Formation of nanostructured LaMg 2Ni by rapid quenching and intensive milling and its hydrogen reactivity

Abstract

The formation of the nanostructured orthorhombic LaMg 2Ni phase using the melt-spinning and the intensive ball milling routes has been studied for the La 25Mg 50Ni 25 and La 20Mg 50Ni 30 compositions. By controlled heat treatment of glassy melt-spun ribbons the formation of the LaMg 2Ni phase proceeds at T A ≥ 220 °C ( T A = annealing temperature) for La 25Mg 50Ni 25 via the initial formation of LaMg 3, but occurs for La 20Mg 50Ni 30 already at T A ≥ 200 °C in a single reaction. Crystallite sizes of LaMg 2Ni of ≈30 nm were achieved. The formation enthalpies of LaMg 2Ni were estimated to Δ H = −320.8 kJ/mol for La 25Mg 50Ni 25 and Δ H = −230.0 kJ/mol for La 20Mg 50Ni 30. 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 LaMg 2Ni. The formation of the complex hydride LaMg 2NiH 7 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 La 20Mg 50Ni 30 and within 7.5 h for La 25Mg 50Ni 25. The hydrogen desorption from intensively milled LaMg 2NiH 7 powder proceeds in two steps. At first, the LaMg 2NiH 7 decomposes into LaH 2–3 and Mg 2NiH 0.2–0.3 at T A > 450 °C with a decomposition enthalpy of Δ H ≈ 48 kJ/mol and the recombination of LaMg 2Ni starts at T A > 540 °C. Above 625 °C the LaMg 2Ni decomposes by the sublimation of Mg.