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Formation of MgCNi3 and Mg–Ni amorphous mixture by mechanical alloying of Mg–Ni–C system
Materials Letters 58 (2004) 2203 – 2206 www.elsevier.com/locate/matlet
Formation of MgCNi3 and Mg–Ni amorphous mixture by mechanical alloying of Mg–Ni–C system L.Z. Ouyang a,*, H. Wang a, C.H. Peng a, M.Q. Zeng a, C.Y. Chung b, M. Zhu a a
b
School of Mechanical Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China Department of Physics and Material Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China Received 1 February 2004; received in revised form 2 February 2004; accepted 10 February 2004 Available online 27 February 2004
Abstract This work provides a new and feasible way to prepare MgCNi3 under ambient conditions by ball milling (BM) Mg, Ni powder and paraffin or graphite. It has been found that paraffin is partially dissociated during ball milling, which resulted in free carbon and hydrogen incorporated within the Mg – Ni alloy powder in increasing amounts with increasing milling time. X-ray diffraction analysis indicates that milled Mg – Ni powders react with the paraffin, producing a mixture of MgCNi3 and amorphous Mg – Ni alloy. The electrochemical experiment results show that the discharge capacity of mechanical alloyed (MA) Mg – Ni-based composites was 415 mA h/g. D 2004 Elsevier B.V. All rights reserved. Keywords: Nickel – metal hydride batteries; Ball milling (BM); MgCNi3; Hydrogen storage
1. Introduction In recent years, more attention was focused on Mg –Nibased alloys for its high intrinsic discharge capacity [1]. It was reported that the initial discharge capacity achieved can be as high as 830 mA h/g [2] and 1082 mA h/g [3], which is much higher than that of the AB5-type alloys. However, the life cycle of Mg – Ni electrode materials needs to be improved because the half life cycle of the discharge capacity were less than 20 cycles [2,4]. This may be due to the oxidation of the alloy surface during the charge– discharge cycling in the alkaline solution [2,4,5]. Various methods such as element substitution [2,5,6], surface modification [7], electro-catalytic element addition [2 –4,8] and multi-phase forming [6,9] were extensively investigated. Orimo et al. [10,11] and Tessir [12] studied the effect of ball milling (BM) under the hydrogen atmosphere and found that the metal hydrides were formed and had fair good electrochemical properties or hydrogen storage properties. Yuan [8] and Orimo et al. [9] investigated the effect of graphite (G) or carbon addition during ball milling of Mg and Ni powder and synthesized
amorphous MgNiCx (x up to 1.31). Imamura et al. [13,14] also studied the Mg/G composites as prepared by ball milling Mg powder and graphite in the presence of organic additive and found the synergetic interactions between magnesium and graphite as a result of mechanical grinding with the organic additives. Furthermore, carbon is a very interesting element for hydrogen storage. During recent decades, an intended combination of carbon nanotubes and metal hydrides was expected to have a potential application in the field of hydrogen storage [15,16]. In the present work, therefore, some C –H compound (here, paraffin or graphite) was added during ball milling to investigate the structure transformation in the Mg – Ni –C( – H) system upon milling, in the expectation of finding new phases, hopefully useful for Ni – MH battery electrode. Accidentally, we found the new way to synthesize MgCNi3 under ambient conditions. It is well known that MgCNi3 is a newly discovered superconductor material [17], which is normally prepared by powder metallurgy at a high temperature up to 1000 jC.
2. Experiment * Corresponding author. Tel.: +86-20-87112762; fax: +86-2087111317. E-mail address: [email protected] (L.Z. Ouyang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.02.002
Mg – Ni-based composites were prepared from mixed powders of Mg and Ni and liquid paraffin mechanically
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alloyed (MA) under argon atmosphere using a Fritsch P-5 planetary mill at the rate of 200 rpm. The weight ratio of Mg/Ni/paraffin and Mg/Ni/graphite is 20:20:1 and 20:20:0.7, respectively. The Mg and Ni powders used were commercial-grade powders with a purity of 99.5% and size of 154 and 74 Am, respectively. The graphite powder used was a commercial-grade powder with a purity of 99.7% and size of 154 m. The milling process was performed after sealing the powder –paraffin or graphite mixture in a stainless-steel vial together with hardened steel balls in a glove box filled with pure argon. The ball to powder weight ratio was 10:1. The XRD analysis was performed in a Philips X’Pert MPD XRD system with Cu Ka radiation. An electrochemical analysis of Guangzhou Qingtian BS9300 equipment was used to characterize the discharge capacity. In the charge– discharge cycle tests, each negative electrode was charge for 10 h at 100 mA/g and discharge at 50 mA/g to 0.45 V vs. NiOOH/Ni(OH)2 counter electrode. After every charging, the circuit was opened for 5 min. The details of electrochemical analysis are given in Ref. [18].
3. Results and discussion Fig. 1 shows the XRD patterns of the mixture of Mg, Ni powder and paraffin milled for different time. It can be seen from Fig. 1(a) and (b) that the diffraction peaks of Mg and Ni broaden and lower with the increasing of milling time from 0 to 10 h, which infer that their grain size was refined by ball milling. The indexing of Fig. 1(c) revealed that Mg2Ni and MgCNi3 phases were formed as the milling process prolonged to 20 h. The broadened peak at about 40j indicated the presence of amorphous Mg – Ni phase. There are still part of Mg and Ni retained at this milling stage. The
Fig. 2. The XRD pattern of annealed materials prepared by ball-milling of Mg, Ni and paraffin for 30 h as shown in Fig. 1(d).
diffraction peaks of Mg, Ni and Mg2Ni disappeared after 30 h of milling, as shown in Fig. 1(d), and the peak at about 40j was much more broader than that in Fig. 1(c). This may be attributed to that more amorphous Mg –Ni phase was formed as the ball milling time increasing to 30 h. From the above observation and analysis, it has been proven that the mixture obtained after 30 h of milling was composed of amorphous Mg – Ni and MgCNi3 phases (designated here after as composites). Since the paraffin was a C –H compound and act as the carbon source in the system milled, the formation of MgCNi3 indicates that the paraffin decomposed to carbon and hydrogen. It means that the ball milling process was carried out with carbon addition and under the hydrogen atmosphere. Although the ball-milling process was carried out at the hydrogen atmosphere, no metal hydride was found in the ball-milled mixture, which may due to the very low hydrogen pressure. Fig. 2 is the XRD pattern of annealed materials prepared by ball milling of Mg, Ni and paraffin for 30 h. The annealed materials
Fig. 1. X-ray diffraction patterns of the mixture of Mg, Ni powder and paraffin milled for different times: (a) milling for 0 h; (b) milling for 10 h; (c) milling for 20 h; and (d) milling for 30 h.
L.Z. Ouyang et al. / Materials Letters 58 (2004) 2203–2206
consisted of Mg2Ni, MgCNi3 and MgO. Comparing Fig. 1(d) with Fig. 2, it is clear that the amorphous phase was Mg –Ni alloy. The lattice parameter for both the ball-milled ˚ , which is a little lower and annealed materials is 3.813 A than what Ren et al. [19] and Huang et al. [20] reported. No MgO exists in the composites before annealing, while some MgO formed during the annealing process as shown in Fig. 2. This because the composites were annealed at 300 jC and the crystallized powder Mg tended to be oxidized by the ambient oxygen. The discharge curves of the electrode made of the composite prepared by 30 h of milling were shown in Fig. 3. The discharge curve had a plateau region during discharge. The discharge capacity of composites electrode is 415 mA h/g at the first cycle. In the second cycle, it is 393 mA h/g, which corresponds to a 5.3% drop. The composite electrode had much larger discharge capacity than that of commercially used AB5-type alloy electrode, which is about 250 mA h/g measured under the same measuring conditions. Inoue et al. [21] studied the Gr-modified Mg2Ni alloy and found that the discharge capacity increased to 290 mA h/g and the life cycle was somewhat improved. Judging from these results, the marked improvement in charge– discharge character by ball milling the Mg and Ni powders with paraffin seems to be ascribable to the formation of homogenous amorphous Mg – Ni alloy and MgCNi3 composites. Fig. 4 shows the XRD pattern of a sample consisting of Mg of 20 g, Ni of 20 g and graphite of 0.7 g milled for different time. After milling for 30 h, it is seen that the reflections of graphite disappear and the diffraction peaks of Mg and Ni are broadened, which is associated with an effect related to grain-size reduction as a consequence of milling procedure. The increase in milling time results in the formation of Mg2Ni, its crystallite size is in nanometric dimensions calculated by the Scherrer formula basing on the broadening effect of Bragg reflection lines. The formation of Mg2Ni indicates that a solid alloying reaction between Mg and Ni happens during milling, while the residual Bragg peaks of f.c.c. Ni show that the Ni content is higher than the stoichiometrical composition of Mg2Ni. The broadening of reflection lines of Ni also indicates that the nanocrystalline
2205
Fig. 4. XRD pattern of samples containing Mg (10 g), Ni (10 g) and graphite (0.7 g) milled for different times.
structure is formed. For the 70-h milled samples, the diffraction intensities of Ni and Mg2Ni are further decreased and their Bragg reflection lines became irregular. In addition, two weak peaks corresponding to magnesium oxide appear, it maybe related to oxygen pollution during frequent sample collection. Increasing the milling time to 80 h, it is observed in the XRD pattern that there exists a group of new diffraction peaks corresponding to the MgCNi3 phase. It indicates that a mechano-chemical reaction takes place between nanocrystalline Mg2Ni, Ni and amorphous graphite. Meanwhile, the diffraction peaks of Mg2Ni are still visible. When the milling time further increases, the Bragg peaks of Mg2Ni disappear gradually and the Bragg peaks of MgCNi3 become sharp. It shows that the mechano-chemical reaction proceeds with milling process. It should be noted that the composition of these samples is not consistent with that of MgCNi3, so the final milled 100-h product is not composed of pure MgCNi3, but MgCNi3 mixed with Mg and graphite. MgCNi3 has a classical perovskite structure with the atomic positions: Mg: 1a (0, 0, 0), Ni: 3c (0, 1/2, 1/2), C: 1b (1/2, 1/2, 1/2). It has a cubic lattice. The lattice constant of this phase ˚, calculated by the XRD data are 3.813 and 3.806 A respectively, which is a little less than that of MgCNi3 (JCPDS No. 41-0903), but a little more than that of MgC0.75Ni3 (JCPDS No. 28-0624). The crystallite size of this phase also reaches nanometric dimensions.
4. Conclusions
Fig. 3. Discharge curve of the composite electrode in the first cycle.
In the present work, Mg and Ni were ball milled with the addition of liquid paraffin or graphite, which could provide a new and feasible way to prepare MgCNi3 under ambient conditions. X-ray diffraction analysis proved that the me-
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L.Z. Ouyang et al. / Materials Letters 58 (2004) 2203–2206
chanically alloyed Mg – Ni-based alloys composed of MgCNi3 and amorphous Mg – Ni were synthesized by 30 or 80 h of milling with paraffin or graphite, respectively, under the present experiment condition. The discharge capacity of the electrode prepared by using the composites is of 415 mA h/g.
Acknowledgements This work was supported by the National Natural Science Foundation under Project Nos. 59925102, 50071022 and 50131040 and a CERG grant from the HKSAR UGC Research Grant Council (Project No. CityU 1316/03E, 9040839).
References [1] V. Guther, A. Otto, Recent developments in hydrogen storage applications based on metal hydrides, Journal of Alloys and Compounds 293 – 295 (1999) 889 – 892. [2] T. Kohno, M. Yamamoto, M. Kanda, Electrochemical properties of mechanically ground Mg2Ni alloy, Journal of Alloys and Compounds 293 – 295 (1999) 643 – 647. [3] C. Iwakura, H. Inoue, S.G. Zhang, et al., Hydriding and electrochemical characteristic of a homogeneous amorphous Mg2Ni – Ni composite, Journal of Alloys and Compounds 270 (1998) 142 – 144. [4] S. Nohara, N. Fujita, S.G. Zhang, et al., Electrochemical characteristics of a homogeneous amorphous alloy prepared by ball-milling Mg2Ni with Ni, Journal of Alloys and Compounds 267 (1998) , 76 – 78. [5] S. Nohara, K. Hamasaki, S.G. Zhang, et al., Electrochemical characteristics of an amorphous Mg0.9V0.1Ni alloy prepared by mechanical alloying, Journal of Alloys and Compounds 280 (1998) 104 – 106. [6] T. Kohno, H. Yoshida, F. Kawashima, et al., Hydrogen storage properties of new ternary system alloys: La 2 MgNi 9 La 5 Mg 2 Ni 23 , La3MgNi14, Journal of Alloys and Compounds 311 (2000) L5 – L7. [7] Q.M. Yang, M. Ciureanu, D.H. Ryan, et al., Composite hydride electrode materials, Journal of Alloys and Compounds 274 (1998), 266 – 273. [8] A. Yuan, N. Xu, A study on the effect of nickel, cobalt or graphite addition on the electrochemical properties of an AB5 hydrogen storage alloys and the mechanism of the effects, Journal of Alloys and Compounds 322 (2001) 269 – 275.
[9] K. Funaki, S.-I. Orimo, H. Fujii, et al., Structural and hydriding properties of amorphous MgNi with interstitially dissolved carbon, Journal of Alloys and Compounds 270 (1998) 160 – 163. [10] S. Hayashi, S.-I. Orimo, H. Fujii, Local structures and hydrogen dynamics in amorphous and nanostructured Mg – Ni – H systems as studied by H1 and H2 nuclear magnetic resonance, Journal of Alloys and Compounds 261 (1997) 145 – 149. [11] Y. Kitano, Y. Fujikawa, N. Shimizu, S. Orimo, Electron microscopy of Mg2Ni – H alloy synthesized by reactive mechanical grinding, Intermetallics 5 (1997) 97 – 101. [12] P. Tessier, H. Enoki, M. Bououdina, et al., Balling-milling of Mg2Ni under hydrogen, Journal of Alloys and Compounds 268 (1998) 285 – 289. [13] H. Imamura, S. Tabata, N. Shigetomi, et al., Composites for hydrogen storage by mechanical grinding of graphite carbon and magnesium, Journal of Alloys and Compounds 330 – 332 (2002) 579 – 583. [14] H. Imamura, N. Sakasai, T. Fujinaga, Characterization and hydriding properties of Mg – graphite composites prepared by mechanical grinding as new hydrogen storage materials, Journal of Alloys and Compounds 253 – 254 (1997) 34 – 37. [15] A. Weidenkaff, S.G. Ebbinghaus, Ph. Mauron, A. Reller, Y. Zhang, A. Zuttel, Metal nanoparticles for the production of carbon nanotube composite materials by decomposition of different carbon sources, Materials Science and Engineering C 19 (2002) 119 – 123. [16] X.P. Gao, X. Qin, F. Wu, H. Liu, Y. Lan, S.S. Fan, H.T. Yuan, D.Y. Song, P.W. Shen, Synthesis of carbon nanotubes by catalytic decomposition of methane using LaNi hydrogen storage alloy as a catalyst, Chemical Physics Letters 327 (2000) 271 – 276. [17] T. He, Q. Huang, A.P. Ramirez, Y. Wang, K.A. Regan, N. Rogado, N.P. Ong, R.J. Cava, Superconductivity in the non-oxide perovskite MgCNi3, Nature 411 (2001) 6833. [18] L.Z. Ouyang, C.Y. Chung, H. Wang, M. Zhu, Microstructure of Mg – Ni thin film prepared by direct current magnetron sputtering and its properties as a negative electrode, Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films 21 (2003) 61905. [19] Z.A. Ren, G.C. Che, S.L. Jia, H. Chen, Y.M. Ni, G.D. Liu, Z.X. Zhao, The structural change and superconductivity in MgCxNi3 (x = 0.5 – 1.55) and MgxCyNi3 (x = 0.75 – 1.55, y = 0.85, 1.0 and 1.45) and the phase diagram of Ni-rich region in Mg – C – Ni ternary system, Physica C 371 (2002) 1 – 6. [20] Q. Huang, T. He, K.A. Regan, N. Rogado, M. Hayward, M.K. Haas, K. Inumaru, R.J. Cava, Temperature dependance of the structural parameters of the non-oxide perovskite superconductor MgCNi3, Physica C 363 (2001) 215 – 218. [21] H. Inoue, T. Ueda, S. Nohara, N. Fujita, C. Iwakura, Effect of ball-milling on electrochemical and physicochemical characteristics of crystalline Mg2Ni alloy, Electrochimica Acta 43 (1998) 2215 – 2219.
Formation of MgCNi3 and Mg–Ni amorphous mixture by mechanical alloying of Mg–Ni–C system L.Z. Ouyang a,*, H. Wang a, C.H. Peng a, M.Q. Zeng a, C.Y. Chung b, M. Zhu a a
b
School of Mechanical Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China Department of Physics and Material Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China Received 1 February 2004; received in revised form 2 February 2004; accepted 10 February 2004 Available online 27 February 2004
Abstract This work provides a new and feasible way to prepare MgCNi3 under ambient conditions by ball milling (BM) Mg, Ni powder and paraffin or graphite. It has been found that paraffin is partially dissociated during ball milling, which resulted in free carbon and hydrogen incorporated within the Mg – Ni alloy powder in increasing amounts with increasing milling time. X-ray diffraction analysis indicates that milled Mg – Ni powders react with the paraffin, producing a mixture of MgCNi3 and amorphous Mg – Ni alloy. The electrochemical experiment results show that the discharge capacity of mechanical alloyed (MA) Mg – Ni-based composites was 415 mA h/g. D 2004 Elsevier B.V. All rights reserved. Keywords: Nickel – metal hydride batteries; Ball milling (BM); MgCNi3; Hydrogen storage
1. Introduction In recent years, more attention was focused on Mg –Nibased alloys for its high intrinsic discharge capacity [1]. It was reported that the initial discharge capacity achieved can be as high as 830 mA h/g [2] and 1082 mA h/g [3], which is much higher than that of the AB5-type alloys. However, the life cycle of Mg – Ni electrode materials needs to be improved because the half life cycle of the discharge capacity were less than 20 cycles [2,4]. This may be due to the oxidation of the alloy surface during the charge– discharge cycling in the alkaline solution [2,4,5]. Various methods such as element substitution [2,5,6], surface modification [7], electro-catalytic element addition [2 –4,8] and multi-phase forming [6,9] were extensively investigated. Orimo et al. [10,11] and Tessir [12] studied the effect of ball milling (BM) under the hydrogen atmosphere and found that the metal hydrides were formed and had fair good electrochemical properties or hydrogen storage properties. Yuan [8] and Orimo et al. [9] investigated the effect of graphite (G) or carbon addition during ball milling of Mg and Ni powder and synthesized
amorphous MgNiCx (x up to 1.31). Imamura et al. [13,14] also studied the Mg/G composites as prepared by ball milling Mg powder and graphite in the presence of organic additive and found the synergetic interactions between magnesium and graphite as a result of mechanical grinding with the organic additives. Furthermore, carbon is a very interesting element for hydrogen storage. During recent decades, an intended combination of carbon nanotubes and metal hydrides was expected to have a potential application in the field of hydrogen storage [15,16]. In the present work, therefore, some C –H compound (here, paraffin or graphite) was added during ball milling to investigate the structure transformation in the Mg – Ni –C( – H) system upon milling, in the expectation of finding new phases, hopefully useful for Ni – MH battery electrode. Accidentally, we found the new way to synthesize MgCNi3 under ambient conditions. It is well known that MgCNi3 is a newly discovered superconductor material [17], which is normally prepared by powder metallurgy at a high temperature up to 1000 jC.
2. Experiment * Corresponding author. Tel.: +86-20-87112762; fax: +86-2087111317. E-mail address: [email protected] (L.Z. Ouyang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.02.002
Mg – Ni-based composites were prepared from mixed powders of Mg and Ni and liquid paraffin mechanically
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L.Z. Ouyang et al. / Materials Letters 58 (2004) 2203–2206
alloyed (MA) under argon atmosphere using a Fritsch P-5 planetary mill at the rate of 200 rpm. The weight ratio of Mg/Ni/paraffin and Mg/Ni/graphite is 20:20:1 and 20:20:0.7, respectively. The Mg and Ni powders used were commercial-grade powders with a purity of 99.5% and size of 154 and 74 Am, respectively. The graphite powder used was a commercial-grade powder with a purity of 99.7% and size of 154 m. The milling process was performed after sealing the powder –paraffin or graphite mixture in a stainless-steel vial together with hardened steel balls in a glove box filled with pure argon. The ball to powder weight ratio was 10:1. The XRD analysis was performed in a Philips X’Pert MPD XRD system with Cu Ka radiation. An electrochemical analysis of Guangzhou Qingtian BS9300 equipment was used to characterize the discharge capacity. In the charge– discharge cycle tests, each negative electrode was charge for 10 h at 100 mA/g and discharge at 50 mA/g to 0.45 V vs. NiOOH/Ni(OH)2 counter electrode. After every charging, the circuit was opened for 5 min. The details of electrochemical analysis are given in Ref. [18].
3. Results and discussion Fig. 1 shows the XRD patterns of the mixture of Mg, Ni powder and paraffin milled for different time. It can be seen from Fig. 1(a) and (b) that the diffraction peaks of Mg and Ni broaden and lower with the increasing of milling time from 0 to 10 h, which infer that their grain size was refined by ball milling. The indexing of Fig. 1(c) revealed that Mg2Ni and MgCNi3 phases were formed as the milling process prolonged to 20 h. The broadened peak at about 40j indicated the presence of amorphous Mg – Ni phase. There are still part of Mg and Ni retained at this milling stage. The
Fig. 2. The XRD pattern of annealed materials prepared by ball-milling of Mg, Ni and paraffin for 30 h as shown in Fig. 1(d).
diffraction peaks of Mg, Ni and Mg2Ni disappeared after 30 h of milling, as shown in Fig. 1(d), and the peak at about 40j was much more broader than that in Fig. 1(c). This may be attributed to that more amorphous Mg –Ni phase was formed as the ball milling time increasing to 30 h. From the above observation and analysis, it has been proven that the mixture obtained after 30 h of milling was composed of amorphous Mg – Ni and MgCNi3 phases (designated here after as composites). Since the paraffin was a C –H compound and act as the carbon source in the system milled, the formation of MgCNi3 indicates that the paraffin decomposed to carbon and hydrogen. It means that the ball milling process was carried out with carbon addition and under the hydrogen atmosphere. Although the ball-milling process was carried out at the hydrogen atmosphere, no metal hydride was found in the ball-milled mixture, which may due to the very low hydrogen pressure. Fig. 2 is the XRD pattern of annealed materials prepared by ball milling of Mg, Ni and paraffin for 30 h. The annealed materials
Fig. 1. X-ray diffraction patterns of the mixture of Mg, Ni powder and paraffin milled for different times: (a) milling for 0 h; (b) milling for 10 h; (c) milling for 20 h; and (d) milling for 30 h.
L.Z. Ouyang et al. / Materials Letters 58 (2004) 2203–2206
consisted of Mg2Ni, MgCNi3 and MgO. Comparing Fig. 1(d) with Fig. 2, it is clear that the amorphous phase was Mg –Ni alloy. The lattice parameter for both the ball-milled ˚ , which is a little lower and annealed materials is 3.813 A than what Ren et al. [19] and Huang et al. [20] reported. No MgO exists in the composites before annealing, while some MgO formed during the annealing process as shown in Fig. 2. This because the composites were annealed at 300 jC and the crystallized powder Mg tended to be oxidized by the ambient oxygen. The discharge curves of the electrode made of the composite prepared by 30 h of milling were shown in Fig. 3. The discharge curve had a plateau region during discharge. The discharge capacity of composites electrode is 415 mA h/g at the first cycle. In the second cycle, it is 393 mA h/g, which corresponds to a 5.3% drop. The composite electrode had much larger discharge capacity than that of commercially used AB5-type alloy electrode, which is about 250 mA h/g measured under the same measuring conditions. Inoue et al. [21] studied the Gr-modified Mg2Ni alloy and found that the discharge capacity increased to 290 mA h/g and the life cycle was somewhat improved. Judging from these results, the marked improvement in charge– discharge character by ball milling the Mg and Ni powders with paraffin seems to be ascribable to the formation of homogenous amorphous Mg – Ni alloy and MgCNi3 composites. Fig. 4 shows the XRD pattern of a sample consisting of Mg of 20 g, Ni of 20 g and graphite of 0.7 g milled for different time. After milling for 30 h, it is seen that the reflections of graphite disappear and the diffraction peaks of Mg and Ni are broadened, which is associated with an effect related to grain-size reduction as a consequence of milling procedure. The increase in milling time results in the formation of Mg2Ni, its crystallite size is in nanometric dimensions calculated by the Scherrer formula basing on the broadening effect of Bragg reflection lines. The formation of Mg2Ni indicates that a solid alloying reaction between Mg and Ni happens during milling, while the residual Bragg peaks of f.c.c. Ni show that the Ni content is higher than the stoichiometrical composition of Mg2Ni. The broadening of reflection lines of Ni also indicates that the nanocrystalline
2205
Fig. 4. XRD pattern of samples containing Mg (10 g), Ni (10 g) and graphite (0.7 g) milled for different times.
structure is formed. For the 70-h milled samples, the diffraction intensities of Ni and Mg2Ni are further decreased and their Bragg reflection lines became irregular. In addition, two weak peaks corresponding to magnesium oxide appear, it maybe related to oxygen pollution during frequent sample collection. Increasing the milling time to 80 h, it is observed in the XRD pattern that there exists a group of new diffraction peaks corresponding to the MgCNi3 phase. It indicates that a mechano-chemical reaction takes place between nanocrystalline Mg2Ni, Ni and amorphous graphite. Meanwhile, the diffraction peaks of Mg2Ni are still visible. When the milling time further increases, the Bragg peaks of Mg2Ni disappear gradually and the Bragg peaks of MgCNi3 become sharp. It shows that the mechano-chemical reaction proceeds with milling process. It should be noted that the composition of these samples is not consistent with that of MgCNi3, so the final milled 100-h product is not composed of pure MgCNi3, but MgCNi3 mixed with Mg and graphite. MgCNi3 has a classical perovskite structure with the atomic positions: Mg: 1a (0, 0, 0), Ni: 3c (0, 1/2, 1/2), C: 1b (1/2, 1/2, 1/2). It has a cubic lattice. The lattice constant of this phase ˚, calculated by the XRD data are 3.813 and 3.806 A respectively, which is a little less than that of MgCNi3 (JCPDS No. 41-0903), but a little more than that of MgC0.75Ni3 (JCPDS No. 28-0624). The crystallite size of this phase also reaches nanometric dimensions.
4. Conclusions
Fig. 3. Discharge curve of the composite electrode in the first cycle.
In the present work, Mg and Ni were ball milled with the addition of liquid paraffin or graphite, which could provide a new and feasible way to prepare MgCNi3 under ambient conditions. X-ray diffraction analysis proved that the me-
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L.Z. Ouyang et al. / Materials Letters 58 (2004) 2203–2206
chanically alloyed Mg – Ni-based alloys composed of MgCNi3 and amorphous Mg – Ni were synthesized by 30 or 80 h of milling with paraffin or graphite, respectively, under the present experiment condition. The discharge capacity of the electrode prepared by using the composites is of 415 mA h/g.
Acknowledgements This work was supported by the National Natural Science Foundation under Project Nos. 59925102, 50071022 and 50131040 and a CERG grant from the HKSAR UGC Research Grant Council (Project No. CityU 1316/03E, 9040839).
References [1] V. Guther, A. Otto, Recent developments in hydrogen storage applications based on metal hydrides, Journal of Alloys and Compounds 293 – 295 (1999) 889 – 892. [2] T. Kohno, M. Yamamoto, M. Kanda, Electrochemical properties of mechanically ground Mg2Ni alloy, Journal of Alloys and Compounds 293 – 295 (1999) 643 – 647. [3] C. Iwakura, H. Inoue, S.G. Zhang, et al., Hydriding and electrochemical characteristic of a homogeneous amorphous Mg2Ni – Ni composite, Journal of Alloys and Compounds 270 (1998) 142 – 144. [4] S. Nohara, N. Fujita, S.G. Zhang, et al., Electrochemical characteristics of a homogeneous amorphous alloy prepared by ball-milling Mg2Ni with Ni, Journal of Alloys and Compounds 267 (1998) , 76 – 78. [5] S. Nohara, K. Hamasaki, S.G. Zhang, et al., Electrochemical characteristics of an amorphous Mg0.9V0.1Ni alloy prepared by mechanical alloying, Journal of Alloys and Compounds 280 (1998) 104 – 106. [6] T. Kohno, H. Yoshida, F. Kawashima, et al., Hydrogen storage properties of new ternary system alloys: La 2 MgNi 9 La 5 Mg 2 Ni 23 , La3MgNi14, Journal of Alloys and Compounds 311 (2000) L5 – L7. [7] Q.M. Yang, M. Ciureanu, D.H. Ryan, et al., Composite hydride electrode materials, Journal of Alloys and Compounds 274 (1998), 266 – 273. [8] A. Yuan, N. Xu, A study on the effect of nickel, cobalt or graphite addition on the electrochemical properties of an AB5 hydrogen storage alloys and the mechanism of the effects, Journal of Alloys and Compounds 322 (2001) 269 – 275.
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