Mechanical alloying and hydrogen absorption properties of the Mg-Ni system

The mechanical alloying process of mixed elemental Mg and Ni was investigated. After milling, we found a mixture of Mg and Mg2Ni in the composition range x>66.67 for MgxNi100-x and a single Mg2Ni phase at the composition x=66.67. The crystallite sizes of Mg and Mg2Ni in the as-milled state were 20 and 10 nm, respectively. The Mg2Ni keeps its nanocrystalline structure when heated to 673 K, whilst Mg grains in the Mg+Mg2Ni composite grow much more rapidly with increasing annealing temperature. The hydrogen storage properties of these nanocrystalline powders were characterized. During the first absorption cycle, we observed that nanocrystalline Mg2Ni absorbs hydrogen more rapidly than the two-phase material. However, after activation, the nanocrystalline Mg+Mg2Ni has better hydriding kinetics at low temperature (423 K) than nanocrystalline Mg2Ni. This is contrary to what was observed in conventional coarse-grained systems. Possible explanations would be that Mg2Ni catalyses the hydrogen chemisorption in the composite and that the phase boundaries enhance hydrogen diffusion. The absorption and desorption enthalpies of Mg and Mg2Ni were evaluated by Van't Hoff plots. The desorption enthalpy increases a little and the absorption enthalpy decreases compared to polycrystalline materials. (C) 1998 Elsevier Science S.A.