REACTION SEQUENCES IN THE SYNTHESIS OF SILICON-NITRIDE FROM QUARTZ

Carbothermal reduction and nitridation of pure silica have been performed to synthesize pure Si3N4 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 Si3N4 powder with predominating beta-phase could be synthesised at 1540-degrees-C. Below this temperature SiC and Si2N2O are the associated phases. SiC is unstable to Si3N4 in nitrogen atmosphere and disappears at 1540-degrees-C or under a longer holding at 1440-degrees-C. There is a critical flow rate of the nitrogen gas of 1.32 x 10(4) litre m-2 h-1, above which the yield on nitridation almost flattens. It has been suggested through thermodynamic arguments that the Si3N4 formation at 1440-degrees-C takes place through the intermediate formation of SiC and Si2N2O as follows: SiO2(s) + 2SiC(s) + 2N2(g) = Si3N4(s) + 2CO(g) SiO2(s) + Si2N2O(s) + 3C(s) + N2(g) = Si3N4(s) + 3CO(g) At 1540-degrees-C the following reaction may predominate: 3SiO2(s) + 6C(s) + 2N2(g) = Si3N4(s) + 6CO(g)