Preparation of beta-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 similar to 4, The
seeds are prepared via heat treatment of a powder mixture that contains
oc-phase-rich Si3N4 and 0.5 wt% Y2O3 at a temperature of 1800 degrees 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.