Thermal conductivity improvement in silicon nitride ceramics via grain purification

Abstract β‐silicon nitride (β‐Si 3 N 4 ) ceramics with an additive oxide system of 1 wt% MgO and 3 wt% Re 2 O 3 (Re = Gd, La, Y, and Yb) were fabricated by liquid phase sintering at 1950°C under nitrogen pressure of 748 kPa. Starting α‐Si 3 N 4 powder with 10 vol% rod‐like β‐Si 3 N 4 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‐Si 3 N 4 ‐MgO‐Gd 2 O 3 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‐Si 3 N 4 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‐Si 3 N 4 ‐MgO‐Re 2 O 3 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.