Self-sustaining reactions induced by ball milling

Ball milling induces self-sustaining reactions in many sufficiently exothermic powder mixtures. The process begins with an activation period, during which size reduction, mixing, and defect formation take place. The MSR (mechanically induced self-propagating reaction) is ignited when the powder reaches a well defined critical state. Once started, the reaction propagates through the powder charge as a combustion process. In this paper, the current knowledge on MSR is reviewed from both experimental and theoretical points of view. Experimental results on a broad variety of systems are examined and compared. The variation of the ignition time with composition and milling conditions is investigated. Some unusual phenomena, such as the mutual suppression of combustion in mixed metal-chalcogen systems, are discussed. The mechanism of MSRs is extremely complex, with important processes on several length and time scales. The key objective is to understand ignition and the changes during the activation process that lead to ignition. Combining models with systematic empirical studies appears to be the most realistic approach to a detailed understanding of MSR processes.