Reaction sequences in the synthesis of silicon nitride from quartz

Carbothermal reduction and nitridation of pure silica have been performed to synthesize pure Si 3N 4 powder. Iron was used to serve as a catalyst. The reduction reaction was studied with respect to different parameters such as temperature, soaking time and gas flow rate, etc. Almost pure Si 3N 4 powder with predominating β-phase could be synthesised at 1540°C. Below this temperature SiC and Si 2N 2O are the associated phases. SiC is unstable to Si 3N 4 in nitrogen atmosphere and disappears at 1540°C or under a longer holding at 1440°C. There is a critical flow rate of the nitrogen gas of 1·32 × 10 4 litre m −2 h −1, above which the yield on nitridation almost flattens. It has been suggested through thermodynamic arguments that the Si 3N 4 formation at 1440°C takes place through the intermediate formation of SiC and Si 2N 2O as follows: SiO 2(s ) + 2SiC(s) + 2N 2(g) = Si 3N 4(s) + 2CO(g) SiO 2(s) + Si 2N 2O(s) + 3C(s) + N 2(g) = Si 3N 4(s) + 3CO(g) At 1540°C the following reaction may predominate: 3SiO 2(s) + 6C(s) + 2N 2(g) = Si 3N 4(s) + 6CO(g)