Thermophysical properties and expansion anisotropy of sintered hafnium hydride compacts

• Thermophysical properties of sintered e-phase HfH 1.86 and δ phase HfH 1.6 are investigated. • Thermal diffusivity values are lower than directly hydrided materials due to sintering-induced porosity. • ε-HfH 2 specific heat exceeds literature HfH 1.83 . which was attributed to its higher H content. • ε-HfH 2 thermal expansion anisotropic, with a-parameter contracting and c-parameter expanding with temperature. • Such anisotropy may induce microstresses at high-angle grain boundaries during thermal cycling. Thermophysical properties are reported on ε-HfH 2 samples fabricated by powder metallurgy. Samples were heat treated in the range 300–550 °C to transform them from ε-HfH 2 to δ-HfH 1.6-x , allowing comparison of the properties of both phases. Higher molar heat capacity was found in stoichiometric ε-HfH 2 compared to literature data on sub-stoichiometric ε-HfH 1.83 . 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 cm 2 s −1 respectively. The 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 data in the literature. The thermal expansion was highly anisotropic, with a negative thermal expansion parallel to the a-axis (R a = −8.7). Such extreme anisotropy has implications in controlling the microstructure for thermal damage tolerance.