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Morphology and composition of Ni–Co electrodeposited powders
M A TE R IA L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 1 7 3–1 1 7 9
Available online at www.sciencedirect.com
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Morphology and composition of Ni–Co electrodeposited powders V.M. Maksimovića,⁎, U.Č. Lačnjevacb , M.M. Stoiljkovića , M.G. Pavlovićc , V.D. Jovićb a
Institute of Nuclear Sciences, “Vinča”, University of Belgrade, 11001 Belgrade, P. O. Box 522, Serbia Institute for Multidisciplinary research, University of Belgrade, P.O. Box 33, 11030 Belgrade, Serbia c Institute of Electrochemistry, ICTM, University of Belgrade, 11000 Belgrade, Njegoševa 12, Serbia b
AR TIC LE D ATA
ABSTR ACT
Article history:
The morphology, phase and chemical composition of Ni–Co alloy powders electrodeposited
Received 3 February 2011
from an ammonium sulfate-boric acid containing electrolyte with different ratio of Ni/Co
Received in revised form
ions were investigated. The ratios of Ni/Co ions were 1/1, 1/2 and 1/3. The morphology,
29 August 2011
chemical composition and phase composition of the electrodeposited alloy powders were
Accepted 1 September 2011
investigated using AES, SEM, EDS and XRD analysis. Composition of the electrolyte, i.e. the ratio of Ni/Co concentrations was found to influence both, the alloy phase composition
Keywords:
and the morphology of Ni–Co alloy powders. At the highest ratio of Ni/Co = 1/1 concentra-
Ni–Co powder
tions typical 2D fern-like dendritic particles were obtained. With a decrease of Ni/Co ions
Electrodeposition
ratio among 2D fern-like dendrites, 3D dendrites and different agglomerates were obtained.
Morphology
X-ray diffraction studies showed that the alloy powders mainly consisted of the facecentered cubic α-nickel phase and hexagonal close-packed ε-cobalt phase and minor proportions of face-centered cubic α-cobalt phase. The occurrence of the latter phase was observed only in the alloy powder with the higher cobalt concentration in electrolyte. The electrodeposition of Ni–Co powders occurred in an anomalous manner. © 2011 Elsevier Inc. All rights reserved.
1.
Introduction
Electrochemically deposited alloys of iron-group metals [1], whether in a form of powders or coatings are very important magnetic materials and also are known as good catalysts for hydrogen evolution [2–5]. The topics of many papers are concerned with compact Ni–Co alloy coatings [1,6,7]. On other side, only a few papers dealing with Ni–Co powder electrodeposition exist in the literature [8–10]. Yur'ev and Golubkov [8] proposed a new method for the electrodeposition of multicomponent powders using a Ni–Co alloy as an example. They used two independent anodes (nickel and cobalt) with separately controlled current densities and used a chloride containing electrolyte (NH4Cl and NaCl) to provide dissolution of nickel and cobalt anodes in order to keep constant concentration of nickel and cobalt ions in the electrolyte. Abd El⁎ Corresponding author. Tel.: +381 11 3408 760; fax: + 381 11 3408 224. E-mail address: [email protected] (V.M. Maksimović). 1044-5803/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2011.09.001
Halim and Khalil [9] investigated Ni–Co powder electrodeposition from an electrolyte containing sulfate and boric acid. It was found that with increasing concentration ratio of Ni/Co ions in the electrolyte the content of nickel in the powder increases non-linearly. Electrodeposition of alloy powders is generally a new area of investigations [10–12]. However, to study this process is more complicated than for pure metals, due the co-deposition of at least two metals and formation of various crystallographic structures arising from the phase diagram. Based on the binary phase diagram Ni–Co, nickel and cobalt build series of solid solutions in the whole range of concentrations [1,13]. The Ni– Co phase diagram shows that several phases exist at room temperature, such as: α-nickel solid solution with fcc lattice (P.S. cF4, S.G. Fm3m), ε-cobalt with hcp lattice (P. S. hP2, S.G. P63/mmc) and α-cobalt with fcc lattice (P.S. cF4, S.G.
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Fm3m). Some literature data show that there is a possibility for superlattice formation at the chemical composition corresponding to CoNi3 and CoNi intermetallic compounds [13–15]. On the other side, under equilibrium conditions, pure cobalt possesses the hcp crystal lattice below the transformation temperature of 417 °C, but exhibits fcc structure above this temperature. However, in electrodeposited form both crystals structures of cobalt may coexist [16,17]. The ability of nickel and cobalt to alloy in various ratios enables the potential uses of their magnetic and the other properties to be explored in a wide range of conditions [7,13]. The electrodeposition of Ni–Co, whether from simple or complex baths, occurs in an anomalous manner [18]. The term anomalous co-deposition as introduced by Brenner [19] refers to the preferential deposition of the less noble metal rather than the more noble metal [5,18,20]. This phenomenon has been extensively reviewed for the iron group binary alloys and numerous models have been developed to predict this behavior [7]. Although many models have been proposed, the mechanism of anomalous co-deposition is not fully understood. One explanation for anomalous co-deposition is the formation of a hydroxide precipitate of the less noble metal at the cathode, caused by a local increase of pH. The hydroxide may suppress deposition of the noble metal [21]. Nickel is always deposited first, then cobalt (II) absorbs onto the freshly deposited nickel and begins to be deposited. The cobalt (II) adsorpstion inhibits subsequent deposition of nickel, although this deposition is not completely obstructed [7]. In the present study the morphology and chemical composition of existing phases of Ni–Co powders galvanostatically deposited from electrolytes with different Ni/Co ions were investigated.
2.
Experimental
All alloy powder samples were electrodeposited at 21 °C in a cylindrical glass cell of a total volume of 1 dm3 with a cone shaped bottom of the cell. Working electrode was a glassy carbon rod of the diameter of 5 mm with the total surface area of 7.5 cm2 immersed in the electrolyte and placed in the middle of the cell. Cylindrical nickel foil placed close to the cell walls was used as a counter electrode providing excellent current distribution in the cell. Electrodeposition of powders was performed with a constant current regime at a current density of 70 mA cm−2 using appropriate power supply. The deposition time was 120 min. During the deposition process powder was removed every 15 min from electrode surface using a brush. The amount of electrodeposited powder was sufficient for SEM and composition analysis. Ni–Co powders were electrodeposited from the electrolyte containing 1 M (NH4)2SO4, 0.4 M H3BO3, 0.2 M Na2SO4 and nickel and cobalt sulfate salts: (1) Ni/Co = 1/1 (0.01 M NiSO4– 0.01 M CoSO4); (2) Ni/Co = 1/2 (0.01 M NiSO4–0.02 M CoSO4); (3) Ni/Co = 1/3 (0.01 M NiSO4–0.03 M CoSO4). pH value of all electrolytes was constant (pH 6). After deposition, powders were washed with EASY pure UV water and alcohol and left to dry in air at room temperature. The morphology and chemical composition of these powders were investigated using XRD, SEM, EDS and AES analysis. X-ray diffraction (XRD) analysis of Ni–Co alloy powders was carried out by a
Siemens D500 diffractometer with a Ni filter and CuKα radiation operated at a tube voltage of 35 kV and a tube current of 20 mA in the theta/2theta mode. The morphology of the electrodeposited powders was examined using scanning electron microscope (SEM) Philips, model XL30 and Tescan VEGA TS 5130MM equipped with an energy dispersive X-ray spectrometer (EDS), and INCAPentaFET-x3 detector, Oxford Instruments. Powder particles were analyzed by EDS in such a way that at least eight powder particles were chosen, and EDS analysis was performed at 3–10 different positions on each particle. EDS analysis was performed in a point. Chemical analysis of the powders was performed by atomic emission spectrometry (AES) using SPECTRO ICP-OES 17.5 MHz spectrometer. Samples for the analysis of about 30 mg were dissolved in 5 cm3 HCl (1:1) at slightly elevated temperature. This procedure was repeated three times and the average value, varying in the range of ± 5% is given in the paper.
3.
Results and Discussion
When recording a polarization diagram during the process of powders deposition from electrolytes, it was determined that the current efficiency in all cases is very low, approximately 1–2%. Further interpretation of the polarization diagrams were not the subject of this work. Detailed explanation of polarization curves was given in previous papers [12,22]. The morphology of the Ni–Co alloy powders is sensitive to the Ni/Co ions ratio in the electrolyte. At the highest investigated nickel content (Ni/Co = 1/1) typical 2D fern-like dendritic particles were obtained (Fig. 1a). Wranglen [23] was the first who reported the appearance of a 2D dendrite. The dendrites denoted as 2D [110] 60° are flat and fern-like with an angle of 60° between the stalk (main branch) and the branches. It is obvious that the dendrites branch along the face diagonal [110] of the unit cube. A higher magnification of an individual particle indicates that the 2D structure is composed of dozens of dendrites. The lengths of the main branches are about a hundred micrometers (main branch, see the full line in Fig. 1b), and each leaf (the first and the second branches, see the dotted and semi-dotted lines in Fig. 1b) is about of a few dozen micrometers. Some particles are branching in the third dimension (marked 2D + branching) (Fig. 2.). With the decrease of Ni/Co ions ratio among 2D branching dendrites (Fig. 3a), compact agglomerates were observed (Fig. 3b). The compact agglomerates have morphology typical for pure Co powder [11]. A further decrease of the ions ratio (Ni/Co = 1/3) dictates the appearance of densely packed 3D dendritic particles (Fig. 4b), together with fern-like 2D dendrites, and agglomerates. 3D dendrites are more compact than 2D dendrites. In the literature [23,24] such dendrites are marked as 3D [100] dendrites or massive dendrites [25] and they were not flat, but threedimensional. Comparing the morphology of various alloy powders it is clear that with the decrease of Ni/Co ions ratio the degree of branching dendrites increases as well as the growing tendency to agglomerate formation. Change of morphology is schematically displayed in Fig. 5. Regardless on the mutual
M A TE R IA L S C H A RAC TE RI ZA T ION 6 2 ( 2 01 1 ) 1 1 7 3–1 1 7 9
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Fig. 1 – SEM microphotographs of Ni–Co powder particles electrodeposited from electrolyte with Ni/Co = 1/1 ions ratio. Secondary electrons; (a) Accelerating voltage 25 kV; (b) accelerating voltage 20 kV.
position of dendrite branches, their morphology is always strictly crystallographically conditioned and associated with the spatial structure. All powders samples were analyzed by EDS in such way that the powder particles of the same morphology were chosen and EDS analysis was performed at several different Fig. 3 – SEM microphotographs of Ni–Co powder particle electrodeposited from electrolyte Ni/Co2 = 1/2 ions ratio; Secondary electrons; Accelerating voltage 20 kV; (a) 2D dendrite with branching, (b) part of agglomerate.
Fig. 2 – SEM microphotograph of Ni–Co powder particle electrodeposited from electrolyte with Ni/Co = 1/1 ions ratio; detail of 2D dendrite with branching in the third dimension. Secondary electrons. Accelerating voltage 20 kV.
positions on the same particle as shown in Table 1. The approximate composition of all alloy powders revealed that the composition not only depends on the Ni/Co ions ratio, but also on the position at which the EDS analysis was performed. Generally, these results indicate the nonhomogeneous distribution of nickel, cobalt and oxygen in the deposited powders. Table 2 illustrates the effect of electrolyte concentration (given as Ni/Co ions ratio) on the chemical composition of Ni–Co alloy powder particles and their morphology. It can be seen that with an increase in the concentration of Co2+ ions in electrolyte and amount of cobalt in powders the degree of branching increases. Fern-like 2D dendrites contain more than 50 wt.% nickel. It may be supposed that such type of particles was formed after a certain time of electrodeposition and a number of particles very quickly fall from the electrode surface as a consequence of intensive hydrogen evolution [11,12]. On the other side, the rest of particles continue to grow. Electrodeposition has two stages, i.e.
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Fig. 4 – SEM microphotographs of Ni–Co powder particle electrodeposited from electrolyte Ni/Co2 = 1/3 ions ratio; Secondary electrons; Accelerating voltage 20 kV; (a) 2D dendrite, (b) part of 3D dendrite and (c) part of agglomerate.
nucleation and growth. In the case of smooth cathodes the growth process is enhanced through nucleation, but the growth process is dominant for powder production where each nucleus is a powder particle. At this stage it is not possible to elucidate the mechanism by which the further deposition will occur. Also, it should be mentioned that the formation of dendrites passes through several stages [23]. Prolonged time of growth creates the conditions for the deposition of branched dendrites with lower nickel content. With the time of growth the disperse deposit is branching in
Fig. 5 – Scheme of the morphological change as a function of Co2+ ions concentration in electrolyte (Ni/Co ions ratio 1/1 → 1/2 → 1/3).
different direction and at the tip of each branch spherical diffusion is taking over the planar one as a consequence of decrease of local current density on the tip of each branch [26]. 2D dendrites with branching have a wide range of concentrations (49% < Co wt% > 76%). The conditions for formation of 3D dendrites and the difference between 2D + branching and 3D dendrites still remain unclear. Agglomerates have more than 80 wt.% cobalt. Also, the presence of a slight peak of oxygen in each Ni–Co powder particle indicates that the particles were covered with a thin oxide layer with a thickness of 1– 1,5 μm due to the passivation of particles. Considering that the experiment was carried out in air, it is not difficult to understand this. [27,28]. The XRD patterns of three samples are shown in Fig. 6a and b. The characteristic peaks of α-nickel phase (△), nickelrich solid solution with fcc lattice, hcp ε-cobalt phase (♦) (Fig. 6a) and fcc α-cobalt phase (▼) may be seen in the Fig. 6a, b and Table 3. With a decrease of the Ni/Co ions ratio, peaks of nickel rich phase (△) become smaller and some of them disappear, while the peaks of hcp ε-cobalt
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Table 1 – The chemical composition (in wt.%) determined by EDS of individual powder particle (agglomerate). Spectrum
Ni
Co
O
(wt.%) S. 1 S. 2 S. 3 S. 4 S. 5 S. 6 Mean ± s.d.
3.39 4.25 5.06 4.49 3.64 4.17 4±1
89.67 97.36 94.38 89.00 89.16 89.43 91 ± 4
5.04 2.72 3.30 4.92 5.11 4.89 4±1
phase (♦) become visible (Fig. 6b). It is interesting that fcc unequilibrium cobalt phase has been detected in the Ni–Co alloy powders electrodeposited from electrolytes containing Ni/Co ions ratio lower than 1/1 based on the existence of a very broad maximum around 2θ near 52° (given as detail in Fig. 6b). Though the hcp structure is the most stable phase of bulk cobalt at room temperature, experimental data reveal the coexistence of both fcc and hcp phases in the samples (Ni/Co = 1/2) and (Ni/Co = 1/3). The fcc and hcp phases of cobalt are closed-packed structures that differ only in the stacking sequence of atomic planes in the cubic [111] direction. Low activation energy for formation of stacking faults could easily lead to formation of both phases in the same sample. This phase transition phenomena is observed in dendritic cobalt nanocrystals synthesized by reduction of Co2+ with hydrazine hydrate in glycerin and glycol [29] and in other materials [16,30]. Table 2 – Influence of electrolyte concentration (given as Ni/Co ions ratio) on the chemical composition a and morphology of Ni–Co alloy powder particles. Ni/Co ions ratio
Ni
Co
O
Type of particles
1/1
53 ± 3 52 ± 4 52 ± 2 50 ± 5 45 ± 3 33 ± 6 29 ± 4 25 ± 4 6±2 14. ± 7 24 ± 2 26 ± 5 4±1 11 ± 2 18 ± 9
46 ± 4 47 ± 5 47 ± 2 49 ± 4 53 ± 5 66 ± 3 68 ± 3 73 ± 4 90 ± 2 84 ± 8 76 ± 3 73 ± 4 91 ± 4 87 ± 2 80 ± 8
1±1 2±1 1 ± 0.3 1±1 1 ± 0.6 1±1 3±1 1±1 3 ± 0.5 1 ± 0.7 1 ± 0.4 1 ± 0.3 4±1 2 ± 0.2 2±1
2D 2D 2D 2D with branching 2D with branching 2D with branching 2D with branching 2D with branching Agglomerate Agglomerate 2D with branching 3D Agglomerate b Agglomerate Agglomerate
1/2
1/3
a b
Determined by EDS and given as mean values. EDS analysis of this powder particle is given in Table 1.
Fig. 6 – An XRD pattern of Ni–Co powders electrodeposited from electrolytes containing: (a) Ni/Co = 1/1; (b) Ni/Co = 1/2 and Ni/Co = 1/3; and detail of XRD patterns for Ni/Co ions ratio 1/2 and 1/3.
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XRD measurements confirmed the crystalline nature of the Ni–Co alloy powder particles. The Ni–Co powders, rich in cobalt exhibited both fcc and hcp peaks (Fig. 6a and b), while those with higher nickel content exhibited fcc peaks of α-nickel (Fig. 6a) and a weak peak of hcp ε-cobalt phase. However, owing to the small difference in the lattice spacing of nickel and cobalt (0.35238 and 0.35477 nm, respectively), the peaks could not be definitively assigned to a solid solution of cobalt and nickel. These results are similar to those obtained by other workers [3]. In the case of powders, which were electrodeposited from the electrolyte with ions ratio 1/2 and 1/3, it could be observed separated peaks, as shown in Fig. 6b. If the morphology of the powder particles and results of XRD analysis are compared, it could be noted when the increased rate of cobalt phases in the powder is accompanied by the reduced rate of fcc nickel rich phase (solid solution of nickel), the shape of dendritic particles changes from fernlike 2D to 3D dendrites. The influence of the electrolyte composition on the alloy powder composition is shown in Fig. 7. Chemical analysis of powders shows that in all cases the atomic percentage of nickel in the powders lies under the reference line. According to Brenner's classification [19], this behavior indicates anomalous type of powder co-deposition.
4.
Conclusion
In summary, alloys powders were successfully electrodeposited. The morphology, chemical and phase composition of codeposited powders depend on the Ni/Co ions concentration ratio. According to the AES and EDS results, composition and
Fig.7 – Atomic percentage of nickel in the alloy powder as a function of the atomic percentage of nickel in the electrolyte.
morphology (SEM) of the powders depend on the Ni/Co ions concentration ratio. X-ray diffraction studies showed that the alloy powders consisted mainly of the face-centered cubic nickel phase and hexagonal close-packed ε-cobalt phase and minor proportions face-centered cubic α-cobalt phase. It is supposed that 2D type of dendrites were formed at after a certain time of electrodeposition. Conditions for formation of 3D dendrites remain still unclear as well as the difference between 2D+branching and 3D dendrites. The electrodeposition of Ni–Co powders occurs in an anomalous manner.
Acknowledgment Table 3 – The precise peak positions of the phases identified using EVA software. Sample
2θ
JCPDS card
Phase
1/1
44.552 47.602 51.841 76.238 92.720 98.239 41.683 44.429 44.461 47.522 51.627 51.893 62.656 75.961 84.208 92.582 41.632 44.428 44.581 47.503 51.519 51.708 76.155 84.180 92.446
04-0850 05-0727 04-0850 04-0850 04-0850 04-0850 05-0727 04-0850 05-0727 05-0727 15-0806 04-0850 05-0727 05-0727 05-0727 05-0727 05-0727 04-0850 05-0727 05-0727 15-0806 04-0850 05-0727 05-0727 05-0727
αNi fcc εCo hcp αNi fcc αNi fcc αNi fcc αNi fcc εCo hcp αNi fcc εCo hcp εCo hcp αCo fcc αNi fcc εCo hcp εCo hcp εCo hcp εCo hcp εCo hcp αNi fcc εCo hcp εCo hcp αCo fcc αNi fcc εCo hcp εCo hcp εCo hcp
1/2
1/3
This work was financially supported by the Ministry of Education and Science of the Republic of Serbia through the Project No. III-45012.
REFERENCES [1] Winand R. Electrodeposition of metals and alloys-new results and perspectives. Electrochim Acta 1994;39:1091–105. [2] German RM. Powder metallurgy science. second ed. Princeton NJ: Metal powder industries federation; 1994. [3] Elumalai P, Vasan HN, Verelst M, Lecante P, Carles V, Tailhades P. Synthesis and characterization of sub-micron size Co–Ni alloys using malonate as precursor. Mater Res Bull 2002;37:353–63. [4] Elumalai P, Vasan HN, Munichandraiah N, Shivashankar SA. Kinetics by hydrogen evolution on submicron size Co, Ni, Pd, Co–Ni alloy powder electrodes by d.c. polarization and a.c. impedance studies. J Appl Electrochem 2002;32:1005–10. [5] Correia AN, Machado SAS. Electrodeposition and characterisation of thin layers of Ni–Co alloys obtained from dilute chloride baths. Electrochim Acta 2000;45:1733–40. [6] Chang LM, An MZ, Shi SY. Corrosion behavior of electrodeposited Ni–Co alloy coatings under the presence of NaCl deposit at 800 °C. Mater Chem Phys 2005;94:125–30. [7] Orinakova R, Turonova A, Kladekova D, Galova M, Smith RM. Recent developments in the electrodeposition of nickel and some nickel-based alloys. J Appl Electrochem 2006;36:957–72.
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[8] B.P. Yur'ev, L.A. Golubkov, Trudy—Leningradskii Politekhnicheskii Institut imeni M.I. Kalinina 14 (1963) 269. (published in Russian). [9] Abd El-Halim AM, Khalil RM. Some characteristics of Ni–Co alloy powders electrodeposition from dilute sulphate baths. Surf Coat Technol 1986;27:103–15. [10] Jović VD, Jović BM, Pavlović MG, Maksimović VM. Morphology and composition of Ni–Co alloy powders electrodeposited from ammoniacal electrolyte. J Solid State Electrochem 2006;10: 959–66. [11] Jović VD, Jović BM, Pavlović MG. Electrodeposition of Ni, Co and Ni–Co alloy powders. Electrochim Acta 2006;51: 5468–77. [12] Jović VD, Jović BM, Maksimović VM, Pavlović MG. Electrodeposition and morphology of Ni, Co and Ni–Co alloy powders Part II. Ammonium chloride supporting electrolyte. Electrochim Acta 2007;52:4254–63. [13] Hasen M, Anderko K. Constitution of Binary Alloys. New York: McGraw-Hill Book Company; 1958. [14] Tianfu J, Jingwu Z, Wantang F, Mackenzie RAD. Co-deposition of Co–Ni alloys and their microstructure analysis by PoSAP. Chin Sci Bull 1997;42:2013–4. [15] Pearson WB. A Handbook of lattice spacings and structures of metals and alloys. London: Pergamon Press; 1958. [16] Zhu Y, Yang Q, Zheng H, Yu W, Qian Y. Flower-like cobalt nanocrystals by a complex precursor reaction route. Mater Chem Phys 2005;91:293–7. [17] Scarni V, Doudin B, Anserment JP. The microstructure of electrodeposited cobalt-based nanowires and its effect on their magnetic and transport properties. J Magn Magn Mater 1999;205:241–8. [18] Gomez E, Ramirez J, Valles E. Electrodeposition of Ni–Co alloys. J Appl Electrochem 1998;28:71–9.
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[19] Brenner A. Electrodeposition of Alloys I. New York: Academic Press; 1963. [20] Zhuang Y, Podlaha EJ. NiCoFe ternary alloy deposition: I an experimental kinetic study. J Electrochem Soc 2000;147:2231–6. [21] Fan C, Piron DL. Study of anumalous nickel-cobal electrodeposition with different electrolytes and current densities. Electrochim Acta 1996;41:1713–9. [22] Jović VD, Jović BM, Pavlović MG. Electrosdeposition of Ni, Co and Ni–Co alloy powder. Electrochim Acta 2006;51:5468–77. [23] Wranglen G. Dendrites and growth layers in the electrocrystallization of metals. Electrochim Acta 1960;2:130–44. [24] Maksimović VM, Pavlović MG, Pavlović LjJ, Tomić MV, Jović VD. Morphology and growth of electrodeposited silver powder particles. Hydrometallurgy 2007;86:22–6. [25] Maksimović VM, Pavlović MG, Pavlović LjJ, Tomić M. Characterization of copper powder particles obtained by electrodeposition as function of different current densities. J Appl Electrochem 2009;39:2545–52. [26] Jović VD, Maksimović V, Pavlović MG, Popov KI. Morphology, internal structure and growth mechanism of electrodeposited Ni and Co powders. J Solid State Electrochem 2006;10:373–9. [27] Zhu LP, Xiao HM, Zhang WD, Yang Y, Fu SY. Synthesis and characterization of novel three-dimensional metallic Co dendritic structures by a simple hydrothermal reduction. Cryst Growth Des 2008;8:1113–8. [28] Lui X, Yi R, Wang Y, Qiu G, Zhang N, Li X. Highly ordered snowflake like metallic cobalt microcrystals. J Phys Chem C 2007;111:163–7. [29] Zhu Y, Zheng H, Yang Q, Pan A, Yang Z, Qian Y. Growth of dendritic cobalt nanocrystals at room temperature. J Cryst Growth 2004;260:427–34. [30] Well AF. Structure Inorganic Chemistry. Oxford: Oxford University Press; 1975.
Available online at www.sciencedirect.com
www.elsevier.com/locate/matchar
Morphology and composition of Ni–Co electrodeposited powders V.M. Maksimovića,⁎, U.Č. Lačnjevacb , M.M. Stoiljkovića , M.G. Pavlovićc , V.D. Jovićb a
Institute of Nuclear Sciences, “Vinča”, University of Belgrade, 11001 Belgrade, P. O. Box 522, Serbia Institute for Multidisciplinary research, University of Belgrade, P.O. Box 33, 11030 Belgrade, Serbia c Institute of Electrochemistry, ICTM, University of Belgrade, 11000 Belgrade, Njegoševa 12, Serbia b
AR TIC LE D ATA
ABSTR ACT
Article history:
The morphology, phase and chemical composition of Ni–Co alloy powders electrodeposited
Received 3 February 2011
from an ammonium sulfate-boric acid containing electrolyte with different ratio of Ni/Co
Received in revised form
ions were investigated. The ratios of Ni/Co ions were 1/1, 1/2 and 1/3. The morphology,
29 August 2011
chemical composition and phase composition of the electrodeposited alloy powders were
Accepted 1 September 2011
investigated using AES, SEM, EDS and XRD analysis. Composition of the electrolyte, i.e. the ratio of Ni/Co concentrations was found to influence both, the alloy phase composition
Keywords:
and the morphology of Ni–Co alloy powders. At the highest ratio of Ni/Co = 1/1 concentra-
Ni–Co powder
tions typical 2D fern-like dendritic particles were obtained. With a decrease of Ni/Co ions
Electrodeposition
ratio among 2D fern-like dendrites, 3D dendrites and different agglomerates were obtained.
Morphology
X-ray diffraction studies showed that the alloy powders mainly consisted of the facecentered cubic α-nickel phase and hexagonal close-packed ε-cobalt phase and minor proportions of face-centered cubic α-cobalt phase. The occurrence of the latter phase was observed only in the alloy powder with the higher cobalt concentration in electrolyte. The electrodeposition of Ni–Co powders occurred in an anomalous manner. © 2011 Elsevier Inc. All rights reserved.
1.
Introduction
Electrochemically deposited alloys of iron-group metals [1], whether in a form of powders or coatings are very important magnetic materials and also are known as good catalysts for hydrogen evolution [2–5]. The topics of many papers are concerned with compact Ni–Co alloy coatings [1,6,7]. On other side, only a few papers dealing with Ni–Co powder electrodeposition exist in the literature [8–10]. Yur'ev and Golubkov [8] proposed a new method for the electrodeposition of multicomponent powders using a Ni–Co alloy as an example. They used two independent anodes (nickel and cobalt) with separately controlled current densities and used a chloride containing electrolyte (NH4Cl and NaCl) to provide dissolution of nickel and cobalt anodes in order to keep constant concentration of nickel and cobalt ions in the electrolyte. Abd El⁎ Corresponding author. Tel.: +381 11 3408 760; fax: + 381 11 3408 224. E-mail address: [email protected] (V.M. Maksimović). 1044-5803/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2011.09.001
Halim and Khalil [9] investigated Ni–Co powder electrodeposition from an electrolyte containing sulfate and boric acid. It was found that with increasing concentration ratio of Ni/Co ions in the electrolyte the content of nickel in the powder increases non-linearly. Electrodeposition of alloy powders is generally a new area of investigations [10–12]. However, to study this process is more complicated than for pure metals, due the co-deposition of at least two metals and formation of various crystallographic structures arising from the phase diagram. Based on the binary phase diagram Ni–Co, nickel and cobalt build series of solid solutions in the whole range of concentrations [1,13]. The Ni– Co phase diagram shows that several phases exist at room temperature, such as: α-nickel solid solution with fcc lattice (P.S. cF4, S.G. Fm3m), ε-cobalt with hcp lattice (P. S. hP2, S.G. P63/mmc) and α-cobalt with fcc lattice (P.S. cF4, S.G.
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Fm3m). Some literature data show that there is a possibility for superlattice formation at the chemical composition corresponding to CoNi3 and CoNi intermetallic compounds [13–15]. On the other side, under equilibrium conditions, pure cobalt possesses the hcp crystal lattice below the transformation temperature of 417 °C, but exhibits fcc structure above this temperature. However, in electrodeposited form both crystals structures of cobalt may coexist [16,17]. The ability of nickel and cobalt to alloy in various ratios enables the potential uses of their magnetic and the other properties to be explored in a wide range of conditions [7,13]. The electrodeposition of Ni–Co, whether from simple or complex baths, occurs in an anomalous manner [18]. The term anomalous co-deposition as introduced by Brenner [19] refers to the preferential deposition of the less noble metal rather than the more noble metal [5,18,20]. This phenomenon has been extensively reviewed for the iron group binary alloys and numerous models have been developed to predict this behavior [7]. Although many models have been proposed, the mechanism of anomalous co-deposition is not fully understood. One explanation for anomalous co-deposition is the formation of a hydroxide precipitate of the less noble metal at the cathode, caused by a local increase of pH. The hydroxide may suppress deposition of the noble metal [21]. Nickel is always deposited first, then cobalt (II) absorbs onto the freshly deposited nickel and begins to be deposited. The cobalt (II) adsorpstion inhibits subsequent deposition of nickel, although this deposition is not completely obstructed [7]. In the present study the morphology and chemical composition of existing phases of Ni–Co powders galvanostatically deposited from electrolytes with different Ni/Co ions were investigated.
2.
Experimental
All alloy powder samples were electrodeposited at 21 °C in a cylindrical glass cell of a total volume of 1 dm3 with a cone shaped bottom of the cell. Working electrode was a glassy carbon rod of the diameter of 5 mm with the total surface area of 7.5 cm2 immersed in the electrolyte and placed in the middle of the cell. Cylindrical nickel foil placed close to the cell walls was used as a counter electrode providing excellent current distribution in the cell. Electrodeposition of powders was performed with a constant current regime at a current density of 70 mA cm−2 using appropriate power supply. The deposition time was 120 min. During the deposition process powder was removed every 15 min from electrode surface using a brush. The amount of electrodeposited powder was sufficient for SEM and composition analysis. Ni–Co powders were electrodeposited from the electrolyte containing 1 M (NH4)2SO4, 0.4 M H3BO3, 0.2 M Na2SO4 and nickel and cobalt sulfate salts: (1) Ni/Co = 1/1 (0.01 M NiSO4– 0.01 M CoSO4); (2) Ni/Co = 1/2 (0.01 M NiSO4–0.02 M CoSO4); (3) Ni/Co = 1/3 (0.01 M NiSO4–0.03 M CoSO4). pH value of all electrolytes was constant (pH 6). After deposition, powders were washed with EASY pure UV water and alcohol and left to dry in air at room temperature. The morphology and chemical composition of these powders were investigated using XRD, SEM, EDS and AES analysis. X-ray diffraction (XRD) analysis of Ni–Co alloy powders was carried out by a
Siemens D500 diffractometer with a Ni filter and CuKα radiation operated at a tube voltage of 35 kV and a tube current of 20 mA in the theta/2theta mode. The morphology of the electrodeposited powders was examined using scanning electron microscope (SEM) Philips, model XL30 and Tescan VEGA TS 5130MM equipped with an energy dispersive X-ray spectrometer (EDS), and INCAPentaFET-x3 detector, Oxford Instruments. Powder particles were analyzed by EDS in such a way that at least eight powder particles were chosen, and EDS analysis was performed at 3–10 different positions on each particle. EDS analysis was performed in a point. Chemical analysis of the powders was performed by atomic emission spectrometry (AES) using SPECTRO ICP-OES 17.5 MHz spectrometer. Samples for the analysis of about 30 mg were dissolved in 5 cm3 HCl (1:1) at slightly elevated temperature. This procedure was repeated three times and the average value, varying in the range of ± 5% is given in the paper.
3.
Results and Discussion
When recording a polarization diagram during the process of powders deposition from electrolytes, it was determined that the current efficiency in all cases is very low, approximately 1–2%. Further interpretation of the polarization diagrams were not the subject of this work. Detailed explanation of polarization curves was given in previous papers [12,22]. The morphology of the Ni–Co alloy powders is sensitive to the Ni/Co ions ratio in the electrolyte. At the highest investigated nickel content (Ni/Co = 1/1) typical 2D fern-like dendritic particles were obtained (Fig. 1a). Wranglen [23] was the first who reported the appearance of a 2D dendrite. The dendrites denoted as 2D [110] 60° are flat and fern-like with an angle of 60° between the stalk (main branch) and the branches. It is obvious that the dendrites branch along the face diagonal [110] of the unit cube. A higher magnification of an individual particle indicates that the 2D structure is composed of dozens of dendrites. The lengths of the main branches are about a hundred micrometers (main branch, see the full line in Fig. 1b), and each leaf (the first and the second branches, see the dotted and semi-dotted lines in Fig. 1b) is about of a few dozen micrometers. Some particles are branching in the third dimension (marked 2D + branching) (Fig. 2.). With the decrease of Ni/Co ions ratio among 2D branching dendrites (Fig. 3a), compact agglomerates were observed (Fig. 3b). The compact agglomerates have morphology typical for pure Co powder [11]. A further decrease of the ions ratio (Ni/Co = 1/3) dictates the appearance of densely packed 3D dendritic particles (Fig. 4b), together with fern-like 2D dendrites, and agglomerates. 3D dendrites are more compact than 2D dendrites. In the literature [23,24] such dendrites are marked as 3D [100] dendrites or massive dendrites [25] and they were not flat, but threedimensional. Comparing the morphology of various alloy powders it is clear that with the decrease of Ni/Co ions ratio the degree of branching dendrites increases as well as the growing tendency to agglomerate formation. Change of morphology is schematically displayed in Fig. 5. Regardless on the mutual
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Fig. 1 – SEM microphotographs of Ni–Co powder particles electrodeposited from electrolyte with Ni/Co = 1/1 ions ratio. Secondary electrons; (a) Accelerating voltage 25 kV; (b) accelerating voltage 20 kV.
position of dendrite branches, their morphology is always strictly crystallographically conditioned and associated with the spatial structure. All powders samples were analyzed by EDS in such way that the powder particles of the same morphology were chosen and EDS analysis was performed at several different Fig. 3 – SEM microphotographs of Ni–Co powder particle electrodeposited from electrolyte Ni/Co2 = 1/2 ions ratio; Secondary electrons; Accelerating voltage 20 kV; (a) 2D dendrite with branching, (b) part of agglomerate.
Fig. 2 – SEM microphotograph of Ni–Co powder particle electrodeposited from electrolyte with Ni/Co = 1/1 ions ratio; detail of 2D dendrite with branching in the third dimension. Secondary electrons. Accelerating voltage 20 kV.
positions on the same particle as shown in Table 1. The approximate composition of all alloy powders revealed that the composition not only depends on the Ni/Co ions ratio, but also on the position at which the EDS analysis was performed. Generally, these results indicate the nonhomogeneous distribution of nickel, cobalt and oxygen in the deposited powders. Table 2 illustrates the effect of electrolyte concentration (given as Ni/Co ions ratio) on the chemical composition of Ni–Co alloy powder particles and their morphology. It can be seen that with an increase in the concentration of Co2+ ions in electrolyte and amount of cobalt in powders the degree of branching increases. Fern-like 2D dendrites contain more than 50 wt.% nickel. It may be supposed that such type of particles was formed after a certain time of electrodeposition and a number of particles very quickly fall from the electrode surface as a consequence of intensive hydrogen evolution [11,12]. On the other side, the rest of particles continue to grow. Electrodeposition has two stages, i.e.
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Fig. 4 – SEM microphotographs of Ni–Co powder particle electrodeposited from electrolyte Ni/Co2 = 1/3 ions ratio; Secondary electrons; Accelerating voltage 20 kV; (a) 2D dendrite, (b) part of 3D dendrite and (c) part of agglomerate.
nucleation and growth. In the case of smooth cathodes the growth process is enhanced through nucleation, but the growth process is dominant for powder production where each nucleus is a powder particle. At this stage it is not possible to elucidate the mechanism by which the further deposition will occur. Also, it should be mentioned that the formation of dendrites passes through several stages [23]. Prolonged time of growth creates the conditions for the deposition of branched dendrites with lower nickel content. With the time of growth the disperse deposit is branching in
Fig. 5 – Scheme of the morphological change as a function of Co2+ ions concentration in electrolyte (Ni/Co ions ratio 1/1 → 1/2 → 1/3).
different direction and at the tip of each branch spherical diffusion is taking over the planar one as a consequence of decrease of local current density on the tip of each branch [26]. 2D dendrites with branching have a wide range of concentrations (49% < Co wt% > 76%). The conditions for formation of 3D dendrites and the difference between 2D + branching and 3D dendrites still remain unclear. Agglomerates have more than 80 wt.% cobalt. Also, the presence of a slight peak of oxygen in each Ni–Co powder particle indicates that the particles were covered with a thin oxide layer with a thickness of 1– 1,5 μm due to the passivation of particles. Considering that the experiment was carried out in air, it is not difficult to understand this. [27,28]. The XRD patterns of three samples are shown in Fig. 6a and b. The characteristic peaks of α-nickel phase (△), nickelrich solid solution with fcc lattice, hcp ε-cobalt phase (♦) (Fig. 6a) and fcc α-cobalt phase (▼) may be seen in the Fig. 6a, b and Table 3. With a decrease of the Ni/Co ions ratio, peaks of nickel rich phase (△) become smaller and some of them disappear, while the peaks of hcp ε-cobalt
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Table 1 – The chemical composition (in wt.%) determined by EDS of individual powder particle (agglomerate). Spectrum
Ni
Co
O
(wt.%) S. 1 S. 2 S. 3 S. 4 S. 5 S. 6 Mean ± s.d.
3.39 4.25 5.06 4.49 3.64 4.17 4±1
89.67 97.36 94.38 89.00 89.16 89.43 91 ± 4
5.04 2.72 3.30 4.92 5.11 4.89 4±1
phase (♦) become visible (Fig. 6b). It is interesting that fcc unequilibrium cobalt phase has been detected in the Ni–Co alloy powders electrodeposited from electrolytes containing Ni/Co ions ratio lower than 1/1 based on the existence of a very broad maximum around 2θ near 52° (given as detail in Fig. 6b). Though the hcp structure is the most stable phase of bulk cobalt at room temperature, experimental data reveal the coexistence of both fcc and hcp phases in the samples (Ni/Co = 1/2) and (Ni/Co = 1/3). The fcc and hcp phases of cobalt are closed-packed structures that differ only in the stacking sequence of atomic planes in the cubic [111] direction. Low activation energy for formation of stacking faults could easily lead to formation of both phases in the same sample. This phase transition phenomena is observed in dendritic cobalt nanocrystals synthesized by reduction of Co2+ with hydrazine hydrate in glycerin and glycol [29] and in other materials [16,30]. Table 2 – Influence of electrolyte concentration (given as Ni/Co ions ratio) on the chemical composition a and morphology of Ni–Co alloy powder particles. Ni/Co ions ratio
Ni
Co
O
Type of particles
1/1
53 ± 3 52 ± 4 52 ± 2 50 ± 5 45 ± 3 33 ± 6 29 ± 4 25 ± 4 6±2 14. ± 7 24 ± 2 26 ± 5 4±1 11 ± 2 18 ± 9
46 ± 4 47 ± 5 47 ± 2 49 ± 4 53 ± 5 66 ± 3 68 ± 3 73 ± 4 90 ± 2 84 ± 8 76 ± 3 73 ± 4 91 ± 4 87 ± 2 80 ± 8
1±1 2±1 1 ± 0.3 1±1 1 ± 0.6 1±1 3±1 1±1 3 ± 0.5 1 ± 0.7 1 ± 0.4 1 ± 0.3 4±1 2 ± 0.2 2±1
2D 2D 2D 2D with branching 2D with branching 2D with branching 2D with branching 2D with branching Agglomerate Agglomerate 2D with branching 3D Agglomerate b Agglomerate Agglomerate
1/2
1/3
a b
Determined by EDS and given as mean values. EDS analysis of this powder particle is given in Table 1.
Fig. 6 – An XRD pattern of Ni–Co powders electrodeposited from electrolytes containing: (a) Ni/Co = 1/1; (b) Ni/Co = 1/2 and Ni/Co = 1/3; and detail of XRD patterns for Ni/Co ions ratio 1/2 and 1/3.
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XRD measurements confirmed the crystalline nature of the Ni–Co alloy powder particles. The Ni–Co powders, rich in cobalt exhibited both fcc and hcp peaks (Fig. 6a and b), while those with higher nickel content exhibited fcc peaks of α-nickel (Fig. 6a) and a weak peak of hcp ε-cobalt phase. However, owing to the small difference in the lattice spacing of nickel and cobalt (0.35238 and 0.35477 nm, respectively), the peaks could not be definitively assigned to a solid solution of cobalt and nickel. These results are similar to those obtained by other workers [3]. In the case of powders, which were electrodeposited from the electrolyte with ions ratio 1/2 and 1/3, it could be observed separated peaks, as shown in Fig. 6b. If the morphology of the powder particles and results of XRD analysis are compared, it could be noted when the increased rate of cobalt phases in the powder is accompanied by the reduced rate of fcc nickel rich phase (solid solution of nickel), the shape of dendritic particles changes from fernlike 2D to 3D dendrites. The influence of the electrolyte composition on the alloy powder composition is shown in Fig. 7. Chemical analysis of powders shows that in all cases the atomic percentage of nickel in the powders lies under the reference line. According to Brenner's classification [19], this behavior indicates anomalous type of powder co-deposition.
4.
Conclusion
In summary, alloys powders were successfully electrodeposited. The morphology, chemical and phase composition of codeposited powders depend on the Ni/Co ions concentration ratio. According to the AES and EDS results, composition and
Fig.7 – Atomic percentage of nickel in the alloy powder as a function of the atomic percentage of nickel in the electrolyte.
morphology (SEM) of the powders depend on the Ni/Co ions concentration ratio. X-ray diffraction studies showed that the alloy powders consisted mainly of the face-centered cubic nickel phase and hexagonal close-packed ε-cobalt phase and minor proportions face-centered cubic α-cobalt phase. It is supposed that 2D type of dendrites were formed at after a certain time of electrodeposition. Conditions for formation of 3D dendrites remain still unclear as well as the difference between 2D+branching and 3D dendrites. The electrodeposition of Ni–Co powders occurs in an anomalous manner.
Acknowledgment Table 3 – The precise peak positions of the phases identified using EVA software. Sample
2θ
JCPDS card
Phase
1/1
44.552 47.602 51.841 76.238 92.720 98.239 41.683 44.429 44.461 47.522 51.627 51.893 62.656 75.961 84.208 92.582 41.632 44.428 44.581 47.503 51.519 51.708 76.155 84.180 92.446
04-0850 05-0727 04-0850 04-0850 04-0850 04-0850 05-0727 04-0850 05-0727 05-0727 15-0806 04-0850 05-0727 05-0727 05-0727 05-0727 05-0727 04-0850 05-0727 05-0727 15-0806 04-0850 05-0727 05-0727 05-0727
αNi fcc εCo hcp αNi fcc αNi fcc αNi fcc αNi fcc εCo hcp αNi fcc εCo hcp εCo hcp αCo fcc αNi fcc εCo hcp εCo hcp εCo hcp εCo hcp εCo hcp αNi fcc εCo hcp εCo hcp αCo fcc αNi fcc εCo hcp εCo hcp εCo hcp
1/2
1/3
This work was financially supported by the Ministry of Education and Science of the Republic of Serbia through the Project No. III-45012.
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