| Description |
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. |
