(458a) Reactive Composites of Boron and Bismuth Fluoride with Varied Fluoride Concentrations

Boron is a thermodynamically attractive fuel that suffers from long ignition delays and burn times. The primary reason for the ignition delay is the nascent oxide/hydroxide layer on the particle that acts as a diffusion barrier at low temperatures. At higher temperatures, the combustion of boron in aerobic environment is kinetically slow. The approach previously found to be effective in addressing these shortcomings was to modify the chemistry of boron oxidation by introducing metal fluorides as oxidizers. Boron-fluoride composites were prepared by arrested reactive milling with 50 wt. % of the fluorides: CoF₂ and BiF_3. The composites exhibited very low ignition temperatures (~ 275 °C at a heating rate of 2500 K/s). Ignition was driven by fluorination reactions involving the boron hydroxide layer. With the active removal of condensed phase oxides, the composite B·BiF₃ was found to burn much faster than pure boron in aerobic conditions while generating favorable gas phase combustion products: BF₃ andBOF

The current research aims to extend the benefits of fluoride oxidizer BiF₃ while lowering its weight fraction in the composite to improve the calorific content for the reactive material combusting in an aerobic environment. The compositions prepared and characterized included boron with 5, 10, 20, 30, and 40 wt. % of BiF₃. The samples with higher weight percentages of BiF_3, 10-40 wt. %_, were prepared by milling_. Chemical deposition was employed to obtain BiF₃ as a surface coating on boron for lower weight percentages of 5 and 10 wt. %. All the prepared samples were found to be reactive when initiated in small quantities by a burning filter paper. A systematic reduction of BiF₃ in the composite resulted in the elevation of ignition temperatures with the 5 wt. % sample having the highest ignition temperature of ~750 °C at a heating rate of 2500 K/s. The ignition and combustion characteristics of these samples in air will be presented. Results of thermo-analytical studies for these materials heated in an oxidizing gas will also be discussed. The plausible reaction pathways in the experimental conditions will be explored.