Effect of Gd3Si2C2 content and sintering time on the microstructure and thermal conductivity of Si3N4 ceramics

Si 3 N 4 ceramics were sintered at 1850 °C and 2 MPa in a nitrogen atmosphere, using Gd 3 Si 2 C 2 -MgO as a sintering aid. The impact of Gd 3 Si 2 C 2 content and sintering dwell time at 1850 °C on the relative density, microstructure, and thermal conductivity of the Si 3 N 4 ceramics was investigated. The results indicate that under 4 h of sintering, increasing Gd 3 Si 2 C 2 content significantly enlarges Si 3 N 4 grain size while reducing microstructural uniformity; however, after 8 h, grain size refines and uniformity markedly improves. This phenomenon is mainly attributed to the increase in Gd 3 Si 2 C 2 content, which raises the number of β-Si 3 N 4 nucleation sites, thereby facilitating grain growth during short-term sintering; whereas during long-term sintering, the further enlarged grains generate a spatial hindrance effect, promoting the homogenization of the microstructure. Furthermore, under the same Gd 3 Si 2 C 2 content, extending the sintering time is beneficial to the growth of silicon nitride grains and the reduction of the glass phase, enhancing the continuity of Si 3 N 4 /Si 3 N 4 interfaces and thereby improving the thermal conductivity of Si 3 N 4 ceramics. The maximum thermal conductivity of 107.25 W·m −1 ·K −1 was achieved with a Gd 3 Si 2 C 2 content of 1 mol% and a sintering period of 8 h at 1850 °C.