The hydriding and dehydriding kinetics of Mg were measured by a method which sweeps through the pressure and reacted-fraction variables. However, a new data-analysis method is employed which permits the derivation of the kinetics at constant pressure, in which form they are much more amenable to interpretation. The eutectic alloy Mg/Mg2Cu was studied at hydrogen pressures such that only the Mg phase was hydrided and dehydrided, so that the Mg2Cu phase played a catalyzer role only. By analysis of the time, pressure, and temperature dependence of the kinetics, it is concluded that the hydriding is rate limited by the diffusion of hydrogen through a growing MgH2 layer, and that dehydriding is rate limited by the diffusion of hydrogen through a growing Mg layer. In addition to the Mg2Cu-catalyzed Mg, a vapor-deposited Mg component developed in the reactor. This complicated the analysis somewhat, but a separation and comparison of the kinetics of both Mg components was achieved. It is concluded that the catalytic role of the Mg2Cu phase is that it provides an external surface that can be reduced of hydriding/dehydriding inhibiting oxides or surface adsorbed gases.
Hydriding and dehydriding kinetics are derived within the framework of the Johnson-Mehl-Avrami equation. Hydriding is considered to be rate limited by hydrogen diffusion through a growing hydride layer, and dehydriding by hydrogen diffusion through a growing metal layer. Incubation time effects due to surface contamination are taken into account by a delayed nucleation function. The dominant composition-dependent terms in the thermodynamics and in the mobility are taken into account to derive normative diffusivity for hydrogen. The temperature dependence of hydriding is defined by an activation energy (QD−ΔH̄H+ΔH12) where QD is the activation energy for diffusion of hydrogen in the hydride, ΔH̄H is the relative partial molar enthalpy of hydrogen in the hydride, and ΔH12 is the enthalpy of the hydrogen-pressure plateau reaction. For dehydriding the activation energy is (QD+ΔH̄H−ΔH12), but now QD and ΔH̄H refer to hydrogen in the metal. The solution thermodynamics contributions to the activation energy are shown to be very important. It is emphasized that in order to determine a meaningful activation energy, it is not the pressure that must be maintained constant but rather a factor T[1−(Pd/P)1/2] in hydriding and a factor [T[1−(P/Pd)1/2] in dehydriding, where Pd is the dissociation pressure of the hydride. Several hydriding studies from the literature are critically disccused in terms of these results.
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