(616c) Combustion of Fuel-Rich Boron – Metal Fluoride Composites

The prospective use of boron as a fuel is often hampered by its extended ignition delays and long particle burn times. This research explores ignition and combustion of boron combined with a condensed, fluorine containing oxidizer and exposed to an oxygenated environment. With both fluorine and oxygen available for boron combustion, it is expected that gaseous boron fluorides and oxyfluorides will be the preferred combustion products. Unlike liquid B₂O₃, such products will not cover boron surface delaying its heterogeneous combustion; they are also expected to benefit such applications as solid propellants, where gas-phase combustion products lead to an increased impulse. Composite boron-metal fluoride powders were prepared using arrested reactive milling. The starting material was 95% pure boron powder by SB Boron. In addition to as-received boron powder, a powder subjected to preliminary washing by acetonitrile and toluene, aimed to remove the natural hydrated oxide layer, was used. Both cobalt (II) fluoride and bismuth (III) fluoride served as oxidizers. Prepared composites included 50 wt. % of metal fluoride. These materials are fuel rich with the equivalence ratios of 24.6 for B·BiF₃ and 13.4 for B·CoF₂. When coated on an electrically heated filament, the prepared materials using as received boron as a starting material ignite at very low temperatures, ca. 300 °C. The ignition temperatures are about 100 °C higher for the composites prepared using washed boron. It is hypothesized that when present, the nascent hydrated boron oxide layer interacts with the metal fluoride oxidizers at low temperatures triggering ignition and yielding gaseous boron-oxy fluorides as products. Combustion of the prepared composites was studied using a custom-built burner operated with air-hydrogen and air-acetylene flames. The particles were injected into the flame axially. The optical emission pulses produced by burning particles were recorded using filtered photomultiplier tubes and an optical spectrometer. In separate experiments, the composite particles were ignited by passing through a focused CO₂ laser beam and burned in air. Similarly, optical emission pulses generated by the burning particles were recorded and analyzed. Burn times as a function of the particle size were obtained for different oxidizing environments correlating the measured statistical distributions of the emission pulse durations with the particle size distribution for the composite powders. The results for all the combustion experiments shall be presented and discussed for the prepared boron-metal fluoride composites.