Experimental evidence for the anisotropic Ostwald ripening of β-silicon nitride

A powder mixture of alpha-Si3N4, Y2O3, and SiO2 was heat-treated in a loose powder state in the temperature range of 1750 degrees-1900 degrees C for 2 h; then, the mixture was acid-rinsed to remove the glassy phase. The widths and lengths of the resulting beta-Si3N4 crystals were analyzed quantitatively. The width-aspect-ratio distribution of the beta-Si3N4 crystals initially showed a strong negative correlation, and then the aspect ratio of crystals with small widths quickly decreased. After a stage in which aspect ratio was almost constant, regardless of the width, the width-aspect-ratio distribution evolved to show a positive correlation in the final stage. This pattern of morphology evolution of the beta-Si3N4 crystals was in good agreement with that predicted by the anisotropic Ostwald ripening model.