Microstructure and thermal conductivity of gas-pressure-sintered Si3N4 ceramic: the effects of Y2O3 additive content

Si3N4 ceramics were sintered at 1900 °C under a nitrogen pressure of 1 MPa using Y2O3-MgO additives. The effects of Y2O3 content (0.5-4 mol%) on microstructure and thermal conductivity were systematically investigated. The increasing Y2O3 content led to increases in amount and viscosity of liquid phase during sintering, which induced a “bimodal to normal” transition in distribution of grain size, decreased Si3N4/Si3N4 contiguity and enhanced devitrification degree of intergranular phase in sintered bulks. Moreover, the decreasing Y2O3 content was found to improve the elimination efficiency of SiO2 impurity during sintering, resulting in lower lattice oxygen content in densified specimens. The microstructure had a strong effect on thermal conductivity. The samples sintered for 3 h gained an increase of thermal conductivity from 65 to 73 W·m-1 K-1 with increasing Y2O3 content, while the samples sintered for 12 h obtained a substantial increase of thermal conductivity from 87 to 132 W·m-1 K-1 with decreasing Y2O3 content.