The effects of phase transformation on the heat conduction of hafnium hydrides

Thermal diffusivity measurements for hafnium hydrides (HfH_x) with different hydrogen concentrations (α + δ'-HfH_0.20, α + δ'-HfH_0.92, α + δ'-HfH_1.15, δ'-HfH_1.48, δ'-HfH_1.57, δ-HfH_1.70, δ + ε-HfH_1.76, ε-HfH_1.88 and ε-HfH_2.02) were carried out within the temperature range of 290-570 K by means of a laser-flash method. The temperature dependence of the thermal diffusivity for α + δ'-HfH_0.20 was the result of a mixture of a hexagonal α-phase and a slightly deformed face-centered cubic (fcc) δ'-phase structure that was similar to that observed in α-Hf. The thermal diffusivities for HfH_0.92-2.02 at temperatures below 373 K drastically changed due to the phase transformation, while those at temperatures above 373 K remained almost the same, 0.06-0.08 cm²/s, which is approximately half that of α-Hf. In particular, the thermal diffusivities for δ'-HfH_1.48 and δ'-HfH_1.57 changed between 360 and 390 K with the conversion to an ideal fcc structure in the δ-phase. The temperature dependence of the thermal conductivities for α + δ'-HfH_0.20, α + δ'-HfH_0.92, δ'-HfH_1.48, δ-HfH_1.70 and ε-HfH_2.02 were evaluated by way of calculations from the experimental thermal diffusivity data and reference data for the specific heat and density.