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, 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, 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 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: hang@itims.edu.vn (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 Experiment La0.8Nd0.2Ni4.9ÀxCoxSi0.1 (x ¼ 0:1; 0.5, 0.75, 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 - ( ) - 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 Results and discussion Selected magnetization curves MðH) are shown in Fig 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 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, and 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 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 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 mA were carried out with all samples and some selected results are shown in Fig 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 Susceptibility (w) at room temperature of La0.8Nd0.2Ni4.9ÀxCoxSi0.1 compounds No Parent samples w (10À6) Tc (K) 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 5000 10000 H (Oe) 379 -10000 -5000 5000 10000 H (Oe) Fig 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 La0.8Nd0.2Ni4.15Co0.75Si0.1 M (emu/g) 0.4 0.2 0.0 -0.2 0h 1h 2h 4h -0.4 -0.6 -15000 -10000 -5000 0h 1h 2h 4h -1 -2 -10000 5000 10000 15000 -5000 5000 10000 H (Oe) H (Oe) Fig 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 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 20 40 60 80 100 Q (C/cm2) 20 40 60 80 100 Q (C/cm2) Fig 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 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 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 have the smallest values and, consequently, are the most suitable This B.T Hang et al / Physica B 327 (2003) 378–381 381 Fig 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 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 All as-prepared samples are paramagnetic at room temperature The susceptibility values of the samples increase with Co content 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 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 ... temperature, with the same rate of temperature change Some results of thermal– magnetization measurements are shown in Fig The small peak on the heating curves at ToTc can be considered as indicating... 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... 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