In this work, the effects of Y2O3/MgO ratio on the densification behavior, phase transformation, microstructure evolution, mechanical properties, and thermal conductivity of Si3N4 ceramics were investigated. Densified samples with bimodal microstructure could be obtained by adjusting the ratio of Y2O3/MgO. It was found that a low Y2O3/MgO ratio facilitated the densification of Si3N4 ceramics while a high Y2O3/MgO ratio benefited the phase transformation of Si3N4 ceramics. Best mechanical properties (flexural strength of 875 MPa, and fracture toughness of 8.25 MPa·m1/2, respectively) and optimal thermal conductivity of 98.04W/(m·K) were achieved in the sample fabricated with Y2O3/MgO ratio of 3:4 by sintering at 1900°C for 4 h.

Silicon nitride with high thermal conductivity was obtained by two-step sintering method using YbH2-MgO as sintering additive. The effect of YbH2-MgO on shrinkage behavior, phase composition, microstructure, thermal conductivity, and flexural strength was investigated. The nitrogen-riched and oxygen-lacked oxynitride liquid phase was generated owing to the elimination of SiO2 by YbH2. Both the removal of lattice oxygen and the growth of β-Si3N4 were stimulated by the liquid phase. Therefore, compared to Yb2O3-MgO doped sample, silicon nitride with enlarged grains, purified lattice and reduced intergranular phase was obtained. Ultimately, the thermal conductivity increased by 13.7% from 115.32 to 131.15 W• m -1 •K -1 after sintering at 1900 ℃ for 24 h by substituting Yb2O3 with YbH2. Although the replacement of Yb2O3 by YbH2 leads to enhanced flexural strength at low temperature, it tends to reduce the flexural strength at high temperature. The optimal flexural strength of (1008 ±35)MPa was achieved after sintering at 1800 ℃ for 4 h. This variation related mainly to the exaggerated bimodal microstructure. This work signifies that YbH2-MgO is effective for obtaining Si3N4 ceramics with both high flexural strength and high thermal conductivity.