Formation of nanostructured LaMg2Ni by rapid quenching and intensive milling and its hydrogen reactivity

The formation of the nanostructured orthorhombic LaMg2Ni phase using the melt-spinning and the intensive ball milling routes has been studied for the La25Mg50Ni25 and La20Mg50Ni30 compositions. By controlled heat treatment of glassy melt-spun ribbons the formation of the LaMg2Ni phase proceeds at T-A >= 220 degrees C (T-A = annealing temperature) for La25Mg50Ni25 via the initial formation of LaMg3, but occurs for La20Mg50Ni30 already at T-A >= 200 degrees C in a single reaction. Crystallite sizes of LaMg2Ni of approximate to 30 nm were achieved. The formation enthalpies of LaMg2Ni were estimated to Delta H = -320.8 kJ/mol for La25Mg50Ni25 and Delta H = -230.0 kJ/mol for La20Mg50Ni30. Intensive ball milling of inductively molten master alloys using a planetary RETSCH mill for 30 h resulted in a crystallite size reduction, e.g. up to 15 nm for LaMg2Ni. The formation of the complex hydride LaMg2NiH7 was studied for both alloy compositions by annealing rapidly quenched and crystallised as well as intensively ball milled powder under hydrogen. However, the most effective method for achieving the complex hydride as single phase was found to be intensive ball milling in a planetary RETSCH mill under a 0.5 MPa hydrogen atmosphere. The hydride formation reaction was completed within 3 h for La20Mg50Ni30 and within 7.5 h for La25Mg50Ni25. The hydrogen desorption from intensively milled LaMg2NiH7 powder proceeds in two steps. At first, the LaMg2NiH7 decomposes into LaH2-3 and Mg2NiH0.2-0.3 at T-A > 450 degrees C with a decomposition enthalpy of Delta H approximate to 48 kJ/mol and the recombination of LaMg2Ni starts at T-A > 540 degrees C.Above 625 degrees C the LaMg2Ni decomposes by the sublimation of Mg. (C) 2009 Elsevier B.V. All rights reserved.