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Kinetics of DyNi2 film growth by electrochemical implantation
Electrochimica Acta 48 (2003) 563 /568 www.elsevier.com/locate/electacta
Kinetics of DyNi2 film growth by electrochemical implantation H. Konishi, T. Nohira, Y. Ito ,1 Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received 8 October 2002
Abstract Electrochemical implantation was performed at Ni electrodes to form DyNi2 films at 0.55 V (vs. Li /Li), 0.62 V, and 0.70 V for 0.5 /5.0 h in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K. It was found that the DyNi2 films grew linearly with time with coulomb efficiency of about 100%. The obtained growth rates were higher at more negative potentials, i.e., 0.47 mm min 1 at 0.55 V, 0.32 mm min 1 at 0.62 V, and 0.14 mm min 1 at 0.70 V. On the analogy of the metal oxide growth, the observed rapid and linear growth of DyNi2 films may be explained by the existence of the outer and inner DyNi2 layers. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: DyNi2 film; Electrochemical implantation; Growth rate; Coulomb efficiency; LiCl /KCl
1. Introduction Rare earth-transition metal alloys attract the great interest of many researchers for their functional properties, e.g., excellent magnetic, hydrogen absorbing or permeating, and catalytic properties. As a new formation method of these alloys, a molten salt electrochemical process, which utilizes the cathodic reduction of rare earth ions (Ce, Dy, Nd, Sm, and Y) on transition metal substrates (Co, Fe, and Ni), has been studied in the authors’ laboratory [1 /9]. Generally, there are a lot of advantages in forming rare earth-transition metal alloys by the molten salt electrochemical process. One of the most promising advantages is that composition and thickness of the alloys can be controlled by electrochemical parameters such as potential and current density. As a typical example, it has been reported by the authors that the formation and phase control of Dy /Ni alloy films are possible by controlling the electrode potential in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K [5]. Through the study, it has been found that potentiostatic electrolysis of a Ni electrode resulted in the rapid formation of a DyNi2 film. Corresponding author E-mail address: [email protected] (Y. Ito). 1 ISE member.
Considering that the experimental temperature (700 K) was lower than half of the melting point of DyNi2 (Tm /1531 K), the growth rate of DyNi2 film, i.e., 30 mm h 1, was extremely high. Moreover, the growth rate was dependent on the applied potential, i.e. higher growth rate at more negative potential. Since the phenomenon was difficult to be explained by ordinary concepts of electrodeposition followed by solid phase diffusion, the process was regarded as ‘electrochemical implantation’ [5]. On the other hand, Xie et al. [2] reported that the YNi2 film of 400 mm was formed merely in 20 h in a molten LiCl /KCl /NaCl /YCl3 system at 773 K. In the report, the diffusion coefficient of Y in a single YNi2 phase region was estimated to be (2.849/0.40) /10 8 cm2 s 1 by an electrochemical transient technique. The growth rate was also strongly affected by electrochemical parameters such as potential and current density. Concerning the growth rate of YNi2, Hachiya and Ito conducted the molecular dynamics simulation and reported that the rapid growth is mainly due to the high-rate self diffusion in and near the grain boundaries [9]. Since the crystal structures of both DyNi2 and YNi2 are Laves phase, there seems to be a common mechanism which accounts for the rapid growth. From this background, kinetics of DyNi2 film growth by electrochemical implantation was investigated in a molten LiCl /KCl/DyCl3 (0.50 mol%) system at 700 K.
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 7 2 3 - 5
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The DyNi2 films were formed at three different potentials, i.e. 0.55 V (vs. Li /Li), 0.62 V, and 0.70 V, and for various periods, from 0.5 to 5.0 h. Thickness of each DyNi2 film was measured by cross-sectional SEM observation to estimate the growth rate of DyNi2 film.
2. Experimental LiCl (99.0%, Wako Pure Chemical Co. Ltd.) and KCl (99.5%, Wako Pure Chemical Co. Ltd.) were mixed to have an eutectic composition (LiCl:KCl /58.5:41.5 mol%), and introduced in a high purity alumina crucible (99.5 wt% Al2O3, SSA-S grade, NIKKATO Co. Ltd.). This was kept under vacuum for more than 72 h at 473 K to remove water. All experiments were performed in the LiCl /KCl eutectic melt in a dry argon atmosphere at 700 K. Anhydrous DyCl3 (99.9%, Kojundo Chemical Laboratory Co. Ltd.) was added directly to the melts. To remove residual water and metal impurities, preelectrolysis was conducted at an electrolysis voltage of 2.80 V with an Al plate cathode and a glassy carbon anode. The pre-electrolysis was terminated when the cathodic current density fell lower than 0.50 mA cm 2. A chromel /alumel thermocouple was used for temperature measurement with an accuracy of 9/1 K. The working electrodes were a Mo wire (5 mm /f1 mm, 99.95% Nilaco Co., Ltd.) and Ni sheets (10 mm /5 mm /0.2 mm or 0.05 mm, 99.7%, Nilaco Co. Ltd.). The reference electrode was a silver wire immersed in a LiCl /KCl containing 1 mol% of AgCl, set in a Pyrex glass tube with thin bottom to maintain electrical contact with the melt. The potential of this reference electrode was calibrated with reference to that of a Li / Li electrode, which was prepared by electrodepositing Li metal on a Mo wire. All the potentials given hereafter are referred to this Li /Li potential. The counter electrode was a glassy carbon rod (50 mm /f5 mm, Tokai Carbon Co. Ltd.) or an Al rod (50 mm /f5 mm, 99.9%, Nilaco Co. Ltd.). A potentio/galvanostat (Solartron: SI1287) was used to obtain the electrochemical data. The samples were prepared by potentiostatic electrolysis, and rinsed with distilled water. The obtained samples were analyzed by XRD (Rigaku Denki Co. Ltd., ultraX 18HB) with Cu Ka line at 50 kV and 300 mA. The surface of the samples was observed by SEM (Hitachi Co. Ltd., S-3500H).
3. Results and discussion 3.1. Formation potential for each Dy /Ni alloy phase To present general information on the Dy /Ni system, the reported Dy /Ni phase diagram [10] is shown in Fig. 1. The Dy /Ni phase diagram shows the presence of ten
intermetallic compounds (Dy3Ni, Dy3Ni2, DyNi, DyNi2, DyNi3, Dy2Ni7, DyNi4, Dy4Ni17, DyNi5, and Dy2Ni17) at the experimental temperature of 700 K. Open-circuit potentiometry was carried out to investigate the formation potential of Dy /Ni alloys. Fig. 2 shows the open-circuit potential transient curve for a Ni electrode after depositing Dy metal by potentiostatic electrolysis at 0.40 V for 120 s in a molten LiCl /KCl / DyCl3 (0.50 mol%) system at 700 K. There are four potential plateaus at (1) 0.78 V, (2) 0.95 V, (3) 1.24 V, and (4) 1.52 V, respectively. From our previous work [5], the observed potential plateaus correspond to coexisting phase states of (DyNi2/DyNi3), (DyNi3/Dy2Ni7), (Dy2Ni7/DyNi5), and (DyNi5/Ni), respectively. Table 1 shows the transformation reactions and the corresponding equilibrium potential values for Dy /Ni phase states obtained from five independent open-circuit potential measurements at 700 K. Combining this result with our previous results [5], the DyNi2 was found to be stable at least in the potential region of 0.55 /0.78 V. 3.2. Growth of DyNi2 films by electrochemical implantation In order to investigate the effect of electrolysis potential on the DyNi2 film growth during electrochemical implantation, potentiostatic electrolysis was conducted at Ni electrodes at 0.55, 0.62, and 0.70 V at 700 K. The electrolysis times were changed from 0.5 to 5.0 h at all potentials. Since all potential values are within the stable potential region of DyNi2, Dy metal does not deposit at these potentials. Fig. 3 shows the obtained current /time curve at 0.55 V as an example. Although large cathodic current of 15 mA cm 2 flowed at the beginning, the currents decreased to the constant value of about 8 mA cm 2 in 1.0 h. Similar current /time curves were observed at 0.62 V and 0.70 V, except the constant current became smaller as the potential value increased. All samples obtained in this study were identified as DyNi2 by XRD analysis. Fig. 4(a) /(c) show cross-sectional SEM images of the samples obtained at 0.55 V for 0.5, 1.5, and 2.0 h, respectively. Coherent and dense DyNi2 films with thickness of about 17, 31, and 60 mm are observed. From the observed film thickness and the quantity of electricity, coulomb efficiency was estimated to be about 111% at 0.55 V. The deviation from 100% is considered to be due to errors in the estimation of film thickness. Therefore, coulomb efficiency can be regarded as almost 100%. Fig. 4(d) /(f) and (g) /(i) show cross-sectional SEM images of the samples obtained at 0.62 and 0.70 V, respectively. Coherent and dense DyNi2 films were also obtained at these potentials. Details of the electrolysis condition and the obtained results are summarized in Table 2. The relationship between thickness of the DyNi2 and the quantity of electricity is shown in Fig. 5. For all cases,
H. Konishi et al. / Electrochimica Acta 48 (2003) 563 /568
565
Fig. 1. Phase diagram of Dy /Ni system [10].
Fig. 2. Open-circuit potential transient curve for a Ni electrode after depositing Dy metal by potentiostatic electrolysis at 0.40 V for 120 s in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K.
Table 1 The transformation reactions and the corresponding equilibrium potential values for Dy /Ni phase state obtained from five independent open-circuit potential measurements in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K Potential value (V vs. Li / Li)
Reaction
0.77690.013 0.94790.004 1.24190.008
2 DyNi3Dy(III)3e 3 DyNi2 3 Dy2Ni7Dy(III)3e 7 DyNi3 7/3 DyNi5Dy(III)3e 5/3 Dy2Ni7 5 NiDy(III)3e DyNi5
1.56190.027
In this paper, uncertainty is given with standard deviation.
Fig. 3. Current /time curve for a Ni electrode by potentiostatic electrolysis at 0.55 V for 5 h in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K.
thickness of the DyNi2 was almost proportional to the quantity of electricity. In addition, the coulomb efficiencies were about 100%. The relationship between thickness of the formed DyNi2 films and the electrolysis time is shown in Fig. 6. Almost linear relation is seen for all potentials. Moreover, the slopes, which mean growth rates, are higher at more negative potential, i.e. 0.47 mm min 1 at 0.55 V, 0.32 mm min 1 at 0.62 V, and 0.14 mm min 1 at 0.70 V. Concerning the observed linear growth of DyNi2 film, the following explanation is proposed. According to the obtained current /time curve in Fig. 3, the growth of film at the initial period (within 15 min) seems to follow
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Fig. 4. Cross-sectional SEM images of the samples obtained by electrochemical implantation in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K: (a) 0.5, (b) 1.5, (c) 2.0 h at 0.55 V, (d) 0.5, (e) 1.5, (f) 2.0 h at 0.62 V, and (g) 0.5, (h) 1.0, (i) 2.0 h at 0.70 V.
the parabolic rate law, but afterwards the growth changes to follow the linear rate law. Such transition from parabolic to linear kinetics is often observed in the
growth of metal oxide films [11,12]. From the analogy to the growth of metal oxides, the observed phenomenon may be explained by the existences of the outer and
Table 2 Summary of the conditions of sample preparation and the obtained results Potential value (V vs. Li /Li)
Electrolysis time (h)
Quantity of electricity (C)
Thickness of DyNi2 film (mm)
0.55
0.5 1.0 1.5 2.0 5.0
17 29 31 54 137
17 25 31 60 140
0.62
0.5 1.0 1.5 2.0 5.0
9 21 24 31 90
9 17 24 28 95
0.70
0.5 1.0 1.5 2.0 5.0
4 5 10 17 40
3 6 10 16 40
H. Konishi et al. / Electrochimica Acta 48 (2003) 563 /568
567
Fig. 5. The relationship between the thickness of DyNi2 films obtained by electrochemical implantation and the quantity of electricity in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K: (a) 0.55 V, (b) 0.62 V, and (c) 0.70 V. The broken line is the theoretical line for the coulomb efficiency of 100%. Fig. 7. Schematic representation of the growth of DyNi2 film: (a) initial parabolic growth stage; (b) linear growth stage.
diffusion in the outer DyNi2 layer is not the rate determining step. Therefore, the growth of film follows the parabolic rate at the initial period, then changes to the linear rate law afterwards. In order to confirm this explanation, however, further metallographic study on microscopic structure is necessary.
4. Conclusion
Fig. 6. The relationship between the thickness of DyNi2 films obtained by electrochemical implantation and the time in a molten LiCl /KCl / DyCl3 (0.50 mol%) system at 700 K: (a) 0.55 V; (b) 0.62 V; and (c) 0.70 V.
inner DyNi2 layers as shown in Fig. 7. In the initial period of the growth, dense DyNi2 film, termed the inner DyNi2 layer, is thought to form. Since the growth is limited by Dy diffusion, the growth follows the parabolic rate law. Then the inner DyNi2 layer is expected to transform to the outer DyNi2 layer at constant rate. Thus, thickness of the inner DyNi2 layer is kept constant at the following period. The outer DyNi2 layer is assumed to have a large Dy diffusivity due to microscopic cracks and/or grain boundaries that serve as fast diffusion paths. This means that the
Kinetics of DyNi2 film growth by electrochemical implantation was investigated in a molten LiCl /KCl / DyCl3 (0.50 mol%) system at 700 K. The conclusions of this study are as follows. (1) When a Ni electrode is cathodically polarized at 0.55 /0.70 V, the DyNi2 film grows linearly with time with coulomb efficiency of about 100%. (2) The growth rates are higher at more negative potentials, i.e. 0.47 mm min 1 at 0.55 V, 0.32 mm min 1 at 0.62 V, and 0.14 mm min 1 at 0.70 V. (3) On the analogy of the metal oxide growth, the observed rapid and linear growth of DyNi2 films may be explained by the existence of the outer and inner DyNi2 layers.
Acknowledgements This work was supported by a grant-in-aid from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
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References [1] [2] [3] [4] [5]
G. Xie, K. Ema, Y. Ito, J. Appl. Electrochem. 23 (1993) 753. G. Xie, K. Ema, Y. Ito, J. Appl. Electrochem. 24 (1994) 321. Y. Ito, T. Nohira, Electrochim. Acta 45 (2000) 2611. T. Iida, T. Nohira, Y. Ito, Electrochim. Acta 46 (2001) 2537. H. Konishi, T. Nohira, Y. Ito, J. Electrochem. Soc. 148 (7) (2001) C506. [6] H. Konishi, T. Nohira, Y. Ito, Electrochim. Acta 47 (2002) 3533.
[7] H. Konishi, T. Nohira, Y. Ito, Electrochem. Solid-State Lett. 2 (12) (2002) B37. [8] H. Konishi, T. Nohira, Y. Ito, Electrochim. Acta, submitted for publication. [9] K. Hachiya, Y. Ito, J. Alloys Compounds 279 (1998) 171. [10] T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, vol. 2, ASM International, 1990, p. 1540. [11] W.W. Webb, J.T. Norton, C. Wagner, J. Electrochem. Soc. 103 (1956) 107. [12] V.P. Zhdanov, P.R. Norton, Appl. Surf. Sci. 99 (3) (1996) 205.
Kinetics of DyNi2 film growth by electrochemical implantation H. Konishi, T. Nohira, Y. Ito ,1 Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received 8 October 2002
Abstract Electrochemical implantation was performed at Ni electrodes to form DyNi2 films at 0.55 V (vs. Li /Li), 0.62 V, and 0.70 V for 0.5 /5.0 h in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K. It was found that the DyNi2 films grew linearly with time with coulomb efficiency of about 100%. The obtained growth rates were higher at more negative potentials, i.e., 0.47 mm min 1 at 0.55 V, 0.32 mm min 1 at 0.62 V, and 0.14 mm min 1 at 0.70 V. On the analogy of the metal oxide growth, the observed rapid and linear growth of DyNi2 films may be explained by the existence of the outer and inner DyNi2 layers. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: DyNi2 film; Electrochemical implantation; Growth rate; Coulomb efficiency; LiCl /KCl
1. Introduction Rare earth-transition metal alloys attract the great interest of many researchers for their functional properties, e.g., excellent magnetic, hydrogen absorbing or permeating, and catalytic properties. As a new formation method of these alloys, a molten salt electrochemical process, which utilizes the cathodic reduction of rare earth ions (Ce, Dy, Nd, Sm, and Y) on transition metal substrates (Co, Fe, and Ni), has been studied in the authors’ laboratory [1 /9]. Generally, there are a lot of advantages in forming rare earth-transition metal alloys by the molten salt electrochemical process. One of the most promising advantages is that composition and thickness of the alloys can be controlled by electrochemical parameters such as potential and current density. As a typical example, it has been reported by the authors that the formation and phase control of Dy /Ni alloy films are possible by controlling the electrode potential in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K [5]. Through the study, it has been found that potentiostatic electrolysis of a Ni electrode resulted in the rapid formation of a DyNi2 film. Corresponding author E-mail address: [email protected] (Y. Ito). 1 ISE member.
Considering that the experimental temperature (700 K) was lower than half of the melting point of DyNi2 (Tm /1531 K), the growth rate of DyNi2 film, i.e., 30 mm h 1, was extremely high. Moreover, the growth rate was dependent on the applied potential, i.e. higher growth rate at more negative potential. Since the phenomenon was difficult to be explained by ordinary concepts of electrodeposition followed by solid phase diffusion, the process was regarded as ‘electrochemical implantation’ [5]. On the other hand, Xie et al. [2] reported that the YNi2 film of 400 mm was formed merely in 20 h in a molten LiCl /KCl /NaCl /YCl3 system at 773 K. In the report, the diffusion coefficient of Y in a single YNi2 phase region was estimated to be (2.849/0.40) /10 8 cm2 s 1 by an electrochemical transient technique. The growth rate was also strongly affected by electrochemical parameters such as potential and current density. Concerning the growth rate of YNi2, Hachiya and Ito conducted the molecular dynamics simulation and reported that the rapid growth is mainly due to the high-rate self diffusion in and near the grain boundaries [9]. Since the crystal structures of both DyNi2 and YNi2 are Laves phase, there seems to be a common mechanism which accounts for the rapid growth. From this background, kinetics of DyNi2 film growth by electrochemical implantation was investigated in a molten LiCl /KCl/DyCl3 (0.50 mol%) system at 700 K.
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 7 2 3 - 5
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H. Konishi et al. / Electrochimica Acta 48 (2003) 563 /568
The DyNi2 films were formed at three different potentials, i.e. 0.55 V (vs. Li /Li), 0.62 V, and 0.70 V, and for various periods, from 0.5 to 5.0 h. Thickness of each DyNi2 film was measured by cross-sectional SEM observation to estimate the growth rate of DyNi2 film.
2. Experimental LiCl (99.0%, Wako Pure Chemical Co. Ltd.) and KCl (99.5%, Wako Pure Chemical Co. Ltd.) were mixed to have an eutectic composition (LiCl:KCl /58.5:41.5 mol%), and introduced in a high purity alumina crucible (99.5 wt% Al2O3, SSA-S grade, NIKKATO Co. Ltd.). This was kept under vacuum for more than 72 h at 473 K to remove water. All experiments were performed in the LiCl /KCl eutectic melt in a dry argon atmosphere at 700 K. Anhydrous DyCl3 (99.9%, Kojundo Chemical Laboratory Co. Ltd.) was added directly to the melts. To remove residual water and metal impurities, preelectrolysis was conducted at an electrolysis voltage of 2.80 V with an Al plate cathode and a glassy carbon anode. The pre-electrolysis was terminated when the cathodic current density fell lower than 0.50 mA cm 2. A chromel /alumel thermocouple was used for temperature measurement with an accuracy of 9/1 K. The working electrodes were a Mo wire (5 mm /f1 mm, 99.95% Nilaco Co., Ltd.) and Ni sheets (10 mm /5 mm /0.2 mm or 0.05 mm, 99.7%, Nilaco Co. Ltd.). The reference electrode was a silver wire immersed in a LiCl /KCl containing 1 mol% of AgCl, set in a Pyrex glass tube with thin bottom to maintain electrical contact with the melt. The potential of this reference electrode was calibrated with reference to that of a Li / Li electrode, which was prepared by electrodepositing Li metal on a Mo wire. All the potentials given hereafter are referred to this Li /Li potential. The counter electrode was a glassy carbon rod (50 mm /f5 mm, Tokai Carbon Co. Ltd.) or an Al rod (50 mm /f5 mm, 99.9%, Nilaco Co. Ltd.). A potentio/galvanostat (Solartron: SI1287) was used to obtain the electrochemical data. The samples were prepared by potentiostatic electrolysis, and rinsed with distilled water. The obtained samples were analyzed by XRD (Rigaku Denki Co. Ltd., ultraX 18HB) with Cu Ka line at 50 kV and 300 mA. The surface of the samples was observed by SEM (Hitachi Co. Ltd., S-3500H).
3. Results and discussion 3.1. Formation potential for each Dy /Ni alloy phase To present general information on the Dy /Ni system, the reported Dy /Ni phase diagram [10] is shown in Fig. 1. The Dy /Ni phase diagram shows the presence of ten
intermetallic compounds (Dy3Ni, Dy3Ni2, DyNi, DyNi2, DyNi3, Dy2Ni7, DyNi4, Dy4Ni17, DyNi5, and Dy2Ni17) at the experimental temperature of 700 K. Open-circuit potentiometry was carried out to investigate the formation potential of Dy /Ni alloys. Fig. 2 shows the open-circuit potential transient curve for a Ni electrode after depositing Dy metal by potentiostatic electrolysis at 0.40 V for 120 s in a molten LiCl /KCl / DyCl3 (0.50 mol%) system at 700 K. There are four potential plateaus at (1) 0.78 V, (2) 0.95 V, (3) 1.24 V, and (4) 1.52 V, respectively. From our previous work [5], the observed potential plateaus correspond to coexisting phase states of (DyNi2/DyNi3), (DyNi3/Dy2Ni7), (Dy2Ni7/DyNi5), and (DyNi5/Ni), respectively. Table 1 shows the transformation reactions and the corresponding equilibrium potential values for Dy /Ni phase states obtained from five independent open-circuit potential measurements at 700 K. Combining this result with our previous results [5], the DyNi2 was found to be stable at least in the potential region of 0.55 /0.78 V. 3.2. Growth of DyNi2 films by electrochemical implantation In order to investigate the effect of electrolysis potential on the DyNi2 film growth during electrochemical implantation, potentiostatic electrolysis was conducted at Ni electrodes at 0.55, 0.62, and 0.70 V at 700 K. The electrolysis times were changed from 0.5 to 5.0 h at all potentials. Since all potential values are within the stable potential region of DyNi2, Dy metal does not deposit at these potentials. Fig. 3 shows the obtained current /time curve at 0.55 V as an example. Although large cathodic current of 15 mA cm 2 flowed at the beginning, the currents decreased to the constant value of about 8 mA cm 2 in 1.0 h. Similar current /time curves were observed at 0.62 V and 0.70 V, except the constant current became smaller as the potential value increased. All samples obtained in this study were identified as DyNi2 by XRD analysis. Fig. 4(a) /(c) show cross-sectional SEM images of the samples obtained at 0.55 V for 0.5, 1.5, and 2.0 h, respectively. Coherent and dense DyNi2 films with thickness of about 17, 31, and 60 mm are observed. From the observed film thickness and the quantity of electricity, coulomb efficiency was estimated to be about 111% at 0.55 V. The deviation from 100% is considered to be due to errors in the estimation of film thickness. Therefore, coulomb efficiency can be regarded as almost 100%. Fig. 4(d) /(f) and (g) /(i) show cross-sectional SEM images of the samples obtained at 0.62 and 0.70 V, respectively. Coherent and dense DyNi2 films were also obtained at these potentials. Details of the electrolysis condition and the obtained results are summarized in Table 2. The relationship between thickness of the DyNi2 and the quantity of electricity is shown in Fig. 5. For all cases,
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Fig. 1. Phase diagram of Dy /Ni system [10].
Fig. 2. Open-circuit potential transient curve for a Ni electrode after depositing Dy metal by potentiostatic electrolysis at 0.40 V for 120 s in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K.
Table 1 The transformation reactions and the corresponding equilibrium potential values for Dy /Ni phase state obtained from five independent open-circuit potential measurements in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K Potential value (V vs. Li / Li)
Reaction
0.77690.013 0.94790.004 1.24190.008
2 DyNi3Dy(III)3e 3 DyNi2 3 Dy2Ni7Dy(III)3e 7 DyNi3 7/3 DyNi5Dy(III)3e 5/3 Dy2Ni7 5 NiDy(III)3e DyNi5
1.56190.027
In this paper, uncertainty is given with standard deviation.
Fig. 3. Current /time curve for a Ni electrode by potentiostatic electrolysis at 0.55 V for 5 h in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K.
thickness of the DyNi2 was almost proportional to the quantity of electricity. In addition, the coulomb efficiencies were about 100%. The relationship between thickness of the formed DyNi2 films and the electrolysis time is shown in Fig. 6. Almost linear relation is seen for all potentials. Moreover, the slopes, which mean growth rates, are higher at more negative potential, i.e. 0.47 mm min 1 at 0.55 V, 0.32 mm min 1 at 0.62 V, and 0.14 mm min 1 at 0.70 V. Concerning the observed linear growth of DyNi2 film, the following explanation is proposed. According to the obtained current /time curve in Fig. 3, the growth of film at the initial period (within 15 min) seems to follow
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Fig. 4. Cross-sectional SEM images of the samples obtained by electrochemical implantation in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K: (a) 0.5, (b) 1.5, (c) 2.0 h at 0.55 V, (d) 0.5, (e) 1.5, (f) 2.0 h at 0.62 V, and (g) 0.5, (h) 1.0, (i) 2.0 h at 0.70 V.
the parabolic rate law, but afterwards the growth changes to follow the linear rate law. Such transition from parabolic to linear kinetics is often observed in the
growth of metal oxide films [11,12]. From the analogy to the growth of metal oxides, the observed phenomenon may be explained by the existences of the outer and
Table 2 Summary of the conditions of sample preparation and the obtained results Potential value (V vs. Li /Li)
Electrolysis time (h)
Quantity of electricity (C)
Thickness of DyNi2 film (mm)
0.55
0.5 1.0 1.5 2.0 5.0
17 29 31 54 137
17 25 31 60 140
0.62
0.5 1.0 1.5 2.0 5.0
9 21 24 31 90
9 17 24 28 95
0.70
0.5 1.0 1.5 2.0 5.0
4 5 10 17 40
3 6 10 16 40
H. Konishi et al. / Electrochimica Acta 48 (2003) 563 /568
567
Fig. 5. The relationship between the thickness of DyNi2 films obtained by electrochemical implantation and the quantity of electricity in a molten LiCl /KCl /DyCl3 (0.50 mol%) system at 700 K: (a) 0.55 V, (b) 0.62 V, and (c) 0.70 V. The broken line is the theoretical line for the coulomb efficiency of 100%. Fig. 7. Schematic representation of the growth of DyNi2 film: (a) initial parabolic growth stage; (b) linear growth stage.
diffusion in the outer DyNi2 layer is not the rate determining step. Therefore, the growth of film follows the parabolic rate at the initial period, then changes to the linear rate law afterwards. In order to confirm this explanation, however, further metallographic study on microscopic structure is necessary.
4. Conclusion
Fig. 6. The relationship between the thickness of DyNi2 films obtained by electrochemical implantation and the time in a molten LiCl /KCl / DyCl3 (0.50 mol%) system at 700 K: (a) 0.55 V; (b) 0.62 V; and (c) 0.70 V.
inner DyNi2 layers as shown in Fig. 7. In the initial period of the growth, dense DyNi2 film, termed the inner DyNi2 layer, is thought to form. Since the growth is limited by Dy diffusion, the growth follows the parabolic rate law. Then the inner DyNi2 layer is expected to transform to the outer DyNi2 layer at constant rate. Thus, thickness of the inner DyNi2 layer is kept constant at the following period. The outer DyNi2 layer is assumed to have a large Dy diffusivity due to microscopic cracks and/or grain boundaries that serve as fast diffusion paths. This means that the
Kinetics of DyNi2 film growth by electrochemical implantation was investigated in a molten LiCl /KCl / DyCl3 (0.50 mol%) system at 700 K. The conclusions of this study are as follows. (1) When a Ni electrode is cathodically polarized at 0.55 /0.70 V, the DyNi2 film grows linearly with time with coulomb efficiency of about 100%. (2) The growth rates are higher at more negative potentials, i.e. 0.47 mm min 1 at 0.55 V, 0.32 mm min 1 at 0.62 V, and 0.14 mm min 1 at 0.70 V. (3) On the analogy of the metal oxide growth, the observed rapid and linear growth of DyNi2 films may be explained by the existence of the outer and inner DyNi2 layers.
Acknowledgements This work was supported by a grant-in-aid from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
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