion resistant (d) All of these 24. Identify the alloy which is more suited for aircraft applications (a) Cu-Si (b) Si-Al (c) Al-Li (d) None 25. Identify the prime constituents of Silicon bonzes (a) Si and Cu (b) Cu and Al (c) Li and Al (d) Si and Al ANSWERS

1. c

2. b

3. a

4. d

5. c

6. a

7. b

8. d

9. d

10. b

11. b

12. c

15. c

16. b

17. c

18. a

19. d

20. b

21. a

22. d

23. d

24. b

25. d

34 Chemistry of Engineering Materials

2 Commodity and Engineering Polymers

The word ‘commodity polymers’ refers to polymers which are used in daily life with exceptional mechanical properties like low creep, stiffness, toughness and are of enhanced thermal properties. These materials are generally used by manufacturers as raw material to achieve the required end product. 2.1 INTRODUCTION TO LIQUID CRYSTALS Matter can be classified as solids, liquid and gases. In The world churns out more solids, molecules are closely packed in a regular fixed than 600 billion pounds of arrangement and with both positional and orientational plastic every year and the interconnected molecules are order. They are anisotropic in nature. Liquids on the other hand, lack intermolecular forces too large for microbes to get and hence the molecules or atoms in them move their biters around. randomly. A distinct ordered liquid state of matter where the molecules are not maintaining any orientational order but have only partial positional order Polypropylene typically used are termed liquid crystals. Hence the extent of molecular for bottle caps, straws and food containers. The material's high order in this state is between highly ordered crystalline heat resistance means it is state (anisotropic), and completely disordered (isotropic) microwave and dishwasher liquid state (Fig. 2.1) and referred to as “mesogenic safe and superstrong for reusable bags. phase”. The molecules in mesogenic phase though are in constant motion, orient in a preferred direction and said to have orientational order which can be referred by a director or a vector denoted by nˆ . However, they lack positional order. Note: Isotropic: Equal physical properties along all axes. Example: Glass, metals, ice crystals

Fig. 2.1: Representation of solid, liquid crystal and liquid state 34

Commodity and Engineering Polymers 35

2.2 STRUCTURAL REQUIREMENT OF A LIQUID CRYSTAL i. Molecules must be elongated and have rigid central part ii. Molecules must have functional groups that exert a force of attraction and hold the molecules parallel to each other (imparts orientational order) iii. Molecules must have flexible ends which twists and wriggles (imparts positional order) iv. Molecules must preferably have aromatic rings and conjugated double bonds imparting rigidity (Fig. 2.2). 2.3 EXAMPLES OF POLYMERIC FRAGMENTS EXHIBITING LIQUID CRYSTAL BEHAVIOUR

2.4 LIQUID CRYSTAL POLYMERS Polymers exist as long chains which can be modified and hence have the potential to be used as liquid crystals. They have unique properties and are capable of replacing metals and ceramics for electronics, aerospace and transportation applications. 2.5 STRUCTURE–ACTIVITY RELATIONSHIP Generally a liquid crystal can be represented by a chemical structure, where R is the side chain alkyl, alkoxy or alkenyl groups which decides the flexibility and phase transition temperature. A and B are aromatic rings which may be same or different on which the polar groups –CN, –F, –Cl attached decides the intermolecular spacing and dielectric properties. Z is the linking group which may be an ester (–CO–O–), ethylene (–CH = CH–), azo (–N = N–) affecting the phase transition temperature. X is the terminal group such as –CN, –OR, –R, –CNO, –CF3, –Cl contributing to dielectric anisotropy.

Fig. 2.2: General representation for the structure of liquid crystal

36 Chemistry of Engineering Materials

2.5.1 Rigid Main Chain High rigidity in the backbone structure with high transition temperature may prevent the compound to behave as a liquid crystal polymer. Thus, it is preferred to have polarizable, planar aromatic rings or semiflexible alkylene or alkyleneoxy segments in the main chain.

2.5.2 Semiflexible Main Chain Alkylene, alkyleneoxy and polymethylene fragments which are nonmesogenic, are introduced to impart flexibility in the main chain. Example:

2.5.3 Role of Substituents Substituents to the main chain are the spacers that affect packing (intermolecular spacing) of polymeric chains. Short polar substituents due to strong intermolecular forces make the liquid crystal more stable, than long flexible n-alkyl side chains. An even number of atoms in the spacer causes high transformation temperature and an odd number causes low transformation temperature. Chiral carbon atoms when introduced in the side chain make the molecule optically active.

2.6. PHASE BEHAVIOR Polymers are normally crystalline before the phase transformation temperature or glass transition temperature is reached where the maximum heat flow occurs. Due to polydispersity of the polymers, a broad temperature range is observed for the phase transformation. The substituents on the main chain greatly affect the phase transformation of the polymeric liquid crystal.

Commodity and Engineering Polymers 37

2.7. CLASSIFICATION OF LIQUID CRYSTALS BASED ON THEIR METHOD OF SYNTHESIS Polymeric liquid crystal can be classified as the following based on their method of synthesis (Fig. 2.3). i. Lyotropic liquid crystals: These are heterogeneous substances formed when appropriate quantity of polymer is dissolved in a suitable solvent to obtain a lyotropic liquid crystal. Generally the solute is amphiphatic with a long chain hydrocarbon bearing a polar group at end and the solvent is water which is sandwiched between the polymeric segments. Example: Kevlar

ii. Thermotropic liquid crystals: These are homogeneous, chemically identical and are formed by thermal processes where a suitable polymer is heated above its glass transition temperature to get a semiliquid melt of thermotropic liquid crystal (Fig. 2.3) Example: Vectran

Fig. 2.3: General classification for the structure of liquid crystal

38 Chemistry of Engineering Materials

Short flexible substituents favour nematic phase while long flexible branches make the polymeric liquid crystal smectic.

Nematic Phase In this phase, molecules are parallel to the director but are not oriented in planes and hence has low viscosity, high fluidity and exhibits a high degree of anisotropy. In this phase, molecules can be classified based on optical activity. a. Normal nematic phase: These are optically inactive molecules with 3 to 7C-atoms in the side chain having a long range orientational order are lacking positional order as well. Example 1: p-methoxybenzylidene-p-n-butylaniline (MBBA)

Example 2: p-azoxyanisole (PAA)

Example 3: p-n-hexyl-p-cyanobiphenyl

Example 4: p-quinquephenyl

b. Cholesteric nematic phase: These are optically active molecules arranged in layers with functional groups protruding out of the molecular plane. In this phase, the orientation of each layer can be altered resulting in a twist of one layer relative to the other towards right or left about the axis perpendicular to the preferred molecular direction. Thus, the director of the liquid crystal traces a helical path resulting in a spiral arrangement of molecules capable of rotating the plane polarized light. The Fig. 2.4: Representation of angle of twist is temperature dependent and is twist of molecules referred to as pitch of the crystal which affects the wavelength of color reflected (Fig. 2.4).

Commodity and Engineering Polymers 39

Example: Cholesteryl nonanoate

Smectic Phase Liquid crystal molecules whose side chain has 8 C-atoms and above exhibit this phase, often characterized by well-defined planes with weak interlayer forces. This permits the crystal to exist in various layered configurations differing in macroscopic textures. They are classified based on molecular arrangement in layers (Fig. 2.5). a. Smectic A: Molecules are oriented along the normal layer upright with high flexibility.

Example: Ethyl p-(-p-phenyl-benzalamino) benzoate b. Smectic B: Layers are rigid with highly ordered periodic arrangement.

Example: Ethyl p-ethoxybenzal-p-aminocinnamate c. Smectic C: Molecules are disorganized and tilted away from the director in each layer.

Example: p-n-octyloxybenzoic acid

Fig. 2.5: Classification of various structures of liquid crystals in semetic phase

40 Chemistry of Engineering Materials

Lamellar Phase Molecules are arranged in bilayer sheets with polar ends facing the solvent sandwiched between the layers. Hexagonal Phase In this phase cylindrical aggregates deposited on hexagonal lattice. 2.8 APPLICATIONS OF LIQUID CRYSTALS Liquid crystal display systems make use of the property of molecular twist in liquid crystals. Hence twisted nematic liquid crystals are widely used. When a small voltage is applied, liquid crystals reorient themselves as they have a permanent dipole moment. For nonpolar molecules an electric dipole is induced. The liquid crystals are sandwiched between two polarizer films whose polarization axes are held at 90º to each other (Fig. 2.6). A ray of plane polarized light is irradiated on the first polarizer film. The linear polarized light passing through the first molecular layer of liquid crystal rotates along the molecules and enters the second molecular layer. The twist of light is proportional to liquid crystal molecular twist. When this occurs repeatedly, the light coming out from the second end of polarizer film shows array of colors. Each liquid crystal may constitute a segment of alpha numeric display system used in watches, dash boards, biochemical meters and analytical instruments.

Fig. 2.6: Representation of liquid crystal display system

Commodity and Engineering Polymers 41

2.9 CONDUCTING POLYMERS These are a class of polymers which can intrinsically conduct electricity, without the aid of fillers and are comparable with metallic conductors. Often referred to as conjugated conductive polymers, the intrinsic conductivity arises due to extended conjugation. The overlap of  electrons leads to delocalised electrons which impart conductivity and show promising scientific and commercial use. Same examples of conducting polymers are shown in the illustration below.

2.9.1. Mechanism of Conduction The first conducting polymers to be synthesised was polyacetylene. It is chemically unstable and hence has no commercial significance. The molecule constitutes a long carbon chain with alternating single and double bond. The chain has sp2 hybridised carbon whose two  orbitals above and below the plane of carbon atoms form  bonds which are responsible for delocalisation. The valence electrons in the individual overlapping orbitals produces valence band ( band). The electronic levels of higher energy produces a conduction band ( band). A gap called band gap (Eg) exists between these energy levels whose magnitude depends on overlapping orbitals. Normally for conduction of a polymer, a band gap of 1.0 eV is required. 2.9.2. The Role of Dopants Oxidation of a polymer to generate positively charged conducting polymer with an attached anion or reduction of a polymer to generate negatively charged conducting polymer with an attached cation by addition of a chemical species is called doping. The oxidants and reductants may be added chemically or electrochemically as dopants.

42 Chemistry of Engineering Materials

The anionic dopants like Cl–, ClO4–, BF4–, PF6–, CH3PhSO3– when added is termed p-type doping. Similarly, when a proton or a sodium ion is added, it is termed cationic or n-type doping. i. Anionic doping (oxidation) (p-type) P(py) + LiClO4  P(py)+ClO4– (Anion has received electron from the polymer) ii. Cationic doping (reduction)(n-type) P(Ac)+NaA  Na+P(Ac)– (Electrons are donated to polymer) These electrons or holes introduced to the conduction band to enhance the conduction of the polymer. Example 1: Polypyrrole [P(Py)] Doped polypyrrole [one electron removed] in polaron form or radical cationic form.

Doped polypyrrole [two electrons removed] in bipolaron form or cationic form.

Example 2: Polyaniline

In polyaniline, the pz orbital of N and C rings form a conjugated system that helps the electrons to delocalise. However, the undoped polyaniline has a very low coductivity of 10–8 mho–1/cm. It is often doped by protonating the quinoid nitrogen atom with p-hydroxy benzene sulphonic acid. The protonation leads to bipolar structure which is unstable due to repulsion of adjacent positive charges. The positive charge of one of the hydrogen will attract electrons from the benzene ring, generating a new positively charged nitrogen group which is more stable. This can equilibrate with the nonconducting form of polyaniline depending on the pH.

We can observe that the above structure has two parts, one having a reduced unit (shown as ‘y’ in left most parentheses) and oxidised unit (shown as 1 – y in the right

Commodity and Engineering Polymers 43

most parentheses). If (1 – y) represents the oxidation state varying from 0.0 to 0.1 and n represents the degree of polymerisation then the above structure can be rewritten as

For 1 – y = 0 which is a completely reduced polymer and cannot be further protonated, the structure can be written as

Leuco-emeraldine form or emeraldine base

with 1 – y = 0.5, we have half-oxidised conductive state, can be illustrated as below.

with 1 – y = 1.0, we have completely oxidised state and is not of any practical use as it is a nonconductive form.

When emeraldine form is protonated, it is referred to as emeraldine hydrochloride (a salt). Polyaniline is one of the most

2.9.3 Applications of Polyaniline studied conducting polymers of the past 50 years and found to i. Used in the construction of actuators, superbe an active material for tissue capacitors and electrochromics engineering ii. Used in the manufacture of electrically conducting yarns iii. Used in preparing antistatic coatings and electromagnetic shielding iv. Used in synthesis of flexible electrodes 2.10 HIGH PERFORMANCE FIBRES These fibers are engineered for specific applications that exhibit a high performance in terms of strength, stiffness, heat resistance or chemical resistance. High performance fibers are mechanically and thermally strong with backbone chain composed of conjugated, aromatic planar sp2 configuration which imparts toughness and singly bonded sp3 carbon which imparts flexibility. Their application in composites is highly important and hence sometimes referred to as high performance reinforcement. These fibres include metallic fibers, glass fibers, carbon fibers, etc.

44 Chemistry of Engineering Materials

Example 1: Polyether ether ketone (PEEK)

Example 2: Nomex

2.10.1 Fibres These are polymers whose length is hundred times its diameter and appear as continuous filaments.

Classification of Fibers i. Classification of fibers based on occurrence Natural fibres: These fibers occur as staples which include cotton, wool, flax and silk. Synthetic fibres: These fibers occur as continuous filaments that can be cut to form short staples which include nylon, polyester, polypropylene and acrylic. ii. Classification of fibers based on application Comfort fibres: These fibers are soft, highly flexible fibers with adequate strength, moisture regain and extensibility which can be dyed. Example: Cotton, silk, wool, polyester Safety fibres: These fibers are highly inflammable and nontoxic and generally contain B, N, Si, P, Cl, Br or Sb in the polymeric chain. On burning, they evolve toxic gases. Example: Aromatic polyamides, polyimides, polyoxy diazoles Industrial fibers: Popularly known as structural fibers, possess a high thermal stability, toughness and durability. These are generally used to reinforce articles such as rigid pipes, tyres and reinforcement in plastics. Example: Carbon fibers, silica fibers Kevlar (Poly-para-phenylene terephtalamide) Popularly known as superstrong plastic, Kevlar was first produced by Dupont in 1972 and still continues to be the best known commercial high performance polymer despite its high cost of processing. These fibers are often combined with conventional polymers to form tough composites with high rigidity modulus and thermal stability. Synthesis The fibre is synthesised by wet spinning, where a viscous hot solution of poly-paraphenylene terephthalamide is forced through a metal sieve to generate long, thin fibres.

Commodity and Engineering Polymers 45

Kevlar is commercially available in various grades of purity as K29, K49, K100, K119, and K129. It is observed that the alternate aromatic rings are separated by -NH groups which try to donate electrons, while the carbonyl groups withdraw electrons and negate the effect.

Advantages i. High strength to weight ratio ii. High chemical resistance iii. High tensile strength iv. Negative coefficient of thermal expansion Disadvantages i. Poor compressive property ii. Very stiff and difficult to cut iii. Expensive iv. Absorb moisture Applications Used in making ship hulls, reinforce tyres, hoses, belts, lightweight materials like bulletproof helmets, surf board, gloves, etc.

Carbon Fibers These are synthetic fibers known for their mechanical strength, stiffness, light weight and are composed of carbon. These are capable of exhibiting excellent stability at extreme conditions, thus are used as reinforcement in polymer, ceramic and metal composites. Carbon fibers are generally synthesized by: Step 1: Pyrolysis—controlled thermochemical decomposition of suitable precursors such as rayon or polyacrylonitrile with carbon as the main constituent. Step 2: Oxidation—the prepared carbon fibers are then heated to 200–400°C in air to achieve stabilization. Step 3: Carbonization—noncarbon elements such as hydrogen, oxygen, nitrogen are removed at about 1000°C. Step 4: Graphitization—the fibres are heated along the direction of fiber to 3000°C to achieve higher carbon content and Young’s modulus. Advantages i. Used as a strongest commercial reinforcing fiber ii. Used in medical equipment due to its property of radiolucence iii. Used as reinforcement in making conducting composites

46 Chemistry of Engineering Materials

Disadvantages i. Poor compression resistance ii. Develops defects when subjected to machining iii. Highly expensive Applications Used in making light weight cylinders and pressure vessels, aircraft materials, turbine blades, sporting goods, etc. 2.11 PHOTONIC POLYMERS These polymers possess periodic structures and display unusual properties on interacting with light. These compounds are referred to as photonic band gap materials since their periodic structural features include band gap for propagation of electromagnetic waves with certain frequencies. The band gap is decided by geometry and dielectric constant. Polymer photonic crystal suppresses propagation of light in a specific wavelength with specific directions and hence is referred to as stop bands. The wavelength of light sb that is scattered by a 3D photonic polymer can be related to material periodicity d by Bragg’s and Snell’s law. Ksb = 2d (n2eff – sin2 )1/2 K = order of differentiation neff = effective refractive index of polymer  = angle of incidence of light Any change in temperature, pH, electrical or mechanical strain induces a change in periodicity and thereby eliciting a shift (measurable) in value of sb which makes photonic polymer to be used as sensors. 2.11.1 Polymethyl Methacrylate (PMMA) The polymethyl methacrylate polymer has the adjacent structure. The process of radical polymerization in bulk or suspension yields an optically active polymer.

Synthesis by Addition Polymerization

Commodity and Engineering Polymers 47

Synthesis by Group Transfer Polymerization In this process an organo-silicon initiator 1-methoxy-1-(trimethylsiloxy)-2-methyl-1propene is used as a group transfer initiated in tetrahydrofuran solvent.

This method has an advantage that it imparts control in producing polymeric chains with terminal functional groups without the requirement of very low temperature.

Glass Transition Temperature of PMMA (Tg ) It is the temperature below which a polymer is hard, brittle and above which it is soft and rubbery. The glass transition temperature of PMMA is relatively high with the presence of –CH3 group which hinders the free rotation around C–C bond thereby reducing the chain mobility. Polythene Tg = –125°C Polysterene Tg = 70°C Polymethylmethacrylate Tg = 100°C Degradation of PMMA PMMA undergoes a chain degradation which is reverse of propagation step in chain polymerization. The monomeric units are successively released from the chain end and hence termed as depolymerization or unzipping. Thermodegradation of PMMA PMMA when heated to 300ºC in vacuum depolymerizes in a quantitative yield. The weak bond in the polymer backbone under the influence of heat generates free radicals. Hence PMMA can be easily recycled with 100% recovery of monomers. Photodegradation The polymer in the molten state on UV irradiation yields a quantitative yield of monomer. During degradation, the molecular weight decreases with evolution of CO, CO2, H2 and CH4 developing reddish brown and yellow color with higher radiation dosage.

48 Chemistry of Engineering Materials

Properties PMMAs are colorless, transparent, plastic-amorphous materials, soluble in ketones, chlorinated hydrocarbons and esters, are optically active, and get depolymerized easily. Applications Used in sign boards, lenses, automobile lightings 2.12 ELASTOMERS These are a group of polymers with high molecular weight which undergoes large reversible elongation. These materials exhibit a combination of local mobility and overall rigidity. They have long flexible chain coils with large interchain free volume and weak interchain cohesion, so that they can uncoil and coil accordingly. When some force is applied, it elongates by virtue of its uncoiling and regains its original shape. An ordered arrangement of the chain molecules leads to decrease in entropy. It is the entropy factor which drives the recoiling on release of force. The interchain cohesive force should be low so as not to hinder the free segmental mobility. 2.12.1 Structural Activity Chain flexibility and segmental mobility is achieved by repeated units having C–C and C–O linkages and avoiding chain stiffening causing aromatic and cycling structures in the backbone chain. Cross linking at repeated intervals with nonpolar groups can hold the chain and prevent slipping. Elastomers are popularly known as rubbers, which can be natural or synthetic. Natural rubber is a classic example while polybutadiene, styrene-butadiene and polyflurocarbon, polyurethane Rubber comes from a tree called and silicone rubbers are good examples of synthetic the Hevea brasiliensis which rubber. originated in South America 2.12.2 Natural Rubber or Raw Rubber Polyisoprene is a naturally occurring compound in tropical plants with formula C5H8 and constitutes the natural rubber. It is formed by individual isoprene molecules by 1,4-addition. Natural rubber has disadvantages such as low heat, chemical resistance, low elastic limit and gets deformed which can be overcome by the method of vulcanization.

Commodity and Engineering Polymers 49

Vulcanization It is the process of heating rubber for 1–4 hours with sulphur at 120–150ºC to introduce sulphur linking between polymeric chains to gain higher strength, stability and elasticity. 2.12.3. Synthetic Rubber Siloxanes or silicone rubber Synthetic rubber is an artificial elastomer synthesized from petroleum byproducts. The most prevalent synthetic rubbers are styrene-butadiene rubbers (SBR) derived from the copolymerization of styrene and 1,3-butadiene. Other synthetic rubbers are prepared from isoprene (2-methyl-1,3-butadiene), chloroprene (2-chloro-1,3-butadiene) and isobutylene (methyl propene) with a small percentage of isoprene for cross-linking. Silicones are polymers having alternate atoms of silicon and oxygen in the main chain with organic side groups attached to silicon atoms. These are artificial elastomers synthesized by petroleum products based monomers. Silicone rubber is one of the important synthetic rubbers constituting silicon. Silicon belongs to the same column of the periodic table as carbon and hence tetravalent and forms silane (SiH4). But Si-H bonds are highly reactive. Si-C is relatively stable whereas Si-H bonds undergo displacement reactions. Unlike carbon, silicon cannot form double bonds but forms Si-Si linkage which is thermally stable. Silicon atoms have a close resemblance to C in its properties of catenation and hence are capable of conjugating with oxygen, forming Si-O-Si linkages which are extremely strong. Silicon forms an extremely strong bond with oxygen. The bonds are highly flexible due to the wide bond angle and high electron density on oxygen hence is used as monomers for polymerization. Polysiloxanes are stable to oxygen upto 200ºC, resistant towards acids, bases, have water repelling and insulating properties. Synthesis of Siloxanes Step 1:

Southeast Asian countries are the largest manufacturers of silicone rubber worldwide and and are safe to recycle.

Step 2:

The polymerisation proceeds to yield a large network of polysiloxanes. Cross linking of silicones forming a 3D-network can change its physical state from liquid to wax and rubber. Cross linking of silicones can be achieved by the process of vulcanisation.

50 Chemistry of Engineering Materials

Vulcanisation of silicones Silicone rubber is vulcanised by free radical mechanism using benzyl peroxide as initiator.

Properties i. High temperature of degradation ii. Light weight due to low density iii. Thermal resistance and water repelling iv. High chemical resistance v. Exists as liquid, waxy and rubbery form depending on cross-linking Applications i. Water repellent surface coating ii. Heat resistant laminates iii. Light weight foams iv. Used as silicone oils in hydraulics and transformers due to high viscocity, low surface tension and chemical inertness. v. Used as silicone greases in gears and shafts. vi. Used as silicone resins in paints and coatings where methyl groups in silicone is replaced by phenyl group

Neoprene Rubber (Polychloroprene) Polychloroprene is an easily crystallisable elastomer with chloroprene monomers. Synthesis of neoprene rubber

Vulcanisation of neoprene rubber This process can be brought about with zinc oxide. The resulting cross-linked polychloroprene has excellent tensile strength and are with good oil resistance.

Commodity and Engineering Polymers 51

Applications of neoprene rubber i. Oil resistant insulation coatings ii. Making solid tyres, gloves and soles and industrial hoses 2.13 INORGANIC POLYMERS A polymer whose backbone chain does not contain C-atoms and can overcome the limitations of organic polymers are termed inorganic polymers. The branches may contain hydrogen, oxygen, nitrogen, etc. Majority of synthetic polymers are organic polymers. Disadvantages i. Poor weather and flame resistance ii. Inflexible and brittle iii. Swell when they come in contact with organic solvents iv. Poor biocompatibility with living tissues v. Emit poisonous gases on heating such as polyanylonitrite emitting hydrocyanic acid 2.13.1 Polyphosphazines Phosphazenes are polymers containing phosphorous and nitrogen atoms arranged alternately and can be represented as The substituents X = Cl or NH2, etc. reduces crystallinity and enhance the elasticity. These compounds form extensive —N = P— bonds with phosphorous and nitrogen atoms arranged alternately, possesing high stability. The phosphazines are represented as

with substituents X=Cl, OR, NH2, etc. reducing crystallinity and enhancing the elasticity. Phosphazines, which are also called as phosphonitrilic polymers, exist in cyclic or chain structures with n = 104. They may be classified as mono, di, tri and polyphosphazines. Monophosphazenes: These contain one X3P= NR, unit with X and R = Cl, OR, NR2, Ar, etc. Diphosphazenes: These contain two P = N units, with P = N and P—N bonds having equivalent bond lengths. Triphosphazenes: Trimers cyclize with alternate single and double bonds in P—N—P skeleton leading to a bond length of ~158 pm, forming benzene like six membered planar cyclic structure. However, unlike benzene, the phosphorous atoms are protonable and resists electrochemical reduction. Polyphosphazenes: Phosphazines containing more than two phosphazine groups are termed as polyposphozines. They have a tendency to form cyclic structures.

52 Chemistry of Engineering Materials

Synthesis

Properties i. Unlike rubber, doesn’t swell when in contact with organic solvents ii. Flexible at low temperature iii. Strongly water repelling especially when X = OCH2CF3 iv. Resistant to hydrolysis, oxidation and flame v. Low Tg = –66ºC and flexible upto MP = 240ºC vi. Depolymerise above 240ºC. Applications i. Used in replacing blood vessels ii. Used in fuel line, foams, gaskets, etc. iii. Used for coatings metals iv. Used as a component in asbestos and glass 2.14 BIODEGRADABLE POLYMERS These are polymers which can be broken into small segments by enzymes produced by micro-organisms. In biodegradable polymers, bonds that can be broken by enzymes are inserted into the polymeric chain. Hence, when buried as waste, the enzymes in the soil can degrade the polymer. Generally, synthetic and biologically derived polymers which degrade hydrolytically or enzymatically through structural modifications are termed biodegradable polymers. These biopolymers are intended to interface, support and replace a biological tissue, function or an organ of the body. A material to undertake such activities must prevent inflammatory foreign body reactions and be chemically, physically, mechanically and biologically compatible in vivo.

Commodity and Engineering Polymers 53

These materials are currently being used for implants, in temporary prostheses, tissue engineering, sensors and drug delivery. Implants: Bone screws, bone plates, sutures and staples. Tissue engineering: A 3D thin membrane or multifilament mesh capable of guiding regeneration or transforming the cells into functioning tissues. Sensors: A biocompatible device converting biochemical response to electric signal Drug delivery: Bioactive molecules are entrapped in a polymer matrix appropriately designed to modulate the release of entrapped molecule.

Properties of Biomaterials i. Should not induce toxic response ii. Must possess a moderate shelf life iii. Must undergo a biocompatible heterogeneous degradation with nontoxic products iv. Must have appropriate permeability to certain ions and biomolecules Classification of Biomaterials (Based on the Mode of Degradation) A. Hydrolytically degradable These biopolymers have hydrolytically susceptible bonds in the backbone chain or functional groups. They include Poly (-esters): These thermoplastic polymers with aliphatic ester linkages susceptible to reverse trans-esterification. They can be further categorized as: • Polyhydroxyalkanoates such as polyglycolic acid, polylactic acid, polycaprolactones and polycarbonates. • Polyalkene dicarboxylates such as polybutylene succinate and polyethylene succinate. Among the two classes mentioned above, glycolides, lactides, polycaprolactones and polycarbonate polymers are popularly used. These undergo degradation through hydrolysis taking place first in the amorphous region followed by crystalline region resulting in fragments with depleted molecular weight. Glycolides: Polymers generally referred to as lactones, resulting from condensation of glycolic acid. These bioactive polymers are generally used as carriers in controlled drug delivery.

Lactides: These are lactones resulting from condensation of lactic acid and are widely used for delivery of steroids, antibiotics and vaccines.

54 Chemistry of Engineering Materials

Polycaprolactones: Caprolactones are polymerized using various initiators under inert conditions at 120°C. Anionic intermediates:

Cationic intermediate:

Free radical intermediate

Polycarbonates: These are linear thermoplastic polyesters of carbonic acid with aliphatic dihydroxy compounds. Carbonate linkage being easily hydrolysed, gets eliminated within 15 days of implantation.

Polyphosphazenes: The special features of polyphosphazenes originates due to the presence of N = P. These polymers are hydrolysed to phosphate and ammonia and are highly useful in in-vitro and in-vivo controlled release of drugs. The polyphosphazenes are synthesized by reacting amines polydichlorophosphazene with nucleophiles such as amines, alkoxides and organometallic molecules.

Polyorthoesters: These are bioerodible polymers with an acid sensitive linkage with 3-alkoxy groups attached to one C-atom. Generally, they are synthesized by transesterification process at 180°C with 0.01 torr. On degradation through hydrolysis they yield a diol and -butyrolactone.

Commodity and Engineering Polymers 55

Polyanhydrides: These are highly hydrophobic, unsatable, reactive and hence suitable for drug delivery applications and unsuitable for textile industries. The synthetic route adopts either melt-polycondensation or solution polymerization Synthesis by melt-polycondensation: This method is adopted to obtain high molecular weight polymers in shorter times using co-ordination catalysts such as Zn-(Et)2-H2O or cadmium acetate. This method fails when monomers are thermolabile.

Synthesis by solution polymerization: This method implements Schotten-Baumann technique between diacid chloride and dicarboxylic acid in the presence of a base such as triethylamine.

The main disadvantage of this method is its demand of reactants in high purity, the catalyst to be in finely ground form and byproducts must be continuously removed. B. Enzymatically degradable These biopolymers are degraded by fragmentation due to microbial driven oxidation or hydrolysis under aerobic or anaerobic conditions. Example: Cellulose—degraded by an enzyme endo-1, 4--glucanase Example: Collagen—degraded by enzyme collagenase Example: Poly (L-glutamic acid)—degraded by the lysosomal enzyme C. Bacterial degradable These biopolymers generally having a melting point between 160°C–180°C undergo hydrolytic cleavage of ester bonds by bacterial resulting in surface erosion. These polymers with a very low in-vivo rate of degradation have lucrative applications in drug release. They can dissolve in wide range of organic solvents and can be shaped into films and fibres. Example: Poly (3-hydroxybutyrate) (PHB) Example: Poly (3-hydroxybutyrate-co-3-hydroxyvalerate P (HB-HV) CONCEPTUAL QUESTIONS

1. Refractive index of liquid crystal changes with temperature. Ans. Rise in temperatures leads to rotation of molecules and changes in van der Waals attractive potential across the molecules and polarizability thereby changing its capacity to diffract light consequently changing the refractive index.

56 Chemistry of Engineering Materials

2. Soap is an example of a lyotropic liquid crystal. Ans. Change in concentration changes the phase, which is a typical behavior of lyotropic liquid crystal. 3. Liquid crystals exhibit unusual optical and electrical properties. Ans. The mesogenic phase with molecules possessing directional order causes unusual optical and electrical properties. 4. Decreased glass transition temperature of polymer changes electrical conductivity. Ans. Decreased glass transition leads to decrease in the free volume of the holes and hence there is decrease in electric conductivity. 5. Liquid crystals are highly useful in display systems. Ans. The molecules possess anisotropy. 6. Polyaniline is a good conductor of electricity but polythene is not. Ans. Due to the presence of delocalized polarons and protons in polyaniline. 7. Photoluminescence and cyclic voltammetry are the best techniques to calculate the energy band gap of conducting polymers. Ans. Both techniques help to correlate between the HOMO-LUMO oxidation potential and also the band gap. 8. The donor-acceptor groups efficiently decrease the band gap of a conducting polymer. Ans. Introduction of donor-acceptor groups enhances extension of conjugation and decreases the HUMO-LUMO separation thereby decreasing the band gap in a conducting polymer. 9. Conjugated polymers display extraordinary and unpredicted magneto-optical properties. Ans. As conjugated organic polymers possess an electric field, it can interact with light which also has dual nature of electrical and magnetic properties. 10. Kevlar and glass tend to be used in similar manner. Ans. As both are composed of the same extent of layers, the partial strength is also the same except the weight of Kevlar is less. 11. In photonic polymers, photoluminescence increases with increase in temperature. Ans. At higher temperature, probability of recombination increases and hence a stronger photoluminescence. 12. Carbon fibres have superior fatigue properties compared to metal. Ans. As the only constituent is carbon, it will not wear out as quickly under the stress with constant use. DESCRIPTIVE QUESTIONS

1. 2. 3. 4. 5. 6.

Differentiate lyotropic and thermotropic liquid crystals with suitable examples. Explain mesogenic phase of liquid crystals with suitable example. List the structural requirements for a molecule to exhibit liquid crystal properties. Explain the optico-electrical properties of liquid crystals. List the applications of liquid crystals. Differentiate conducting and photonic polymers.

Commodity and Engineering Polymers 57

7. Elaborate on the role of dopants in conducting polymers. 8. Define fibers. Outline the classification of fibers. 9. Summaries photo-degradation of PMMA with relevant reactions. 10. Define elastomers and outline their classification. 11. Describe the steps involved in the synthesis of siloxanes. 12. Define vulcanization. Explain the mechanism of vulcanization of neoprene. 13. Define biodegradable polymers and classify them based on the mode of degradation. 14. Differentiate between organic and inorganic polymers. 15. Outline the synthesis of phosphazenes. 16. Outline the synthesis of silicones, PMMA, Kevlar and neoprene 17. List the properties of: i. Polyaniline ii. Kevlar iii. PMMA iv. Silicone v. Neoprene vi. Polysiloxanes vii. Polyphosphazenes 18. List the applications of: i. Polyaniline ii. Kevlar iii. PMMA iv. Silicone v. Neoprene vi. Polysiloxanes vii. Polyphosphazenes MULTIPLE CHOICE QUESTIONS

1. The shape of the molecules in liquid crystals are (a) Circular (b) Rod like (c) Irregular (d) Triangular 2. By which the orientation of molecules in a layer of liquid crystals can be changed? (a) Magnetic field (b) Electric field (c) Electromagnetic field (d) Gallois field 3. Electro-optical effect is produced in which of the following? (a) LED (b) LCD (c) OFC (d) OLED 4. Different arrangement of molecules in liquid crystal are (a) Smectic (b) Nematic (c) Cholestric (d) All of these 5. Choose the conducting polymer among the following. (a) Polyester (b) Polythene (c) Polyaniline (d) Polyurethane 6. Choose an example for high performance fibres. (a) Carbon fibre (b) Silicon fiber (c) Kevlar (d) None of these 7. Which one of the followings are not an inorganic polymer? (a) Polyphospazenes (b) Polysiloxanes (c) Polythene (d) Polysilanes 8. Name the composition of inorganic polymers form the following. (a) Carbon (b) Carbon and Hydrogen (c) Si, P, N (d) Only S

58 Chemistry of Engineering Materials

9. Identify the group in the backbone of silicone polymer. (a) Si-Si (b) Si-C (c) Si-O (d) C=O 10. Common name for PMMA is (a) Polyacrylonitrile (b) Polymethylmethacrylate (c) Polyvinyl fluoride (d) Perfluoroalkoxy ethylene 11. Recognise the state of liquid crystals (a) Molten (b) Mesogenic (c) Hydrophobic (d) Vapour 12. Recognise the application of liquid crystals in electrical circuit (a) Viscosity variation (b) Potential failure (c) Temperature change (d) Gas released 13. Silicone is a polymer of (a) Tetraalkyl silane (b) Silicon tetrachloride (c) Dialkyl dichloride silane (d) Silane 14. Identify the reason for polyaniline to be conducting (a) Extended conjugation (b) Molecular weight (c) Stability (d) Linear structure 15. Silocone is referred to as inorganic polymer due to absence of (a) Nitrogen (b) Oxygen (c) Carbon (d) Hydrogen 16. Recognize the polymer used for making contact lenses (a) Bakelite (b) Nylon6,6 (c) PMMA (d) PTFE 17. Which of these is not an inorganic polymer (a) Polyphospazenes (b) Polysiloxanes (c) Polythene (d) Polysilanes 18. Enzymatic degradation of polymers is driven by (a) Oxidation (b) Hydroxylation (c) Both (a) and (b) (d) None 19. The most efficient way to dope polyaniline is by (a) Hydrogenation (b) Protonation (c) Deprotonation (d) Hydration 20. Protonation of polyaniline leads to (a) Bipolar structure (b) Monohydrate structure (c) Amphiphatic molecule (d) None of the mentioned ANSWERS

1. b

2. b

3. b

4. d

5. c

6. c

7. c

8. c

9. c

10. b

11. b

12. b

13. c

14. a

15. c

16. c

17. c

18. c

19. b

20. a

Electronic Materials 59

3 Electronic Materials

3.1 INTRODUCTION Materials can be classified based on energy band theory as conductors, insulators, superconductors and semiconductors, which differ in room temperature and conductivity properties. In all these materials, the atoms overlap with the electronic energy levels generating electronic bands. Conductors: Materials with no forbidden gap and has overlapping valence and conduction band are termed conductors. Hence, they have large number of electron available for conduction and without any additional energy supply acts as carrier of electricity. Example: Al and Cu Insulators: Materials with large forbidden gap in between the conduction and valence band with no possibility of electron to jump from valence band to conduction band and thereby resist the flow of heat, sound and electricity are termed insulators. Example: Diamond with a forbidden gap of 7 eV. They are broadly classified as 1. Electrical insulators: These materials fail to pass electric current through them, due to a very small number of free electrons and are often referred to as dielectrics when used for charge storage. They have resistivity of about 109–1020  cm at room temperature. However, when a large voltage called ‘breakdown voltage’ is applied, they become conductive and polarised depending upon the electric susceptibility. Example: • Ceramics constituting clay • Plastics such as polyvinyl chloride, cresyl pthalate • Glass containing silica, soda ash and limestone • Rubber and oils containing carbon backbone chain • Diamond with a forbidden gap of 7 eV Factors affecting insulators • Temperature: Insulation resistance coefficient decreases exponentially with increase in temperature • Humidity: Humidity is proportional to the insulation resistance • Impurities: Some impurities introduces ions which impart conductivity 59

60 Chemistry of Engineering Materials

Properties of electrical insulators • High impedance to electric current • High breakdown voltage • High air permeability • Resistance to flammability, water, oxygen and chemicals • High mechanical strength, flexibility, shear strength, abrasion resistance with high tear and tensile strength Applications of electrical insulators • Natural dielectric air provides insulation between the overhead transmission lines free of cost • Nitrogen acts as dielectric in electrical capacitor working under high pressure • Electronegative gases such as SF6 and CCl4 used in capacitors and wires are chemically stable and hence act as insulators up to 800°C. • Liquid insulating materials such as silicon oils, vegetables oils and synthetic hydrocarbon liquids have high electric strength and good heat dissipating properties can act as insulating cum heat transfer medium in cables, capacitors, transformers and circuit breakers. • Solid insulating materials such as cotton, silk, paper, polyester, epoxides, mica, fiber-glass and asbestos can act as insulators over a wide range of temperatures in washers, spacers and tubes. 2 Thermal insulators: They resist the flow of heat by conduction, convection and radiation and hence are popularly used to limit heat loss. Example: Organic insulators—air, cork, sawdust, wood Inorganic insulators—slag wool, plastic, asbestos, glass wool and foamed polystyrene slabs. Properties of thermal insulators • Thermal, chemical and dimensional stability • High air permeability and low specific heat • Resistant to vibration and shock • Non-inflammable, water repellent, odourless and lightweight Applications of thermal insulators • Spacecraft: Coating of carbon composites and silica fibre helps the aircraft withstand temperature changes while escaping from atmosphere. • Construction material: Keeps the building warm in severe cold climate. • Clothing: Wool reduces convective heat loss and cotton fabrics facilitate the evaporation of sweat and enables cooling effect. 3. Sound insulators: Materials which resist sound transmission through them are termed sound insulators. Air borne sound insulators and impact sound insulators are popularly used. Example: Mineral wool, rock wool, and fiberglass Superconductors: Materials which can conduct electricity with vanishing resistance at finite low temperatures are termed superconductors. Every material has a critical temperature Tc at which it becomes a superconductor.

Electronic Materials 61

Some practical applications for superconducting devices are cryotron switches, memory storage elements in electronic, computers and superconducting wires wound around solenoid coils generating high field electromagnets. Most materials have very low Tc and hence huge amount of energy must be used in the cooling process thus making superconductors inefficient and uneconomical. Semiconductors: Materials with a narrow forbidden gap in between the conduction band andvalence band are termed as semiconductors. When subjected to high voltage or high temperatures, the electrons jump from valence band to the conduction band and conducts electricity. Example: Silicon with a forbidden gap of 1.1 eV. 3.1.1 Energy Levels and Bands Electrons in the outer shell of an atom are the farthest and referred to as valence electrons which determine the electrical and chemical properties of an atom. When atoms are close together in solids, these electrons are influenced by the nucleus of other atoms. Hence the energy levels occupied by electrons merge into bands. All the energy levels of the valence electrons overlap to produce a valence band while the electronic levels above these levels combine to produce a conduction band. The energy gap separating these two bands is termed forbidden gap, where no electrons exist and is represented as Eg. The energy levels of electrons are measured in electron volts (eV). In insulators, the forbidden gap is huge and hence no electrons are available in the conduction band. In conductors, there is no forbidden gap and valence band merges with conduction band making a large number of mobile electrons available. Electrons in the conduction band are not strongly bound to nucleus and thus vibrate within the material. Electrons in the valence band are under the strong influence of electrons and are not mobile. If the forbidden gap is small, i.e. about 1 eV, the electrons in the valence band can be excited from valence band to conduction band by means of thermal excitation, vibrational excitation or photoexcitation and made mobile and are called semiconductors. 3.2 CONDUCTION IN SEMICONDUCTORS Semiconductors can be insulators at low temperatures and conductors at high temperatures. As they are used in the fabrication of electronic devices, semiconductors play an important role in our lives. These materials are the foundation of modern day electronics such as radio, computers and mobile phones. Conduction results when electrons are moved in a 50 V of static electricity is desired direction by application of the desired voltage. This enough to compromise a may be achieved either by motion of electrons or by hole small electronic device transfer. During motion of electrons, the free electrons in the conduction band move under the influence of applied electric field creating an electric current. When sufficient energy is supplied, these electrons jump from one atom to a hole in another atom. During this process, it leaves a hole behind it. Hence, holes are positively charged particle moving in opposite direction to that of electrons.

62 Chemistry of Engineering Materials

Electrons and holes are charge carriers but only free electrons require less energy than holes to move as they are not strongly held by the nucleus. Hole: It is defined as the gap formed in the absence of an electron. Example: In silicon, it is seen that the outermost shell can accommodate only eight electrons but has only four. Thus there are four holes in Si atom (Fig. 3.1). 3.2.1 Doping The process of adding impurities to intrinsic semiconductors to alter their properties is called as Fig. 3.1: Atomic structure of silicon doping. Depending on the nature of the dopant, semiconductors can be classified as intrinsic and extrinsic semiconductors. 3.2.2 Intrinsic Semiconductors Pure semiconductor materials are known as intrinsic semiconductors. These materials are not of much use as they have a few mobile electrons. To increase the availability of free electrons impurity to atoms are added. This process is termed doping. A doped semiconductor is called as extrinsic semiconductor which is used in device manufacturing. Drawbacks i. Number of holes is equal to number of electrons ii. Small conduction under normal temperature to increase the conduction constant, heating is required iii. When thermal energy is given, all the electrons move from valence band to conduction band and hence the current is controlled by temperature. 3.2.3 Extrinsic Semiconductor A semiconductor into which the impurities are added is called as dopants and when added to modify the conductivity of the material is called extrinsic semiconductor. These impurities introduce new energy levels near the conduction band and valence band. The dopants can be classified as ntype and p-type based on the tendency of the impurity to donate or accept electrons. Fig. 3.2: n-type semin-type: Semiconductor is doped with donor atoms conductor (donor energy) (Fig. 3.2) Example: Pentavalent phosphorus atom is substituted for Si when one of the outer valence electron with phosphorus is not bound to a neighbouring Si atom. The unbound electron can move freely in the material and thus said to introduce a new energy level near the conduction band. Fig. 3.3: p-type semip-type: Semiconductor is doped with acceptor atoms conductor (acceptor energy) (Fig. 3.3).

Electronic Materials 63

Example: Trivalent aluminium is substituted for Si where only 3 electrons exist in the outermost atomic orbital. Thus space is created in US semiconductor companies lead the Si matrix called hole. This is said to introduce the global semiconductor market a new energy level near the valence band. share, accounting for about 51% of the total global semiconductor sales. Energy Bands and Fermi Level In semiconductors and insulators, electrons are confined to a small region represented by a band and are forbidden from the rest. The energy difference between valence and conduction bands across which the electrons would be able to jump is termed band gap. The energy bands in a semiconductor are denoted by Fermi levels EF, it is the energy level where probability of occupancy of electron is the maximum.

3.3 FERMI LEVEL IN INTRINSIC SEMICONDUCTOR For an intrinsic semiconductor with no impurities, the concentration of electrons equals to the concentration of holes. The intrinsic carrier concentration in equilibrium is called ni, where n = p = ni or np = ni2 The Fermi level in intrinsic material is referred to as intrinsic Fermi level and is represented by EF = (Ec + Ev)/2 3.3.1 Fermi Level in Extrinsic Semiconductor Case 1: Fermi level in n-type extrinsic semiconductor—when the n-type impurity is added, a large number of electrons are added and a new energy level called additional donor energy level ED is formed near the conduction band. Fermi level now shifts towards the conduction band, i.e. above intrinsic level ni. Mathematically, we can determine the electron concentration in the conduction band by integrating the product of two functions over the full range of energy levels. Function 1: Number of available states at a given energy is called density of states D(E). Function 2: Probability of an energy state being occupied within a distinct range of values is called as distribution function f(E). Thus, electron concentration n is given by n=

 D(E) f (E) DE

Applying Pauli’s exclusion principle, we have the density of state

1 2  2m     2  h2  Fermi-Dirac distribution expression,

3/2

D(E) =

f(E) =

E

1 ( E  E f )/kT

1 e Substituting and simplifying, the effective electron concentration in valance band is given by   Ec E f  n = Nc e KBT where Nc = effective density of states in conduction band.

64 Chemistry of Engineering Materials

and electron concentration in conduction band is given by 

 E f  Ev



KBT

p = Nv e where KB is Boltzmann constant, T is the absolute temp of intrinsic semiconductor, Nv is effective density of states in valence band. For the n-type material n > p (Ec – Ef) > (Ef – Ev) or Ec – Ef < Ef – Ev Case 2: Fermi level in p-type extrinsic semiconductor—here, the acceptor impurities are added and hence a new energy level namely acceptor energy level EA near valence band is formed. Now Fermi level shifts towards valence band below the intrinsic level ni. The effective electron concentration in valence band is given by n = ni eEf – Ei/KBT or Ef = Ei + KBT ln [n/ni] The effective electron concentration in conduction band is given by p = ni eEi – Ef /KBT or Ef = Ei + KBT ln [p/ni] The total electronic concentration in a semiconductor is given by np = NcNv e [–(Ec – Ev)/KT] np = NcNv e [–(Eg)/KT] where, Nc and Nv are densities of states, Eg is the energy gap between valence band and conduction band. 3.4. E-K DIAGRAMS It is a plot between kinetic energy vs momentum, which serves as a pictorial representation of electric potential, possible state, wave function allowed to exist in a semiconductor (Fig. 3.4). These potentials will restrict the movement and energy of the electron. Any mass is associated with kinetic energy E = 1/2 mv2 and momentum P = mv. Similarly electrons possess a momentum K and energy E, thus E vs K is a dispersion diagram. But K is an electronic wave vector represented by k(x) which

Fig. 3.4: E–K diagram

Electronic Materials 65

changes its direction or coordinates with the change in momentum of an electron during its journey from valence band to conduction band or vice versa. The E–K curve for any semiconductor consists of many discrete points corresponding to a possible state for the electron in a semiconductor. Each point on the curve corresponds to a particular state and the points are so close that we normally draw E – K relationship or a continuous curve, comprises many discrete points each corresponding to a possible state. But the points are so close that they appears as a continuous curve. 3.5 BAND GAP ENGINEERING The range of energy where the electron states do not exist is called a band gap. The process by which this range of energy is modified to obtain desired electronic properties is called ‘band gap engineering’. This is typically done to semiconductors by controlling the composition of alloys or constructing layered materials with alternating compositions. Example: Consider two compound semiconductors, gallium arsenide (GaAs) and aluminium arsenide (AlAs). They can be combined at the crystal growth stage to form the alloy semiconductor Al-GaAs which has better semiconducting properties. 3.6 APPLICATIONS OF SEMICONDUCTORS IN OPTOELECTRONIC DEVICES Devices which can convert light into an electric signal or vice versa through the lightmatter interactions are called optoelectronic devices. These are also termed photocells, which can sense the presence or absence of light. These devices are also used where the intensity of light needs to be measured. In all these devices, a flow of current results in irradiation of light. However, the thermally generated minority carrier results in a generation of small electric current even in the absence of irradiated light, called the “dark current”. pn-junction is the active site where the electronic action of devices takes place. These are building blocks of many devices such as diodes, transistors, LED and solar cells. In a semiconductor material, n-side consists of excess of electrons and p-side consists of excess of holes and pn-junction is a boundary or interface between these two types. After joining, electrons from n-region diffuse to p-region leaving behind positive charges. Similarly, holes receive electrons and become negatively charged, thus at the interface a neutral region is formed called as the ‘depletion layer’. Biasing is the process of connecting semiconductor to a potential difference. Forward bias Positive terminal of battery is connected to the p-type and –ve terminal to n-type. This establishes an easy path for the flow of current called as forward current, due to majority carriers (Fig. 3.5). Reverse bias n-type material is connected to +ve terminal and p-type material is connected to –ve terminal. The potential difference increases the strength of the barriers which prevents the charges to move across the junction. This gives a high resistive path to the flow of current through the circuit and the magnitude of resistance it offers is called as ‘reverse current’ (Fig. 3.6).

66 Chemistry of Engineering Materials

Fig. 3.5: Representation of forward bias

Fig. 3.6: Representation of reverse bias

However, some electrons flow from n-type to p-type against the potential applied, called the minority carriers. 3.6.1 Light Emitting Diode (LED) These devices work on the principle of electroluminescence. It is forward biased pn-junction diode which on energizing can emit visible light (Fig. 3.8). When a junction diode is forward biased, free electrons exist in conduction band and hole in valence band with electrons from n-side move towards depletion region and combine with the holes. The free electrons with higher energy are moving from high energy conduction band to low energy holes. While moving, the energy is released proportional to semiconductor band gap in the form of visible light radiation. Generally mixtures of gallium, arsenic and phosphorous are used in LED. The wavelength or colour of the light depends on the forbidden energy gap. Eg = hƒ = hc/ where

 = hc/Eg

Various kinds of LEDs i. GaAs LED: Emits red and IR light ii. AlGaP LED: Green light

Electronic Materials 67

iii. GaN LED: Bright blue light iv. GaP LED: Yellow light Construction: n-type layer is grown on a substrate followed by p-type (Fig. 3.7). The light escapes from outer edge of p-type layer. This is symbolically represented as

Fig. 3.7: Construction of light emitting diode

LED in 7-segment Display Seven LEDs are arranged in a rectangular fashion where each LED is called a segment as it forms a part of the digit being displayed. By forward biasing for different LEDs, we can display the digits from 0 to 9. Depending upon the segment to be displayed, a particular set of LEDs are forward biased. Advantages i. Brightness can be modulated by varying the current ii. Consumes low energy and inexpensive iii. Lightweight, compact and fast operating iv. Operates quickly and have long lifetime v. Ecofriendly Applications i. Numeric displays, calculators and traffic signals ii. Aviation lighting and camera flash iii. Burgler’s alarm sensors and autocouplers 3.6.2 Solar cell It is a voltage producing semiconductor device often referred to as photovoltaic cell that can convert light energy to electrical energy by photoelectric effect. Example: Arsenide, indium, cadmium, silicon, selenium and gallium Photoelectric effect Photons when incident on semiconductor energy system ejects electrons called photoelectrons, and the phenomenon is called as photoelectric effect (Fig. 3.8). Construction A photovoltaic cell consists of forward biased pn-junction. These kinds of interconnected solar cells constitute a solar panel on solar array. When light is incident upon a material, it promotes the electrons in valence band to conduction band creating a hole in the valence band. The vacant site and the promoted electrons are referred to as electron hole pair. The electrons liberated generates a DC current which later can

68 Chemistry of Engineering Materials

be converted into 240 V AC current by an inverter, when a device which makes use of current or which can store is connected to the solar cell. Advantages Clean nonpolluting renewable long-lasting, long-lifetime with little maintenance. Disadvantages Cannot work at night, expensive installation and requires large space.

Fig. 3.8: Forward biased photovoltaic cell

3.6.3 Phototransistors It is a 3-layered semiconductor light sensitive device which converts light energy into electrical energy, used in circuits to amplify the signals. A phototransistor is an npn or pnp transistor where light is irradiated at pn-junction and forward current is triggered and amplified. The region from where electrons flow is emitter and towards which it moves is collector. Junction JE is forward biased and JC is reverse biased. The radiant energy causes the excitation and increases the mobility of electrons, hence the potential difference between the emitter and collector increases. This is symbolised as in Fig. 3.9. The resulting flow of current is termed as base or collector or photocurrent proportional to luminance. The flow of current depends on wavelength of incident light and the area of base (Fig. 3.10). Advantages i. High current gain ii. Immune to noise interference iii. Economical and simple Disadvantages i. Poor conversion efficiency ii. Very slow iii. Conversion is poor at high frequency light

Fig. 3.9: Phototransistor symbolised

The resulting flow of current is termed the base current or collector or photocurrent which is proportional to luminance. The factors affecting the flow of current depend on the wavelength of incident light and area of base (Fig. 3.11).

Electronic Materials 69

Fig. 3.10: Representation of phototransistor

Fig. 3.11: Graphical representation of variation of current with varying voltage generated by radiant energy

3.6.4 Photoconductors In a semiconductor, the conduction band is partially filled at room temperature. If light is irradiated, covalent bonds are cleaved leading to the formation of electrons and holes. These act as current carriers causing transitions from valence band to conduction band. Photoconduction can happen in two ways: i. Intrinsic excitation: In an intrinsic semiconductor, photons release an electron hole pair and both contribute to the current. ii. Extrinsic excitation: In an extrinsic semiconductor, only one type namely the electrons for n-type and holes for p-type are generated. This is also termed impurity excitation. Cutoff wavelength The minimum energy required for excitation is equal to the energy gap of the semiconductor (Eg) Eg = h Eg = hc/ or wavelength () is too high, energy is too low then no transition from valence to conduction band occurs. Hence, every material has a cutoff threshold wavelength for photoconduction.

70 Chemistry of Engineering Materials

For

Si, Eg = 1.1 eV;

For Ge, Eg = 0.72 eV;

at = 1.13 m at = 1.73 m

Advantages i. Can be used to work with UV, visible, IR and X-rays ii. High reliability and stability Disadvantages Cooled by expensive liquid N2 to minimise noise. 3.6.5 Photodiodes Semiconductor devices convert light to electric current, nearly similar to photovoltaic cells but are tailored to work as reverse biased pn-junction with light falling on the depletion layer of the junction (Fig. 3.12). These are designed to work in reverse biased because when reverse biased, minority charge carriers which are less in number control the current flow (Fig. 3.13). Thus light generated carriers are significant to reverse current. Whereas when forward biased, applied voltage takes control over the current and existence of majority charge carriers subsides light generated charge carriers and the effect of light becomes negligible. Construction A lens is fixed over the junction to focus the light. When radiation hits the depletion layer region, electrons move towards n-region and holes towards p-region generating current Ip (Fig. 3.14). The more the intensity of light, more is the number of electronhole pairs and more is the photocurrent. Since, electrons are –vely charged, n-region is connected to +ve end. Thus, they are attracted to n-region. The intensity of light is measured in Lumen meter per square meter (Lm/m2). However, in the absence of light, a small amount of thermally generated current called dark current IO is observed. Therefore, on illumination I = IP + IO If IO is reduced, sensitivity of the device increases. Advantages i. Low noise and lightweight ii. Low cost and compact iii. 60% to 80% quantum efficiency iv. Excellent linearity of output current as a function of incident light Disadvantages i. Does not work for long distance ii. It is only used as a light sensor and not as a source of power

Fig. 3.12: Symbol of photodiode Fig. 3.13: Reverse biased pn-junction

Electronic Materials 71

Fig. 3.14: Representation of generation of photocurrent

iii. Poor temperature stability iv. Amplification may be necessary due to low current intensity Applications i. Used as photodetectors due to fast operation ii. Used in alarm systems and optical receivers iii. Used as a cell and variable resistance device iv. Used in high speed logic circuits 3.7 SAFE DISPOSAL OF ELECTRONIC MATERIALS e-Waste represents 2% of

Electronic waste or e-waste refers to the electronic America’s trash in landfills, products such as cathode ray tubes (CRTs), printed but it equals to 70% of overall board assemblies, capacitors, mercury switches and toxic waste relays, batteries, liquid crystal displays (LCDs), cartridges from photocopying machines, selenium drums (photocopier) and electrolytes that have It takes 530 lb of fossil fuel, 48 lb become unwanted, nonworking or obsolete, and have of chemicals, and 1.5 tons of water essentially reached the end of their useful life. These to manufacture one computer and monitor materials are disposed by conventional landfill or incineration methods. The following table lists some of the toxic substances in the electronic products. Electronic materials 1. 2. 3. 4. 5. 6. 7. 8. 9.

Circuit boards Printed circuit boards Monitor of cathode ray tubes (CRTs) Flat screen monitors Computer batteries Capacitors and transformers Plastic casings and insulation Cooling units Older photocopy machines

Toxic constituents Pb and Cd Brominated flame retardants and antimony oxide Lead oxide and Cd Mercury Cadmium Polychlorinated biphenyls Dioxins and furans Chlorofluorocarbons (CFCs) Selenium (Se)

72 Chemistry of Engineering Materials

Disposing of gadgets and devices can affect the environment in the following ways: 1. Contaminates the soil 2. Leaches into the water bodies and contaminates the underground water 3. Occupational exposure to toxic substances Some chemical processes adapted to process electronic waste component: 1. Used electronic components are desoldiered 2. Chemical stripping by acid bath and burning of removed chips 3. Chemical stripping of elements The ecofriendly waste disposal techniques that can be adopted are: 1. Batteries must be removed from the gadgets before disposing 2. Gadgets must be upgraded rather than replacing 3. Encourage recycling 4. Use ecofriendly materials in electronic gadgets 5. Raise awareness of the impact of e-waste pollution 3.8 BIOIMPLANTS Biosynthetic material embedded into a living system to support, control or regularize physiological functions. With the progress in medical technology, bioimplant designing and manufacturing has received immense importance. Bioimplants have a great impact on treating degenerative and inflammatory diseases and improving the life span. These are materials introduced into the living system, made of materials that are accepted by living tissues to replace, supplement, stabilise or to support the healing process of a damaged part of the body (Table 3.1). Table 3.1: Criteria for a material to be used as bioimplant Compatability

Mechanical properties

Manufacturing

Must not induce adverse tissue reactions Minimum bone resorption Chemically inert to physiological environment Must have fine texture

Elasticity

Easily available raw materials

Ductility Fatigue strength

Withstand sterilization procedures Inexpensive

Corrosion resistance

Easy fabrication procedure

Classification Bioimplants can be classified based on composition as: i. Metal bioimplants Example: Stainless steel used in joint prostheses (Co-Cr-Mo) ii. Ceramic bioimplants Example: Hydroxyapatite [Ca10 (PO4)6(OH)2], Ca3(PO4)2 iii. Polymeric bioimplants Example: Polydimethylsiloxane, polytetrafluoroethylene iv. Tissue engineered bioimplants Example: Artificial collagen

Electronic Materials 73

3.8.1 Metal Bioimplants Metal implants are used to replace or stabilise bone tissues generally in the form of plates, rods and screws. Metals having high modulus yield point and ductility make them suitable to be used as implants and they can bear large load without large deformations. Metallic implants are classified based on usage: i. Prosthesis devices: These devices are to replace some portion of the body such as joints, long boxes and skull plates. ii. Fixation devices: These devices are temporary implants which join two pieces of tissues together and stabilize it such as bone plate, rods, screws and sutures. The metals used for implants include stainless steel, cobalt based alloys and Ti-based alloys. Many metals such as Fe, Cr, Co, Ti, Ni, Mo and W can be tolerated in minute quantities also. These implant materials are corrosion resistant because of the formation of oxide on hydroxide layer on the surface. Some of the popular metal bioimplant alloys are discussed below. Stainless steel It is a predominant implant alloy due to its excellent mechanical and corrosion behaviour. Vanadium-steel-chromium, steel-nickel-steel and manganese-steel are the alloys of steel which are typically used in the implants. Cr and Mo imparts corrosion resistance while Ni acts as a stabilizer. Lowering the carbon content improves corrosion resistance. Example 1: 316-L steel. 316-L steel Since 1950, improved steel called 316-L steel where the carbon content is reduced from 0.08% to 0.03% and 11% molybdenum is added. It cannot be hardened by heat treatment but can be hardened only by cold working. It is nonmagnetic and highly corrosion resistant and hence recommended by American Society for Testing and Materials (ASTM). Surface modifications like anodising and passivation decreases the rate of corrosion also. However 316 L may corrode inside the body under high stress, oxygen depletion and hence used for temporary implants. Example 2: 18Cr–8Ni steel: Has good corrosion resistance and longevity. Stainless Steel: Compositions of 316-L Element

Composition (%)

Carbon

0.03 max

Manganese

2.00 max

Phosphorus

0.03 max

Sulfur

0.03 max

Silicon

0.75 max

Chromium

17.00–20.00

Nickel

12.00–14.00

Molybdenum

2.00–4.00

74 Chemistry of Engineering Materials

Mechanical properties of 316-L stainless steel for implants Condition

Ultimate tensile strength, min. (MPa)

Yield strength (0.2% offset), min. (MPa)

Elongation 2 in. (50.8 mm), min(%)

Rockwell hardness

Annealed Cold worked

485 860

172 690

40 12

95HRB —

Types of stainless steel alloys in use Devices

Alloy type

Jewitt hip nails and plates Intramedullary pins Mandibular staple bone plates Heart valves Stapedial prosthesis Mayfield clops (neurosurgery) Scwhartz clips (neurosurgery) Cardiac pacemaker electrodes

316L 316L 316L 316 316 302 420 304

Stainless steel used in implants is of 2 types: Wrought steel

Forged steel

Uniform microstructure with fine grains Hard and brittle cannot be used for high tension application Low mechanical strength Undergoes hot or cold working

Highly ductile and flexible Highly tough and used for high tension applications Used as cashing with high mechanical strength Used after hammering with hydraulic hammers that forces atoms and molecules of steel into a particular alignment

Representation of stress and strain in different materials are given in Fig. 3.15.

Fig. 3.15: Representation of stress and strain in different materials

Electronic Materials 75

Cobalt Chromium Alloys Cobalt is a transition metal placed between Fe and Ni with atomic number 27. It is soluble in acids and is passivated by strong oxidising agents. Cobalt exists in two allotropic forms: E-phase: Closely packed hexagonal form till it reaches 417°C. -phase: Face centred cubic form about 417°C to MP and 1495°C. The density of Co is 8.85 g/cc3 at room temperature with Young’s modulus of elasticity 220 to 234 GPa which is higher than most grades of steel. Metallic cobalt in the pure form is not particularly ductile and corrosion resistant hence chromium is added in calculated quantity as the basic elements forms a solid solution upto 65%. It produces finer grains resulting in highest strength and enhances corrosion resistance. The two types of cobalt chromium alloys used are: i. Co-Cr-Mo: Co exists in hcp lattice form and Cr in bcc lattice form and hence a low density alloy. ii. Co-Ni-Cr-Mo: The high tensile strength of this composition is highly popular with its increased strength and decreased ductility. Cobalt chromium alloys for medical implants are used in the following forms: • Cast alloy is a material obtained by forcing molten metal under high pressure into a mould cavity. However, these alloys have large grains metallurgical imperfections and leads to low mechanical strength. The heat treatment also provides only limited benefits but the properties can be improved by isostatic pressing. Cast alloy is generally made by investment casting. Investment casting: It is a wax model of implant, built with ceramic shell around the wax model. The wax model is melted away so that the ceramic shell has the shape of the implant to which metal is poured. The ceramic shell is removed to obtain a metal implant. • Wrought alloy is an alloy processing a uniform microstructure with fine grains and can be further strengthen by cold work. • Forged alloy is produced by a process called as hot forging process. Advantages of Co-Cr alloys i. Good corrosion resistance ii. Modulus of elasticity of these alloys are higher than steel iii. Low wear compared to other alloys Disadvantages of Co-Cr alloys The particulate of Co and Cr in large quantities is toxic to human tissues and inhibits the synthesis of Type 1 collagen. However, Cr is less toxic than Ni and Co Applications of Co-Cr alloys i. Co-Cr alloys are used in dentistry or as an alternative for too expensive gold alloys ii. Used in fabrication of artificial heavy loaded femoral hip joints iii. Porous coated Co-Cr implants are used in bone in growth applications iv. Gravity sintered Co-Cr alloy pads are extensively used in orthopaedic implants

Titanium Based Alloys Titanium is used in metal implant for its lightness and flexibility with density of 4.505 kg/m3 at 25°C with excellent mechano-chemical properties, its ability to form a

76 Chemistry of Engineering Materials

stable solid oxide layer of TiO2 in-vivo. There are 4 grades of unalloyed commercially available Ti for surgical implants. Element

Grade 1

Grade 2

Grade 3

Grade 4

Ti6A14V

Nitrogen Carbon Hydrogen Iron Oxygen

0.03 0.10 0.015 0.20 0.18

0.03 0.10 0.015 0.30 0.25

0.05 0.10 0.015 0.30 0.35

0.05 0.10 0.015 0.50 0.40

0.05 0.08 0.0125 0.25 0.13

Titanium exists as an allotropic material. i. -form: Closely packed hexagonal crystal structure with c/a = 1.587 ii. -form: Body centered cubic structure The addition of alloy to Ti gives a wide range of properties. The main alloying elements for Ti are aluminium (6%), vanadium (4%) and nickel (8%). Titanium alloys are generally available in two forms: Wrought form is obtained by annealing at 730°C for 1–4 hours and air cooled to room temperature. Forged form requires processing before usage as Ti alloys are very reactive. Commercially available pure form of Ti is very dilute than titanium oxygen alloy. Oxygen is soluble in -titanium at room temperature to about 14 weight% forming a solution with single phase. The oxides of titanium used in medical implants undergo one of the following treatments where the surface is modified: i. Oxide layer may be enhanced by suitable treatments like anodizing ii. Surface can be hardened by diffusion of interstitial atoms into surface layer iii. Flame spraying of metal oxide onto the surface iv. Other metals may be electroplated. The British standard specification for Ti-alloys for medical implants is listed below. Elements

Composition

Nitrogen Carbon Iron Oxygen Aluminium Vanadium Other Elements Titanium

0.05 max 0.08 max 0.25 max 0.18 max 5.5–6.5 3.5–4.5 0.1 Remainder

Aluminium It tends to stabilize the -phase and increases the transformation temperature from - to -phase. Aluminium content gives good mechanical strength and oxidation resistance. -alloy has a single phase microstructure promoting good weldability and produces local stream field capable of absorbing DE formation energy. -face cannot be strengthened by heat treatment.

Electronic Materials 77

Vanadium It tends to stabilize -phase resulting in a 2-phase system. When vanadium reaches 13%, it can be strengthened by heat treatment. The commercially available alloy Ti6Al4V is popularly known for its sterility and high mechanical strength. Nickel The Ti-Ni alloy called nitinol is composed of equi-atomic intermetallic compound with highly symmetric cubic crystalline arrangement which is thermodynamically stable at 650°C. As the alloy is cooled, the coordinated atoms gets displaced and metastable phase of law of crystal symmetry called martensite is formed. The equiatomic Ti-Ni or Ni-Ti shows special properties such as shape memory effect, and superelasticity. Two main additional properties possessed by metal implants are discussed below. Shape memory effect is the phenomenon where the default material can gain back its previous shape at room temperature. This property arises due to diffusion-less phase transformation caused by molecular ordering and parent and martensitic phases. This property makes it highly useful for orthodontic dental archwire, orthopaedic staple, rapid wedging or separation of teeth. Superelasticity is also called pseudoelasticity, where the metal springs back to its original shape when they formed slightly above its transformation temperature. This is a reversible response to stress caused by phase transformation and instability in structure. This property is used in making vascular stent and catheter wires. 3.8.2 Ceramic Bioimplants These are mechanically strong and chemically inert materials composed of bioacceptable mineral crystal molecules. They are known to have excellent compatibility as they do not release ions during degradation. The most popular ceramic bioimplants are: i. Alumina (Al2O3): Highly compressive and inert but has a low tensile strength and are highly brittle. It is highly biocompatible due to its lubricating properties and high hydrophilicity, thereby gets coated by a thin layer of water in-vivo. ii. Hydroxyapatite [Ca10(PO4)6(OH)2]: The implanted hydroxyapatite binds to the bone forming a monosystem promoting osseointegration. Hydroxyapatite requires sintering at 800°C and generally demonstrates gradual degradation. It is widely used in fabricating artificial bone replacement and bone defect filler. Ceramic bioimplants can be classified based on properties as follows: i. Bioinert ceramic bioimplant Example: Alumina [Al2O3], zirconia [ZrO2] Biologic reactivity to specific ii. Bioactive ceramic bioimplant materials is species-, site-, and Example: Bioglass [Na 2O.CaO.P2O 3.SiO], time-dependent. In humans, it can vary substantially from one Hydroxyapatite [Ca10(PO4)6(OH)2] individual to another. iii. Biodegradable ceramic bioimplant Example: Carbonateapatite [Ca10(PO4)6CO3], tricalcium phosphate [Ca3(PO4)2]

78 Chemistry of Engineering Materials

3.8.3 Polymeric Bioimplants Many organs of the body such as skin, tendons, cartilages and blood vessels are composed of natural polymers and have properties resembling synthetic polymers. Some examples of polymers with biological applications are listed in the table below: 1. 2. 3. 4. 5. 6.

Polymer

Properties

Applications

Polymethyl methacrylate Polyethylene Polytetrafluoroethylene Polyvinylchloride Polydimethylsiloxane Polyurethane

Hydrophobic, transparent Low density, and flexible Thermal and chemically stable Mechanically soft Highly flexible and inert Semicrystalline and low Tg and hydrophobic

Intraocular lenses Catheters Vascular graft material Transfusion tubes Insulator for pace maker Phase segregating membrane

3.8.4 Tissue Engineering The synthesis of a biocompatible material using natural tissues and artificial methods under suitable biochemical and physicochemical factors to improve or replace biological tissues is termed tissue engineering. This process includes extracting necessary tissues, culturing them under suitable conditions, imparting biological functions and transplanting it at the necessary location. Example: 1. Extracellular matrix synthesis 2. Protein synthesis 3. Collagen synthesis

Ancient scriptures report art of malleable gold plates to repair skull defects. Overwhelming majority of materials used in medical devices and implants are standard commodity substances developed for clinical laboratory standards.

3.8.5. Biocompatibility It is the ability of a material to display nontoxic harmonious contact with tissues and body fluids around medical implants. The surface tissue interaction takes place at different stages depending on the physical nature and chemical composition of surface of the implant which causes different types of bonding to biomolecules. During the first few seconds after implantation there will be only water, dissolved ions and free biomolecules around the implant. As inflammation proceeds, the composition of adsorbed biomolecules changes and finally a fibrous capsule is formed around the implant. Depending on the interfacial response of tissues, biomaterials can be classified as. Type 1: Inert (smooth surface)—does not react with surrounding tissues after being placed in the living system. Example: Titanium and zirconia. Type 2: Nearly inert (microporous surface)—exerts minimum interaction with surrounding tissues with a very low rate of degradation. Example: Aluminum and stainless steel.

Electronic Materials 79

Type 3: Controlled reactive surface—these contain immobilized bioactive species on surface that trigger specific cell signalling by adhesion, proliferation, and differentiation. Example: Chitosan nanofibers Type 4: Resorbable—porous materials that can absorb, assimilate and reabsorb which is generally used in bone graft substitutes. Example: Calcium sulfate hemihydrate or plaster of Paris 3.9 BIOSENSORS The name bio refers to a biological element such as enzyme and sensor refers to sensitivity to such elements. Both bio and sensor must be bound to each other by membrane entrapment, physical adsorption, van der Waals force, hydrophobic forces, hydrogen bonds, ionic forces, matrix entrapment or covalent bonding to obtain the response in the form of a signal. Definition: A biosensor is defined as a measuring device that includes a probe with a detecting material made of biological molecules and a transducer to detect the presence of chemicals. A device comprising receptor and transducer is capable of providing selective quantitative analytical information using a biological recognition method or element. Biosensors provide rapid real time accurate and reliable information about the analyse

Fig. 3.16: Working of a biosensor

80 Chemistry of Engineering Materials

without disturbing the analyte. Biosensor consists of three basic components: bioreceptor, transducer, and microelectronics (Fig. 3.16). i. Bioreceptor is a biological component of all the molecular species which binds to the analyte and utilizes biochemical reaction for recognition. Example: Protein, enzyme, antibody, receptor, and nucleic acid, etc. ii. Transducer is the part of biosensor that converts the biochemical reaction response to detectable signals. This is called transduction. iii. Microelectronics is a part of biosensor which consists of microchips and micro circuits that helps and decoding the signals and converting it to a graphical output. 3.10 CLASSIFICATION OF BIOSENSORS Biosensors can be classified based on the transduction methods that are employed. 3.10.1 Optical Biosensors Optical biosensors, exploit the interaction of an optical field with biochemical reaction response and produces a signal proportional to the concentration of the analyte using calorimetric, fluorescence or luminescence interferometry. Construction The biological component like enzyme or a receptor is immobilised on a gold foil. The analyte is introduced by microfluid equipment. Now the light is irradiated and the intensity or reflection or transmission is measured which is proportional to analyte-receptor interaction. 3.10.2 Resonant Biosensor An acoustic wave transducer is coupled with a bioelement (antibody or enzyme) to be detected or quantified. The change in mass changes the resonant frequency of the transducer and the change in frequency is then measured. 3.10.3 Electrochemical Biosensors These biosensors report the changes in the form of electrical signal caused by the binding of the analyte and the biological component of the sensor. The output may be given as current or voltage (Fig. 3.17).

Fig. 3.17: General representation of electrochemical biosensors

Construction A typical electrochemical sensor comprises sensing or working electrode, reference electrode and counter electrode respectively. The system is dipped in an analyte where the binding of the analyte leads to ionic discharge measured in the form of current or voltage. The variations in electrochemical biosensors are: i. Conductometric: The binding of the biocomponent and bioreceptor produces ions or electrons, and thus the conductivity or resistivity of the solution changes. ii. Potentiometric: The binding of the biocomponent and bioreceptor changes the concentration of the ions setting up the potential on the indicator electrode.

Electronic Materials 81

3.10.4. Mass-based Biosensors These biosensors reports the response to a change in mass caused by the binding of the analyte and the biological component (Fig. 3.18). The schematic below shows the biomolecular response to molecules and their mechanism.

Fig. 3.18: Measurement of change displayed as a change in mass

3.10.5 Enzymatic Biosensors If the enzymes are used as bioreceptors, they are called enzymatic receptors. Example: Enzyme based glucose sensor 3.10.6 Thermal-detection Biosensors The heat of the reaction in binding of biocomponent to the bioreceptor changes the temperature of the medium which is compared to the initial temperature of the analyte. Thermal detectors can also be used to detect pesticides and pathogenic bacteria. Example: Molecularly imprinted polymers to detect biotin 3.10.7 Ion-sensitive Biosensors The presence of the ions can be detected, as they diffuse through a semipermeable membrane causing a potential difference across the barrier. Example: Ta2O5/ZnO bilayer to detect -D glucose 3.11 GLUCOSE BIOSENSORS Diabetes or hyperglycemia is a chronic disease where blood glucose increases. Many biosensors are used which can link themselves to insulin and monitor the glucose levels. 3.11.1 Enzymatic Glucose Biosensors Glucose oxidase enzyme based glucose sensor is a biocatalyst based amperometric biosensor. Working Glucose biosensor is based on the fact that the enzyme glucose oxidase catalyses the oxidation of -D-glucose by molecular oxygen producing gluconic acid and hydrogen peroxide. This process reduces the cofactor associated with the enzyme which is later restored by oxidation process coupled with reduction of molecular oxygen to H2O2.

82 Chemistry of Engineering Materials

In this process, the active sites of the cofactor are converted to inactive reduced state. But again the enzyme is activated by transferring the electrons to the molecular oxygen leading to H2O2 production. H2O2 is proportional to the analyte (glucose). Mechanism Glucose oxidase Gluconic acid + H 2 O2 Step 1: Glucose + H2  O2  

Step 2: H2O2  2H+ + O2 + 2e– The concentration of analyte is determined by the measurement of current I by the following equation. I = K.Ke {d[H2O2]}Dt I = K.Ke {–d [glucose]}Dt Construction This device consists of an indicator electrode made of platinum strip attached to a bag comprising the enzyme glucose oxidase. The bag is made of thin membrane permeable to glucose and H2O2. The analyte diffuses through the membrane and it is the enzyme glucose oxidase which gets oxidized to glucuronolactone where the cofactor of enzyme gets reduced. The active site is oxidised by reduction of molecular oxygen to H2O2 (Fig. 3.19).

Fig. 3.19: Working of enzymatic glucose biosensor

Advantages i. Simple and economical ii. No interference of coexisting molecules like ascorbic acid and paracetamol Disadvantages i. Cannot be implemented in vivo ii. High fluctuations with variation in analyte concentration 3.12 CHOLESTEROL BIOSENSORS Cholesterol is a steroid metabolite found in cell membranes that plays a role in producing bile acids, vitamins and functioning of many biomolecules. However, they clog in arteries causes many ill effects that are life threatening and require a constant monitoring. It is an important indicator in human blood for many myocardial infraction, hypertension and arteriosclerosis. Among all the biosensors available, electrochemical biosensors are found to be economical, portable, and fast which gives a reliable response for wide concentration ranges.

Electronic Materials 83

Working The enzyme cholesterol oxidase is a water-soluble enzyme which converts cholesterol to unstable cholest-5-en-3-one and cholest-4-en-3-one. During the process, the flavin cofactor of the enzyme gets reduced and inactivated. The reduced cofactor is oxidized back to form H2O2 while transferring electrons to O2. The transfer of electrons leads to change in current (Figs 8.20 to 8.22).

Fig. 3.20: Graphical representation of variation of current with analyte concentration in a cholesterol biosensor

Fig. 3.21: Convertion of cholesterol to cholest-4-en-3-one

Fig. 3.22: Representation of enzyme and cholesterol interaction

84 Chemistry of Engineering Materials

3.13 DNA BIOSENSORS Genetic information is stored in a cell in the form of DNA comprising 2-stranded double helix structure. The strands consist of 4 nucleotide bases and different combinations like cytosine, thymine, adenine, and guanine. The information is coded in the sequential arrangement of nucleotides. DNA biosensors sequence in nucleotides comprises a DNA molecule. The determination of nucleic acid sequence can help in detection of genetic disorders, forensic studies, tissue matching test for mutation, etc. Among various methods of DNA detection used for the electrochemical technique is popular because of its simplicity, low cost, rapidness and high sensitivity. Principle: Single stranded RNA or DNA fastened to the recognition layer interacts with its complementary sequence strand to form a stable double stranded RNA or DNA. This process referred to as ‘hybridization’ is transduced to generate an electronic signal (Fig. 3.23).

Fig. 3.23: Sequence of events in electrochemical DNA biosensors

Electronic Materials 85

3.13.1 Electrochemical DNA Biosensors Electrochemical genosensors make use of the hybridisation event to detect the target DNA sequence. Hybridization is a phenomenon where a single stranded DNA or RNA molecule combines to a complementary DNA or RNA strand (Fig. 3.24). Steps involved in electrochemical DNA biosensors Step 1: Formation of DNA recognition layer Step 2: Hybridization event Step 3: Transformation of hybridisation event into electric signal Genetic information is stored in a cell in the form of DNA comprising two stranded double helix. The strands consist of 4 nudeotide bases in different combinations; cytosine (C), thymine (T), adenine (A) and guanine (G). The information is coded in the sequential arrangements of nucleotides. The steps involved in sensing are: Step 1: A single stranded DNA which has inherent affinity for target DNA is used as a probe and is immobilized on a recognition layer Step 2: Target DNA is captured at the recognition layer and hybridization occurs Step 3: Interaction is transduced to signal generation. There are two different pathways for signal generation. The two types of instruments available are: Type 1: Label-free—Guanine and adenine are intrinsically electroactive bases of DNA as they are oxidisable and easily absorbed on carbon electrodes. On binding to the complementary base during hybridization, the signals diminish. Type 2: Labelled—Metal complexes such as cobalt phenanthroline, cobalt bipyridine, ruthenium bipyridine, redox active daunomycin, methylene blue dye are used as labels. These compounds selectively bind with DNA. The changes and the peak potential or current of the labels during hybridisation forms the basis of signal generation. The low Young’s modulus of 200 Cn/m2 which is one half of stainless steel implies great flexibility and significantly important in orthopaedic devices.

Fig. 3.24: Representation of steps involved in sensing

86 Chemistry of Engineering Materials

CONCEPTUAL QUESTIONS

1. Conductivity of the semiconductor increases with the increase in temperature. Ans. As the temperature increases more and more covalent bonds break resulting in the release of charged particles and hence conductivity of the semiconductor increases. 2. Gallium arsenide solar cells are preferred over silicon solar cells. Ans. GaAs can operate with visible energy whereas the silicon diodes work with infrared energies and hence gallium arsenide solar cells are preferred. 3. An extrinsic semiconductor gets permanently damaged if temperature is increased beyond a certain temperature. Ans. If temperature is increased beyond a limit, large number of covalent bonds break in the structure resulting in release of charge carriers and making it highly conducting and semiconductor gets permanently damaged. 4. Potential barrier across a pn-junction cannot be measured using voltmeter. Ans. The depletion layer has no free charges present and thus offers an infinite resistance to the flow of current through it. Therefore, potential barrier across a pn-junction cannot be measured using voltmeter. 5. Thickness of depletion region in a pn-junction diode increases with increase in reverse bias. Ans. When a pn-junction is formed, a small potential difference is set up across the depletion layer. But when reverse biased, the charges move away from the junction thus increasing the width/thickness of the depletion region. 6. The resistance of the pn-junction is low when forward biased is high when it is reverse biased. Ans. The resistance of pn-junction is low when it is forward biased because force acts on charge carriers to move them across the junction whereas the force in reverse biasing acts in opposite direction which opposes the motion of charge carriers across the junction. 7. Energy band represented in E-K diagram is not continuous. Ans. Electrons have energy values only in allowed energy bands and these allowed energy bands are separated by forbidden gap and therefore energy band is discontinuous. 8. Solar panels work even when covered with snow. Ans. Solar panels dark glass accelerates and show ability to melt and slide off before it hampers performance. The smooth white surface of snow reflects light, almost like a mirror. This albedo effect of snow enables panels to produce even more electricity in the cold. 9. The problem of effective waste segregation and management of waste management is an important economical requirement. Ans. Because waste management is largely concerned with the issues of ecology, environmental protection and the possibility of implementation into national economies of sustainable development according to the concept of a new, green economy. 10. Development of new alloys for bioimplant is continuously experimented. Ans. As most metallic materials show toxicity and are fractured because of corrosion and mechanical damages, therefore development of new alloys for bioimplants is continuously experimented.

Electronic Materials 87

11. Ti and Ta are popularly used for bio implants. Ans. Ti and Ta are highly resistant to corrosion and therefore used in bioimplants. 12. During the working of glucose biosensor, oxidation current and glucose concentration is not detected as soon as the electrode surface is saturated. Ans. The saturation of electrode decreases the extent of hydrogen and hydroxide ion adsorption which influences the chemisorption and oxidation of glucose. Thus, a direct correlation between the oxidation current and glucose concentration is lost with saturation of electrode surface. DESCRIPTIVE QUESTIONS

1. Represent Fermi level in n-type and p-type extrinsic semiconductor. 2. Define band gap engineering and list its applications. 3. Define E-K diagram. Explain the importance of E-K diagrams in band gap engineering. 4. Explain the construction and working of light emitting diode and photovoltaic cell. 5. Explain the construction and working of phototransistors and photodiodes. 6. Appraise the importance of safe disposal of electronic materials. 7. Define bioimplant. Mention the criteria for a material to be used as a bioimplant. 8. Explain the role of cobalt-chromium alloys in the construction of efficient bioimplants. 9. Explain the different components of a biosensor with a neat diagram. 10. Classify biosensors based on the transduction methods. 11. Differentiate prosthesis devices and fixation devices with suitable examples. 12. Explain the construction and working of enzymatic glucose biosensors. 13. Outline the steps involved in electrochemical DNA biosensing. 14. Explain the reactions involved in cholesterol biosensing. 15. Define biocompatibility. Outline the classification based on interfacial response of tissues. MULTIPLE CHOICE QUESTIONS

1. Which of the followings is a characteristic feature of intrinsic semiconductors? (a) Number of holes is equal to number of electrons (b) Number of holes are greater than electrons (c) Number of electrons are greater than holes (d) None of these 2. Identify the charge carriers in p-type semiconductors. (a) Electrons and holes (b) Electrons (c) Holes (d) None of these. 3. The color emitted by LED is determined by (a) Conduction band (b) Band gap (c) Valence band (d) None of these

88 Chemistry of Engineering Materials

4. Identify the biomaterial used in the replacement of bones. (a) Hydroxyapatite (b) Tungsten carbide (c) Silicon carbide (d) None of these 5. Identify the process involved in the production of extrinsic semiconductors. (a) Annealing (b) Doping (c) Quenching (d) Heating 6. The region where electrons and holes diffuse is called (a) Depletion region (b) Depletion junction (c) Depletion boundary (d) Depletion space 7. Doping of boron into silicon results in generation of (a) n-type (b) p-type (c) p-n type (d) None of these 8. In glucose biosensor, glucose is converted to (a) Glucokinase (b) Gluconic acid (c) Sucrose (d) None of these 9. Which of these materials are not used in photovoltaic cells (a) Rhodium (b) Silicon (c) Selenium (d) Gallium 10. Titanium is popularly used in metal implant for its (a) Easy availability (b) Lightness and flexibility (c) Lustre (d) Electrical conductivity 11. Conversion of biochemical reaction response to detectable signals is referred to as (a) Transfusion (b) Transmittance (c) Termination (d) Transduction 12. Glucose biosensors have gained popularity because it is (a) Simple (b) Economical (c) Noninterference with other biomolecules (d) All of these 13. Devices converting light into an electric signal or vice versa are called (a) Optical devices (b) Optoelectronic devices (c) Electronic devices (d) None of these 14. Which of these is not an e-waste (a) Printed circuit board (b) Capacitors (c) LCD (d) All of the mentioned 15. Photoelectrons are referred to the (a) Electrons ejected by semiconductor energy system (b) Electrons moving in a semiconductor (c) Electrons moving against the field (d) None of these 16. Light emitting diode works on the principle of (a) Electroluminescence (b) Electron resonance (c) Electron capturing (d) None of these

Electronic Materials 89

17. Ceramic bioimplants gain its biocompatibility due to (a) High porosity (b) Absence of ions during degradation (c) Presence of ions during degradation (d) None of these 18. The biological component and sensor must be bound to each other in a biosensor by (a) Membrane entrapment (b) Physical adsorption (c) van der waals forces (d) All of these 19. Glucose biosensor is generally used to detect hyperglycemia; a chronic disease where (a) Blood glucose increases (b) Blood glucose decreases (c) Blood glucose fluctuates (d) Blood glucose remains constant 20. DNA biosensors detect the target DNA sequence by (a) Hybridization event (b) Conjugation event (c) Propagation event (d) None of these 21. The energy gap between the conduction and the valence band is called (a) Conduction band (b) Valence band (c) Forbidden gap (d) Band gap 22. Identify an examples for metallic biomaterials (a) Co-Cr alloys (b) Al2O3 (c) ZrO2 (d) None of these 23. The enzyme involved in the glucose biosensor is (a) Glucose oxidase (b) Cholesterol oxidase (c) Amylase (d) Maltase 24. Identify the charge carriers in semiconductors (a) Electrons and holes (b) Electrons (c) Holes (d) None of these 25. Identify the type of operation of diode in light emitting diode (a) Reverse (b) Forward (c) Combination of both (d) None of these 26. Metal bioimplants are used to replace or stabilize (a) Muscles (b) Nerves (c) Bones (d) Cartilage 27. Which of the following ‘biocompatability’ depends on (a) Physical factors (b) Chemical factors (c) Biological factors (d) All of these ANSWERS

1. a

2. c

3. b

4. b

5. b

6. a

7. b

8. b

9. a

10. b

11. d

12. d

13. b

14. d

15. a

16. a

17. b

18. a

19. a

20. a

21. d

22. a

23. a

24. a

25. a

26. c

27. d

90 Chemistry of Engineering Materials

4 Characterization of Materials

4.1 INTRODUCTION Resources that are used as raw materials for any kind of construction or manufacturing in an organized way of engineering application are known as engineering materials. Over the last three decades, material science has played a central role in industries, as the usage of appropriate materials is crucial while designing the material systems. Chemical composition has a direct impact on properties of engineering materials, Properties such as chemical, mechanical, thermal, electrical and optical, etc. strongly influence the quality of the material and thereby the technological progress. An appropriate choice and efficient use of engineering materials require an understanding of fundamental principles of chemistry. Hence characterisation of materials is becoming more relevant and numerous analytical characterization tools are available. Characterization of a material can be defined as description of features of the composition, structure and defects of a material that are significant to the preparation, study of properties, usage, and reproduction of the material. The following table gives a brief overview of characterization techniques (Table 4.1). Solids

Table 4.1: Characterization levels of various structures Macrostructure Microstructure Nanostructure

Scale Magnification Common techniques Characteristic features

2 billion nm –5 million nm ×1 Visual inspection, ultrasonic inspection Production defects, porosity, cracks and inclusions

80000 nm –2000 nm ×104 Optical microscopy

100 nm –0.1 nm ×106 X-ray diffraction, SEM, TEM, AFM, STM Grain and particle size, Grain and phase boundphase morphology and aries, crystal, interface anisotropy structure, point defects

4.2 ELECTRON MICROSCOPE The most powerful microscope existing presently is electron microscope which can be used to analyze nanoscale materials (Fig. 4.1). Many of the principles of electron diffraction have been exploited to make electron microscope one the valuable instruments available to the metallurgist, can be used in many different ways to provide structural details to a size of about 10 Å. It can provide information on dislocations, stacking faults, grain boundary, fine precipitates, strain centers and small clusters of point defects. 90

Characterization of Materials 91

As the wavelengths of electron are far shorter than light, they can detect smaller particles with a better resolution than light waves. When electrons interact with the analyte, emission of radiation occurs proportionate to the spacing of energy levels in an atom thereby analytes to be characterized and also quantified. They yield information on: i. Morphology: Shape and size ii. Topography: Surface features like texture, hardness, etc. iii. Crystallography: Arrangement of atoms in an object The following table shows the major differences between the optical and electron microscopes. Optical microscope 1. Uses light rays 2. Tungsten/xenon lamps are used as a source 3. Vacuum is not required 4. Glass lenses are used to focus the beam 5. Resolving power is limited by wavelength of light 6. 7. 8. 9. 10.

Magnification 2000X is maximum Image is visible to eye and can be photographed Analyte can be solid of any shape Analyte need not be a conductor Reveals grain size and microconstituents

11. Economic 12. Magnification up to 2 × 10–7 m 13. No display required

Electron microscope Uses electron beam Electron gun having tungsten filament is used as a source. Vacuum is a must Magnetic coils are used to focus the beam Resolving power is very high order of 10 Å Magnification 1000X to 100000X Image is projected on a fluorescent screen Thin layer of analyte is required Analyte has to be a conductor Reveals minute details of structure, identifies dislocation, grain boundaries, elemental composition and provides 3D image Highly expensive Magnification upto 1 × 10–10 m Digital display or imaging required

Fig. 4.1: Representation of components in an electron microscope inside the vacuum chamber

92 Chemistry of Engineering Materials

An electron microscope can be represented as the following arrangement of components inside the vacuum chamber. The magnetic lenses are made of DC coils of soft iron whose focal lengths are changed by regulating the current through the coil. The two condenser lenses collimate the beam, which passes through the specimen and the objective lens to form the first image, magnified about X40 by the first projector lens. A small area from this image is then projected on to the fluorescent viewing screen, with further magnification of about X50 by second projector lens. Total magnification in the range of X20000 to X100000 is attained. There are mainly two types of electron microscopes: 1. Transmission electron microscope (TEM) 2. Scanning electron microscope (SEM) 4.3 TRANSMISSION ELECTRON MICROSCOPE (TEM) This is the first electron microscope to be designed that closely resembles the light transmission microscope. Here an electron beam is transmitted through an electrolytically polished thin foil of the specimen. 4.3.1 Principle A beam of electrons are accelerated onto a sample that interactions occur inside the sample which affects the electron beam, thus some electrons are transmitted after the interaction. The transmitted electrons form an image on fluorescent screen or photography film. Darker areas represent areas of sample where only a few electrons are transmitted. Light areas are those where more electrons are transmitted. Transmission of electrons depends on properties of the analyte. 4.3.2 Construction (Fig. 4.2) Electron gun: A sharp pointed filament of tungsten potential produces stream of electrons. Condenser lens: The lenses are magnetic, made of DC coils in soft iron and their focal length can be changed by regulating the current through the coil. Direct electron detector: Transmitted electrons are received on a phosphor screen or a fluorescent viewing screen comprises particle size of 10–100 m of ZnS with a further

Fig. 4.2: Representation of components in a transmission electron microscope

Characterization of Materials 93

magnification of about X50, by the second projector lens. Total magnification in the range X20000 to X100000 is attained. 4.3.3 Advantages i. Magnification of 400000 can be obtained ii. Sharper images than SEM can be obtained iii. It can be attached to EDS (energy dispersive spectrometer) iv. Has a better resolution than an optical microscope 4.3.4 Disadvantages i. Electron beam can damage the sample by exciting the electrons inside the sample thereby causing chemical reactions ii. Electron transparent thin layer of sample is required 4.3.5 Applications i. Identifies faults and damages microsized objects ii. Gives high magnification for forensic studies iii. Identifies defects in silicon chips iv. Provides topographical information of objects 4.4 SCANNING ELECTRON MICROSCOPE (SEM) SEM is used to analyze the surface topography of the analyte with a magnification of the image upto 100000 times their normal size. 4.4.1 Principle A beam of electrons are scanned through the sample. The electrons from the beam hit the nucleus and get deflected due to the attractive force. This deflection of electron path is referred to as Rutherford elastic scattering whose scattering angle depends on atomic number of the nucleus, the re-emerging electrons from the surface of the sample after deflection are called secondary electrons or secondary particles and they carry a lot of information on shape and structure of the sample and hence serve as imaging data (Fig. 4.3).

Fig. 4.3: Beam of electrons scanning through the sample

4.4.2 Construction (Fig. 4.4) Electron gun: Emits a beam of electrons Objective lens: Contains a set of coils creating an electromagnetic field of 20 kV which focuses the electron beam on to the specimen Detector: Secondary electrons cause a voltage in a detector which is converted to an amplifier signal 4.4.3 Advantages i. SEM can produce 3D image of great clarity with less sample ii. Wide range of magnification and good resolution

94 Chemistry of Engineering Materials

Fig. 4.4: Representation of components in a scanning electron microscope

4.4.4 Disadvantages i. Sample giving off vapour cannot be analysed as vapour would interact with electrons ii. Specimen must be conductive 4.4.5 Applications i. To investigate the texture of nanotubes and nanofibres ii. To investigate high temperature superconductors iii. To investigate surface morphology of semiconductors iv. To investigate bank note for authenticity based on fabrication Comparison of SEM and TEM SEM 1. Based on scattering of electrons 2. Provides information on surface morphology 3. Sample can be in any dimension 4. Generally poor resolution 5. Any quantity of sample can be analysed 6. Image is displayed on an electronic output device. 7. Finds application in polishing, etching microstructures and chemical segregation 8. 3-dimensional image is obtained using SEM

TEM Based on transmission of electrons. Provides information on crystallisation and magnetic domains Sample is taken as thin film Relatively higher resolution image is obtained. Only small samples can be analysed. Image is obtained on a fluorescent screen Finds application in imaging of dislocations, tiny precipitates, grain boundaries and other defect structures in solids 2-dimensional image is obtained using TEM

4.5 SCANNING PROBE MICROSCOPY The techniques of scanning are extensively used to study surfaces using a sharp probe called tip to monitor the interaction between probe and the surface. The most popularly used techniques in scanning probe microscopy are scanning tunneling microscopy (STM) and atomic force microscopy (AFM).

Characterization of Materials 95

4.6 SCANNING TUNNELING MICROSCOPE (STM) STM is a nonoptical microscope to generate real space images of surface with atomic resolution and give information on roughness, defects and surface reactions. 4.6.1 Principle The probe is the tip on to which voltage is passed. When the tip scans the surface of the analyte, the electrons tunnel from tip to the sample or vice versa leading to a current called tunneling current is obtained. The process is called tunneling. With the variation of surface tip to sample spacing varies which in turn varies with the current there by creating an STM image. For tunneling to take place, both the sample and tip must be conductors and semiconductors. 4.6.2. Construction (Fig. 4.5) Tip: A sharp tip of ~50 nm made of gold, tungsten, platinum-iridium or carbon nanotube is used. Tunneling current: Maintaining a constant tip, the sample distance of several angstroms, a small sweeping current of ~10 pA to 1 nA is applied using a bias voltage. Detector diode: A small difference in the signal is recorded by the diode and translated to DC voltage signal. A schematic of working of STM in shown in Fig. 4.6. 4.6.3 Advantages i. Gives 3D profile of surface ii. High resolution in atomic level iii. Usable in high vacuum, air, water and other liquids iv. Operated over a wide range of temperatures, from 0 K to 100°C. 4.6.4 Disadvantages i. Requires expertise to handle the instrument ii. Clean surface of the analyte required iii. Sample must be conductive and not oxidisible iv. Instrument is fragile and expensive.

Fig. 4.5: Generation of tunneling current from tip of the probe to the analyte surface

96 Chemistry of Engineering Materials

Fig. 4.6: Working of scanning tunneling microscope (schematic)

4.6.5 Applications i. Allows analysis of surface in range of 0.1 nm ii. Surface roughness determination of industrial compounds iii. Resolution studies for understanding electrochemical processes and biological molecules iv. Study the electrokinetics of the reaction 4.7 ATOMIC FORCE MICROSCOPY (AFM) It is a powerful tool for studying 3D topography of nanomaterials, the surface measurement with high resolution can be carried out with minimum sample information on properties such as height, friction and magnetism of the surface. 4.7.1 Principle A tip scans over the surface with a close range attractive force between surface and the tip, and deflects with vertical movement depending upon the topography of the sample. The force present on the tip is kept constant and the deflection of the cantilever is detected by a laser beam. 4.7.2 Construction Tip: A sharp tip made of silicon or silicon nitride of 3–6 m tall pyramid with 15 to 40 nm radius is attached to the cantilever. Cantilever: A cantilever made of Sb or Ni whose backside is coated with gold for better reflection is made to move on the surface of the analyte. As the cantilever moves up and down over the irregularities on the surface, the position of the laser on the photodetector also moves up and down. Detector: A deflection of the laser beam incident on the cantilever is converted into an electrical signal. As the cantilever deflects towards or away from the surface, a laser beam is incident on flat surface of cantilever whose direction changes with deflections in cantilever. Imaging: A position sensitive photodiode is used to track the deflections. Imaging can be done by various modes are described hereafter.

Characterization of Materials 97

Contact mode AFM: The tip scan through the surface by contact. The image of the surface is obtained by cantilever deflection. It is suitable for rough surfaces and is fast acting, however, can damage soft surfaces. Intermittent mode AFM: The tip oscillates with fixed oscillation amplitude such that it can contact the surface intermittently. The taps on the surface of sample produces an image. It is ideal for biological samples but needs slow scan speed. Noncontacts mode AFM: The tip contacts the absorbed fluid layer on the surface. It has a long probe lifetime, but needs low resolution. 4.7.3 Applications i. To distinguish cancer cells from normal cells ii. To analyse thin films iii. To distinguish composition of nanocomposites iv. To detect the faults in energy storage device 4.8 X-RAY METHODS X-rays are high energy electromagnetic radiations with wavelength ranging from 0.01 to 10 nm and hence is highly valuable in analysis of materials. Due to their high energy, they have the ability to penetrate and interact with the material. Analyses of materials are carried out using X-ray techniques, such as X-ray fluorescence (XRF) spectrometry, proton-induced X-ray emission (PIXE) spectrometry, X-ray photoemission spectroscopy and X-ray diffraction (XRD). Here X-ray diffraction (XRD) and X-ray photoemission spectroscopy are discussed in detail. 4.8.1 X-ray Photoemission Spectroscopy (XPS) It is a surface sensitive spectroscopic technique often referred to as electron spectroscopy for chemical analysis and is used in elemental identification, determining chemical state of an element, constituents on surface region and valance bond structure (Fig. 4.7).

Fig. 4.7: Representation of electronic states and photoelectrons

98 Chemistry of Engineering Materials

Principle When a material is irradiated with X-rays, electrons are emitted called photoelectrons. The kinetic energy of emitted electron is correlated with electronic state and composition of surface region of sample (Fig. 4.8).

Fig. 4.8: Generation and emission of photoelectrons

The kinetic energy of the atomic level from where electrons are ejected is given by: KE = hvPh –Eion KE = Energy of photoelectron emitted-ionization energy of the atomic energy level. These emitted electrons are passed through a velocity analyser which allows electrons with narrow range of velocities to pass through a trajectory. The electrons are coming out of the trajectory are focussed towards the detector which measures the number of electrons having a given kinetic energy. Construction Sample: A thin film of the sample is introduced into the sample holder. Source: Fixed energy X-ray is generated by bombarding a metallic plate with high energy electrons. Electron analyser/detector: The emitted photoelectrons are analysed under high vacuum conditions according to their kinetic energy and elecrons of a particular energy is measured (Fig. 4.9).

Fig. 4.9: Velocity analyser in a X-ray photoemission spectrometer

Characterization of Materials 99

4.8.2 Analysis of the Spectrum The energy state in the valance band of an atom has characteristic ionization energy which is a reflection of the surrounding lattice environment. Hence, ionization energy or binding energy measurement at which the peak appears can identify the presence and the state of an atom. a. The position of the peak defines the characteristic of the element Example: Consider the XPS of the Pd metal using Mgk -radiation. The main peak occurs at kinetic energy of 330 eV, 690 eV, 720 eV, 910 eV and 920 eV. Since the X-ray radiation energy is always known, it is preferable to plot intensity of signal against binding energy (BE) and not against kinetic energy (KE) (Fig. 4.10).

Fig. 4.10: Plot of intensity against binding energy

The peaks are assigned as: i. Valance bond (4d, 5s) emission occurring at a binding energy of 0–8 eV. ii. Emission from the 4p and 4s levels gives rise to weak peaks at 54 eV and 88 eV respectively. iii. 3p and 3s levels give rise to 534 eV, 561 eV and 673 eV respectively. iv. An intense peak at 335 eV is due to emission from 3d levels of Pd atom. v. All the remaining peaks are not XPS peaks but Auger peaks caused due to X-ray induced Auger emission. b. Peak area ratios and sign orbital splitting are characteristic of an element The separating distance between two peaks are named spin orbital splitting. This remains same for an element in different compounds. Example: For Au, the peak area ratios and spin orbital splitting are designed on the basis of L-S coupling using the equation j=l+s where l = Azimuthal quantum number l = 0 (s-orbital) s = Spin quantum numbers l = 1 (p-orbital) s= 

1 2

l = 2 (d-orbital) l = 3 (f-orbital)

100 Chemistry of Engineering Materials

Hence, the peaks are designed by the following symbols (Fig. 4.11).

Fig. 4.11: Spin orbital splitting based on J-S coupling

Advantages i. It is a nondestructive technique ii. Highly surface sensitive iii. Quantitative measurements with high accuracy can be achieved iv. Determines the elemental composition v. Provides information on nature of chemical bonding Disadvantages i. A highly expensive technique ii. Instrumental handling expertise required iii. Vacuum environment and slow processing time required iv. H and He cannot be identified Applications i. Used in the analysis of stability of the surfaces of polymers ii. To detect the extent of corrosion iii. Useful in thin film coating iv. Useful in failure analysis 4.9 X-RAY POWDER DIFFRACTION (XRD) This technique makes use of the periodicity of the crystals and the diffraction of waves by a lattice. X-ray diffraction reveals the internal structure of material to an order of 10–9 m in size. XRD is an analytical technique used for the rapid identification of the phase (atomic rearrangement), where information is obtained on unit cell dimensions in a material using the X-rays in the range of 0.5 to 2.5A. AW Hull stated (1919), in a paper titled ‘A new method of chemical analysis’, that every crystalline substance gives a pattern. A same substance always gives the same pattern and in a mixture of substances each produces its pattern independently of others. Solids can be classified as crystalline and amorphous. Amorphous: Atoms randomly arranged as in liquid, and hence XRD cannot be used.

Characterization of Materials 101

Crystalline: Atoms are arranged in a regular pattern with repeated 3D-arrangement. 95% of solid matters are crystalline. Crystalline substances can be classified as: i. Single crystal: Has only 1 orientation and is anisotropic. Can be identified and analysed by single crystal diffractometer. The spectra obtained are a discrete function of intensity as a function of diffraction angle. ii. Polycrystal: Often referred to as mixture, it has different orientations and is isotropic and can be identified and analyzed by powder XRD technique. Single crystal and powder XRD have different methodology instrumental design for hkl data collection. However, the principle behind both remains the same. 4.9.1 Principle When X-rays are irradiated on an atom, the electrons around the atom starts oscillating with the same frequency as the incoming beam, the atoms arranged in a regular pattern of a crystal, diffract the X-ray in all directions. Rays which are in phase, mutually reinforce each other leading to a constructive interference whereas the destructive interference occurs with no resultant energy leaving the sample. The monochromatic rays leaving the sample due to constructive interference are collected and processed (Fig. 4.12). Sometimes X-ray reflection and diffraction are used as synonyms. 4.9.2 Lattice Planes The surface of the crystal is described by a crystal plane or lattice plane, and the orientation of this plane is defined by how the plane intersects the axes of the solid, the intersection is given by the assignment of Miller Indices hkl, a set of number which quantifies the intercepts (Fig. 4.13). A given set of planes with indices hkl cut the axis of the unit cell in h sections, the b axis in k section and the c axes in l section. A zero indicates that the plane is parallel to the corresponding axes. The three indices hkl become the order of diffraction along the unit cell of axes a, b and c respectively. Example: The 2, 2, 0 planes cut the b axis in half but are parallel to the c axis. When a beam of monochromatic X-ray is incident on a set of planes, hkl with spacing d, it will be diffracted through an angle .

Fig. 4.12: Monochromatic rays undergoing constructive and destructive interference

102 Chemistry of Engineering Materials

Fig. 4.13: Intercepts in a crystal plane quantified

Bragg's law William Lawrence Bragg and William Henry Bragg, father and son team formulated for the first time that electromagnetic radiation incident with angle  upon a crystalline sample with parallel planes separated by distance “d”, undergoing reflection with constructive interference would leave the crystal according to n = 2d sin  Let us consider an X-ray beam incident on a pair of parallel planes P1 and P2, separated by an interplanar spacing d. Both incident and reflected rays make an angle  with the fixed crystal plane. Reflection occurs from the plane which is set at  to the incident beam and generates reflected beam at 2 from incident beam, therefore, the 2 values depend on unit cell dimensions. It is given by 2d sin , where  = wavelength of X-ray and d = distance between planes. The intensity of the reflected or diffracted rays depends on nature of atom and location of these atoms in a unit cell. Highest electron density is found around atoms and the planes going through these areas with high electron density will reflect strongly.

Bragg’s law is applied in analysis of the results by two techniques

Fig. 4.14: X-ray beam incident on a pair of parallel planes

Characterization of Materials 103

i. X-ray spectroscopy: A crystal with planes of known spacing ‘d’, is analyzed to detect  and the wavelength of radiation used is determined. This instrument is referred to as X-ray spectrometer. ii. Structural analysis: A known wavelength of X-rays is irradiated and  is measured to determine the d spacing of planes in a crystal using an instrument referred to as a ‘diffractometer’. Construction X-rays are produced by bombarding a metal target with fast electrons in a vacuum tube. Below a certain voltage for the acceleration of these electrons, a smooth spectrum of white X-rays is produced over a wide range of wavelengths. Above about 50000 V, however, intense monochromatic X-rays are emitted at particular wavelengths, characteristic of the target metal. X-rays are produced because bombarding electrons have enough energy to knock electrons out of the low energy level in the atoms of the target. Other electrons in the higher energy levels in these atoms drop down into the empty quantum states so created and, thus, release energy in the form of X-rays according to the equation E = h. X-ray tube: X-ray tube is about 10 mm long and 1 mm wide with power capacity 200 W/mm2. It is a cathode ray tube made of copper, ceramic material having a filament which can produce electrons. i. Source of electrons: An electric Fig. 4.15: Components hot metal filament at about in a diffractometer 3 amps which produces electrons. ii. Accerelating voltage: Voltage is applied across the electron source and a metal target. iii. Metal target: Accelerated electrons hit the atoms of the target and dislodge inner shell electrons. During this process X-rays are produced. The X-rays emitted disperse in all the directions and escapes from a window made of beryllium or aluminum which is transparent to X-rays. The electrons are accelerated towards a target by applying a voltage. These electrons hit the atoms of the analyte and dislodge inner shell electrons. During this process X-rays are produced. Sample: The X-ray tube is kept at an angle theta. The instrument used to maintain the angle and rotate the sample is called goniometer. Generally plate is collected at 2 range from 5º to 70º. Detector: Typically an argon or xenon filled detector (where Ar is ionised to Ar+ and e– by fast moving electrons to generate signal) is placed at an angle of 2 with respect to the incident beam. The diffraction pattern is collected by moving the detector at a constant angular velocity with increasing value of 2 till a complete angular range is scanned. The result

104 Chemistry of Engineering Materials

is a graph with counts per second plotted against diffraction angle 2. The following are the popularly used detectors. i. Ionisation devices: Typically an argon or xenon filled detector (where argon is ionised to Ar+ and e– by fast moving electrons generate signal) is placed at an angle of 2 with respect to incident beam. ii. Photographic film: Used for intensity measurements, it records various diffracted beams at one time according to their relative positions. iii. Fluorescent screens: It is made of thin layer of ZnS with traces of Ni. The diffracted beams are generally too weak to be detected on the screens and hence adjoined with a phototube which enhances the sensitivity and is often referred to as scintillation counter. 4.9.3 Analysis of Diffractogram The detector moves in a circle around the sample. Mechanism • The detector position is recorded at an angle 2 • The detector records the number of X-rays observed at each angle 2 • The X-ray intensity is usually recorded as counts or as counts per second

Fig. 4.16: X-rays intensity versus angles of diffraction

Fig. 4.17: Comparison of X-ray diffraction pattern in pure phase material and mixture

Characterization of Materials 105

The position of a diffraction line on a diffractometer is located. For a known , the d spacing of the reflected planes in a lattice producing the lines is calculated. It is observed that the planes with larger spacing across the planes produces lowest 2 value. An X-ray diffraction pattern is a plot of the intensity of X-rays vs angles of diffraction. Note: Some spectra indicate ka1 and ka2 which represent two characteristic energies of X-ray photon during transition within the electron shell. i. Each phase produces a unit diffraction pattern ii. A phase is a specific chemistry and atomic arrangement iii. Quartz cristobalite and glass are different phases of SiO2 iv. They are chemically identical but the atoms are arranged differently v. X-ray diffraction pattern is distinct for each different phase material vi. Amorphous materials like glass do not produce sharp diffraction peak vii. The diffraction pattern of a mixture in a sample and some of the diffraction patterns of each individual phase. From the XRD pattern one can determine: 1. The crystalline phases in a mixture 2. The degree of each crystalline phase in the mixture 3. If any amorphous material is present in the mixture For qualitative analysis, experimental XRD data are compared with the reference pattern and to determine the phase ‘International Centre of Diffraction Data (ICDD)’ previously called ‘Joint Committee on Powder Diffraction Standard (JCPDS)’ releases the spectral data of all organic and inorganic compounds and yearly release data is available online. An example of such a sample is shown below, where the vertical lines represent this standard. The experimental data must consist of the major peaks listed in reference pattern.

Fig. 4.18: Sample diffractogram with standard values represented by vertical lines matching with JCPDS

106 Chemistry of Engineering Materials

viii. Errors analysis a. Systematic displacement or shift: If all the peaks are proportionately displaced either in 2 values or in intensity indicate misalignment of the sample or overfilled and underfilled sample holder. b. Doublets and singlets: Peak splitting is generally caused due to formation of new phase. However, k1 and k2 doublet will almost always be present overlaping heavily at low angles and slightly separated at high angles and requires expensive procedures to remove k2 line. Example: Cubic phase sharp singlet peaks and tetragonal phase will have peak splitted. c. Amount of each phase is not proportional to intensity of diffraction peak. Example: An equal amount of TiO2 and Al2O3 mixed would give a spectra where TiO2 diffract X-ray more efficiently.

Fig. 4.19: Comparison of efficiency of diffraction in TiO2 and Al2O3

Peak broadening: Can be caused due to small crystallite size, defect arising in crystal due to strain, inhomogeneous composition in material or due change of instrument. Very small crystals are not with enough planes to give complete destructive interference and hence broadening occurs.

Fig. 4.20: Representation of peak broadening in diffractogram

Characterization of Materials 107

4.9.4 Calculation of Unit Cell Parameters from Diffraction Peak Positions The peak position 2 is converted to dhkl values using Bragg’s law dhkl = /2d sin . The individual hkl values are determined using published reference pattern (ICDD). For new compounds indexing of pattern has to be done to determine the value of hkl lattice. A small amount of NaCl is finely powdered and press into the sample holder and put into XRD scan where X-ray used were Cu-Ka1 ( = 1.5406 Å) and Cu-Ka2 ( = 1.544 Å). It is established that NaCl has a cubic lattice structure with the following variants. i. Primitive ii. bcc iii. fcc iv. Diamond or ZnS But the diffraction pattern matches with fcc. For any cubic crystal with lattice constant ‘a’ and distance between lattice place (hkl) is ‘d’. Since for both cases, the values of ‘a’ is same and l matches with real values 0.564 nm. hkl

2

d

H2 + K2 + L2

200 220

31.5 45.83

0.284 0.198

4 8

H 2 + K2 + L2 2 2.828

A 0.568 0.560

Fig. 4.21: Diffraction pattern of NaCl matching with FCC cubic lattice

4.9.5 Debye-Scherrer Equation to Calculate Particle Size Particle size serves as an important parameter as it decides most of the observed behavior of the nano particle structure. The application of Debye-Scherrer equation on the XRD pattern helps in calculating the particle size. The calculation is based on the peak width of the dominating peak which generally depends on the number of lattice planes participating in diffracting the incident X-rays. The mean size of the crystallite ‘D’ is calculated in nanometers using the equation D = K/ cos  where K = crystallite shape factor (approximated to 0.9)  = X-ray wavelength,

108 Chemistry of Engineering Materials

 = width of the peak at half the maximum of the X-ray diffraction peak measured in radians  = Braggs’ angle in degrees. Note: Fourier transformation is applied on the XRD pattern if the instrument provides only a small range of angle of scattering with a large background noise. This approach can lead to determination of average distribution of interatomic distance in an aggregate without assuming the periodicity in planes. 4.10 THERMAL ANALYSIS This process comprises a group of thermo-analytical techniques in which a physical property of a substance or a product is measured as a function of temperature when the substance is subjected to a controlled temperature program. Thermogravity (TG) and differential thermal analytic (DTA) are important methods which provide information on chemical nature of the sample. 4.11 THERMOGRAVIMETRIC ANALYSIS (TGA) It is a technique where the temperature of the sample is increased linearly and the mass of the sample is recorded as a function of temperature or time at a controlled rate in the environment of N2, CO2, He or Ar. The plot of mass or mass percent as a function of time is called a thermogram or thermal decomposition curve. Principle A change in weight of a substance is recorded as a function of temperature or time under a defined and controlled environment with respect to heating rate, gas atmosphere, flow rate, crucible type, etc. 4.11.1 Instrumentation Furnace: The furnace made of corrosion resistant material changes the temperature slowly up to 2000 °C linearly with time. A water-cooled jacket provides the microfurnace with fast cooling and therefore allows for high sample throughput. Thermobalance: A known weight of the sample is taken in the ceramic crucible placed on the recording balance. A ray of light is irradiated on the pan whose deflection is proportional to the change in weight of the sample. Thermocouple: Pt/Pt-Rh thermocouple measures the sample temperature.

Fig. 4.22: Schematic representation of thermogravimetric analysis

Characterization of Materials 109

Recorder: Thermogram is plotted by recorder with temperature on x-axis and change in weight on y-axis. Example:  Ag + NO2 + O2 1. Decomposition of AgNO3 

2. Decomposition of oxalate   monohydrate Step1: Loss of water begins at 100°C  CaC2O4 (s) + H2O (g) CaC2O4OH2O (s) 

Fig. 4.23: Thermogravimetric decomposition curves of oxalate

Step 2: Loss of CO begins at 400°C  CaCO3 (s) + CO (g) CaC2O4 (s)  Step 3: Loss of CO2 begins at 680°C  CaO (s) + CO2 (g) CaCO3 (s) 

4.11.2 Factors Affecting TGA 1. Instrumental factors i. Heating rate of 3.5ºC per minute when maintained shows reliable and reproducible TGA ii. Purity of inert gas inside furnace ensures a neat thermogram. 2. Sample i. A small quantity (0.5 mg to 2.0 mg) of a sample eliminates the possibility of temperature gradient across the sample. ii. A small particle size is preferred as large particles result in rapid weight loss and heat liberation during decomposition.

110 Chemistry of Engineering Materials

4.11.3 Application of TGA i. ii. iii. iv. v.

To study the kinetics of the trace decomposition To determine the composition of a mixture To study the sublimation, vapourisation and desorption behaviour To establish different chemical state of the catalyst To analyse the oxidation patterns of polymers, pharmaceuticals, clay, minerals, metals and alloys

4.12 DIFFERENTIAL THERMAL ANALYSIS (DTA) 4.12.1 Principle It is a technique in which the difference in temperature (T) between a substance (Ts) and a reference material (Tr) is measured as a function of temperature (Fig. 4.24). 4.12.2 Procedure Identical cavities of sample and reference are placed in the ceramic oven which is pressure controlled and temperature programmed. A thermocouple is placed directly with sample and reference and the difference in temperature T is noted. T is an indication of physical or chemical change, a sharp peak indicates crystallinity or fusion while a broad peak indicates dehydration reaction. 4.12.3 Factors Affecting DTA i. Sample weight: A small quantity of standardized sample mass with uniform particle size must be taken to minimize change in peak size and position. ii. Atmospheric condition: Inert gas must be filled.

Fig. 4.24: Schematic representation of differential thermal analysis

Characterization of Materials 111

iii. Heating rate: A low heating rate must be maintained to avoid changes in peak size and position. iv. Thermocouple: Standardized thermocouple location is preferred to get a reproducible curve. A plot between Ts – Tr vs furnace temperature is shown in Fig. 4.25.

Fig. 4.25: Plot of (Ts – Tr) vs furnace temperature

Advantages i. Analysis at a very high temperature can be carried out. ii. Instrument is highly sensitive. iii. Reaction temperature and characteristic transition temperature can be determined accurately. iv. Physical changes like fusion, vapourisation, sublimation, absorption, adsorption, desorption and chemical changes like dehydration, reduction and decomposition can be maintained by DTA. Example: 1. DTA of dolomite sample (Fig. 4.26)

Fig. 4.26: Representation of DTA of dolomite sample

112 Chemistry of Engineering Materials

2. DTA of polymer Initial decrease in T is due to glass transition. The maxima represents exothermic process. Crystal formation leads to first exothermic peak and the area becomes larger with slow leading rate so that a large number of crystal can grow. Second peak is endothermic peak involved in melting of microcrystals. The third peak is exothermic oxidation of polymer. At the end, –ve T results from endothermic decomposition (Fig. 4.27).

Fig. 4.27: Representation for DTA of a polymer

Applications i. Quantitative purity assessment and identification. ii. Impurities are detected by lowering the melting point. CONCEPTUAL QUESTIONS

1. Gravity separation followed by magnetic separation is mandatory before exposer of the analyte to XRD. Ans. Trace amounts of other minerals containing iron oxides and sulfides show up as XRD peaks and hence separation of metals is mandatory. 2. A filter made of a material whose K-absorption lies between the K and K wavelengths of the target metal will absorb the K component much more strongly than the K component. Ans. An abrupt change in absorption coefficient of filter happens between these two wavelengths. 3. The metal analyte, subjected to XRD must be water-cooled to prevent its melting. Ans. Most of the kinetic energy of the electrons hitting the metal target is converted into heat and hence water cooling is necessary. 4. Rotating-anodic tube is preferred in XRD analysis. Ans. Rotating-anodic tube brings fresh target metal into the focal-spot area and so allows a greater power input without excessive heating of the anode. 5. Most X-ray films do not resolve fine details and cannot withstand much enlargement. Ans. X-ray films are made of thick layers of emulsion and made grainy on both sides in order to increase the total absorption. Hence, these films do not resolve fine detail.

Characterization of Materials 113

6. Gas tubes and filament tubes are often mounted so that their cathode end is absolutely inaccessible to the user during operation by placing it below a table top, in a box or behind a screen. Ans. The anode end of most X-ray tubes is usually grounded and therefore safe, but the cathode end operated at high voltages is a source of danger. 7. The X-rays used in XRD apparatus are particularly harmful to human tissues. Ans. X-rays used in XRD have relatively long wavelengths and are therefore easily absorbed by the body. 8. Back-scattered electrons provide valuable information in the scanning electron microscopy. Ans. Intensity of the back scattered electron signals is strongly related to the atomic number (Z) of the specimen. However, these images can provide information about the distribution, but not the identity of different elements in the sample. 9. Analytes subjected to SEM are often carbon-coated, but metals are not coated prior to imaging in the SEM. Ans. Metals do not require a coating as they are conductive and provide their own pathway to ground. 10. Electron gun in SEM consists of tungsten. Ans. Tungsten has the highest melting point and lowest vapor pressure of all metals. DESCRIPTIVE QUESTIONS

1. Explain the principle of usage of X-ray diffraction in analysis of crystals. 2. Summarize the instrumentation involved in X-ray diffraction with a neat diagram. 3. Outline the components and working involved in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) with detailed illustration. 4. Differentiate the technique and working of components involved in instrumentation of thermal gravimetric analysis and differential thermal analysis with a neat diagram. 5. Compare an optical and electron microscope. 6. Explain the construction and working of transmission electron microscope with a neat diagram. 7. Compare the advantages and disadvantages of X-ray diffraction in analysis of crystals. 8. Explain the principle of X-ray photoemission spectroscopy. 9. List the applications of STM and AFM. 10. List the advantages and disadvantages of XPS. 11. Differentiate between the single and polycrystal analysis of X-ray diffraction. 12. What do singlets, doublets and peak broadening signify in X-ray diffraction analysis. 13. List the factors affecting TGA and DTA analysis. 14. Compare the instrumental methods of analysis with chemical methods of analysis. 15. List the applications of XRD and SEM.

114 Chemistry of Engineering Materials

MULTIPLE CHOICE QUESTIONS

1. X-rays have larger wavelengths than which of the following? (a) Gamma rays (b) Beta rays (c) Microwave (d) Visible light 2. Minimum interplanar spacing required for Bragg’s diffraction is: (a) /4 (b) /2 (c)  (d) 2 3. In Bragg’s equation  is the angle between: (a) Specimen surface and incident rays (b) Normal to specimen surface and incident rays (c) Parallel lattice surfaces at distance d apart and incident rays (d) Normal to parallel lattice surfaces at d distance apart and incident rays 4. Bragg’s equation is expressed as (a) n = 2.d. sin  (b) n/2 = 2.d. sin  (c)  = d. sin  (d) 2n = d. sin  5. X-ray diffractometers are not used to identify the physical properties of which of the following? (a) Metals (b) Liquids (c) Polymeric materials (d) Solids 6. Which of these techniques are used to detect the inherent crystal structure. (a) XRD (b) SEM (c) TEM (d) TGA 7. Which of these techniques are used to detect surface morphology of materials? (a) XRD (b) SEM (c) TEM (d) TGA 8. Which of these techniques are used to define characterization of nanomaterials? (a) XRD (b) SEM (c) TEM (d) All of these 9. The secondary electrons radiated back in scanning microscope is collected by (a) Specimen (b) Anode (c) Cathode (d) Vacuum chamber 10. The changes in weight of oxysalts is measured by (a) Elemental analysis (b) Wagner analysis (c) Stockbarger analysis (d) Thermal analysis 11. Identify which of these happens in a scanning electron microscope (SEM). (a) Electrons interact with analyte (b) Electrons hit the nucleus and gets deflected (c) Electrons reduce the analyte (d) None of the above mentioned

Characterization of Materials 115

12. Which of the followings is a sample destructive technique? (a) SEM (b) TGA (c) XRD (d) All of these 13. Identify the component in electron microscope to focus electron beam. (a) Optical lens (b) Prism (c) Grating (d) Magnetic coils 14. Which of these is preferred for analyzing cconducting materials? (a) XRD (b) DTA (c) STM (d) SEM 15. Which of these is true with TGA? (a) Traces decomposition pattern (b) Traces both decomposition and oxidation (c) Traces oxidation pattern (d) None of these 16. Which of these is a major disadvantage of electron microscopes? (a) Not lightweight (b) Not ecofriendly (c) Not accurate (d) Expensive 17. The ionization energy of an atom depends on the surrounding: (a) Lattice environment (b) Lattice coordination number (c) Lattice size (d) None of these 18. The distance between the two peaks in XPS is referred to as (a) Spin orbital splitting (b) Spin orbital coupling (c) Spin covalent splitting (d) None of these 19. The instrument used to maintain the angle and rotate the sample in XRD is called (a) Goniometer (b) Galvanometer (c) Galiometer (d) None of these 20. Which of the following is significant in DTA analysis? (a) Catalytic properties of enzyme (b) Elasticity of crystals (c) Enthalpy of substances (d) Line positions of phases ANSWERS

1. a

2. b

3. b

4. a

5. b

6. a

7. b

8. d

9. b

10. d

11. b

12. b

13. d

14. c

15. a

16. d

17. d

18. a

19. a

20. c

116 Chemistry of Engineering Materials

5 Nanotechnology

5.1 INTRODUCTION Nanotechnology is a group of techniques having ability to manipulate the properties of matter measurable with the scale of 10–9 and generally of size between 1 to 100 nm. These techniques, being highly interdisciplinary, have gained popularity over the decades as they have led to a revolution in research and development of material science, water technology, drug delivery techniques, Analysts say the global market energy storage, communication and computing, etc. for manufactured goods using It refers to the conception and creation of functional nanomaterials could hit $5.6 structures, devices, and systems in the scale length of trillion by 2030. 1–100 nm, where new functionalities and properties of matter are observed and harnessed for a wide Shirt that charges your cell phone as range of applications. you stroll, or an implanted device for measuring blood pressure that’s Example: 1. Bulk copper: These are highly malleable and powered by your own heartbeat is ductile but nano-Cu particles, are superhard possible with nanotechnology with no malleability and ductility. 2. Ferromagnetic materials in Bulk: These can be used as memory storage devices but nanoparticles switch their magnetization direction using room temperature and thermal energy thus making them unsuitable for memory storage. 3. Gold nanoparticles appears deep red to black: These are smaller than 100 nm, have many properties, often referred to as size dependent properties which differ from corresponding bulk material thereby making them attractive for many electronic, optical or magnetic applications. Some of the interesting size dependent properties are: i. Electrical properties: Some metals which are good conductors in bulk become semiconductors or insulators as their size decreases to nano level. When particle size decreases, the energy bands Studies show that nanoparticles can gradually convert into discrete molecular work their way into the bloodstream, electronic levels. penetrate cells, and get pass the bloodii. Surface area: When a bulk material is brain barrier. Research has linked such replaced by a nano- material, the total particles to lung damage; the brain may volume remains the same. Thus the be affected too. 116

Nanotechnology 117

collective surface area is greatly increased. Properties like catalytic activity, gas adsorption, and chemical reactivity which depend on surface area show specific surface related properties. iii. Optical properties: Size of metal nanomaterials are very much less than wavelength of visible light. Therefore, it cannot absorb visible light. However, nano-particles are found to exhibit optical absorption due to a phenomenon called surface plasmon resonance (SPR). When light hits the surface of metal particle, the electrons on the surface starts oscillating back and forth creating an electron cloud which is larger than the wavelength of light, it can capture radiation at different wavelengths. Therefore, nano-particles show unique absorption and reflection properties causing different colorations. iv. Lattice parameters: The lattice parameters decreases with the decrease in size of nanoparticles. This phenomenon can be explained using continuous media model which is based on the shape of the material which decides the surface area and the following assumptions: a. A nano particle is taken out from the bulk material with no changes in the structure. b. Surface tension will elastically shrink and increase the surface energy of particle on pulling it out of the bulk. c. A nano particle will be formed in equilibrium conditions. Taking the shape of nanoparticle into consideration, generally all nanoparticles are assumed to be spherical. However, when a particle is non-spherical, its volume is assumed to be the same of spherical nano particle. To alter the difference in shape between the spherical and non-spherical nanoparticles, a factor ‘’ is introduced which is expressed as  = S/S1 where S = surface area of spherical nanoparticle given by S = 4r2 (5.1) 1 and S = surface area of non-spherical nano particle whose volume is same as spherical nanoparticle,  = 1 and  > 1 for any spherical and non-spherical nano particles respectively. Surface energy ‘’ is given by the equation (5.2)  = 0 + T (d/dT) where 0 = surface energy per unit area at 0 K, d/dT is the coefficient of surface free enrgy to temperature which is