Influence of alumina on the formation process and morphology of silicon nitride whisker

& beta;-Si3N4 whiskers were synthesized by heat treatment of & alpha;-Si3N4 and Y2O3 mixed powder, and different content of Al2O3 powder was introduced to adjust the synthesis condition and L/D. Introducing Al2O3 significantly accelerated the phase transformation from & alpha;-Si3N4 to & beta;-Si3N4. The phase transformation can be completed at 1650 degrees C by introducing only 0.3 wt% Al2O3, and the phase transformation temperature was reduced by about 100 degrees C after 1.8 wt% Al2O3 was introduced. Y2O3 orients & beta;-Si3N4 grain growth, while the role of Al2O3 is to reduce liquid viscosity, promote mass transfer, and accelerate grain growth. Introducing a small amount of Al2O3 and appropriately extending the holding time at a lower treatment temperature has been proven to be an effective method to obtain & beta;-Si3N4 whiskers with higher length and diameter ratio (L/D) under mild conditions. High purity & beta;-Si3N4 whiskers (99 wt%) were proved to be synthesized at low temperature (1650 degrees C) without pickling.