2Mg–Fe alloys processed by hot-extrusion: Influence of processing temperature and the presence of MgO and MgH 2 on hydrogenation sorption properties

Abstract

2Mg–Fe alloy powder produced by high-energy ball milling was processed by hot extrusion at temperatures of 200 °C and 300 °C to produce bulk samples. The alloys were hydrogenated for 24 h under hydrogen pressures of 24 bar (to produce the Mg 2FeH 6 phase) and 15 bar (to produce a mixture of MgH 2 + Mg 2FeH 6 phases), respectively. After the hydrogenation treatments, the complex hydride Mg 2FeH 6 was identified in both conditions, while the MgH 2 and MgO phases were observed only after extrusion at 200 °C. Desorption temperatures varied with the extrusion conditions; extrusion at 300 °C resulted in a desorption onset temperature about 68 °C lower than that of samples extruded at 200 °C, and about 200 °C lower than that of commercial MgH 2. Extrusion at the lower temperature did not change the number of stored defects (point defects, dislocations, voids, stacking faults, vacancies and others) produced in the milling process and increased the preferential sites for hydride nucleation, increasing the hydrogen storage capacity. The presence of MgO produced the beneficial effect of grain boundary pinning, but delayed the onset temperature of desorption. The combined presence of MgH 2 and Fe after hydrogenation at 15 bar seems to play a catalytic role that considerably hastened the Mg–H reactions and increased the desorption kinetics. However, the desorption kinetics in both conditions was still low.