Thermophysical properties and reversible phase transitions in yttrium hydride

The thermophysical properties of yttrium hydrides were investigated as a function of temperature (room temperature [RT] to 700°C) and hydrogen concentration (H/Y ratio ranges from 1.52 to 1.93). The results indicate that at the temperatures below 300°C, the hydrogen content did not have a significant influence on the thermal expansion, while the specific heat capacity, the thermal diffusivity, and the calculated thermal conductivity were slightly higher for the larger H/Y ratio. Between 300°C and 700°C, a reversible second-order endothermic transition in all measured thermal properties was observed. It was also found that the onset temperatures of the observed transition varied, with the composition having inverse dependence on the hydrogen content. The phase transition from (α-Y + δ-YH_x) mixed phase to the δ-YH_x single phase was excluded from the main contributors to the observed transition. An attempt was made to explain the behavior of the thermophysical properties at higher temperatures by considering the order–disorder transition as a result of hydrogen redistribution.