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

Abstract

The present work reports a feasibility study of the direct mechano-chemical synthesisby controlled reactive mechanical alloying (CRMA) in a magneto-ball mill ofthe nanostructured magnesium tetrahydroaluminate (magnesium alanate)Mg(AlH4)2 complex hydride. Three stoichiometric Mg–2Al mixtures, (a) elemental Mg and Alpowders, (b) elemental Al powder and commercial AZ91 alloy (Mg–Al–Zn alloy) and(c) powder of as-cast Mg–2Al alloy, have been used. No successful synthesis ofMg(AlH4)2 has been achieved. The only nanocrystalline hydride formed up to 270 h of CRMA isβ-MgH2, and it does notreact with Al and H2 to form Mg(AlH4)2. It has been found that there is strong competition between formation of Al(Mg) solid solution and theβ-MgH2 hydride occurring toa various extent up to ∼10 h of CRMA in all three Mg–2Al mixtures. It is hypothesized thatthe presence of Al(Mg) solid solution inhibits the reaction ofβ-MgH2, Aland H2 toform Mg(AlH4)2. Furthermore, despite the fact that after prolonged milling the Al(Mg) solution eventuallydecomposes into secondary Al(s) (derived from solid solution), the latter retains itsphysico-chemical characteristics of the former solid solution which still inhibits the reaction to formMg(AlH4)2. Experimental evidence from DSC measurements shows increasing ranges of the meltingenthalpy with increasing amounts of Al(Mg) solid solution and consequently the secondaryAl(s) for all the three Mg–2Al mixtures. This strongly supports the hypothesis about thedifferent nature of Al(Mg) and the secondary Al(s) as compared to the primary elementalAl powder.