Thermodynamics of the Si-C-O system for the production of silicon carbide and metallic silicon

Abstract

The predominance diagrams of the Si-C-O system for 1350 to 2200 °C have been constructed thermodynamically by taking into account the presence of liquid silicon monoxide. The high-temperature behavior of various feeds, such as the SiO₂-C and SiO₂-SiC systems, have been calculated as functions of mixing ratio and reaction temperature by using the method of equilibrium mass-balance. For an efficient SiC-making, a charge having a C/Si ratio of 2.91 should be heated between 2035 and 2045 °C, where SiC crystals can be grown mainlyvia gas-phase reactions of SiO(g). Metallic silicon cannot be produced unless a charge of the Si-C-O system is heated above 2037 °C. In Si-making, liquid SiO plays an important role in that the yield can be improved substantially by enhancing the condensation of SiO gas in the upper cooler part of the furnace. This work also lends thermodynamic proof and support to the double-reactor model, in which a greater fixation of ascending SiO gas on carbon as SiC is considered essential for obtaining a higher silicon yield in industrial furnaces. Some fundamental strategies useful to the improvements of commercial Si- and SiC-making operations have been described quantitatively.