A thermal analysis investigation of the hydriding properties of nanocrystalline Mg-Ni based alloys prepared by high energy ball milling

A thermal analysis study of the hydrogen loading characteristics of nanocry stalline Mg-Ni alloys (Ni content ranging from 0.1 at% to 10 at%) has been carried out in 3 MPa hydrogen, employing the techniques of differential sca nning calorimetry and thermogravimetric analysis (TGA). The measurements co nfirmed the nonequilibrium state of the samples as prepared by the mechanic al alloying technique. An enthalpy associated with the stabilisation of the alloys on first heating in hydrogen was found for all the samples studied. The magnitude of this enthalpy increased with the nickel content of the al loy. All the samples showed rapid uptake of hydrogen at 3 MPa pressure, ind icating that the nickel was thus playing a very active role at the ahoy sur face in dissociating hydrogen and so enabling more rapid hydride formation by the alloy. This catalytic activity of the nickel decreased with temperat ure cycling over the range 80 degrees C to 500 degrees C. Although TGA anal ysis, carried out at the end of the cycling period, gave the hydrogen conte nt as 1.1 wt% to 1.7 wt% for the alloys, this is well short of the theoreti cal amounts expected (7.6 wt% for MgH2), indicating that the samples had be come deactivated during cycling. No evidence was found of the intermetallic Mg,Ni prior to or after hydriding. (C) 2000 Elsevier Science S.A. All righ ts reserved.