MECHANICAL ALLOYING AND HYDROGEN ABSORPTION PROPERTIES OF THE MG-NI SYSTEM

The mechanical alloying process of mixed elemental Mg and Ni was inves tigated. After milling, we found a mixture of Mg and Mg2Ni in the comp osition range x>66.67 for MgxNi100-x and a single Mg2Ni phase at the c omposition x=66.67. The crystallite sizes of Mg and Mg2Ni in the as-mi lled state were 20 and 10 nm, respectively. The Mg2Ni keeps its nanocr ystalline structure when heated to 673 K, whilst Mg grains in the Mg+M g2Ni composite grow much more rapidly with increasing annealing temper ature. The hydrogen storage properties of these nanocrystalline powder s were characterized. During the first absorption cycle, we observed t hat nanocrystalline Mg2Ni absorbs hydrogen more rapidly than the two-p hase material. However, after activation, the nanocrystalline Mg+Mg2Ni has better hydriding kinetics at low temperature (423 K) than nanocry stalline Mg2Ni. This is contrary to what was observed in conventional coarse-grained systems. Possible explanations would be that Mg2Ni cata lyses the hydrogen chemisorption in the composite and that the phase b oundaries enhance hydrogen diffusion. The absorption and desorption en thalpies of Mg and Mg2Ni were evaluated by Van't Hoff plots. The desor ption enthalpy increases a little and the absorption enthalpy decrease s compared to polycrystalline materials. (C) 1998 Elsevier Science S.A .