Magnetic Field Influence On Limiting Current Of Cobalt

ISSN 1978-5933 The Second International Conference On Green Technology and Engineering (ICGTE) 2009 Faculty of Engineering Malahayati University

Magnetic Field Influence on Limiting Current of Tin Electrodeposition Sudibyo1, A. B. Ismail2, M. H. Uzir1, M. N. Idris2 and N. Aziz1* 1

School of Chemical Engineering, Universiti Sains Malaysia School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Engineering campus, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia 2

Abstract Since microelectronics has developed to one of the most important branches of industry, the manufacture of thin metal layers is a very important process. Electrodeposition can be an alternative process to produce these layers, which is often more productive and cheaper. However, the problem of obtaining a uniform, dense and compact deposition had plague researchers, thus various methods have been devised to address this problem. One of the methods of tackling this problem is magneto electrodeposition (MED). As this technology is not widely being investigated, this work is to focus on the limiting current under magnetic field effects (MFE) on tin electrodeposition. Measuring the limiting current using linear sweep voltametry is important in order to know the influence of MFE on the mass transport phenomena of electrodeposition process. The effects of magnetic fields on tin electrodeposition are investigated in terms of variations in the magnetic field strength, the electrode area, the concentration of the electro active species, the diffusion coefficient of the electroactive species and the kinematic viscosity of the electrolyte. The effect of a uniform magnetic field with flux density up to 0.3 T on the electrodeposition of tin from sulphate electrolyte has been investigated. Results achieved show that, when the magnetic field is applied parallel to the electrode surface, the limiting current density is increased due to the magnetohydrodynamic effect. As the magnetic field strength is increased, the limiting current increases significantly. The increment in the working electrode area, the bulk concentration and diffusion coefficient of electro active species also leads to the increase the limiting current. Different limiting currents are observed when there are variations in the kinematic viscosity of the electrolytes. Significant influence on the limiting current was observed when the kinematic viscosity of the electrolytes was varied. Keywords: Limiting current; Tin electrodeposition; magnetic field; magnetoelectrodeposition; coefficient diffusion; kinematic viscosity 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 [1]. It is also used to 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) [2]. 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. MED is an electrodeposition phenomena occurring under the influence of a magnetic field or the formation of a substance layer on an appropriate substrate in externally imposed magnetic fields [3].

It is now well established that the currents observed in electrochemical processes are modified in MED process [4]. This effect, called “magnetohydrodynamic effect” (MHD), is generally explained by the appearance of a Lorentz force. It leads to a convective movement of the species to the electrode surface, and for the electrochemical systems limited by the mass transfer; it induces an increase of the electrolytic currents [4]. Limiting current density is the maximum current density that can be achieved for an electrode reaction at a given concentration of the reactant in the presence of a large excess of supporting electrolyte. The mass transport occurs exclusively through diffusion in the diffusion layer, driven by the concentration difference of the reactant between the edge of the diffusion layer and the electrode surface [5]. For this MED technology, many researchers have used the fundamental hydrodynamic equations as a guide to the system parameters that should control the mass transport limited current. According to fundamental hydrodynamic equation these parameters should include the magnetic field strength, B, the number of electrons of the redox process, n, the electrode area, A, the diffusion

pdfMachine Is a pdf writer that produces quality PDF files with ease! Produce quality PDF files in seconds and preserve the integrity of your original documents. Compatible across nearly all Windows platforms, if you can print from a windows application you can use pdfMachine. 1 Get yours now!

coefficient of the electroactive species, D, the concentration of the electroactive species, C, and the kinematic viscosity of the electrolyte, v [6]. Thus, those parameters can be studied meticulously. In this work, the study on the effect of magnetic strength, the electrode area, the concentration of the electro active species, the diffusion coefficient of the electro active species, and the kinematic viscosity of the electrolyte towards the limiting current of tin electrodeposition will be carried out. Experimental 1. Apparatus The magnetic field was generated by an electromagnet (Lake Shore EM 4, USA). The pole pieces were of 50 mm diameter and 50 mm apart. The induction was uniform and equal to 0.3 T in the magnet gap. The coils temperature was controlled by a water flow. Chronoamperometric, and linear sweep voltammetric studies were performed using a PGP 201 potentiostat monitored by the Voltamaster 4.0 software (Radiometer analytical S.A., France). The absolute viscosity of each electrolyte was measured using a viscosimeter (Model DV-III, Brookfield programable rheometer). 2. Cell and electrodes The narrow gap between the pole pieces required the design of a special three electrodes cell with 45 mm inner diameter as shown in Fig. 1. Fourth platinum plates with areas of 0.32285, 0.5, 0.57485, and 0.95285 cm2 were used as working electrodes (WE). The counter electrode (CE) was a platinum wire (0.95285 cm2 area) and the reference electrode (RE) was an Ag/AgCl electrode.

mV/s and were plotted in a Tafel plot. The effect of a magnetic field on the diffusion was investigated

Fig. 1. Schematic illustration of electrochemical cell Result and Discussion 1. Effect of Bulk Concentration of Electro active Species

3. Reagents Tin (II) Sulphate (≥99%), Sodium sulphate (≥99%) and Sulphuric acid (≥99%) were purchased from R & M (Malaysia). All solutions were diluted with distilled water. For each experiment, 40 ml electrolyte solutions were used. 4. Procedure The cell containing 40 cm3 of the solution was placed in the field cavity for the experiments performed under the influence of the magnetic field. The cell was placed in the field cavity so that the working electrode surface faced downward and was parallel to the lines of the magnetic flux that run horizontally. The magnetic field was then applied perpendicular to the electric field (Fig. 1). The measurements were carried out at 25-27 ◦C. Linear sweep voltammetry (LSV) technique was used to investigate the effect of magnetic field, working electrode area, additive material and electrolyte concentration on the limiting current density. Linear sweep voltammetry (LSV) were performed from +2 to -2 V at a sweep rate of 10

Fig. 2 Variation of the limiting current iB under magnetic field as a function of the SnSO4 concentration C. 0.5 cm2 working electrode area. T = 25-27◦C. B = 0.3 T. [H2SO4] = 0.5 M using chronoamperometry. The limiting current was recorded at slow sweep rates (10 mV/s) for every compound at concentrations in the range from 0.025 to 0.1 M in H2SO4 0.5 (M). The magnetic field was kept constant at 0.3 T. From Fig. 2, it is found that the limiting current increases as the concentration of SnSO4 increases. The increment of limiting current indicated the increasing mass transport on electrodeposition process. This increment of limiting current can be linked to the increasing concentration of the SnSO4 in the electrolyte. As the concentration increases, the free cations (Sn4+) available in the electrolyte will

pdfMachine Is a pdf writer that produces quality PDF files with ease! 2 quality PDF files in seconds and preserve the integrity of your original documents. Compatible across Produce nearly all Windows platforms, if you can print from a windows application you can use pdfMachine. Get yours now!

ISSN 1978-5933 The Second International Conference On Green Technology and Engineering (ICGTE) 2009 Faculty of Engineering Malahayati University also increase. These higher concentrations cause more efficient stirring. It will also induce a turbulent flow within the electrolyte, which this will increase the flux of the species [6]. As a result, the thickness of the Nernst diffusion layer will gradually be reduced and this will decrease the screening effect. As the Nernst diffusion layer decrease, the limiting current density will also increase [7, 8]. The MHD effect caused by the magnetic field will also decrease the screening effect at the deposition site [9]. This MHD effect also will create mixing in the diffusion area and reduce the thickness of its Nernst diffusion layer in front of the electrode effectively. As the Nernst diffusion layer decrease, the limiting current density will increase [7]. This will increase the deposition rate. 2. Effect of working electrode area (A)

Fig. 3. Variation of the limiting current iB under magnetic field as a function of the working electrode area, A. T = 25-27◦C. B = 0.3 T. [H2SO4] = 0.5 M and [SnSO4] = 0.1 M. The effect of the electrode area (A) on limiting current under magnetic field (iB), was determined with three different electrodes in in H2SO4 0.5 (M) under a constant magnetic field of 0.3 T. From Figure 3, it is found that the limiting current increases as the working electrode area increases. The increment of limiting current is due to increment of the current which subsequently increase the electrode reaction [5]. A larger area also will cause more effective magnetic stirring in diffusion area. This reduces the thickness of its Nernst diffusion layer. This will cause the limiting current and deposition rate increase [6]. 3. Effect of diffusion coefficients of Electro active Species (D) and kinematic viscosity of electrolyte (v) The influence of the diffusion coefficient (D), was determined in SnSO4 0.01 M solutions using Na2SO4 as supporting electrolytes. The electrolyte concentration was varied from 10 mM to saturated

solutions. Limiting currents were measured using the 0.5 cm2 platinum plate as working electrode. The effect of a magnetic field on the diffusion of Sn4+ were investigated using chronoamperometry. A potential step is a cathodic potential in the diffusion

Fig. 4. The current response at a Pt working electrode in SnSO4 0.01 M, Na2SO4 0.075 M, B = 0.3 T, a cathodic potential step of 700 mV.

Fig. 5. The Cottrell plot at a Pt working electrode in SnSO4 0.01 M, Na2SO4 0.075 M, B = 0.3 T, a cathodic potential step of 700 mV. -limited current plateau.It was applied to take the working electrode from the rest potential, where no faradaic reaction occurs, to a final value where all electroactive species that reach the electrode are instantaneously reduced. This corresponds to a cathodic potential in the diffusion- limited current plateau of tafel plot, where the electrode kinetics are significantly faster than the rate of mass transport. In quiescent solution, the rate of reaction, and hence the measured current response, is solely determined by the rate of diffusion. For a reduction reaction on the electrodeposition system, such as the electrodeposition of tin, the current I, is given by the Cottrell equation: I (t ) 

nFAD1/ 2 c ( t )1/ 2

(1)

pdfMachine Is a pdf writer that produces quality PDF files with ease! 3 Produce quality PDF files in seconds and preserve the integrity of your original documents. Compatible across nearly all Windows platforms, if you can print from a windows application you can use pdfMachine. Get yours now!

Where A is the electrode area. Equation 1 shows that the diffusion-limited current decreases with t-1/2. This is due to the decrease in concentration gradient with time as the diffusion layer thickness grows. However, natural convection arising from density differences in the solution, eventually perturbs the concentration gradient and prevents further growth of the diffusion layer. This results in a steady-state current at long times. A plot of I against t-1/2 is known as a Cottrell plot. Under conditions of semiinfinite linear diffusion, such a plot will be linear, enabling the determination of the combination nAD1/2 from equation 1[8]. This part of the graph can be extrapolated to the origin, thereby demonstrating the expected behavior, and the slope yields a diffusion coefficient. One example of the choroamperometry graph and the Cottrell plot for this experiment is shown in Fig. 4 and Fig. 5, respectively. Table 1. Variation of the limiting current iB on the effect of diffusion coefficient (D) and Kinematic viscosity (v) of the electroactive species. SnSO4 Na2SO4 v Ib D (M) (M) (Stoke) (mA) (cm2/s) 0.01 0.075 0.0178 1.3544 0.000001 0.01 0.1 0.0179 1.3224 0.00000098 0.01 0.15 0.0184 1.2585 0.00000095 0.01 0.2 0.0188 1.2273 0.00000094 0.01 0.3 0.0192 1.1715 0.00000091

It seems that the decrease of the mass-transportlimited current is due to the friction forces becoming more effective as the electrolyte viscosity increases, preventing magnetohydrodynamic convection of the solution [6].

Fig. 7. Variation of the limiting current iB under magnetic field as a function of the diffusion coefficient (D). SnSO4 0.01 M, 0.5 cm2 working electrode area, T = 25-27◦C. B = 0.3 T. 4.

Effect of Magnetic strength (B)

Fig. 8. Variation of the limiting current iB under magnetic field as a function of the Magnetic strength (B). SnSO4 0.01 M, 0.5 cm2 working electrode area, T = 25-27◦C. B = 0.3 T.

Fig. 6. Variation of the limiting current iB under magnetic field as a function of the kinematic viscosity (v). SnSO4 0.01 M, 0.5 cm2 working electrode area, T = 25-27◦C. B = 0.3 T. From Table 1, it is found that the limiting current decreases as the concentration of Na2SO4 increases. This decrease of limiting current can be linked to the increment kinematic viscosity of the Na2SO4 in the electrolyte as shown in Fig. 6. The variation of the electrolyte kinematic viscosity leads to a variation of the electroactive species diffusion coefficient [4]. The increasing of the electroactive species diffusion coefficient leads to the increment of limiting current as shown in Fig. 7.

The influence of the magnetic strength was determined in 0.01 M SnSO4 solutions. Limiting currents were measured using the 0.5 cm2 platinum plates as working electrode and Na2SO4 1 M as supporting electrolyte. The change in the magnetic strength leads to the variation of the limiting current, iB, but the value of D (diffusion coefficient) is unchanged by the magnetic field [6, 8]. Fig. 8 showed that the limiting current increases as the magnetic strength (B) increases. The increment of limiting current indicated the increasing of mass transport on electrodeposition process. This increment of limiting current can be linked to the presence of magnetohydrodynamic effect. When the MHD effect is present, the convective flow will create mixing in the diffusion area and reduce the thickness of its Nernst diffusion layer in

pdfMachine Is a pdf writer that produces quality PDF files with ease! 4 quality PDF files in seconds and preserve the integrity of your original documents. Compatible across Produce nearly all Windows platforms, if you can print from a windows application you can use pdfMachine. Get yours now!

ISSN 1978-5933 The Second International Conference On Green Technology and Engineering (ICGTE) 2009 Faculty of Engineering Malahayati University front of the electrode effectively. As the Nernst diffusion layer decrease, the limiting current density will increase [7]. possible force which could be responsible for the enhancement of mass transfer is known as the Lorenz force [2, 10]. The Lorenz force acts on migration of charged ions inside the electrolyte and induce a convective flow of electrolyte close to the electrode surface. This convective effect on electrodeposition process is known as MHD effect [11]. Conclusions The effect of the magnetic field on limiting current of tin electrodeposition has been studied using a three electrode electrochemical cell. It is found that the limiting current of tin electrodeposition strongly influenced by applied magnetic field. Under the influence of magnetic field, the limiting currents were observed to be higher due to the presence of MHD effect. The value of the magnetic strength, working electrode area, the bulk concentration, diffusion coefficient of electroactive species and the kinematic viscosity of the electrolyte also found to be significantly effect on the mass transport of electrodeposition. The higher mass transport of electrodeposition can be achieved when the lowest of kinematic viscosity of electroactive species and the highest magnetic field strength, working electrode area and concentration of electroactive species were applied with the magnet field placed horizontally. The increment kinematic viscosity leads to decrease of limiting current. This decrease of limiting current can be linked to the decrease of the diffusion coefficient of electroactive species. As the kinematic viscosity is increased, the the diffusion coefficient of electroactive species decreases significantly.

This phenomenon could happen because the magnetic field could increase the rate of transport of electroactive species to or from the electrode. The Acknowledgment Financial supports from Ministry of Higher Education Malaysia through FRGS grant No. 607113 is greatly acknowledged. References: [1] Sudibyo, M.B. How, N. I. Basir and N. Aziz, Study of magnetic field effects on copper electrodeposition. RSCE – SOMCHE 2008, Kuala Lumpur, Malaysia, pp. 961 - 964 2nd -3rd December 2008. [2] Matsushima, J.T., Trivinho-Strixino, F., Pereira, E.C., Electrochimica Acta. 51 (2006) 1960– 1966. [3] Ackland, G.J. And Tweedie, E.S., Microscopic model of diffusion limited aggregation and electrodeposition in the presence of levelling molecules. School of Physics, the University of Edinburgh, Scotland, United Kingdom, 2007. [4] Legeai, S., Chatelut, M., Vittori, O., Chopart, J.P., Aaboubi, O., Electrochimica Acta. 50 (2004) 51-57. [5] James, A.M, Electrochemistry Dictionary. John Wiley & Sons, Ltd, 1984. [6] Leventis, N., M. Chen, X. Gao, M. Canalas, and P. Zhang, Journal of Physical Chemistry B, 102 (1998) 3512-3522. [7] Fahidy, T.Z., Progress in Surface Science 68 (2001)155 -158. [8] Hinds, G., Spada, F. E., Coey, J. M. D., Ni Mhiocha´in, T. R., Lyons, M. E. G., Journal of Physical Chemistry B. 105(2001) 9847-9502. [9] Mogi, I., Kamiko, M., Journal of Crystal Growth. 166 (1996) 276-280 [10] Bund, A., Koehler, S., Kuehnlein, H.H., Plieth, W., Electrochimica Acta. 49 (2003) 147-152. [11] Coey, J. M. D., Hinds, G., Journal of Alloys and Compounds. 326(2001) 238 – 245

pdfMachine Is a pdf writer that produces quality PDF files with ease! 5 Produce quality PDF files in seconds and preserve the integrity of your original documents. Compatible across nearly all Windows platforms, if you can print from a windows application you can use pdfMachine. Get yours now!

ISSN 1978-5933 The Second International Conference On Green Technology and Engineering (ICGTE) 2009 Faculty of Engineering Malahayati University

Study of Magnetic Field Effect in Cobalt Electrodeposition Sudibyo1, A. B. Ismail2, M. H. Uzir1, M. N. Idris2 and N. Aziz1* 1

School of Chemical Engineering, Universiti Sains Malaysia School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Engineering campus, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia 2

Abstract Electrodeposition is used to reflect material properties and to improve contact resistance and friction properties. It is also used to improve corrosion resistance or particular desired physical or mechanical properties on the metal surface. Roughening is the problem in electrochemical deposition. There are numerous studies that had been carried out to reduce it. One of the methods of tackling this problem is magneto electrodeposition (MED). Cobalt electrodeposition is an important technology to produce magnetic alloys, cutting-wear resistant alloys and super alloys. As the cobalt electrodeposition is very useful for technical applications, this has been choosen as a case of study. The effects of the magnetic field strength, the electrode area, the concentration of the electro active species, the diffusion coefficient of the electroactive species, the kinematic viscosity of the electrolyte and the additive concentration on cobalt electrodeposition have been carried out. The electrodeposition of Cobalt in CoSO4 solutions were studied on platinum electrodes in a three electrode electrochemical cell. The magnetic fields were varied from 0 to 0.3 Tesla which has been introduced externally. Based on the experimental results, the presence of magnetic field would increase limiting current. As the magnetic field strength is increased, the limiting current increases significantly. The increment in the working electrode area, the bulk concentration and diffusion coefficient of electro active species also leads to the increase of the limiting current. It also observed that the presence of boric acid as an additive was lead to the smaller amount of limiting current the decrease of limiting current. However, it is acting as a buffer to avoid the local pH rise caused by parallel hydrogen evolution reaction (HER). Keywords: Limiting Current; Cobalt electrodeposition; magnetoelectrodeposition; coefficient diffusion; kinematic viscosity; Boric Acid additive Introduction Cobalt is an element with excellent ferromagnetic properties. Besides, it is relatively stable against corrosion and easily to handle, which makes it even more useful for technical applications [1]. Since microelectronics has developed to one of the most important branches of industry, the manufacture of thin metal layers, nanowires, dot array and nanocontacts are a very important process [2]. These products are mostly in a nanoscale thickness range. Electrodeposition can be an alternative process, which is often more productive and cheaper [1, 2]. Roughening is the problem in electrochemical deposition. There are numerous studies that had been carried out to reduce it. One of the methods of tackling this problem is magneto electrodeposition (MED) [3]. MED is an electrodeposition phenomena occurring under the influence of a magnetic field or the formation of a substance layer on an appropriate substrate in externally imposed magnetic fields [4]. The MFE have significant influence on electrodeposition. The most striking effects on magneto electrodeposition are the impressive

increase in the limiting current and a dramatic change in morphology of the fractal electrodeposits [3, 5]. Both phenomena could happen because the magnetic field could increase the rate of transport of electroactive species to or from the electrode [3, 6]. This effect, called magnetohydrodynamic (MHD) effect, is generally explained by the appearance of a Lorentz force. It leads to a convective movement of the species to the electrode surface, and for the electrochemical systems limited by the mass transfer; it induces an increase of the electrolytic currents [7]. Limiting current density is the maximum current density that can be achieved for an electrode reaction at a given concentration of the reactant in the presence of a large excess of supporting electrolyte. The mass transport occurs exclusively through diffusion in the diffusion layer, driven by the concentration difference of the reactant between the edge of the diffusion layer and the electrode surface [8]. 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

1

parameters such as the working electrode area, concentration of the electroactive species and the concentration of supporting electrolyte can be studied meticulously [9, 10]. 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 [5, 9]. Moreover, in the electrodeposition technology, the additive concentration is also a crucial factor in determining the electrodeposits with a smooth surface. Boric acid was considered as cobalt systems additive. It had being found that the presence of boric acid in electrolyte will avoid the local pH rise caused by parallel hydrogen evolution reaction (HER). The results showed that the buffer contribution of boric acid is effective in the cobalt electrodeposition at 25 ◦ C [11]. In this work, the effects of supporting and additive electrolyte concentration are studied as a function of the kinematic viscosity and difussion coefficient. The effect of magnetic strength, the electrode area, the concentration of the electro active species, the diffusion coefficient of the electro active species, the kinematic viscosity of the electrolyte, and the additive concentration towards the limiting current of cobalt electrodeposition was also been studied. 1.

B

Fig. 1. Schematic illustration of electrochemical cell sweep rate of 10 mV/s. Diffusion coefficients of electroactive species were determined using chronoamperometry. A potential step of choroamperometry is a cathodic potential at limiting current. Limiting current is then given by the Cottrell equation:

Experimental

Cobalt (II) sulfate heptahydrate (≥99%) was purchased from Merck. Sodium sulphate (≥99%), and Boric Acid (≥99.8%), were purchased from R & M (Malaysia). All solutions were prepared using distilled water. The magnet employed in this study was a superconducting water-cooled electromagnet (Lake Shore EM 4, USA) with the pole pieces were of 5cm diameter and 5 cm apart. The intensity of the magnetic field could be adjusted with the rheostat of the control panel of the power supply from 0 to 0.3 T. All electrochemical experiments were carried out in a three electrodes cell (4.5 cm inner diameter) as shown in Fig. 1, which containing 40 mL of the appropriate electrolytic solution. The cell was placed in the field cavity so that the working electrode surface faced downward and was parallel to the lines of the magnetic flux that run horizontally. The measurements were carried out at 25-27 ◦C in the field cavity. Four platinum wires (0.1 cm diameter) with areas of 0.32285, 0.48035, 0.63785 and 0.95285 cm2 were used as working electrodes (WE). The counter electrode (CE) was a platinum wire (0.95285 cm2 area) and the reference electrode (RE) was an Ag/AgCl electrode. Electrochemical experimentation (chronoamperometry and linear sweep voltameter) were carried out with a PGP 201 potentiostat (Radiometer analytical S.A., France) and were recorded and monitored by Compaq 6510 b Laptop

2

with the Voltamaster 4.0 software. The absolute viscosity of each electrolyte was measured using a viscosity meter (Model DV-III, Brookfield). Linear sweep voltammetry (LSV) technique was used to investigate the effect of magnetic field, working electrode area, additive material and electrolyte concentration on the limiting current density. LSV were performed from +2 to -2 V at a

I (t ) 

nFAD1/ 2 c ( t )1/ 2

(1) Where A is the electrode area. Equation 1 shows that the diffusion-limited current decreases with t-1/2. A plot of I against t-1/2 is known as a Cottrell plot. Under conditions of semi-infinite linear diffusion, such a plot will be linear, enabling the determination of the combination nAD1/2 from equation 1 [12, 13]. This part of the graph can be extrapolated to the origin, thereby demonstrating the expected behavior, and the slope yields a diffusion coefficient. 3.

Result And Discussion

3.1 Presence of magnetic field and the influences of its Strength

ISSN 1978-5933 The Second International Conference On Green Technology and Engineering (ICGTE) 2009 Faculty of Engineering Malahayati University Fig. 2. Variation of the limiting current iB under magnetic field as a function of the Magnetic strength (B). CoSO4 0.01 M, 0.9529 cm2 working electrode, T = 25 - 27◦C. B = 0.3 T. Based on the results obtained from the experiment, the the limiting current increases proportionately to the magnitude of the magnetic strength (B) as shown in Fig. 2. The increment of limiting current indicated the increasing of mass transport on electrodeposition process. This increment of limiting current can be linked to the presence of MHD effect. When the MHD effect is present, the convective flow will create mixing in the diffusion area and reduce the thickness of its Nernst diffusion layer in front of the electrode effectively. As the Nernst diffusion layer decrease, the limiting current density will increase [13]. This phenomenon could happen because the magnetic field could increase the rate of transport of electroactive species to or from the electrode. The possible force which could be responsible for the enhancement of mass transfer is known as the Lorenz force [3, 6]. The Lorenz force acts on migration of charged ions inside the electrolyte and induce a convective flow of electrolyte close to the electrode surface. This convective effect on electrodeposition process is known as MHD effect [14]. The change in the magnetic strength leads to the variation of the limiting current, iB, but the value of D (diffusion coefficient) is unchanged by the magnetic field [7, 10]. 3.2 The influence of bulk concentration of electroactive

varied between 0.001 and 0.05 M using Na2SO4 0.5M as supporting electrolyte at a constant magnetic field (0.3T). Limiting currents were measured using a platinum wire (0.9529 cm2) as working electrode. Linear sweep voltammetry (LSV) were performed from +2 to -2 V at slow sweep rates (10 mV/s) for every compound. The limiting current values of Fig. 3 show that at higher concentrations of the electroactive species magnetic stirring is more effective. This increment of limiting current can be linked to the increasing concentration of the CoSO4 in the electrolyte. As the concentration increases, the free cations (Co2+) available in the electrolyte will also increase. These higher concentrations cause more efficient stirring. It will also induce a turbulent flow within the electrolyte, which this will increase the flux of the species [10]. As a result, the thickness of the Nernst diffusion layer will gradually be reduced and this will decrease the screening effect. As the Nernst diffusion layer decrease, the limiting current density will also increase [12, 13]. In other words, typical data are shown in Fig. 3 for confirming that higher concentrations cause more efficient mixing in the diffusion area. This is certainly due to the fact that higher bulk concentrations cause steeper concentration gradients and therefore higher currents, which in turn increase the circulation producing even higher currents and so on until the magnetic force is counterbalanced by friction [10]. The MHD effect caused by the magnetic field will also decrease the screening effect at the deposition site. When the screening effect at the Nernst diffusion layer decreases, the limiting current density also will increase [9]. 3.3 The influence of working electrode area (A)

Fig. 3. Variation of the limiting current iB under magnetic field as a function of the CoSO4 concentration C. 0.9529 cm2 working electrode. T = 25-27◦C. B = 0.3 T. [Na2SO4] = 0.5 M. To study the effect of concentrations of the electroactive species on limiting current under magnetic field, the concentration of CoSO4 was

Fig. 4. Variation of the limiting current iB under magnetic field as a function of the working electrode area, A. T = 25-27◦C. B = 0.3 T. [Na2SO4] = 0.5 M and [CoSO4] = 0.01 M.

3

To study the effect of the electrode area on limiting current under magnetic field, the concentration of CoSO4 was kept constant at 0.01 M using Na2SO4 0.5 (M) under a constant magnetic field of 0.3 T. Limiting currents were measured using the four platinum wires (0.32285, 0.48035, 0.63785 and 0.95285 cm2 area) as working electrode. The effect of the electrode area (A) on limiting current under magnetic field (iB), was determined with three different electrodes in in Na2SO4 0.5 (M) under a constant magnetic field of 0.3 T. From Fig. 4, it is found that the limiting current increases as the working electrode area increases. The increment of this limiting current due to the increment of current which subsequently increase the electrode reaction [8]. A larger area also will cause more effective magnetic stirring in diffusion area. This reduces the thickness of its Nernst diffusion layer. This will cause the limiting current and deposition rate increase [10]. 3.4 The influence of supporting electrolyte concentration Table 1. Variation of diffusion coefficient (D) and Kinematic viscosity (v) of the electroactive species on the effect of supporting electrolyte (Na2SO4) concentration CoSO4 (M)

Na2SO4 (M)

D x 106 (cm2/s)

v (Stoke)

0.01

0.05

6

0.01313

0.01

0.1

5.6

0.01402

0.01

0.2

4.9

0.01500

0.01

0.3

4.4

0.01598

0.01

0.4

4.25

0.01679

Fig. 6. Variation of the limiting current iB under magnetic field as a function of the diffusion coefficient (D). CoSO4 0.01 M, [Na2SO4] = 0.05 – 0.4 M, Boric acid 0 – 0.12 M, 0.9529 cm2 working electrode, T = 25-27◦C. B = 0.3 T. The influence of supporting electrolytes, was determined by keeping the electroactive species constant (CoSO4) and by changing the s supporting electrolytes (Na2SO4). The concentration of CoSO4 was 0.01 M in 0.05 – 0.4 M Na2SO4. The change of supporting electrolyte concentration leads to the variation of difussion coefficient and electrolyte kinematic viscosity. Rheometer is used to measure the viscosity of electrolyte and it was divided by its density to yield the kinematic viscosity; the results are tabulated in Table 1. Limiting currents were measured using the 0.9529 cm2 platinum wire as working electrode. The diffusion coefficient value for Co2+ were investigated using chronoamperometry. From Fig. 4, it is found that the limiting current decreases as the concentration of Na2SO4 increases. The decrease of limiting current can be linked to the increment kinematic viscosity of the Na2SO4 in the electrolyte as shown in table 1. The variation of the electrolyte kinematic viscosity leads to a variation of the electroactive species diffusion coefficient [7]. The increasing of the electroactive species diffusion coefficient leads to the increment of limiting current as shown in Fig. 5. It seems that the decrease of the mass-transport-limited current is due to the friction forces becoming more effective as the electrolyte viscosity increases, preventing MHD convection of the solution [10]. 3.5

Fig. 5. Variation of the limiting current iB under magnetic field as a function of the kinematic viscosity (v). CoSO4 0.01 M, [Na2SO4] = 0.05 – 0.4 M, 0.9529 cm2 working electrode, T = 25-27◦C. B = 0.3 T.

4

The influence concentration

of

additive

electrolyte

The effect of additive electrolyte concentration on magnetic electrodeposition was determined in CoSO4 0.01 M solutions by adding different amounts of boric acid, from 0 to 0.12 M. Limiting currents were measured using the 0.9529 cm2 platinum wire as working electrode. The effect of a magnetic field on the diffusion of Co2+ were investigated using chronoamperometry. The change in the electrolyte composition leads to the variation of difussion coefficient and electrolyte kinematic viscosity. Fig. 7 shows the variation of the limiting current iB under magnetic field as a function of the kinematic viscosity (v) by adding different amounts of boric

ISSN 1978-5933 The Second International Conference On Green Technology and Engineering (ICGTE) 2009 Faculty of Engineering Malahayati University acid. The influence of additive on limiting current is give linear graph; it curves downward confirming that higher concentrations of boric acid cause decrease stirring effect of MHD. This is certainly due to the fact that higher additive concentrations cause steeper kinematic viscosity of electrolyte gradients. The increment of the electrolyte kinematic viscosity leads to the decreasing of the diffusion coefficient of electroactive species. This causes the decreasing of limiting current as shown in Fig. 8. [7]. It seems that the decrease of the mass-transport-limited current is due to the friction forces becoming more effective as the electrolyte viscosity increases, preventing MHD Table 2. Variation of diffusion coefficient (D) and Kinematic viscosity (v) of the electroactive species on the effect of Additive concentration CoSO4 (M) 0.01 0.01 0.01 0.01 0.01 0.01

Na2SO4 (M) 0.5 0.5 0.5 0.5 0.5 0.5

H3BO3 (M) 0 0.01 0.04 0.05 0.08 0.12

v (Stoke) 0.0171 0.0173 0.0175 0.0179 0.0182 0.0192

D x 106 (cm2/s) 4.0 3.6 3.3 3.2 2.9 2.5

Fig. 7. Variation of the limiting current iB under magnetic field as a function of the kinematic viscosity (v). CoSO4 0.01 M, [Na2SO4] = 0.5 M, Boric acid 0 – 0.12 M, 0.9529 cm2 working electrode, T = 25-27◦C. B = 0.3 T.

Fig. 8. Variation of the limiting current iB under magnetic field as a function of the diffusion coefficient (D). CoSO4 0.01 M, [Na2SO4] = 0.5 M, Boric acid 0 – 0.12 M, 0.9529 cm2 working electrode, T = 25-27◦C. B = 0.3 T. convection of the solution [10]. Although the presence of boric acid leads to decrease the limiting current, it is useful as a buffer to avoid the local pH rise caused by parallel hydrogen evolution reaction (HER) [11]. The Increment of HER caused a roughness on the surface metal of electrodeposits. 4.

Conclusions

A homogeneous magnetic field has been applied in a three electrode electrochemical cell and oriented perpendicular to the electrode surface causes the increment of the limiting current. The effects of the magnetic field strength, the electrode area, the concentration of the electro active species, the diffusion coefficient of the electroactive species, the kinematic viscosity of the electrolyte and the additive concentration on cobalt electrodeposition have been studied. The experimental results showed that those parameters also found to be significantly effect on the mass transport of cobalt electrodeposition. Higher limiting current of electrodeposition was achieved when higher magnetic field strength, working electrode area and concentration of electroactive species and also when lower concentrations of supporting and additive electrolyte were applied. The increment concentrations of supporting or additive electrolyte lead to the increase of the kinematic viscosity of electrolyte and also lead to the decrease of the diffusion coefficient of electroactive species. It also observed that the presence of boric acid as an additive was lead to the smaller amount of limiting current the decrease of limiting current. However it is acting as a buffer to avoid the local pH rise caused by parallel hydrogen evolution reaction (HER) which results to the smoother surface of electrodeposits. 5.

Acknowledgment

5

Financial supports from Ministry of Higher Education Malaysia through FRGS grant No. 607113 is greatly acknowledged. References: [1]

Krause, A., M. Uhlemann, A. Gebert, L. Schultz. Electrochimica Acta. 49 (2004) 4127– 4134. [2] Sudibyo, M.B. How, N. I. Basir and N. Aziz, Study of magnetic field effects on copper molecules. School of Physics, The University of Edinburgh, Scotland, United Kingdom, 2007. [5] Mhiochain, T.R., Hinds, G., Martin, A., Chang, E., Lai, A., Costiner, L., Coey, J.M.D.,.Electrochimica Acta. 49, (2004) 48134828 [6] Bund, A., Koehler, S., Kuehnlein, H.H., Plieth, W., Electrochimica Acta. 49 (2003) 147-152. [7] Legeai, S., Chatelut, M., Vittori, O., Chopart, J.P., Aaboubi, O., Electrochimica Acta. 50 (2004) 51-57. [8] James, A.M, Electrochemistry Dictionary. John Wiley & Sons, Ltd, 1984. [9] Mogi, I., Kamiko, M., Journal of Crystal Growth. 166 (1996) 276-280

6

electrodeposition. RSCE – SOMCHE 2008, Kuala Lumpur, Malaysia, pp. 961 - 964 2nd 3rd December 2008. [3] Matsushima, J.T., Trivinho-Strixino, F., Pereira, E.C., Electrochimica Acta. 51 (2006) 1960– 1966. [4] Ackland, G.J. and Tweedie, E.S., Microscopic model of diffusion limited aggregation and electrodeposition in the presence of levelling

[10] Leventis, N., M. Chen, X. Gao, M. Canalas, and P. Zhang, Journal of Physical Chemistry B, 102 (1998) 3512-3522. [11] Santos, J.S. , Matos, R., Trivinho-Strixino, F., Pereira, E.C. Electrochimica Acta. 53 (2007) 644–649. [12] Hinds, G., Spada, F. E., Coey, J. M. D., Ni Mhiocha´in, T. R., Lyons, M. E. G., Journal of Physical Chemistry B. 105(2001) 9847-9502. [13] Fahidy, T.Z., Progress in Surface Science 68 (2001)155 -158. [14] Coey, J. M. D., Hinds, G., Journal of Alloys and – 245. Compounds. 326(2001) 238