Magnesiothermic Combustion Synthesis of Nanocrystalline Silicon and Oxidation Kinetics of Magnesium Particles

Magnesium (Mg) is a well-known reduction agent in metallurgy (more specifically, in metallothermy). Recently, magnesiothermic reduction of oxides has been used in self-propagating high-temperature synthesis (SHS) of advanced materials such as ultrahigh-temperature ceramics (UHTC). The magnesiothermic SHS has also been used for the conversion of silica (SiO2) to silicon-based materials for thermoelectric applications. However, since oxidation of Mg releases a lot of heat, the direct use of it as the reduction agent leads to excessively high temperatures and undesirable sintering of the products. We have proposed to use magnesium silicide (Mg2Si) instead of Mg in the SHS of nanostructured silicon. In this approach, the products remain the same, magnesium oxide (MgO) and Si, but the process temperature is lower, thus potentially improving properties of the fabricated silicon. Therefore, one objective of the present work was to investigate the SHS of silicon using Mg2Si for the reduction of silica and to examine the microstructure of the obtained silicon. Magnesium is also used in energetic materials. Since its specific energy and energy density are lower than those of aluminum, it is not used so widely in propulsion and explosives. However, in some pyrotechnic applications, Mg is the preferred metal fuel. Recently, it has been proposed to use combustion of Mg powders in power systems for space missions where the use of solar or nuclear energy is impractical. Specifically, Mg powder would be placed in a combustor, and oxygen would infiltrate, thus maintaining, upon ignition, a self-sustained propagation of the combustion wave. For this and other pyrotechnic applications, the knowledge of Mg oxidation is critically essential. However, the oxidation mechanisms of Mg particles are not well understood. Therefore, the second objective of the present work was to clarify the mechanism and kinetic parameters of the oxidation of Mg particles in oxygen. Specifically, we conducted non-isothermal and isothermal thermogravimetric experiments with Mg powders of different sizes and shapes (spherical and flakes) and analyzed the obtained data with model-free and model-based methods