Isothermal growth of β-Si3N4 crystals dispersed in a series of rare-earth (RE = Y, Gd, Nd and La) oxynitride liquids (RE-Mg-Si-O-N) was studied after heat treatment at 1773K for 1 to 8 h. The microstructural characterization of α- and β-Si3N 4 crystals was quantitatively analyzed by electron microscopy. The number of α-Si3N4 crystals in oxynitride glasses decreased and that of β-Si3N4 crystals gradually increased with heat treatment time. And the mean grain size of β-Si 3N4 crystals also increased. It is determined that the rate of α/β transformation in the oxynitride liquid with Y 2O3 additive can be faster than that in oxynitride liquids with other additives. Likewise, the radii of rare-earth elements make significant differences in the mean grain size and aspect ratio. In all cases mentioned above, a marked change was observed in the order of ionic radius from La to Y. The mean diameter of the needlelike β-Si3N4 crystals heat-treated with La2O3 additive was almost constant in the time range of heat treatment in this study. However, the aspect ratio of β-Si3N4 crystals with La2O 3 additive was found to be much larger than that in oxynitride liquids with other additives, indicating negligible growth of β-prism plane.
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