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Formation and phase control of Co–Gd alloy films by molten salt electrochemical process
Journal of Alloys and Compounds 379 (2004) 256–261
Formation and phase control of Co–Gd alloy films by molten salt electrochemical process Tetsuro Kubota, Takahisa Iida, Toshiyuki Nohira∗ , Yasuhiko Ito1 Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received 30 January 2004; accepted 16 February 2004
Abstract The electrochemical formation of Gd–Co alloys was investigated in a molten LiCl–KCl–GdCl3 (0.5 mol%) system at 723 K. A CoGd3 film (12 m thickness) was formed on a Co substrate by cathodic electrolysis at 0.43 V (versus Li+ /Li) for 1 h. The formed CoGd3 alloy phase changed to other alloy phases by anodic dissolution of Gd. The formation potentials of CoGd3 , Co2 Gd and Co7 Gd2 were found to be 0.55, 0.80 and 0.96 V, respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Electrochemical reaction; Intermetallics; Magnetic films; Scanning electron microscopy; X-ray diffraction
1. Introduction Rare earth (RE)–transition metal (TM) alloys are of great interest due to their excellent magnetic, hydrogen absorbing or permeable, and catalytic properties. Among RE–TM alloys, Co–Gd alloys are especially interesting because of its magnetic or magneto-optical properties. For instance, Lee et al. [1,2] reported on magneto-optical property of single crystalline Co2 Gd alloy. Generally, these alloy films are formed by dry processes, such as RF sputtering. As a novel formation method of RE–TM alloy films, molten salt electrochemical process has been investigated in the authors’ laboratory [3]. So far, Ni–Y [4,5], Ni–Sm [6], Ni–Dy [7–9], Ni–Yb [10], Fe–Dy [11], and Co–Sm [12] alloy films have been successfully formed. This process has the following advantages: (i) Phases of the alloy films can be controlled by electrochemical parameters (e.g., potential and current density). (ii) The alloy films can be formed on various shapes of substrates (e.g., small to large surface area, complex shape, etc.). ∗ Corresponding author. Tel.: +81-75-753-4817; fax: +81-75-753-5906. E-mail addresses: [email protected] (T. Nohira), [email protected] (Y. Ito). 1 Co-corresponding author.
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.02.041
(iii) Mass production is easy. In this study, we have investigated the formation of Co–Gd alloy films by molten salt electrochemical process. Fig. 1 shows the schematic drawing of the expected Co–Gd alloy formation by molten salt electrochemical process. When GdCl3 is added to molten LiCl–KCl, GdCl3 dissociates to form Gd(III) and Cl− ions. And by cathodic electrolysis, Gd(III) ions are reduced to Gd(0) to form Co–Gd alloys on a Co substrate. In addition, the Gd concentration of formed alloy is expected to be controlled by anodic dissolution of Gd.
2. Experimental Experimental apparatus has been shown in our previous paper [6]. The LiCl–KCl eutectic (LiCl:KCl = 58.5:41.5 mol%; Wako Pure Chemical Co. Ltd.) was contained in a high purity alumina crucible (99.5 wt.% Al2 O3 ; SSA-S grade, Nikkato Corp.) and kept under vacuum for more than 72 h at 473 K to remove water. All experiments were performed in a LiCl–KCl eutectic melt under a dry Ar atmosphere at 723 K. GdCl3 (99.9%, Kojundo Chemical Laboratory Co. Ltd.) was added directly to the melt as a Gd(III) ion source. A Mo plate (5 mm × 20 mm × 0.2 mm (thickness); 99.95%, Nilaco Co. Ltd.) and a Co plate (5 mm × 20 mm × 0.5 mm (thickness); 99.9%, Nilaco Co. Ltd.) were used as working electrodes. The reference
T. Kubota et al. / Journal of Alloys and Compounds 379 (2004) 256–261
Fig. 1. Principle of formation of Co–Gd alloy films by molten salt electrochemical process.
257
Fig. 2. Cyclic voltammograms at Mo (broken curve) and Co (solid curve) electrodes in a molten LiCl–KCl–GdCl3 (0.50 mol%) system at 723 K. Scanning rate: 1.0 × 10−2 V s−1 .
electrode was an Ag wire immersed in LiCl–KCl containing 1 mol% AgCl, placed 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 Ni wire. All the potentials in this paper were referred to this Li+ /Li potential. The counter electrode was a glassy carbon rod (50 mm × Ø5 mm, Tokai Carbon Co. Ltd.) as an anode, or an Al plate (10 mm × 20 mm × 0.2 mm (thickness), Nilaco Corp.) as a cathode. A potentio/galvanostat (Hokuto Denko, HA501G) was used for cyclic voltammetry, chronopotentiometry, and potentiostatic electrolysis. The samples were prepared by potentiostatic electrolysis, and were rinsed with cooled water and then kept in Ar atmosphere, because Co–Gd alloys are easily oxidized. Phase of the samples was analyzed by X-ray diffraction (XRD) (Rigaku Corp., Multiflex) with Cu K␣ line at 40 kV and 40 mA. The cross-sections of the samples were analyzed by scanning electron microscopy (SEM) (S-2600H, Hitachi Co. Ltd.) and EPMA (EX-200, Horiba Co. Ltd.).
3. Results and discussion 3.1. Cyclic voltammetry Cyclic voltammetry was conducted in a molten LiCl–KCl– GdCl3 (0.5 mol% added) system at 723 K. The broken curve in Fig. 2 shows the result obtained at a Mo electrode at a scanning rate of 1.0 × 10−2 V s−1 . Since no alloys or intermetallic compounds exist for Mo–Gd binary system at 723 K [13], the cathodic currents can be regarded as Gd metal deposition. In this voltammogram, there is only one couple of anodic and cathodic waves around 0.49 V (versus
Fig. 3. (a) XRD pattern and (b) cross-sectional SEM image of the sample electrolyzed at 0.43 V for 1 h using a Co substrate. CoGd3 : rhombohedral unit cell with a = 0.703 nm, b = 0.951 nm, and c = 0.630 nm.
258
T. Kubota et al. / Journal of Alloys and Compounds 379 (2004) 256–261
Fig. 4. Phase diagram of the Co–Gd system [13].
Li+ /Li), which corresponds to deposition and dissolution of Gd metal, respectively. This potential value agrees with the potential of Gd(III)/Gd reported by Yang et al. [14] and Lantelme et al. [15]. The solid curve in Fig. 2 shows the voltammogram at a Co electrode. In the cathodic scan, the current deviation from the Mo electrode is observed from 0.55 V, which is thought to correspond to the Co–Gd alloy formation. When the scanning direction is reversed, three peaks are observed around 0.75, 0.92, and 1.42 V, which indicates the dissolution of Gd from the different Co–Gd alloy phases.
3.3. Phase control of Co–Gd alloy films According to the phase diagram of the Co–Gd system (Fig. 4 [13]), there are eight Co–Gd intermetallic compounds at 723 K. When Gd is anodically dissolved from CoGd3 electrode, other alloy phases having lower Gd concentrations than CoGd3 are expected to be formed. To confirm this possibility, chronopotentiometry was conducted for
3.2. Electrochemical formation of CoGd3 Based on the result of cyclic voltammetry, a sample was prepared by potentiostatic electrolysis at 0.43 V for 1 h using a Co electrode. This potential is more negative than deposition potential of Gd metal (0.49 V). Sample was identified as CoGd3 from XRD (Fig. 3a). Since the deposited Gd metal was easy to flake off, no Gd metal was detected by XRD. Fig 3b shows the cross-sectional SEM image and the concentration profile of Co and Gd by EPMA line analysis. The current efficiency for the CoGd3 formation was estimated to be 63% from the average thickness of alloy layer (about 12 m). The residual current was thought to be used for the deposition of Gd metal.
Fig. 5. Chronopotentiogram at a CoGd3 electrode at 1.0 mA cm−2 in a molten LiCl–KCl–GdCl3 (0.50 mol%) system at 723 K. The CoGd3 electrode was prepared by cathodic potentiostatic electrolysis at 0.43 V for 10 min using Co plate.
T. Kubota et al. / Journal of Alloys and Compounds 379 (2004) 256–261 Table 1 Electrolysis potentials and identified phases for the samples prepared by anodic electrolysis of the CoGd3 electrode for 0.5 h Sample no.
Electrolysis potential (V)
Identified phase
1 2 3 4 5 6 7 8 9 10
0.63 0.85 0.94 0.96 0.98 1.00 1.02 1.04 1.06 1.30
Co2 Gd Co7 Gd2 Co7 Gd2 ␣-Co, -Co ␣-Co, -Co ␣-Co, -Co ␣-Co, -Co ␣-Co, -Co ␣-Co, -Co ␣-Co, -Co
the CoGd3 alloy electrode at an anodic current density of 1.0 mA cm−2 . The CoGd3 alloy electrode was previously prepared by potentiostatic electrolysis at 0.43 V for 10 min. The obtained chronopotentiogram is shown in Fig. 5. There are four potential plateaus at 0.55, 0.80, 0.96, and 1.03 V, respectively. These plateaus possibly correspond to coexisting phase states of Co–Gd alloys.
Fig. 6. (a) XRD pattern and (b) cross-sectional SEM image of sample 1, prepared by anodic electrolysis of the CoGd3 at 0.63 V for 0.5 h. Co2 Gd: cubic unit cell with a = 0.726 nm.
259
Based on this result, samples were prepared by the following procedures. Firstly, CoGd3 electrodes were made from Co plates by potentiostatic electrolysis at 0.43 V for 1 h. Then, anodic dissolutions of Gd were conducted for 0.5 h by potentiostatic electrolysis at various potentials, as summarized in Table 1. These potential values were basically chosen as intermediate values between the adjoining plateaus. Additionally, since plateaus at 0.96 and 1.03 V are very close to each other, potential values were set every 0.02 V steps from 0.94 to 1.06 V to carefully examine this potential region. Sample 1 (obtained at 0.63 V) was identified as Co2 Gd by XRD (Fig. 6a), which shows that the CoGd3 phase completely changed to a Co2 Gd phase. Therefore, the potential plateau at 0.55 V corresponds to the following reaction: 1 5 Co2 Gd
+ Gd(III) + 3e− 25 CoGd3
(1)
Fig. 6b shows the cross-sectional SEM image and the result of EPMA line analysis. The alloy layer became porous and the thickness of alloy layer was approximately 10 m. Samples 2 (0.85 V) and 3 (0.94 V) were identified as Co7 Gd2 by XRD. Fig. 7a shows the XRD pattern of sample 2. Therefore, the potential plateau at 0.80 V corresponds to
Fig. 7. (a) XRD pattern and (b) cross-sectional SEM image of sample 2, prepared by anodic electrolysis of the CoGd3 at 0.85 V for 0.5 h. Co7 Gd2 : hexagonal unit cell with a = 0.502 nm, c = 3.632 nm.
260
T. Kubota et al. / Journal of Alloys and Compounds 379 (2004) 256–261 Table 2 Formation reactions and corresponding potentials for the Co–Gd alloys in a molten LiCl–KCl–GdCl3 (0.50 mol%) system at 723 K Reaction
Potential (V) vs. Li+ /Li
1/5 Co2 Gd + Gd(III) + 3e− = 2/5 CoGd3 2/3 Co7 Gd2 + Gd(III) + 3e− = 7/3 Co2 Gd 7/2 Co + Gd(III) + 3e− = 1/2 Co7 Gd2
0.55 0.80 0.96
On the other hand, the phase diagram of Co–Gd system shown in Fig. 4 suggests possibilities of formation of Co17 Gd2 , Co5 Gd, Co3 Gd, Co3 Gd4 , and Co7 Gd12 . However, they were not identified in any samples under our experimental conditions. This might be explained by that the formation rates of these phases were very low at 723 K. Accordingly, formation reactions of Co–Gd alloys and the corresponding potentials can be summarized as Table 2.
4. Conclusions Electrochemical formation and phase control of Co–Gd alloy films was studied in a molten LiCl–KCl–GdCl3 (0.5 mol%) system at 723 K. The results are summarized as follows:
Fig. 8. (a) XRD pattern and (b) cross-sectional SEM image of sample 10, prepared by anodic electrolysis of the CoGd3 at 1.30 V for 0.5 h.
(1) A CoGd3 alloy film (∼10 m) was formed on a Co substrate at 0.43 V for 1 h. (2) The CoGd3 layer changed to Co2 Gd, Co7 Gd2 , and Co layer by anodic potentiostatic electrolysis depending on the potentials. (3) Formation reactions and potentials for the CoGd3 , Co2 Gd, and Co7 Gd2 have been clarified.
the following reaction: 2 3 Co7 Gd2
+ Gd(III) + 3e− 73 Co2 Gd
(2)
According to the cross-sectional SEM image and EPMA line analysis for the sample 2 shown in Fig. 7b, the thickness of alloy layer was approximately 10 m. Samples 4–10 (0.96–1.30 V) were identified as ␣-Co and -Co. As a typical result, Fig. 8a shows the XRD pattern of sample 10. The potential plateau at 0.96 V thus corresponds to the following reaction: − 7 2 Co + Gd(III) + 3e
21 Co7 Gd2
(3)
Fig. 8b shows the cross-sectional SEM image and the result of EPMA line analysis for sample 10, the thickness of Co layer was approximately 10 m. The cause of the plateau at 1.03 V may be explained as follows. After the plateau at 0.96 V in Fig. 5, anodic dissolution of Gd proceeds from the surface Co layer, where Gd is supplied from the inside Gd-rich layer by the diffusion. Around 1.03 V, the supply of Gd may become easier due to the decrease of the surface Gd chemical potential, which results in the potential stagnation.
Acknowledgements This work was supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research (B), 12555244, 2003 and Grant-in-Aid for “Establishment of Center of Excellence on Sustainable-Energy system” program. We greatly appreciate Dr. Gert Nolze for sending the PowderCell program.
References [1] S.J. Lee, K.J. Kim, P.C. Caneld, D.W. Lynch, J. Magn. Magn. Mater. 213 (2000) 312. [2] S.J. Lee, K.J. Kim, Solid State Commun. 118 (2001) 269. [3] Y. Ito, T. Nohira, Electrochim. Acta 45 (2000) 2611. [4] G. Xie, K. Ema, Y. Ito, J. Appl. Electrochem. 23 (1993) 753. [5] G. Xie, K. Ema, Y. Ito, J. Appl. Electrochem. 24 (1994) 321. [6] T. Iida, T. Nohira, Y. Ito, Electrochim. Acta 46 (2001) 2537. [7] H. Konishi, T. Nohira, Y. Ito, J. Electrochem. Soc. 148 (2001) C506.
T. Kubota et al. / Journal of Alloys and Compounds 379 (2004) 256–261 [8] H. Konishi, T. Nohira, Y. Ito, Electrochem. Solid-State Lett. 5 (2002) B37. [9] H. Konishi, T. Nohira, Y. Ito, Electrochim. Acta 48 (2003) 563. [10] T. Iida, T. Nohira, Y. Ito, Electrochim. Acta 48 (2003) 1531. [11] H. Konishi, T. Nohira, Y. Ito, Electrochim. Acta 47 (2002) 3533. [12] T. Iida, T. Nohira, Y. Ito, Electrochim. Acta 48 (2003) 901.
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[13] T.B. Massalski, J.L. Murray, L.H. Benett, H. Baker, Binary Alloy Phase Diagrams, American Society for Metals, 1990. [14] L. Yang, R.G. Hudson, et al., Trans. Met. Soc. AIME 215 (1959) 589. [15] F. Lantelme, Y. Berghoute, J. Electrochem. Soc. 146 (1999) 4137.
Formation and phase control of Co–Gd alloy films by molten salt electrochemical process Tetsuro Kubota, Takahisa Iida, Toshiyuki Nohira∗ , Yasuhiko Ito1 Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received 30 January 2004; accepted 16 February 2004
Abstract The electrochemical formation of Gd–Co alloys was investigated in a molten LiCl–KCl–GdCl3 (0.5 mol%) system at 723 K. A CoGd3 film (12 m thickness) was formed on a Co substrate by cathodic electrolysis at 0.43 V (versus Li+ /Li) for 1 h. The formed CoGd3 alloy phase changed to other alloy phases by anodic dissolution of Gd. The formation potentials of CoGd3 , Co2 Gd and Co7 Gd2 were found to be 0.55, 0.80 and 0.96 V, respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Electrochemical reaction; Intermetallics; Magnetic films; Scanning electron microscopy; X-ray diffraction
1. Introduction Rare earth (RE)–transition metal (TM) alloys are of great interest due to their excellent magnetic, hydrogen absorbing or permeable, and catalytic properties. Among RE–TM alloys, Co–Gd alloys are especially interesting because of its magnetic or magneto-optical properties. For instance, Lee et al. [1,2] reported on magneto-optical property of single crystalline Co2 Gd alloy. Generally, these alloy films are formed by dry processes, such as RF sputtering. As a novel formation method of RE–TM alloy films, molten salt electrochemical process has been investigated in the authors’ laboratory [3]. So far, Ni–Y [4,5], Ni–Sm [6], Ni–Dy [7–9], Ni–Yb [10], Fe–Dy [11], and Co–Sm [12] alloy films have been successfully formed. This process has the following advantages: (i) Phases of the alloy films can be controlled by electrochemical parameters (e.g., potential and current density). (ii) The alloy films can be formed on various shapes of substrates (e.g., small to large surface area, complex shape, etc.). ∗ Corresponding author. Tel.: +81-75-753-4817; fax: +81-75-753-5906. E-mail addresses: [email protected] (T. Nohira), [email protected] (Y. Ito). 1 Co-corresponding author.
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.02.041
(iii) Mass production is easy. In this study, we have investigated the formation of Co–Gd alloy films by molten salt electrochemical process. Fig. 1 shows the schematic drawing of the expected Co–Gd alloy formation by molten salt electrochemical process. When GdCl3 is added to molten LiCl–KCl, GdCl3 dissociates to form Gd(III) and Cl− ions. And by cathodic electrolysis, Gd(III) ions are reduced to Gd(0) to form Co–Gd alloys on a Co substrate. In addition, the Gd concentration of formed alloy is expected to be controlled by anodic dissolution of Gd.
2. Experimental Experimental apparatus has been shown in our previous paper [6]. The LiCl–KCl eutectic (LiCl:KCl = 58.5:41.5 mol%; Wako Pure Chemical Co. Ltd.) was contained in a high purity alumina crucible (99.5 wt.% Al2 O3 ; SSA-S grade, Nikkato Corp.) and kept under vacuum for more than 72 h at 473 K to remove water. All experiments were performed in a LiCl–KCl eutectic melt under a dry Ar atmosphere at 723 K. GdCl3 (99.9%, Kojundo Chemical Laboratory Co. Ltd.) was added directly to the melt as a Gd(III) ion source. A Mo plate (5 mm × 20 mm × 0.2 mm (thickness); 99.95%, Nilaco Co. Ltd.) and a Co plate (5 mm × 20 mm × 0.5 mm (thickness); 99.9%, Nilaco Co. Ltd.) were used as working electrodes. The reference
T. Kubota et al. / Journal of Alloys and Compounds 379 (2004) 256–261
Fig. 1. Principle of formation of Co–Gd alloy films by molten salt electrochemical process.
257
Fig. 2. Cyclic voltammograms at Mo (broken curve) and Co (solid curve) electrodes in a molten LiCl–KCl–GdCl3 (0.50 mol%) system at 723 K. Scanning rate: 1.0 × 10−2 V s−1 .
electrode was an Ag wire immersed in LiCl–KCl containing 1 mol% AgCl, placed 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 Ni wire. All the potentials in this paper were referred to this Li+ /Li potential. The counter electrode was a glassy carbon rod (50 mm × Ø5 mm, Tokai Carbon Co. Ltd.) as an anode, or an Al plate (10 mm × 20 mm × 0.2 mm (thickness), Nilaco Corp.) as a cathode. A potentio/galvanostat (Hokuto Denko, HA501G) was used for cyclic voltammetry, chronopotentiometry, and potentiostatic electrolysis. The samples were prepared by potentiostatic electrolysis, and were rinsed with cooled water and then kept in Ar atmosphere, because Co–Gd alloys are easily oxidized. Phase of the samples was analyzed by X-ray diffraction (XRD) (Rigaku Corp., Multiflex) with Cu K␣ line at 40 kV and 40 mA. The cross-sections of the samples were analyzed by scanning electron microscopy (SEM) (S-2600H, Hitachi Co. Ltd.) and EPMA (EX-200, Horiba Co. Ltd.).
3. Results and discussion 3.1. Cyclic voltammetry Cyclic voltammetry was conducted in a molten LiCl–KCl– GdCl3 (0.5 mol% added) system at 723 K. The broken curve in Fig. 2 shows the result obtained at a Mo electrode at a scanning rate of 1.0 × 10−2 V s−1 . Since no alloys or intermetallic compounds exist for Mo–Gd binary system at 723 K [13], the cathodic currents can be regarded as Gd metal deposition. In this voltammogram, there is only one couple of anodic and cathodic waves around 0.49 V (versus
Fig. 3. (a) XRD pattern and (b) cross-sectional SEM image of the sample electrolyzed at 0.43 V for 1 h using a Co substrate. CoGd3 : rhombohedral unit cell with a = 0.703 nm, b = 0.951 nm, and c = 0.630 nm.
258
T. Kubota et al. / Journal of Alloys and Compounds 379 (2004) 256–261
Fig. 4. Phase diagram of the Co–Gd system [13].
Li+ /Li), which corresponds to deposition and dissolution of Gd metal, respectively. This potential value agrees with the potential of Gd(III)/Gd reported by Yang et al. [14] and Lantelme et al. [15]. The solid curve in Fig. 2 shows the voltammogram at a Co electrode. In the cathodic scan, the current deviation from the Mo electrode is observed from 0.55 V, which is thought to correspond to the Co–Gd alloy formation. When the scanning direction is reversed, three peaks are observed around 0.75, 0.92, and 1.42 V, which indicates the dissolution of Gd from the different Co–Gd alloy phases.
3.3. Phase control of Co–Gd alloy films According to the phase diagram of the Co–Gd system (Fig. 4 [13]), there are eight Co–Gd intermetallic compounds at 723 K. When Gd is anodically dissolved from CoGd3 electrode, other alloy phases having lower Gd concentrations than CoGd3 are expected to be formed. To confirm this possibility, chronopotentiometry was conducted for
3.2. Electrochemical formation of CoGd3 Based on the result of cyclic voltammetry, a sample was prepared by potentiostatic electrolysis at 0.43 V for 1 h using a Co electrode. This potential is more negative than deposition potential of Gd metal (0.49 V). Sample was identified as CoGd3 from XRD (Fig. 3a). Since the deposited Gd metal was easy to flake off, no Gd metal was detected by XRD. Fig 3b shows the cross-sectional SEM image and the concentration profile of Co and Gd by EPMA line analysis. The current efficiency for the CoGd3 formation was estimated to be 63% from the average thickness of alloy layer (about 12 m). The residual current was thought to be used for the deposition of Gd metal.
Fig. 5. Chronopotentiogram at a CoGd3 electrode at 1.0 mA cm−2 in a molten LiCl–KCl–GdCl3 (0.50 mol%) system at 723 K. The CoGd3 electrode was prepared by cathodic potentiostatic electrolysis at 0.43 V for 10 min using Co plate.
T. Kubota et al. / Journal of Alloys and Compounds 379 (2004) 256–261 Table 1 Electrolysis potentials and identified phases for the samples prepared by anodic electrolysis of the CoGd3 electrode for 0.5 h Sample no.
Electrolysis potential (V)
Identified phase
1 2 3 4 5 6 7 8 9 10
0.63 0.85 0.94 0.96 0.98 1.00 1.02 1.04 1.06 1.30
Co2 Gd Co7 Gd2 Co7 Gd2 ␣-Co, -Co ␣-Co, -Co ␣-Co, -Co ␣-Co, -Co ␣-Co, -Co ␣-Co, -Co ␣-Co, -Co
the CoGd3 alloy electrode at an anodic current density of 1.0 mA cm−2 . The CoGd3 alloy electrode was previously prepared by potentiostatic electrolysis at 0.43 V for 10 min. The obtained chronopotentiogram is shown in Fig. 5. There are four potential plateaus at 0.55, 0.80, 0.96, and 1.03 V, respectively. These plateaus possibly correspond to coexisting phase states of Co–Gd alloys.
Fig. 6. (a) XRD pattern and (b) cross-sectional SEM image of sample 1, prepared by anodic electrolysis of the CoGd3 at 0.63 V for 0.5 h. Co2 Gd: cubic unit cell with a = 0.726 nm.
259
Based on this result, samples were prepared by the following procedures. Firstly, CoGd3 electrodes were made from Co plates by potentiostatic electrolysis at 0.43 V for 1 h. Then, anodic dissolutions of Gd were conducted for 0.5 h by potentiostatic electrolysis at various potentials, as summarized in Table 1. These potential values were basically chosen as intermediate values between the adjoining plateaus. Additionally, since plateaus at 0.96 and 1.03 V are very close to each other, potential values were set every 0.02 V steps from 0.94 to 1.06 V to carefully examine this potential region. Sample 1 (obtained at 0.63 V) was identified as Co2 Gd by XRD (Fig. 6a), which shows that the CoGd3 phase completely changed to a Co2 Gd phase. Therefore, the potential plateau at 0.55 V corresponds to the following reaction: 1 5 Co2 Gd
+ Gd(III) + 3e− 25 CoGd3
(1)
Fig. 6b shows the cross-sectional SEM image and the result of EPMA line analysis. The alloy layer became porous and the thickness of alloy layer was approximately 10 m. Samples 2 (0.85 V) and 3 (0.94 V) were identified as Co7 Gd2 by XRD. Fig. 7a shows the XRD pattern of sample 2. Therefore, the potential plateau at 0.80 V corresponds to
Fig. 7. (a) XRD pattern and (b) cross-sectional SEM image of sample 2, prepared by anodic electrolysis of the CoGd3 at 0.85 V for 0.5 h. Co7 Gd2 : hexagonal unit cell with a = 0.502 nm, c = 3.632 nm.
260
T. Kubota et al. / Journal of Alloys and Compounds 379 (2004) 256–261 Table 2 Formation reactions and corresponding potentials for the Co–Gd alloys in a molten LiCl–KCl–GdCl3 (0.50 mol%) system at 723 K Reaction
Potential (V) vs. Li+ /Li
1/5 Co2 Gd + Gd(III) + 3e− = 2/5 CoGd3 2/3 Co7 Gd2 + Gd(III) + 3e− = 7/3 Co2 Gd 7/2 Co + Gd(III) + 3e− = 1/2 Co7 Gd2
0.55 0.80 0.96
On the other hand, the phase diagram of Co–Gd system shown in Fig. 4 suggests possibilities of formation of Co17 Gd2 , Co5 Gd, Co3 Gd, Co3 Gd4 , and Co7 Gd12 . However, they were not identified in any samples under our experimental conditions. This might be explained by that the formation rates of these phases were very low at 723 K. Accordingly, formation reactions of Co–Gd alloys and the corresponding potentials can be summarized as Table 2.
4. Conclusions Electrochemical formation and phase control of Co–Gd alloy films was studied in a molten LiCl–KCl–GdCl3 (0.5 mol%) system at 723 K. The results are summarized as follows:
Fig. 8. (a) XRD pattern and (b) cross-sectional SEM image of sample 10, prepared by anodic electrolysis of the CoGd3 at 1.30 V for 0.5 h.
(1) A CoGd3 alloy film (∼10 m) was formed on a Co substrate at 0.43 V for 1 h. (2) The CoGd3 layer changed to Co2 Gd, Co7 Gd2 , and Co layer by anodic potentiostatic electrolysis depending on the potentials. (3) Formation reactions and potentials for the CoGd3 , Co2 Gd, and Co7 Gd2 have been clarified.
the following reaction: 2 3 Co7 Gd2
+ Gd(III) + 3e− 73 Co2 Gd
(2)
According to the cross-sectional SEM image and EPMA line analysis for the sample 2 shown in Fig. 7b, the thickness of alloy layer was approximately 10 m. Samples 4–10 (0.96–1.30 V) were identified as ␣-Co and -Co. As a typical result, Fig. 8a shows the XRD pattern of sample 10. The potential plateau at 0.96 V thus corresponds to the following reaction: − 7 2 Co + Gd(III) + 3e
21 Co7 Gd2
(3)
Fig. 8b shows the cross-sectional SEM image and the result of EPMA line analysis for sample 10, the thickness of Co layer was approximately 10 m. The cause of the plateau at 1.03 V may be explained as follows. After the plateau at 0.96 V in Fig. 5, anodic dissolution of Gd proceeds from the surface Co layer, where Gd is supplied from the inside Gd-rich layer by the diffusion. Around 1.03 V, the supply of Gd may become easier due to the decrease of the surface Gd chemical potential, which results in the potential stagnation.
Acknowledgements This work was supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research (B), 12555244, 2003 and Grant-in-Aid for “Establishment of Center of Excellence on Sustainable-Energy system” program. We greatly appreciate Dr. Gert Nolze for sending the PowderCell program.
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