Elevated Temperature Thermal Properties of ZrB₂ with Carbon Additions

The thermal properties of zirconium diboride (ZrB ₂ ) ceramics with carbon additions of up to 3 wt% were characterized up to 2000⁰C. Carbon contents were selected to produce ZrB ₂ that was nominally pure, contained dissolved carbon, or contained carbon inclusions. The microstructure and density changes that resulted from the carbon additions affected the thermal behavior of ZrB ₂ at room and elevated temperatures. Thermal diffusivity at 200⁰C increased from 0.150 cm ² /s for nominally pure ZrB ₂ to 0.175 cm ² /s for ZrB ₂ with 3 wt% carbon. The thermal diffusivity decreased with increasing temperature, reaching a value of 0.143 cm ² /s at 2000⁰C for ZrB ₂ with 3 wt% carbon. In addition, thermal diffusivity changed irreversibly during the first thermal cycle after densification due to changes in the microstructure that started between 1550⁰C and 1650⁰C. Heating resulted in the formation of a new phase, growth of ZrB ₂ grains, changes in the morphology of carbon inclusions, and migration of W impurities from the ZrB ₂ matrix into the new phase. Heat capacity, unlike thermal diffusivity, did not change during thermal cycling. Thermal conductivity, which was calculated from thermal diffusivity, heat capacity, and density, was as high as 64.2 W·(m·K) ^-1 at 2000⁰C for ZrB ₂ with 3 wt% carbon. The phonon contribution to thermal conductivity decreased to nearly zero with the addition of 3 wt% carbon due to the presence of elongated carbon inclusions around ZrB ₂ grains.