Effect of lattice impurities on the thermal conductivity of beta-Si3N4

Effect of impurities in the crystal lattice and microstructure on the thermal conductivity of sintered Si3N4 was investigated by the use of high-purity beta-Si3N4 powder. The sintered materials were fabricated by gas pressure sintering at 1900 degreesC for 8 and 48 h with addition of 8 wt.% Y2O3 and 1 wt.% HFO2. A chemical analysis was performed on the loose Si3N4 grains taken from sintered materials after the chemical treatment. Aluminum was not removed from Si3N4 grains, which originated from the raw powder of Si3N4. The coarse grains had fewer impurities than the fine grains. Oxygen was the major impurity in the grains, and gradually decreased during grain growth. The thermal conductivity increased from 88 Wm(-1) K-1 (8 h) to 120 Wm(-1) K-1 (48 h) as the impurities in the crystal lattice decreased. Purification by grain growth thus improved the thermal conductivity, but changing grain boundary phases might also influence the thermal conductivity. (C) 2002 Elsevier Science Ltd. All rights reserved.