| Description |
Boron is a promising material for energetic applications because of its high gravimetric and volumetric heats of combustion. However, its relatively slow oxidation kinetics at high temperatures and its high cost compared to metallic fuels limit its performance and feasibility in most applications. Metal fuels with lower heats of combustion such as aluminum and magnesium have typically been used in place of boron because of their increased reaction rates in energetic systems, but a recent push towards insensitive energetic materials has reopened investigations into boron based fuels. Metal boride compounds attempt to incorporate the high potential of boron with the proven performance of metals. The work described in this thesis compares the energetic performance of the borides MgB2, AlB2, Al0.5Mg0.5B2, AlMgB14, SiB6 and AlB3C, as well as stoichiometric mixtures of the corresponding elemental powders for each boride. X-ray diffraction (XRD) was used to determine the composition of the powders. Multipoint gas adsorption surface area analysis and laser light scattering were used to calculate the surface area and particle size of the powders, and scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to analyze particle morphology and phase distribution. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were used to assess oxidation characteristics in air. Through these techniques it was confirmed that adding magnesium or aluminum to boron a mixture increased the oxidation efficiency of boron by forming metal-borate crystals (9B2O3·Al2O3 and 3B2O3·MgO, respectively) which removed liquid B2O3 from the surface of oxidizing particles. Aluminum also greatly increased the oxidation efficiency of B4C by a similar mechanism. The powder mixtures were not significantly less sensitive than the aluminum standard, but the reacted compounds exhibited much greater sensitivity. However, AlB12, AlB3C, AlMgB14 and SiB6 did not react completely or quickly, making them poor energetic materials. AlB2, MgB2 and Al0.5Mg0.5B2 were the only materials that reacted to above 85% of their theoretical values while exhibiting increased insensitivity, making them top candidates for further energetic testing. |
