DSpace at VNU: Studies on AB(5) metal hydride alloys with Co additive

Physica B 327 (2003) 378–381

Studies on AB5 metal hydride alloys with Co additive
Bui Thi Hanga,*, Luu Tuan Taia,b, Le Xuan Quec, Mai Duc Hanha,
Nguyen Phu Thuya,b, Than Duc Hiena, Le Thi Hai Thanha
a

International Training Institute for Materials Science (ITIMS), ITIMS Building, Hanoi University of Technology,
1 Dai Co Viet Road, Hanoi, Viet Nam
b
Faculty of Physics, College of Natural Science, Viet Nam National University, 334 Nguyen Trai Road, Hanoi, Viet Nam
c
Institute for Tropical Technology, NCST, Hoang Quoc Viet Road, Hanoi, Viet Nam

Abstract
Some effects of Co additive on the magnetic and electrochemical properties of the alloy series La0.8Nd0.2Ni4.9ÀxCox Si0.1 (x ¼ 0:1; 0.5, 0.75, 1 and 1.5) have been studied. The results of magnetic measurements indicate that the
susceptibility (w) and the Curie-temperature (Tc ) of the samples increases with Co addition. The milling and the charge–
discharge process change the magnetic properties of the as-prepared samples. Electrochemical measurements show that
small additions of Co improve the performance of metal hydride alloy electrodes as charge transfer facilities.
r 2002 Published by Elsevier Science B.V.
Keywords: AB5 alloy; LaNi5; Metals hydride; Charge–discharge process; Charge transfer

1. Introduction
In the past few years, metal hydride battery
materials have become increasingly popular. The
principal materials used in these batteries are
LaNi5-based compounds. In order to obtain
optimum performance, other transition metals
and rare earth elements are substituted for Ni
and La, respectively. Sakai et al. [1] studied alloys
with different ternary solutes including Mn, Cr,

Al, Co and Cu, and found that the cycle-life was
improved in the order MnoNioCuoCroAloCo.
A substitution of the rare-earth elements such as
Nd and Ce may enhance the cycle-life. Schlapbach
[2] studied the magnetic properties of pure LaNi5
*Corresponding author. Tel.: +84-4-8692518; fax: 84-48692963.
E-mail address: (B.T. Hang).

and found that after hydrogen cycling Ni clusters
are formed on its surface. In this work, the effect
of partly replacing Ni by Co on the magnetic and
electrochemical properties of La0.8Nd0.2Ni4.9ÀxCoxSi0.1 compounds is investigated and the
obtained results are discussed.

2. Experiment
La0.8Nd0.2Ni4.9ÀxCoxSi0.1 (x ¼ 0:1; 0.5, 0.75, 1
and 1.5) compounds were prepared by arc-melting
under argon atmosphere. All starting materials
were of 99.9 wt% purity. The single phase of the
samples was checked by X-ray diffractometry.
Approximately 0.4 g of the powder was compacted
into a pellet (4 mm in diameter) by a pressure of
7000 kg/cm2 for electrochemical measurements.

0921-4526/03/$ - see front matter r 2002 Published by Elsevier Science B.V.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 4 9 - 0


B.T. Hang et al. / Physica B 327 (2003) 378–381

A three electrodes system was used for electrochemical measurement. The magnetic measurements were carried out on the vibrating sample
magnetometer (VSM) system at various applied
magnetic fields and in the temperature range from
300 to 700 K.

3. Results and discussion
Selected magnetization curves MðH) are shown
in Fig. 1. It can be seen that all of the as-prepared
samples are paramagnetic. But after being milled
and charged–discharged the samples are ferromagnetic. This can be due to the ferromagnetism of a
Ni and/or Co phase, which was decomposed from
the alloys during the milling and charging–
discharging process [3]. From these curves, the
susceptibility values (w) of the as-prepared samples
were calculated. They are listed in Table 1. It is
clear that the susceptibility of La0.8Nd0.2Ni4.9ÀxCoxSi0.1 compounds increases with Co content.
This can be understood by taking into account the
fact that in the same valence state of the metals the
paramagnetic moment of a Co ion is larger than
that of a Ni ion [4].
The influence of the milling time on the
magnetic properties was studied with milling times
of 1, 2 and 4 h. Some results, presented in Fig. 2,
show that the ferromagnetism increases with
milling time.
The thermal–magnetization measurements were
carried out, for all cycled samples, by heating from
300 to 700 K, followed by cooling from 700 K

M (emu/g)

La0.8Nd0.2Ni4.8Co0.1Si0.1

down to room temperature, with the same rate of
temperature change. Some results of thermal–
magnetization measurements are shown in Fig. 3.
The small peak on the heating curves at ToTc can
be considered as indicating the re-crystallization
temperature of nearly amorphous Ni and/or Co,
which was decomposed by the cycling process [3].
From the cooling curves, the Curie temperatures
were determined. The values are listed in Table 1.
It can be seen that, for all samples, the Curie
temperature (Tc ) increases with Co content. It
proves that the ferromagnetism is not only due to
Ni atoms but also due to Co atoms.
The galvanostatic charge–discharge studies with
current 3 mA were carried out with all samples and
some selected results are shown in Fig. 4. We have
found that Co stabilizes the charge–discharge
process. Just after several initial cycles, evidently
a stable state is reached. Furthermore, the
discharge potential observed on the discharge
curves of these samples falls slowly, and in certain
interval of the discharge electric quantity Q it
appears to be nearly constant, meaning the best
Table 1
Susceptibility (w) at room temperature of La0.8Nd0.2Ni4.9ÀxCoxSi0.1 compounds
No.

Parent samples

w (10À6)

Tc (K)

1
2
3
4
5

La0.8Nd0.2Ni4.8Co0.1Si0.1
La0.8Nd0.2Ni4.4Co0.5Si0.1
La0.8Nd0.2Ni4.15Co0.75Si0.1
La0.8Nd0.2Ni3.9Co1Si0.1
La0.8Nd0.2Ni3.4Co1.5Si0.1

6.03
10.64
14.27
23.69
31.41

585
650
667
675
>700

0.4

0.8

0.2

0.4

0.0

0.0

-0.2
-0.4

bulk sample
powder sample
cycled sample

La0.8Nd0.2Ni4.15Co0.75Si0.1

0.4

bulk sample
powder sample
cycled sample

0.8
-10000 -5000 0 5000 10000
H (Oe)

379

-10000 -5000 0 5000 10000
H (Oe)

Fig. 1. Magnetization curves of La0.8Nd0.2Ni4.9ÀxCoxSi0.1 samples before and after being cycled.


B.T. Hang et al. / Physica B 327 (2003) 378–381

380

0.6

La0.8Nd0.2Ni4.8Co0.1Si0.1

2

La0.8Nd0.2Ni4.15Co0.75Si0.1

M (emu/g)

0.4
1

0.2

0

0.0
-0.2

0h
1h
2h
4h

-0.4
-0.6
-15000 -10000 -5000

0

0h
1h
2h
4h

-1
-2
-10000

5000 10000 15000

-5000

0

5000

10000

H (Oe)

H (Oe)

Fig. 2. Magnetization curves of La0.8Nd0.2Ni4.9ÀxCoxSi0.1 samples, showing the dependence of the ferromagnetic behavior on milling
time.

La0.8Nd0.2Ni4.4Co0.5Si0.1

M (emu/g)

2.4

2.4

La0.8Nd0.2Ni4.15Co0.75Si0.1

2.0

2.0

1.6

1.6

1.2

1.2

0.8

0.8

0.4
300

400

500 600
T (K)

700

300

400

500
T (K)

600

700

Fig. 3. The temperature dependence of the magnetization of the cycled samples.

-E (mV/SCE)

1200

1200
La0.8Nd0.2Ni4.15Co0.75Si0.1

1100

C-D1
C-D5
C-D7
C-D10

1000

La0.8Nd0.2Ni0.2Co1Si0.1

1100

C-D1
C-D5
C-D7
C-D10

1000

900

900
0

20

40

60

80 100

Q (C/cm2)

0

20

40

60

80 100

Q (C/cm2)

Fig. 4. Galvanostatic charge and discharge curves of La0.8Nd0.2Ni4.9ÀxCoxSi0.1 samples. The legends C-D1, C-D5, C-D7 and C-D10
indicate the number of charge–discharge cycles.

quality of the discharge process. From charge–
discharge studies we have found that the yield and
the stable discharge potential of the samples with
x ¼ 0:75 and 1 are higher than in samples with

other Co content (not shown here). They are good
for application as negative electrode material in
rechargeable Ni-MH batteries.

From impedance spectra at various applied
potentials, for all samples Rct and ÀZimax were
determined. The values are given in Fig. 5. Both
Rct and ÀZimax decrease with increasing Co
content x except for x ¼ 1:5: The samples with
Co content x ¼ 0:75 and 1 have the smallest values
and, consequently, are the most suitable. This


B.T. Hang et al. / Physica B 327 (2003) 378–381

381

Fig. 5. Variation of Rct ; and ÀZimax ; determined at 1300 and 1350 mV as a function of Co content.

result is in good agreement with the results of the
galvanostatic charge–discharge studies.

sitions are good for application as metal
hydride electrode material in rechargeable NiMH batteries.

4. Conclusions
In this work, the magnetic and electrochemical
properties of La0.8Nd0.2Ni4.9ÀxCoxSi0.1 samples
have been studied. The main results can be
summarized as follows.

1. All as-prepared samples are paramagnetic at
room temperature. The susceptibility values of
the samples increase with Co content.
2. The La0.8Nd0.2Ni4.9ÀxCoxSi0.1 compounds
change from paramagnetic into ferromagnetic
after milling and charging–discharging process.
The Curie temperature values of the samples
increase with Co content.
3. Our electrochemical studies have pointed out
the best performance for La0.8Nd0.2Ni4.15Co0.75Si0.1 and La0.8Nd0.2Ni3.9Co1Si0.1. These compo-

Acknowledgements
This work is supported by the National Research Program under the Grant KC02/13/02 and
by the State Program on Fundamental Research of
Viet Nam under the Grant No. 421001.

References
[1] T. Sakai, K. Oguru, H. Miyamura, N. Kuriama, A. Kato,
H. Ishikawa, J. Less-Common Metals 161 (1990) 193.
[2] L. Schlapbach, J. Phys. F 10 (1980) 2477.
[3] A.H. Boonstra, T.M.N. Bernards, J. Less-Common Metals
161 (1990) 355.
[4] David Jiles, Introduction to Magnetism and Magnetic
Materials, Chapman & Hall, London UK, 1991.