Feasibility study of the direct mechano-chemical synthesis of nanostructured magnesium tetrahydroaluminate (alanate) [Mg(AlH4)2] complex hydride

The present work reports a feasibility study of the direct mechano-chemical synthesis by controlled reactive mechanical alloying (CRMA) in a magneto-ball mill of the nanostructured magnesium tetrahydroaluminate (magnesium alanate) Mg(AlH4)(2) complex hydride. Three stoichiometric Mg-2Al mixtures, (a) elemental Mg and Al powders, (b) elemental Al powder and commercial AZ91 alloy (Mg-Al-Zn alloy) and (c) powder of as-cast Mg-2A] alloy, have been used. No successful synthesis of Mg(AlH4)2 has been achieved. The only nanocrystalline hydride formed up to 270 h of CRMA is beta-MgH2, and it does not react with Al and H-2 to form Mg(AlH4)(2). It has been found that there is strong competition between formation of Al(Mg) solid solution and the beta-MgH2 hydride occurring to a various extent up to similar to 10 h of CRMA in all three Mg-2Al mixtures. It is hypothesized that the presence of A](Mg) solid solution inhibits the reaction of beta-MgH2, Al and H-2 to form Mg(AlH4)(2). Furthermore, despite the fact that after prolonged milling the AI(Mg) solution eventually decomposes into secondary Al(s) (derived from solid solution), the latter retains its physico-chemical characteristics of the former solid solution which still inhibits the reaction to form Mg(AlH4)(2). Experimental evidence from DSC measurements shows increasing ranges of the melting enthalpy with increasing amounts of AI(Mg) solid solution and consequently the secondary Al(s) for all the three Mg-2Al mixtures. This strongly supports the hypothesis about the different nature of Al(Mg) and the secondary Al(s) as compared to the primary elemental Al powder.