Control of beta-Si3N4 crystal morphology and its mechanism (Part 1) - Effect of SiO2 and Y2O3 ratio

The effect of Y2O3/SiO2 ratio on grain growth of Si3N4 was systematically i nvestigated using the "seed" fabrication method. The aspect ratio of beta-S i3N4 crystals heat-treated with an additive composition of Y2O3 : SiO2 = 1 : 2 was found to be much smaller than that of Y2O3 : SiO2 = 2 : 1. It was c onfirmed that this was due to the difference in solubility of Si3N4 caused by the change of Y2O3/SiO2 ratio of the liquid phase. The number of a-cryst als originally contained in the raw Si3N4 powder changed very little in the Y2O3 : SiO2 = 1 : 2 composition, while it significantly decreased in the Y 2O3 : SiO2 = 2 : 1 composition, which resulted in a considerable difference in aspect ratio. A mechanism for this enhanced solubility of Si3N4 in the liquid phase with high Y2O3/SiO2 ratio was proposed.