Self-sustaining reactions as a tool to study mechanochemical activation.

Ever since Faraday reported on the reduction of AgCl with Zn and Sn the "dry way", the study of Mechanically induced Self-propagating Reactions (MSRs) has been an important area within mechanochemistry. An interesting phenomenon is the mutual suppression of ignition in some mixed metal-chalcogen systems, such as in (1 - x)(Sn + Se) + x(Zn + Se) powders. Here both the Sn + Se and Zn + Se mixtures react in a self-sustaining manner after some activation time, but when they are combined, the reaction is gradual in the middle of the concentration scale. Mechanically induced metal-chalcogen combination reactions were studied by Chakurov et al. in the 1980s, using a low-energy vibratory mill. Similar measurements were carried out in our laboratory using the more energetic SPEX 8000 shaker mill. The results show qualitative similarities, but the details are different. It is suggested that the loss of MSR in mixed systems is the consequence of the very different properties of the binary systems, so that either one of the components (Zn) or a product formed gradually without ignition (e.g. SnSe) can act as an inert component relative to the rest of the system. Several examples are presented and the effect of the milling conditions is discussed. Finding new systems with similar behaviour and detailed studies of the activated state are needed to understand MSR in these systems.