Figure 15 illustrates the temperature dependence of the heat capacity of zirconium hydride. It is clear that the heat capacity of the metal hydride is higher than that of the pure metal, particularly at higher temperatures. This behavior can be explained by the observation that while lattice vibrations are dominated by the acoustic mode at low temperatures, the contribution ofthe optical mode increases steadily as the temperature increases beyond ambient values.

2.11.3.5 Thermal Conductivity of Metal Hydrides23

Figure 16 shows the thermal conductivity of zirco­nium hydride, which is seen to be almost identical to that of the pure metal, and shows no dependence on temperature.

In order to analyze the thermal conductivity results, we expressed thermal conductivity as the sum of a lattice-vibration contribution (1lat) and an electronic contribution (1el). The electronic

contribution was evaluated from the Wiedemann — Franz relation as follows:

1el = LaT

Here, L is the Lorentz constant and a is electrical conductivity. Figure 17 plots the values of each con­tribution to the thermal conductivity of zirconium hydride. Here, the electronic contribution is greater than that of lattice vibrations, and the former increases with temperature, whereas the latter decreases as the temperature rises.

2.11.3.6 Comparison of Thermal Conductivity of Zirconium Hydride with those of the Hydrides of Titanium and Yttrium

Figure 18 shows the thermal conductivity of tita­nium hydride.24 The thermal conductivity of the hydride is approximately equal to that of the pure metal, but in this case, it increases slightly with hydrogen content. Figure 19 shows the thermal con­ductivity of yttrium hydride.25 In this case, the ther­mal conductivity of the hydride is higher than that of

Figure 18 Temperature dependence of the thermal conductivity of titanium hydride.

Temperature, T (K)

the metal, and it decreases with increasing hydrogen content. Additionally, the hydride’s thermal conduc­tivity decreases with decreasing temperature, whereas that of the metal remains more or less con­stant. Figure 20 plots the values of a lattice-vibration contribution and an electronic contribution to the thermal conductivity of titanium hydride. The results for titanium hydride are not much different from
those for zirconium hydride, as shown in Figure 17. However, as shown in Figure 21, the results for the case of yttrium reveal that both the lattice-vibration and electronic contributions to thermal conductivity are greater for yttrium hydride than for the pure metal. This indicates that the thermal conductivity characteristics of yttrium are different from those of zirconium and titanium.

Temperature, T (K)