Effect of lattice defects on the thermal conductivity of β-Si3N4

Two types of β-Si3N4 were sintered at 1900 °C one for 8 h and the other for 36 h by using Yb2O3 and ZrO2 as sintering additives. The latter specimen was further annealed at 1700 °C for 100 h to promote grain growth. The microstructures of the sintered materials were investigated by SEM, TEM, and EDS. The thermal conductivities of the specimens were 110 and 150 Wm−1K−1, respectively. The sintered material which possessed 110 Wm−1K−1 had numerous small precipitates that consisted of Yb, O and N elements and internal dislocations in the β-Si3N4 grains. In the sintered material with 150 Wm−1K−1 neither precipitates nor dislocations were observed in the grains. The microscopic evidence indicates that the improvement in the thermal conductivity of the β-Si3N4 was attributable to the reduction of internal defects of the β-Si3N4 grains with sintering and annealing time as the grains grew.