PREPARATION OF SILICON-NITRIDE WHISKERS FROM DIATOMACEOUS-EARTH .1. REACTION CONDITIONS

Whiskers and powder of silicon nitride were prepared by the carbothermal reduction and nitridation of diatomaceous earth in the presence of flowing N-2 and NH3. The optimum temperature for the formation of Si3N4 whiskers was 1350 degrees C and the yield reached almost 20% after 24 h. The alpha-Si3N4 content decreased with increasing nitridation temperature. Yields of the whiskers were dependent on NH3 concentration and the total gas feed rate. The maximum yield of inside whiskers was obtained for a 25 vol% NH3/N-2 mixture, while the maximum quantity of outside whiskers was produced for 75 vol% NH3/N-2. The sum of the yield of the inside and outside whiskers increased with decreasing total gas feed rate. However, no nitridation of SiO2 was observed at a feed gas rate below 0.18 mmol.min(-1). The yield of the inside whiskers increased gradually with increasing reaction time up to 36 h, whereupon a constant value was attained. Although the amount of outside whiskers produced was relatively small, the quantity seemed to increase until 60 h.