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

A successful synthesis of the Mg(AIH(4))(2) + 2NaCl mixture by the mechano-chemical activation synthesis (MCAS) for 5 and 10 h has been achieved. The nanocrystallineMg(AlH4)(2) hydride in the mixture has the grain size on the order of 18nm. Even relatively short milling for just 10h seems to result in a partial decomposition of the initially formed Mg(AIH4)2 into the nanocrystalline beta-MgH2, elemental Al (grain size similar to 26 nm) and hydrogen gas. A number of DSC tests at the scan rate of 4 degrees C/min of the Mg(AlH4)(2)+2NaCl mixture synthesized for 5 and 10h show either endothermic or exothermic heat flow changes at the similar to 125-180 degrees C range due to the decomposition of Mg(AlH4)(2). Endothermic peaks with the maxima at similar to 271 and 316 degrees C due to the decomposition of beta-MgH2 and the formation of Al3Mg2 intermetallic as well as AI(Mg) solid solution are present. The peak at -452 degrees C is due to the entectic melting of the Al3Mg2 and Al(Mg) mixture. Prolonged milling for 40 It results in a complete decomposition of the Mg(AlH4)(2) into two nanocrystalline solid phases such as beta-MgH2 (grain size similar to 11 nm) and the elemental Al (grain size similar to 20 nm), and hydrogen gas. DSC tests at the scan rate of 4 degrees C/min up to 500 degrees C of the 40 h milled powder, which contains beta-MgH2, results in two endothermic peaks with the maxima at -292 degrees C due to the decomposition of beta-MgH2 and -452 degrees C due to the eutectic melting. TGA tests for the 5 and 10h 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. (c) 2006 Elsevier B.V. All rights reserved.