Carbothermal synthesis of silicon nitride

Carbothermal synthesis of silicon nitride Si3N4 followed by decomposition of Si3N4 is a novel approach to production of solar-grade silicon. The aim of the project was to study reduction/nitridation of silica under different conditions and to establish mechanism of silicon nitride formation.Carbothermal reduction of quartz and amorphous silica was investigated in a fixed bed reactor at 1300-1650 °C in nitrogen at 1-11 atm pressure and in hydrogen-nitrogen mixtures at atmospheric pressure. Samples were prepared from silica-graphite mixtures in the form of pellets. Carbon monoxide evolution in the reduction process was monitored using an infrared sensor; oxygen, nitrogen and carbon contents in reduced samples were determined by LECO analyses. Phases formed in the reduction process were analysed by XRD. Silica was reduced to silicon nitride and silicon carbide; their ratio was dependent on reduction time, temperature and nitrogen pressure. Reduction products also included SiO gas which was removed from the pellet with the flowing gas. In the experiments, reduction of silica started below 1300 °C; the reduction rate increased with increasing temperature. Silicon carbide was the major reduction product at the early stage of reduction; the fraction of silicon nitride increased with increasing reaction time. Maximum silicon nitride to carbide ratio (Si3N4/SiC) in the reduction of silica in nitrogen at atmospheric pressure was observed at 1450 °C. Further increase in temperature decreased Si3N4/SiC ratio. Increase in hydrogen content to 10 vol% favoured SiO2 reduction. Further increase in H2 content led to decreased N2 partial pressure, which had a negative effect on the nitridation process.Elevated nitrogen pressure increased silicon nitride yield and stability. When nitrogen pressure was 11 atm, maximum Si3N4/SiC ratio was observed at 1550-1600 °C. Increasing nitrogen pressure increased reduction and nitridation rates and suppressed SiO loss under otherwise the same conditions.Synthesis of silicon nitride proceeded through silicon carbide. In the beginning of the reduction/nitridation process, silica was predominantly reduced to silicon carbide which was converted to silicon nitride. This mechanism of silicon nitride synthesis was supported by kinetic modeling of reduction/nitridation process. Feasibility of production of solar silicon was demonstrated in a study of decomposition of Si3N4 The decomposition rate increased with increased temperature