Ignition Mechanisms of Reactive Nanocomposite Powders Combining Al, B, and Si as Fuels with Metal Fluorides as Oxidizers

Micron-sized nanocomposite powders combining 50 wt% of Al, B, or Si as fuels with a metal fluoride, specifically BiF₃, CoF₂, or NiF₂, were prepared by arrested reactive milling. The reactions occurring upon heating such composite powders in both inert and oxidizing environments were studied using thermal analysis. Reaction products were collected at different temperatures and examined using electron microscopy and x-ray diffraction. Prepared composites were coated on an electrically heated filament for ignition experiments in air. Fluorination substantially accelerates oxidation for all fuels in the prepared composites heated in an oxidizing environment. Respectively, ignition temperatures are substantially reduced compared to those of fuel powders. For all composites with BiF₃ as oxidizer, ignition is rate-limited by its thermal decomposition. Thermal decomposition of other fluorides rate-limits ignition for boron and silicon, for which the fluorination products are volatile. For Al-based materials, a condensed AlF₃ layer forms prior to ignition, and diffusion through this layer limits the rate of reaction. For boron coated with boric acid, the fluorination is catalyzed by release of water upon heating, occurring at lower temperature than other reactions.