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SYNTHESIS, CHARACTERIZATION AND ENERGETIC PERFORMANCE OF METAL BORIDE COMPOUNDS FOR INSENSITIVE ENERGETIC MATERIALS by Michael L. Whittaker A thesis submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Master of Science Department of Materials Science and Engineering University of Utah May 2012 Copyright © Michael L. Whittaker 2012 All Rights Reserved The University of Utah Graduate School STATEMENT OF THESIS APPROVAL The thesis of Michael L. Whittaker has been approved by the following supervisory committee members: Raymond A. Cutler , Chair 03/09/2012 Date Approved Anil V. Virkar , Member 03/09/2012 Date Approved Gerald B. Stringfellow , Member 03/09/2012 Date Approved and by Feng Liu , Chair of the Department of Materials Science and Engineering and by Charles A. Wight, Dean of The Graduate School. ABSTRACT Six metal boride compounds (AlB2, MgB2, Al0.5Mg0.5B2, AlB12, AlMgB14 and SiB6) with particle sizes between 10-20 m were synthesized for insensitive energetic fuel additives from stoichiometric physical mixtures of elemental powders by high temperature solid state reaction. B4C was also investigated as a lower cost source of boron in AlB2 synthesis and showed promise as a boron substitute. Thermal analysis confirmed that the formation of boride compounds from physical mixtures decreased sensitivity to low temperature oxidation over the aluminum standard. Both Al+2B and AlB2 were much less sensitive to moisture degradation than aluminum in high humidity (10-100% relative humidity) and high temperature (20-80°C) environments. AlB2 was determined to be safe to store for extended periods of time in cool, dry environments. Impact, friction and shock sensitivity testing indicated that AlB2 and MgB2 were less sensitive than aluminum. The activation energies for the oxidation of Al, B, Al+2B and AlB2 were determined through an isothermal, isoconversional method in N2-20%O2 and O2 at one atmosphere. An activation energy of 413 ± 20 kJ/mol was calculated for AlB2 in O2. The incorporation of magnesium and/or aluminum with boron increased its oxidation rate and overall conversion through the formation of metal-borate crystals (2Al2O3·B2O3 and 3MgO·B2O3) which removed liquid B2O3 from the surface of oxidizing particles. Aluminum also increased the oxidation efficiency of B4C by a similar mechanism. AlB2, MgB2 and Al0.5Mg0.5B2 oxidized to greater than 85% of their theoretical values while exhibiting decreased sensitivity to low temperature oxidation, making them top candidates for further energetic testing. Cylinder expansion testing of AlMgB14 showed little reaction of the boride material within seven volume expansions, corresponding to poor energetic performance. Detonation calorimetry of AlB2 and Al + 2B using proprietary energetic mixtures in an argon atmosphere showed that AlB2 reacted almost completely while Al + 2B did not. Future work should focus on testing the diboride materials and synthesizing and testing similar materials made from B4C. iv Dedicated to my family, and the Flying Spaghetti Monster TABLE OF CONTENTS ABSTRACT.......................................................................................................................iv ACKNOWLEDGEMENTS...........................................................................................viii 1. INTRODUCTION.........................................................................................................1 1.1 Background....................................................................................................................1 1.2 Material Selection..........................................................................................................4 1.3 References ....................................................................................................................16 2. SYNTHESIS AND CHARACTERIZATION OF BORIDES..................................15 2.1 Characterization Procedure..........................................................................................15 2.2 Starting Powder Characterization................................................................................16 2.3 Reacted Powder Characterization ................................................................................25 2.4 Summary......................................................................................................................31 2.5 References ....................................................................................................................36 3. HIGHER PURITY AlB2..............................................................................................39 3.1 In-House Synthesis......................................................................................................39 3.2 Commercial Powder.....................................................................................................58 3.3 Conclusions..................................................................................................................59 3.4 References ....................................................................................................................60 4. SENSITIVITY AND OXIDATION BEHAVIOR.....................................................62 4.1 Oxidation Characteristics in Air..................................................................................62 4.2 Conclusions..................................................................................................................87 4.3 References ....................................................................................................................89 5. KINETIC ANALYSIS.................................................................................................91 5.1 Isothermal Oxidation...................................................................................................91 5.1.1 Model-Free Method I, Air.........................................................................................91 5.1.2 Model-Free Method II, O2......................................................................................109 5.2 Discussion ..................................................................................................................119 5.3 References .................................................................................................................124 6. MOISTURE SENSITIVITY.....................................................................................126 6.1 Experimental Procedures...........................................................................................126 6.2 Results and Discussion..............................................................................................128 6.3 Conclusions................................................................................................................139 6.4 References ..................................................................................................................140 7. SUMMARY AND CONCLUSIONS........................................................................141 APPENDICES A: PARTICLE SIZE HISTOGRAMS.........................................................................147 B: MICROSTRUCTURE, MECHANICAL PROPERTIES AND PERFORMANCE OF MAGNESIUM ALUMINUM BORIDE (MgAlB14)......155 C: BORIDE BASED MATERIALS FOR ENERGETIC APPLICATIONS...........168 vii ACKNOWLEDGEMENTS First and foremost I would like to thank Dr. Raymond Cutler, who has been a teacher, boss, advisor and role model. He challenged me to live up to my potential, and taught me not what to think, but how. Dr. Paul Anderson provided part of the funding for this work. I am indebted to Dr. Anil Virkar and Dr. Gerald Stringfellow, not only for advising me through the thesis process, but for inspiring me to learn as much as they have learned in order to make a real difference in the world, as they have. I could not have produced Chapter 5, possibly the most significant portion of this thesis, without the help of Dr. Hong Yong Sohn. This thesis would not have been on time or in print without the help of Ashley Quimby, who keeps me focused and punctual with a smile on her face. Marc Finders offered advice and encouragement on many weekends and late nights. Lyle Miller helped collect TGA data and allowed me to temporarily take over part of his characterization lab. Finally, thanks to the faculty, staff, and students of the Materials Science department, who keep me on my toes and challenge me every day. 1. INTRODUCTION 1.1 Background Energetic materials are generally characterized as materials that contain a significant amount of chemical energy that can be released quickly.1 Metals are commonly used fuels and fuel additives in binary energetic systems (a combination of fuel and oxidizer) due to their relatively high heats of combustion and fast reaction kinetics.1 Boron is a promising energetic fuel because of its high volumetric and gravimetric heats of combustion,2-6 but unlike metals the slow oxidation kinetics of boron limit its ability to perform adequately in most energetic systems.2-8 The poor performance of boron can be attributed to the unique relationship between boron and its oxides. Elemental amorphous boron melts at a temperature of 2077⁰C and boils at 3867⁰C.9 Because boron does not readily liquefy, the combustion of boron particles proceeds as a heterogeneous reaction controlled by diffusion of an oxidizing species from the surroundings to the boron particle surface.3,
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