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Metal hydride thin film electrodes prepared by r.f. sputtering
Journal of Alloys and Compounds, 192 (1993) 182-184 JALCOM 2136
182
Metal hydride thin film electrodes prepared by r.f. sputtering T. Sakai, H . Y o s h i n a g a , H . M i y a m u r a , N. K u r i y a m a , H. I s h i k a w a a n d I. U e h a r a Government Industrial Research Institute, Osaka, Midorigaoka 1-8-31, Ikeda, Osaka 563 (Japan)
Abstract T h i n films of m i s c h m e t a l - b a s e d h y d r o g e n storage alloys were p r e p a r e d by r.f. sputtering. It was f o u n d t h a t m a n g a n e s e c o n t a i n e d in t h e alloy significantly c h a n g e d t h e crystallographic a n d e l e c t r o d e p r o p e r t i e s in good a g r e e m e n t with t h e results o b t a i n e d for the b u l k alloys.
1. Introduction
Mischmetal (Mm) based hydrogen storage alloys are very useful negative electrode materials for a nickel-metal hydride (Ni-MH) battery. The solidification rate of the melted alloy greatly influenced the electrode performance [1]. Very rapid solidification such as vapour deposition of LaNi5 [2] gave an amorphous alloy which was not pulverized by the hydrogen absorption. We [3] have conducted electrochemical characterization on thin films of LaNi5 and LaNi2.sC025 which were prepared by magnetron r.f. sputtering. In the work reported in this paper, thin films of mischmetal-based alloys MmNi3.sCOo.TAlO.S and MmNi3.sCoo.sMno.4Alo.3as usual MH electrode materials were prepared by r.f. sputtering in order to examine the effect of solidification rate on crystallographic and electrode properties.
nation of the film was conducted by X-ray diffraction. Electrode properties were measured for the 2--4 /zm thick films (area, 7 cm 2) deposited on the nickel sheet in the current range from 0.2 to 1 mA and at a cutoff voltage of - 0 . 6 V v s . Hg-HgO (6 M KOH).
3. Results and discussion
Table 1 shows alloy compositions (by weight per cent) of target alloy and film alloy. The EPMA indicated a higher nickel content than the ICP analysis because the analytical depth in EPMA was only several micrometres on the surface, confirming that the nickel was enriched on the surface region which suffered from oxidation. More nickel-enriched alloy was formed by sputter deposition. Charge--discharge cycles caused further enrichment of nickel because the oxidized deT A B L E 1. Alloy compositions of targets and thin films on nickel sheet for MmNi3.sCO0.TA10.8 (A) and MmNi3.5Co0.sMn0.4A10.3 (B)
2. Experimental details
Thin films were prepared with a magnetron r.f. sputtering machine under a 0.6 Pa atmosphere of argon gas at an r.f. power of 400 W on substrates of soda-lime slide glass or nickel sheet [3]. Alloy plates (100 mm in diameter, 5 mm in thickness) of MmNi35Co0.7Alo.8 (A) and MmNi35Coo.sMno4Alo.3 (B) were used as targets. The chemical compositions of the targets were examined by inductively coupled plasma (ICP) emission spectroscopy analysis. The deposition rate was about 1200/~ min-1 under the above conditions. The surface morphology of the deposited film was observed by scanning electron microscopy (SEM). The alloy compositions of the films were examined by electron probe X-ray microanalysis (EPMA). Crystallographic exami0925-8388/93/$6.00
Alloy composition (wt.%) La
Ce
Pr
Nd
Ni
Co
Mn
A1
Target A a 7.94 Target A b 8.52
18.04 16.69
2.22 1.01
5.94 5.63
50.48 54.71
10.04 10.37
5.32 3.07
Film A b Film A c
5.17 4.35
12.57 10.18
1.32 0.34
5.41 3.84
59.94 64.45
12.33 13.41
3.26 3.04
Target B a Target B b
7.50 7.91
16.69 17.05
2.10 1.19
5.68 5.64
49.40 51.80
11.30 11.05
5.38 3.05
1.95 2.31
Film B b Film B c
6.42 5.88
14.84 15.79
0.82 0.94
5.73 4.64
54.84 57.54
11.15 9.84
4.78 4.35
1.41 1.02
aValues for bulk alloys by ICP analysis. bValues for alloys and thin films by EPMA, i.e. values on the surface region to a depth of several micrometres. ~Values for thin films after cyclic tests by EPMA.
© 1 9 9 3 - Elsevier Sequoia. All rights reserved
T. Sakai et al. / Metal hydride thin film electrodes
composition of the alloy could proceed. Crystallographic data for the film alloys are summarized in Table 2 in comparison with those of powder alloys. Film A exhibited no sharp peak on X-ray diffraction patterns, suggesting an amorphous structure as shown in Fig. 1 (a). With increasing number of charge-discharge cycles the amorphous film was more crystallized. This behaviour is in good agreement with the previous result [4] for the bulk alloy MmNi3.fCO0.Tmlo. 8 that the X-ray diffraction peaks became sharper after cyclic tests. The hydrogen absorption-desorption would facilitate atomic T A B L E 2. Lattice constants a and c and cell volume V of bulk alloy and thin film for MmNi3.fCo0.TAlo.8 (A) and MmNi3.sCo0.sMn0AAl0.3 (B) Sample
a (/~)
c (/~)
V(/~ 3)
Alloy A"
4.070
87.02
Film A c
4.969 Amorphous 4.888
4.228
87.47
Alloy B a Film B b Film B ~
4.984 4.990 4.964
4.063 4.081 4.075
87.41 88.02 86.95
Film A b
183
rearrangement, releasing the lattice strain. We have called this behaviour "hydrogen-induced homogenization". On the contrary, film B was obtained as only a polycrystalline alloy as shown in Fig. l(b). This is in good agreement with the result for bulk alloy [4] that the very quick nucleation of manganese caused only the equiaxial structure in the ingot. This feature distinguishes the manganese-containing alloy from the former alloy. The charge-discharge cycles caused a decrease in cell volume (Table 2) because the aluminium and manganese were dissolved out from the film alloy into the KOH solution (Table 1). The discharge capacity of film A (42 mA h g-t) at 0.2 mA was less than half that of film B (about 110
22 1.O
10mA (12mAh/g)
0.5
"Values for powder alloys prepared by induction melting. bValues for film alloys deposited on nickel sheet. ~¢'alues for film alloys after cyclic tests.
~"""
(}SmA (27mAh/g
MmNi3 5Co0 7AI0. 8 1
0 2n/,\ (42n/,\h,'g)
[~i[111( 411 I]1)
2
3 4 Disuhatgc linlu / Ii
{a)
5
6
r 8k
MmNi3.5Coo.7Alo.a
(111)
after cycle test ~j ~2
1.0
E 2
0.5
(100)
1 ()mA (44nlAh/gl
( I H)m,\h/!!)
05mA 1(~] nlAll/gl
4k e~fter
cycle test
MmNi~ SCoU 8Mn0.4AI0. 3 Film {2pml 0
1
2
(b)
before eyete test
3
4
5
Di~d~:.g~
6
7
B
9
,c, h
Fig. 2. Discharge curves at 0.2, 0.5 and 1.0 m A for (a) film A and (b) film B. a
10
40~
20
20
60
70
10k
(111) after cyde
M mNh.sCoo.aMno.4AIo.3
test
NI (200) (100)
(101)
(110)
(002)201
I I]
(211) (202) (3011
5k before cycle test
lO
20
40
20
60
70
Fig. 1. X-ray diffraction patterns before and after cyclic tests for (a) film A and (b) film B.
a
Mm Ni35Coo 7A10.8
b
Mm Ni~ sC00 eMno ~P-Io3
Fig. 3. SEM images of thin films after cyclic tests for (a) film A (MmNia.fCOo.TAl0.8) and (b) film B (MmNi3.sCo0.sMn0.4Al0.3.
184
T. Sakai et al. / Metal hydride thin film electrodes
mA h g-a) as shown in Fig. 2. Figure 3 compares surface morphology of the electrodes after 20-30 cycles. Film B had many cracks on the surface in good agreement with the previous result [5] that the manganesecontaining alloys were more easily pulverized by the hydrogen absorption-desorption cycles. The pulverization would facilitate the dissolution of aluminium and manganese into the KOH solution (Table 1). On the contrary, very few cracks were observed for the film A, depressing the dissolution of the aluminium after cyclic tests (Table 1). The higher capacity of film B could be ascribed to the higher electrode area than that of film A. The capacities of the thin films (film A, 42 mA h g-a; film B, 110 mA h g-a) were considerably smaller than those of powder alloys (alloy A, 250 mA h g-a;
alloy B, 270 mA h g-a), which might be ascribed to smaller reaction area of the thin film than that of the powder.
References
1 T. Sakai, T. Hazama, H. Miyamura, N. Kuriyama, A. Kato and H. Ishikawa, J. Less-Common Met., 172-174 (1991) 1175. 2 G. Adachi, K. Niki and J. Shiokawa, Z Less-Common Met., 81 (1981) 345; 88 (1982) 213. 3 T. Sakai, H. Ishikawa, H. Miyamura, N. Kuriyama, S. Yamada and T. Iwasaki, J. Electrochem. Soc., 138 (1991) 908. 4 T. Sakai, H. Yoshinaga, H. Miyamura, N. Kuriyama and H. Ishikawa, Z Alloys Comp., 180 (1992) 37. 5 T. Sakai, K. Oguro, H. Miyamura, N. Kuriyama, A. Kato, H. Ishikawa and C. Iwakura,J. Less-Common Met., 161 (1990) 193.
182
Metal hydride thin film electrodes prepared by r.f. sputtering T. Sakai, H . Y o s h i n a g a , H . M i y a m u r a , N. K u r i y a m a , H. I s h i k a w a a n d I. U e h a r a Government Industrial Research Institute, Osaka, Midorigaoka 1-8-31, Ikeda, Osaka 563 (Japan)
Abstract T h i n films of m i s c h m e t a l - b a s e d h y d r o g e n storage alloys were p r e p a r e d by r.f. sputtering. It was f o u n d t h a t m a n g a n e s e c o n t a i n e d in t h e alloy significantly c h a n g e d t h e crystallographic a n d e l e c t r o d e p r o p e r t i e s in good a g r e e m e n t with t h e results o b t a i n e d for the b u l k alloys.
1. Introduction
Mischmetal (Mm) based hydrogen storage alloys are very useful negative electrode materials for a nickel-metal hydride (Ni-MH) battery. The solidification rate of the melted alloy greatly influenced the electrode performance [1]. Very rapid solidification such as vapour deposition of LaNi5 [2] gave an amorphous alloy which was not pulverized by the hydrogen absorption. We [3] have conducted electrochemical characterization on thin films of LaNi5 and LaNi2.sC025 which were prepared by magnetron r.f. sputtering. In the work reported in this paper, thin films of mischmetal-based alloys MmNi3.sCOo.TAlO.S and MmNi3.sCoo.sMno.4Alo.3as usual MH electrode materials were prepared by r.f. sputtering in order to examine the effect of solidification rate on crystallographic and electrode properties.
nation of the film was conducted by X-ray diffraction. Electrode properties were measured for the 2--4 /zm thick films (area, 7 cm 2) deposited on the nickel sheet in the current range from 0.2 to 1 mA and at a cutoff voltage of - 0 . 6 V v s . Hg-HgO (6 M KOH).
3. Results and discussion
Table 1 shows alloy compositions (by weight per cent) of target alloy and film alloy. The EPMA indicated a higher nickel content than the ICP analysis because the analytical depth in EPMA was only several micrometres on the surface, confirming that the nickel was enriched on the surface region which suffered from oxidation. More nickel-enriched alloy was formed by sputter deposition. Charge--discharge cycles caused further enrichment of nickel because the oxidized deT A B L E 1. Alloy compositions of targets and thin films on nickel sheet for MmNi3.sCO0.TA10.8 (A) and MmNi3.5Co0.sMn0.4A10.3 (B)
2. Experimental details
Thin films were prepared with a magnetron r.f. sputtering machine under a 0.6 Pa atmosphere of argon gas at an r.f. power of 400 W on substrates of soda-lime slide glass or nickel sheet [3]. Alloy plates (100 mm in diameter, 5 mm in thickness) of MmNi35Co0.7Alo.8 (A) and MmNi35Coo.sMno4Alo.3 (B) were used as targets. The chemical compositions of the targets were examined by inductively coupled plasma (ICP) emission spectroscopy analysis. The deposition rate was about 1200/~ min-1 under the above conditions. The surface morphology of the deposited film was observed by scanning electron microscopy (SEM). The alloy compositions of the films were examined by electron probe X-ray microanalysis (EPMA). Crystallographic exami0925-8388/93/$6.00
Alloy composition (wt.%) La
Ce
Pr
Nd
Ni
Co
Mn
A1
Target A a 7.94 Target A b 8.52
18.04 16.69
2.22 1.01
5.94 5.63
50.48 54.71
10.04 10.37
5.32 3.07
Film A b Film A c
5.17 4.35
12.57 10.18
1.32 0.34
5.41 3.84
59.94 64.45
12.33 13.41
3.26 3.04
Target B a Target B b
7.50 7.91
16.69 17.05
2.10 1.19
5.68 5.64
49.40 51.80
11.30 11.05
5.38 3.05
1.95 2.31
Film B b Film B c
6.42 5.88
14.84 15.79
0.82 0.94
5.73 4.64
54.84 57.54
11.15 9.84
4.78 4.35
1.41 1.02
aValues for bulk alloys by ICP analysis. bValues for alloys and thin films by EPMA, i.e. values on the surface region to a depth of several micrometres. ~Values for thin films after cyclic tests by EPMA.
© 1 9 9 3 - Elsevier Sequoia. All rights reserved
T. Sakai et al. / Metal hydride thin film electrodes
composition of the alloy could proceed. Crystallographic data for the film alloys are summarized in Table 2 in comparison with those of powder alloys. Film A exhibited no sharp peak on X-ray diffraction patterns, suggesting an amorphous structure as shown in Fig. 1 (a). With increasing number of charge-discharge cycles the amorphous film was more crystallized. This behaviour is in good agreement with the previous result [4] for the bulk alloy MmNi3.fCO0.Tmlo. 8 that the X-ray diffraction peaks became sharper after cyclic tests. The hydrogen absorption-desorption would facilitate atomic T A B L E 2. Lattice constants a and c and cell volume V of bulk alloy and thin film for MmNi3.fCo0.TAlo.8 (A) and MmNi3.sCo0.sMn0AAl0.3 (B) Sample
a (/~)
c (/~)
V(/~ 3)
Alloy A"
4.070
87.02
Film A c
4.969 Amorphous 4.888
4.228
87.47
Alloy B a Film B b Film B ~
4.984 4.990 4.964
4.063 4.081 4.075
87.41 88.02 86.95
Film A b
183
rearrangement, releasing the lattice strain. We have called this behaviour "hydrogen-induced homogenization". On the contrary, film B was obtained as only a polycrystalline alloy as shown in Fig. l(b). This is in good agreement with the result for bulk alloy [4] that the very quick nucleation of manganese caused only the equiaxial structure in the ingot. This feature distinguishes the manganese-containing alloy from the former alloy. The charge-discharge cycles caused a decrease in cell volume (Table 2) because the aluminium and manganese were dissolved out from the film alloy into the KOH solution (Table 1). The discharge capacity of film A (42 mA h g-t) at 0.2 mA was less than half that of film B (about 110
22 1.O
10mA (12mAh/g)
0.5
"Values for powder alloys prepared by induction melting. bValues for film alloys deposited on nickel sheet. ~¢'alues for film alloys after cyclic tests.
~"""
(}SmA (27mAh/g
MmNi3 5Co0 7AI0. 8 1
0 2n/,\ (42n/,\h,'g)
[~i[111( 411 I]1)
2
3 4 Disuhatgc linlu / Ii
{a)
5
6
r 8k
MmNi3.5Coo.7Alo.a
(111)
after cycle test ~j ~2
1.0
E 2
0.5
(100)
1 ()mA (44nlAh/gl
( I H)m,\h/!!)
05mA 1(~] nlAll/gl
4k e~fter
cycle test
MmNi~ SCoU 8Mn0.4AI0. 3 Film {2pml 0
1
2
(b)
before eyete test
3
4
5
Di~d~:.g~
6
7
B
9
,c, h
Fig. 2. Discharge curves at 0.2, 0.5 and 1.0 m A for (a) film A and (b) film B. a
10
40~
20
20
60
70
10k
(111) after cyde
M mNh.sCoo.aMno.4AIo.3
test
NI (200) (100)
(101)
(110)
(002)201
I I]
(211) (202) (3011
5k before cycle test
lO
20
40
20
60
70
Fig. 1. X-ray diffraction patterns before and after cyclic tests for (a) film A and (b) film B.
a
Mm Ni35Coo 7A10.8
b
Mm Ni~ sC00 eMno ~P-Io3
Fig. 3. SEM images of thin films after cyclic tests for (a) film A (MmNia.fCOo.TAl0.8) and (b) film B (MmNi3.sCo0.sMn0.4Al0.3.
184
T. Sakai et al. / Metal hydride thin film electrodes
mA h g-a) as shown in Fig. 2. Figure 3 compares surface morphology of the electrodes after 20-30 cycles. Film B had many cracks on the surface in good agreement with the previous result [5] that the manganesecontaining alloys were more easily pulverized by the hydrogen absorption-desorption cycles. The pulverization would facilitate the dissolution of aluminium and manganese into the KOH solution (Table 1). On the contrary, very few cracks were observed for the film A, depressing the dissolution of the aluminium after cyclic tests (Table 1). The higher capacity of film B could be ascribed to the higher electrode area than that of film A. The capacities of the thin films (film A, 42 mA h g-a; film B, 110 mA h g-a) were considerably smaller than those of powder alloys (alloy A, 250 mA h g-a;
alloy B, 270 mA h g-a), which might be ascribed to smaller reaction area of the thin film than that of the powder.
References
1 T. Sakai, T. Hazama, H. Miyamura, N. Kuriyama, A. Kato and H. Ishikawa, J. Less-Common Met., 172-174 (1991) 1175. 2 G. Adachi, K. Niki and J. Shiokawa, Z Less-Common Met., 81 (1981) 345; 88 (1982) 213. 3 T. Sakai, H. Ishikawa, H. Miyamura, N. Kuriyama, S. Yamada and T. Iwasaki, J. Electrochem. Soc., 138 (1991) 908. 4 T. Sakai, H. Yoshinaga, H. Miyamura, N. Kuriyama and H. Ishikawa, Z Alloys Comp., 180 (1992) 37. 5 T. Sakai, K. Oguro, H. Miyamura, N. Kuriyama, A. Kato, H. Ishikawa and C. Iwakura,J. Less-Common Met., 161 (1990) 193.