Mixtures of magnetite and zinc powders were milled for up to 540 minutes and the development of the system was followed using X-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. The process takes place in two over lapping steps. During the first hour of milling, a nonmagnetic intermediate mixed oxide phase forms which decomposes into ZnO and Fe upon further milling. The freshly formed iron particles are supersaturated with Zn. Ball milling induces self-sustaining reactions in sufficiently exothermic powder mixtures.
Combustion occurs after some activation time, when the powder reaches a well defined critical state.
Investigating the nature of this state and the processes leading to ignition are used as a vehicle to learn about the mechanism of mechanochemical reactions in general. A possible framework is proposed to describe the process. The effects of a single collision play the central role. The more fundamental atomic scale events and the global kinetics of the milling process are also considered. Experimental investigations of the ignition of combustion are reviewed with emphasis on systematic studies relating the reaction parameters and the ignition time.