Hydrogenation properties of Mg2AlNi2 and mechanical alloying in the Mg-Al-Ni system

Samples with MgmAlNin composition (m, n <= 3) were synthesized by ball milling in form of crystalline nanoparticles, and were found to be a single phase with partially disordered CsCl-type cubic structure. For compositions with large m and small n, minor quantities of Mg and Mg2Ni are segregated. XRD Rietveld refinements indicate that the two Pm (3) over barm sublattices basically have (Mg, Al) and (Mg, Al, Ni) occupations, respectively. Hydrogenation experiments on the Mg2AlNi2 sample by the PCI technique showed a reversible absorption/desorption of 1.4 wt% H-2 with formation/decomposition of the MgH2 and Mg2NiH4 hydrides. The van't Hoff plot gives Delta H-r = 67.7 kJ mol(-1) (enthalpy of dehydrogenation), and T-0 = 524 K (temperature of dehydrogenation at p = 0.1 MPa). A kinetic study of the gas release at 535 K showed a satisfactory behaviour, with a first fast step followed by a slower one in both the absorption and desorption processes. (C) 2008 Elsevier B.V. All rights reserved.