ht. J. Hydrogen Energy! Vol. 10. No. l/s. pp. 523-529, 1985. Printed in Great Britain.

0360-3199/85 53.00 + o.cul Pcrgamon Press Ltd.

@ 1985 International Association for Hydrogen Energy.

INVESTIGATIONS ON THE STRUCTURAL AND HYDROGENATION CHARACTERISTICS OF LaN&, HoNiS, GdNis, SmN&, MmNis AND

CFMmNi4.sAlo.s THIN FILMS

S. K. SINGH, A. K. SINGH, K. RAMAKRISHNA and 0. N. SRIVASTAVA

Physics Department, Banaras Hindu University, Varanasi-221005, India

(Receioed for publication 4 October 1984)

Abstract-This communication deals with the investigations made on synthesis, structural characteristics, hydrogen absorption behaviour and electrical resistivity variation with hydrogenation in thin films of rare earth metal pentanickelides-RN& where R = La, Sm, Gd, Ho and Mm (Mischmetal) and a related phase cerium-free MmNi.sAk,.s. The synthesis of the above mentioned intermetallia was accomnlished bv suitabk solid-state interdiffusion and homogenization. The as-synthesized flux were characterized by k-ray dii&ion. The thin glms prepared by vacuum thermal vapour deposition were characterized by transmission electron microscopy in various modes. These films were amorphous and transformed to crystalline form on annealing. The hydrogen absorption behaviour (both in bulk and thin films) was studied by employing electrolytic method. It was found that amorphous forms absorb more hydrogen than their crystalline counterparts. The variation of electrical resistivity with hydrogen absorption in thin-film form of various intermetallics was also studied. Plausible explanations for the above hydrogenation characteristics have been outlined.

With the recognition of the fact that hydrogen can be

INTRODUCT’ION

an effective substitute for fossil fuels, there has been a significant spurt in the studies seeking to investigate various aspects of hydrogen storage [l-3]. It has been established that storage in the form of metal/interme- tallic hydrides has several special characteristics which are more suitable for storage media than liquid and gas storage systems [4,5]. The hydrides can store hydrogen at densities higher than either liquid or solid hydrogen [6]. Yet another remarkable feature of hydrides is that these not only store hydrogen but thermal energy as well (71. Generally the intermetallics for hydrogen stor- age correspond to materials which can absorb hydrogen reversibly, exothermically, non-stoichiometrically and without imposing any stringent condition with regard to the purity of the absorbing material or the gas.

It is well-established by now that the various hydrogen absorbtion features are crucially influenced by the elec- tronic band and crystal structure characteristics of the intermetallics [g-lo]. Both these can be significantly different in thin film forms than their corresponding bulk counterparts. Thus, investigations of hydrogena- tion and its correlation with electronic and structural behaviours in thin films are of obvious importance. In passing it may be mentioned that in spite of their obvious importance, studies covering thin films are rather sparse. The present communication embodies investi- gations of thin films of rare-earth pentanickelides in regard to their structural and hydrogenation behaviours. Variation of structural and electronic characteristics as a result of hydrogenation have also been studied. Some important results emerging from the present investi- gations relate to the fact that the thin films (as deposited) unlike bulk forms are amorphous and that this amor-

phous form has a higher hydrogen capacity. In order to have some insight into the details of the hydrogen- ation process and also the variation of electronic band structure, investigations on variation of electrical con- ductivity, as a function of time, on hydrogenation were also carried out.

EXPERIMENTAL TECHNIQUES AND RESULTS

Synthesis and electron microscopic characterization of RN& and related phase

The rare-earth metal pentanickelides (RN&, R = La, Sm, Gd, Ho, Mm) and CFMmNir.5Alo..c were syn- thesized by sealing the stoichiometric mixture of rare- earth metal and Ni in silica tube in a vacuum ( 10e6 torr) and then melting the mixture with the help of RF (radio frequency) induction furnace. The solid ingot so formed was remelted several times and then homogenized at a temperature of 900°C for about 12 h. whereas the RN& (R = Sm, Gd, Ho and Mm) were synthesized in the laboratory, the LaNis and CFMmNi4.5Alo.s corre- sponded to the material synthesized by ERGENICS (Wyckoff, New Jersey). The agglomerates produced in this way were checked by X-ray diffraction using a Guinier focussing camera in transmission mode. This characterization revealed that the synthesized material corresponded to the known CaCus-type structural phase. Figures l(a,b,c,) show characteristic Guinier photographs of the alloys LaN&, SmNi5 and MmNis. respectively. The indexing of the X-ray diffraction lines is indicated. Thin films (thickness ranging between 300 and 1000 A) were prepared by the thermal evaporation technique in a vacuum of the order of 10e6 torr.

The thin films prepared as mentioned above were investigated by employing the techniques of electron

523

524

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S. K. SINGH et al.

microscopy in various modes viz transmission imaging and diffraction, surface scanning and analytical-EDAX. The results obtained therefrom are described below:

(1) The thin.films grown were invariably found to be amorphous. This was derived from the electron micro- scopic characterization--the microstructures were always featureless and the diffraction patterns contained haloes [11]. Representative examples of these are brought out by Figs 2(a) and 2(b). In some cases micro- crystals embodied in the amorphous matrix were also observed. An illustrative example of this is manifested through Fig. 3. Scanning transmission electron micro- scopic mode was employed to investigate the surface characteristics of the films. Figure 4 reveals a represen- tative example of surface structure of the LaNis film.

2(:; 2C

21

3(]

3C

Fig. 1. Guinier X-ray powder diffraction patterns for the syn- thesized (a) LaNis, (b) SmNis and (c) MmNis. Several powder

lines have been indexed.

Fig. 2. (a) Transmission electron micrograph of the CFMmNi4.sAJo.5 thin film, bringing out the amorphosity of the film. (b) Electron diffraction pattern of the thin film of CFMmNi4.sAIo.5 exhibiting haloes and hence revealing the

amorphosity.

INVESTIGATIONS OF RARE-EARTH PENTANICKELIDES 525

Fig. 3. Transmission electron micrograph of the thin film of LaNi5 showing the formation of an isolated crystal.

genation treatment, no such investigation seems to be available to date. In view of this an attempt was made to monitor the changes in the mierostruetural charac- teristics ensuing from the hydrogenation of RNis. The results of such a study are outlined in Figs 5(a) and (b). These figures bring out the microstruetural character- istics of LaNi5 before and after hydrogenation. As can be noted these figures reveal the interesting result that the hydrogenation leads to enhancement of amorphosity which has been brought out by the decrease of the density of the tiny scattering regions. rhis is expected since the hydrogen absorption presumably distorts the local bondings.

(4) The thin films transformed to the crystalline phase when heated by the focussed electron beam at about 6000C for about 5-15 s. Figure 6 shows the consequence of electron-beam heating in situ. The diffraction pattern from the transformed phase exhibited the powder rings corresponding to RNis. The powder diffraction patterns for SmNi5 and MmNi5 are shown in Figs 7(a,b), respec- tively. The indices of few reflections are also shown.

It can be noted that the surface is not planar but instead contains several protrusions. In passing it may be men- tioned that surfaces of these alloys play a very crucial role in the hydrogenation characteristics and there is a need to investigate the details of the surface structure more extensively than has been done hitherto.

(2) Analytical electron microscopy embodying EDAX revealed that the chemical composition in gen- eral corresponded to RNis. In some eases where the film was found to be slightly nickel deficient, the RNi5 composition was restored on annealing.

(3) In spite of the obvious importance of the possible variation in microstructure resulting from the hydro-

Fig. 4. Scanning transmission electron micrograph of LaNi~ bringing out the surface feature of the thin film.

i Fig. 5. Transmission electron micrographs of the LaNi5 thin films (a) before hydrogenation and (b) after h~,drogcnation

(see text).

526 S. K. SINGH et ~.

(a)

12

102 101

Fig. 6. Transmission electron micrograph corresponding to the transformed crystalline phase CFMmNh.sAIo.5 thin film con-

sequential to electron beam heating.

The fact that amorphous to crystalline transformation leads to the formation of known CaCus-type phase evidences that the films were indeed RNis-type phases.

Hydrogenation characteristics of the present RNi5 and related phase

An interesting outcome of the extensive studies in regard to the structural characteristics of the thin films employing TEM techniques corresponds to the fact that it is possible to have genuine amorphous phases of RNis. It appears that such thin films of RNis phases have not been reported earlier. It would thus be of obvious importance to investigate the hydrogenation characteristics of this amorphous phase and to establish the differences in this as compared to the known behav- iour of their crystalline counterparts.

In view of the special form of the material the geo- metrical configuration being that of thin film; two dif- ferent techniques were resorted to bring out the hydro- genation behaviour of the films. One of these was to monitor the hydrogenation behaviour through an elec- trolytic cell [12, 13]. The other corresponded to the monitoring of the hydrogen characteristics indirectly through the variation in the important electronic characteristic--the resistivity----commencing from the hydrogenation of the as grown thin films.

(a) Low-pressure hydrogen absorption character- istics: This was accomplished by employing the electro- lytic cell [12, 13] consisting of platinum electrodes. The electrolytic solution was distilled water made conducting by adding sulphuric acid in traces. On electrolysis (d.c. voltage ~ 10 V, distance between electrodes ~ 10 cm), the hydrogen and oxygen liberated at the cathode and

(b)

]-,,2 --102 --I01

Fig. 7. The electron diffraction pattern of the transformed phases of (a) SmNis (b) MrnNi5.

anode, respectively, were in the ratio of 2:1. In order to investigate the low-pressure absorption, known quan- tity of rare-earth pentanickelides were placed at the cathode. The quantity of absorbed hydrogen was esti- mated by the following relation:

Volume of hydrogen absorbed = 2 x vol. of Oz liberated at the anode.

- vol. of H2 liberated at the cathode.

The above procedure was carried out with all the RNi5 and CFMmNi4.sAI0.5 in the thin film (as grown) and bulk (synthesized and quenched) forms. Some of the illustrative results of this investigation are shown

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Fig. 8. Hydrogen absorption in: (a) thin film (amorphous) of GdNis; (b) synthesized bulk (polycrystalline) GdNis (c) thin film (amorphous) of HoNis; (d) synthesized btflk (polycrys- talline) HoNi_~; (e) synthesized bulk (polycrystalline) MinNie;

and (f) quenched synthesized bulk MmNis.

528 S. K. SINGH et al.

in Figs 8 (a-f). The salient features of these are described below.

The total absorption per unit mass of RNis in thin films (amorphous) is comparatively more than the cor- responding bulk (polycrystalline) counterparts (see Figs 8(a--d)). Since the amorphous thin films cannot be pre- pared easily in large amounts, it was thought to simulate the amorphous structure in bulk. Tfiis was accomplished by the rapid quenching of the melted RNi5 bulk ingot. The quenching was done in liquid nitrogen by dropping vacuum-sealed quartz capsule containing the melted RNi~ flux. X-ray diffraction characterization of the quenched materials revealed that they did not contain sharp Bragg reflection and had characteristics rep- resenting phases varying from microcrystalline to amor- phous. The hydrogenation characteristics of the quenched phases were also carried out in the present investigation. A striking characteristic of this study is that the amorphous and nearly amorphous phases have larger hydrogen absorption capacity as compared to their crystalline counterparts [14]. This appears to be an important characteristic. The larger capacity of amorphous phase is presumably due to the possibility of larger number of voids per unit volume for the occupation of hydrogen. This is because in the amor- phous phase the positions of various atoms are less strictly defined than in the crystalline phase.

(b) Resistivity variation on hydrogenation: as indi- cated earlier hydrogenation effects in these films were also monitored through the resistivity variation of the films [15]. This was accomplished by measuring the resistivity of the thin films of CFMmNi~.sA10.s, LaNi5 and MmNis deposited on glass slides. The resistivity variations were found to be similar in nature for all the intermetallic phases studied in the present investigation. A representative example of the resistivity variation with time on hydrogenation at 200 psi is shown in Fig. 9 for the case of CFMmNi4~AI05. The 'p--t' curves in other systems such as LaNis, MmNis, etc. were very similar to the one shown in Fig. 9. It can be seen that the resistivity increases slightly when the thin films are exposed to hydrogen (AB of Fig. 9). This initial rise is followed by a curious resistivity region where it decreases (BC). It then rises again and obtains a satu- ration value (EF of Fig. 9).

As is well known, 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 as a result of hydrogenation [10, 16]. In the present study also we can invoke the effect of variation in DOS due to hydrogenation to explain the curious characteristics of the variation of resistivity with time (see Fig. 9). We can take the low (PQ), medium (QR) and high (RS) time interval regions as the low, medium and high concentration regions of hydrogen in the starting intermetallic phase. The pres- ence of low hydrogen concentration is known to let the initial DOS structures of the parent intermetallic phase remain nearly unchanged. Thus in this region the elec- trons from hydrogen will fill the unoccupied states near

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Fig. 9. Resistivity variation on hydrogenation CFMmNh.sAIo.5 thin-film at 200 psi.

of

the Fermi level and the DOS would be reduced and the resistivity should increase. We believe the initial rise in the resistivity (see Fig. 9, region AB) is due to this reduction in DOS. In the medium, hydrogen concen- tration region corresponding to medium time interval region in our p--t curves, the band structure starts chang- ing due to onset of M-H interaction and the reduction in the unit-cell volume. The DOS gets lowered down on energy scale and the new structure in the DOS providing additional states for conduction starts appear- ing. In this region the conductivity (resistivity) is expected to increase (decrease). The drop in resistivity in the curve (see Fig. 9 region BC) is expected to be due to the forgoing reasons. As the hydrogenation proceeds further we enter the high hydrogen-concen- tration region. Here the M-H, H-H interactions and the reduction in the unit cell volumes affect the DOS drastically. It is known that these changes incorporate considerable reduction in the density of energy states available for conduction and the changes may be thought of leading to a situation where electronic con- duction would require interband transition [16]. The conductivity (resistivity) would therefore get reduced (increased) drastically. This is thought to be the reason for the significant rise in resistivity (see Fig. 9 region DE) corresponding to high hydrogen-concentration region. The resistivity variation in the present case shows a curious characteristic which reflects the onset, progress and near completion of the hydrogenation process.

INVESTIGATIONS OF RARE-EARTH PENTANICKELIDES 529

CONCLUSIONS 2. D. G. Westlake, C. B. Satterthwaite and J. H. Weaver.

(1) The thin films (as grown) prepared through vac- uum thermal vapour evaporation are invariably amorphous.

(2) These films underwent amorphous--~ crystalline transformation on annealing.

(3) The hydrogen absorption capacity of these thin films (amorphous) is comparatively higher than the corresponding bulk (crystalline) counterparts. Quenched bulk forms (varying from amorphous to microcrystalline) also have higher hydrogen absorption capacity as compared to the crystalline phase.

(4) Hydrogenat ion leads to a curious variation of electrical resistivity of these thin films. This variation is thought to arise due to changes in the density of energy states consequent to hydrogen absorption.

Acknowledgements--The authors are grateful to Professor A. R. Verma and Professor M. V. C. Sastri for their interest in the work. They are thankful to Dr G, D. Sandrock, Dr S. Suda, Dr G. G. Libowitz and Dr A. R. Miedema for useful discussions. The present work was supported financially by the Department of Non-Conventional Energy Sources (New Delhi), Government of India.

R E F E R E N C E S

1. Proc. World Hydrogen Energy Conference IV, Pasadena, California, U.S.A. 13-17 June (eds T. N. Veziroglu, W. D. Van Vorst and J. H. Kelly), Vol. 3, Ch. 7, Pergamon Press, Oxford (1982).

Physics Today November issue, pp. 32-39 (1978). 3. G. G. Libowitz and A. J. Maeland "Hydrides" in Hand

Book on the Physics and Chemistry of Rare Earths (eds K. G. Gsehneider Jr and L. Erying), Vol. 3, Ch. 26 (1979).

4. Hydrogen in Metals (eds G. Alefeld and J, Volkl). Sprin- ger, Berlin (1977).

5. K. H. J. Buschow, P. C. P. Bouten and A. R, Miedema, Rep. Prog. Phys. 45,937-1039 (1982).

6. G. D. Sandrock, Proc. 2nd World Hydrogen Energy Con- ference, p. 1625 Zurich. Pergamon Press, OxfOrd (1978).

7. S. Suda and Y. Komazaki, J. Less. Com. Metals 89, 127-132 (1983). K. A. Gschneider Jr, T. Takeshita, Y. Chung and O. D. McMasters, J. Phys. F, 12, L1-6 (1982), W. E. Wallace and E. B. Boltich, J. Sol. Chem. 33, 435--437 (1980). A. C. Switendick, Solid State Commun. 8, 1463-1467 (1970). Ajay Kumar Singh, Anand Kumar Singh an d O. N. Sri- vastava, J. Less. Common Metals, 88, 97--105 (1982). Ajay Kumar Singh, Ph.D. Thesis, Banaras Hindu Uni- versity, Varanasi, India. (December 1983). K. N. Rai, S. Gupta, R. Rani and J, Kumar, Proc. World Hydrogen Energy Conference IV, Pasadena, California, U.S.A. (eds T. N. Veziroglu, W. D. Van Vorst and J. H. Kelley), Vol. 3, pp. 1245--1254, Pergamon Press, Oxford (1982). K. Suzuki, J. Less. Com. Metals 89, 183-195 ~(1983). G. Adachi, K. Niki, H. Nagai and J. Shiokawa, J. of Less. Corn. Metals 88, 213-216 (1982). J. H. Weaver and D. T. Peterson, J. Less. Com. Metals, 74, 207-216 (1980).

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14, 15,

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