Electrophoretic Deposition of Al2O3 And ZnO on

The Iraqi Journal For Mechanical And Material Engineering, Special Issue (C)

Electrophoretic Deposition of Al2O3 And ZnO onto Copper Substrate Ghanim A., N. College of Engineering/Babylon University Electrochemical Engineering Department E-mail: [email protected]

ABSTRACT The aim of this study is to fabricate ZnO and Al2O3 coated ceramic material onto copper sheet substrate with the aqueous liquid suspensions and with the organic liquid suspensions using the electrophoretic deposition technique. A 10-wt% of suspension prepared as a starting suspensions using a constant potential of 30 V and current density of 25-30 mA/cm2 by a suitable power supply. Electrode surface of the copper substrates were coated with EPD technique gives a uniform ceramic film coating increased subsequently with time over the various solutions. Regressions that explain the deposition weight for each ceramic material per the electrode surface is as a function of time deposition duration were formulated at fixed conditions. Coated samples were dried and sintered at 1000°C for two hours. Characteristics of the sintered coated film were investigated with SEM technique. INTRODUCTION Electrophoretic deposition (EPD) is a process in which ceramic particles, suspended in a liquid medium, migrate in an applied electric field and deposit on an electrode. It is a well-known processing technique for the deposition of films and coatings. EPD is a combination of electrophoresis and deposition, in which a particle film is deposited onto an electrode. In the first step the particles suspended in a liquid are forced to move toward an electrode by applying an electric field that is the electrophoresis. In the second step, the particles deposited at the electrode surface forming coherent deposit via particle coagulation. The phenomenon of electrophoresis has been known since the beginning of the 19th century and it has found applications during the past 40 years mainly in traditional ceramic technology. The fabrication of thick films of titanium and oxides of titanium (i.e. titania) for micromachined structures was investigated by Marquordt [1]. In this colloidal process, charged particles are moved in a fluid due to an electric dc field to an oppositely-charged electrode, where they coagulate into a dense film. In an engineering sense, this deposition method is similar to electroplating, except that a colloidal suspension of particles as opposed to an ionic solution is the electrolyte. Hamaker [2] reported that there is no reaction of either the cathode or the anode with the particles involved. The electrodes only provide the electric field to move the charged particles in the fluid. In addition, the electrodes presumably act as sources and sinks of electrons. It is most likely that the positively charged particles and the negatively charged ions of the solution are the carriers of the current. To obtain a satisfactory structure with EPD, the following process steps are required :(1) creation of a stable solution; (2) application of an appropriate electric field; (3) drying of the deposited film; and (4) sintering in a suitable atmosphere to anneal and densify the film. An application of abrasion or corrosion resistant glass ceramic coating materials on metal substrate by electrophoretic deposition technique in an aqueous medium has been described by Datta [3]. He concluded that the rate of electrophoretic deposition of glass-ceramic coating materials from aqueous bath is dependent on the total number of particles present in the bath, the current passing through the electrodes and the time. Dogan et al [4] demonstrated that the

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Electrophoretic Deposition of Al2O3 And ZnO onto Copper Substrate

Ghanim A. N.

EPD technique can be utilized to form ZnO films with 10μm thickness for sensor applications. A stable suspension is necessary for a successful EPD process. The stable suspension of submicron ZnO particles can be achieved by chemically aided milling approach. They stated that the particles in chemical aided milling form sediment within about 10 minutes and have a diameter of approximately 120nm. The prepared suspension had a conductivity of 0.87mS/cm and a pH of 8.4 so it was stable enough for EPD. The problems with the mineral insulated, metal sheathed lead wires used to instrument aircraft engines have become more severe at the increased operating temperatures of today’s engines. Electrophoretic deposition using pure alumina onto platinum wires where three coating methods have been studied by Kreidler and Bhallamudi [5], electrophoretic deposition (EPD) from ethanol suspensions, slurry coating, and EPD from aqueous suspensions. The deposition rates of the coatings were studied as functions of the variables in the processes. Although all three coating methods were sufficiently rapid to serve as the basis for a commercial process, aqueous EPD coatings are not recommended for this application. The electrical resistivity of the sintered coatings was studied as a function of temperature from 800 to1050oC. The resistivities of the coatings at 800oC were 3.4-7.5 x 108 Ω-cm depending upon coating type. The shape SiC tubes were prepared by electrophoresis process. Nobre et al [6] studied the anodic deposits with thickness from 0.35 to 1.10 mm using low electric field from alcoholic slurry. The SiC dispersion in ethanol with solid loading of 10 vol. % was used. Slurry dispersion degree is a function of SiC surface oxidation degree. High surface oxidation degree give rises a more stable suspension in alcoholic medium. The engineering of the surface allowed creating appreciable amount of free silano groups (Si–OH). They concluded that the stabilization of SiC suspension in alcohol medium is possible by surface modifications. This process is performed via physical-chemistry surface changing and surface-active substance containing nitrogen, which adsorb chemically on SiC surface. SiC surface oxidation leads indirectly to the development of free silanol groups, which are the adsorption centers for acrylic acid–acrylate copolymer. Meng et al [7] stated a deposit application of higher voltages (>200 V) for periods longer than 10 seconds and reported bigger porous particles deposit. Increasing the electric field resulted in increased rate of deposition, but the deposited particles had shorter time to rearrange and therefore these coatings had a more porous microstructure. The present work was conducted to develop a thin coated ceramic film on a copper sheet. Two types of solutions have been studied: one that contains suspension of 10-wt% aqueous alkaline solution (pH=9), and one that contains suspension of 10-wt% a binder-anion dispersantsolvent. All electrophoretic deposition was done on thin copper electrode at room temperature for 100 minutes for alumina and 20 min for zinc oxide. ATTEN DC power supply type APR 3002A with 0-30 V of voltage and 0-2 A of current was used. A fixed potential of 30 V and 2530 mA/cm2 current density were tested for all experiments. The suspensions were agitated with a magnetic stirrer to minimize settling of the ceramic particles. The microstructure and composition of the deposits films were characterized using a Union ME-3145 scanning electron microscope (SEM) supplied with digital camera and equipped with a computer. As such, coatings produced using EPD is governed by electrical parameters as well as the properties of the suspension. During deposition, one of the most important parameters is the electric field, which is applied either through constant current density or constant voltage across the electrodes in the suspension. Most EPD processes are carried out under constant voltage conditions. Under low voltages (≤30 V), small particles are deposited regularly and smooth, uniform deposits was observed. The aim of this work is to fabricate a uniform ceramic coating material onto copper sheet using the electrophoretic technique with the two mentioned suspension solutions, to evaluate the ceramic coated film constructed by alumina or zinc oxide particles and also to find the best characteristics and properties of the coated film by the SEM technique.

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The Iraqi Journal For Mechanical And Material Engineering, Special Issue (C) EXPERIMENTAL In this study, the suspension for EDP consisted of magnetically agitated glass cell of 250 cm3 equipped with two copper counter electrodes were used for electrophoretic deposition. The working area of the electrodes was approximately 3.25 cm2, and a separation distance between the working electrode and counter electrode was fixed to 1 cm using a rigid positioning tool. The schematic diagram of electrode arrangement in the electrophoretic cell, which was used for coating small copper sheet of (13×25×5 mm) dimension, is shown in (Fig.1). Two types of solvent have been tested during the firm of experiments e.g. ethanol and water. The solvents and the other chemicals used were of laboratory reagent grade. The properties and characterization of ceramic coating compositions used in this work are as listed: Ceramic Melting point ο material C ZnO 1800 Al2O3 2000

Sp.Gr

Solubility(g/100g)

Particle size

5.47 4.00

4.2×10-4 in water insoluble

90%>+325 mesh 95%>+325 mesh

The glass beaker container was used as a deposition cell. A regulated D.C. power supply source of capacity 200 mA and 30 V was used to supply the necessary electric power. The built-in resistances in the power unit were controlled, in order to obtain constant current or constant voltage. The pH of the suspension was determined by a pH meter (pHM84), using a standardized glass electrode. Specimen preparation: each specimen cleaned by sand blast and dipped in 5% H2SO4 bath (5 min.) then rinsing with tap water and distilled water and drying in oven. Dispersant preparation: for both ZnO, and Al2O3 each powder is milled for1hr in porcelain jar, weight 10 gm using digital balance and the added to 100 ml ethyl alcohol with use of 2% PVA (poly vinyl alcohol) as a binder and 3% polyacrylic acid as dispersant.

Figure (1) Schematic diagram of as experimental setup.

Poly vinyl alcohol (PVA) was added to the suspensions to increase the adhesion of the deposited coatings and strength of the deposited material, as well as to prevent cracking. The used polyacrylic acid as dispersant has the following properties: Mwt 72.02

density 1.22g/cm3

solvent Water or ethanol

partial specific volume 0.648 cm3/gm in water at 25 οC

Polyacrylic acid is a vinyl polymer improves the suspension stability by the action of anionic dissociation. Other applications of this polymer are the pigment suspension in paint and flocculating agent for particles suspension in water. Aqueous suspension preparation: for both ZnO, and Al2O3 each powder is milled for 1hr in porcelain jar, weight 10 gm using digital balance and the added to 100 ml distilled water and 3% polyacrylic acid as dispersant, pH adjusted by the addition of concentrated caustic solution. Green deposit treatment: the specimen was air dried carefully and heated in air oven at 120οC. A weight of the deposit is measured before and after deposition. Specimens were heated

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Ghanim A. N.

in electrical furnace at 1000οC to achieve adhesive ceramic coating. Subsequently, the coatings were examined for surface quality, using SEM technique. RESULTS AND DISCUSSION Water based suspensions for electrophoretic deposition (EPD) contained 10 wt% ceramic alumina or zinc oxide. Distilled water was used for all preparation and an alkaline solution with pH=9. These suspensions were tested with and without dispersants. Aqueous solutions have been more frequently investigated than non-aqueous media, they have some disadvantages. One disadvantage is that water electrolyzes at applied voltages above a few volts, thereby limiting the range of deposition conditions and potentially introducing gas bubbles onto the electrode at higher voltages. Another potential problem is corrosion of the anode. The structure of the dried coating produced by EPD from an aqueous suspension is shown in Fig. 4. The coating contains holes where it grew around oxygen bubbles attached to the anode surface. When the bubbles break away, the coating tends to fill in the holes but this process is often incomplete and the surface of the coating has irregular undulations which are relicts of the incompletely filled holes. In aqueous media with organic dispersant the latter dissociates to give R-COO- ions and these ions are adsorbed onto the ceramic surface and give the particles a negative charge. The coatings produced by EPD from alcohol-based suspensions behave differently than those made from aqueous suspensions. Disadvantages of organic liquids can be toxicity, safety aspects, costs, and moderate dielectric constants, which lead to higher deposition voltages. The suspensions used for organic solvent EPD were prepared with ethanol. When pure ethanol was used alone, a deposition was not observed even at 30 Volts, the limit of the power supply. The additions of organic dispersant such as the polyacrylic acid lower the required voltage for EPD coating and to facilitate the formation of negative charge particles to be moved to anode. Kreidler et al [5] proposed a mechanism for the charging of A12O3 in pure ethanol without dispersant included the adsorption of ethanol molecules at basic surface sites as first step. The adsorbed molecules then dissociate into protons and C2H5O- ions. The C2H5O- ions dissolve into the liquid phase and the protons remain adsorbed on the ceramic surface. The pH of the suspensions was measured as well as the thickness of coatings deposited during the total time of deposition at fixed potential of 30 V and current density of 25-30 mA/cm2. Alkaline solutions were used in order to obtain negatively charged particles. Electrophoretic deposition of the alumina powder onto the copper sheet substrate has been examined as a function of deposition time and the applied voltage. A Value of pH = 9 was chosen, since the isoelectric point of Al2O3 is at pH=9. An alkaline solution (NaOH) was therefore used to produce negatively charged particles that can be deposited on the anode. A pH sufficiently far from this point was desirable to guarantee the stability of negatively charged alumina particles that can be deposited on the anode. The coatings can be dried without cracking but when sintered, these coatings develop regularly spaced cracks similar to those found in aqueous EPD coatings. Figure (2) shows deposit weight per unit area of electrode versus time dependencies for alumina deposits. It is seen that deposit weight increases with time at a constant 30 V potential and 25-30 mA/cm2 current density variation but the organic system (b) is more effective for alumina deposition. Prepared suspensions exhibited high stability, and a relatively high deposition rate could be achieved due to the use of an effective binder. The obtained deposits adhered well to the substrates and exhibited enhanced stability against cracking. It seems clearly that the deposition rate of alumina is significantly high when using the solvent-binderdispersant system in comparison to the alkaline solution (a), besides that a uniform thin films laminates deposit layer could be obtained.

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The Iraqi Journal For Mechanical And Material Engineering, Special Issue (C) b.Deposition Weight of alumina vs. Time for 10-wt% EtOH and PA polymer, 30V 30mA

a.Deposition Weight of Alumina vs. Time, for 30V and 30mA, Alkaline solution PH=9 y = 0.0027x 2 + 0.4805x - 0.5328 R2 = 0.9994

80

Deposition Weight (mg/cm^2)

70

y = 0.0112x 2 + 1.3532x + 5.115 R2 = 0.9971

300


60 50 40 30 20 10 0

250 200

150 100 50 0

0

50

100 Time (min)

0

150

50

100

150

Time (min)

Figure (2) Deposition weight of alumina per unit area versus time with (a) alkaline solution (b) organic solvent-binder-dispersant system Figure (3) shows deposit weight per unit area of electrode versus time dependencies for zinc oxide deposits. Also it is seen that deposit weight increases with time at a constant 30 V potential and 25-30 mA/cm2 current density variation but the deposition rates are approximate similar in both cases (a) and (b), showing that the differ medium of zinc oxide EPD had practically less influence during rather the short time of deposition. a.Deposition Weight of Znic Oxide vs.Time for 10wt% aqueous sol. with PA polymer 30V,30mA y = 1.2853x 2 + 3.8879x + 2.7407 R2 = 0.9974

160

y = 1.2603x 2 - 4.6184x + 12.978 R2 = 0.9842

500

140


Deposition Weight (mg/cm2)

b.Deposition Weight of Zinc Oxide vs. Time for 10-wt% EthOH solution, 30V and 30mA and PA polymer

120 100 80 60 40 20

450 400 350 300 250 200 150 100 50 0 0

0 0

5

10

15

5

10

15

20

25

Time (min)

Time (min)

Figure (3) Deposition weight of zinc oxide per unit area versus time with (a) aqueous solventbinder-dispersant system (b) organic solvent-binder-dispersant system. The deposit layer of zinc oxide was nonuniform with thik films and the rate of deposition is much higher than the rate of alumina for both two systems. The electric field drives ceramic particles toward the electrode and exerts a pressure on the deposited layer. It is desirable to maintain a high potential difference between the anode and the cathode. The use of high voltages has the advantage of smaller deposition times and higher deposit thickness, so that for alumina deposition higher electric field should be used. The dispersant must be added before the binder to prevent competitive adsorption for organic solvent-binder-dispersant. Comparison between water and polar solvents shows that solvent

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Ghanim A. N.

with polar character exhibits more advantage based on the detrimental of the electrolyze process, a typical phenomenon of water. Otherwise, the polar solvents present relatively high dielectric constant, which allows use of high electric fields. Therefore, the low electrophoretic mobility can be compensated either by high electric field or by surface particle modification via adsorption of dispersant molecules on the surface particle. Experiments also indicate that the ethanol-polyvinyl alcohol-polyacrylic acid system is an effective mixture for the deposition of various ceramic materials. The experimental data presented in Figures (2) and (3) demonstrate a manner in which the amount of deposited material can be controlled. This is especially important for deposition of consecutive ceramic layers of controlled thickness in multilayer processing. Figure (4) shows the surface features of alumina thin film deposited onto copper substrate. The photograph clearly show that the films were uniform and conformal. It was difficult to control the particle size of suspension. It was believed that coatings with symmetrical particle size suspension gives uniform deposits and a continuous deposition rate could be produced. The coating layer closest to the substrate was observed to be dense whereas the outer coating layers porous. The production of dense coatings with initial voltage of 10 V suggested that only fine particles had enough speed to arrive at the anodic copper substrate. Figure.4 Photograph for EPD of dried coated alumina on copper substrate. Figure (5) shows SEM micrograph of sintered coatings for 2 hours at 1000°C. As it is seen on the figure the morphology of the sintered alumina with a constant voltage create a porous structure and the deposit distribution is not completely homogeneous, but for some applications uniform porosity is required. The spacing of the deposits is fairly regular over the length of the coated area and the structure of the sintered material has a smooth surface and is free of cracks. Figure.5 SEM micrograph of deposit structure by EPD after sintering. CONCLUSION EPD being simple and inexpensive process that provides an attractive valuable method for producing films for a variety of application such as the design of solid-oxide fuel cells, solar cells, microelectronic devices, fiber-reinforced composites and batteries . EPD technique can be utilized to form Al2O3 and ZnO films onto copper substrate. The rate of EPD of alumina from organic solvent-binder-dispersant gives a better uniform deposit layer than the aqueous solution. The deposit from the suspension containing a dispersant had a higher deposition rate, improved surface morphology and more uniform particle distribution. EPD technique enable less deposition time for ZnO suspension in comparison with the Al2O3 suspension using a similar condition. The conditions for deposition suggest nonlinear regressions that explain the deposition weight of ZnO and Al2O3 as a function of time with a

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The Iraqi Journal For Mechanical And Material Engineering, Special Issue (C) constant applied electric field, suspension concentration, and approximate electrophoretic mobility of particles. FUTURE WORK Depending on the suspension concentration and size, thin films of 100 µm up to more than few mm's can be directly fabricated on an electrically conductive substrate. As EPD process can actually be controlled as a function of time, additionally the current, potential and temperature can be varied during the EPD process. The effect of the average current density on the deposition rate and the film structure should be studied with the contribution of particle size, concentration and dispersant effect. REFERENCES [1] C. Marquordt , M.G. Allen, Fabrication of Micromechanical Structures of Titania and Titanium with Electrophoretic Deposition, The 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, June 10 – 14, 2001 [2] H.C. Hamaker, Formation of Deposition by Electrophoresis, Transactions Faraday Society, 36, p.279-287, 1939 [3] S. Datta, Application of design of experiment on electrophoretic deposition of glassceramic coating materials from an aqueous bath, Bull. Mater. Sci., Vol. 23, No. 2, April 2000, pp.125–129, Indian Academy of Sciences. [4] A. Dogan, et al, Electrophoretic depostion of ZnO thin film in an aqueous media, Anadolu University, Department of Materials Science and Engineering, Eskisehir/Turkey (2003) [5] E. R. Kreidler and V. P. Bhallamudi, Application of ceramic insulation on high temperature instrumentation wire for turbine engines, Journal of Ceramic Processing Research. Vol. 2, No. 3, pp. 93-103 (2001) [6] M. Nobre, R. Castro, D. Gouvea, Engineering surface and electrophoretic deposition of SiC powder, Materials Letters 50, 2001,115–119 August 2001www.elsevier.comrl [7] X. Meng et al, Hydroxyapatite coating by electrophoretic deposition at dynamic voltage, Dental Materials Journal 2008; 27(5): 666-671 [8] I. Zhitomirsky, Ceramic Films Using Cathodic Electrodeposition, JOM-e, 52 (1) (2000) http://www.tms.org/pubs/journals/JOM/0 [9] G. Knornschild, Electrophoretic Deposition of Aluminum on an Mg-Alloy, Revista Matéria, v. 10, n. 3, pp. 497 – 501, 2005 [10] Abdul Razak and Zainal, Electrophoretic Deposition and Characterization of Copper Selenide Thin Film, The Malaysian Journal of Analytical Sciences, Vol 11, No 1 (2007): 324330 [11] Chunfen Han, Qi Liu and Douglas G. Ivey, Development of Simple Electrolytes for the Electrodeposition and Electrophoretic Deposition of Pb-free, Sn-based Alloy Solder Films, CS MANTECH Conference, May 14-17, 2007, Austin, Texas, USA [12] I. Zhitomirsky, L. Gal-Or, Formation of hollow fibers by electrophoretic deposition, Mater. Lett. 38(1999)17

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