Hydrogen absorption properties of γ-Mg17Al12 phase

Complete text of publication follows: In the present work, the Mg-Al system was studied by mechanical alloying. The two following points are developed: the homogeneity of the extended range of the γ-phase and its hydrogen absorption properties. For the first point, several MgxAl100?X compounds from X=47.5 to 70 by incremental step of 2.5 were prepared by ball-milling with post-annealing. Different experimental measurements were performed and are in mutual agreement with properties of existing phases. They show that deviation from the ideal composition of Mg17Al12 (X=58.62) leads to a decrease of the homogeneity (morphologic and chemical composition) . Rietveld refinements on XRD patterns show that the single γ-phase range is extended at room temperature and is identified for initial 55≤X≤62.5 atomic % of Mg. In order to investigate the hydrogenation properties, two compounds at the nominal composition X=50 and X=58.6 were prepared by the bulk mechanical alloying technique. The hydrogen absorption are presented at 350 C. Samples X=50 and X=58.6 show a maximum hydrogen capacity of 3.5 and 3.7 wt% respectively. For these two compounds, the pressure-composition isotherm (PCT) curves show two absorption plateaus, with equilibriums around 10 bar which are higher than that of pure MgH2. The shape discontinuity corresponds to the transformation of γ-Mg17Al12 to the intermediate β-Al3Mg2 phase, before the completed decomposition in MgH2 hydride and pure Al-fcc in a second step. After final desorption, the initial compounds are recovered, indicates that the reactions are reversible