Gradual and Self-Sustaining Processes in the Sn-Zn-Se System

Ball milling induces self-sustaining reaction in binary Sn-Se and Zn-Se powder mixtures. But if such mixtures are blended, the ignition time increases at either end of the concentration scale and the suppression of ignition can take place in an intermediate concentration interval. This phenomenon was investigated in (1-x)(Sn+Se)+x(Zn+Se) and (1-x)(Sn+2Se)+x(Zn+Se) mixtures, by measuring the ignition time as a function of both composition and milling conditions and investigating activated and reacted mixtures using X-ray diffraction and scanning electron microscopy. At the Sn-rich compositions of the first system, ignition happened as soon as the mill was started, in spite of the rather low adiabatic temperature of the reaction. Simultaneous local melting of Sn and Se is suggested as a possible explanation for immediate ignition. It can also explain the asymmetry of the properties of the binary reactions, namely that Sn+Se is less exothermic but easy to ignite, while Zn-Se is more exothermic but difficult to ignite. Similar asymmetry is considered as the reason for the increase of the ignition time and the loss of ignition in other mixed metal-chalcogen systems.