DSpace at VNU: Magnetic properties of LaNi5-based compounds

Journal of Magnetism and Magnetic Materials 262 (2003) 485–489 Magnetic properties of LaNi5-based compounds L.T Taia,b,*, B.T Hangb, N.P Thuya,b, T.D Hienb a Cryogenic Laboratory, Faculty of Physics, College of Natural Science, Vietnam National University, 334 Nguyen Trai Road, Hanoi, Viet Nam b International Training Institute for Materials Science (ITIMS), ITIMS Building, Dai Hoc Bach Khoa, Dai Co Viet Road, Hanoi, Viet Nam Abstract Magnetic properties of the La(Ni4.5M0.5), (La1ÀxRx)Ni5 (M=Fe, Co, Mn, Cu, Cr, Si and R=Nd, Pr) and their hydride compounds have been studied by magnetization measurement using a VSM The susceptibility (w) of the samples increases with Co, Mn, Fe, Nd, Pr but decreases with Cu, Cr, Si additives The hydrogenation changes magnetic properties of the La(Ni4.5M0.5), (La1ÀxRx)Ni5 parent compounds Specially, the milling process changes magnetic properties of LaNi5 This work indicates that besides electrochemical measurements, the study of magnetic properties also gives important information on the quality of rechargeable sealed nickel–metal hydride batteries r 2003 Published by Elsevier Science B.V PACS: 82.47.Cb; 75.20.En Keywords: Metal hydrides; Chemical hydrogenation; Magnetic properties Introduction The intermetallic RT5 compounds (R: rare earth, T: transition metals) can absorb and desorb a large hydrogen quantity at atmospheric pressure and near room temperature This property makes them attractive to be used as negative electrode for nickel-metal hydride (Ni–MH) rechargeable batteries [1,3] The use of a hydride-forming electrode has enabled the development of rechargeable Cd-free batteries, which have a long *Corresponding author Cryogenic Laboratory, Faculty of Physics, College of Natural Science, Vietnam National University, 334 Nguyen Trai Road, Hanoi, Viet Nam Tel.: +84-4585281; fax: +84-4-8584438 E-mail addresses: lttai50@yahoo.com (L.T Tai), hang @itims.edu.vn (B.T Hang) life and high-energy storage capacity (30–50% higher than Ni–Cd batteries) [1,2] Many studies have been performed on RT5 compounds for battery application Main attention, however, has been focused on electrochemical measurements [4–7], whereas the magnetic properties of hydride material have been insufficiently studied In this paper, we present results of magnetic measurements on La(Ni4.5M0.5), (La1ÀxRx)Ni5 (M=Fe, Co, Mn, Cu, Cr, Si and R=Nd, Pr) compounds and their hydrides Experimental All samples were prepared by conventional arcmelting under argon atmosphere All starting 0304-8853/03/$ - see front matter r 2003 Published by Elsevier Science B.V doi:10.1016/S0304-8853(03)00082-9 L.T Tai et al / Journal of Magnetism and Magnetic Materials 262 (2003) 485–489 0.08 M (emu/g) materials (La, Nd, Pr, Ni, Co, Fe, Mn, Cu, Cr and Si) are of 99.9 wt % or more purity A slight excess of La, Nd, Pr was used to compensate the weight loss during the melting process Prior to melting the samples, Ti–metal ingot was melted as a getter The ingots were turned over and remelted several times in order to ensure the homogeneity Powder samples with an average particle size of about 50 mm were obtained by pulverizing the as-melted compounds in an agate mortar The samples were checked by X-ray diffraction, which reveals the samples to be of single phase of CaCu5 structure Hydride samples were prepared by a chemical hydrogenation method The parent alloy powders were mixed with NaBH4 in a proper ratio This mixture was put in a spherical jar and soaked in 50 ml of distilled water The whole system was tightly sealed for at least 24 h for the following chemical reaction to complete LaNi5 powder 0.00 -10000 10000 H (Oe) La0.75Nd0.25Ni5 La0.75Nd0.25Ni5Hx -2 -10000 LaðNi2MÞ5 þ NaBH4 þ 3H2 O 10000 H (Oe) ¼ LaNi-Mị5 Hx ỵ NaH2 BO3 ỵ 4-x=2ịH: 0.04 LaNi4.5Co0.5 M (emu/g) LaNi4.5Co0.5Hx 0.00 -0.04 -10000 10000 H (Oe) 0.1 LaNi4.5Mn0.5 LaNi4.5Mn0.5Hx M (emu/g) The produced powder was then washed several times by distilled water in order to get rid of the reacted by products Finally it was washed in alcohol and dried in air Magnetic properties of the samples have been studied by measuring magnetization curves and the thermomagnetization using a sensitive vibrating sample magnetometer (VSM) system Magnetization curves at room temperature were measured on the bulk samples, powder samples and their hydrides The maximum applied field was 1.3 T The temperature dependence of the magnetization MðTÞ of the hydride samples was measured in a nitrogen gas environment The temperature range was from 300 K to above 700 K The maximum applied field was 0.1 T During the measurement, the temperature was raised at a rate estimated to be about 5 C/min LaNi5 LaNi5Hx -0.08 M (emu/g) 486 0.0 -0.1 -10000 10000 H (Oe) Results and discussion The magnetization curves of the as-prepared samples are shown in Fig for comparison with those of the hydride samples As can be clearly seen from the magnetization curves, all parent Fig Magnetization curves of La(Ni4.5M0.5) and (La1ÀxRx)Ni5 (M=Fe, Co, Mn and R=Nd, Pr) and their hydrides, at room temperature L.T Tai et al / Journal of Magnetism and Magnetic Materials 262 (2003) 485–489 487 Table Susceptibility (w) at 300 K of parent samples and Curie temperature (Tc ) of hydride samples No Parent samples w (10À6) Hydride samples Tc (K) 10 11 12 13 14 LaNi5 LaNi4.5Co0.5 LaNi4.5Fe0.5 LaNi4.5Mn0.5 LaNi4.5Cu0.5 LaNi4.5Cr0.5 LaNi4.5Si0.5 La0.99Nd0.01Ni5 La0.95Nd0.05Ni5 La0.90Nd0.1Ni5 La0.99Pr0.01Ni5 La0.95Pr0.05Ni5 La0.90Pr0.1Ni5 3.80 7.40 35.06 11.05 3.01 2.35 2.49 5.02 5.43 6.14 4.58 5.13 5.88 LaNi5Hx LaNi4.5Co0.5Hx LaNi4.5Fe0.5Hx LaNi4.5Mn0.5Hx LaNi4.5Cu0.5Hx LaNi4.5Cr0.5Hx LaNi4.5Si0.5Hx La0.99Nd0.01Ni5Hx La0.95Nd0.05Ni5Hx La0.90Nd0.1Ni5Hx La0.99Pr0.01Ni5Hx La0.95Pr0.05Ni5Hx La0.90Pr0.1Ni5Hx LaNi5 powder 570 755 700 565 485 495 465 570 570 570 570 570 570 570 samples are paramagnets From these curves the susceptibility (w) was determined Results are shown in Table The susceptibility of the LaNi5 compound increases when La is partly replaced by Nd, Pr and Ni is partly replaced by Co, Fe, Mn, but it decreases when Ni is partly replaced by Cu, Cr and Si This can be understood by taking into account that, in the same valence state of the metals, the paramagnetic moments of the Co, Fe and Mn ions are all larger than that of a Ni ion [8], and that Nd, Pr are magnetic while Cu, Cr and Si are nonmagnetic In contrast to those of the parent samples, the magnetization curves of all hydride samples show a ferromagnetic behavior This can be explained as being due to the ferromagnetism of pure Ni (or/and Co, Fe, Mn) phases that were decomposed from the compounds during the hydrogenation process The difference in ferromagnetic behavior of the hydride samples, with different element substitutions, can also be seen in Fig The thermomagnetization measurements were performed on all hydride samples in two different ways At first, the magnetization of the asprepared samples was measured with increasing temperature from 300 K to above 700 K After that, the samples were cooled down to room temperature Then the magnetization was measured again with increasing temperature in the same temperature interval Results are shown in Fig In Fig the filled symbols are results taken on as-hydrided samples and the empty symbols are those for the same samples in the second measuring cycle One can easily see a clear difference between the two magnetization curves The thermal cycle thus has a pronounced effect on the thermal magnetization behavior of the materials The anomaly observed in the curves of the asprepared samples can be explained as being due to the presence of free Ni (or/and Co, Fe, Mn) metal phases that were decomposed by the hydrogenation process and exist in a nearly amorphous or cluster form [9,10] The onset temperatures of the maxima at ToTc observed for all hydride samples can be considered as recrystallization temperatures From the thermomagnetic curves obtained in the second measurement cycle, the Curie temperatures (Tc ) were determined for the hydride compounds (see Table 1) It is clear that the Curie temperatures of La(Ni4.5M0.5) compounds differ very remarkably for different M metals In contrast, the Tc values of (La1ÀxRx)Ni5 compounds with R=La, Nd and Pr are all the same This indicates that in the hydrogenation process only the transition metals were decomposed from the compounds L.T Tai et al / Journal of Magnetism and Magnetic Materials 262 (2003) 485–489 488 0.8 LaNi5Hx LaNi5 powder 0.6 0.06 M (emu/g) M (arb.units) 0.12 0.00 0.4 0.2 200 400 600 800 T (K) 0.0 0.18 300 M (arb.units) La0.75Nd0.25Ni5Hx 600 700 Fig The temperature dependence of the magnetization of LaNi5 powder measured at H ¼ kOe 0.09 200 400 600 800 T (K) 0.24 M (arb.units) 500 T (K) 0.00 LaNi4.5Co0.5Hx 0.12 0.00 200 400 600 800 T (K) 0.30 LaNi4.5Mn0.5Hx M (arb.units) 400 powder has the same behavior as that of the hydride sample (see Fig 3) It can be due to the fact that during the milling process Ni atoms were also decomposed from the sample We note that the effect of the chemical hydrogenation process of the (La1ÀxRx)Ni5 and La(Ni4.5M0.5) compounds is quite similar to that of the charging process with negative electrode in Ni–MH batteries We observed the same results in all samples, when they are subjected to a charging– discharging process The transition metals are decomposed by the hydrogenation process and precipitated in cluster form on the surface of particles, leading to the increase of the reactivity of these surfaces This effect can be related to the larger value of the double-layer capacitance as found in electrochemical measurements 0.15 Conclusion 0.00 In this work, the magnetic properties of LaNi4.5M0.5 and LaNi4.5M0.5Hx have been studied The main results can be summarized as follows: 200 400 600 800 T (K) Fig The temperature dependence of the magnetization of hydride samples measured at H ¼ kOe Specially, after milling, the LaNi5 sample changes from paramagnetic to ferromagnetic (see Fig 1), and the thermomagnetic curves of LaNi5 (1) The susceptibility of LaNi5-based compounds increases when partly replacing Ni by Co, Fe, Mn and La by Nd, Pr and decrease upon replacing by Cr, Cu, Si additive (2) The LaNi5-based compounds change from paramagnetic to ferromagnetic after chemical hydrogenation and milling L.T Tai et al / Journal of Magnetism and Magnetic Materials 262 (2003) 485–489 (3) The magnetic measurements on La(Ni–M)5 powders provide valuable information on the nature of the hydrogenation during the electrochemical process in a Ni–MH rechargeable battery Acknowledgements This work is supported by National Research Program under the Grant KC02/13/02 and the State Program on Fundamental Research of Vietnam under the Grant No 421001 References [1] G Sandrock, in: P.D Bennett, T Sakai (Eds.), Proceedings of the Symposium on Hydrogen and Metal Hydride Batteries, Battery Division, Proc., Vol 94-27, The Electrochemical Society, Inc., Pennington, NJ, 1994, pp 1–15 [2] P.H.L Notten, R.E.F Einerhand, J.L.C Daams, J Alloys Comp 210 (1994) 221 489 [3] H Uchida, M Sato, O Moriwaki, J Alloys Comp 253–254 (1997) 235 [4] B.V Ratnakumar, S Surampudi, S di Stefano, G Halpert, in: P.D Bennett, T Sakai (Eds.), Proceedings of the Symposium on Hydrogen and Metal Hydride Batteries, Battery Division, Proc., Vol 94-27, The electrochemical Society, Inc., Pennington, NJ, 1994, pp 57–67 [5] C.K Witham, B Fultz, in: P.D Bennett, T Sakai (Eds.), Proceedings of the Symposium on Hydrogen and Metal Hydride Batteries, Battery Division, Proc., Vol 94-27, The electrochemical Society, Inc., Pennington, NJ, 1994, pp 68–75 [6] Z Ye, T Sakai, Noreus, in: P.D Bennett, T Sakai (Eds.), Proceedings of the Symposium on Hydrogen and Metal Hydride Batteries, Battery Division, Proc., Vol 94-27, The Electrochemical Society, Inc., Pennington, NJ, 1994, pp 184–195 [7] A Percheron-Guegan, M Latroche, J.C Achard, Y Chabre, J Bouet, in: P.D Bennett, T Sakai (Eds.), Proceedings of the Symposium on Hydrogen and Metal Hydride Batteries, Battery Division, Proc., Vol 94-27, The electrochemical Society, Inc., Pennington, NJ, 1994, pp 196–218 [8] D Jiles, Introduction to Magnetism and Magnetic Materials, Chapman & Hall, London, 1991, p 243 [9] T.P Blach, E Mac, A Gray, J Alloys Comp 253–254 (1997) 336 [10] L Schlapbach, J Phys F 10 (1980) 2477  ... hydrides, at room temperature L.T Tai et al / Journal of Magnetism and Magnetic Materials 262 (2003) 485–489 487 Table Susceptibility (w) at 300 K of parent samples and Curie temperature (Tc ) of hydride... taking into account that, in the same valence state of the metals, the paramagnetic moments of the Co, Fe and Mn ions are all larger than that of a Ni ion [8], and that Nd, Pr are magnetic while Cu,... the same This indicates that in the hydrogenation process only the transition metals were decomposed from the compounds L.T Tai et al / Journal of Magnetism and Magnetic Materials 262 (2003)