Effect of the addition of β-Si3N4 nuclei on the thermal conductivity of β-Si3N4 ceramics

Various microstructures of β-Si3N4 were fabricated, with or without the addition of β-Si3N4 seed particles to high-purity β-Si3N4 powder, using Yb2O3 and ZrO2 as sintering additives, by gas-pressure sintering at 1950 °C for 16 h. The thermal conductivity of the specimen without seeds was 140 W·(m·K)−1, and the specimen exhibited a bimodal microstructure with abnormally grown grains. The thermal conductivity of the specimen with 24 vol.% seed addition was 143 W·(m·K)−1, and this specimen had the bimodal microstructure with finer grain size than that without the seeded material, but maintained the same amount of large grains (⩾2 μm in diameter) as in the specimen without the seeds. This finding indicates that the thermal conductivity of β-Si3N4 is controlled by the amount of reprecipitated large grains, rather than by the grain size of the β-Si3N4.