Int. J. ttvdro,¢en Energy. Vo[. 14. No. 8. pp. 573-577. 1989, Printed in Great Brilain.

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International Association |or Ilydrogen Energy.

THE HYDROGENATION BEHAVIOUR OF RNi5 TYPE MATERIALS IN THIN FILM AND BULK FORM

S. K. S1NC;H, K. RAMAKRISHNA, A. K. StN(;H and O. N. SRIVASTAVA

Physics Department, Banaras Hindu University, Varanasi 221005, India

( Received jbr publication 16 November 1988)

Abstract--The present communication deals with the electrical resistivity variation in RNis (R = rare earth, mischmetal) thin films on hydrogenation, the differential thermal analysis and application of RNi~ in vehicular transport using hydrogen as a fuel. Explanation for the observed behaviour of resistivity variation of RNi~ on hydrogenation has been advanced on the basis of the band structure of LaNi~ and hydride of LaNi~. The differential thermal amdysis of the amorphous MmNi4 ~AI~ 5 has been discussed.

1. INTRODUCTION

Rare earth pentanickelides are now established as one of the key materials for hydrogen storage. Whereas the intermetallics themselves have been studied quite exten- sively [1-4], the influence of hydrogenation on physical properties has been studied rather sparsely. The main cause for this is the fact that the material decrepitates and breaks into a fine powder due to large lattice expansion leading to internal stress on hydrogenation. This difficulty is avoided to a large extent in thin films [5, 6]. Therefore, investigations were made to reveal the variation of electrical resistivity on hydrogenation. The electrical resistivity vs time (~)-t) of the hydrogenation curve was observed. The surface characteristics are known to affect the hydrogenation behaviour and hence it has also been taken into account for explaining the observed (~)-t) curve. The proposed mechanism to explain the observed (0-t) curve is based on the recently reported band structure of LaNi5 and LaNis.H¢, [7].

In addition to fundamental studies relating the varia- tion of resistivity with time, the hydrogen characteristics through differential thermal analysis were also studied. This sheds light on the thermal stability and phase transformation on hydrogenation/dehydrogenation. Yet another aspect which has been described in this communication relates to the application of hydrogen/ hydride in a two wheeler vehicular road transport system.

2. RESISTIVITY VS HYDROGENATION TIME MEASUREMENTS

Thin films of RNi5 intermetallics were made from either synthesized materials or the other ones supplied from Ergenics (U.S.A.). The synthesis was accom-

plished by melting the stoichiometric mixture of pow- dered R and Ni conforming to the composition RNi~ in a vacuum sealed silica tube. The melting was done by radio frequency induction furnace. In order to homoge- nize the RNi5 ingot so formed, the above procedure was repeated several times.

The synthesized materials were tested by X-ray dif- fraction employing Philips PWI710 powder diffracto- meter. The results confirmed that the synthesized mate- rial possessed the CaCu.s type structure. The films (thin and thick) of RNi5 intermetallics were prepared by vacuum thermal vapour deposition technique under the vacuum of - 1 0 ~ torr using suitable masks. The subs- trate used was ultrasonically cleaned glass plates. The evaporation was carried out through a tungsten boat. The thickness of the films was monitored in two ways: (a) by measuring the mass of the sample and the distance between source and substrate and (b) by multiple beam interferometry method. These films were charcterized by electron microscopy (imaging and diffraction) which revealed that the as-deposited films are amorphous [5]. The chemical composition of the films was checked by EDAX technique in the electron microscope and was found to confirm to RNis composition. Then these films were transferred into the reactor vessel for the measure- ment of resistivity. The reactor was filledwith hydrogen gas up to a pressure of 25.25 x 105 nt m -. The variation of resistivity as a function of hydrogenation was moni- tored for the films of the two thickness ranges i.e. (a) thick film (100-150 nm) and (b) thin film (30-50 nm) at 300 K temperature.

The resistivity variation as a function of hydrogena- tion time for a large number of films corresponding to the above two categories was investigated. Representa- tive curves revealing the resistivity variations with respect to hydrogenation time for thick and thin films of LaNi5 are shown in Figs 1 and 2, respectively. The significantly different characteristics of LaNi~H, thick

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Fig. 1. Resistivity variation of LaNi5 thick film (~150 nm) on hydrogenation.

and thin films (Figs 1 and 2), particularly the decrease (BC) of resistivity in thin films, are rather surprising. The hydrogena.tion behaviour is known to be crucially dependent on the physical nature and surface structure of the storage material. It may, therefore, appear logical that the difference in the characteristics may be germane in the details of the surface characteristics of the two categories of the films. In fact the observed (~-t) behaviour in MmNi5 (Mm: mischmetal, typically con- tains 49% Ca, 32% La, 13% Nd, 4% Pr and 2% other rare earths) which is less susceptible to surface oxidation than LaNi5 is consistent with this proposition.

The growth, synthesis, structural characteristics and resistivity variation of RNi5 films (100 nm thick) on hydrogenation have been reported. In the present communication the observed LO-t behaviour is ex- plained. Two main points affecting the ~o-t behaviour merit emphasize here: (a) the change in the band structure due to hydrogenation, i.e. hydrogen does not merely donate (protonic model) or absorb (ionic model) electrons but also gives rise to additional states and hence changing the total density of states [8] and (b) the surface characteristics which are known to affect the hydrogenation. The recently reported electronic band structure of LaNis and hydride of LaNi5 is invoked here.

Keeping in view the foregoing points, we have put forward an explanation by taking into account both the expected surface characteristics and the variation of the density of state, for making the observed p- t behaviour

S. K. SINGH et al.

explicable. It has been reported by different workers that the surface of LaNi5 consists of La203/Ni channel through which hydrogen diffuses into bulk [9, 10]. In the case of LaNi5 films it is expected that the thicker films ( -150 nm) would have a bulk-like behaviour, since the surfilce would get contaminated similar to the bulk. On the other hand, in the case of thin film ( - 3 0 nm), the surface is expected to be severely contaminated through oxide formation. The surface of the thin film would, therefl)rc, have high coverage by oxide formation. The higher the oxide coverage, the greater would be the extent of free nickel on the surface since oxide formation leads to the reactions

4 LaNi5 + 3 O2 ~ 2 La20~ + 20 Ni.

The availabilit'y of large nickel concentration would lead to the formation of large nickel nuclei. The surface would therefore have lower density of La2OJNi chan- nels which would bc available for hydrogen diffusion leading to absorption/desorption. In addition to this, the density of interstitial voids in thin films arc also likely to be lower than that of the bulk counterparts.

In the light of this, the hydrogen concentration in thin films would be less. On the other hand, thick films (LaNis) have higher hydrogen capacity due to less oxide contamination. So the surface as well as the hydrogen capacity plays a dominant role in governing the O-t variations of these films. In the following the hydrogena- tion characteristics are first outlined, i.e. ()-t variation for thick and thereafter for the thin films. In order to

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Fig. 2. Resistivity variation of LaNi5 thin film (~30 nm) on hydrogenation.

RNi~ TYPE MATERIALS 575

explain the basic differences of the two thickness ranges of films we have considered the effects of surface structure on the hydrogenation and resistivity.

Figure 1 shows the resistivity (e) curve of LaNi5 thick film on hydrogenation. Initially the resistivity decreases (BC of Fig. 1) then "~" increases to CD and eventually attains a saturation value DEK. The variation in the electronic and optical properties of metals/intermetallics on hydrogen absorption becomes intelligible in terms of the changes in the density of states (DOS) consequent to the hydrogen concentration. As already mentioned the hydrogen absorption in the thicker film is analogous to the bulk material. The resistivity variation of thick film on hydrogenation can easily be understood based on the wlriation of the density of states. Resistivity characteris- tics in material with transition metal (and related systems) is decided by the scattering process in which electron makes a transition from s to d band; the probability of such a transition is proportional to the density of states in the d band. Density of states is proportional to the cube root of the number of holes in the d band [11]. Hence the conductivity is inversely proportional to the number of holes in the d band. Keeping this in view we find our observation on "'resistivity variation" with hydrogenation is explicable. The suggested explanation is given on the basis of the band structure of LaNi5 and LaNisHT, recently marked out by Michele Gupta [7] and reproduced in Fig. 3. The marked difference in the DOS for the parent and hydrogenated material is that whereas in LaNi5 the Fermi level lies at the falling trend of DOS, in LaNisH7 it is at the increasing trend of DOS (see figure).

The variation of resistivity as observed to the present

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investigation is compatible with the above two facts. The region BC, namely the variation of resistivity with hydrogenation at the initial stages, where very little hydrogen has diffused in and only metal hydrogen (M-H) interaction becomes effective corresponds to the DOS region XY in Fig. 3 where the DOS decreases. This should cause the decrease in the resistivity as in the observed case (BC of Fig. 1). The range CD (increase in the resistivity) is also compatible with the range YZ of the band structure where the DOS increases. This corresponds to the region where significant hydrogen has diffused in and besides M-H interaction t{-H interaction also becomes effective. The conductivity (resistivity) would therefore get reduced (increased) drastically. Consequently the 0 (CD of Fig. 1 ) increases and attains saturation with time.

Figure 2 shows the ~2-t behaviour of the thin fihn of LaNi~ which is different from that of thick film. It may be mentioned that due to the dominant role of surface scattering in thin films, the resistivities of thin films are always higher (several times to a few orders of magni- tude) than thick film. in the present case the resistivity of thin film is approximately three times higher than that for thick flinT. From the figure it is apparent that the resistivity decreases from B' to C' thereafter it attains a saturation C 'F ' . The decrease in the resistivity of LaNi~ thin film like thick film presumably originates due to M-H interactions in which the hydrogen atoms interact dominantly with metal atoms in the intermetallic matrix. This corresponds to the situation of low hydrogen concentration region (BC region of Fig. I). Due to the lower density of La:O3/Ni interfaces and lower hyd- rogen concentration in the solid solution, there is a highly decreased probability of interaction among the H atoms, i.e. H-H interaction. So the resistivity in Fig. 2. (B 'C ' ) decreases initially and never increases thereafter. This is because unlike in the thicker films the H-H interaction responsible for the variation in DOS leading to an increase in resistivity (region CD of Fig. 1) will not switch on in the thin film case. The slow rate of decrease of resistivity with time may be due to the high oxide coverage, leaving a lower density of channels for hydrogen diffusion. Since the density of channels is known to be the rate controlling factor, the switching of M H interaction in the film would be accordingly slow. After an optimum hydrogen uptake, the resistivity decrease would attain a saturation value. Similar inves- tigations have been carried out on MmNi5 films as well. The observed behaviour is intelligible on the basis of the above explanation.

3. DTA STUDIES ON RNi5 HYDRIDES

The present investigation was undertaken with a view to obtain information on thermodynamical properties of the RNi5 and its related phases. There has been great interest in the thermodynamical properties of interme- tallic hydrides because of their importance in various

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DTAcurves for the dehydriding behavioursof Mm Ni4 5AI0.sHx ,~ 73°C Ii ii :, 75oC

Mm Ni 45AI05 Hx

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Fig. 4. I)TA thermogram of the hydrogen containing MmNi4 ~AI ~.

(West Germany) up to 650°C with a heating rate of 3.3°C rain -~. From Fig. 4 it can be noticed that initially the hydrogen loss from MmNi4.5Al0.5 began at 28°C and proceeded until the total hydrogen has been desorbed. The decomposition of the hydrides were seen by the endothermic peak, evident in the DTA curve (see Fig. 4). The temperature of endothermic reactions were observed at 73 and 75°C for material supplied from Ergenics (U.S.A.) and laboratory synthesized MmNi4 >AI, 5, respectively. The two exothermic peaks at 307, 310 and 490,495°C in the DTA curve in Fig. 4 are presumably due to the complete dehydrogenation of MmNi~.sAIo.5 resulting in the crystallization [17] of the amorphous phase.

4. APPLICATIONS OF MmNi4 sAl, 5 HYDRIDE AS A H Y D R O G E N / H Y D R I D E F U E L FOR TWO

WHEELER ROAD TRANSPORT

One of the ainls of the studies of the synthesis of rare carth based pentanickclides was to use its hydride as a sourcc of hydrogen for fuelling the I.(7. engine of a common Indian road transport vehicle,

Kecping in view the fact that a 4 stroke 1.C. engine is easier to adopt for hydrogen fuel, we selected a 4 stroke 1{)0 c.c., 1 H.P. two wheeler motorcycle for conversion to run on hydrogen fuel.

The hydride tank consisted of six, 1" diameter, 1 ~/a' length aluminium hollow rods joined together at the

technical purposes as well as for thc physical under- standing of metal hydrogen reactions. Numerous ex- 10 perimental [2, 12] investigations on the thermodynamic aspects of interaction of hydrogcn with metals, alloys 9 and intermetallic compounds have been reported. Ex- perimentally the thermodynamic quantities can be 8 obtained from pressure-composition isotherms (p-c isotherm) [13] or from the related equivalent approach ~ 7 of electrode potential composition temperature rcla- =

8 tionship or from heat capacity measurements [14] or o from calorimetric methods [151. Shilov et al. [16] have -fi m S suggested an alternative method based on an approxi- mate enthalpy balance equation in conjunction with g diffcrential thermal analysis (DTA). Motiwlted by' the ~ 4

e> foregoing factors we have studied the thermodynamical :~ 3 behaviour of the RNi5 (MmNi4.sAI .s) phase.

DTA thermograms of the hydrogen containing 2 MmNi4 sAl~ sH, (2 ~< x ~< b) were obtained and the results are shown in Fig. 4. The samples used in the present study were either synthesized in the laboratory or were the corresponding materials synthesized by o Ergenics (Wyckoff, New Jersey). Characterization of the samples were carried out with the help of X-ray diffractometer (Philips-PW1710) using CuK radiation and hydrogen contents of the samples were determined by p-c isotherms (Fig. 5). DTA were performed under 1 Fig. 5. atm argon pressure using DTA equipment L62, Linseis

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Pressure-hydrogen concentration isot hcrlllS MmNi4 ~AI¢ ~ (l~C desorption isotherm).

t)f

RNi 5 TYPE MATERIALS 577

the help of a cam injector system. The total weight of hydride employed by us is about 5 kg. The range of the vehicle is about 8 km at present. It is proposed to utilize twin and improved hydride tanks to achieve a higher vehicle range (~20 kin) and results on this aspect will be forthcoming.

Acknowledgements--The present work was carried out under a Dcpl. of Non-Conventional Energy Sources (Government of India) sponsored project. Helpful discussions with Prof. A. R. Verma, Prof. V. G. Bhide, Prof. M. V. C. Sastri and Prof. A, P. B. Sinha are gratefully acknowledged. One of the aulhors (K. Ramakrishna) is grateful to CSIR, New Delhi for financial support.

Fig. 6. Hydride/hydrogen fuelled motorcycle developed by DNES research group at the Physics Department, B.H.U., India. The basic fuel MmNi4,AI os (H,) synthesized by the research group is mounted inside a special hydride tank (see tcxt for details), The hydride lank is mounted in place of the exhaust-silencer tank of the conventional vehicle and serves as a hydride container cure silencer. The hydride gets automatical- ly heated by the exhaust gas. The tank carries up to about 5 kg of hydride. The motorcycle has actually been test driven on

road and presently has a range of about six k.

hydrogen inlet and also exhaust outlet points. The rods were mounted inside another aluminium envelope (8" diameter , 3' long) (see Fig. 6). The hollow aluminium cylinders were filled with synthesized and pretested M m N i 4 5 A I ~ storage alloy. In several cases modified mischmetal (enriched up to 10% weight by La and Nd) pentanickelide Mm'Ni45AI05 were employed. The hyd- rides were obtained by hydrogenating the pentanick- elides through the hydrogen inlet port on the hydride tank (see Fig. 6). The hydride tank was mounted at the same place where the exhaust-silencer is mounted in the conventional vehicle (see Fig. 6). One of the advantages of such a mount ing would be that the heating of the hydride tank up to a tempera ture of about 80°C, necessary to ensure smooth and continuous supply of hydrogen to the engine through hydride desorption, would take place automatically by the hot exhaust gas passing through the envelope tank. The envelope serves as a kind of exhaust-cum silencer. Hydrogen emanat ing from the hydride tank is injected into the IC engine with

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