(616f) Transition Metal Catalysts for Boron Ignition and Combustion

Boron is an attractive fuel additive for explosives and propellants due to its high gravimetric and volumetric combustion energy densities. The limitations of using boron in many practical formulations are associated with its relatively slow reaction kinetics leading to long combustion times and ignition delays. Additionally, in hydrogen containing environments, it forms thermodynamically unfavorable but kinetically preferred intermediates such as HOBO. Past studies showed that boron ignition is delayed by its inhibiting oxide layer. It was also observed that boron burns at temperatures substantially below its boiling point and the combustion may be rate-limited by its heterogeneous surface reaction with gaseous oxidizers. Recently, it was shown that the complete or partial removal of the hydrated surface oxide from boron powders by treating them in a polar solvent helps reducing boron particle ignition delays. It was also found that doping boron with a small, less than 5% additive of iron results in reduced particle burn times in oxygen and hydrogen containing oxidizing gases. It was hypothesized that the iron dopant has a catalytic effect on the heterogeneous oxidation of burning boron particles. This work expands these recent efforts to address effect of a broader range of transition metals with high oxygen affinity and multiple oxidation states as potential catalysts improving kinetics of boron ignition and combustion. The candidate transition metals include cobalt, molybdenum, and zirconium. Selected are metals with relatively high boiling points to ensure their lasting effect on heterogeneous combustion of molten boron particles. It is expected that during combustion, metal dopants will be preferentially oxidized by surrounding gaseous oxidizers on surface of the burning particles. It is further expected that boron will readily reduce transition metal oxides. This mechanism is expected to accelerate heterogeneous reactions and thus minimize the chances of generating HOBO as a gas phase intermediate. The boron-metal composite powders are prepared by high-energy mechanical milling using liquid process control agents. Prepared samples are characterized for particle size distributions, elemental distribution of the dopant on the particle surface and interior. Combustion studies are conducted for samples laser ignited in air and in steam created by hydrogen-air diffusion flame. Measured burn times, emission traces, and temperatures will be presented and compared for doped and as received boron powders.