Int. J. Hydrogen Energy, Vol. 16, No. 2, pp. 101-104, 1991. Printed in Great Britain. 0360-3199/91 $3.00+ 0.00 Pergamon Pressplc. '(© 1991 International Association for HydrogenEnergy. THE EFFECT OF A SULFUR LAYER ON FeTi THIN FILMS OBLIQUELY DEPOSITED FOR HYDROGEN STORAGE M. SINGH, Y. K. VIJAY and I. P. JAIN* Department of Physics and *University Science Instrumentation Center, University of Rajasthan, Jaipur 302004, India (Received for publication 9 October 1990) Akstract--FeTi thin films of thickness of 730/~ were obliquely deposited at different deposition angles 0 (0 = 0', 30°, 45°, 60° and 75°) under 5 x 10-5 torr pressure by thermal evaporation onto the glass substrate at room temperature. A layer of sulfur of thickness 115 A was deposited on the FeTi thin film. It was found that the resistance of the FeTi thin films increases with deposition angle and with the absorption of hydrogen, and decreases with desorption. The sulfur layer on the FeTi thin film was found to produce a marked improvement in the properties of thin films for hydrogen storage, and charging and discharging rate becomes faster in comparison to FeTi thin films. INTRODUCTION It is well known that the intermetallic compound FeTi has favorable characteristics for hydrogen storage applications [1]. The control process of hydrogen ab- sorption and desorption is a very important problem. However, only a few investigations of the hydriding kinetics of FeTi thin films have been reported [2-3]. The intermetallic compound FeTi is known to be the best material for hydrogen storage owing to its large hydro- gen storage capacity and low material cost. However, the activation of the compound is a very difficult process, which is one of the fundamental problems that need to be solved. It has been reported that the partial substi- tution of iron in FeTi by Mn [4] or A1 [5] improves the activation process. The substitution of iron b~, aluminum is particularly useful for reducing the material weight when it is to be used for mobile applications. The Brookhaven National Laboratory, U.S.A., has demonstrated a greatly in- creased versatility for Fe-Ti by incorporating various substituent transition metals (e.g. Mn, Cr, Ni, Co and V) into the alloy by many workers [6-8]. The Mn and Cr were found to be mild stabilizers of the Fe2Ti phase and therefore, for the maximum hydrogen storage capacity Mn and Cr modified alloys should be melted with a slight excess of Ti. The Mn modified alloys are of the greatest practical interest because 4.5% Mn results in an alloy that can be activated at room temperature and does not need the first stage heating procedure. The quantity of hydrogen absorbed was measured as a function of time in the alloy FeTi and LaNi5 for several successive cycles using hydrogen contaminated with nitrogen, CH4, CO2, CO or H2S in various concen- trations by Block et al. [9]. They observed that when H 2S was added for the first time the saturation was un- changed but the storage capacity was reduced to a hydrogen to metal ratio of about 2.8%. This leads to the conclusion that a part of the alloy FeTi was fully passivated by surface layer of H2S while the unpassi- rated portion continued to absorb hydrogen. The sub- sequent absorption cycle with impure hydrogen resulted in the complete passivation of the samples. The samples could not be reactivated by intensive heat treatment and cycling with pure hydrogen. Further analysis of the sample after heat treatment revealed that the sulfur deposition had vanished and the powder had become brittle. Auger Electron Spectroscopy showed that the surface may still be covered by a sulfur layer which inhibited hydrogen absorption and caused poison- ing of the sample. However, LaNi5 regained almost all of its original hydrogen storage capacity if samples were heated up to 700 K in vacuum. These results are contra- dictory to that obtained by Suzuki et al. [10]. They observed that the addition of sulfur to FeTi alloy was found to produce a marked improvement in the proper- ties of the alloy for hydrogen storage. The inhibition of the hydrogen absorption by tanta- lum at room temperature can be reduced appreciably by coating the surface with thin metal films, e.g. with palladium films [11-17]. The catalytic effect by overlayer coating found for unpoisoned nickel and iron overlayers on the titanium and the slight impediment caused by the manganese layer has been observed by Uchida et al. [18]. The result of the study of the activation characteristics and charging and discharging behavior of the FeTi thin films coated by sulfur are presented in this paper. EXPERIMENTAL The FeTi thin films were obliquely deposited at an angle 0 (0 =0 °, 30 °, 45 °, 60 ° and 75 °) onto a glass substrate by thermal evaporation of bulk FeTi at 101