Effect of Vapor—Liquid—Solid and Vapor—solid Silicon Carbide Whiskers on the Effective Thermal Diffusivity/Conductivity of Silicon Nitride Matrix Composites

Abstract

A study was conducted of the relative effect of vapor—liquid—solid (VLS) and vapor—solid (VS) SiC whiskers on the effective thermal diffusivity and conductivity of pressed‐densified silicon nitride. It was found that VLS whiskers cause an increase in the thermal diffusivity/conductivity, whereas the opposite effect was found for the VS‐SiC whiskers. Comparison with composite theory suggests that the VS‐SiC whiskers have a thermal conductivity as low as 25 to 30 W/(m·K). In contrast the VLS‐SiC whiskers appear to have a value for the thermal conductivity of at least about 100 W/(m·K) to as high as 250 W/(m·K). These large differences in thermal conductivity for these two types of SiC whiskers are attributed to the much larger density of structural defects in the VS‐SiC whiskers, which act as phonon scatterers, thereby lowering the thermal conductivity.