JOURNAL OF MATERIALS SCIENCE 21 (1986) 2335-2338 Surface studies on activated and hydrided CaNi 5 alloy M. P. SRIDHAR KUMAR, B. VISWANATHAN, C. S. SWAMY, V. SRINIVASAN Department of Chemistry, Indian Institute of Technology, Madras 600 036, India X-ray photoelectron spectroscopic studies on the activated pure CaNi 5 alloy as well as the hydride formed at 1 atm pressure and room temperature, CaNi5 H48, showed that on activation calcium segregates to the surface and forms the oxide, whereas most of the nickel on the surface is present in the metallic form. This indicates that the coexistance of the oxide phase and the metal phase is essential for the facile hydriding behaviour of CaNis. 1. Introduction Of the available alloy materials for hydrogen storage applications as "rechargeable metal hydrides", AB 5 type systems have received considerable attention [1] because of their ability to form hydrides at modest hydrogen pressures and room temperature. Among the AB5 type systems, CaNi 5 appears to be one of the promising materials for hydrogen storage. There are various conflicting reports on the CaNis-H system [2-5]. Van Mal et al. [3] reported a plateau pressure of 15 atm for absorption at room temperature but have not identified the composition of the hydride. Sandrock et al. [2] have observed two plateau pressures at 0.5 and 25 atm for hydrogen sorption at 25 ~C with the corresponding compositions approxim- ating to CaNisH s and CaNisH 6. Shinar et al. [5] also report a hydride with the composition CaNisH 8. Although there are numerous studies on LaNi 5, a prototype of AB s alloys, studies pertaining to CaNis are rather limited. The aspects on which information is particularly required include: 1. the exact composition of the hydride formed at low pressures; 2. the surface segregation effects [6, 7], since in the case of LaNis it is known that lanthanum diffuses to the surface and superparamagnetic nickel precipitates are formed; and 3. the process of activation of CaNis and the valid- ity of models involving the separation of surface oxides or different surface segregation effects. The present communication, therefore, concerns these aspects with respect to CaNi 5 alloy by means of equilibrium hydrogen sorption mesurements as well as by X-ray photoelectron spectroscopic method. 2. Experimental procedure The sample CaNi 5 (supplied by Ergenics Corporation, USA as HYSTOR 201) was activated by the following procedure. The alloy was heated at 300 ~C under vacuum for a period of 90min. Then keeping the temperature the same, hydrogen gas was admitted to a pressure of 150 torr. The sample was left in contact with hydrogen for 2 h and then evacuated completely at 300~ until a pressure of 10 .6 torr was obtained. It was cooled and the absorption measurements were carried out. The X-ray photoelectron spectroscopic analysis was performed on a Vacuum Generators ESCALAB spec- trometer at a base pressure of l0 8torr. The absorp- tion measurements were carried out in an all-glass volumetric unit which can operate up to 1 atm pressure. 3. Results and discussion 3.1. Equilibrium hydrogen sorption measurements Fig. ! shows the hydrogen absorption/desorption hysteresis loop at 25~ for CaNi~ up to latin pressure. The plateau pressure (0.5atm at 25~ agrees with the report of Sandrock et al. [2] who have reported three absorption plateau pressures around 0.03 to 0.04, 0.5 and 25 arm at 25 ~C. Fig. 2 shows the pressure-composition-temperature (P-C-T) behaviour of the system for absorption isotherms in the temperture range 0 to 35~ Sandrock et al. [2], from a complete analysis of the P-C-T isotherms, deduced that the pseudo-binary system CaNis-H 2 contains four phases, an a-solution of hydrogen in CaNis, and three compound hydrides of average compositions approximating to CaNisH, CaNisH5 and CaNisH6, and designated them as ]3, 7 and fi phases, respectively. In the present experiment, because of the restriction of the upper limit to the pressure range, the phases that can be identified are c~, /Yand y. The e-solution, with small values ofx (~ 0.2), should be caused by initial chemisorption and trap- ping regions. However, the t3 and 7 phases exhibit considerable hydrogen solubility with compositions at 25~ extending for the /~-phase between CaNisH0. 6 to CaNisH0.9, while the 7-phase composition range is CaNisH4. 4 to CaNisH4. 8. The values of plateau pressures were used in the plots of lnp against lIT (Fig. 3) for obtaining the values of standard enthalpies of formation. The values of standard enthalpies for the formation offl and 7 hydrides are 50 and 32 kJ (mol H2) -1, respectively, These values are comparable with those reported in literature [2]. 0022-2461/86 $03.00 + .12 9 1986 Chapman and Hall Ltd. 2335