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 nanocrystalline 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 scanning calorimetry and thermogravimetric analysis (TGA). The measurements confirmed the nonequilibrium state of the samples as prepared by the mechanical 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 alloy. All the samples showed rapid uptake of hydrogen at 3 MPa pressure, indicating that the nickel was thus playing a very active role at the alloy surface in dissociating hydrogen and so enabling more rapid hydride formation by the alloy. This catalytic activity of the nickel decreased with temperature cycling over the range 80°C to 500°C. Although TGA analysis, carried out at the end of the cycling period, gave the hydrogen content as 1.1 wt% to 1.7 wt% for the alloys, this is well short of the theoretical amounts expected (7.6 wt% for MgH2), indicating that the samples had become deactivated during cycling. No evidence was found of the intermetallic Mg,Ni prior to or after hydriding.