Controlled preparation of silicon and magnesium silicide on silica glass substrate through magnesiothermic reduction

Abstract

Magnesiothermic reduction has received attention as a method to produce silicon retaining the morphology of the silicon dioxides that serve as the starting materials of the reduction. We performed the reduction of silica glass substrates with magnesium deposited on them under various conditions, including changing the reaction temperature from 500 to 700°C, changing the reaction time from 15 to 420 minutes, and changing the thickness of the deposited magnesium film from 6.6 to 12.1 μm. Crystalline products and their amounts were investigated using X‐ray diffraction. In these reactions, the silica glass substrates were reduced to silicon and/or magnesium silicide depending on the conditions. Under the conditions where the reaction temperature is higher, the reaction time is longer, and the thickness of the magnesium is thinner, silicon was generated more predominantly than magnesium silicide, and vice versa. Under the intermediate conditions, both crystalline silicon and magnesium silicide were generated. Furthermore, it was revealed that the magnesium silicide was rapidly generated within 15 minutes at temperatures higher than 600°C. Then, as the reaction continued, the amount of magnesium silicide gradually decreased, and that of silicon increased. From these results, a two‐step reaction model was proposed: At first, silicon dioxide is reduced to magnesium silicide with the complete consumption of the deposited magnesium; and subsequently, the magnesium silicide reacts with unreacted silicon dioxide and silicon in the intermediate oxidation states to produce silicon. Based on this model, silicon and magnesium silicide were selectively prepared under well‐controlled conditions.

Porous silicon prepared from monolithic porous silica glass using a two‐step magnesiothermic reduction

“…45,46 We have investigated the reaction process of the magnesiothermic reduction, in which a silica glass substrate is reduced by a magnesium film that is deposited on the substrate surface. 47,48 The reduction was performed at various conditions by varying the reaction temperature, duration, and thickness of the deposited magnesium film. The relationship between the reaction conditions and the reaction products was closely examined.…”

“…From these experiments, we concluded that the complete reduction of silicon dioxide to magnesium silicide shown in Equation (3) quickly proceeds in the magnesiothermic reduction at the surface of the silica glass substrate. Thus, we hypothesized that the silicon is generated from the reaction between the magnesium silicide produced in the reduction reaction and the unreacted silica glass, as shown in Equation (4), 48 Magnesium silicide is not stable when in contact with silicon dioxide. Since the reaction in Equation (4) has a negative Gibbs free energy change, ΔG 4 , −195 kJ/mol at 600°C, the reaction proceeds spontaneously.…”

“…We successfully prepared silicon from a silica glass substrate through this two-step reaction. 48 In the present work, the two-step magnesiothermic reduction was applied to monolithic porous silica glass samples that were prepared from a borosilicate glass through the heat treatment for phase separation and acid leaching of the borate-rich phase. The mixture was melted in a platinum crucible at 1400°C for 1 hour, and was then poured into carbon molds for vitrification.…”

“…From these experiments, we concluded that the complete reduction of silicon dioxide to magnesium silicide shown in Equation () quickly proceeds in the magnesiothermic reduction at the surface of the silica glass substrate. Thus, we hypothesized that the silicon is generated from the reaction between the magnesium silicide produced in the reduction reaction and the unreacted silica glass, as shown in Equation (), 48 …”

“…In the subsequent second step, the produced magnesium silicide reacts with the unreacted silicon dioxide to produce silicon. We successfully prepared silicon from a silica glass substrate through this two‐step reaction 48 …”