Boron-metal fluoride reactive composites: Preparation and reactions leading to their ignition

Composite powders combining boron with BiF₃ and CoF₂ were prepared using mechanical milling. As-received amorphous boron and boron initially washed with acetonitrile to remove the hydrated surface oxide were used as starting materials. In the prepared composite powders, each particle was a dense agglomerate of primary, nano-sized boron particles coated with metal fluorides. Reactions occurring upon heating in both aerobic and anaerobic environments were characterized using thermoanalytical measurements and mass spectroscopy. Powders were ignited on an electrically heated wire. Ignition sensitivity of the powders to electrostatic discharge was found to be low. For composite powders prepared using commercial boron, low-temperature reactions catalyzed by hydrated boron oxide occurred upon heating, leading to formation of gaseous boron fluorides. At high heating rates, such reactions led to ignition at very low temperatures. The exothermic reaction kinetics and ignition temperatures for such powders were not affected by the fluoride oxidizer. For composites using boron washed in acetonitrile, exothermic reactions began at higher temperatures. Respectively, such powders ignited at higher temperatures. For powders using washed boron, both kinetics of the exothermic reactions and ignition temperatures were affected by the type of fluoride: materials using BiF₃ reacted and ignited at temperatures lower than those using CoF₂.