Article ID: 1003-6326(2005)05-1190-04
Electrochemical studies on La-Co alloy film in acetamide-urea-NaBr melt system
GUO Cheng-yu(郭承育), WANG Jian-chao(王建朝),
CHEN Bi-qing(陈必清), WANG Jing-gui(王金贵)
(Department of Chemistry, Qinghai Normal University, Xining 810008, China)
Abstract: The kinetics of La-Co alloy film in acetamide-urea-NaBr molten salt electrolyte at 353K was investigated. It is shown that the reduction of Co(Ⅱ) to Co is irreversible reaction with the transfer coefficient of 0.28 and the diffusion coefficient of 7.46×10-5 cm2/s. While La(Ⅲ) cannot be reduced to La directly; but can be codeposited with cobalt. The content of La in the uncrystallized La-Co alloy film increases with increasing cathodic overpotential, molar ratio of La3+ to Co2+ and electrolysis time as well, and reaches the maximum of 66.32%.
Key words: La-Co alloy film; acetamide-urea-NaBr molten salt electrolyte; electroreduction; inductive co-deposition; rare earth elements CLC number: O641.4
Document code: A
1 INTRODUCTION
The magnetic rare earth elements bring significant changes to the magnetic material industry due to their excellent magnetic and magneto-optical properties[1]. Since La and Co are rich elements in nature, in addition to their relatively cheap prices, it would be very prospective to prepare magnetic functional materials using La-Co alloy. Compared with traditional techniques of spray and vacuum spattering or sputtering, electrodeposition is an economical method to develop alloy film due to a simple procedure and low expense. Because of the active chemical property of RE and Co, it is difficult to deposit RE-Co alloy in aqueous solution and studies on electrodeposition of RE-Co alloy film in organic electrolytes have been reported[2-5].
Owing to the lower melting point(up to 315.2K) and the higher electric conductivity of molten salt electrolyte of acetamide-urea-alkali halide[6-8] in the certain mixture, the acetamide-urea-NaBr melt was chosen as electrolyte for depositing La and Co from La(NO3)3 and CoCl2. In Refs.[8-14], the deposition of RE-Co alloy film from urea-NaBr and urea-NaBr-KBr molten salt electrolytes was reported. In this paper, by using cyclic voltammetry, the results related to the reduction behaviors of La(Ⅲ) and Co(Ⅱ) in acetamide-urea-NaBr molten salt electrolyte and the relationship between alloy composition and molar ratio of La(Ⅲ) to Co(Ⅱ), electrolysis time and electrode potential were discussed. At the same time, the deposited film surface morphology was studied.
2 EXPERIMENTAL
Water free La(NO3)3 and CoCl2 were made from A.R. La(NO3)3·6H2O (Shanghai Chemical Co. China) and CoCl2 (Beijing Red Star Chemical Co. China) by vacuum dehydration. Dried urea (Beijing Chemical Co. China), Acetamide (56%, mass fraction, Beijing Chemical Co. China) and NaBr (7%, mass fraction, Xian Chemical Co. China) were uniformly mixed and then put into a cell with three electrodes. All measurements were carried out under the protection of argon gas at 353K. The reference electrode was Ag/acetamide-urea-NaBr, to which all potentials in this paper were referred. Counter electrode was a platinum plate with high purity of 99.9%. The working electrode was a platinum (99.9%, 0.015cm2 ) and a copper wire (99.9%, 0.35cm2). Respectively CV techniques were employed using an Potentiostat HDV-7C and a HD-1A low & ultraslow frequency response analyzer (Sanming Instrumental Co. Fujian, China), and handled by 3086 X-Y data recorder (Sichuan Fourth Instruments, China).
3 RESULTS AND DISCUSSION
3.1 Electrochemical behavior of La(Ⅲ) in melt of acetamide-urea-NaBr
The experiment is carried out with the melt of acetamide-urea-NaBr as supporting electrolyte and scanning rate is varied in 20, 50, 100 and 200 mV/s. In order to investigate the electrochemical behavior of the melt of acetamide-urea-NaBr, a cyclic voltammetry is recorded in a wide potential region of -1.5-0V with a copper wire as working electrode (see Fig.1(a)). And then La(NO3)3 (324.9g/mol) is added in the cell and CV curves (Fig.1(b)) are recorded. Compared with Fig.1(a), there is an anodic current in Fig.1(b) at potential of -1.10V but no evident peak. In the meantime, there is no deposition after electrolysis in the potential range of -1.10--1.40V, which indicates that La(Ⅲ) cannot be reduced to La in the melt and this coincides with McManiss report[7].
Fig.1 Cyclic voltammograms of Cu electrode in different solutions
3.2 Electrochemical behavior of Co(Ⅱ) in melt of acetamide-urea-NaBr
When the previous experiment is repeated in the melt containing 0.06mol/L CoCl2, a peak appears in the negative scanning at -0.82V (Fig.1(c)).This peak corresponds to the reduction of Co(Ⅱ) and it is an irreversible reaction because of no oxidation peak in the backward scanning. After 20min controlled-potential electrolysis at -0.80V, a gray color and uniform deposited film appears on the copper plate. Furthermore, the deposit is confirmed to be Co by EDS.
Fig.2 shows a set of cyclic voltammograms recorded with increasing scanning rate in the region of Co(Ⅱ) reduction. It is evident that the peak position shifts towards more negative potential values with the increase in scanning rate, indicating that the Co(Ⅱ) reduction reaction is under diffusion control. In addition, as already discussed above, the Co(Ⅱ) reduction reaction is shown to be highly irreversible, which can be seen from the linear relationship of peak potential (φp) and logarithm of scanning rate. By using relation for irreversible electrochemical reaction at 353K: [JB(|]φp/2-φp[JB)|]=1.857RT/(αnF), the average transfer coefficient can be calculated as α=0.28(n=2).
Fig.2 Cyclic voltammograms of Pt electrode in CoCl2-acetamide-urea-NaBr
recorded at various scanning rates
In order to calculate the diffusion coefficient, the curve of the peak current (Ip) versus square root of scanning rate is plotted (Fig.3). By using Ip=0.4958nF(αnFD0υRT)1/2Ac[15], the diffusion coefficient is calculated from the slope of line in Fig.3 : D0=7.46×10-5cm2/s at 353K and 0.06mol/L of CoCl2. Here A is surface area of electrode, D0 is diffusion coefficient, and c is concentration.
Fig.3 Dependence of peak current on scanning rate
3.3 Co-deposition of La (Ⅲ) and Co(Ⅱ) in melt of acetamide-urea-NaBr
3.3.1 Electrochemical behavior
Fig.4 shows the cyclic voltammetry curve of La(NO3)3 (0.03mol/L)-CoCl2 (0.06mol/L) in the melt of acetamide-urea-NaBr on Cu electrode. Compared with Fig.1(c), the peak potential shifts toward more negative value, and the current increases evidently. It would be an inductive codeposition of Co and La. Under an electrolysis at cathodic potential of -0.8V, a black deposit on Cu surface is got and it consists of La and Co according to EDS analysis, which further confirms that La(Ⅲ) can be co-deposited with Co(Ⅱ).
Fig.4 Cyclic voltammogram of Cu electrode in La(NO3)3-CoCl2-acetamide-urea-NaBr
3.3.2 Effect of electrodeposition condition on alloy composition
3.3.2.1 Effect of electrode potential
Table 1 shows the content of La-Co alloy film from 20min controlled potential electrolysis at various potentials when keeping the molar ratio of La(Ⅲ) to Co(Ⅱ) at 1∶1 in 0.06mol/L La(NO3)3-0.06mol/L CoCl2-acetamide-urea-NaBr system. It can be seen that the content of La in alloy film increases with negative shift of cathodic potential and reaches the maximum value of 50.08% at -0.95V. However, at more negative potential, the content of La decreases, which will be due to the polarization caused by concentration difference under these potentials. Scanning electron microscope (SEM) image of deposited film shows that the film is uniform and well-distributed on the surface of Cu substrate.
Table 1 Content of La in La-Co alloy film at various electrolysis potentials
3.3.2.2 Effect of molar ratio of La(Ⅲ) to Co(Ⅱ)
After the completion of electrolysis at potential of -0.850V for 20min, the content of La in the deposited film increases with increasing molar ratio of La(Ⅲ) to Co(Ⅱ) (see Table 2). Fig.5 shows the SEM image of deposited La-Co alloy film.
Table 2 Content of La in La-Co alloy film with different molar ratios of La(Ⅲ) to Co(Ⅱ)
Fig.5 SEM image of deposited film after heat treatment at 600℃
3.3.2.3 Effect of time in controlled-potential electrolysis
In order to investigate the relationship between the content of La in deposited alloy film and electrolysis time, the experiment is performed under molar ratio of La(Ⅲ) to Co(Ⅱ) of 2∶1 and cathodic potential of -0.850V (Table 3). The results show that the content of La increases at first and then decreases with time. The same trend is also observed from deposition of Gd-Co in melt of acetamide-urea-NaBr, which may be attributed to faster deposition speed of the iron triad elements compared with that of rare earth elements and the increasing extend of rare earth element content decreases with time.
3.3.2.4 Effect of time in controlled-current electrolysis
Table 4 shows that the content of La formed in the melt of La(NO3)3 (0.12mol/L)-CoCl2 (0.06mol/L)-acetamide-urea-NaBr under cathodic current of 2.50mA increases at first and then decreases with time lasting. Moreover, the content of La deposited by controlled-potential electrolysis is much higher than that by controlled-current electrolysis at the same time.
Table 3 Content of La in La-Co alloy film under controlled-potential electrolysis
Table 4 Content of La in La-Co alloy film under controlled-current electrolysis
3.4 Analysis of structural image of deposited film
A set of disordered peaks are found in X-ray diffusion pattern of La-Co alloy film obtained from electrolysis in the system of La(NO3)3 (0.12mol/L)-CoCl2 (0.06mol/L)-acetamide-urea-NaBr, which suggests the deposited film is uncrystallized. However after 6h heat treatment at 600℃ under argon gas protection, only diffraction peaks are observed in XRD pattern, which means that the alloy is crystallized during heat treatment. It can also be confirmed by the scanning electron microscope(SEM) image of La-Co alloy film after heat-treatment at 600℃(see Fig.5).
4 CONCLUSIONS
1) Co(Ⅱ) can be reduced to Co directly in the melt of acetamide-urea-NaBr and the reduction is an irreversible electrochemical reaction.
2) La(Ⅲ) cannot be reduced to La in the melt of acetamide-urea-NaBr, but can be reduced inductively in the presence of Co.
3) The content of La in La-Co alloy film increases with increasing molar ratio of La(Ⅲ) to Co(Ⅱ), negative shift of cathodic potential and time lasting as well. The maximum value is 66.32%.
4) La-Co alloy film is uncrystallized when deposition from the melt of acetamide-urea-NaBr, but it becomes crystal after heat treatment.
REFERENCES
[1]Strnat K J. Modern permanent magents for application in electro-technology [J]. Proc of the IEEE, 1990, 78: 923-946.
[2]Sato Y, Ishida H, Kobayakawa K, et al. Electrodeposition of Sm-Co forrmamide solution [J].Chemistry Letters, 1990, 224(8): 1471-1474.
[3]LIU Guan-kun, TONG Ye-xiang, LIU Jian-ping, et al. Electrodeposition of rare earth alloy in organic bath [J]. Journal of Chinese Rare Earth Society, 1998, 16: 857-863.
[4]LIU Guan-kun, TONG Ye-xiang, GU Li-ping, et al. Electrodeposition of Nd-Co alloy film in dimethyl sulphoxide(DMSO) [J]. Journal of Chinese Rare Earth Society, 1999, 17: 91-94.
[5]LIU Peng, YANG Qi-qin, TONG Ye-xiang, et al. Electrodeposition of Gd-Co film in organic bath [J]. Electrochemical Acta, 2000, 45(13): 2147-2152.
[6]Gambino M, Bros J P. Capacity calorifique de 1uree et de quelques mélanges eutectiques a base dduree entre 30 et 140℃ [J]. Thermochemica Acta, 1988, 127: 223-236.
[7]McManis G E, Fletcher A N, Bliss D E, et al. Electrochemical characteristics of simple nitrate+amide melts at ambient temperature [J]. J Electroanal Chem, 1985, 190(1/2): 171-183.
[8]LIU Li-zhi, LIU Peng, TONG Ye-xiang, et al. Electrodeposition on formation of Er-Co alloy in urea-NaBr molten [J]. Acta Scientiarum Naturalium Universitatis Sunyatseni, 1999, 38(3): 119-120.
[9]WANG Jian-chao, XU Chang-wei, LIU Peng, et al. Preparation of Y-Co alloy film in acetamide-urea-NaBr melt [J]. Trans Nonferrous Met Soc China, 2002, 12(6): 1191-1194.
[10]YANG Qi-qin, QIU Kai-rong, ZHU De-rong, et al. Electrodeposition of Nd-Co, Tb-Co and Y-Co in urea-NaBr-KBr Melt [J]. Acta Physico-Chemica Sinica, 1995, 11(5): 965-967.
[11]YANG Qi-qin, QIU Kai-rong, ZHU De-rong, et al. Electrodeposition of cobalt and rare earth-cobalt in urea-NaBr melt [J]. Electrochemistry, 1995, 1(3): 274-277.
[12]LIU Li-zhi, TONG Ye-xiang, YANG Qi-qin. Electroreduction Co(Ⅱ), Ni(Ⅱ), and codeposition with La(Ⅲ) in urea-NaBr melt [J]. Rare Metal, 2000, 19(3): 237-241.
[13]TONG Ye-xiang, LIU Li-zhi, XU Chang-wei, et al. Preparation of Gd-Ni alloy film alloy in urea-NaBr melt [J]. Trans Nonferrous Met Soc China, 2001, 11(3): 451-454.
[14]ZHU De-rong, YANG Qi-qin, QIU Kai-rong. Electrodeposition of Co and La-Co alloy in nonaqueous system [J]. 1994, 13(4): 1-4.
[15]Bard A J, Faulkner L R. Electrochemical Methods: Fundamental and Applications [M]. New York: John Wliey & Sons, 1980. 222-223, 253.
Foundation item: Project(50264004) supported by the National Natural Science Foundation of China
Received date: 2004-10-18; Accepted date:2005-04-04
Correspondence: GUO Cheng-yu, Associate professor; Tel: +86-971-6307635; E-mail: wangjc@qhnu.edu.cn
(Edited by YANG Bing)
