Ignition of iron-coated and nickel-coated aluminum particles under normal- and reduced-gravity conditions

In aluminized solid propellants, the use of Al particles coated by certain transition metals may improve engine performance characteristics, owing to decreased agglomeration and lower ignition temperature of the particles. For this application, a detailed knowledge of particle ignition is required. Earlier, we reported the ignition mechanism of single similar to 2.5-mm Ni-coated Al particles heated by a laser in Ar and CO2 atmospheres. In the present work, using the same apparatus, we investigate the ignition mechanism of Fe-coated At particles. The results show that intermetallic reactions contribute to the heating rate upon At melting at 660 degrees C, but the particles ignite at 1350-1500 degrees C under normal-gravity conditions. The ignition mechanism includes the formation of a solid Fe2Al5 phase at the interface between liquid Al and the coating, with subsequent melting of Fe2Al5, formation of a solid FeAl layer, and conversion of the Fe coating to a Fe-Al solid solution. Microgravity (10(-3)-10(-2) g) experiments with Fe-coated and Ni-coated At particles were conducted to reduce convection inside the particles, which may influence phase mixing during ignition. In microgravity, both Ni-coated and Fe-coated At particles ignited at 1250-1400 degrees C. The significantly lower ignition temperature, compared with conventional oxide-coated Al, suggests that Fe-coated and Ni-coated Al particles are promising candidates for propulsion applications.