Cyclic charging and discharging of obliquely deposited FeTi thin films

0360 3199/90 $3.00 + 0.00 Pergamon Press plc. InternationalAssociationfor HydrogenEnergy

Int. J. Hydrogen Energy, Vol. 15, No. 12, pp. 867 870, 1990. Printed in Great Britain.

CYCLIC C H A R G I N G A N D D I S C H A R G I N G OF OBLIQUELY DEPOSITED FeTi THIN FILMS K. S. UPADHYAY, 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 13 June 1990)

Abstract--The charging of FeTi thin films with hydrogen of 1000 ~ thickness deposited at different angles (0 = 0 , 30°, 45°, 60° and 75°) was carried out at 1 atm hydrogen pressure and at room temperature. The discharging was carried out at 10 5Torr pressure by heating. The resistance of FeTi films on charging with hydrogen increases and decreases on discharging. The change in resistance in subsequent charging cycles was found to decrease, indicating saturation stage.

INTRODUCTION The electrical resistivity of bulk metal hydrides has been studied [1-6], but the literature concerning absorption of hydrogen in thin films is still not very rich [7-12]. The studies covering absorption of hydrogen in FeTi thin films are rare [8-10, 13]. The studies related to change in the hydrogen characteristics of the metal hydrides under cyclic conditions are available in the literature [14-16]. Adachi et al. [11, 12] have studied the effect of hydrogen absorption on the electrical resistivity of LaNi 5, LaC% and MmNi45 Mn0.5 films (where Mm is Misch metal) deposited by flash evaporation at 90°C under vacuum for 30 min for several cycles. Wulz et al. [9] studied the hydrogen absorption by FeTi, LaNi5 and TiNi films at low pressure and 300 K temperature. Uchida et al. [17] have investigated the hydrogen absorption in tantalumwire specimen and used the resistivity change to determine the content of hydrogen absorbed. Welter et al. [18] have investigated the resistivity of alloy F e T i - H system and have monitored the concentration of hydrogen by resistivity measurements. In this paper we have investigated the cyclic charging and discharging of obliquely deposited FeTi thin films by measuring the change in the electrical resistance of FeTi thin films. E X P E R I M E N T A L TECHNIQUE The FeTi thin films of 1000/~ thickness were deposited at 5 × 10 5 Torr pressure at room temperature and at different angles (0 = 0 °, 30 °, 45 °, 60 ° and 75 °) by oblique deposition technique described elsewhere [10]. After deposition all the sample were transferred to the measurements chamber for hydrogenation one by one. The FeTi film samples were of size 1 cm x 1 cm and thickness 1000/~ on glass substrate of size

2.54 cm × 2.54 cm × 1 mm. The thickness of the films were measured with the help of a quartz crystal thickness monitor and by using the weighing method the thickness of the films were found to be approximately 1000 A. The FeTi film samples after activation were hydrogenated at 1 atmosphere hydrogen pressure and at room temperature. For discharging, the samples were heated slowly and gradually by varying power in the heater. The temperature of the hydrogenated FeTi film samples was measured with the help of copper-constantan thermocouple having an accuracy of 2°C. The hydrogen (99.9% pure) was flushed into the chamber for charging of the samples for second cycle. Each sample was charged with hydrogen and then discharged by heating under vacuum. For each subsequent charging cycle, all samples in turn were exposed to hydrogen under similar conditions. The charging and discharging of FeTi film samples were monitored by measuring change in their resistance with the help of a digital multimeter (HIL 1000, India) having an input impedance of 10 Mohm. By X R F analysis the deposited FeTi film samples were found to be of same composition as bulk. The samples were found to have single phase as confirmed by M6ssbauer spectroscopy measurements of thin film samples. RESULTS A N D DISCUSSION The charging and discharging of FeTi thin films deposited at different angles (0 = 0 °, 30", 45 c', 6ff ~ and 75 °) of 1000 A_ thickness were studied for four hydrogen charge and discharge cycles as shown in Figs 1-5. The ratio RH/Ro is plotted as a function of temperature (°C) where R n is the resistance of FeTiHx films and R 0 represents the initial resistance of FeTi films before hydrogenation. The charging of all samples was carried out at 1 atmosphere hydrogen pressure and at room temperature and discharging was carried out while vary867

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K.S. UPADHYAY et al.

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Fig. 1. RH/Ro vs temperature (°C) for 0 = 0 ° film.

Effect of hydrogen charge and discharge cycles ing temperature. The charging is shown in Figs 1-5 by the relative change in resistance (i.e. RH/Ro) on the left hand side by a vertical line marked with points A - H and discharging by the points W-Z including the curves. The resistance of the hydrogenated films RH is always found to be greater than R0. The increase in resistance of FeTi on charging with hydrodgen is due to H - anion formation. The hydrogen atom takes one electron from the conduction band of FeTi and forms a hydrogen ion. Thus the value of RH/Ro is always > 1.0. This is also supported by Nakamura et al. [1].

The curves obtained as a result of charging and discharging for four hydrogen cycles on samples deposited at angles (0 = 0 °, 30 °, 45 °, 60 ° and 75 °) are given in Figs 1-5. Although the behavior of variation in resistance with temperature in various samples is more or less same, the sample deposited at 0 = 30 ° shows a different hysteresis loop. 1-Cycle. On hydrogenation of the films the relative resistance RH/Ro increases from the initial value indicated by point I up to point A, but with increase in temperature the resistance ratio RH/R o decreases. On CHARGING

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Fig. 5. RH/Rovs temperature (°C) for 0 = 75° film.

cooling the samples, the resistance is stabilized which is shown by point B. H-Cycle. In the second hydrogenation cycle, increase in the ratio RH/R o is found to be more than first charging cycle indicated by the portion BC. On cooling the samples up to room temperature the resistance gets stabilized and is indicated by point D. Ill-Cycle. In the third hydrogen charging cycle, the increase in relative resistance RH/R. is found to be less in comparison to the second cycle indicated by the portion DE. On subsequent cooling of the samples, the resistance gets stabilized at point F. IV-Cycle. In the fourth charging cycle, the increase in the ratio RH/R o is found to be less than the second and third hydrogen charging cycles. In the first hydrogen charging cycle, the smaller increase in the ratio Rn/R o could be due to the formation of an over layer of oxygen which was formed during the transfer of samples from one chamber to another for hydrogenation, and that may not have been removed completely. This causes less hydrogen to be absorbed in the first cycle, but it seems that with the removal of oxide layer in the second cycle, more hydrogen gets absorbed in the film indicated by comparatively large change in the resistance. In the fourth cycle, it appears that the samples tend to acquire stable state. Similar behaviour is obtained in most of the samples investigated, however, in some samples the saturation effect could be in more than four cycles. The measurements described above for four hydrogen charge and discharge cycles on FeTi obliquely deposited films are supported by another plot of percentage change in resistance (Rn) due to hydrogen absorption with angle of deposition (0) as shown in Fig. 6. The

percentage change in R H for films deposited at larger angles of deposition is more than that of normal deposited films. This shows that the large angle deposited films can absorb more hydrogen than the normally deposited films. The explanation for this is that due to oblique deposition the growing films are porous because of the "Self Shadowing Effect" [19, 20] and thus provide large surface area for hydrogen absorption. The effect of cyclic charging and discharging of films due to hydrogen tends to saturate the samples with hydrogen. The plausible explanation for the variation of resistance of hydrogenated samples has been given by many workers [5, 7, 10, 12]. However, the variation of resistance of hydrogenated samples with temperature could be explained on the basis of desorption of hydrogen due to heating. At elevated temperatures some of the hydrogen comes out of the samples causing decrease in resistance. However, in the cooling cycle, the increase in resistance of FeTi films with a little hydrogen in it, is due to the granular nature of the films [13, 19]. In granular films the electrical conduction is due to the hopping of electrons from one grain to another [23] which is easier at higher temperatures. Thus, on heating the samples, the resistance decreases. The different hysteresis curves obtained on heating and subsequent cooling for films deposited at different angles (0 = 0% 3if, 45', 60 ° and 75 °) could be explained on the basis of the Gorsky effect [21]. According to which the inhomogenous spatial hydrogen distribution in a sample causes strain gradients leading to deformities in the shape of the samples, probably giving rise to different shapes of the curves. The results, in general, are in agreement with the results of Doyle et al. [22] on Pd-8 at % Y alloy, where they have also observed hysteresis type behavior in the resistivity curves.

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Acknowledgements--The authors are thankful to Mr A. S. Banthia for helpful discussions. This work was financially supported by the Department of Non-Conventional Energy Sources, New Delhi, India.

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