Electrochemical characteristics of nanocrystalline ZrCr2 and Mg2Ni type metal hydrides prepared by mechanical alloying

The mechanical alloying process (MA) has been introduced to produce nanocry stalline ZrCr2 and Mg2Ni type metal hydride. MA has recently emerged as a n ovel technique for producing alloy powders whose structures are nanocrystal line. The Zr-Cr-Ni and Mg-Ni systems were prepared using a planetary hall m ill, starting from mixtures of elemental powders. Tn the Zr-Cr-Ni system, n anocrystalline C14 structure could not be obtained after ball-milling but c ould be obtained fi-om heat-treatment of ball-milled powders. The heat-trea ted Zr-Cr-Ni powders showed a similar discharge capacity to the arc-melted alloy, but the activation was much easier. In contrast to Zr-Cr-Ni system, nanocrystalline Mg2Ni alloy could be formed by ball-milling. In comparison with the polycrystalline one, nanocrystalline Mg3Ni showed a lower temperat ure for hydrogenation and a much higher discharge capacity at 30 degrees C. The partial substitution of Zr for Mg resulted in a microstructure differe nt from mechanically alloyed Mg,Ni and greatly improved the discharge capac ity at 30 degrees C. The discharge capacity of 120 h-milled 1.8Mg-0.2Zr-Ni electrode was 465 mAh g(-1). (C) 1999 Elsevier Science S.A. All rights rese rved.