Thermal Properties of Hf-Doped ZrB₂ Ceramics

The effect of Hf additions on the thermal properties of ZrB ₂ ceramics was studied. Reactive hot pressing of ZrH ₂ , B, and HfB ₂ powders was used to synthesize (Zr _1-x ,Hf _x )B ₂ ceramics with Hf contents ranging from x = 0.0001 (0.01 at.%) to 0.0033 (0.33 at.%). Room-temperature heat capacity values decreased from 495 J·(kg·K) ^-1 for a Hf content of 0.01 at.% to 423 J·(kg·K) ^-1 for a Hf content of 0.28 at.%. Thermal conductivity values decreased from 141 to 100 W·(m·K) ^-1 as Hf content increased from 0.01 to 0.33 at.%. This study revealed, for the first time, that small Hf contents decreased the thermal conductivity of ZrB ₂ ceramics. Furthermore, the results indicated that reported thermal properties of ZrB ₂ ceramics are affected by the presence of impurities and do not represent intrinsic behavior.