The Effect of Embedding Conditions on the Thermal Conductivity of .BETA.-Si3N4

Abstract

Dense β-Si3N4 ceramics were prepared from the α-Si3N4 raw powder by sintering at 1900°C for 12 h at a nitrogen pressure of 1 MPa, using a mixture of Y2O3 and MgSiN2 as the sintering additives. The effect of embedding conditions on the thermal conductivity of β-Si3N4 ceramics was investigated by changing the number of samples and the embedded fraction in BN packing powder with BN crucible. It was found that the embedment had no effect on the densification, but had an effect on the weight loss, crystalline secondary phase and microstructure. The thermal conductivity was found to increase linearly with the weight loss, but independent of the grain size. The complete embedment and the increased number of sintered samples tended to suppress the weight loss and thus lowered the thermal conductivity. The optimum condition led to a significant improvement in the thermal conductivity from 96 to 117 Wm-1 K-1. It is proposed that the enhanced thermal conductivity is attributed to the removal of lattice oxygen by the reaction Si3N4(s)+3 SiO2(l)⇔6 SiO(g)+2N2(g). In addition, the decreased amount of secondary phases also contributes to the thermal conductivity. This work suggests that the increased weight loss may also be an important strategy for enhancing the thermal conductivity of β-Si3N4 ceramics by controlling the sintering atmosphere.