Synthesis of Silicon Nitride
Xiaohan Wan, Guangqing Zhang (University of Wollongong) and Oleg Ostrovski (UNSW Australia)
Silicon nitride is an advanced ceramic material; it has excellent high temperature properties: high strength and hardness, resistance to creep, oxidation, and thermal shock. This project studied synthesis of silicon nitride by carbothermal reduction/nitridation. Samples were prepared from silica-graphite mixtures in the form of pellets. Carbothermal reduction of 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.
The extent of reduction (xSiO2) of silica was defined as the fraction of oxygen removed from SiO2 by the reduction. The extent of nitridation, defined as Si3N4 yield (ySi3N4), was calculated as the fraction of Si in the form of Si3N4. Similarly, the extent of carburisation, defined as SiC yield (ySiC), was calculated as the fraction of Si in the form of SiC. Phase development in the process of carbothermal synthesis of silicon nitride is shown in Figure 1.
Figure 1: Si distribution between SiO2, Si3N4, and SiC in fumed silica reduced in the 10 vol% H2-90 vol% N2 gas mixture at 1450°C. Gas flow rate was 1 L/min; C/SiO2 molar ratio was 4.5.
Production of Si3N4 by carbothermal reduction/nitridation has a relatively narrow temperature interval: at high temperatures, SiC has higher thermodynamic stability than Si3N4; at low temperatures, reaction is too slow to be feasible. The highest yield of silicon nitride in reduction in nitrogen at 1 atm was obtained at 1450oC. Increase in nitrogen pressure can increase silicon nitride stability at high temperatures with a positive effect on its synthesis. Table 1 shows the elemental composition, weight loss, extent of reduction and yield of silicon nitride and silicon carbide in reduction of silica in nitrogen at 11 atm pressure. The reduction rate of SiO2 increased with increasing temperature; the highest silicon nitride yield was obtained at 1600oC.
Table 1: Carbothermal synthesis of silicon nitride in nitrogen under pressure 11 atm
It was also established that hydrogen addition to nitrogen promotes the silica conversion rate; H2 can be directly involved into the reduction or/and by forming CH4 by reacting with C. The maximum Si3N4 to SiC ratio was obtained with addition of 10 vol% H2 at 1450°C. Higher H2 addition led to lower N2 partial pressure with a negative effect on Si3N4 formation.
Silicon nitride can be decomposed to silicon and nitrogen in an inert gas atmosphere and/or under reduced pressure at temperatures above 1600oC. Synthesis of silicon nitride and its decomposition can be a promising technology for production of solar grade silicon.