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Effect of Y2O3 additive on phase transition,aspect ratio and microstructure of β-Si3N4 seed prepared by heat treatment under 35KPa and 1atm nitrogen pressure using α-Si3N4 powder as raw materials was studied.Results have shown that Y2O3 additive could completely promote the Si3N4 phase transition at 1750 ℃.With the introduction of 0.5wt%Y2O3 additive,ideal β-Si3N4 seed with average aspect ratio of 4-5 could be obtained in 35KPa.However,the high nitrogen pressure would lead to the increasing aspect ratio and agglomeration of β-Si3N44 seed.