Comparative study of mechanical alloying of (Mg plus Al) and (Mg plus Al plus Ni) mixtures for hydrogen storage

Mg is a desirable hydrogen-storage material of high capacity, but suffers from low kinetics and is difficult to activate. Addition of appropriate alloying elements, such as Al and Ni, and a fine microstructure can ease the problems. In order to increase the solubility of Al into Mg and to refine the powder structure, a (Mg+10 at.% Al) mixture was investigated by mechanical alloying. It was noted that the amount of elemental Al decreases with increasing milling time. After 20 h of milling, an h.c.p. (Mg, Al) solid solution was obtained with a volume contraction of 1.2%. An energy dispersive X-ray (EDX) analysis confirmed its formation and showed its chemical composition as (Mg+12 at.% Al). When 10 at.% Ni was further added to the (Mg, Al) solid solution as a catalytic element, a new Mg12Al17 phase was formed, in addition to a modified (Mg, Al, Ni) solution. When the (Mg+10 at.% Al) mixture milled for 2 h was annealed at 400degreesC for 2 h, all the Al dissolved into Mg and formed a single (Mg, Al) solid solution. When the (Mg+10 at.% Al+10 at.% Ni) mixture was sintered at the same condition, a new AlNi phase was formed, in addition to the presence of a (Mg, Al, Ni) solid solution. (C) 2002 Elsevier Science BY All rights reserved.