Phase composition and the effect of thermal cycling for VHx, V0.995C0.005Hx, and V0.975Zr0.020C0.005Hx

X-ray diffraction (XRD) studies were performed on hydride phases formed by vanadium and its carbon substituted alloys. It was previously found that th ermal cycling of VHx across the beta-gamma mixed phase region changed the r eversible hydrogen storage capacity and other properties. The present mater ials were compared where annealing was one prior treatment and the material s were cycled from 3 cycles to over 1000 cycles and one sample was cycled 6 182 times. The effects on phase composition, c/a unit cell axial ratios, un it cell volume and crystallite particle size and BET surface area measureme nts were studied. Earlier studies placed the beta-gamma phase boundary at x = 0.82 for XRD studies. The phase boundaries of the substituted alloys are found to be virtually the same. Thermal cycling produced similar behavior for all the materials. (C) 1999 Elsevier Science S.A, All rights reserved.