Combustion Synthesis of Nanocrystalline Silicon from Silica and Magnesium Silicide

Nanostructured silicon is a promising material for various emerging applications, and there is an urgent need to develop effective and inexpensive methods for its large-scale production. Silicon can be obtained by magnesiothermic reduction of silica (SiO2) via self-propagating high-temperature synthesis (SHS), which consumes low energy and uses simple, scalable equipment. However, the use of magnesium (Mg) as the reduction agent leads to excessively high temperatures and undesirable sintering of the products. In the present work, magnesium silicide (Mg2Si) was proposed to reduce silica in the SHS of silicon. To ensure a self-sustained propagation of the combustion wave, Mg2Si/SiO2 mixtures were mechanically activated in a shaker ball mill. It was determined that milling for several minutes effectively improved the reactivity of Mg2Si/SiO2 mixtures through the formation of composites, consisted of submicron and micron-sized particles. The use of Mg2Si instead of Mg decreased the temperature during SHS and, after leaching, produced micron-scale agglomerates of Si nanoparticles, in contrast with sponge-like Si particles obtained in the case of using Mg. The addition of 15-25 wt% sodium chloride further decreased the combustion temperature, leading to the formation of nanocrystalline silicon powder with crystallite size of about 30 nm.