48. Study Of Magnetic Field Effects On Copper Electrodeposition...961

RSCE-SOMCHE 2008 Edited by Daud et al.

961

STUDY OF MAGNETIC FIELD EFFECTS ON COPPER ELECTRODEPOSITION

Sudibyo, M.B. How, N. I. Basir and N. Aziz* School of Chemical Engineering, Universiti Sains Malaysia 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia *Corresponding author, Tel: +6(04)5996457 Fax: +6(04) 5941013 Email: [email protected]

Keywords: copper deposition, fractal, magnetic field, magnetoelectrodeposition, electrodeposition

ABSTRACT Magnetoelectrodeposition (MED) plays a vital role in electrodeposition process to synthesize metal alloy, thin film, multilayer, nanowires, multilayer nanowires, dot array and nanocontacts which are the technology of the future to build the next generation of computing devices. As this technology is not widely being investigated, this work is to focus on the fundamental study of the magnetic field effects on electrodeposition. Copper were chosen as the makeup material for the anode and cathode for the electrodeposition unit. The effects of magnetic fields on copper electrodeposition are investigated in terms of variations in the magnetic field strength, the voltage potential, the electrolyte concentration and the magnetic field alignment. The experiments are conducted in a simple electrodeposition unit consisting of a central cathode and a circular anode ring. The magnetic field is introduced externally. Based on the experimental results, the mere presence of magnetic field would results in a compact deposit. As the magnetic field strength is increased, the deposit grows denser. The increment in electrical potential also leads to the increase the deposited size. Different compact deposited metal structures are observed when there are variations in the magnetic field alignments. INTRODUCTION The control of surface microstructure of transition metal thin film has both scientific and technological importance. Electrodeposition is one of convenient techniques that can control the surface morphology and the crystal orientation of thin metal films. Electrodeposition is used to improve contact resistance, reflection properties of material and to impart friction properties. It is also used to

962 impart corrosion resistance or particular desired physical or mechanical properties on the surface metal. Obtaining a uniform, dense and compact deposition is one of the major problems in electrodeposition. There are numerous studies that had been carried out to reduce it. One of methods available to overcome this problem is magnetoelectrodeposition (MED) (Matsushima et al., 2006). MED plays a vital role in electrodeposition process to synthesize metal alloy, thin film, multilayer, nanowires, multilayer nanowires, dot array and nanocontacts which are the technology of the future to build the next generation of computing devices. For this MED technology, the parameters that can be studied are somehow similar to conventional electrodeposition. This is due to the fact that magnetoelectrolytic deposition itself is actually based on electroplating technology. Thus, similar parameters such as plating bath concentration, bath temperature, pH of the bath and electrodepositing current can be studied meticulously. In addition to that, the effects of variations in field strength in magneto-electrolytic deposition technology also can be observed. It had being found that different strength of magnetic field will effect the electrodeposited metal to a certain extend (Mogi & Kamiko, 1996; Mhiochain et al., 2004). Moreover, in the MED technology, the alignment of the magnetic field is also a crucial factor in determining the growth pattern of the deposited metal. A magnetic field exposed at a certain degree to the plating site may induce a certain special pattern in the deposited metallic and it is interesting to observe the growth as it may provide a promising alternatives in improving the MED (Coey & Hinds, 2001). In this work, the objectives are to study the effect of magnetic strength, the voltage supplied and the alignment of the magnetic field towards growth fractal electrodeposits. EXPERIMENTAL PROCEDURE The copper fractal electrodeposits were grown in a flat circular cell (see Figure 1 for a schematic representation) with a copper wire cathode (1 mm in diameter) at the center and a copper ring anode (thickness 0.5 mm, outer diameter 70 mm and inner diameter 50 mm). Copper sulfate solutions were made up using CuSO4.5H2O in 0.5 M H2SO4. The depth of the aqueous film can be varied by regulating the total volume of the CuSO4 solution. The copper was electrodeposited with a series of voltage; from 4 volt to 6 volt and finally to 8 volt. A Ferrite and neodymium permanent magnets were used to provide a weak (18 gauss) and strong (31 gauss) magnetic field, respectively. In the experiment with variations in orientation of magnetic field effect, the magnet used was aligned at a series of degree at each turn of experiment. The degrees are 0, 45, 90, 135 and 180. The resulting fractal patterns were photographed using a digital camera and than analysed using matlab image processing. A mass microbalance was used to measure the mass of fractal electrodeposits.

963 RESULTS AND DISCUSSION Presence of Magnetic Field and the Influences of its Strength When electrodeposition is carried out conventionally without the presence of any magnetic field, it is observed that the deposited copper formed a diffusion limited aggregation (DLA)-like structure (Figure 2a). The pattern become denser and compact as magnetic field is exposed to the unit. From the DLA-like structure in Figure 2a, the aggregate pattern changed to dendritic–like (Figure 2b) and to compact dendrite (Figure 2c) as the magnetic field strength increases. RECTIFIER

MULTIMETER

EXTERNAL A FLAT CIRCULAR CELL

Figure 1: Schematic diagram for the magnetoelectrodeposition system

(b)

(c)

( )

Figure 2: Copper electrodeposits (applied voltage 6 V, CuSO4 0.2 M, time duration 20 minutes) : (a) without magnet, ( b) in weak magnetic field (ferrite magnet), (c) in strong magnetic field (neodium magnet) Meanwhile, under the influence of magnetic field, the clusters or aggregates are observed to be much denser and more compact due to the presence of magnetohydrodynamic effect (Figure 2b and Figure 2c). This magneto-hydrodynamic (MHD) effect is actually generated by the magnetic force and as the magnetic field strength increases, the strength of the MHD effect increases as well. Consequently, the MHD effect is much stronger in the case of Figure 2c than Figure 2b (Mogi & Kamiko, 1996). MHD effect acts by reducing the thickness of the Nernst diffusion layer. When the thickness is reduced, the screening effect within the layer is reduced as well (Mogi et al., 1995). Subsequently, more copper ions are able to discharge at a given point in the deposition area. Thus, the overall aggregate will become more compact. Besides that, before the branches starts to elongate, the random walker or free ion will also tend to discharge at a higher rate within the fjord area. Consequently, it will cause the aggregate to grow at a more uniform pattern as shown in Figure 2b and Figure 2c.

964 The MHD effect also will cause a turbulent flow near the deposition area which will enhance the ionic mass transfer. As more charged ions are present within the vicinity of the deposition area, the cluster or aggregate is somehow more uniform in shape as the turbulent flow is able to provide sufficient metal ion to each branch during the growing process. The additional metal ion introduced by the turbulent flow will also encourage more side branches to develop beside the main branch (Hinds et al., 2001). Effect of Variation in Magnitude of Electrical Potential on Magnetic Electrodeposition (b)

(a)

(c)

Figure 3: Copper electrodeposits (neodium magnet, CuSO4 0.2 M, time duration 20 minutes): (a) applied voltage 4 V, (b) applied voltage 6 V, (c) applied voltage 8V Mas s de posite d vs . Voltage

Mass Deposited (gr)

0.05 0.04 0.03 0.02 0.01 0 4

6 Voltage (V)

8

Figure 4: Mass copper electrodeposits in variation an applied voltage Based on the results obtained from the experiment, the size of the copper agregates increase proportionately to the magnitude of the electrical potential. From the tiny dendritic structure in Figure 3a, the aggregate structure increases in its size to a larger dendritic structure (Figure 3b) as the magnitude of the electrical potential increases. When the electrical potential supplied reaches 8V, the dendritic structure had already became a large, dense and compact dendrite (Figure 3c). Besides, the mass of the copper deposited is also found to be increasing along with the increment in electrical potential supplied. This can been seen in the Figure 4. The increase in the size of the aggregate and the mass deposited is contributed by the increment in the electrical potential supplied. When the electrical potential is increased, it will induce a higher charge at the electrode. Thus, the anode and

965 cathode will become more positively and negatively charged respectively. A highly charged anode will oxidize at a quicker rate, thus producing more free ions (anion). On the other hand, a highly charged cathode will attract more cations from the electrolyte to diffuse towards it. Hence, this will also increase the driving force of cations towards the cathode. At the cathode, these influxes of cations will be reduced to copper with the electron supplied by the electrical potential. Consequently, more copper will be deposited and the deposited aggregate will also increase in its size and grows at a faster pace. Effect of Magnetic Field Placement on Magnetic Electrodeposition

(a)

(d)

(b)

(c) (e)

Figure 5 : Copper electrodeposits (applied voltage 6 V, time duration 20 minutes, CuSO4 0.2 M, neodium magnet): the magnet placed (a) at 0o from vertical plane, (b) at 45o from vertical plane, (c) at 90o from vertical plane, (d) at 135o from vertical plane, (e) at 180o from vertical plane. From the results obtained, the aggregate pattern is the most compact when the magnet is placed at 0o from vertical plane and as the magnet is tilted; the aggregate becomes more random branched. The branching is most apparent when the magnet is tilted to 90o from vertical plane as seen from Figure 5b. However, as the magnet is tilted ever further from the horizontal plane, again we notice the pattern become less branching. The aggregate also returns to its original pattern which is denser and compact (Figure 5e). At this point, the magnet is at 180o from vertical plane. The aggregate is most compact and dense when the magnet is placed at 0o from vertical plane as the magnetic force is parallel to the cathode surface. Consequently, the magnetic field is oriented directly perpendicular to the direction of the ion fluxes at the deposition site. When the external magnetic force is directly perpendicular to the direction of the ion flux, the magnetic force is the strongest (Nikolic et al., 2004, Bund et al., 2003). Thus, the MHD effect is also the strongest at this placement CONCLUSIONS The effect of the magnetic field on copper electrodeposition has been studied using a flat electrochemical cell. It is found that the growth of fractal

966 electrodeposits or agregates was strongly influenced by applied magnetic field. Under the influence of magnetic field, the fractal electrodeposits were observed to be much denser and more compact due to the presence of magnetohydrodynamic (MHD) effect. The value of the magnetic strength, applied voltage and the allignmnet of the magnetic field also found to be significantly effect on the growth pattern of electrodeposits. The most compact and dense electrodeposits achieved when the strongest magnetic field and the highest voltage (8V) were applied with the magnet placed at 0° from vertical plane. ACKNOWLEDGMENT Financial supports from Ministry of Higher Education Malaysia through FRGS grant No. 607113 is greatly acknowledged. REFERENCES Bund, A., Koehler, S., Kuehnlein, H.H., Plieth, W. (2003). Magnetic field effects in electrochemical reactions. Electrochimica Acta 49, pp 147-152. Coey, J. M. D., Hinds, G. (2001). Magnetic electrodeposition. Journal of Alloys and Compounds 326: 238 – 245. Hinds, G., Spada, F. E., Coey, J. M. D., Ni Mhiocha´in, T. R., Lyons, M. E. G. (2001) Magnetic field effects on copper Electrolysis. J. Phys. Chem B 105: 9847-9502. Matsushima, J.T. , Trivinho-Strixino, F., Pereira, E.C. (2006). Investigation of cobalt deposition using the electrochemical quartz crystal microbalance. Electrochimica Acta 51: 1960–1966. Mhiochain, T.R., Hinds, G., Martin, A., Chang, E., Lai, A., Costiner, L., Coey, J.M.D., (2004). Influence of magnetic field and gravity on the morphology of Zinc fractal electrodeposits. Electrochimica Acta 49: 4813-4828. Mogi, I., Kamiko, M., (1996). Striking effects of magnetic field on the growth morphology of electrochemical deposits. Journal of Crystal Growth 166: 276280. Mogi, I., Kamiko, M., (1996). Striking effects of magnetic field on the growth morphology of electrochemical deposits. Journal of Crystal Growth 166, pp 276-280. Mogi, I., M. Kamiko, S. Okubo, (1995). Magnetic field effects on fractal morphology in electrochemical deposition, Physica B 211 pp 319-322. Nikolic, N.D., Wang, H., Cheng, H., Guerrero, C.A., Garcia, N. (2004). Influence of the magnetic field and magnetoresistance on the electrodeposition of Ni nanocontacts in thin films and microwires. Journal of Magnetism and Magnetic Materials 272-276: 2436-2438.