Thermal Conductivity of Vapor‐Liquid‐Solid and Vapor‐Solid Silicon Carbide Whisker‐Reinforced Lithium Aluminosilicate Glass‐Ceramic Composites

The thermal conductivities of two lithium aluminosilicate glass‐ceramic matrix composites reinforced with 30 vol% of either SiC VS (rice hull) whiskers or SiC VLS (vapor‐liquid‐solid) whiskers were determined from room temperature to 500°C. Because of the preferred alignment of the whiskers, the thermal conductivity values normal to the hot‐pressing direction were found to be significantly higher than those in the parallel direction. The composites with the VLS whiskers exhibited higher thermal conductivity values than those with the VS whiskers. An analysis of the room‐temperature data showed that the thermal conductivity values parallel to the hot‐pressing direction were higher than those predicted from theory, even for whiskers with infinite thermal conductivity and perfect interfacial thermal contact. This effect was attributed to a significant contribution of percolation to the total heat flow as a result of direct whisker‐to‐whisker contact. For both types of whiskers, the interfacial thermal conductance and thermal conductivity values (at ∼6.5 × 10 5 W/(m 2 ‐K) and 200 W/(m·K), respectively) inferred from the composite thermal conductivity values perpendicular to the hot‐pressing direction were essentially the same. It was concluded that the order of magnitude difference in thickness for the two whisker types was primarily responsible for the differences in thermal conductivity measured for these two composites.