Effect of Purity, Surface Modification and Iron Coating on Ignition and Combustion of Boron in Air

Combustion characteristics of boron powders with varying purity, surface treatment and iron dopant loadings were investigated. Particles were ignited by a CO₂ laser and burned in air. Commercial 95% and 99% pure boron powders were used as the starting materials; the former powder was also washed with acetonitrile to reduce the inhibiting oxide layer. All powders were coated with iron by thermal decomposition of iron pentacarbonyl. Optical emission of single burning particles was captured in order to determine combustion temperatures and burn times as a function of particle size. All materials ignited at approximately the same CO₂ laser beam energy. For all materials, two-stage combustion patterns were detected for longer emission pulses produced by particles burning in air, which became difficult to resolve for shorter burn times. It was observed that higher purity boron particles burn faster. A reduction in the natural oxide layer thickness achieved by acetonitrile wash did not appreciably change the burn time of boron particles. However, it did lead to a brighter emission during second stage combustion, to oscillatory emission patterns, and to formation of nitride caps on the surface of quenched boron particles. Coating boron with iron was effective for commercial 95% pure boron. It was less effective for washed boron and even less effective for boron of higher purity. Boron coated with iron burned faster than un-coated boron; the effect appeared to scale with the amount of iron present. Additionally, boron coated with iron burned at a higher temperature than the un-coated powders. The increase in the burn rate and temperature caused by the iron coating are consistent with the proposed reaction mechanism, in which iron serves as a shuttle catalyst that is oxidized by external oxygen and then reduced by boron.