Combustion of Aluminum-Metal Fluoride Reactive Composites in Different Environments

Combustion of two nanocomposite powders comprising equal mass fractions of aluminum and a metal fluoride, BiF₃ or CoF₂, is characterized experimentally. The powders were prepared by arrested reactive milling; earlier work showed that they are readily ignited upon heating and are insensitive to ignition by electrostatic discharge. Here, powder particles were ignited in air by passing through a focused CO₂ laser beam. They were also ignited and burned in oxygen-starved environments produced in the combustion products of an air-acetylene and air-hydrogen flames. Burn times and color temperatures of the particles were measured optically. Combustion products were collected and examined using electron microscopy. Correlations of burn times with particle sizes showed that the composites containing BiF₃ burned faster than those containing CoF₂. In air, the burn rates of both composite powders exceeded that of pure Al; in the oxygen-starved environments the burn rates of the prepared composites were comparable to that of Al. The flame temperatures of the CoF₂-containing composite were limited by the Al boiling point while BiF₃ containing composites burned at higher temperatures. Combustion mechanisms for the prepared materials are discussed qualitatively.