Hydriding and dehydriding kinetics of Mg in a Mg/Mg2Cu eutectic alloy: Pressure sweep method

Abstract

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.