Thermal conductivity improvement in silicon nitride ceramics via grain purification.

β‐silicon nitride (β‐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°C under nitrogen pressure of 748 kPa. Starting α‐Si3N4 powder with 10 vol% rod‐like β‐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 β‐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 β‐Si3N4 grains (≥ 2 μm 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 β‐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. [ABSTRACT FROM AUTHOR]