Mechanically activated combustion synthesis and shockwave consolidation of magnesium silicide

Abstract

Magnesium silicide (Mg2Si) is a promising intermetallic compound for applications such as light-weight composite materials and thermoelectric energy conversion. It is difficult, however, to synthesize high-quality Mg2Si on a large scale. Self-propagating high-temperature synthesis (SHS) is an attractive pathway, but it is difficult to ignite the low-exothermic Mg/Si mixture and achieve a self-sustained propagation of the combustion wave. In the present paper, mechanical activation was used to facilitate the ignition. Magnesium and silicon powders were mixed and then milled in a planetary ball mill in an argon environment. The mixtures were compacted into pellets and ignited at the top in a reaction chamber filled with argon. Depending on the pellet dimensions and diameter-to-height ratio, two modes of combustion synthesis, viz., thermal explosion and SHS, were observed. In both modes, Mg2Si product was obtained. Thermocouple measurements have revealed that the exothermic reaction stages include two self-heating events separated by a long period of relatively slow interaction. To clarify the reaction mechanisms, differential scanning calorimetry was used, which also revealed two peaks of exothermic reaction in the milled Mg/Si mixture. The first peak is explained by the effect of mechanical activation. Explosive-based shockwave consolidation was used to increase the product density. Thermophysical properties of the obtained material were determined using a laser flash apparatus.