Boride-Based Materials for Energetic Applications
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Boride-Based Materials for Energetic Applications Michael L. Whittaker,1 Raymond A. Cutler,2 and Paul E. Anderson3 1
University of Utah, 122 S. Central Campus Drive, Salt Lake City UT, 84112 Ceramatec, Inc., 2425 South 900 West, Salt Lake City, UT 84119 3 Explosives Research and Development Branch, ARDEC, Picatinny Arsenal, NJ 07806 2
ABSTRACT Metal borides (AlB2, MgB2, Mg0.5Al0.5B2, AlB12, SiB6 and MgAlB14) and boron carbide (B4C) reacted with Al were compared to B, Mg, Al, Mg-Al and Si as potential energetic fuel additives. Stoichiometric physical mixtures of powders corresponding to unreacted boride compounds (Al+2B, Mg+2B, Mg-Al+2B, Al+12B, Si+6B, Mg-Al+14B, and B4C+2Al) were also investigated in comparison to the compounds. Submicron boron was used, which resulted in very fine particle sizes for all materials studied. It was demonstrated that boride compounds were less sensitive to low-temperature oxidation in flowing air than physical mixtures or metallic fuels. Compounds with high mole fractions of boron were generally less sensitive, but their high temperature oxidation behavior showed no improvement over boron. Cylinder expansion testing of MgAlB14 exposed its poor performance in an energetic mixture. However, aluminum and magnesium diborides (AlB2, MgB2 and Mg0.5Al0.5B2) also had relatively low sensitivity and exhibited mechanisms to increase the rate of boron oxidation at high temperatures, showing promise as insensitive high-energy-density fuel additives. Detonation calorimetry of mixtures with AlB2 or Al+2B suggested that the AlB2 mixture released approximately 50% more heat per gram than Al +2B and underwent complete reaction. These results warrant further testing of the diboride compounds in energetic formulations. Due to the high cost of boron and acceptable performance of B4C-Al mixtures, B4C should also be investigated as a lower-cost alternative to boron. INTRODUCTION Boron has long been recognized as fuel for rocket boosters and other energetic applications where high energy density is required.1,2 The heat of combustion for the oxidation of boron to boron oxide is highly exothermic on both a volumetric and gravimetric basis. The main problems with using boron have been obtaining complete combustion due to slow oxidation kinetics1 and the high cost of the material. Metals like Al, Mg and Mg-Al have typically been used despite lower enthalpies of combustion and higher sensitivity to accidental discharge due to more favorable oxidation kinetics. Mitani and Izumikawa3 showed that the addition of micron sized Al to B increases its combustion efficiency in simple strand burner studies. Flower et al.4 demonstrated a similar improvement in performance by bomb calorimetry for mechanically alloyed boron and Al powders. Hsia2 measured ignition delay and burning time for 30-75 m Al, Mg and Li borides in air using optical techniques and came to the conclusion that the metal borides are superior to B for use in rocket propulsion systems due to faster ignition and complete combustion. Mixtures of metal powde
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