Thermal Properties of HIP Sintered Silicon Nitride

Abstract

Thermal diffusivity and conductivity as well as specific heat were studied for capsule-HIP sintered Si3N4. Yttria and alumina additives with a total amount of 6mol% were used. For a given additive percentage, specimens of higher β phase fraction had higher thermal diffusivity and conductivity, as expected However, samples with larger amounts of Y2O3 additive had higher thermal diffusivity and conductivity in spite of a lower β phase fraction than those with more Al2O3 additive. The highest thermal conductivity was 70W/(m·K) with 6mol% Y2O3 additive alone. The reason is that aluminium and oxygen ions are soluble in β-Si3N4 particles, in the case for Al2O3 addition.Oxygen, presented as an impurity, also had a profound effect on the thermal diffusivity and conductivity. Higher oxygen contents reduced the thermal diffusivity and conductivity. This may be due to the amount of grain boundary glassy phase.The specific heat of sintered Si3N4 was 0.67±0.02kJ/(K·g). The crystalline structure (α or β-Si3N4), additive composition, and raw material particle size did not show any effect on the specific heat.