Hydrogen desorption behavior from magnesium hydrides synthesized by reactive mechanical alloying

The beta- and gamma -phases of MgH2 were synthesized by reactive mechanical alloying (RMA) at room temperature under hydrogen atmosphere. The structural and desorption properties of the products obtained were examined by X-my diffraction (XRD). differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and scanning electron microscopy (SEM). Reactive mechanical alloying of Mg leads to the formation of about 50 wt.% of MgH2 (beta- and gamma -phases) after 50 h of milling. Different thermal behaviors are observed depending on the milling time. Hydrogen thermal desorption shows a sharp endothermic peak for MgH2, milled up to 30 h associated with beta -MgH2. For longer milling times, two endothermic peaks (or a double peak) are observed and are associated with hydrogen desorption from the gamma -MgH2-beta -MgH2 mixture and the beta -MgH2 phase. The presence of gamma -MgH2 destabilizes the beta -MgH2 phase, reducing its desorption temperature. The results of this study are consistent with hydrogen desorption from gamma -MgH2 prior to the gamma --> beta -MgH2 transformation. (C) 2001 Elsevier Science BN, All rights reserved.