by
C. Madore and D. Landolt
Featured Content
Laboratoire de Metallurgie Chimique
Lausanne, Switzerland
Originally published as:
Plating & Surface Finishing, 80 (11), 7378 (1993).
Editor’s Note: A printable PDF version is available by clicking HERE.
ABSTRACT
A new cell geometry, the RCH cell, can replace the classical Hull cell in applications where mass transport is of importance. The cell includes a rotating cylinder electrode and an asymmetrically placed counterelectrode with a suitably placed concentric separating wall. The primary current distribution resembles that of the classical Hull cell. The usefulness of this cell design is demonstrated by studying the reaction distribution on the cathode during deposition of a copper nickel alloy from a citratebased model electrolyte.
Introduction
The Hull cell^{1} is a trapezoidal structure in which the cathode is placed at an oblique angle with respect to the anode. The cell allows exploration of the variation in the appearance of an electrodeposit over a wide range of current densities along the cathode surface in order to determine optimal electroplating conditions. It is commonly used for maintenance of plating solutions to insure continuous production of satisfactory deposits and for the development of new electrolytes. A shortcoming of the traditional Hull cell is the absence of controlled masstransport conditions. Indeed, for many plating processes, mass transport is a crucial step; examples include metal plating in the presence of leveling agents or alloy plating with one component being present in low concentration. In such cases, interpretation of a Hull cell plate is difficult and the cell cannot be used effectively.^{2}
To overcome this problem, several authors have proposed Hulltype cells that include controlled hydrodynamic conditions. Graham and Pinkerton built a rotating cylinder placed coaxial with a stationary conical anode.^{3} The current distribution along the cathode examined in their studies, however, was significantly more uniform than that obtained in the traditional Hull cell. Kadija, et al. developed a hydrodynamically controlled Hull cell using a rotating cylinder with the anode placed below the cathode on the cylinder.^{4} Using a partially submerged cathode and insulator baffles, they obtained a current distribution similar to that of the Hull cell. In this cell, however, a welldefined current distribution is obtained only if effects resulting from a vortex can be avoided. Lu proposed several designs using conical and cylindrical electrodes.^{5} The described cell configurations, however, were either empirical or of complex geometry and therefore had limited applicability.
In the absence of masstransport effects, uniformity of the current distribution in electrochemical cells depends on the ratio of the polarization resistance at the electrode surface and the ohmic resistance of the electrolyte. This ratio is expressed by the Wagner number. For Tafel kinetics, the Wagner number is given by Equation 1:
(1)
where η is the overpotential, ρ_{e} is the electrolyte resistance, κ is the electrolyte conductivity (κ = 1/ρ_{e}), i_{avg} is the average current density, and L is a dimensionless length, here the cathode length h. The most nonuniform current distribution, the socalled primary current distribution, corresponds to Wa 0. The empirical Eq. (2) used in practice for the determination of the current distribution i on the classical Hull cell corresponds to this case.^{6}
(2)
where z is the dimensionless cathode length measured from the low current end and i_{avg} the average current density in the cell. The insensitivity of practical Hull cell experiments to kinetic parameters is readily understood with Eq. (1) because the evaluated average current density is usually quite high, leading to a value of Wa close to zero.
The goal of the present study is to design an alternative Hull cell having a primary current distribution close to that of the classical Hull cell, but with wellcharacterized masstransport conditions. The masstransfer rate should be uniform over the entire cathode surface and the magnitude of the limiting current should be easily varied. The new cell should be relatively simple to build and to use, without the need for elaborate pumping systems or of a large electrolyte volume. Its primary and secondary current distribution should be well known from numerical simulation.
Cell geometry
Theoretical and practical design considerations of a cylindrical Hull cell with controlled convection have been published in a recent paper.^{7} In the present investigation, a simple cell design is described and its performance tested with two model systems  electrodeposition of copper at almost primary current distribution, and electrodeposition of a coppernickel alloy involving masstransfercontrolled deposition of copper. The cell consists of a rotating cylinder electrode with a counterelectrode placed at one end. An outer stationary insulating separator is placed coaxially with the rotating cylinder electrode and its diameter determines the current distribution along the cylinder electrode. The primary current distribution was simulated for the above geometry with a software based on a boundary element method (BEM).^{8,9} The cell geometry was optimized in order to yield a current distribution similar to that of the Hull cell.^{7} The optimal geometry was found to correspond to a ratio of the electrode length over the distance between the electrode and the insulating separator equal to three. Moreover, the calculations showed that position of the anode, placed outside the insulating separator, has no influence on the current distribution.
Figure 1 compares the primary current distribution for the optimal geometry of the RCH cell described above and the Hull cell. The two distributions are quite similar. For small values of z, the current density in the RCH cell varies slightly less than in the Hull cell because the current density at the far end does not go to zero. The simulated primary current distribution for the RCH cell is described by the following expression:^{7}
(3)
where z is the dimensionless distance along the cathode from the lowest current density side (z = l  x/h in Eq. [9] of Ref. 7), i(z) the current density along the cylinder cathode and i_{avg} the applied current density.
Figure 1  Primary current distribution for the RCH cell given by Eq. (3) and for the Hull cell, Eq. (2).
An experimental cell that fulfills the above geometric considerations was built and is shown schematically in Fig. 2. The cylindrical cell, made of Plexiglas™, is 135 mm in height and 120 mm in diameter. A cylindrical Plexiglas separator, 85 mm in height and 55 mm in inner diameter is fixed concentrically at the bottom endplate. The motor shaft and the rotating cylinder assembly are held concentric with the separator by a Teflon guide attached to the top endplate. This design facilitates the removal of the electrode. The rotating assembly is supported from below with a guide pin in order to avoid eccentric motion. The electrode is submerged about 4 cm below the electrolyte surface and its top end is located 1 cm below the upper edge of the cylindrical separator. In this configuration, all the current between anode and cathode flows through the open space at the top of the separator. The current density, therefore, is maximum at the top end of the cathode and diminishes continuously toward its lower end. In a previous design, the insulating separator had been placed so that the maximum current density was reached at the bottom of the cathode.^{10} An advantage of the present design is that gas bubbles, which sometimes form at the cathode at high current density, can leave the electrode surface without disturbing the reactions proceeding on the lowcurrentdensity part of the surface. Figure 3 shows a photograph of the present cell assembly, including motor, rotating cathode and stationary counterelectrode. The latter is an oxidecoated commercial titanium mesh placed concentrically, outside the cylindrical separator. Other anode materials in the form of sheets of meshes can also be employed.
Figure 2  Schematic of the RCH cell. The cylinder cathode (A) is connected electrically through a silver friction contact (B) and is attached to the motor (C). Insulating separator (D) is fixed concentrically with the cylinder cathode. Concentric anode (E) is placed outside the insulating separator. Dimensions in mm.
The rotating cylinder assembly is shown in more detail in Fig. 4. It consists of an aluminum shaft 13 cm in length. The electrical contact between the rotating electrode and the current source is provided by a silver friction contact. The electrode is a hollow metal cylinder 6 cm in length and 1.5 cm in diameter, which fits on the shaft. The cathode surface is recessed by 0.25 cm with respect to the surface of the insulating sleeves that cover the rotating shaft above and below the active electrode surface. The recess prevents the primary current distribution at the near edge going to infinity. Mass transport is not measurably changed by the presence of a recess.^{10} The electrode surface area is 28.3 cm^{2}. The upper part of the shaft is covered with Teflon™ insulation 5.5 cm in length. A1.5cm Nylon™ part is screwed onto the lower end of the shaft. It holds the electrode in place and permits its easy removal. The upper end of the rotating cylinder is attached to the motor shaft. The motor velocity is electronically controlled with an optical feedback sensor and can be operated in the range of 50 to 3600 rpm. The velocity is stable within 5 percent, at rotations below 300 rpm and within 2 percent at higher speeds.
Figure 3  RCH cell.
Figure 4  Rotating electrode assembly.
Verification of current distribution
To verify the primary and secondary current distributions at the cathode in the absence of masstransport effects, copper was deposited much below its limiting current density. Deposition was carried out from an acid sulfate bath containing 0.6M CuSO_{4} and 1.0M H_{2}SO_{4}, prepared from reagentgrade chemicals and doublydistilled water. A high copper concentration was used to minimize the influence of mass transport and a high acid concentration used to increase bath conductivity and prevent oxide formation at the anode. All deposits were plated at a constant temperature of 25°C, using a cylinder rotation speed of 900 rpm. The cathode was a nickel cylinder and the anode was a pure copper sheet. Prior to deposition, the nickel cylinder was polished with 600 grit sandpaper, washed with soap and rinsed with distilled water.
Deposition was performed galvanostatically at an average current density of 100 mA/cm^{2}. The total charge passed through the cell was 693°C, corresponding to an average deposit thickness of 9.3 µm. The current density employed corresponded to about 20 percent of the limiting current density. Consequently, the effect of mass transport could be safely neglected. Current efficiency for copper deposition as measured by weight gain experiments was found to be approximately 100 percent for all deposits. Copper thicknesses along the electrode length were measured by xray fluorescence (XRF).
Simulation of the secondary current distribution was performed with a BEM method, using Tafel kinetics as boundary conditions at the cathode.^{9,10} A cathodic Tafel constant of β_{c} = 45 mV, an exchange current density i_{o} of 2 mA/cm^{2}, and an electrolyte resistance of 0.35 Ωcm^{2} were measured for a 0.5M copper sulfate acid electrolyte at 25°C.^{10} These values were used in the calculations. For the cell used, these deposition conditions correspond to a Wagner number of 0.02.
In Fig. 5, the experimentally measured dimensionless thickness µ(z)/µ_{avg} and the calculated dimensionless current density i(z)/i_{avg} are represented as a function of the dimensionless length z along the cathode. The deposit thickness is related to the current density through Faraday's law:
(4)
where M is the atomic weight of copper and ρ its density. For a current efficiency of 100 percent, the dimensionless thickness µ(z)/µ_{avg} corresponds exactly to the value of i(z)/i_{avg}. Good agreement was found between the computed secondary current distribution and the experimental data showing the variation of deposit thickness with distance from the upper edge. The slight difference in the computed curves for primary and secondary current distribution suggests that kinetic effects were not entirely negligible at the average current density of 100 mA/cm^{2}. However, the secondary current distribution is very close to the primary case and the metal distribution can be approximated by Eq. (3). For many practical applications of the RCH cell at low Wa, Eq. (3) should be sufficiently accurate for estimation of the local current density.
Figure 5  Experimental and theoretical current distribution along the cylinder cathode. The secondary distribution is calculated for a Wagner number of 0.02; the primary distribution is calculated by Eq. (3). Current densities and lengths are dimensionless.
Alloy deposition involving a mass transportlimited step
To demonstrate the usefulness of the RCH cell for alloy deposition applications, coppernickel alloys were deposited from a citratemodel electrolyte, containing a large excess of nickel. The bath was composed of 0.25M Na_{3}C_{6}H_{5}O_{7}•2H_{2}O, 0.7M NiSO_{4}•6H_{2}O and 0.025M CuSO_{4}•5H_{2}O dissolved in doublydistilled water. This electrolyte composition contains a low copper concentration in order to facilitate masstransport control of the copper deposition reaction. The copper content in the electrolyte was monitored by atomic absorption and was kept constant within 10 percent. The cylindrical cathode was made of stainless steel. Because copper and nickel did not adhere well to the substrate, a 0.5µmthick gold electroplate was deposited on the base metal to produce adherent deposits. A DSA mesh anode was used in order to avoid a change in the electrolyte composition by anode dissolution.
Deposition was performed galvanostatically at 25°C at an average current density of 25 mA/cm^{2}, and the average deposit thickness was 9.3 µm. The thicknesses and compositions of the alloys were analyzed by XRF and the partial current densities for Cu and Ni were deduced from the analysis. The partial current for hydrogen was negligible on most parts of the cathode. The effect of mass transport on the alloy composition was investigated by varying the rotation speed from 300 to 1500 rpm and the copper content in the electrolyte from 0.01M to 0.038M.
Results
A wide range of alloy compositions was obtained in a single experiment because of nonuniform current distribution at the cathode. The change in composition could be readily seen from the color of the deposit, which varied from red at the lowcurrentdensity end to gray at the highcurrentdensity end. Compact deposits were obtained for copper concentrations less than 80 percent. At higher copper contents, darkred and slightly powdery deposits were formed. A black powdery deposit was obtained near the highcurrentdensity edge of the cathode where some hydrogen evolution occurred during deposition. This region of the cathode could not be analyzed by XRF.
The variation in alloy composition along the cylinder is shown in Fig. 6. The copper content in the electrolyte was 0.025M and the cathode was rotated at 600 rpm. The deposit thickness increased from 2 µm to 20 µm with increasing current density. The corresponding variation in composition was 75 percent Cu to 10 percent Cu.
Figure 6  Variation in thickness and composition along the cylinder cathode for a coppernickel alloy. Deposition performed at 25 mA/cm^{2} average current density, 600 rpm and 0.025M Cu^{+2}.
The alloy composition is also dependent on masstransport conditions. The dependence of copper current density with rotation speed is shown in Fig. 7 for a copper concentration of 0.025M. The copper partial current density follows a straight line when plotted against the rotation speed. A linear relation is also observed when the copper current density is plotted against the copper concentration in the bath (Fig. 8). This shows that copper deposition is masstransport limited and follows the Eisenberg, Tobias and Wilke correlation^{12} at a rotating cylinder electrode in a turbulent flow regime:
(5)
where C_{Cu}_{+2} is the copper concentration in the electrolyte (mol/cm^{3}), n the rotation speed (rpm) and K is a proportionality factor given by:
(6)
where i_{lim} is the limiting current density, d the cylinder diameter, D the diffusion coefficient and ν the kinematic viscosity. Linear regression of the above relation yields a slope (averaged for the four electrolytes) of K = 1.4. Based on a kinematic viscosity of 0.01 cm^{2}/sec, a diffusion coefficient is computed for the coppercitrate complex of 1 × 10^{6} cm^{2}/sec. This value is lower than the diffusion coefficient of copper sulfate (5 × 10^{6} cm^{2}/sec) and suggests that Cu^{+2} is probably complexed.
Figure 7  Variation of limiting current density for Cu^{+2} with rotation speed for a copper concentration of 0.025M.
Figure 8  Variation of limiting current density with concentration of Cu^{+2} for a rotation speed of 900 rpm.
Figure 9 shows dimensionless partial currents for nickel and for copper along the cathode as determined from the data of Fig. 6. The copper current is constant and equal to its masstransferlimited value over the entire length of the cathode, while the nickel current is a function of the position along the cathode. The total current is obtained by summing the measured current for nickel and the limiting current for copper. The partial current for hydrogen was neglected. The current for nickel was calculated assuming Tafel kinetics because the nickel ion concentration in the electrolyte was high, and therefore, masstransport effects could be neglected. The simulated curve is obtained by summing the calculated current for nickel and the limiting current for copper. For nickel deposition, a conductivity of 0.04 S/cm and a Tafel slope of 0.13 V/decade (β_{c} = 57 mV) were assumed in the calculations. These values were measured in independent experiments. For L = 6 cm (cathode height) and i_{avg} = 21 mA/cm^{2}, the corresponding Wagner number is 0.02. Therefore, the partial current distribution for nickel is almost primary. As a result of the small copper concentration in the electrolyte, the assumption that copper, the more noble metal, deposits under masstransport control is justified. The results of Fig. 9 show excellent agreement, confirming the assumptions made.
Figure 9  Measured partial currents for copper and nickel deposition along the cathode for an average current density of 25 mA/cm^{2}. Also shown is the total current density calculated with the measurements (points) and by numerical simulation (line). Copper content, 0.025M; rotation speed, 600 rpm.
The model involving a more noble metal (copper), depositing at its limiting current density, and a less noble metal (nickel), depositing following Tafel kinetics is schematically presented in Fig. 10. In such a case, the alloy composition depends on both the applied current density and agitation of the bath. For the RCH cell, the variation of alloy composition with position can be theoretically predicted for a given average current density and rotation rate. Because of unknown masstransport conditions, this is not possible for the classical Hull cell. In the present study, it has been assumed that codeposition proceeds independently for the two metals and that the partial current densities can simply be added. In many alloy plating processes, for example, those involving anomalous codeposition, this assumption is not valid. The RCH cell should nevertheless be useful for the study of such processes because of its controlled masstransport conditions.
Figure 10  Variation of the partial currents for copper and nickel deposition with potential (schematic).
Summary and conclusions
A new cell design, the RCH cell, has been described and tested, which can advantageously replace the classical Hull cell for electrodeposition applications involving one or several masstransportcontrolled reactions. The RCH cell consists of a rotating cylinder cathode and a counterelectrode placed asymmetrically. Using numerical simulation, the RCH cell geometry has been optimized in such a way that the primary current distribution corresponds closely to that of the conventional Hull cell. The use of a rotating cylinder cathode gives uniform and wellcontrolled masstransport conditions. Owing to its simple design, the RCH cell can be built easily in the laboratory and is relatively inexpensive. The deposition of a coppernickel alloy from a citrate bath has been used to demonstrate the importance of mass transport for the reaction distribution on the cathode. For this type of application, the RCH cell is clearly superior to the classical Hull cell.
Acknowledgment
This work was financially supported by OPEN, Bern, and by Fonds National Suisse, Bern, Switzerland. The authors thank M. Matlosz for helpful discussions and R. Dabbous for experimental assistance.
References
1. R.O. Hull, AES Monthly Review, 26, 753767 (Oct. 1939).
2. W. Hohse, Die Untersuchung galvanischer Bader in der Hull Zelle, 3rd ed., Eugen G. Leuze Verlag, Saulgau/ Wurtt, Germany, 1984.
3. A.K. Graham and H.L Pinkerton, "A Cell for HighSpeed Plating Evaluations," Tech. Proc. Am. Electroplaters Soc., 50, 135138 (1963).
4. I. Kadija, J.A. Abys, V. Chinchankar and H.K. Straschil, Plating and Surf. Fin., 78, 6067 (July 1991).
5. PoYen Lu, Plating and Surf. Fin., 78, 6265 (Oct. 1991).
6. Norm 50957 DIN (Deutsches Institut fur Normung), GalvanisierungsprufungmitderHullZelle, TAB 175, Beuth Verlag GmbH, Berlin, Koln (1983).
7. C. Madore, A.C. West, M. Matlosz and D. Landolt, Electrochemica Acta, 37, 6974 (1992).
8. C.A. Brebbia, J.C.F. Telles and LC. Wrobel, Boundary Element Techniques, Theory and Applications in Engineering, SpringerVerlag, Berlin, 1984.
9. M. Matlosz, C. Creton, C. Clerc and D. Landolt, J. Electrochem. Soc., 134, 30153021 (1987).
10. C. Madore, A.C. West, M. Matlosz and D. Landolt, J. Appl. Electrochem., 22, 11551160 (1992).
11. D.P. Barkey, R.H. Mullerand C.W. Tobias, J. Electrochem. Soc., 136, 21992206 (1989).
12. M. Eisenberg, C.W. Tobias and C.R. Wilke, J. Electrochem. Soc., 101, 306320 (1954).
About the authors (1993)
At the time of publication in 1993, Dr. Charles Madore was a Doctoral student at the Ecole Polytechnique Federal de Lausanne, in the Laboratoire de Metallurgie Chimique, MXC Ecublens, CH1015 Lausanne, Switzerland. There, he worked on current distribution in alloy electroplating. He holds a B.Sc. in physics and an M.Sc.A. in engineering physics from the Ecole Polytechnique de Montréal.
Dr. Dieter Landolt was Professor of Materials Science and Engineering at the Swiss Federal Institute of Technology in Lausanne (EPFL) where he headed the Laboratory of Metallurgical Chemistry (Laboratoire de métallurgie chimique) until his retirement in 2003. He is Professor emeritus (professeur honoraire) at EPFL. His research focused on electrochemical metal dissolution and deposition processes, as well as surface chemistry and corrosion. He has been the recipient of many honors and awards, including the AESF Scientific Achievement Award in 2000.
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