Thermophysical properties and expansion anisotropy of sintered hafnium hydride compacts

Thermophysical properties are reported on ε-HfH2 samples fabricated by powder metallurgy. Samples were heat treated in the range 300–550 °C to transform them from ε-HfH2 to δ-HfH1.6-x, allowing comparison of the properties of both phases. Higher molar heat capacity was found in stoichiometric ε-HfH2 compared to literature data on sub-stoichiometric ε-HfH1.83. The δ-phase undergoes a vacancy order–disorder transformation at ∼130 °C with a transformation enthalpy of ∼1.4 kJ mol−1. The room-temperature thermal diffusivity of the ε and δ phases were 0.11 and 0.09 cm2 s−1 respectively. These values are lower than those for literature bulk hydride materials, which is accounted for by pore-phonon scattering. Thermal expansion of ε and δ phases was measured by high-temperature X-ray diffraction to be 9.2 and 11 x10-6 K−1, respectively. The data on the ε phase is the first known in the literature. The thermal expansion was highly anisotropic, with a negative thermal expansion parallel to the a-axis (Ra = −8.7). Such extreme anisotropy has implications in controlling the microstructure for thermal damage tolerance.