Fuel-rich aluminum–nickel fluoride reactive composites

Abstract

Composite powders comprising aluminum and nickel fluoride are prepared by arrested reactive milling. All compositions are fuel-rich, with 50–90 wt% aluminum. Adequate mixing between components and enhanced reactivity is achieved for up to 70 wt% Al. Materials were characterized using electron microscopy, thermal analysis, as well as custom ignition and combustion experiments. Ignition temperatures were lower for powders with 50 and 70 wt% of Al compared to pure Al. Correlation of thermal analysis and ignition tests suggests that exothermic fluorination of Al with an activation energy of ca. 80 kJ/mol likely governs ignition at high heating rates. In air, the composites with 50 and 70 wt% of Al burn faster than spherical aluminum with comparable particle sizes. The flame temperature for 50Al·NiF2 composite does not exceed the Al boiling point suggesting a purely heterogeneous reaction. For 70Al·NiF2 composites, higher flame temperatures are measured suggesting the presence of vapor-phase reactions. In constant volume explosion experiments, combustion of 70Al·NiF2 occurred with shortest ignition delays and resulted in the pressures matching or exceeding those of pure Al, while also essentially matching the pressures predicted for this composite by equilibrium calculations. The burn times observed for the composite powders injected into an air–acetylene flame are similar to those of pure aluminum; it is hypothesized that side reactions of NiF2 with CO, CO2 and H2O diminish its effectiveness as an oxidizer for Al in these environments.