Boron-based fuel-rich propellant: properties, combustion, and technology aspects 9780367141660, 0367141663

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Boron-Based Fuel-Rich ­Propellant Properties, Combustion, and Technology Aspects

Boron-Based Fuel-Rich ­Propellant Properties, Combustion, and Technology ­Aspects

WeiQiang Pang Luigi T. De Luca, XueZhong Fan, Oleg G. Glotov, FengQi Zhao

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2019 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed on acid-free paper International Standard Book Number-13: 978-0-367-14166-0 (Hardback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the ­copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged, please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, ­including ­photocopying, microfilming, and recording, or in any information storage or retrieval system, without written ­permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http:// www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Contents Preface............................................................................................................................................ xiii Foreword.........................................................................................................................................xv Summary...................................................................................................................................... xvii Authors.......................................................................................................................................... xix Abbreviations............................................................................................................................... xxi 1. Basic Formulations and Components of Ramjet Propellants.........................................1 1.1 Fuel-Rich Propellants....................................................................................................1 1.2 Main Components of Boron-Based Fuel-Rich Solid Rocket Propellant................. 2 1.2.1 Binders................................................................................................................2 1.2.2 Metal Fuels.........................................................................................................2 1.2.2.1 Boron in Nature.................................................................................4 1.2.2.2 Amorphous and Crystalline Boron................................................4 1.2.2.3 Preparation of Amorphous Boron Powder.................................... 5 1.2.2.4 Physicochemical Properties of Boron Powders.............................6 1.2.2.5 Boron–Aluminum or Boron–Magnesium Alloy Powder............7 1.2.3 Oxidizers and Solid Fillers..............................................................................7 1.2.4 Combustion Catalysts and Stabilizers...........................................................8 1.2.5 Combustion Stabilizer......................................................................................9 1.2.6 Bonding Agents.................................................................................................9 1.2.7 Antioxidants.................................................................................................... 12 1.3 Main Properties of Boron-Based Fuel-Rich Solid Rocket Propellant................... 13 1.3.1 Energetic Properties....................................................................................... 13 1.3.2 Processibility (Rheological and Surface-Interfacial Properties)............... 14 1.3.2.1 Effect of Content.............................................................................. 16 1.3.2.2 Impact of Particle Properties on Viscosity................................... 16 1.3.3 Combustion Properties.................................................................................. 18 1.3.4 Performance Characteristics of Boron-Based Fuel-Rich Propellants...... 19 1.4 Main Problems of Boron-Based Fuel-Rich Solid Rocket Propellant..................... 20 1.5 Recent Progress of Boron-Based Fuel-Rich Solid Rocket Propellant....................22 1.5.1 Development of Modification of Boron Powder.........................................22 1.5.2 Current Research Situation of Boron-Based Fuel-Rich Solid Rocket Propellants������������������������������������������������������������������������������������������������������ 23 1.6 Prospects of Boron-Based Fuel-Rich Solid Rocket Propellant............................... 28 References................................................................................................................................ 29 2. Surface Modification and Characterization of Boron Powder..................................... 33 2.1 Introduction.................................................................................................................. 33 2.2 Deterioration of Propellant Manufacturing Process Caused by Boron and Its Mechanism�������������������������������������������������������������������������������������������������������������34 2.2.1 XPS Analysis....................................................................................................34 2.2.2 R heological Analysis......................................................................................34

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2.2.3 2.2.4

Infrared Analysis............................................................................................ 36 Mechanism of the Deteriorated Process in HTPB Fuel-Rich Propellant������������������������������������������������������������������������������������������������������ 37 2.3 Preparation Process of Agglomerated Boron Particles........................................... 38 2.3.1 Pretreatment of Amorphous Boron Powder............................................... 39 2.3.2 Coating of Amorphous Boron Powder........................................................ 40 2.3.3 Agglomeration and Prills of Amorphous Boron Powder.........................43 2.4 The Particle Size Distribution and Fractal Dimension Characterization of Agglomerated Boron Particles���������������������������������������������������������������������������������� 50 2.4.1 Measuring Principle of Fractal Dimension................................................. 50 2.4.2 The Particle Size Fractal Dimension of Solid Additives in Solid Propellants������������������������������������������������������������������������������������������������������ 51 2.4.3 Fractal Dimension of Surface Roughness and Size Distribution of Agglomerated Boron Particles and Its Relationship with Rheological Properties of Fuel-Rich Propellant���������������������������������������� 56 2.5 Bulk Density of Agglomerated Boron Particles....................................................... 59 2.5.1 Bulk Density Measurement of Agglomerated Boron Particles................ 60 2.5.2 Two Types of Bulk Density Comparison of Agglomerated Boron Particles����������������������������������������������������������������������������������������������������������� 60 2.5.3 Dispersion Property of Agglomerated Boron Particles............................. 62 2.5.4 Effects of Vibration Times on the Tap Bulk Density of Agglomerated Boron Particles���������������������������������������������������������������������63 2.6 Robustness of Agglomerated Boron Particles..........................................................64 2.6.1 Robustness Test Principle of Agglomerated Boron Particles...................64 2.6.2 Basic Physical Parameters of Agglomerated Boron Particles...................65 2.6.3 Effects of Different Factors on the Robustness of Agglomerated Boron Particles�����������������������������������������������������������������������������������������������65 2.7 Surface Properties of before and after Agglomeration Boron Particles............... 68 2.7.1 Morphology of Agglomerated Boron Particles........................................... 68 2.7.2 Acidity Analysis of Agglomerated Boron Particles................................... 68 2.7.3 X-Ray Analysis of Agglomerated Boron Particles...................................... 71 2.7.4 Coating Degree of Agglomerated Boron Particles..................................... 71 2.8 Crystal Boron Powder................................................................................................. 72 2.8.1 The Physicochemical Properties of Crystal Boron Powder...................... 72 2.8.2 XRD Analysis of Crystal Boron Powder...................................................... 74 2.8.3 XPS Analysis of Crystal Boron Powder....................................................... 74 2.8.4 Surface Acid Property of Crystal Boron Powder....................................... 76 2.9 Boron/Magnesium (Aluminum) Composite Powders........................................... 76 2.9.1 Morphology and Particle Size Distribution of B/Mg (Al) Composite Powders��������������������������������������������������������������������������������������� 76 2.9.2 Density of B/Mg (Al) Composite Powders.................................................. 78 2.9.3 X-Ray Analysis of B/Mg (Al) Composite Powders..................................... 78 2.9.4 Surface Acid Degree of B/Mg (Al) Composite Powders........................... 79 References................................................................................................................................80 3. The Surface-Interfacial Properties of Boron and Binder System................................83 3.1 Problem Statement.......................................................................................................83 3.2 Measuring Principle and Methods............................................................................83 3.2.1 Surface-Interfacial Chemical Principle........................................................83

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3.2.2 Measuring Principle of Contact Angle........................................................85 3.2.3 Calculating Principle of Adhesion Work and Spread Coefficient........... 86 3.2.4 Measuring Method......................................................................................... 86 3.3 Surface-Interfacial Properties of ABPs and Binder System.................................... 87 3.3.1 Surface Properties of ABPs............................................................................ 88 3.3.2 Calculation of Contact Angle and Surface Free Energy............................ 89 3.3.3 Calculation of Adhesion Work and Spread Coefficient............................90 3.3.4 Surface Modification Mechanism of ABPs.................................................. 91 3.4 Surface-Interfacial (Multi-Interfacial) Properties of Fuel-Rich Solid Propellant Containing ABPs������������������������������������������������������������������������������������� 96 3.5 Effects of ABP on the Microstructures of Fuel-Rich Solid Propellant.................. 97 References................................................................................................................................ 98 4. Rheological Properties....................................................................................................... 101 4.1 Boron Propellant Slurries.......................................................................................... 101 4.2 Characterization and Measurement of Rheological Properties of Propellant......101 4.2.1 Characterization of Rheological Properties.............................................. 101 4.2.2 Preparation of Samples................................................................................ 102 4.2.3 Measuring Methods..................................................................................... 103 4.3 Effects of Different Elements on the Rheological Properties of Fuel-Rich Solid Propellants������������������������������������������������������������������������������������������������������� 103 4.3.1 R heological Properties of HTPB Binder.................................................... 103 4.3.2 R heological Properties of ABP/HTPB Slurry........................................... 104 4.3.3 Effects of Different Ingredients on the Rheological Properties of Fuel-Rich Solid Propellant�������������������������������������������������������������������������� 107 4.4 Processibility of Boron-Based Fuel-Rich Solid Rocket Propellants.................... 117 4.4.1 Designing Principle of Boron-Based Fuel-Rich Solid Rocket Propellant������������������������������������������������������������������������������������������������������ 118 4.4.2 Processibility of Fuel-Rich Solid Propellant............................................. 118 4.4.3 Formulations of Fuel-Rich Solid Propellant Containing Agglomerated Boron Particles������������������������������������������������������������������� 118 4.4.4 Preparation of Boron Fuel-Rich Solid Propellant Samples..................... 119 4.4.5 P  rocessibility of Boron Fuel-Rich Solid Propellant Samples.................. 119 References.............................................................................................................................. 121 5. Energetic Properties............................................................................................................ 123 5.1 General Description................................................................................................... 123 5.2 Estimation of Energetic Properties of Boron-Based Fuel-Rich Solid Propellant������������������������������������������������������������������������������������������������������������������� 123 5.2.1 C  alculation of Energetic Properties of Fuel-Rich Solid Propellant....... 125 5.2.2 T  heoretical Calculation and Measurement Methods of Combustion Heat����������������������������������������������������������������������������������������� 128 5.2.3 P  ropellant Density Measurements............................................................. 129 5.3 Technical Approaches to Increase the Energetic Properties of Fuel-Rich Solid Propellant��������������������������������������������������������������������������������������������������������� 129 5.3.1 Addition of Metal Fuels............................................................................... 129 5.3.2 Effective Additives........................................................................................ 130 5.4 Effect of Formulation Factors on the Energetic Properties of Boron-Based Fuel-Rich Solid Propellant��������������������������������������������������������������������������������������� 131

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5.4.1 Binder Type.................................................................................................... 131 5.4.2 Boron Fuel...................................................................................................... 133 5.4.3 Oxidizers and Solid Fillers.......................................................................... 138 References.............................................................................................................................. 145 6. Combustion Properties....................................................................................................... 147 6.1 Introduction................................................................................................................ 147 6.2 Characterization and Measurement of Combustion Properties of Fuel-Rich Solid Propellant��������������������������������������������������������������������������������������� 148 6.2.1 Characterized Parameters........................................................................... 148 6.2.1.1 Burning Rate.................................................................................. 149 6.2.1.2 Burning Rate Pressure Exponent................................................ 149 6.2.1.3 Burning Rate Temperature Sensitivity....................................... 149 6.2.2 Measuring Methods..................................................................................... 150 6.3 Technical Approaches to Improve the Combustion Properties of Boron-Based Fuel-Rich Solid Propellant��������������������������������������������������������������� 151 6.3.1 Addition of Agents....................................................................................... 151 6.3.2 Surface Coating of Boron Powder............................................................... 155 6.3.2.1 Surface Coating by Means of LiF, Fluorine Rubber, and Silane���������������������������������������������������������������������������������������������� 155 6.3.2.2 Surface Coating by Means of Oxidizers or Energetic Binders������������������������������������������������������������������������������������������� 156 6.3.2.3 Replacing Inert Binder by Energetic Binders............................ 156 6.4 Ignition and Combustion of Boron Powder........................................................... 158 6.4.1 Ignition Model of Boron Powder................................................................ 159 6.4.2 Ignition Model of Agglomerated Boron Particles.................................... 159 6.4.3 Ignition Properties of Boron Powder in Different Atmosphere............. 161 6.4.3.1 Combustion in the Air.................................................................. 161 6.4.3.2 Ignition of Boron in an Environment Containing Water Vapor���������������������������������������������������������������������������������������������� 162 6.4.3.3 Ignition of Boron Particles in Cl2................................................ 163 6.4.3.4 Ignition of Boron Particles in an Environment Containing Fluorine������������������������������������������������������������������������������������������ 164 6.4.3.5 Ignition of Boron Particles in an Environment Containing Nitrogen����������������������������������������������������������������������������������������� 165 6.4.4 Emission Spectrum and Flame Morphology of Boron Powder in Different Atmosphere��������������������������������������������������������������������������������� 165 6.4.4.1 Pure N2/Pure O2............................................................................ 167 6.4.4.2 H2O/O2 Mixed Atmosphere......................................................... 168 6.4.4.3 N2/O2 Mixed Atmosphere............................................................ 168 6.5 Effects of Boron on the Combustion Properties of Fuel-Rich Solid Propellant����171 6.5.1 C  ombustion Properties of Boron-Based Fuel-Rich Solid Propellant..... 172 6.5.2 T  hermal Decomposition of Surface-Modified Boron Particles.............. 173 6.5.3 C  ombustion Properties of Fuel-Rich Solid Propellant Containing Crystal Boron Powder��������������������������������������������������������������������������������� 176 6.5.4 T  hermal Decomposition of Fuel-Rich Solid Propellant Containing Crystal Boron Particles�������������������������������������������������������������������������������� 177 6.5.5 C  ombustion Properties of Fuel-Rich Solid Propellant Containing Boron (Magnesium) Composite Particles������������������������������������������������� 179

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6.5.6 Solid Motor Tests of Fuel-Rich Solid Propellant Containing Agglomerated Boron Particles������������������������������������������������������������������� 180 6.6 Combustion Mechanism of Fuel-Rich Solid Propellants..................................... 183 6.6.1 Combustion Flame Structures.................................................................... 183 6.6.1.1 The Combustion Flame Structure of Boron-Based Fuel-Rich Propellant�������������������������������������������������������������������� 184 6.6.1.2 Effect of Agglomerated Boron Particles on the Combustion Flame Structure of Fuel-Rich Propellant����������� 185 6.6.1.3 Effect of Coated Materials of AP and LiF on the Combustion Flame Structure of Fuel-Rich Propellant����������� 185 6.6.2 Combustion Wave Temperature................................................................. 186 6.6.2.1 The Combustion Wave Structure of Boron-Based Fuel-Rich Propellant�������������������������������������������������������������������� 187 6.6.2.2 Effect of Agglomeration on the Combustion Wave Structure of Boron-Based Fuel-Rich Propellant���������������������� 189 6.6.2.3 Effect of AP Coating on the Combustion Wave Structure of Boron-Based Fuel-Rich Propellant���������������������������������������� 192 6.6.2.4 Effect of LiF Coating on the Combustion Wave Structure of Boron-Based Fuel-Rich Propellant���������������������������������������� 194 6.6.3 Flameout Surface Analysis.......................................................................... 196 6.6.4 Primary Combustion Product Analysis.................................................... 198 6.6.4.1 Collection of Primary Combustion Products............................ 199 6.6.4.2 Morphology and Particle Size of Primary Condensed Combustion Products������������������������������������������������������������������ 202 6.6.4.3 Chemical Analysis of Condensed Phase Products Qualitative and Quantitative Aspects��������������������������������������� 203 6.6.4.4 Preliminary Analysis of Organic and Gaseous Products in the Primary Combustion Products��������������������������������������� 212 6.6.5 Combustion Residue Analysis.................................................................... 215 6.6.5.1 Experimental Principle of Chemical Analysis of Combustion Residue�������������������������������������������������������������������� 215 6.6.5.2 T  he Main Components and Determination of Combustion Residues of Boron-Based Fuel-Rich Propellants����������������������������������������������������������������������������������� 216 6.6.5.3 Reliability Verification of Method...............................................223 6.6.5.4 Effect of Coat on the Combustion of Boron in Fuel-Rich Propellant�������������������������������������������������������������������� 224 6.7 Influencing Factors on the Primary Combustion Ejection Efficiency of Boron-Based Fuel-Rich Propellant������������������������������������������������������������������������� 226 6.7.1 Propellant Formulation and Experimental Scheme................................ 226 6.7.2 Effects of Ingredients on the Efficiency of Fuel-Rich Propellant........... 226 6.7.3 Ejection Apparatus on the Efficiency of Fuel-Rich Propellants............. 229 6.8 T he Primary Combustion Model of Boron-Based Fuel-Rich Solid Propellant���������������������������������������������������������������������������������������������������� 229 6.8.1 Establishment of Physical Model in the Primary Combustion.............. 230 6.8.2 Establishment of Mathematical Model in the Primary Combustion.... 231 6.8.1.1 AP/HTPB System.......................................................................... 232 6.8.1.2 Agglomerated Boron/AP/HTPB System.................................... 233 References.............................................................................................................................. 236

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7. Combustion Performance of Model Propellant with Boron and Boron-Containing Compounds........................................................................................ 243 7.1 Boron as Ingredient for Composite Propellant...................................................... 243 7.2 Boron–Aluminum Mechanical Alloy as a Metallic Fuel...................................... 244 7.2.1 Production and Characterization of B-Al Mechanical Alloys............... 245 7.2.1.1 Milling Regimes and X-Ray Data............................................... 245 7.2.1.2 Granulometric Data...................................................................... 247 7.2.1.3 Analytical Chemistry of Powders and Chemical Analyses Data������������������������������������������������������������������������������� 250 7.2.1.4 Summary of Al/B Mechanical Alloy Properties......................254 7.2.2 Experimental Procedures............................................................................254 7.2.2.1 Propellant Formulations and Samples.......................................254 7.2.2.2 Experimental Techniques............................................................ 255 7.2.3 Results and Discussion................................................................................ 256 7.2.3.1 Preliminary Remarks on Parameters Characterizing the Properties of Alloy as a Propellant Component���������������������� 256 7.2.3.2 Burning Rate and Flame Temperature....................................... 257 7.2.3.3 Ignition Delay................................................................................ 262 7.2.3.4 Study of CCP.................................................................................. 262 7.2.4 Summary on Combustion Parameters of Model Propellant with B-Al Mechanical Alloys������������������������������������������������������������������������������ 267 7.3 Reactivity of Boron and Boron–Magnesium Alloy-Based Propellants.............. 268 7.3.1 Experimental Procedures............................................................................ 269 7.3.1.1 Materials......................................................................................... 269 7.3.1.2 Boron-Filled Energetic Binders................................................... 270 7.3.1.3 Thermal Gravimetric Analysis.................................................... 271 7.3.1.4 X-Ray Photoelectron Spectroscopy............................................. 271 7.3.2 Results and Discussion................................................................................ 271 7.3.2.1 The Effect of Boron Particles Coated with GAPm.................... 272 7.3.2.2 The Effect of Adding Mg into Boron Particles Coated with GAPm����������������������������������������������������������������������������������� 272 7.3.2.3 TGA Experiments.......................................................................... 275 7.3.2.4 The Effect of Milling Time........................................................... 276 7.3.3 Summary on Combustion Parameters of Model Propellant with B-Mg Alloys�������������������������������������������������������������������������������������������������� 276 7.4 Formation of CCP of Boron-Containing Solid Propellants and Combustion Efficiency��������������������������������������������������������������������������������������������� 278 7.4.1 Boron Agglomeration, CCP, and Combustion Efficiency........................ 278 7.4.2 Mechanism of Solid Residue Formation................................................... 280 7.4.3 Modeling of the Boron Particles Agglomeration..................................... 282 7.4.4 Critical Conditions for the Slag Formation...............................................284 7.4.5 Dependence of the Mass of Residues on Mean Pressure in Gas Generator������������������������������������������������������������������������������������������������������� 287 7.5 Combustion Efficiency of Propellants Containing Boron Fuels......................... 289 7.5.1 General Remarks........................................................................................... 289 7.5.2 Components, Formulations, and Experimental Program....................... 290 7.5.3 Propellant Specimens and Sampling Technology and Treatment........ 291 7.5.4 Experimental Results................................................................................... 293 7.5.4.1 Burning Rate.................................................................................. 294

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7.5.4.2 CCP of Propellants with Boron................................................... 294 7.5.4.3 CCP of the Propellants with Boron and Aluminum................ 296 7.5.4.4 The Energy Release Efficiency.................................................... 296 7.5.5 Summary on Combustion Efficiency of Model Propellant with B and AlB2����������������������������������������������������������������������������������������������������� 298 References.............................................................................................................................. 299 8. Application Perspectives and Development Trends of Boron-Based Fuel-Rich Propellants......................................................................................................... 307 8.1 Introduction................................................................................................................ 307 8.2 Requirements of Boron-Based Fuel-Rich Solid Rocket Propellants................... 307 8.2.1 Requirements of New Air-to-Air Missile.................................................. 307 8.2.2 Requirements of Supersonic Velocity Missile in Future.........................308 8.2.3 Requirements of Ground–Air Missile against High Mobility Targets��������������������������������������������������������������������������������������� 308 8.2.4 Requirements of Large Caliber Artillery Lift...........................................309 8.3 Development Progress and Trends of Fuel-Rich Solid Rocket Propellant........ 309 8.3.1 Low Signature...............................................................................................309 8.3.2 Insensitive Characteristics...........................................................................309 8.3.3 High-Pressure Exponent..............................................................................309 8.3.4 Using Nanosized Fuels in Fuel-Rich Solid Propellant............................ 310 8.4 Advice of Experts to the Development of Fuel-Rich Solid Propellants............. 311 References.............................................................................................................................. 312 Index.............................................................................................................................................. 315

Preface Solid rocket ramjets featuring lightweight, small volume, fast velocity, large operative range, and good maneuvering performance are the most suitable power devices to adapt the needed quickness and flexibility technical requirements to the highly ­supersonic ­aircrafts to be used as future new-generation missiles. In this respect, the combination of different aspects of aerospace propulsion technology is the best choice. In recent years, with the development of science and technology, solid rocket ramjets have drawn a ­widespread attention by worldwide researchers, and fuel-rich solid propellants to be burned in solid rocket ramjets have undergone a rapid development. Boron powder, due to its very high gravimetric and volumetric heat of combustion, has become the most promising metal fuel candidate of fuel-rich solid propellants, ­attracting much attention from researchers all over the world. Boron-Based Fuel-Rich Propellant: Properties, Combustion, and Technology Aspects is a professional book designed to systematically introduce the latest research progress in the areas of boron-based fuel-rich solid propellants. This publication covers the surface modifications, coating techniques, granulation, and characterization of amorphous boron powder as well as its application to fuelrich solid rocket propellants. Moreover, technologies to control the processing ­methods and combustion performance of fuel-rich propellants are also touched. At the end, the book summarizes the research in boron-based fuel-rich solid propellants and looks ­forward to the foreseeable development trends of military applications. This ­volume ­constitutes a collective scientific research achievement by a team of authors through referring to and summarizing recent domestic as well as international research findings in the relative field. Thus, it has great value both as a theoretical research accomplishment, thanks to the wide scope of its compilation, and as a reference document, thanks to the large amount of reported experimental data. The publication of this book is an important contribution to the international ­technical literature and will certainly play an important role in the theoretical as well as technological development of boron-based fuel-rich solid rocket propellants. Professor Luigi T. De Luca SPLab, Space Propulsion Laboratory, Politecnico di Milano, Italy September 16, 2016

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Foreword Solid rocket ramjet features possess strong penetration ability and strike force, which make them the best choices for air-to-air missiles, antiship missiles, and so on. Since solid rocket ramjets use air as the oxidizer, their energy is much higher than that of solid rocket motors. When engines are of the same volume and weight, ramjets can provide a range at least two times higher than that of solid rocket motors. Such excellent performance is based on the high-energy properties of fuel-rich propellants. The more energy they have, the greater the potential of ramjets will be. They can provide high specific impulse, ­realize powered flight over the entire range, and have a small volume, long working time, and good maneuvering performance, which satisfy the requirements of air-to-air, air-to-ground, antiship, and antiradiation missiles, thus becoming the most suitable power devices worthy of research in terms of extending the range of a projectile. In recent years, developing solid rocket ramjets using fuel-rich propellants became a hot topic again. After decades of development, their application technology has seen a trend of the increase of performance and subdivision of varieties. As an important power source for ramjets, boron-based fuel-rich solid propellants are currently one of the fuel-rich propellants that have the highest energy and one of the most ideal propulsion energy sources for ramjets, grabbing extensive attention across the world. However, a book d ­ edicated to the systematic description of boron-based fuel-rich propellant technology has not yet appeared. There is only a limited introduction to the fuel-rich propellant technology in certain chapters of some books. Therefore, this book aims to give a systematic summary and improvement to the published articles and papers on the basic research of this field, serving as a useful technical reference document for engineers and technicians ­engaging in solid-propellant research and applications. This publication focuses on the surface modifications and agglomeration of boron powder as well as the energy characteristics, processing properties, and combustion performance of boron-based fuel-rich propellants, reflecting the current research and development level of boron-based fuel-rich propellants and providing important guidance for relevant research and development techniques. With 8 chapters, this book discusses the properties and application requirements of boron-based propellants in a systematic way. Chapter 1 introduces the main ­composition, properties, and latest research development of boron-based fuel-rich propellants and outlines the performance characteristics of and high energy weapons’ requirement for boronbased fuel-rich propellants as well as the pending problems. Chapter 2 covers the surface modifications and agglomeration of amorphous boron powder and compares the status ­ roperties of before and after modifications. Chapter 3 touches on the surface/interface p boron-based fuel-rich propellants, analyzes the surface/interface properties of boron power before and after modifications and of propellant compositions, as well as the surface/ interface mechanism of agglomerated boron particles and binders from the perspective of contact angle, surface free energy, and adhesive function. Chapter 4 introduces the rheological properties of the boron-based fuel-rich propellant slurry that are analyzed and explained from the perspective of the binder system, boron powder before and after modifications, as well as binder suspension and boron-based fuel-rich propellant slurry, and explores the preparation method of boron-based fuel-rich propellants. Chapter 5 analyzes the energy performance and factors affecting boron-based fuel-rich propellants both theoretically and practically. Chapter 6 studies the combustion performance  of  boron-based xv

xvi

Foreword

fuel-rich≈propellants and establishes the ignition model of boron powder. It introduces the influence of different types and particle sizes of boron powder on the combustion performance of boron-based fuel-rich propellants, and analyzes the influences of different factors on their combustion efficiency in terms of combustion residues. Chapter 7 introduces the combustion performance of boron-metal alloy, with an emphasis on the combustion properties and the condensed combustion products characteristics of boron–aluminum and boron–magnesium alloys and pure boron in solid propellants. Chapter 8 introduces the requirement for boron-based fuel-rich propellants and provides suggestions for their development. The completion of this book is attributed to support and help from all sides. Our special thanks go to Prof. Li BaoXuan, Prof. Hu SongQi, and Prof. Zhang JiaoQiang at Northwestern Polytechnical University for their guidance and help in the scientific experiments; to Prof. Shen RuiQi and Prof. Zhou WeiLiang at the School of Chemical Engineering of Nanjing University of Science and Technology, and Prof. Liu ZongHuai and Prof. Zhang GuoFang at the School of Materials Science and Engineering of Shaanxi Normal University for their invaluable suggestions; to research fellows Zhang XiaoHong, Xu HuiXiang, Yu HongJian, and Zhang Wei, associate research fellows including Li YongHong, and Sun ZhiHua, as well as leaders at all levels of Xi’an Modern Chemistry Research Institute for their great support and help in the completion of this publication. Some of the contents of this book is from international collaboration, and the authors give their warmest thanks to Prof. V. E. Zarko, V. N. Simonenko, G. S. Surodin, and A. B. Kiskin at the Voevodsky Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, Novosibirsk, Russia; R. K. Tukhtaev, T. F. Grigor’yeva at the Institute of Solid State Chemistry and Mechanochemistry, Russian Academy of Sciences, Novosibirsk, Russia; T. D. Fedotova at Novosibirsk State University, D. A. Yagodnikov at Bauman Moscow State Technical University, Moscow, Russia; Prof. Alon Gany at Israel Institute of Technology, Israel; Prof. Luciano Galfetti, Dr. Filippo Maggi, and Dr. Christian Paravan at the Space Propulsion Laboratory (SPLab), Politecnico di Milano, Italy; Prof. Sergey A. Rashkovskiy at the Institute for Problems in Mechanics of the Russian Academy of Sciences, Russia; Prof. Charles Dubois at École Polytechnique de Montréal, Canada, for their support, ­suggestions, and English revisioin. Defects may exist in the book due to the ever-changing science and technology and the limitations of the authors’ knowledge. Readers are welcome to offer suggestions. WeiQiang Pang, Luigi T. De Luca, XueZhong Fan, Oleg G. Glotov, and FengQi Zhao October 30, 2018 Xi’an Modern Chemistry Research Institute Space Propulsion Laboratory (SPLab), Politecnico di Milano Voevodsky Institute of Chemical Kinetics and Combustion Siberian Branch of the Russian Academy of Sciences

Summary The book is intended for readers with a technical or scientific background, active in ­governmental agencies, research institutes, trade, and industry, concerned with the ­procurement, development, manufacture, investigation, and use of energetic materials, such as high explosives, propellants, pyrotechnics, fireworks, and ammunition. The book serves both as a daily reference for the experienced as well as an introduction for the ­newcomers to this field.

xvii

Authors

Dr. WeiQiang Pang, professor, 2014–2015 Visiting Scholar, Politecnico di Milano. For years he is engaged in combustions and application of energetic materials in composite and fuel-rich solid propellants, modeling and simulation, nanosized technology, and the ­performance of metallized and dual metal formulations, agglomerations, and a­ ggregations. He has ­published more than 100 papers in the international and national reviewed journals, holds more than 50 patents, and 5 books in recent years. Address: Xi’an Zhangba East Road No.168, Xi’an Modern Chemistry Research Institute/ Tel: +86 029 88291063/Fax: +86 029 88220423/E-mail: [email protected], pangwq204@ gmail.com. Dr. Luigi T. De Luca, 1990–2015 Professor, Politecnico di Milano; 2013–2017 guest professor, Nanjing University of Science and Technology (NUST), Nanjing, China; 2010 honorary ­professor, Omsk State Technical University, (OmSTU), Omsk, Russia; and 1998 visiting scholar, Kyushu Institute of Technology, Japan. He is an Associate Fellow at American Institute of Astronautics and Aeronautics. From 2013 to 2018, he was on the editorial boards of Chinese Journal of Explosives and Propellants and Chinese Journal of Defense Technology. From 2010 to 2015, he was on the editorial board of International Journal of Energetic Materials and Chemical Propulsion. De Luca has engaged in many international and Italian national ­projects and also published more than 200 papers and 10 books. He has directed ­several international collaborative research efforts with former Soviet Union countries (CNR, RAS, INTAS, ISTC) and organized several international workshops attended by the most ­qualified international scientists. In recent years, his interests have moved to the combustion of innovative high-energy condensed ­materials, nanoenergetic for propulsion, performance of metallized formulations, agglomeration and aggregation, dual metal formulations, quasi-steady regression rates, solid and hybrid rocket motors, space launchers, and in-space propulsion. Address: Via La Masa 34, Space Propulsion Laboratory (SPLab), Politecnico di Milano/ Tel: +39 3460557442/E-mail: [email protected], [email protected]. Dr. XueZhong Fan, professor, doctor supervisor. He is engaged in applied chemistry and solid ­propellants. Fan has published more than 300 papers in international reviewed ­journals and six books during recent years. Address: Xi’an Zhangba East Road No.168, Xi’an Modern Chemistry Research Institute/ Tel: +86 029 88291138/Fax: +86 029 88220423/E-mail: [email protected]. Dr. Oleg G. Glotov is the head of the Laboratory of Condensed Systems Combustion at Voevodsky Institute of Chemical Kinetics and Combustion Siberian Branch of the Russian Academy of Sciences (ICKC SB RAS). He is engaged in metal combustion, aluminum agglomeration, condensed combustion products of metalized compositions, ignition of solids, combustion of titanium and combined metal fuels, and condensed combustion products of combined metal fuels. In 2006 he was a guest lecturer at Beijing Institute of Technology, China. In 2018 he was a guest lecturer at Xi’an Modern Chemistry Research xix

xx

Authors

Institute, China. He has published more than 50 papers in international reviewed journals and books during recent years. Address: Institutskaya street, 3, Novosibirsk 630090, Voevodsky Institute of Chemical Kinetics and Combustion Siberian Branch of the Russian Academy of Sciences (ICKC SB RAS)/Tel: +7 383 3304847/Fax: +7 383 3307350/E-mail: [email protected]. Dr. FengQi Zhao, professor, doctor supervisor, the director of the National Defense Key Laboratory of Science and Technology on Combustion and Explosives. He is engaged in energetic materials and thermal decomposition and the combustion mechanism of solid propellant. He has published more than 400 papers in international reviewed journals and 10 books during recent years. Address: Xi’an Zhangba East Road No.168, Xi’an Modern Chemistry Research Institute/ Tel: +86 029 88291663/Fax: +86 029 88220423/E-mail: [email protected].

Abbreviations ABP ADN Al AN AP ATR B BAMO BA series BPS CCP CL-20 DOS DTA CTPB DSC DTG EDS GAP GFP HMX HNF HP HP2 HTPB KP MAPO Mg μAl nAl NMMO NPBA NP O/F PSAN RDX RN SFRJ TDI TEA TGA

agglomerated boron particles ammonium dinitramide aluminum powder ammonium nitrate ammonium perchlorate air-turbo rockets boron powder 3,3′-bis(azidomethyl)oxetane boric acid ester bonding agent series bis-propargyl succinate condensed combustion products hexanitrohexaazaisowurtzitane dioctyl sebacate differential thermal analysis carboxyl-terminated polybutadiene differential scanning calorimeter derivative thermogravimetry energy disperse spectroscopy glycidyl azide polymer catocene cyclotetramethylene tetranitramine (High Melting Explosive) hydrazine nitroform hydrazine perchlorate hydrazine diperchlorate hydroxyl-terminated polybutadiene potassium perchlorate methylaziridinyl phosphine oxide magnesium powder microsized aluminum powder nanosized aluminum powder 3-nitratomethyl-3-methyl oxetane neutral polymer bonding agent nitryl perchlorate air/fuel ratio phase-stabilized ammonium nitrate cyclotrimethylene trinitramine (Royal Demolition Explosive) reducing number solid fuel ramjet toluene-2,4-diisocyanate triethylaluminum; triethanolamine thermal gravimetric analysis

xxi

xxii

TMP trimethylol propane TNT trinitrotoluene Viton A, it is one trade designations for vinylidene type of Viton series   fluoride-­perfluoropropylene copolymer XPS X-ray photoelectron spectroscopy XRD X-ray diffraction

Abbreviations

1 Basic Formulations and Components of Ramjet Propellants

1.1  Fuel-Rich Propellants The solid rocket ramjet propulsion system features fast velocity, lightweight, small volume, high specific impulse, long range, simple structure, and reliability. It has a strong defense penetration power and strike force, and will be one of the best propulsion schemes for modern air-to-air missiles and anti-ship missiles.1–3 Compared with traditional solid rocket motors, solid rocket ramjets use fuel-rich propellants, taking oxygen from air as the main oxidizer for the secondary combustor, which obviously increases the range of the weapon. The solid ramjet mainly includes Solid Rocket Ramjet and Solid Fuel Ramjet, where fuelrich propellants are mostly used in the solid ramjet. Besides, in recent years, fuel-rich solid propellants are also used in solid fuel ramjets and air-turbo rockets with better theoretical performance. Fuel-rich propellants mainly include boron-based fuel-rich propellants, ­aluminum-based fuel-rich propellants, and magnesium-based fuel-rich propellants.4,5 Among them, the calorific value released by the full combustion of “medium energy” Mg-based and Al-based fuel-rich propellants is around 19–22 MJ·kg−1, the specific impulse can reach 5,000–8,000 N·S·kg−1, while the calorific value released by the full c­ ombustion of high-energy boron-based fuel-rich propellants is 30–34 MJ·kg−1, with a s­ pecific impulse of about 1.6–1.8 times of that of Al-based fuel-rich propellants and about 3 times that of solid propellants.6–9 The earlier data indicate that high-energy boron-based fuel-rich propellants have aroused widespread attention from researchers as one of the most ideal propulsion energy sources for solid motors. More than 40% mass fraction of boron powder in the fuelrich propellants has been successfully developed,10,11 with 40–42 MJ·kg−1 of calorific value and 13,994 N·s·kg−1 of specific impulse. Compared with the researches on boron-based fuel-rich propellants conducted abroad,12 the researchers in China mainly concentrated on the fuel-rich propellants with around 35% of boron, whose performance was far from meeting the requirements of high-energy fuel-rich propellants. How to increase boron quantity in fuel-rich propellants poses a challenge to the ­preparation techniques of fuelrich propellants, becoming a key technology that must be tackled in the development of such ­propellants. According to theoretical analysis, ­applying ­high-energy boron-based fuel-rich propellants to the solid ramjets of the new generation for medium- and longrange air-to-air missiles and cruise missiles can meet the performance requirement of high speed and long range.13

1

2

Boron-Based Fuel-Rich Propellant

1.2 Main Components of Boron-Based Fuel-Rich Solid Rocket Propellant Boron-based fuel-rich solid rocket propellants are a kind of composite solid propellants with binders as the matrix and solid fillers. Its main components are macromolecule binders (e.g., hydroxyl-terminated polybutadiene [HTPB]), oxidizers (e.g., ammonium ­perchlorate [AP]), metal fuels (e.g., boron powder B, Mg powder or aluminum powder), ballistic modifiers, plasticizers, and curing agents, which are used to render certain performance to the fuel-rich propellants. In the propellant formulation, additives with different functions and performance modifiers are also added, such as combustion catalysts, bonding agents, and antioxidants. 1.2.1 Binders Binder is a polymeric compound that can bind other components in the propellant into ensemble with ideal performance. Although binders only take 10%–15% of mass fraction in solid propellants, it plays an important role. On the one hand, it can be taken as a highenergy fuel to produce CO2 and H2O by reacting with oxidizers, thus producing thrust; on the other hand, it can enable metal additives, oxidizers, and combustion modifiers to be evenly bound and form solid grains that possess certain strength under both high and low temperatures, form a continuous matrix in boron-based fuel-rich ­propellants, and ­provide combustible elements such as C and H during combustion to release energy. Binders widely used in recent years include high polymers such as HTPB and glycidyl azide ­polymer (GAP), which are cost effective, have the mobility close to Newtonian fluid with low viscosity, and enables more solid components to be added. Besides, modern binders are compatible with highly active chemicals such as Mg powder. HTPB, known as hydroxygum, is one kind of telechelic liquid rubber, including free radical type and anionic type, with polybutadiene as the backbone. Based on the number of terminal hydroxyl, HTPB can be divided into difunctional hydroxygum and polyfunctional hydroxygum. The hydroxygum is an important variety of liquid rubber that has good transparency, low viscosity, oil resistance and antioxidation, good low temperature characteristics, and good processing performances. It can produce in bulk with three-dimensional network ­structures by reacting with a chain extender under room and high temperatures. This material has excellent mechanical properties; water, wear, acid, and alkali resistance; and electrical insulation properties. It is stable at indoor temperatures and does not produce any tiny crack when used. The structural formula is as follows:

1.2.2 Metal Fuels In recent years, an important development way is to add metal fuels. Boron powder, ­magnesium powder, and aluminum powder with high calorific value can be used as metal fuels for propellants. Since boron powder has a series of necessary physical and chemical features such as heavy weight and volumetric calorific value, it becomes the prime metal fuel choice, especially for the weaponry system that is restrained in weight and volume.14 High-energy boron-based fuel-rich propellants meet the requirement of engine

3

Ramjet propellants

performance, and are an ideal choice for solid rocket ramjets15 in terms of their theoretical design or development practice. Fuel-rich propellants have higher density specific impulse because of the added metal fuels with high calorific value and higher density. As well-known, beryllium (Be) has higher calorific value, but beryllium and its combustion products are highly toxic substances; lithium has very small density and very high activity, and Li is not compatible with many propellant components. Both cannot be put into use. The main properties of three common metals and their oxides are summarized in Tables 1.1 and 1.2. Here after, we consider B as a “metal” fuel. From Tables 1.1 and 1.2, it can be seen that B, Al, and Mg powder fuels have their own advantages and disadvantages. In terms of energy, aluminum has the highest density of 2.70 g·cm−3, boron the second, and magnesium the lowest density of 1.74 g·cm−3. The o ­ xygen consumption of Mg is the lowest, having a value of 0.66 g; yet, boron ranks first with 2.22 g of oxygen consumption, which is three times that of Mg and Al. In terms of combustion, the melting points and boiling points of magnesium, aluminum, and boron increase successively, and combustion difficulties also increase. The melting points of magnesium and aluminum are low, at around 660°C, while the melting point of boron can be as high as 2,074°C. The ­boiling points of aluminum and boron are similar, and the boiling point of boron can reach 2,550°C, while Mg has a minimum boiling point of 1,117°C. The gasification heat of boron can reach 535.81 kJ·mol−1 and that of aluminum is about half that of boron, and that of magnesium is one-third less than that of boron. In terms of heat, magnesium has the lowest, only half of that of boron, and boron’s heat is 400 kJ·mol−1 less than aluminum. Yet since the atomic mass of aluminum is twice that of boron, the quality calorific value and volume calorific value of boron are both larger than those of aluminum. The TABLE 1.1 Basic Properties of Three Types of Metals16–18

Elements B Al Mg

Elements B Al Mg

Atomic Weight

Density (g·cm−3)

Melting Point (°C)

Boiling Point (°C)

Gasification Heat (kJ·mol−1)

2,550 2,447 1,117

535.81 284.44 136.13

10.81 26.98 24.3

2.34 2.70 1.74

2,074 660 650

Oxygen Consumed Quantity (g)

Heat of Combustion (kJ·mol−1)

Mass Heat of Combustion (MJ·kg−1)

2.22 0.88 0.66

1,264.17 1,670.59 602.11

58.30 31.02 24.75

Volume Heat of Combustion (kJ·cm−3) 136.44   83.75   43.06

TABLE 1.2 Basic Characteristics of Three Types of Metal Oxides16–18 Metal Oxidizers B2O3 Al2O3 MgO

Molecular mass   69.62 101.96   40.30

Density (g·cm−3) 2.46 3.97 3.58

Melting Point (°C)

Boiling Point (°C)

460 2,045 2,800

1,860 2,980 3,580

4

Boron-Based Fuel-Rich Propellant

quality calorific value of aluminum is 31.02 MJ·kg−1, while that of boron is 58.30 MJ·kg−1. In terms of metal oxide, B2O3 has the lowest melting point of 460°C, MgO has the highest melting point of 2,800°C, and that of Al2O3 reaches 2,045°C; the melting point of MgO is the highest, reaching 3,580°C, and the boiling point of Al2O3 is 2,980°C and that of B2O3 is 1,860°C. After comprehensively considering factors such as energy performance and combustion properties, boron powder is selected as the main metal fuel additive; meanwhile, a small amount of magnesium powder or aluminum powder was added to enable fuel-rich propellants to possess good ignition performance. 1.2.2.1 Boron in Nature In nature, boron mainly exists in the form of borate, such as boric acid, alkali metal, and alkaline-earth metal, and the element boron does not exist by itself in nature. In China, boron minerals are sparsely distributed, with few high-graded boron minerals with ­exploitation values. There is natural borax in places such as Qinghai and Tibet, and endogenic boron ore with szaibelyite as the main ore in Northeast China, and a little reserve of boron minerals in other places. Most Chinese manufacturers of boron powder are located in Yingkou City, taking local boron ore as the raw material and producing amorphous boron powder with different specifications. 1.2.2.2 Amorphous and Crystalline Boron Boron powder can be divided into amorphous boron and crystalline boron. Amorphous boron powder is widely used in the fireworks industry as an explosion initiator, in solid rocket motors to increase thrust, in nuclear industry as neutron absorber, or in the air bags of automobiles. In addition, amorphous boron powder is also widely used for the smelting of molten steel, solid fuel, ceramic composition, aerospace studies, etc. Crystalline boron powder is also widely used in metallurgy and glass industry. Some physical properties of boron are given in Table 1.3. Under normal temperature, crystalline boron powder and amorphous boron powder are stable. When heated to 300°C, the amorphous boron powder will be oxidized. Around 700°C, it will be ignited. Yet, crystalline boron is heat resistant and does not easily burn. Boron can be dissolved in sulfuric acid, nitric acid, and molten metal (copper, iron, ­aluminum, and calcium), and cannot dissolve in water, hydrochloric acid, ethanol, and diethyl ether. When dissolved in cold concentrated alkaline solutions, hydrogen will be released. Under high temperature, boron can react with oxygen, nitrogen, sulfur, h ­ alogen, and carbon, and react with organic compounds to produce B-C compounds or B-O-C compounds. Boron belongs to the third main group in the periodic table of elements. Its atomic number is 5, and atomic mass 10.81. Boron has two forms: wet amorphous brown powder with active nature and wet gray crystal with stable nature, whose Moh’s hardness reaches 9.3, and not easily made into fine-grained particles. In recent years, China has developed crystalline boron powder with 99% purity. Since there is a small amount of B2O3 and H3BO3 on the surface, this powder can be directly applied in the formulation of HTPB boronbased propellants. This powder will not only avoid the necessary treatment required in the case of the application of amorphous boron powder but also help raise the combustion ­properties and energy properties of the propellants. As early as 1970s, other countries had studied the application of crystalline boron powder.20 After comparing the influence of 10 μm crystalline boron powder and 1–2 μm amorphous boron powder on the burning rate

5

Ramjet propellants

TABLE 1.3 Physical Characteristics of Elementary Boron19 Characteristics

Values

Melting point (°C) Boiling point (°C) Density (g·cm−3) Molar heat capacity Cp (J/(mol·K))

Standard free energy of formation ΔGf (298 K), (J·mol−1) Standard free energy of formation ΔGf (298 K), (J·mol−1) Molar entropy (298 K) (J·mol−1) Specific resistance (Ω·cm) Evaporation heat (kJ·mol−1) Heat conductivity coefficient (J s−1·cm−2) Burning point (°C) Heat of combustion (MJ·kg−1) Boron powder grade\amorphous boron powder d50 (μm) Specific surface area (m2·g−1)

2,300 2,550 (sublimation) 2,500 2.30–2.40 2.46–2.52 11.095 11.958 33.955 39.063 406.7 362.8 1.7 6.548 5.875 7.5 × 102 7 × 105 1,281.16 125.6 780 59.3 0.8 13

State Amorphous β-diamond Amorphous Amorphous (25°C) High pressure model β-diamond Amorphous (25°C) Solid (melting point) Liquid (melting point) Gaseous state Gaseous state Amorphous Amorphous β-diamond Amorphous β-diamond Amorphous (20°C–750°C) Amorphous (20°C–80°C) Amorphous Amorphous Amorphous Amorphous

of fuel-rich propellants, it was shown that upheld that particle size did not have an obvious influence on the burning rate. Of course, there have been different research results on the influence of granularity of boron powder on the burning rate of fuel-rich ­propellants. Pang21 studied the influence of boron particle size and purity on the combustion properties of boron-based fuel-rich propellants, and the results indicated that the burning rate of ­crystalline boron powder (≥99% purity) propellants containing granulous (0.04–0.15 mm) was higher than that of amorphous boron powder propellants (0.8–1.0 μm, 95.5%–96.5% purity). Therefore, it can be seen that boron powder with different particle size and purity has an obviously different influence on the burning rate of propellants. 1.2.2.3 Preparation of Amorphous Boron Powder In the industry, boron oxide reduction by magnesium is normally used to prepare amorphous boron powder via the reaction

B 2 O 3 + 3Mg → 2B + 3MgO (1.1)

The technological process is as follows: under decompression conditions, heating the boric acid in a tubular reacting furnace at a constant rate of 250°C, dehydrating boric acid to get boron oxide, smashing to 80 mesh, mixing with Mg powder according to the mass proportion of 3:1 and adding them into the reaction tube to undergo reduction reaction

6

Boron-Based Fuel-Rich Propellant

at a temperature of 850°C–900°C, soaking the reactant in water for two days, boiling it in hydrochloric acid for 4 h, removing the magnesium oxide, washing away the acid, and then getting an amorphous boron powder with purity less than 90%. To obtain boron ­powder with higher purity, the amorphous boron powder needs to react with Mg for 3–4 h at 800°C–850°C, and the excess boron oxide needs to be washed away. Finally, the amorphous boron powder with boron content over 90% can be gained after filtering and drying. 1.2.2.4 Physicochemical Properties of Boron Powders Based on the magnesium thermic reduction method, one company in Tangshan city (China) used atomized magnesium powder as reducing agent and developed a new technology of producing amorphous boron powder. The atomized magnesium powder is the most ideal reducing agent for producing amorphous boron powder in terms of indicators such as Mg content, apparent density, and mobility, which can ensure maximum surface reaction. The results are shown in Table 1.4 after analyzing the purity of boron powder using energy disperse spectroscopy. According to the analysis result of Tangshan’s boron powder, the content of boron in the sample is 91.4%. Using low-purity powder in boron-based fuel-rich propellants will not only lead to enormous treatment problems but also to a reduction of propellant energy. The result is shown in Table 1.5 after analyzing the content of micro-components of two boron powders from a company in Yingkou with a high-precision fluorescence spectrophotometer. TABLE 1.4 Surface Element Distribution of the Boron Powder of Tangshan22 Samples No. 1 Elements

No. 2

No. 3

Element Fraction (%)

Atomic Fraction (%)

Element Fraction (%)

Atomic Fraction (%)

Element Fraction (%)

Atomic Fraction (%)

85.10 3.18 11.46 0.15 0.11 100

92.10 2.33 5.52 0.03 0.02 100

84.56 4.02 11.03 0.19 0.21 100

91.66 2.94 5.31 0.04 0.04 100

82.80 2.76 13.79 0.24 0.42 100

91.07 2.05 6.74 0.05 0.09 100

B O Mg Mn Fe Total

TABLE 1.5 Microelements and Their Distribution in Different Boron Powders Elements Mass fraction (%)

Crystal boron Amorphous boron

Elements Mass fraction (%)

Crystal boron Amorphous boron

B

O

Mg

Al

Si

P

S

98.2 91.8

0.70 5.3

0.89 2.52

– 0.0278

– 0.111

0.0043 0.0211

0.00034 0.0861

Cl

K

Ca

Ti

Mn

Fe

0.00264 0.00538

0.00621 0.00130

– 0.0218

0.00355 0.00554

– 0.0271

0.0333 0.0326

Ramjet propellants

7

One can see in Table 1.5 that the main impurity elements in crystalline boron powder are O and Mg, and microelements include Al, Cl, Ca, Mn, Fe, etc. Regardless of other elements, since there is no peak for hydrogen, all the impurities are calculated as B2O3, and the percentage composition of boron in crystalline boron powder is 97.88%. For the boron powder of Yingkou, the main impurities are O and Mg as well as many microelement impurities. After converting O into B2O3, the percentage composition of boron is 89.4%. Therefore, the percentage composition of boron in crystalline boron powder is much higher than that of ordinary boron powder. There are not only less quantities of impurities such as B2O3 and H3BO3 but also less varieties of other impurities. The influence of impurities on the processing properties and combustion properties of fuel-rich propellants will be reduced. 1.2.2.5 Boron–Aluminum or Boron–Magnesium Alloy Powder Metal composite powder is the mixture of two or more metal powder particles in ­certain proportions, which has superior characteristics to individual metal powder. It is well known that aluminum and boron have high combustion enthalpy, and aluminum has long been applied in the formulation of propellants and explosives and in the initiation of explosive devices.9 However, its final combustion products, i.e., condensed-state aluminum oxide, can cause many problems to rocket engines. Since Mg powder featuring low melting point and easy ignition has wide applications, the adding mode of Mg powder and combination mode of solid fillers have an obvious influence on the features of p ­ ropellants. Boron is taken as a promising energetic substance that can be applied in motors with nozzles. Boron needs more oxidizers during combustion, but under certain conditions, it produces less condensed-state substances compared with aluminum. Combining the advantages of aluminum and boron, the boron–aluminum (B/Mg) alloyed powder can be produced through ball-milling method. Researches demonstrated that boron–aluminum alloy has a high combustion efficiency during the combustion of solid propellants, which means that boron–aluminum alloy powder has certain advantages in fuel-rich propellants. 1.2.3 Oxidizers and Solid Fillers Oxidizer is an important component of fuel-rich propellants. It can provide the oxygen needed for combustion and control the burning rate of propellants through gradation of particle size. The selection of oxidizers is based on the principle of high content of ­effective oxygen, high specific weight, and high production enthalpy, gaseous ­decomposition ­products, and physical and chemical stability during processing and storage. The ­oxidizers mostly used are inorganic salts, such as AP, potassium perchlorate (KP), ammonium nitrate (AN), and cyclic nitramines, such as cyclotetramethylenetetranitramine (HMX) and cyclotrimethylenetrinitramine (RDX).23–25 Currently, AP is a widely used oxidizer that has superior comprehensive performance (e.g., heat of formation, effective oxygen content, density, etc.), and its disadvantage is that it produces HCl gas as combustion products, forming a white smog in the exhaust plume of rocket engines which exposes the flight trajectory of missiles. AN is another oxidizer with application values, featuring low price and wide sources; meanwhile, its combustion products do not contain HCI gas, so it is normally used in propellants with low ­requirement for energy performance, low burning rate, and low flame temperature. The biggest ­disadvantage of AN is that it tends to absorb moisture and agglomerate, and it may cause crystal transformation when the temperature is changed. To refrain AN from moisture absorption and crystal transformation, a small amount of compounds, such as

8

Boron-Based Fuel-Rich Propellant

Ni, Cu, and Zn, are added to make eutectic and form a relatively stable phase stabilized ammonium nitrate (PSAN). Currently, in terms of effective oxygen content, LiClO4 ranks first and Mg(ClO4)2 ranks second, but their moisture absorption is obvious. The performance of AP is stable with a relatively low moisture absorption capacity and high heat formation, can be safely treated, and is thus widely used in rocket propellants. HMX and RDX are the high-energy nitramine explosives widely used in p ­ ropellants. They feature high formation enthalpy, large density, and produce smokeless gases, which can markedly increase the energy level of propellants and reduce exhaust plume. In recent years, the promising high-energy oxidizers also include the compounds 4,6,8,10,12-hexanitro2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and ammonium ­dinitramide (ADN) that have high energy and density. The formation enthalpy, density, and gas production of these new types of energetic additives are all superior to the existing oxidizers. Both have good application values in fuel-rich propellants. For the basic performance of solid oxidizers, please refer to Table 1.6. From the Table, it can be seen that ADN and HNF are both oxidizers with application prospects. Yet, from the aspects of melting point, density, and heat formation, HNF is obviously better than ADN. By replacing a part of AP with AN and HMX, HCl can be reduced. For the decrement of HCl, please refer to Table 1.7. 1.2.4 Combustion Catalysts and Stabilizers Combustion catalyst, also named as burning rate law modifiers, is a kind of substance used to adjust the combustion properties of solid propellants. Based on the roles of increasing TABLE 1.6 Basic Performance of Solid Oxidizers23–25 Oxidizers AN AP HP HP2 NP RDX HMX ADN HNF

Molecular Formula

Melting Point (°C)

Heat of Formation (kJ·mol−1)

Density (g·cm−3)

NH4NO3 NH4ClO4 N2H5ClO4 N2H6(ClO4)2 NO2ClO4 C3H6N6O6 C4H8N8O8 NH4N(NO2)2 N2H5C(NO2)3

170 130 170 170 120 204 278 90 295

−365 −296 −178 −293 37 71 75

1.72 1.95 1.94 2.20 2.22 1.82 1.96 1.82 1.90

−150 −72

Oxygen Balance (%) 20 34 24 41 66 −21.6 −21.6 25.8 13.1

TABLE 1.7 Decrement of HCl after AN and HMX Replaces Parts of AP26–28 Ingredients

Composition #1

Composition #2

Composition #3

Composition #4

Composition #5

AN AP HMX Al HCl release

– 69.7 – 16 21

36 10 – 18 2.8

31 15 – 18 4.2

41 10 – 18 2.8

41 10 18 10 3.0

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and decreasing propellant burning rate, they can be divided into positive combustion catalysts and negative combustion catalysts, and the function mechanism is to adjust burning rate by changing the combustion wave structure of the propellants. Combustion catalysts can also reduce the influence of pressure on burning rate and gain the so-called ­“plateau” burning rate law with low-pressure exponent (within a certain pressure range, the ­burning rate is only slightly influenced by pressure). Combustion catalysts often used in fuel-rich propellants include following types: 1. Organic metal compounds such as catocene, copper salicylate, and adipic acid. 2. Inorganic metal compounds such as C·C, Fe2O3, CuO, Co2O3, and Co3O4. 3. Supported catalysts such as carbon nanotube loaded metal oxides, carbon fiber loaded metal oxide, graphene-loaded metal oxide. 1.2.5  Combustion Stabilizer A combustion stabilizer is a substance used to prevent the unstable combustion of solid propellants. Adding metal additives to composite solid propellants can prevent unstable combustion. Aluminum powder is a commonly used combustion stabilizer. When ­aluminum-based propellant is burned, aluminum particles and the produced condensed Al2O3 particles suppress gas oscillation in the combustion chamber. 1.2.6  Bonding Agents Composite solid propellant is a kind of energetic composite polymer materials taking polymers as matrix, oxidizer solid particles as fillers, along with various additives. Mechanical properties are important performance parameters of solid propellants. There are many factors influencing the mechanical properties of propellants. Due to the high content of solids in this kind of propellants, its mechanical properties are reduced and the processing property lowered. Since nitramine oxidizers are added, the crystal surface is smooth and easily “dehumidified”, making the mechanical properties of propellants unable to meet the requirements.29 Thus, the effective bonding between solid particles and binder interface is the necessary condition for composite solid propellants to gain necessary mechanical properties. Researchers found that the mechanical properties of propellants can be effectively improved by selecting suitable bonding agents. Therefore, normally 0.01 to 0.1% bonding agents are added in the propellant formulation. Bonding agents are mostly molecular polar compounds with one end linking to inorganic oxidizers and polymerization reaction going on its surface, forming a high-modulus tearing resistance layer, and with the other end linking to the binder matrix and becoming a whole through chemical reaction. Bonding agents may also be combined on the surface of fillers in the form of monolayer to add the moisture-absorbing ability and affinity of fillers and matrix, or directly linked to the fillers and matrix in the form of “bond bridge” to increase the bonding ability of the interface, or transferred to the interface to integrate into a high-modulus transition layer to prevent microcracks from further extending to the interface. In this case, when solid propellants under load, microcracks will be propelled deeper into the binder matrix instead of reaching the surface of filler’s particles so that the filler particles can continue to bear load without “dehumidification,” thus strengthening the bonding capability of the interface layer and raising the mechanical properties of propellants. Along with the advancements in science, there are several macromolecular polymer bonding agents.

10

Boron-Based Fuel-Rich Propellant

For years, wide-ranging and in-depth studies on the influencing factors of HTPB ­propellant’s mechanical properties and the mechanical property adjustment technologies have been carried out in China and other countries, with many effective bonding agents being developed. The bonding agents can be divided into neutral polymer bonding agents (NPBAs), organosilane compounds, organic titanates, hydramines, hydantoin triazine bonding agents, organic boric acid ester bonding agents, etc.,30–32 of which the latter three kinds of bonding agents are widely used in HTPB composite propellants at present. When glycolylurea triazine bonding agents are added to HTPB-based, four-component propellant formulation, the mechanical properties of propellants under normal temperature and low temperature can be improved, but with poor aging-resistant performance; organic boric acid ester bonding agents can improve the high-­temperature ­performance and aging-resistant performance of propellants, but the performance under normal temperature is ordinary, and the processing properties of slurry needs to be improved. Since the interfacial modification result of components of HTPB composite propellants has a great influence on the performance of propellants, the structures and function mechanisms are varied. At present, there are different types of functional mechanisms of bonding agents used for composite solid propellants. Hydramine and its derivatives, and hydramine compounds such as triethanolamine (TEA), are common bonding agents. Their functional mechanism is as follows. First of all, alkanolamine and AP form the ionic bond of ammonium salt through chemical reactions that are firmly adsorbed on the surface; then, the hydroxy in ammonium salt reacts with isocyanate curing agents to enter the binder system, strengthening the adhesive strength between AP and binders. Since ammonia gas is released during the reaction, the solid propellant becomes porous. Thus, the commonly used bonding agents are the complex with BF3 or modified alkanolamine derivatives.33 Yet when TEA coexists with BF3 in propellant slurry, with the catalytic effect of BF3, TEA reacts with aluminum powder and AP, forms a hydroxyl layer around aluminum powder and AP particles, and creates a hydrogen bond complex, reducing the shear strain rate exponent n of the slurry; on the other hand, HTPB interacts with BF3 (aluminum powder can accelerate and enhance this effect), which enables HTPB macromolecules to cross-link, increasing the viscosity coefficient K of the slurry. The combination of earlier two effects rapidly increases the apparent viscosity of TEA·BF3 HTPB propellant slurry under shear strain, worsening the leveling property of the slurry’s motion.24 Organosilane compounds are normally added to propellants with AP coating, whose general formula is X3Si(CH2) nY. In the formula, X is the hydrolysable group, and Y is the active group that can react with the binder matrix. Since AP tends to absorb moisture and the existence of water can worsen the viscosity of the interface between AP and binders, the mechanical properties of propellants will be worsened. Adding silane bonding agents can eliminate the weak boundary layer caused by water around filler particles and strengthens the bonding between the fillers and matrix. The functional mechanism is that the organosilane X3Si(CH2) nY undergoes hydrolysis with microamounts of water on the surface of AP and turns into Y (CH2)nSi(OH)3; then the hydroxyl group forms a strong hydrogen-bond interaction with AP and is adsorbed on its surface and is autoagglutinated into high-modulus tearing resistance layer; the active group at the other side creates chain tangles with the binders or directly cross-links with the matrix to enter the binder phase, thus strengthening the bonding between interfaces.12 In terms of organic titanates, it is normally upheld that the functional mechanism of this kind of bonding agent is that Ti(OR)4 undergoes hydrolysis by absorbing water from the air on the surface of AP oxidizers, forms polymers, and links to binders through chemical effects or physical adsorption so as to raise the adhesive strength within the interior interfaces of propellants. Compared with ordinary titanate bonding agents, the stability

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of chelated titanate bonding agents synthesized34 is largely increased and is not easily hydrolyzed under general conditions. The strong polarity of the part next to cyclic annular of chelated titanates in polyether propellants works with HMX to form hydrogen bond or other strong physical adsorption, which acts on the surface of HMX; the other side has better compatibility with binders since it has same chain link with the molecular chain of polyether, fully disperses in the binders, and tangles with macromolecular chain to strengthen the interaction with binders. The terminal hydroxyl group of bonding agents reacts with curing agents and enters into the cross-linked network where cross-linking density increases, further strengthening the interaction with the binder matrix, forming high-density cross-linked modulus layer on HMX surface, and increasing the strength of propellants, but slightly reducing the extensibility. As bonding agents of composite solid propellants, hydantoin compounds are suitable for many filler/binder system; by means of a scanning electron microscope, it was found11 that after adding hydantoin bonding agents in propellants, oxidizers are not stripped when there is bending deformation in the propellant, and the high-modulus tearing resistance layer formed on the surface of filler particles strengthens the bonding of the interface and obviously improves the processing properties. In terms of NPBAs, the bonding agents applied in composite solid propellants are ­normally small molecular polarity compounds. Yan-ping35 introduced polymer macromolecule into bonding agents and put forward the concept of NPBA. Since a large amount of polarity plasticizers are added in high-energy propellant formulation, a part of polarity fillers are dissolved, forming a soft interface between the fillers and binder matrix and worsening the mechanical properties. The ordinary micromolecule bonding agents can hardly migrate to the interface and cannot play bonding roles. Since NPBAs have many action points, it had a strong affinity with polarity fillers. Its active hydroxyls form highrelevant transition layer on the surface of fillers and chemically cross-link with the polymer matrix, thus increasing the bonding strength of the interface. Polyfunctional oxazolines are newly developed bonding agents of solid rocket propellants. When acid oxidizer AP exists, they will undergo polymerization reaction, thus forming a high-modulus tearing resistance layer on AP surface, and the active group in the polymerizate has a chemical cross-link with the binder system and enters the polymer matrix, increasing the bonding strength between the oxidizer particles and binders.36 Organic boron acid ester bonding agents were developed as a new type of bonding agent.34 The functional mechanism of this kind of bonding agent is as follows: the boron’s outer electron structure is S2P1, and three SP2 hybridized orbitals are formed through SP2 hybridization, in the shape of a regular triangle, forming a σ-covalent bond with single electron orbital, respectively, with the remaining vacant orbital perpendicular to the SP2 hybridized orbital plane and becoming the acceptor of electron pairs. In the molecules of nitramine oxidizers RDX and HMX, there is a large amount of N atoms that have SP3 hybridization, and has a lone pair of electrons after forming three σ—covalent bonds, which is the donor of electron pairs. B atoms and N atoms form N → B coordinate bonds, and since B atom has small radius and strong electron-withdrawing ability, and the c­ oordinate bond formed by B and N is also strong. In addition, the active group carried by bonding agents can react with curing agents and fillers to form hydrogen bonds and enter the cross-linked network. Therefore, this kind of bonding agent can closely combine the fillers and matrix, in small dosage and with high efficacy.10 Therefore, when selecting nitramine bonding agents, the following requirements should be met: nitramine HMX and RDX having good wettability; being able to produce chemical bonds or physical adsorption or complex with nitramines; the coupling reagent having two

12

Boron-Based Fuel-Rich Propellant

or more solidifiable groups, and forming network structures with binders through curing agents, thus forming tearing resistance high-modulus layer on the surface of HMX and RDX: the new coupling reagent has no negative effects on other major properties of propellants.18 Organic boric acid ester bonding agents (BA series)34 can closely combine RDX fillers with the matrix, thanks to its sound coupling effects and mechanical properties in small dosage and with high efficacy. They can obviously improve the high-temperature and antioxidation performance of HTPB four-component solid rocket propellants. They have been widely used in composite solid propellants. The BA series of bonding agents has good effect because strong coordinate bonds with fillers can be formed, as well as its active hydrogen groups which enable it to react with curing agents and enter the cross-linked network, thus combining the matrix with fillers. Therefore, since BA series of bonding agents has active hydrogen groups, it has an obvious influence on the cross-linked network and the mechanical properties of the curing system during the curing of HTPB composite propellants. 1.2.7 Antioxidants The aging of HTPB propellants is a complicated physical and chemical process, affected by environmental temperature, humidity, radiation, light, heat, oxygen, and other various factors. In the propellant, the oxidizer AP is slowly decomposed, creating active oxidative decomposition products that attack the carbon–carbon double bond-a weak link in HTPB propellants that is easily eroded, leading to the physical and chemical changes of the binder system and causing the aging of HTPB propellants.15 The chemical aging of HTPB propellants may reduce their mechanical properties. Technical approaches to improve the aging performance are as follows: improving the heat stability of AP, restraining the oxygenolysis of AP, strengthening the bonding effects on the interface between oxidizers and binders, and increasing the antioxidant ability of binders. Improving the heat stability of AP, the granularity and purity of the oxidizer of AP have an important influence on the thermal decomposition of solid propellants. At high ­temperature and pressure, according to the crack position of propellants tested by s­ canning electron microscope, the broken coarse oxidizer particle can cause cracking of propellants. The purity of oxidizer has a larger influence on the aging of propellant, and the catalyzed impurities such as ClO3− can speed up the decomposition of propellants; the negatively catalyzed impurities such as phosphate can increase the heat stability of AP propellant. Therefore, recrystallization increases purity, reduces AP particle sizes, and some additives that restrain AP heat decomposition can improve the heat stability of AP.15 Strengthening the bonding effect of AP/HTPB interface, adding a small amount of bonding agents can markedly increase the bonding effect, which can not only improve the mechanical properties of propellants but also raise the antioxidation performance of propellants.20 In HTPB propellants, bonding agents can have physical and chemical adsorption with AP particles; at the same time, its active functional group chemically integrates into the binder system, becoming a part of the binder network. In this way, the bonding agent forms a semilinked hard and tough shell with high modulus around AP particles. When the oxidizer is decomposed under external factors, the released oxidative products can hardly break through the surrounding hard shell of macromolecules to attack the weak link of carbon–carbon double bonds of the binder’s macromolecules. Meanwhile, since the oxidative decomposition products are not easy to diffuse, further decomposition of the oxidizer is restrained, thus improving the storage aging process of HTPB propellants.15

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Enhancing the antioxidation ability of HTPB binder, the aging of HTPB propellants is mainly caused by the oxidative decomposition products of AP attacking carbon–carbon double bonds. Therefore, saturation of double bonds through the hydrogenation of binders is also an effective way to raise the stability of HTPB propellants and prolong its storage life. Yet, solid rubber is obtained through 100% hydrogenation of HTPB. Adding single or composite antioxidants can effectively restrain the aging of HTPB propellants. The amine and phenolic antioxidants are often used in HTPB propellants. The antioxidation of amine antioxidants is better than hindered phenols; at the same time, there is an optimum content of antioxidant.15 Normally, its content is 1% of binders, which is not enough to reach the best protective effect; if it is overused, the surplus antioxidant normally assists oxidization.21 Antioxidant H (N,N’-diphenyl-para-phenylene diamine) is a commonly used antioxidant. By absorbing the primary free radicals or oxides produced by oxide decomposition and polymer chain degradation, the aging of HTPB can be prevented. Yet, the antioxidant H has two active hydrogens of tertiary amine, which can react with curing agents and then cross-link into the HTPB network, affecting the mechanical properties of HTPB. Antioxidant H can be taken as a kind of chain extender in HTPB propellants. There are two conditions for it to function as a chain extender. First, the temperature needs to be higher than the glass transition temperature (Tg) in the hard segment domains formed by chain extenders. Second, the bonding strength of the interface between curing fillers and binders must be higher than the stress needed by plastic slip deformation in these hard segment domains. When the temperature is lower than Tg, the hard segment domains formed by antioxidant H will play the role of physical cross-linking, raising the strength of continuous phase of binders, which leads to the dehumidification of propellants. The higher the content of antioxidant H, the more prominent the roles will be. The antioxidant H slightly improves the mechanical properties under normal temperature, and ­prominently improves the mechanical properties at high temperature. This is the result of antioxidant H’s role as a chain extender. When the content of antioxidant H is higher than 0.15%, the extension rate at low temperature will obviously decrease, which is the result of interface “dehumidification.”22 It should be noted that although antioxidant H functions as chain extender and attracts much harder segment domains which largely increase the mechanical properties of HTPB propellants, it has a large amount of solid fillers and interface factors affecting the mechanical properties of propellants. Therefore, it can be expected that the rule of antioxidant H’s influence on HTPB propellants may be different from the earlier phenomenon.9

1.3 Main Properties of Boron-Based Fuel-Rich Solid Rocket Propellant 1.3.1 Energetic Properties Energetic properties are the main properties of boron-based fuel-rich solid rocket propellants. It is also the parameter for missile armament, aerospace propulsion system, and carrying capacity. The main parameters for boron-based fuel-rich solid rocket propellants include gravimetric calorific value, volumetric calorific value, specific impulse, characteristic velocity, density, density specific impulse, gas specific volume, average relative

14

Boron-Based Fuel-Rich Propellant

molecular mass of gas, etc., among which gravimetric calorific value and density are the most important and most frequently adopted parameters for fuel-rich propellants. A higher energy level has always been the goal in developing boron-based fuel-rich solid rocket propellants. It is also an effective way to extend missile range or to make engines smaller when the comprehensive performance can be guaranteed to maximize the energy level. To get propellants with high-energy characteristics, analysis must be done on the formulation of propellants as well as the compatibility between thermodynamic properties of their combustion products and energy characteristics in the design of energy properties of boron-based fuel-rich solid propellants, thus oxidation–reduction can be fully processed to maximize the combustion efficiency of propellants. Theoretical calculation of energetic properties is significant in selecting high-energy propellant formulations, based on solving the gas equilibrium compositions in the combustion chamber of solid propellant rocket engines, which is usually done by two methods, the equilibrium constant method and the minimum free energy method. The fuel-rich propellant composed of HTPB, AP, and B has a theoretical combustion heat of 25–41 MJ·kg−1, a density of 1.45–1.63 g·cm−3, and a flame temperature of 1,800–2,100 K. Currently, the energy level of fuel-rich propellants is commonly measured by testing the combustion heat of fuel-rich propellants in oxygen bombs. 1.3.2 Processibility (Rheological and Surface-Interfacial Properties) Boron-based fuel-rich solid rocket propellant slurry is a composite fluid featuring a high solid-loading level and thermal curing. The good or bad rheological characteristics of propellant slurry directly determine whether boron-based fuel-rich solid rocket propellants can be produced or not. Therefore, investigating the rheological characteristic of ­propellant slurry is the first step to study the processing properties of boron-based fuelrich solid rocket propellants. In the past, studies mostly focus on rheological parameters and processing properties under shear condition, reflecting the macrorheological features; today, dynamic methods are employed to study the structure of composite materials, which facilitates the understanding of rheological properties of propellant slurry and its laws, thus guiding the adjustment and improvement of processing properties of propellants,6–11 and to some extent, predicting and adjusting the mechanical properties of propellant grains. Rheology is extremely important to the study of polymers and polymer matrix. The ­rheological properties of polymers are complicated because they usually shows characteristics of non-Newtonian fluids, including involved shear viscosity, flexibility, normal stress difference, extensional viscosity, etc. All these rheological actions rely on the structure, molecular mass, molecular mass distribution, contents of all additives, particle shape and size distribution, shear rate, and temperature. The production of composite solid propellants and double-base propellants involve processes covering mixing, casting, calendering, compression molding, and so on. In the processes, the rheological properties of polymers greatly influence the processing difficulty and product quality. Figure 1.1 is the property relationship diagram of composite materials, showing the significance of ­rheology (processing properties) in a composite material study. Solid propellant is a kind of macromolecule energetic material filled with a great amount of solid particles. Surface/interfacial properties are the characteristics of the surface/­ interface between its components (solid–solid, solid–fluid, and fluid–fluid). It influences many aspects of solid propellants, including rheological properties, structural integrity, and mechanical properties.10 According to some processes and combustion properties,

15

Ramjet propellants

Elastic mechanics

Fluid mechanics High polymer material processing

Machineshaping theory Rheology

Plasticity

Polymer physics

Physical properties physics Materials morphology

polymeric materials application

Fracture mechanics FIGURE 1.1 Property relationship of composite materials.

amorphous boron powder needs to go through surface treatments in the production process of boron-based fuel-rich propellants, including purification, inactivation, coating, agglomeration, and so on.3–7 Different treatments play an important role in the properties of the surface/interface between the boron powder produced in different ways and the binder system as well as solid fillers. As the surface properties of boron powder changes, its other properties change as well, such as surface tension and surface free energy. Thus, the tension of the interface with binders and the adhesion work change as well. Therefore, the surface/interfacial phenomenon is the key area in the study of material surface/­ interface field.8,9 Currently, there are many methods to study surface/interfacial properties, such as capillary rise method, maximum bubble pressure method, drop weight method, drop volume method, contact angle method, thin capillary osmosis, etc., and contact angle method has become one of the standard methods to characterize solid surface/interface because of its convenience and maturity.11–13 The regulation of processing properties of solid propellants usually refers to the ­regulation of propellant slurry’s rheological properties. Propellant slurry consists of two parts: continuous phase and dispersed phase. Continuous phase is composed of liquid components including binders and plasticizers; besides, it exerts an effect on propellant slurry by influencing the bulk viscosity of continuous phase and the volume fraction it takes in propellant slurry. In terms of dispersed phase, particle properties and particle gradation of rigid fillers, like oxidizers and metal powders, have a great influence on the rheological properties of propellant slurry. However, besides the nature of continuous phase and disperse phase, the properties of the interface between the two also have a significant effect on the rheological properties of propellant slurry. Surfactant material is added to improve the processing properties of propellants. It coats on the surface of the solid component particles to better moisturize the surface of solid particles (like AP, metal powders) in the propellant slurry and destroy the bonding between solid particles and binders, thus improving interfacial properties. Besides, the surface tension of fluid can be greatly reduced, so the slurry viscosity goes down.37 Regular surface, kneading temperature, and time as well as feeding sequence would be considered. Among those, solid particle active agents include sodium dodecyl sulfate, lecithin, and so on. Basically, the influence of rheological properties of boron-based fuel-rich propellants are twofold: First, the influence of formulation variables, including the binder system, curing agents, oxidizers, metal powder, processing aids, and other components; second, the influence of processing conditions, including kneading intensity, kneader structure, and size properties, significantly affect the rheological properties of propellant slurry.

16

Boron-Based Fuel-Rich Propellant

1.3.2.1 Effect of Content Many scholars have done studies on the relationship between suspension concentration and suspension viscosity. Einstein’s (1906) equation is an early well-known example:

η = η0 ( 1 + kEφ ) (1.2)



In the equation, η and η 0 are the viscosity of suspend agent and slurry, respectively, k E is the Einstein coefficient, 𝜙 is bulk fraction of fillers, that is:

φ = Vs (Vs + Vl ) (1.3)



In this equation, suffix s is solid, l is liquid. This equation applies to spherical particles and the situation when the suspension concentration is below 2%. Hundreds of equations have been invented to estimate medium concentration and the suspension concentration of spherical particles, among which the most commonly used and satisfactory ones are Robinson equation:  φ φm  ηsp = 2.5  (1.4)  φ φm 

Landel equation:

ηr = ( 1 − φ φm ) (1.5) 2.5

Mooney equation:

ln ηr =



kEφ (1.6) 1 − φ φm

Koch equation: 1.5



 2.24φ  ηr =  1 +  (1.7) − φ φm  1 

Here 𝜙m is the maximum bulking fraction, namely, the ratio of the filler’s real volume to the apparent volume after packing. Different packing methods bring different m ­ aximum packing fractions. When the volume fraction of the solid in the suspension gradually reaches 𝜙m, its viscosity will soar to a plateau at a maximum value. 1.3.2.2 Impact of Particle Properties on Viscosity The particle size of solid fillers plays a significant role in determining the ballistic parameters of fuel-rich solid propellants. Also the coarse and fine particle size and their gradation have much influence to the viscosity of propellant slurry.

1. Influence of particle size Particle size is often represented in three ways: micrometer (μm), sieve mesh, and pneumatic milling pressure. Currently spherical AP particles made in China

Ramjet propellants





are in different sizes including 40–60 mesh (300–450 μm), 60–80 mesh (200–300 μm), 80–100 mesh (154–200 μm), and 100–140 mesh (105–154 μm). Propellant factories need to do the smashing of particles to get AP below 140 mesh (105 μm). Koch et al.12 conducted a study on the relationship between the RDX size and the viscosity of the suspension RDX in TNT. The result shows that, in the range of coarse particle size (over 200 μm), the particle size has little influence on ­viscosity; in the range of 100–200 μm of particle size, viscosity increases as particle size decreases; when particle size is below 50 μm, viscosity goes up rapidly. 2. Influence of particle shape AP particle shape is divided into two types: spherical and nonspherical. From the perspective of process, spherical AP is the best, for spherical particles suffer less from moving resistance and have a smaller specific surface area, and thus propellant slurry made up by spherical particles has a higher loading density and a lower level of viscosity; besides, spherical AP shows good anticaking ability. Many researchers also introduced some parameters to characterize particle shape in terms of its influence on suspension viscosity, among which shape coefficient, condition factor, and length-to-diameter ratio are frequently used. 3. Influence of particle aggregation The k E in equation 1.2 for dispersed spherical particle is 2.5. However, if many spherical particles gather into a rigid aggregation, then some fluid is fixed in the agglomeration in the gaps between the particles. Thus, the amount of free l­ iquid that can flow reduces and the suspension viscosity goes up and k E increases. Coefficient kE of the aggregation is related to both particle number and particle arrangement. Aggregation mainly affects 𝜙m and k E in Mooney equation, which means it makes 𝜙m lower and kE higher. The combination of the two will make the relative viscosity of propellant slurry to increase sharply as the particle number in aggregation increases. Therefore, AP aggregation shall be avoided as much as possible in the process of propellant production. Besides, the composite properties of solid particle, namely the gradation of ­particle size, also have a great effect on slurry viscosity. This was described in the closest packing theory and the interference theory.17 The core concept of ­closest packing theory is choosing proper sets and particle size ratio and calculating the volume percentage of particles to get the maximum loading density (namely, leaving the least space). When the solid content stays the same, the viscosity of the closest packing solid particle slurry is the lowest. The key point of interference theory: the average diameter of small particles is D2 plus the gap among particles whose average diameter is less than D1 on the adsorption layer; otherwise, small particles will not fill the gap among big particles, or small particles will squeeze the big ones away, leading to a decrease of apparent density of granular ­mixtures and the rise of suspension viscosity. A new concept based on two theories was raised.19 This is the so-called rolling gradation method, in which a flowing ­particle system has twofold physical properties: the pure spatial packing (static ­property) and the acceleration (dynamic property). The closest packing theory and the interference theory only emphasize the static property and ignore the dynamic property. The rolling gradation method reveals the nature of how to bring down the viscosity of suspended particle system with high solid content, which lies in gaining good group static and dynamic properties (spatial packing and rolling properties). In fact, the closest packing theory is a special case of the

17

18

Boron-Based Fuel-Rich Propellant

rolling gradation method. When guiding the design of composite solid propellant formulations, rolling gradation method has more mobility and flexibility than the closest packing theory and can meet specific requirement for broad burning rate and mechanical properties. 1.3.3 Combustion Properties Combustion properties are an important factor directly influencing the ballistic properties of rocket motors. Burning rate determines the working time and flying velocity of rockets. Subjected to testing pressure and temperature, the propellant burning rate directly affects the stability and performance of rocket motors. Therefore, it is necessary to control and adjust propellant combustion properties. The combustion properties of fuel-rich propellants include many aspects, such as ignition performance, stable combustion, unstable combustion, combustion efficiency, combustion residue, flame-out performance, exhaust plume parameters, etc. Stable combustion is the key to solid rocket engine performance, whose main parameters include burning rate (r), burning rate pressure exponent (n), burning rate temperature sensitivity (σp), and motor pressure temperature sensitivity (πk). Propellant combustion is a complicated process of mass and heat transfer, which in essence is an exothermic chemical reaction featuring high t­ emperature, and high pressure. So, the combustion rate of propellants is determined by the chemical reaction rate as well as the speed of mass and heat transfer. As the rate of a chemical reaction is determined by the nature of reactants and reaction ­conditions, the burning rate of a propellant is determined by the nature of the propellant, such as its components and their respective content as well as the working conditions of the engine. Methods are adopted to adjust the combustion properties of boron-based fuel-rich propellants, ­including adding combustion catalysts, changing boron powder content, and adjusting particle size gradation as well as oxidizer ­content and ­gradation. Ways to test combustion rate include lab-scale combustion rate testing method (strand burner method) and motor combustion rate testing method (reduced-scale or full-scale motor method). Boron-based fuel-rich propellants is an ideal fuel for solid rocket ramjets because of its excellent energetic properties and its high potential energy is released through combustion, so its combustion properties determine the applicability of boron-based fuel-rich propellants. For unchoked rocket ramjets whose propellant combustion is at the grain end, boron-based fuel-rich propellant is required to burn steady below 0.2–1.0 MPa with a relative fast burning rate and high-pressure exponent. Compared with normal composite solid propellants, boron-based fuel-rich propellants feature high metal fuel content and low oxidizer content, etc., but their composition characteristics and working environment determine the following difficulties during their combustion.

1. Low combustion rate. On the one hand, due to the low oxidizer content in boronbased fuel-rich propellants, it is harder for the propellants to burn fully at one time, reducing heat release and thus reducing the propellant combustion rate; meanwhile, limited by the processing properties, a large amount of fine-grained oxidizer cannot be added to realize high combustion rate; on the other hand, because boron-based fuel-rich propellants works at low pressure, especially in unchoked gas generators, which usually works at a pressure of 0.2–1.0 MPa. Such low pressure is not conducive to the combustion of fuel-rich propellants.

Ramjet propellants

19



2. Low burning rate pressure exponent. The ramjet must regulate the flow in the gas generator to maintain high combustion efficiency when the air flow changes during the flight. The regulation range of fuel-rich gas flow depends on the combustion rate pressure exponent of propellant. The higher the combustion rate pressure exponent is, the wider the regulation range and the better the engine’s adaptability. Therefore, the propellant’s burning rate pressure exponent is usually above 0.5. But at a pressure 4–8), midhigh altitude (H > 15–40 km), superlow altitude (H < 100–300 m), and medium-long range (L > 100 km), and also the key supporting technology for hybrid rocket engines with adjustable thrust. In terms of property adjustment of fuel-rich propellants, the combustion properties ­(especially to boron-based propellant formulations) are undoubtedly the priority. Combustion property adjustment is a complicated and systematic work, covering key technologies such as selection, proportion, compatibility, and processing technology of propellant components. Once the problems regarding the application of high-energy boron-based fuel-rich propellants are solved, the combustion efficiency will be largely improved and ramjets with fuel-rich propellants will have remarkable advantages in ­competition with other engines.

References

1. Wang Bo-xi, Feng Zeng-guo, Yang Rong-jie. Combustion Theory of Propellants and Explosives. Beijing: Beijing Institute of Technology, 1997. 2. Li Bao-xuan, Wang Ke-xiu. Properties of Solid Propellants. Xi’an: Northwestern Polytechnical University, 1990, pp. 16–19. 3. Zang Ling-qian. The use of boron as a fuel component of propellant. Journal of Propulsion Technology, 1990, 11(4):56–62. 4. Pang Wei-qiang, Zhang Jiao-qiang, Guo Ji-ying, et al. Study and development trend of f­ oreign solid propellants in twenty-first century. Chemical Propellant and Polymer Materials, 2005, 3(3):16–20. 5. Xu Hui-xiang, Fan Xue-zhong, Zhao Feng-qi. Development and prospect of fuel rich ­propellant. Winged Missile, 2005, 1:48–53. 6. Pang Wei-qiang, Fan Xue-zhong. Application progress of metal fuels in solid propellants. Chemical Propellant and Polymer Materials, 2009, 7(2):1–5. 7. Zhang Yuan-jun. Progress of metal fuels propellants. Journal of Propulsion Technology, 1981, 3:66–68. 8. Zhang Wei, Zhu Hui, Fang Ding-qiu. Technical approaches to improve the combustion ­characteristics of high energy fuel rich propellants with boron particles. Chinese Journal of Energetic Maerials, 1998, 6(4):179–182. 9. Pang Wei-qiang. Study of Fuel Rich Propellants with High Mass Fraction of Boron Particles. Xi’an: Xi’an Modern Chemistry Research Institute, 2011. 10. Wang Yong-shou. Combustion of aluminum and boron of solid propellants. Winged Missile, 1987, 3:41–43. 11. Fan Hong-jie, Wang Ning-fei, Guan Da-lin. Study on the combustion characteristics of boron solid propellant coated with GAP. Journal of Propulsion Technology, 2002, 23(3):262–264. 12. Koch H. W., Scattergood R. O., Youssef K. M., Chan E., Zhu Y. T. Nanostructured materials by mechanical alloying: new results on property enhancement. Journal of Materials Science, 2010, 45:4725–4732.

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Boron-Based Fuel-Rich Propellant

13. Pei Yang. Research on the combustion of metal fuel in solid ramjet. Journal of Propulsion Technology, 1986, 5:76–79. 14. Pang Wei-qiang, Zhang Jiao-qiang, Zhang Qiong-fang, et al. Coating of boron particles and combustion residue analysis of boron-based solid propellants. Journal of Solid Rocket Technology, 2006, 12(2):204–207. 15. Yoshio Oyumi. Urethane reaction mechanism on the amorphous boron surface in GAP ­propellant. Propellants, Explosives, Pyrotechnics, 1992, 17(5):278–282. 16. Liehmann W. Combustion of boron-based slurries in a ramburner. Propellants, Explosives, Pyrotechnics, 1992, 17(1):14–16. See also: International Journal of Energetic Materials and Chemical Propulsion, doi:10.1615/IntJEnergeticMaterialsChemProp.v2.i1-6.200, pp. 353–359. 17. Pang Wei-qiang, Fan Xue-zhong, Yu Hong-jian, Zhang Wei, Xu Hui-xiang, Li Ji-zhen, Li Yonghong. Application of amorphous boron agglomerated with hydroxyl terminated ­polybutadiene in fuel rich solid propellant. Propellants, Explosives, Pyrotechnics, 2011, 36:360–366. 18. Pang Wei-qiang, Fan Xue-zhong, Xu Hui-xiang, Wang Guo-qiang, Li Yong-hong. Study on a standard substance for magnesium aluminum poor oxygen propellant. Journal of Astronautic Metrology and Measurement, 2009, 29(2):57–60. 19. King M. K. Ignition of boron particles and clouds. Journal of Spacecraft, 1982, 19(4):294–296. 20. Maček A., Semple J. M. Combustion of boron particles at atmospheric pressure. Combustion Science and Technology, 1969, 1:181–183. 21. Pang Wei-qiang. Agglomeration Technique of Boron and Its Application to Fuel Rich Propellants. Xi’an: Northwestern Polytechnical University, 2006. 22. Kuwahara T., Kubota N. Role of boron in burning rate augmentation of AP composite ­propellants. Propellants, Explosives, Pyrotechnics, 1989, 14:43–45. 23. Hsich, W.H. Combustion behavior of boron based BAMO/NMMO fuel rich solid propellants, AIAA 89-2884. 24. Gao Dong-lei. Primary Combustion Performance of Boron Rich Fuel Propellant. Changsha: National University of Defense Technology, 2009. 25. Hu Song-qi. Primary Combustion Performance of Boron Rich Fuel Propellant. Xi’an: Northwestern Polytechnical University, 2004. 26. Mao Cheng-li. Combustion of Boron-Based Poor Oxygen Propellant. Xi’an: Northwestern Polytechnical University, 2001. 27. Wei Qing. Study on the Process and Combustion Properties of Fuel Rich Solid Propellant with High Boron Content. Xi’an: Northwestern Polytechnical University, 2005. 28. Xiao Xiu-you. Study on Magnesia–Aluminum Fuel Rich Propellant and Combustion at Low Pressure Ranges. Xi’an: Northwestern Polytechnical University, 2005. 29. Pang Wei-qiang, Fan Xue-zhong, Xu Hui-xiang, Zhao Feng-qi, Li Yong-hong. Study on ­surface modification of amorphous boron powder by chemical substances. Journal of Solid Rocket Technology, 2010, 33(2):196–200. 30. Pang Wei-qiang, Xu Hui-xiang, Wang Guo-qiang, Fan Xue-zhong. Study on the law of low pressure burning rate of magnesium-aluminum fuel rich propellant. Journal of Measuring Technique, 2008, 28(6):16–19. 31. Wang Gui-lan, Zhao Xiu-yuan. Application of boron powder in the propellants. Journal of Solid Rocket Technology, 1998, 21(2):46–49. 32. Li Shu-fen. Improvement of combustion properties of propellants containing boron. Journal of Solid Rocket Technology, 1995, 18(2):39–42. 33. Li Feng-sheng, Haridwar Singh, Guo Xiao-de, Himanshu Shekhar, et al. Preparation Technology and Application of Special Ultrafine Powder. Beijing: National Defense Industry Press, 2008. 34. Wu Wan-e, Mao Gen-wang, Hu Song-qi, et al. Influence factors of pressure exponent of boron fuel rich propellant. Chinese Journal of Explosives and Propellants, 2007, 30(3):62–65. 35. Chen Yan-ping. Study on High Energy Boron Fuel Rich Propellant. Changsha: National University of Defense Science and Technology, 2005. 36. Zhang Ji-hua. Physicochemical Properties of Propellants and Explosives. Beijing: Beijing Institute of Technology, 1997.

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31

37. Zheng Jian, Wang Ai-hua, Pang Ai-min. Study on process deterioration mechanism of boron based HTPB fuel rich propellant. Journal of Propulsion Technology, 2003, 24(3):282–285. 38. Zheng Jian. Study on High Energy Boron Fuel Rich Propellant Technology. Beijing: Beijing Institute of Technology, 2004. 39. Pan Zu-ren. Polymer Chemistry. Beijing: Chemical Industry Press, 1992. 40. Wang Ying-hong. Low Pressure Combustion of Boron Fuel Rich Propellant. Xi’an: Northwestern Polytechnical University, 2004.

2 Surface Modification and Characterization of Boron Powder

2.1 Introduction The B2O3 and H3BO3 compounds on the surface of boron will undergo esterification ­reaction with the hydroxyl in the hydroxyl-terminated polybutadiene (HTPB) binder, which makes the preparation process of fuel-rich propellant severely difficult. Therefore, to improve the rheological properties of boron-based fuel-rich propellant slurry, B2O3 and H3BO3 should be removed or reduced by necessary surface modification. The recent research results1–3 show that the agglomerated modification of boron is one of the most important approaches to improve the processing property of boron-based fuel-rich propellant; meanwhile, the investigations in the area of boron surface modification are of particular increasing ­i nterest. By using agglomerated particles,4–7 not only different sizes of boron particles can be obtained but also the problem occurring in propellant casting with increased processing performance can be efficiently solved. Generally, the agglomeration (pelleting) of powders can be conducted in either dry or wet condition. In the dry agglomeration method, the raw powder material is added continuously into a rotating cylinder. The powder particles have “spherical” or “nearspherical” shapes owing to the physical adhesive force. The dry method uses a simple device and has low energy consumption/production cost. However, the agglomerated particles made by means of the dry method are fragile and hence is difficult to maintain the shape. In the wet agglomeration method, raw powder material is mixed with a certain weight percentage of agglomeration promoter and solvent with stirring. After aggregation (pelleting) and dry process, the solvent will be removed and spherical particles will be obtained. By using this approach, the powders adhere strongly. The additional advantage is also decreasing the pollution of final product during transportation and application.8 The aim of this c­ hapter is to consider the manufacturing issue of boron-based fuel-rich (>40 wt. %) propellant and several premodification processes of amorphous boron using different chemicals. By adjusting the types and percentages of binders and optimizing the manufacturing process parameters, spherical agglomerated boron particles with controllable and uniform size distribution were prepared by mechanical stirring method, which provides technical support and lays the foundation for manufacturing boronbased fuel-rich propellants.

33

34

Boron-Based Fuel-Rich Propellant

2.2  Deterioration of Propellant Manufacturing Process Caused by Boron and Its Mechanism As to the mechanism of propellant manufacturing process deterioration caused by boron, there are two different viewpoints. (1) The impurities of B2O3 on the boron surface form centers with two electron bonds with C=C double bond from HTPB, which produces a relatively strong adhesion force between the particles thus increasing the viscosity of the whole system and hurting the manufacturing process of boron–HTPB propellant.9 (2) The impurities on the boron surface condenses with HTPB,10 producing a highly viscous macromolecular compound, which increases the viscosity of the whole system and thus ­deteriorates the propellant slurry during the manufacturing process. Besides that, there are other impurities like Mg, MgCl2, MgO, MgB2, MgB4, B2O3, BO6, BO7, etc., and some of them easily absorb moisture from the atmosphere. The water formed on the surface of boron makes it incompatible to other composites in the propellants.11 Therefore, the gelatinization reaction is related to the impurities in boron. 2.2.1 XPS Analysis Generally, there are impurities like Mg, Al, Fe, Mn, Si, Ca, Ni, Pb, etc. in boron.12 We have done measurements on the physical–chemical properties of boron, such as overall boron content, water-soluble boron compound content, pH, nonsoluble materials (by using H2O2), and size distribution13; however, we still cannot make sure what the impurities are. X-ray photoelectron spectroscopy (XPS) is an efficient way to determine the atom type and valence-electron configuration of solid surface; therefore, not only the elemental ­component and elemental chemistry status of materials surface can be determined but also some quantitative information can be obtained. Two different batches (labeled as boron A and B) of boron powders from two different factories were analyzed by XPS, and the results are shown in Table 2.1 and Figure 2.1. As Table 2.1 shows, the major elements found on the boron surface are C and O. However, it is because of high surface energy of the superfine boron powder (1–2 μm), which absorbs some oxycarbide or other compound. The “nonfresh” surface contributes to the high value of C and O in the measurement, while the true surface condition was hidden. As the ­binding energy in Figure 2.1 indicates, the boron element in boron A exists as pure boron in boronbased compounds like H3BO3 and B2O3, and the same is true to boron B listed in Table 2.1.14,15 2.2.2 Rheological Analysis Crystalline boron (active boron >98%) or amorphous boron (active boron −90%) powders were mixed with HTPB at a mass ratio of 4:6. Then the mixtures were heated at 50°C in an oil bath. Gelation was detected in both situations, and the viscosities and yield values of the two mixtures were also measured and listed in Tables 2.2 and 2.3. TABLE 2.1 Elemental Analysis of Boron A and B Elements

C (%)

O (%)

B (%)

Fe (%)

Cr (%)

Boron A Boron B

43.38 48.96

21.28 18.70

34.91 32.05

0.22 –

0.22 0.29

35

Characterization of Boron Powder

FIGURE 2.1 The boron element analysis of boron A.

TABLE 2.2 Viscosity Comparison Slurry t/h Crystal boron/HTPB Amorphous boron/HTPB

1

2

3

4

5

228.5 293.4

325.8 412.0

367.9 497.1

389.2 501.9

424.5 581.6

TABLE 2.3 Yield Stress Comparison Slurry t/h

1.5

2.5

3.5

4.5

5.5

Crystal boron/HTPB Amorphous boron/HTPB

23.6 24.9

29.8 35.6

32.9 39.4

36.0 43.1

38.3 48.8

As shown in Tables 2.2 and 2.3, the viscosity and yield value of crystalline boron/HTPB increase much more slowly in comparison with the amorphous boron/HTPB, which ­indicate that, with the increase of impurities in boron powder, the manufacture of boronbased propellant will be more difficult. The XPS results show that there are H3BO3 and B2O3 in boron powders, which correspond to the preparation process of amorphous boron by magnesium reduction method,16 which indicates that the major impurities should be raw materials like H3BO3, B2O3, by-product of the reaction like MgO, MgCl2 and ­intermediate products like B2O3, etc. Simulation experiments were designed to examine the reaction between existing impurities (H3BO3 and B2O3) or possible impurities (Mg and MgO) and HTPB. The experimental procedure is as follows: 5 wt. % of Mg, MgO, H3BO3, and B2O3 were added into HTPB/Al (1:1 by mass), respectively. Each sample was then kept at 50°C for a certain time. The viscosities of different samples were measured, and the results are shown in Table 2.4. It was found that only the samples with H3BO3 and B2O3 have ­gelation after heating for 90 and 25 h, respectively. In summary, it is the H3BO3 and B2O3 on the surface of boron inducing the gelation reaction between boron and HTPB. To confirm whether the gelation is specifically induced by C=C double bond from HTPB, CTPB/Al (1:1 by mass) was used to replace HTPB/Al (1:1 by mass) in the earlier experiments. The results are shown in Table 2.5. The results indicate that no rapid increase of viscosities can be observed in this system. This means C=C double bond is not the key parameter.

36

Boron-Based Fuel-Rich Propellant

TABLE 2.4 The Effect of H3BO3, B2O3, MgO, and Mg on the Viscosity of HTPB/Al Slurry Slurry t/h HTPB/Al HTPB/Al/H3BO3 HTPB/Al/B2O3 HTPB/Al/MgO HTPB/Al/Mg

1

6

9

14

25

32

48

56

72

12.8 27.8 58.9 22.3 16.5

12.6 37.2 117.0 23.0 16.8

12.4 43.3 258.0 24.3 17.3

11.2 59.7 497.6 25.2 16.9

12.6 85.1

15.4 173.4

14.4 309.8

14.2 436.6

25.8 20.5

25.3 17.0

13.7 278.7 gel 29.4 16.5

29.7 17.3

29.1 18.9

TABLE 2.5 The Effect of H3BO3 and B2O3 on the Viscosity of CTPB/Al Slurry Slurry t/h CTPB/Al CTPB/Al/B2O3 CTPB/Al/H3BO3

1

3

8

15

24

32

48

72

35.8 48.5 46.5

35.5 45.1 46.7

35.7 42.9 45.8

36.2 43.3 47.9

37.2 46.6 50.5

37.6 50.4 48.0

38.4 51.2 49.5

38.9 51.8 51.3

2.2.3 Infrared Analysis Two grams of H3BO3 and B2O3 were mixed with 10 g of HTPB, respectively, and heated to 50°C in an oil bath for further reaction, and then the formed gel was taken out for infrared analysis. Compared with the infrared spectrum of pure HTPB, the gelation products from H3BO3/HTPB and B2O3/HTPB have new peaks at 1,348 and 1,336 cm−1,15 which are in the range of the characterization infrared vibration band (1,350–1,310 cm−1) of B-O in boric acid ester.17 Therefore, it is indicated that the reaction product of H3BO3/HTPB and B2O3/HTPB might be the boric acid ester. Two grams of B2O3 and boron powders were mixed with 10 g of HTPB, respectively. The reaction process (1 h) of the mixtures was examined by an in situ infrared method, in which four data points were selected and shown in Figures 2.2 and 2.3. As shown in Figure 2.2, with the increase of reaction time, the intensity of -O-H peak (3,350 cm−1) decreases. And a new peak at 1,340 cm−1 shows up and its intensity increases

FIGURE 2.2 The infrared spectrum of B2O3/HTPB changes with reaction time.

37

Characterization of Boron Powder

FIGURE 2.3 The infrared spectrum of B/HTPB changes with reaction time.

with reaction time. Therefore, it can be concluded that the -O-H (hydroxy) in HTPB and B2O3 have esterification reaction and produce boric acid ester. As to the system of boron/ HTPB, Figure 2.3 shows inconspicuous changes about the -O-H (hydroxy) peak while there is a new peak at 1,340 cm−1, which confirms the formation of boric acid ester. These results ­indicate that the impurities in boron powders, such as H3BO3 and B2O3, undergo esterification reaction with -O-H (hydroxy) in HTPB. It is also worthy to note that the appearance of peak at 1,340 cm−1 indicates the beginning of the reaction for B/HTPB case. This can be explained by the fact that the large specific area of boron powder highly increases the ­contact area between H3BO3/B2O3 and HTPB, thus promoting the reaction rate. 2.2.4 Mechanism of the Deteriorated Process in HTPB Fuel-Rich Propellant In the esterification reaction between HTPB and H3BO3/B2O3, H3BO3 has trifunctionality and B2O3 has more than trifunctionality even hexafunctionality because of its special structure (specific analysis can be found15), while HTPB only has two functionalities. When H3BO3 undergoes esterification reaction with HTPB with a functionality more than two, the reaction process18 must be started with the formation of branched chains, then further reactions will induce cross-linking to form a three-dimensional polymer. In such system, when the polymerization reaction proceeds to a certain extent, cross-linking will be triggered, the viscosity of the system would be dramatically increased, and the bubbles will stay in the system. Then gelation will happen, and a gel will be formed. Therefore, it can be concluded that the esterification reaction between H3BO3, B2O3, B, and HTPB is a crosslinking-gelation process. The critical reaction extent18 when gelation happens:

Pc = 2 f . (2.1)

where f is the average functionality of the system. When unequal amounts of monomer with two functionalities have a gelation reaction, the average functionality of the system f can be calculated by the following equation:

f = 2 f2 ⋅ N A ( N A + N B ) . (2.2)

38

Boron-Based Fuel-Rich Propellant

where f2 is the functionality of less amount of the reactant, NA and NB are the amounts of molecules of reactants. From the earlier equations, the value of Pc is dependent on f : larger f results in smaller Pc. As for the esterification reaction between H3BO3, B2O3, and HTPB, the HTPB is excess.15 The average functionality of the system can be calculated by equation (2.2). When we study the effect of H3BO3 and B2O3 on the viscosity of HTPB/Al, the added amount of both H3BO3 and B2O3 is 5 g. The molecular weights of H3BO3 and B2O3 are 61.8 and 69.6, respectively, thus we hypothesize that the molecule amount of two is equal, that is, N H3 BO3 = N B2 O3 . As mentioned earlier, the functionality of H3BO3 ( fH3 BO3 ) is 3 and the functionality of B2O3 ( fB2 O3 ) is higher than 3, i.e., fH3 BO3 < fB2 O3 . According to equation (2.1): PC(B2 O3 ) < PC( H3 B3 O3 ) . (2.3)



Equation (2.3) indicates that the gelation reaction between B2O3 and HTPB is more rapid and earlier than that between H3BO3 and HTPB. This coincides with the simulation results. The critical reaction extent of gelation process between boron powder and HTPB can also be calculated by the equations (2.1) and (2.2). Let there be 100 g of boron powder and 100 g of HTPB. There are 3.79 g H3BO3 and 0.76 g B2O3 in the powder according to the ­composition of boron powder.15 The average functionality of the system can be calculated by equation (2.1).

f =

2 ( fH3 BO3 ⋅ N H3 BO3 + fB2 O3 ⋅ N B2 O3 ) (2.4) N H3 BO3 + N B2 O3 + N HTPB

The hydroxyl value of HTPB is 0.482 mmol·g−1, and the relative average molecular weight is 4,150. Set the functionality degree of B2O3 as 3. According to equation (2.4), f is 4.50. Thus, Pc = 2/f = 44.5% . If we set the functionality degree of B2O3 as 6, f is 5.18 and Pc = 2/f = 38.6% . The calculation results indicate that when boron and HTPB are mixed as a mass ratio of 1, the critical reaction extent of gelation is 44.5%, that is, when H3BO3 and B2O3 on the surface of boron have a reaction extent of 44.5%, the system will be gelling. The contact area between H3BO3, B2O3, and HTPB is large because of the small size, which results in a rapid cross-linking reaction. Thus, it is not difficult to reach the reaction extent of 44.5%. Moreover, in the real suspension, the weight percentage of boron is even higher, and the relative amount of HTPB is less. So, the functionality of the whole system is even higher, resulting in smaller Pc. The strong mixing from kneader machine also highly increases the contact between reactants, thus promoting the cross-linking reaction rate. Therefore, from the earlier results, it can be concluded that it is the esterification reaction between impurities H3BO3/B2O3 in boron powder and HTPB inducing the gelation, increasing the processing difficulties of boron/HTPB-based propellants.

2.3  P reparation Process of Agglomerated Boron Particles The spherical pelleting process of boron particles is shown in Figure 2.4. Pelleting process of different types of boron-based composites is shown in Figure 2.5.

39

Characterization of Boron Powder

Pretreated

agglomerated

Coat

Amorphous boron powder

solidify

Spherical agglomerated boron particles

FIGURE 2.4 Spherical pelleting process of boron particles.

Binder agglomeration Mannitol Amorph ous boron powder pretreat ment

Glycerol TMP Pre tre at

Agglomerate

TEA

Formulation

Combustion

Ethanol NaOH APǃGAP B-Mg alloy B-Al alloy

FIGURE 2.5 Pelleting process of different types of boron-based composites.

2.3.1 Pretreatment of Amorphous Boron Powder The pretreatment setup of amorphous boron powders is shown in Figure 2.6. Three different chemicals, such as mannitol, trimethylolpropane (TMP), and triethanolamine (TEA), were used to pretreat amorphous boron powders, respectively. The three chemicals were dissolved in 150 mL solvents (absolute ethyl alcohol was used to dissolve mannitol and TEA while acetone was used for TMP). A certain amount of ­a morphous boron powder was then added to the solutions. After stirring for 24 h at 50°C water bath, the samples were dried at 70°C for 72 h to remove all the solvents. The final surface modified boron powders were obtained. The earlier three pretreated boron ­powders were labeled as B-1, B-2, and B-3, respectively. For NaOH-treated boron, the procedure is slightly different. Three hundred milliliters of NaOH solution (2 mol·L−1) and 50 g amorphous boron powders were mixed in a beaker and was then stirred at 60°C for 4 h. The products were then washed using water until pH = 7.0. The powders were then filtered from the solution and then dried at 70°C. The final product pretreated by NaOH was labeled as B-4, and the boron powder without treatment was labeled as B-0.

40

Boron-Based Fuel-Rich Propellant

FIGURE 2.6 Schematic view of pretreatment setup of amorphous boron powders. 1. Mechanical stirring; 2. Thermal couple; 3. Three-mouth flask; 4. Return line; 5. Iron stand; 6. Water bath with constant temperature.

Note: After treatments by the earlier three chemicals, the final products of boron also have gelation reaction with HTPB. Thus, there is no follow-up work on it. 2.3.2 Coating of Amorphous Boron Powder When boron is ignited, there is a molten oxide layer on the surface of boron. The layer can only be vaporized at very high temperature to promote inside burning. Therefore, the ignition and combustion performance of boron are poor. Moreover, the impurities of B2O3 and H3BO3 on the surface of boron have poor compatibility with HTPB.19,20 Therefore, coating of boron with different materials may promote the ignition and combustion of boron. In this section, different coating materials such as ammonium perchlorate (AP), 3,3’-­perazidomethyl oxybutyl ring and tetrahydrofuran copolyether (PBT), HTPB, and LiF were employed to coat boron using different methods. Nonsolvent and deposition ­methods were commonly used to coat boron with AP. Neutralization precipitation method was used to coat boron with LiF. The functional groups in PBT and HTPB can react on the surface of AP to achieve coating. Transmission electron microscopy (TEM) and acidimeter were used to evaluate the coating results and the effects of the coated powders on the ­pelleting process of boron-based fuel-rich propellant.

1. Experimental principle Nonsolvent and deposition methods are commonly used to coat boron with AP. Nonsolvent methods employ the fact that the solubility of AP in two solvents (which cannot dissolve each other) is different. AP in saturated solution will crystallize on the surface of boron, thus form a coating layer. Deposition method also uses crystallization of AP from saturated solution after solvent evaporation. Previous experimental results show that the evaporation rate of solvent plays a significant role on coating results. When evaporation rate is low (10 g·h−1), AP can

Characterization of Boron Powder

be coated uniformly on the surface of boron. PBT is a polymer with -OH groups. The functional groups in PBT react with the oxide on the boron surface and thus achieve the coating. Optimum coating can be performed using tetrahydrofuran as the solvent and having a surface reaction of at least 16 h. Before coating, silane coupling agent is used to pretreat boron powders. The coating principle of HTPB is like that of PBT. The HTPB will react with the acidic functional group on boron, thus forming a coating (HTPB content ≤10 wt. %). Neutralization precipitation method is used to coat boron with LiF. Based on the reaction LiOH (aq) + HF (aq) = LiF (s) + H2O, the formed LiF precipitation will deposit on the surface of boron making the coating. 2. Attentions in experiments a. Combining the ultrasonication and mechanical stirring methods improves the uniformity of coating and particle dispersion of boron. b. When coating boron powder with AP, the evaporation rate of solvent is ­relatively slow, which also slows down the crystallization rate of AP. Therefore, AP deposits uniformly on the surface of boron. c. When coating boron powder with LiF, the stirring speed affects the d ­ ispersion homogeneity of precursor. Experimental results show the best coating ­performance with a stirring speed of 750 r·min−1. d. When coating boron powder with PBT, the coating layer of PBT is becoming thicker with the increase of reaction time. Thicker coating layer has lower infrared transmission and higher absorption. The IR spectrum of a sample with 24 h reaction is like that of a 16 h case, suggesting that the reaction is ­completed within 16 h. e. When coating boron powder with HTPB, both the adding amount of HTPB and reaction time affect the coating performance. When the adding amount of HTPB is >15 wt. %, the coated boron is easy to agglomerate. Furthermore, ­adding HTPB will also decrease the burning rate and energy density of ­propellant. Therefore, the adding amount of HTPB was fixed at 10 wt. %. By comparison of the acidity of coated boron powder for 8, 14, and 16 h, it is found that boron was completely coated after 14 h. 3. Effects of coating agents on the coated boron particles The TEM images of boron particles modified by AP, PBT, HTPB, and LiF are illustrated in Figure 2.7. As Figure 2.7a shows, there is an obvious coating layer on boron surface, indicating a successful coating of AP on boron using precipitation method. It is worth to note that the dispersion of coated boron is much better in the solution compared with that in the scanning electron microscope (SEM) images shown. Figure 2.7b shows the boron particles coated with LiF, which is a transparent layer on the boron particles. Figure 2.7c also shows a transparent layer on boron particles, which is supposed to be the reaction product of PBT and boron oxide layer. Figure 2.7d shows the agglomeration of boron particles with a transparent layer of HTPB. 4. Effects of coating agents on the acid degree of boron particles The previous studies show that boron can be compatible with HTPB. However, for the amorphous boron particles with low purity, there are impurities of B2O3, BO6, and BO7 formed on the boron surface. These impurities will react with water vapor in atmosphere and produces H3BO3, which is an acid.21–25

41

42

Boron-Based Fuel-Rich Propellant

FIGURE 2.7 TEM images of boron particles coated with different materials.

The reaction equation is B2O3(s) + 3H2O = 2H3BO3. The key reason for worsening the compatibility between HTPB and boron is the condensation reaction between H3BO3 and HTPB molecules, which is induced by the electron-deficient B2O3 and acid H3BO3. When the condensation reaction proceeds, large molecule weight polymer chains are formed with increasing viscosity. Therefore, the acid materials are the culprit affecting the propellant manufacturing process. It is necessary to investigate the effects of coating agents on propellant manufacturing process via the research on acidity. Because of the low solubility of B2O3 in water and high ­solubility in ­methanol, we cannot directly measure the acidity.26,27 Thus, to measure the ­acidity, a mixture of water and methanol (mass ratio 4:1) was prepared and used to ­disperse boron particles. After sufficient sonication (10–20 min) and stirring, H3BO3 was fully dissolved in the mixture. Then, the pH value of the mixture was measured by a precision acidity meter (PHS 2C). The pH value curves changing with time of different coated boron particles suspension (12 wt. %) are shown in Figure 2.8. As Figure 2.8 shows, the pH values decrease with time at first, and then keep constant. For AP-coated boron suspension, the pH drops until 35 min, which means that all B2O3 on the boron surface have already dissolved in the suspension after 35 min. The timings for PBT-, LiF-, and HTPB-coated boron suspension are 70, 30 and 65 min, respectively.

FIGURE 2.8 The pH value curves changing with time for different coated boron particle suspension.

Characterization of Boron Powder

43

FIGURE 2.9 The pH value curves changing with suspension concentration for different coated boron particle suspension.

The pH values of coated boron (by AP, LiF, PBT, and HTPB) dispersed in pure water was also measured, as illustrated in Figure 2.9. As Figure 2.9 shows that uncoated boron has a pH value of 3–4, which belongs to medium strong acid. The boron coated by AP is also acidic, and when the particle content in the suspension is ≤ 7.5 wt. %, the pH value is larger than that of uncoated boron. However, with the increase of particle content, the pH value of coated boron decreases rapidly. This is because of the hydrolysis of AP in water (note: there is no hydrolysis of AP in propellant bulk because of very little water inside). The reaction is as follows28 NH +4 + H 2 O ↔ NH 4 OH + H + . However, the pH value of boron coated by PBT and LiF s­ uspension is higher than 6.5, especially for the boron coated by LiF, whose pH value is higher than 7.4. The pH value of boron coated by HTPB suspension is just raised slightly to 5.5. In summary, all the coated boron suspensions were found to have a higher pH value, which will efficiently improve the manufacturing process of boron-based propellant. 2.3.3 Agglomeration and Prills of Amorphous Boron Powder In this section, mechanical stirring method was mainly used to prepare agglomerated ­pellets from liquid. The procedure is as follows. First, dissolving a certain amount of binder into organic solvent forms a homogenous solution, then adding amorphous boron powder into the solution forms a suspension. The fine powders in the suspension form a velocity gradient from the fixed stirring center to the side. The spin stirrer will rotate around the fixed stirring center and thus agglomerate the powders with the evaporation of the ­solvent. The evaporation rate and stirring rate can be adjusted in the setup to evenly breakup agglomeration. After agglomeration, the formed particles need to be dried in ­vacuum. This pelleting process can be completed in one step with high efficiency.29–32



1. The selection of agglomeration agent. The main binders we use in this section are glycidyl azide polymer, HTPB, and polyvinyl butyral, etc. 2. The control of agglomeration temperature. The temperature was controlled by a circulating water bath. If the temperature is too high, the evaporation rate of solvent will be too rapid, making the agglomeration harder. But if the temperature is too low, the agglomeration rate and the evaporation rate of solvent are much slow, making it difficult to form spheres.

44

Boron-Based Fuel-Rich Propellant

The experimental results show that the best water bath temperature is 30°C ± 2°C for a good yield of agglomerated spheres. 3. The control of coating during pelleting. The coating layer of the agglomerated spheres was done by a domestic manpowered atomizer. A certain mass ratio of AP and ethanol was mixed to form the coating solution. The solution was sprayed by the atomizer during the stirring process, thus creating a thin oxidizer layer on the agglomerated spheres.











4. The formulation and ratio. To prepare agglomerated boron spheres with uniform size distribution with a good compactness, raw materials of boron with small size and high density need to be selected. The boron and binder ratio of B/HTPB is from 95/5 to 80/10. The detailed formulation is shown in Table 2.6. 5. Postpelleting process. The agglomerated spheres have relatively wide size distribution and high humidity with weak strength. Thus, it is necessary for postpelleting processing, such as vacuum dry and graiding granulation. 6. Vacuum dry. The agglomerated spheres were dried in a vacuum drying oven at 50°C for a day. Then, increase the temperature to 70°C and dry it for another 3 days. 7. Grading granulation. Standard sieves with different mesh apertures were used to do sizing screen for the dried pellets. There are five size ranges: 0.074–0.104 mm (140–200 mesh), 0.104–0.150 mm (100–140 mesh), 0.178–0.25 mm (60–80 mesh), 0.25–0.84 mm (20–60 mesh), and 0.84–2.0 mm (10–20 mesh). Take the size range of 0.178–0.250 mm as an example. Use the sieve with a mesh aperture of 0.178 mm to select the p ­ articles with size ≥0.178 mm and then use the sieve with a mesh aperture of 0.250 mm to select the particles with size ≤0.250. Then we get the particles with a size range from 0.178 to 0.250 mm. Five hundred grams of raw powder was agglomerated into pellets, then the ­pelleting yield of the particles with a size range of 0.074–2.0 mm (10–200 mesh) is: Total pelleting yield =



Total massof spherical particles = 87.2% Powder mass

Previous studies showed that a certain size distribution (0.104–0.84 mm) of boron agglomerations has better manufacturing properties compared with others. The pelleting yield of particles with a size distribution of 0.104–0.84 mm is TABLE 2.6 Detailed Formulation of Agglomeration of Boron Spheres No. 1 2 3 4

Boron (g)

Binder (g)

Ethyl Acetate (mL)

AP (g)

Curing Agent (g)

Curing Catalyst

80 85 90 95

9.4 7.4 4.6 4.6

200 150 100 50

10 7 5 –

0.6 0.6 0.4 0.4

2 drops 2 drops 1 drops 1 drops

Note: Dibutyltin dilaurate (T12) was used as the curing catalyst. The mass is 1/15 (wt. %) in ethyl acetate.

45

Characterization of Boron Powder

  Total pelleting yield =



Total mass  of spherical particles ( 0.104 − 0.84 mm ) = 63.08% Powder mass

The raw amorphous boron powder for pelleting is 500 g. After adding a ­pelleting agent, controlling the humility of the powder, and with proper handling time, 436 g of boron pellets with different size distributions was made. The overall ­pelleting yield is 87.2%. 1. Optimization of boron particles agglomeration process In consideration of so many varying factors affecting the pelleting process, five typical factors (stirring speed, process temperature, B/binder ratio, curing temperature, and curing time) were selected based on the experimental results. The orthogonal factor experiment plan was designed for the optimal pelleting ­condition. Four levels were selected for each factor, as shown in Table 2.7. The earlier orthogonal experiments show that all the factors, such as stirring speed, process temperature, B/binder ratio, curing temperature, and curing time, have a significant impact on the quality of pelleting. As Table 2.8 shows, the impact high to low is: B/binder ratio, process temperature, curing temperature, curing time, and stirring speed. The optimal experimental condition is A2B1C2D3E4, that is, stirring speed is 90 r·min−1, process temperature is 30°C, B/binder ratio is 9/1, curing temperature is 70°C, and curing time is 7 days. The pellet prepared by the earlier optimal condition was characterized by the SEM. The images show spherical shape, good dispersion, and narrow size distribution (80% of the particles are in the size of 0.10–0.20 mm). The experimental results reveal that the curing of pellets can be achieved not only by high temperature (90°C) and short curing time (3 days) but also by low temperature (50°C) and long curing time (7 days). Raising the curing temperature yields benefits equal to increasing the curing time, which corresponds to the time– temperature equivalence principle. However, when the curing temperature is 70°C and the curing time is 7 days, the S value is the highest. Both the physical diffusion and chemical reaction have the time–temperature equivalence principle; however, the synthetic effect of the two makes the inconsistence between the theoretical and experimental results. This might occur because the time–temperature ­equivalence principle was obtained by the Arrhenius activation energy equation, which is based on the Eyring general equation about reaction rate.33–35 In the e­ quations, the activation energy does not change when it describes the relaxing movement of the molecule chain. But in the pelleting process of boron powder, when the ­reaction proceeds, the molecular weight in the interaction point decreases with the change

TABLE 2.7 Details of the Designed Orthogonal Experiments Level 1 2 3 4

Stirring Speed A (rpm)

Process Temperature B (°C)

B/Binder Ratio C

Curing Temperature D (°C)

Curing Time E (day)

50 90 70 110

30 20 40 50

95/5 90/10 85/15 80/20

50 60 70 90

1 3 5 7

46

Boron-Based Fuel-Rich Propellant

TABLE 2.8 The Results of the L16(4)5 Orthogonal Experiments

No.

Stirring Speed A (rpm)

Process Temperature B (°C)

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #15 #16 K1 K2 K3 K4 R

1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 120.5 131.7 122.8 126.3 11.2

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 123.3 115.2 146.6 116.3 31.4

B/Binder C

Curing Temperature D (°C)

Curing Time E (hour)

Comprehensive Evaluation Value S

1 2 3 4 2 1 4 3 3 4 1 2 4 3 2 1 106.6 154.6 126.2 114.0 48.0

1 2 3 4 3 4 1 2 4 3 2 1 2 1 4 3 119.6 114.1 140.8 126.9 26.7

1 2 3 4 4 3 2 1 2 1 4 3 3 4 1 2 119.8 127.0 122.5 132.1 12.3

22.1 32.5 38.8 27.1 45.3 25.4 34.4 26.7 31.2 27.8 30.2 33.6 24.7 29.5 43.2 28.9 T = 501.4

Note: K is the mean difference, and R is the variance.







of activation energy, which is different from the theoretical relationship explained by Arrhenius equation. Therefore, through the analyses of the earlier orthogonal experiments, the optimal experimental condition is A2B1C2D3E4. 2. Effects of different factors on the agglomerated boron particles The impact factors of pelleting, such as the solvent, stirring speed, binder ­content, processing temperature, curing temperature, and time, will be introduced in the following sections. 1. The effect of solvent This section investigates the effect of different solvents on pelleting, and the results are shown in Figure 2.10. As seen in Figure 2.10b, with the coating solvent of styrene (commonly used in propellant formulation as a diluted agent), the particles show a smooth ­surface. The disadvantages include the toxic properties of styrene, and some styrene residue will be coated on the formed boron pellet. However, as shown in Figure 2.10a, by using ethanol as the coating agent, the surface becomes pretty rough. 2. The effect of stirring speed This section investigates the effect of stirring speed on pelleting, and the results are shown in Figure 2.11.

47

Characterization of Boron Powder

(a)

(b)

FIGURE 2.10 Effect of solvent on the agglomeration of boron particles. (a) ethanol, (b) styrene.

(a)

(b)

FIGURE 2.11 Effect of stirring speed on the agglomeration of boron particles. (a) 70 rpm, (b) 90 rpm.



As shown in Figure 2.11, the higher the stirring speed, the smaller the ­pellets. With the increase of stirring speed, the molecular movement is enhanced and the solvent is easier to evaporate. The pelleting processing time was reduced and the shearing force on the suspension was enhanced. Therefore, the agglomerated boron pellets were broken and the growth of boron pellets was limited. On the contrary, with the decrease of stirring speed, the average size will increase. 3. The effect of binder content The effect of different binder content (5% and 10%) on pelleting was ­investigated, and the results are shown in Figure 2.12. The agglomeration ­temperature and processing time were fixed at 50°C and 3 h, respectively. If the binder content is too high, that is the B/binder ratio is too low, the ­particles will be adhered to each other and is not easy to be dispersed. The size will be eventually larger than required, and the pelleting efficiency will be low. On the contrary, the higher the B/binder ratio, the higher is the

48

Boron-Based Fuel-Rich Propellant

(a)

(b)

FIGURE 2.12 Effect of binder content on the agglomeration of boron particles. (a) 5.0%, (b) 10.0%.





­ iscosity, which causes difficulties in the increase of energy density. A proper v B/binder ratio is beneficial to form spherical and uniform boron pellets. As seen in Figure 2.12, when the binder content decreases from 10% to 5%, the adhesion between the boron particles is weak and makes the specific area of pellets larger. The particle size distribution is uniform with 10% binder content, and the strength of the particles is high under a certain high pressure. However, the adhesion of boron particles with 5% binder content is weak, and the formed spherical p ­ ellets ratio is low. The experimental results also show a smoother surface when using 10% binder. 4. The effect of process temperature The smoother the particle surface is, the lower the specific area, that is, the lower is the resistance to motion of particle movement in propellant slurry. Therefore, the propellant slurry with smooth pellets has higher loading of ­particles and lower viscosity. Moreover, in the pelleting process, the solvent was rapidly evaporated via capillary attraction in intervals of particles. With the evaporation of solvent, the dissolved binder deposits on the boron particles to adhere the particles into pellets. In this process, the curing temperature is an important factor, and this section investigates the effect of processing temperature on the pelleting process, and the results are shown in Figure 2.13. As seen in Figure 2.13, with the increase in processing temperature, ­molecule movement enhanced and solvent evaporation accelerated. The deposition speed also increased and the binder cannot fill in the spaces among p ­ articles evenly and densely. This will create defects in the coating and make the ­surface rough. Furthermore, with the increase in temperature, the particle size will increase but the quantity is decreased, which makes the quality of pellets difficult to control. 5. The effect of curing temperature The binder in the boron-based pellets needs to be cured for being solid. Different curing temperatures have a significant impact on the pelleting ­performance. This section studies the effect of curing temperature (50°C and 70°C) on the pelleting process, and the results are shown in Figure 2.14.

49

Characterization of Boron Powder

(a)

(b)

FIGURE 2.13 Effect of process temperature on the agglomeration of boron particles. (a) 50°C, (b) 30°C.

FIGURE 2.14 Effect of curing temperature on the agglomeration of boron particles. (a) 50°C, (b) 70°C.



As seen in Figure 2.14, the lower the curing temperature, the slower the curing reaction speed and longer the curing time. Low curing temperature will eventually increase the preparation time of boron-based fuel-rich propellant. The higher the curing temperature, the lower the solidifying time. However, curing will not be complete if the curing is too fast. 6. The effect of curing time In the pelleting process, the curing time is very important for the shapes of pellets. This section studies the effect of curing time of 3 and 7 h on the pellets, and the photos are shown in Figure 2.15. As shown in Figure 2.15, the longer the curing time, the more complete the curing will be. The solidified particles are stronger and not easy to break. Moreover, the shape of the pellets is tendentially spherical. With the increase in curing time, the percentage of pellets, that is the required size distribution, is higher.

50

Boron-Based Fuel-Rich Propellant

(a)

(b)

FIGURE 2.15 Effect of curing time on the agglomeration of boron particles. (a) 3 h, (b) 7 h.

Therefore, after the pelleting process of amorphous boron powders, the particle ­morphology is highly improved and the particles are “spherical” or “semispherical”, and the size ­distribution can be easily controlled.36–38

2.4 The Particle Size Distribution and Fractal Dimension Characterization of Agglomerated Boron Particles For a long time, the ratio of a certain component size distribution was used to judge the size distribution of components in a composition. This method is not beneficial to the characterization of particles size and the application of simulation model, especially for the input of parameters in neural network calculation. In fact, the size distribution of solid components can be better characterized using the direct and accurate method of fractal analysis.39–42 2.4.1 Measuring Principle of Fractal Dimension Generally, the size distribution is described by the particle mass fraction with different size ranges.43–45 If the size distribution has fractal property, that is

Yn ( x) ∝ − x − D (2.5)

where Yn(x) is the ratio of particles number with size