Transition Metal Catalysts for Boron Combustion

Composite powders comprising 95 wt% boron and 5 wt% metal additives including Fe, Co, Ni, Hf, and Zr, were prepared by high energy mechanical milling in a planetary mill using hexane as a process control agent (PCA). Additionally, composites with Fe were prepared using acetonitrile and stearic acid as PCA. Single particles of boron and all prepared materials were fed into the combustion products of a pre-mixed air-acetylene flame, in a diffusion air-hydrogen flame, and ignited in air using a CO₂ laser beam. Infrared emission pulses of burning particles were recorded. Distribution of burn times for each powder was obtained based on the durations of these pulses. Correlating the time distributions with measured particle size distributions yielded trends showing the effect of particle size on its burn time for all materials. Color temperatures were also recovered from the recorded emission signals. Additionally, constant volume explosion experiments were performed with all materials aerosolized in air. Results show that the morphology of the prepared composite powders is similar to that of boron. Single particle combustion experiments show that adding Hf leads to the most significant acceleration of combustion compared to boron. A positive effect of Hf as an additive is noted in different oxidizing environments. Adding Fe leads to an improved burn rate in air; however, there is no clear advantage in other oxidizing environments. No clear improvement is observed when other metal additives are used. Preparing B-Fe composites using different PCAs suggests that both a greater refinement of iron and a stronger acceleration in the burn rates of the prepared powders particles in air occur when acetonitrile and stearic acid are used as PCA. The results of the present cloud combustion experiments were found to be affected by multiple parameters and processes making it difficult to offer their meaningful interpretation.