Thermal conductivity of hot-pressed ZrB2-ZrC ceramics

Abstract

The thermal conductivity of ZrB2 with 0–10 vol.% ZrC was studied from 25 to 2000 °C. The ZrB2-ZrC ceramics were prepared by attrition milling the starting powders and adding 0.5 wt.% C as a sintering aid. The ceramics were hot-pressed to full density at 1900 °C for 30 min in a flowing Ar atmosphere. The ceramics contained graphitic carbon as a secondary phase, with ZrB2 and ZrC grain sizes of approx. 3.5 and 1.5 μm, respectively. At room temperature, thermal conductivity was 75 W/m·K for monolithic ZrB2, increasing to 79 W/m·K for 2.5 vol.% ZrC, and decreasing to 72 W/m·K for 10 vol.% ZrC. At 2000 °C, thermal conductivity was 72 W/m·K for ZrB2, decreasing to 69 W/m·K for 10 vol.% ZrC. Electrical resistivity behavior was similar to thermal conductivity, decreasing with the addition of ZrC up to 2.5 vol.% (9.9 µΩ·cm at 25 °C), then increasing with further ZrC additions. The increase in thermal conductivity with ZrC addition up to 2.5 vol.% was the result of preferential segregation of W from the ZrB2 phase to the ZrC phase. The reduction in thermal conductivity with greater ZrC additions was due to the lower thermal conductivity of ZrC.