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

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.