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Electrochemical characteristics and microstructures of rapidly quenched Ti0.5Zr0.5Mn0.2Cr0.5V0.2Ni0.95 alloy
Journal of Alloys and Compounds 364 (2004) 289–294
Electrochemical characteristics and microstructures of rapidly quenched Ti0.5Zr0.5Mn0.2 Cr0.5V0.2 Ni0.95 alloy Yang-Huan Zhang a,b,∗ , Ping Li a,c , Xin-Lin Wang a , Yu-Fang Lin a , Xuan-Hui Qu c a
Department of Functional Material Research, Central Iron and Steel Research Institute, 76 Xuanyuannan Road, Haidian District, Beijing 100081, China b Department of Material Science and Engineering, Baotou Iron and Steel University of Technology, Baotou 014010, China c The Power Metallurgy Institute, University of Science and Technology, Beijing 100083, China Received 7 March 2003; received in revised form 21 May 2003; accepted 21 May 2003
Abstract The microstructures of the as-cast and quenched Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy is composed of the C15 main phase and traces of the C14 phase and Zr7 Ni10 phase. The ratio of the three phases, which could be varied by different quenching rates, influences the electrochemical characteristics of the alloys. An amorphous phase was formed easily in the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy, a larger quantity of the amorphous phase being formed in the as-quenched alloy. The amorphous phase could significantly enhance the cycle stability of the alloy and strongly decrease the discharge capacity and the activation performance of the alloy. The cycle life of the as-quenched alloy obtained with a quenching rate of 26 m/s is larger than 1000 cycles, which is five to six times as large as that of the as-cast alloy. Its maximum discharge capacity is less than 200 mAh/g, and the alloy could be activated completely in about 180 charge–discharge cycles. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Metal hydrides; Electrode materials; Electrochemical reactions
1. Introduction In the last years, much attention was focused on the electric vehicle. The battery used in electric vehicles forms the key for this technology. Therefore, investigations of the Ni/MH battery have become a major topic in the field of battery research. This kind of battery is required to have a large capacity and a long cycle life as well as a current density of over 60–70 Wh/kg. The energy density of the battery depends on the materials used for the positive and negative electrodes and the structure of the battery. It is very difficult to enhance the discharge capacity of the positive electrode of the Ni/MH battery, because its practical discharge capacity has reached over 90% of the theoretical one. However, the discharge capacity of the negative electrode made of a metal hydride still has much room for further improvement. The investigation of the alloy electrode made of AB5 type hydrogen storage alloys has attained a mature stage, its practical capacity approaching the theoretical one. At present, ∗ Corresponding author. Tel.: +86-10-6218-7570; fax: +86-10-6218-7570. E-mail address: [email protected] (Y.-H. Zhang).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00551-6
investigations of hydrogen storage alloys with high capacity are focused on Zr-based, Mg-based and Ti-based alloys [1–4]. Mg-based and Ti-based hydrogen storage alloys are not used actually because the investigations on the cycle stability of these alloys have not yet reached a significant breakthrough. Zr-based hydrogen storage alloys have led to large scale industrialization. The main defects of Zr-based hydrogen storage alloy are higher cost of the production, poor activation performance and low rate discharge capability. The investigations on these alloys concentrate on how to improve their synthetic routes. The cycle stability of the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy prepared by rapid quenching was enhanced extremely, but the rest of the electrochemical characteristics needs further improvement.
2. Experimental 2.1. Preparation of alloys An alloy of the nominal composition Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 was melted in a vacuum induction furnace in an argon atmosphere. After induction melting, the melt was
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rapidly cooled by pouring it into a copper mould cooled by water. Part of the cast alloy was re-melted and quenched by melt spinning with a rotating copper wheel. Flakes of the rapidly quenched alloy were obtained with quenching rates of 2, 8, 14, 20, 26 and 32 m/s, the quenching rate being expressed by the linear velocity of the copper wheel. 2.2. Electrochemical measurement The alloy samples were ground mechanically into powder to below 250 meshes. Electrode pellets (15 mm in diameter) were prepared by mixing 1 g alloy powder and fine nickel powder in a weight ratio of 1:1 together with a small amount of polyvinyl alcohol (PVA) solution as binder. Then pressing at 3500 kg cm−2 was applied. After drying for 2 h at room temperature, the sample electrodes were immersed in 6 M KOH solution for 1 day in order to fully wet the electrode before the electrochemical measurements. The electrochemical measurements were carried out in a tri-electrode open cell with Ni(OH)2 /Ni(OOH) as the positive electrode, Hg/HgO as the reference electrode, and a 6 M KOH solution as electrolyte. The voltage between the negative electrode and the reference electrode was defined as the discharge voltage. In every cycle the electrode was charged with a constant current of 100 mA/g for 5 h, resting 15 min, then discharged with constant current of 100 mA/g, the cut-off voltage was −0.500 V. The environment temperature of the measurement was kept at 30 ◦ C. 2.3. Microstructure determination and morphology observation The samples of the as-cast alloy were directly polished and pieces of flake of the as-quenched alloy were fixed on a grid made of stainless steel after melt spinning. The samples thus prepared were etched with a 60% HF solution. The morphologies and the micro zone compositions of the alloy were observed and analyzed by SEM. The phase structures of the alloy were detected by XRD. The type of X-ray diffractometer used in this experiment is D/max/2400, the diffraction was performed with Cu K␣1 and the rays were filtered by graphite. The experimental parameters for determining the phase composition were: 160 mA, 40 kV and 10◦ /min. The alloy samples were pulverized by mechanical grinding. The obtained samples were dispersed in absolute alcohol for observing the grain morphology with TEM, and for determining with SAD whether an amorphous phase existed in the samples.
illustrated in Fig. 1. Fig. 2 represented the quenching rate dependence of the activation number. It can be derived from Figs. 1 and 2 that the activation capabilities of the as-cast and quenched Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloys are very poor. The cast alloy was completely activated after 25 cycles and it needed at least 40 cycles for the quenched alloy. The activated number increased with increase of the quenching rate. The quenched alloy obtained with quenching rate of 26 m/s was activated not fully even after 170 cycles. Obviously, the activation mechanism of the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy was the same as that of the Zr-based hydrogen storage alloy. The compact oxidation film, which was formed on the surface of the Zr-based alloy submersed in the electrolyte, could prevent the hydrolysis and the percolation of hydrogen, so the activation performance of the alloy was very poor. Surface treatment is an effective method of improving the activation performance of Zr-based alloys [5], and the multi-element alloying is another one. Zr can enhance the cycle life and reduce the activation capability of the alloy, but the effect of Ti is contrary to that of Zr [6,7]. Therefore, the activation performance of the alloy can be improved by increasing the Ti/Zr ratio. We can conclude that the poor activation performance of the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy is due to the relatively high Zr content. Because the surface to bulk ratio of the as-quenched alloy is rather large and the surface activation of the alloy is poor, so the activation performance of the alloy has decreased sharply [8,9]. The quenching rate dependence of the maximum discharge capacity is illustrated in Fig. 3. It can be derived from Fig. 1 and Fig. 3 that the discharge capacity of the as-quenched alloy is much lower than that of the as-cast alloy. The maximum discharge capacity of the as-cast alloy is 315 mAh/g, and that of the as-quenched alloy obtained with a quenching rate of 5 m/s is 242 mAh/g. When the quenching rate is 26 m/s, the maximum discharge capacity of the as-quenched alloy is lower than 200 mAh/g. The discharge capacity of the alloy decreased sharply with increase of the quenching rate.
3. Results and discussion 3.1. Electrochemical characteristics of the alloy The cycle number dependence of the discharge capacity of the alloy obtained with different quenching rates is
Fig. 1. The relationship between the cycle number and the discharge capacity of alloys obtained with different quenching rates.
Y.-H. Zhang et al. / Journal of Alloys and Compounds 364 (2004) 289–294
291
Fig. 2. The relationship between the quenching rate and the activation number.
The cycle lives of the as-cast and quenched alloys are compared in Fig. 4. The cycle stability of the quenched alloy is superior to that of the as-cost alloy. It can be seen from Fig. 4 that the cycle life of the as-cast alloy is less than 200 cycles, but the discharge capacity of the as-quenched alloy obtained with a quenching rate of 26 m/s is only reduced by 10 mAh/g (5% of the maximum discharge capacity) after 300 charge–discharge cycles.
Measurements of the cycle life of the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy was not carried out beyond 300 cycles. The cycle stability of the alloy was appraised from the ratio between the discharge capacity after 300 cycles and the maximum discharge capacity. Generally, the cycle life is defined by the number of cycles after which the discharge capacity reached with constant current charge–discharge at 300 mAh/g is reduced to 60% of the greatest capacity. The cycle lives of the quenched alloys obtained with quenching rates of 8, 14, 20 and 26 m/s were larger than 300 cycles. Therefore the following method was used for estimating the cycle lives of the quenched alloys. The discharge capacity of the alloy reached the maximum value after the alloy was activated fully and, after that point, decreased almost linearly with increase of the cycle number. The decrease of the capacity after a single cycle, which is known as the capacity decay rate and indicated as D, can be calculated by following formula: D =
Fig. 3. The relationship between the quenching rate and the maximum discharge capacity.
C100,max − C100,300 300 − n
where C100,max is the maximum capacity at discharge rate 100 mA/g, C100,300 the capacity after 300 cycles at discharge rate 100 mA/g, n the cycle number at which the maximum capacity was reached. The cycle life N could be calculated by following formula: N = n+
Fig. 4. The comparison of the cycle lives of the as-cast and quenched alloys.
(1)
C100,max × (1–60%) D
(2)
The calculated results are listed in Table 1. It can be seen from Fig. 4 that decay of capacity is not completely linear. The decay of the discharge capacity was large at the beginning and then became small. So the cycle lives that were calculated by formulas (1) and (2) are smaller than the ones that were measured practically. Anyhow, Table 1 shows that the cycle life of the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy can be enhanced significantly by rapid quenching.
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Table 1 The cycle lives of as-cast and quenched Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloys Quenching rate Cycle life a
As-cast 172a
5 m/s 224a
8 m/s 339
14 m/s 552
20 m/s 581
26 m/s 1267
Represents the values measured practically, the rest were values calculated theoretically.
3.2. Microstructure and morphology of the alloys 3.2.1. Analysis of the phase structures Fig. 5 presents X-ray diffraction diagrams of the as-cast and quenched alloys. The phase structures of the as-cast and quenched Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloys are composed of the C15 main phase and small amounts of the C14 phase and the Zr7 Ni10 phase. The ratios of three phases are
different for different quenching rates. Because the phase structures of the alloy are closely related to electrochemical characteristics [10], it is clear that the different ratios of three phases might have a strong influence on the electrochemical properties of the alloys. The activation capabilities and the discharge capacities as well as the cycle lives of the as-cast and quenched alloys are compared in Fig. 1, Fig. 3 and Table 1. We found that the electrochemical character-
Fig. 5. X-ray diffraction diagrams of the as-cast and quenched alloys.
Y.-H. Zhang et al. / Journal of Alloys and Compounds 364 (2004) 289–294
Fig. 6. The morphologies of the as-cast Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy (SEM).
istics of the as-quenched alloys strongly changed with the increase of the quenching rate. However, it is very difficult to explain the mechanism of the tremendous change of the electrochemical properties on the basis of the ratio of C15, C14 and Zr7 Ni10 phases. By means of an analysis of the microstructure of the alloy, we were able to draw more realistic conclusions.
293
3.2.2. Observation of the microstructures and morphologies The morphologies of the as-cast Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy are composed of two kinds of grains, as shown in Fig. 6. It can be seen from Fig. 6 that the morphologies of the cross-section and the longitudinal profile of the grains show little differences. So we can conclude that the grains of the main phase are almost equi-axial crystals. Rapid quenching produced finer grains and some amorphous structure in the alloy. An amorphous phase is present in the alloy prepared with a quenching rate of 5 m/s (Fig. 7a), and Fig. 7b shows that the amorphous phase and the crystalline phase co-exist in the alloy obtained with quenching rate of 5 m/s. When using a quenching rate of 26 m/s, a larger amount of amorphous phase existed in the alloy. So we can conclude that the amount of the amorphous phase in the quenched alloy increased with increase of the quenching rate. The main reason for the as-quenched alloy having longer cycle life is that an amorphous phase exists in the alloy. The effects on the electrochemical characteristics of the alloy, which were assumed to be produced by different ratios of the C15, C14 and Zr7 Ni10 phases, are obscured by the large amount of amorphous phase present. When no amorphous phase or only a small quantity would have existed in the alloy, the effect of different ratios of these phases might have become apparent.
4. Conclusions (1) The as-cast and quenched Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloys are composed of the C15 main phase and small amounts of the C14 phase and Zr7 Ni10 phase. The ratios of three phases, which can be changed by adjusting
Fig. 7. The morphology and SAD of the as-quenched alloy Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 (5 m/s) (SEM).
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the rapid quenching rate, have a significant influence on the electrochemical characteristics of the alloys. (2) An amorphous phase can be formed easily in the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy when the quenching rate is higher than 5 m/s. The higher the quenching rate, the larger the quantity of the amorphous phase. (3) The amorphous phase can markedly enhance the cycle life, and strongly reduce the discharge capacity and the activation capability of the alloy. The cycle life of the alloy prepared with a quenching rate of 26 m/s is larger than 1000 cycles which is five to six times as large as that of the as-cast alloy. However, the maximum discharge capacity of this alloy is lower than 200 mAh/g, and it could be only fully activated after about 180 charge–discharge cycles.
Acknowledgements This work is supported by National Natural Science Foundations of China (50131040 and 50071050).
References [1] J. Chen, S.X. Dou, H.K. Liu, J. Alloys Comp. 256 (1997) 40– 44. [2] F.J. Liu, H. Ota, S. Okamoto, S. Suda, J. Alloys Comp. 253–254 (1997) 452–458. [3] T. Kohno, H. Yoshida, F. Kawashima, T. Inaba, I. Sakai, M. Yamamoto, M. Kanda, J. Alloys Comp. 311 (2000) L5–L7. [4] D. Cracco, A. Percheron-Guégan, J. Alloys Comp. 268 (1998) 248– 255. [5] D. Sun, M. Latroche, A. Percheron-Guégan, J. Alloys Comp. 257 (1997) 302–305. [6] A. Züttel, D. Chartouni, K. Gross, M. Bachler, L. Schlapbach, J. Alloys Comp. 253–254 (1997) 587–589. [7] M. Jurczyk, W. Rajewski, W. Majachrzycki, G. Wojcik, J. Alloys Comp. 274 (1998) 299–302. [8] X. Gao, D. Song, Y. Zhang, Z. Zhou, W. Zhang, M. Wang, P. Shen, J. Alloys Comp. 229 (1995) 268–273. [9] C. Hong, J. Yang, Y. Yang, J. Chin. Phys. Chem. Soc. 14 (2) (1998) 154–157. [10] B. Knosp, C. Jordy, Ph. Blanchard, T. Berlureau, J. Electrochem. Soc. 5 (145) (1998) 1478–1482.
Electrochemical characteristics and microstructures of rapidly quenched Ti0.5Zr0.5Mn0.2 Cr0.5V0.2 Ni0.95 alloy Yang-Huan Zhang a,b,∗ , Ping Li a,c , Xin-Lin Wang a , Yu-Fang Lin a , Xuan-Hui Qu c a
Department of Functional Material Research, Central Iron and Steel Research Institute, 76 Xuanyuannan Road, Haidian District, Beijing 100081, China b Department of Material Science and Engineering, Baotou Iron and Steel University of Technology, Baotou 014010, China c The Power Metallurgy Institute, University of Science and Technology, Beijing 100083, China Received 7 March 2003; received in revised form 21 May 2003; accepted 21 May 2003
Abstract The microstructures of the as-cast and quenched Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy is composed of the C15 main phase and traces of the C14 phase and Zr7 Ni10 phase. The ratio of the three phases, which could be varied by different quenching rates, influences the electrochemical characteristics of the alloys. An amorphous phase was formed easily in the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy, a larger quantity of the amorphous phase being formed in the as-quenched alloy. The amorphous phase could significantly enhance the cycle stability of the alloy and strongly decrease the discharge capacity and the activation performance of the alloy. The cycle life of the as-quenched alloy obtained with a quenching rate of 26 m/s is larger than 1000 cycles, which is five to six times as large as that of the as-cast alloy. Its maximum discharge capacity is less than 200 mAh/g, and the alloy could be activated completely in about 180 charge–discharge cycles. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Metal hydrides; Electrode materials; Electrochemical reactions
1. Introduction In the last years, much attention was focused on the electric vehicle. The battery used in electric vehicles forms the key for this technology. Therefore, investigations of the Ni/MH battery have become a major topic in the field of battery research. This kind of battery is required to have a large capacity and a long cycle life as well as a current density of over 60–70 Wh/kg. The energy density of the battery depends on the materials used for the positive and negative electrodes and the structure of the battery. It is very difficult to enhance the discharge capacity of the positive electrode of the Ni/MH battery, because its practical discharge capacity has reached over 90% of the theoretical one. However, the discharge capacity of the negative electrode made of a metal hydride still has much room for further improvement. The investigation of the alloy electrode made of AB5 type hydrogen storage alloys has attained a mature stage, its practical capacity approaching the theoretical one. At present, ∗ Corresponding author. Tel.: +86-10-6218-7570; fax: +86-10-6218-7570. E-mail address: [email protected] (Y.-H. Zhang).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00551-6
investigations of hydrogen storage alloys with high capacity are focused on Zr-based, Mg-based and Ti-based alloys [1–4]. Mg-based and Ti-based hydrogen storage alloys are not used actually because the investigations on the cycle stability of these alloys have not yet reached a significant breakthrough. Zr-based hydrogen storage alloys have led to large scale industrialization. The main defects of Zr-based hydrogen storage alloy are higher cost of the production, poor activation performance and low rate discharge capability. The investigations on these alloys concentrate on how to improve their synthetic routes. The cycle stability of the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy prepared by rapid quenching was enhanced extremely, but the rest of the electrochemical characteristics needs further improvement.
2. Experimental 2.1. Preparation of alloys An alloy of the nominal composition Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 was melted in a vacuum induction furnace in an argon atmosphere. After induction melting, the melt was
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rapidly cooled by pouring it into a copper mould cooled by water. Part of the cast alloy was re-melted and quenched by melt spinning with a rotating copper wheel. Flakes of the rapidly quenched alloy were obtained with quenching rates of 2, 8, 14, 20, 26 and 32 m/s, the quenching rate being expressed by the linear velocity of the copper wheel. 2.2. Electrochemical measurement The alloy samples were ground mechanically into powder to below 250 meshes. Electrode pellets (15 mm in diameter) were prepared by mixing 1 g alloy powder and fine nickel powder in a weight ratio of 1:1 together with a small amount of polyvinyl alcohol (PVA) solution as binder. Then pressing at 3500 kg cm−2 was applied. After drying for 2 h at room temperature, the sample electrodes were immersed in 6 M KOH solution for 1 day in order to fully wet the electrode before the electrochemical measurements. The electrochemical measurements were carried out in a tri-electrode open cell with Ni(OH)2 /Ni(OOH) as the positive electrode, Hg/HgO as the reference electrode, and a 6 M KOH solution as electrolyte. The voltage between the negative electrode and the reference electrode was defined as the discharge voltage. In every cycle the electrode was charged with a constant current of 100 mA/g for 5 h, resting 15 min, then discharged with constant current of 100 mA/g, the cut-off voltage was −0.500 V. The environment temperature of the measurement was kept at 30 ◦ C. 2.3. Microstructure determination and morphology observation The samples of the as-cast alloy were directly polished and pieces of flake of the as-quenched alloy were fixed on a grid made of stainless steel after melt spinning. The samples thus prepared were etched with a 60% HF solution. The morphologies and the micro zone compositions of the alloy were observed and analyzed by SEM. The phase structures of the alloy were detected by XRD. The type of X-ray diffractometer used in this experiment is D/max/2400, the diffraction was performed with Cu K␣1 and the rays were filtered by graphite. The experimental parameters for determining the phase composition were: 160 mA, 40 kV and 10◦ /min. The alloy samples were pulverized by mechanical grinding. The obtained samples were dispersed in absolute alcohol for observing the grain morphology with TEM, and for determining with SAD whether an amorphous phase existed in the samples.
illustrated in Fig. 1. Fig. 2 represented the quenching rate dependence of the activation number. It can be derived from Figs. 1 and 2 that the activation capabilities of the as-cast and quenched Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloys are very poor. The cast alloy was completely activated after 25 cycles and it needed at least 40 cycles for the quenched alloy. The activated number increased with increase of the quenching rate. The quenched alloy obtained with quenching rate of 26 m/s was activated not fully even after 170 cycles. Obviously, the activation mechanism of the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy was the same as that of the Zr-based hydrogen storage alloy. The compact oxidation film, which was formed on the surface of the Zr-based alloy submersed in the electrolyte, could prevent the hydrolysis and the percolation of hydrogen, so the activation performance of the alloy was very poor. Surface treatment is an effective method of improving the activation performance of Zr-based alloys [5], and the multi-element alloying is another one. Zr can enhance the cycle life and reduce the activation capability of the alloy, but the effect of Ti is contrary to that of Zr [6,7]. Therefore, the activation performance of the alloy can be improved by increasing the Ti/Zr ratio. We can conclude that the poor activation performance of the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy is due to the relatively high Zr content. Because the surface to bulk ratio of the as-quenched alloy is rather large and the surface activation of the alloy is poor, so the activation performance of the alloy has decreased sharply [8,9]. The quenching rate dependence of the maximum discharge capacity is illustrated in Fig. 3. It can be derived from Fig. 1 and Fig. 3 that the discharge capacity of the as-quenched alloy is much lower than that of the as-cast alloy. The maximum discharge capacity of the as-cast alloy is 315 mAh/g, and that of the as-quenched alloy obtained with a quenching rate of 5 m/s is 242 mAh/g. When the quenching rate is 26 m/s, the maximum discharge capacity of the as-quenched alloy is lower than 200 mAh/g. The discharge capacity of the alloy decreased sharply with increase of the quenching rate.
3. Results and discussion 3.1. Electrochemical characteristics of the alloy The cycle number dependence of the discharge capacity of the alloy obtained with different quenching rates is
Fig. 1. The relationship between the cycle number and the discharge capacity of alloys obtained with different quenching rates.
Y.-H. Zhang et al. / Journal of Alloys and Compounds 364 (2004) 289–294
291
Fig. 2. The relationship between the quenching rate and the activation number.
The cycle lives of the as-cast and quenched alloys are compared in Fig. 4. The cycle stability of the quenched alloy is superior to that of the as-cost alloy. It can be seen from Fig. 4 that the cycle life of the as-cast alloy is less than 200 cycles, but the discharge capacity of the as-quenched alloy obtained with a quenching rate of 26 m/s is only reduced by 10 mAh/g (5% of the maximum discharge capacity) after 300 charge–discharge cycles.
Measurements of the cycle life of the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy was not carried out beyond 300 cycles. The cycle stability of the alloy was appraised from the ratio between the discharge capacity after 300 cycles and the maximum discharge capacity. Generally, the cycle life is defined by the number of cycles after which the discharge capacity reached with constant current charge–discharge at 300 mAh/g is reduced to 60% of the greatest capacity. The cycle lives of the quenched alloys obtained with quenching rates of 8, 14, 20 and 26 m/s were larger than 300 cycles. Therefore the following method was used for estimating the cycle lives of the quenched alloys. The discharge capacity of the alloy reached the maximum value after the alloy was activated fully and, after that point, decreased almost linearly with increase of the cycle number. The decrease of the capacity after a single cycle, which is known as the capacity decay rate and indicated as D, can be calculated by following formula: D =
Fig. 3. The relationship between the quenching rate and the maximum discharge capacity.
C100,max − C100,300 300 − n
where C100,max is the maximum capacity at discharge rate 100 mA/g, C100,300 the capacity after 300 cycles at discharge rate 100 mA/g, n the cycle number at which the maximum capacity was reached. The cycle life N could be calculated by following formula: N = n+
Fig. 4. The comparison of the cycle lives of the as-cast and quenched alloys.
(1)
C100,max × (1–60%) D
(2)
The calculated results are listed in Table 1. It can be seen from Fig. 4 that decay of capacity is not completely linear. The decay of the discharge capacity was large at the beginning and then became small. So the cycle lives that were calculated by formulas (1) and (2) are smaller than the ones that were measured practically. Anyhow, Table 1 shows that the cycle life of the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy can be enhanced significantly by rapid quenching.
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Table 1 The cycle lives of as-cast and quenched Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloys Quenching rate Cycle life a
As-cast 172a
5 m/s 224a
8 m/s 339
14 m/s 552
20 m/s 581
26 m/s 1267
Represents the values measured practically, the rest were values calculated theoretically.
3.2. Microstructure and morphology of the alloys 3.2.1. Analysis of the phase structures Fig. 5 presents X-ray diffraction diagrams of the as-cast and quenched alloys. The phase structures of the as-cast and quenched Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloys are composed of the C15 main phase and small amounts of the C14 phase and the Zr7 Ni10 phase. The ratios of three phases are
different for different quenching rates. Because the phase structures of the alloy are closely related to electrochemical characteristics [10], it is clear that the different ratios of three phases might have a strong influence on the electrochemical properties of the alloys. The activation capabilities and the discharge capacities as well as the cycle lives of the as-cast and quenched alloys are compared in Fig. 1, Fig. 3 and Table 1. We found that the electrochemical character-
Fig. 5. X-ray diffraction diagrams of the as-cast and quenched alloys.
Y.-H. Zhang et al. / Journal of Alloys and Compounds 364 (2004) 289–294
Fig. 6. The morphologies of the as-cast Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy (SEM).
istics of the as-quenched alloys strongly changed with the increase of the quenching rate. However, it is very difficult to explain the mechanism of the tremendous change of the electrochemical properties on the basis of the ratio of C15, C14 and Zr7 Ni10 phases. By means of an analysis of the microstructure of the alloy, we were able to draw more realistic conclusions.
293
3.2.2. Observation of the microstructures and morphologies The morphologies of the as-cast Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy are composed of two kinds of grains, as shown in Fig. 6. It can be seen from Fig. 6 that the morphologies of the cross-section and the longitudinal profile of the grains show little differences. So we can conclude that the grains of the main phase are almost equi-axial crystals. Rapid quenching produced finer grains and some amorphous structure in the alloy. An amorphous phase is present in the alloy prepared with a quenching rate of 5 m/s (Fig. 7a), and Fig. 7b shows that the amorphous phase and the crystalline phase co-exist in the alloy obtained with quenching rate of 5 m/s. When using a quenching rate of 26 m/s, a larger amount of amorphous phase existed in the alloy. So we can conclude that the amount of the amorphous phase in the quenched alloy increased with increase of the quenching rate. The main reason for the as-quenched alloy having longer cycle life is that an amorphous phase exists in the alloy. The effects on the electrochemical characteristics of the alloy, which were assumed to be produced by different ratios of the C15, C14 and Zr7 Ni10 phases, are obscured by the large amount of amorphous phase present. When no amorphous phase or only a small quantity would have existed in the alloy, the effect of different ratios of these phases might have become apparent.
4. Conclusions (1) The as-cast and quenched Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloys are composed of the C15 main phase and small amounts of the C14 phase and Zr7 Ni10 phase. The ratios of three phases, which can be changed by adjusting
Fig. 7. The morphology and SAD of the as-quenched alloy Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 (5 m/s) (SEM).
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the rapid quenching rate, have a significant influence on the electrochemical characteristics of the alloys. (2) An amorphous phase can be formed easily in the Ti0.5 Zr0.5 Mn0.2 Cr0.5 V0.2 Ni0.95 alloy when the quenching rate is higher than 5 m/s. The higher the quenching rate, the larger the quantity of the amorphous phase. (3) The amorphous phase can markedly enhance the cycle life, and strongly reduce the discharge capacity and the activation capability of the alloy. The cycle life of the alloy prepared with a quenching rate of 26 m/s is larger than 1000 cycles which is five to six times as large as that of the as-cast alloy. However, the maximum discharge capacity of this alloy is lower than 200 mAh/g, and it could be only fully activated after about 180 charge–discharge cycles.
Acknowledgements This work is supported by National Natural Science Foundations of China (50131040 and 50071050).
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