Phase structures and hydrogen storage properties of Ca–Mg–Ni alloys prepared by mechanical alloying

Abstract

Binary Ca–Ni and ternary Ca–Mg–Ni alloys have been synthesized by mechanical alloying. The phase structures and hydrogen storage properties of the as-milled nanocrystalline and the thermally annealed alloys have been investigated. The phase structure changes with the (Ca+Mg)/Ni and the Mg/Ca ratios in the Ca–Mg–Ni ternary system. A too high Mg/Ca ratio leads to the formation of a MgNi2 phase. The plateau pressure increases with Mg content in the CaxMg1−xNi2 and CaxMg1−xNi3 pseudo-binary alloys, however, the hydrogen storage capacity decreases. Mg does not substitute Ca in the CaNi5 and Ca2Ni7 compounds. Adding or substituting a small amount of Mg in these compounds destroys the A2B7 as well as the AB5 structures and results in an AB3 phase and an accompanying nickel phase.