THE EFFECT OF INDUCED DISORDER ON THE HYDROGENATION BEHAVIOUR OF THE PHASE ZrCo

Abstract

The hydrogenation behaviour of the CsCl-type phase ZrCo has been investigated and the P-C-T relationships and the partial phase diagram have been established over the pressure range 0.2 to 560τ and temperature range 423 to 773K. These studies show that there are three phases in the ZrCo-H system over these ranges of temperature and pressure; the α-solid solution based on the CsCl-type structure, a broad intermediate phase designated β1 which has a metal lattice with the orthorhombic CrB-type structure and at greater hydrogen concentrations, another phase β2 which again has a metal lattice with the CrB-type structure. Under the experimental conditions employed in these studies the β2 phase has an easily obtainable maximum hydrogen concentration of 1.25 H/M i.e. ZrCoH2.5. The appreciable volume expansion on forming the orthorhombic hydrides resulted in the disintegration of the bulk material with the production of rather coarse flakes rather than discrete powder particles and SEM studies of the surface of these flakes indicated a mixed ductile-cleavage type fracture. This observation indicated that a significant amount of plastic deformation had occurred as a result of the hydrogenation process and X-ray diffraction measurements on the dehydrogenated b.c.c. phase (hydrogen removed at ~ 423K) indicated increasing departures from perfect order with successive hydrogen cycling across the (β1 + β2) phase boundary. This cycling also resulted in a progressive decrease in the magnitude of both ▪ and ▪ as well as the appearance of a ferromagnetic contribution to the magnetic susceptibilities of both the hydride phases and the dehydrogenated b.c.c. phase. All these effects have been attributed to the disordering phenomenon and the hydrogenation behaviour of the ZrCo phase has been contrasted with that of previously investigated phases such as ZrNi and LaNi5. The wider aspects of the effect of hydrogen cycling on the pressure characteristics of ZrCo-H could have an important bearing on the selction and design of practical hydrogen storage devices.