Thermal conductivity improvement in silicon nitride ceramics via grain purification

Abstract

Abstractβ‐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.

SiC skeleton‐reinforced highly oriented graphite with good thermophysical and electromagnetic shielding properties

“…The TC value in the x – y plane first increased from 277 W/m/K for the GF@4SiC composite to 322 W/m/K for the GF@9SiC composite and then decreased to 223 W/m/K for the GF@27SiC composite. Previous studies have shown that the TC of composites is significantly influenced by pores, defects, and interfaces 32–34 . With an increase in the SiC volume fraction, an increase in the number of interfaces leads to a reduction in the TC of the GF@SiC composites.…”

“…and interfaces. [32][33][34] With an increase in the SiC volume fraction, an increase in the number of interfaces leads to a reduction in the TC of the GF@SiC composites. However, our study showed that the density of the GF@SiC composites increased and the porosity decreased as the SiC content increased from 4 to 9 vol.%, resulting in an improvement in the TC of the composites.…”

Impurity Oxygen-Triggered α- → β-Si3N4 Phase Transformation at 1900 °C

“…On the other hand, recently, we proposed a facile method to produce rod-like β-Si 3 N 4 seed crystallites by the oxide additive-free single-step heat treatment at 1900 • C of commercial α-Si 3 N 4 powder [27][28][29]. Subsequently, bi-modal structure controlling was successfully achieved for the liquid phase-sintered β-Si 3 N 4 -MgO (1 wt%)-Re 2 O 3 (3 wt%) (R = Gd, Yb, Y, La) ceramics by the addition of 10 vol% rod-like β-Si 3 N 4 seeds produced at our laboratory, and the resulting ceramics sintered at 1900 • C for 40 h exhibited improved thermal conductivity of higher than 110 Wm −1 K −1 [30]. Moreover, in our previous study [29], the correlation between α-→ β-Si 3 N 4 phase transformation behavior and the changes in the amount of oxygen and carbon impurities, and the α-→ β-Si 3 N 4 phase transformation kinetics at 1900 • C were investigated under a nitrogen pressure of 980 kPa.…”