Effect of ball-milling on the properties of Mg2Cu hydrogen storage alloy

Abstract

We investigated by differential scanning calorimetry the effect of ball milling on the hydrogenation properties of Mg2Cu, a hydrogen storage alloy, prepared by two methods: One is mechanically alloyed Mg2Cu using Mg and Cu powders as starting materials. Eight hours (= 28.8 ks) of ball milling transformed the Mg : Cu = 2 : 1 mixture into Mg2Cu single phase which reacts reversibly with hydrogen. The other is prepared by ball milling a cast Mg2Cu alloy. While the as-cast Mg2Cu undergoes neither hydrogenation nor dehydrogenation under 3 MPa of hydrogen in the temperature range of 300-773 K, just an hour of ball milling activates the inert Mg2Cu to react with hydrogen reversibly. Examining the milling period dependency of the particle size, crystallite size, activation energy and the apparent heat of dehydrogenation which reflects the fraction of activated part in a specimen, we found that the ball milling firstly influenced the particle size and the activated fraction of the sample, and then followed the effect on the crystallite size and kinetic properties. We also found that a trace amount of oxygen could significantly spoil the benefits brought by the ball milling tor longer period or milling. © 2008 The Japan Institute of Metals.