Thermal conductivity improvement in silicon nitride ceramics via grain purification

beta-silicon nitride (beta-Si3N4) ceramics with an additive oxide system of 1 wt% MgO and 3 wt% Re2O3 (Re = Gd, La, Y, and Yb) were fabricated by liquid phase sintering at 1950 degrees C under nitrogen pressure of 748 kPa. Starting alpha-Si3N4 powder with 10 vol% rod-like beta-Si3N4 seed crystallites and an extended sintering time from 5 to 40 h resulted in the formation of bimodal microstructure composed of fine matrix grains and large grains. The 40 h-sintered specimens of pseudo ternary beta-Si3N4-MgO-Gd2O3 system exhibited enhanced thermal conductivity of 127.2 +/- 2.5 W m-1 K-1 associated with a degradation of the fracture strength from 1008.0 +/- 38.0 to 491.7 +/- 32.0 MPa, which was due to the formation of coarse-grained aggregates that acted as both fracture origin as well as a thermal conductive pathway. The theoretical thermal conductivity was predicted for the sintered specimens by using equations based on a mean-field micromechanics model to estimate the effective thermal conductivity of each component of binary composites. The calculation results suggested that the thermal conductivity of the large beta-Si3N4 grains (>= 2 mu m in diameter) was relatively high and estimated to be in the range of 175 to 191 Wm-1 K-1. The improved thermal conductivity of the 40 h-sintered specimens was further discussed for the series of beta-Si3N4-MgO-Re2O3 systems based on the nanostructure characterization results obtained by the high-resolution transmission electron microscopy and scanning transmission electron microscopy-energy dispersive X-ray spectrometry analyses. The thermal conductivity of beta-silicon nitride (beta-Si3N4)-1 wt%MgO-3 wt%Re2O3 (Re = Ga, La, Y, and Yb) ceramics greater than 110 Wm-1 K-1 achieved by the 1950 degrees C-sintering for up to 40 h are attributed to that the positive effect on the thermal conductivity by the formation of high purity large beta-Si3N4 grains is sufficiently high to exceed the negative effect caused by the formation of lattice defects, intragranular glassy nanoprecipitates.image