Thermal conductivity of reaction bonded Si 3N 4

Abstract

To investigate the effect of α- and β-Si3N4 content on the thermal conductivity of silicon nitride ceramics, six samples containing various quantities of α-Si3N4 were prepared by the reaction bonding process and the thermal conductivity of the reaction bonded Si3N4 (RBSN) was measured by the transient hot wire method at various temperatures from room temperature to 1000°C. The RBSN samples containing 1, 25, 33, 45, 55 and 66% α-Si3N4 were produced by nitriding the compacts of pure silicon powder in a nitrogen atmosphere at 1480°C for prescribed periods. Bulk density, true density and total porosity of samples were 2.51±0.02g/cm3, 3.08±0.03g/cm3 and 18.6±1.0%, respectively, regardless of the α-content. No significant change in microstructure of RBSN samples was observed by scanning electron microscopy With an increase in the content of α-Si3N4, the thermal conductivity (λ) of BRSN samples decreased from λ=17.4Wm-1K-1 for the sample with α-Si3N4 of 1% (viz. consisted mainly of β-Si3N4) to λ=9.3Wm-1K-1 for that of 66%. It was considered that the α-Si3N4 solid solutions by which the thermo-elastic wave carrying the heat energy in the non-electro-conductive solid materials such as Si3N4 ceramics is scattered, might be formed with the impurities included in silicon powder as a starting material. The thermal conductivity of all RBSN samples decreased simply with increasing temperature due to the increase of the mutual scattering of the thermo-elastic wave, and the decreasing rate of thermal conductivity for the RBSN with lower α-contents was found to be larger than that for those with higher α-contents.