Mechanical alloying and hydrogen storage properties of CaNi5-based alloys

CaNi5-based hydrogen storage materials have been synthesized by mechanical alloying of the elemental Ca and Ni powder blend, followed by an isothermal annealing. It was found that high energy ball milling of a Ca and Ni blend does not result in the formation of the CaNi5 intermetallic compound. However, DSC measurements show that the CaNi5 phase forms upon heating the mechanically milled powders to 395 degreesC. Replacement of Ca by Ce or Mm (Mischmetal), and of Ni by Zn leads to direct formation of nanocrystalline alloy phases with CaCu5 structure. The as-milled nanocrystalline powder does not absorb much hydrogen. A post-milling isothermal annealing at 640 degreesC leads to grain growth and release of microstrain, and to an improvement of the hydrogen storage properties. Replacement of Ca by Ce or Mm increases the plateau pressure, while replacement of Ni by Zn reduces the desorption plateau pressure. (C) 2001 Elsevier Science BN. All rights reserved.