Bismuth fluoride-coated boron powders as enhanced fuels

By double displacement reaction in aqueous media, bismuth fluoride, BiF₃, was coated onto micron-sized boron particles to target two compositions with 5 and 10 wt.% of BiF₃, referred to as 95B·5BiF₃ and 90B·10BiF₃, respectively. The actual mass fractions obtained were estimated to be 3 and 7 wt.%, respectively. The deposited BiF₃ particles had narrow size distributions centered around 60 nm. For each of the two prepared powders, the coated boron particles of different sizes contained different mass fractions of BiF₃, which scaled with the particle's specific surface area. Both coated samples exhibited a low-temperature oxidation starting at 450 °C, similar to B·BiF₃ composite powders prepared by arrested reactive milling. The low-temperature oxidation was sufficiently rapid to sustain ignition of 90B·10BiF₃ powder placed on an electrically heated wire. Single particle combustion experiments in air showed a detectable increase in the burn rate for 90B·10BiF₃ powder compared to the starting boron; however, no similar increase was noted for 95B·5BiF₃ sample. In constant volume explosion tests in air, combustion of 90B·10BiF₃ powder generated significantly greater peak pressures and rates of pressure rise than observed for both commercial boron and fine aluminum powders. The improvements were less pronounced, but clearly detectable for 95B·5BiF₃ sample.