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

Thermal diffusivity measurements for hafnium hydrides (HfHx) with different hydrogen concentrations (α + δ’-HfH0.20, α + δ’-HfH0.92, α + δ’-HfH1.15, δ’-HfH1.48, δ’-HfH1.57, δ-HfH1.70, δ + ɛ-HfH1.76, ɛ-HfH1.88 and ɛ-HfH2.02) 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’-HfH0.20 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 HfH0.92–2.02 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’-HfH1.48 and δ’-HfH1.57 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’-HfH0.20, α + δ’-HfH0.92, δ’-HfH1.48, δ-HfH1.70 and ɛ-HfH2.02 were evaluated by way of calculations from the experimental thermal diffusivity data and reference data for the specific heat and density.