Preparation of β-Silicon Nitride Seeds for Self-Reinforced Silicon Nitride Ceramics

This paper describes a method for the preparation of silicon nitride (Si₃N₄) seeds that have an average aspect ratio of ～4. The seeds are prepared via heat treatment of a powder mixture that contains alpha-phase-rich Si₃N₄ and 0.5 wt% Y₂O₃ at a temperature of 1800°C and a nitrogen pressure of 35 kPa. A Y-Si-O-N liquid forms during heat treatment; this liquid acts as a flux for seed precipitation. During cooling, the Y-Si-O-N liquid transforms to a thin intergranular grain-boundary phase and causes strong agglomeration of the seeds. The seeds can be isolated by dissolving the grain-boundary phase in hot phosphoric acid, followed by an ultrasonic treatment (for 30 min). The method can be used to produce large quantities of seeds.