Hydrogen absorption and kinetic studies in Zr<inf>0.2</inf>Ho<inf>0.8</inf>Fe<inf>2</inf>

The hydrogen absorption and the kinetics of hydrogen absorption are studied on the C15 type Laves phase pseudo-binary alloy Zr0.2Ho0.8Fe2 in the temperature and pressure ranges RT ≤ T ≤ 150 °C (RT = room temperature) and 0 ≤ P ≤ 70 bar respectively. The maximum hydrogen concentration of 8.1 hydrogen atoms per formula unit is attained at the equilibrium pressure of 67 bar at RT, which is the highest value observed so far in Zr-based C15 Laves phase alloys. From the dependence of thermodynamics of dissolved hydrogen on the hydrogen concentration, the two-phase region in the isotherm corresponding to the equilibrium pressure of about 67 bar at RT is identified as the γ-phase region. This is further confirmed through the X-ray powder diffractogram of the hydrogenated samples on the two-phase regions and from the kinetics of hydrogen absorption. The relative partial molar enthalpy (ΔH̄H) and entropy (AS̄H) of hydrogen at infinite dilution are found to be in the ranges -(3-13) kJ (mol H)-1 and -(14-33) J K-1 (mol H)-1 respectively. The unit cell volume expansion (ΔV/V) upto 26% is found at the maximum hydrogen concentration at RT without change in its cubic Laves phase structure. The average activation energies (Ea) in the (α + β) and β phases are found to be 24 kJ mol-1 and 2 kJ mol-1 respectively.