Influences of annealing heat treatment on phase structure and electrochemical properties of Zr(MnVNi)(2) hydrogen storage alloys

The influences of annealing treatment on the phase structure and the electr ochemical properties of Zr(Mn0.25V0.20Ni0.55)(2) and Zr(Mn0.05V0.40Ni0.55)( 2) hydrogen storage alloys were investigated by means of XRD analysis and e lectrochemical tests. As-cast alloys were comprised of Laves phases (i.e., C15 and C14) dispersed Zr-Ni phases (Zr9Ni11 and ZrNi) and exhibited good o verall electrochemical properties. When the as-cast alloys were annealed, t he Zr-Ni phases and part of C14 phase decomposed completely with increasing annealing time. The thermodynamically stable phase structure of the anneal ed alloys was a mixture of C15 and C14 Laves phase. The decomposition of Zr -Ni phases and part of C14 phase resulted in a remarkable decrease in capac ity and activation property as compared to those of the as-cast alloy. A sy nergistic effect due to the presence of Zr-Ni phases in the Laves phases is inferred. The deterioration in the overall electrochemical properties is a ttributed to the decomposition of the Zr-Ni phases. (C) 1999 Published by E lsevier Science S.A. All rights reserved.