Grain Growth of [beta]-Silicon Nitride in RE(RE=Y, Gd, Nd and La)-Mg-Si-O-N Melts

Isothermal growth of β-Si3N4 crystals dispersed in rare-earth (RE=Y, Gd, Nd and La) oxynitride melts (RE-Mg-Si-O-N) was studied during heat treatment at 1773 K under 0.9 MPa of nitrogen pressure for 64 to 512 min. The microstructure of β-Si3N4 crystals was characterized by electron microscopy. It is found that the rate of α-β transformation increases with decreasing ionic radius of the rare-earth element. Likewise, the radii of rare-earth elements make significant differences in the morphology and mean aspect ratio of β-Si3N4. Especially, the aspect ratio of β-Si3N4 crystals with the additive of La2O3 was found to be much larger than that in oxynitride melts with additives of others.