Mechano-chemical activation synthesis (MCAS) of nanocrystalline magnesium alanate hydride [Mg(AlH4)2] and its hydrogen desorption properties

A successful synthesis of the Mg(AlH4)2 + 2NaCl mixture by the mechano-chemical activation synthesis (MCAS) for 5 and 10 h has been achieved. The nanocrystalline Mg(AlH4)2 hydride in the mixture has the grain size on the order of 18 nm. Even relatively short milling for just 10 h seems to result in a partial decomposition of the initially formed Mg(AlH4)2 into the nanocrystalline β-MgH2, elemental Al (grain size ∼26 nm) and hydrogen gas. A number of DSC tests at the scan rate of 4 °C/min of the Mg(AlH4)2 + 2NaCl mixture synthesized for 5 and 10 h show either endothermic or exothermic heat flow changes at the ∼125–180 °C range due to the decomposition of Mg(AlH4)2. Endothermic peaks with the maxima at ∼271 and 316 °C due to the decomposition of β-MgH2 and the formation of Al3Mg2 intermetallic as well as Al(Mg) solid solution are present. The peak at ∼452 °C is due to the eutectic melting of the Al3Mg2 and Al(Mg) mixture. Prolonged milling for 40 h results in a complete decomposition of the Mg(AlH4)2 into two nanocrystalline solid phases such as β-MgH2 (grain size ∼11 nm) and the elemental Al (grain size ∼20 nm), and hydrogen gas. DSC tests at the scan rate of 4 °C/min up to 500 °C of the 40 h milled powder, which contains β-MgH2, results in two endothermic peaks with the maxima at ∼292 °C due to the decomposition of β-MgH2 and ∼452 °C due to the eutectic melting. TGA tests for the 5 and 10 h milled sample give the total weight loss of ∼2.45 and 2.16 wt.%, respectively. The hydrogen desorption kinetics of the as-synthesized powders directly after milling were also evaluated using a Sieverts-type apparatus.