Electrophoretic Deposition of Materials

Annu. Rev. Mater. Sci. 1999. 29:327–52 c 1999 by Annual Reviews. All rights reserved Copyright 

ELECTROPHORETIC DEPOSITION OF MATERIALS Omer O. Van Van der Biest and Luc J. Vandeperr Vandeperree Departement Metaalkunde en Toegepaste Materiaalkunde, Katholieke Universiteit Leuven, De Croylaan 2, 3001 Heverlee, Belgium; e-mail: [email protected] KEY WORDS: mechanism, mechanism, kinetics, applications, applications, suspensions

ABSTRACT The electrophoretic deposition of materials is reviewed. Numerous applications of electrophoretic deposition are described, including production of coatings, free-standing objects, and laminated or graded materials, infiltration of porous materials, materials, and fabricati fabrication on of woven fiber preform preforms. s. The preparation preparation of electrophoretic suspensions is discussed as are a number of mechanisms of deposition that have been proposed proposed elsewhere. In discussing the kinetics of the process, primary attention is given to the relation between the evolution of the current and the electric field strength.

INTRODUCTION Electrophore Electrophoretic tic deposition deposition (EPD) is essentially essentially a two-step two-step process. In the first step, particles suspended in a liquid are forced to move toward an electrode by applying an electric field to the suspension suspension (electrophore (electrophoresis). sis). In the second second step, the particles collect at one of the electrode and form a coherent deposit on it. It should should be noted noted that the process process yields yields only a powder powder compact, compact, and therefore electrophoretic deposition should be followed by a densification step such as sintering or curing in order to obtain a fully dense material. The particles in suspension will move only in response to the electric field if they they carr carry y a char charge ge.. Four Four mech mechan anis isms ms have have been been ident identifie ified d by whic which h the the char charge ge on the particles can develop (1): ( a) selective adsorption of ions onto the solid particle from the liquid, ( b) dissociation of ions from the solid phase into the liquid, (c) adsorption or orientation of dipolar molecules at the particle surface,

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and (d ) electron electron transfer between the solid and liquid phase due to differences differences in work function. A charged particle in a suspension is surrounded by ions with an opposite charge in a concentration higher than the bulk concentration of these ions; this is the so-called double-layer (Figure 1). When an electric field is applied, these ions and the particle should move in opposite directions. However, the ions are also attracted attracted by the particle, particle, and as a result, result, a fraction fraction of the ions surrounding the particle will not move in the opposite direction but move along with the













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particle. Hence, the speed of a particle is not determined by the surface charge but by the net charge enclosed in the liquid sphere, which moves along with the particle. The potential at the surface of shear is termed the zeta-potential or electrokine electrokinetic tic potential. In principle, a particle particle with a negativ negativee surface charge can show a positive positive zeta-potentia zeta-potential. l. The latter occurs, occurs, for example, example, when the charge of specifically adsorbed ions is higher than the surface charge. In fact, the equilibrium speed of the particle is determined by four forces acting on the particle. The first, which accelerates the particle, is the force caused by the interaction interaction of the surface surface charge with the electric electric field. All other forces slow slow the the particle: particle: These are viscous drag from the liquid following following Stoke’s Stoke’s law law,, the the forc forcee exert xerted ed by the the elec electr tric ic field field on the the coun counte terr-ion -ionss in the the doub double le laye layerr (re(retardation) and, when a particle moves, the distortion in the double layer caused by a displacement between the center of the negative and positive charge, (relaxation). A more complete treatment of these forces, along with information on calculations of their magnitudes, can be found, for example, in Reference 2. In this paper, we first present a number of applications of electrophoretic deposition. deposition. Next, Next, the preparation preparation of suspensions for electrophore electrophoretic tic deposition is discussed, followed by a synthesis of the available information on the mechanisms of electrophoretic deposition and the kinetics of the process.

APPLICATIONS Electrophoretic deposition can be applied to any solid that is available in the form of a fine powder ( < 30 µm) or a colloidal colloidal suspension. suspension. Examples Examples of electrophoretic deposition of materials of almost any material class can be found, includi including ng metals metals,, polymer polymers, s, carbide carbides, s, oxides, oxides, nitride nitrides, s, and glasse glassess (see (see Table able 1). Furt Furthe herm rmor ore, e, the the proce process ss can can be used used for for produc producing ing coat coatin ings, gs, for for shap shapin ing g monomonolithic, laminated and graded free-standing objects, and for infiltration of porous materials and woven fiber preforms for composite production.

Coatings Electrophoretic deposition of coatings has already gained a world-wide acceptance for automotiv automotive, e, appliance, appliance, and general general industrial industrial (organic) coatings (3). For example, the construction and operation of an 800 bodies-per-hour cat-













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A non-exhaustive non-exhaustive list of examples from a broad range of material classes that have been electrophoretically electrophoretically depositeda Table 1

Material class

Examples

Acids Acids and hydroxides hydroxides

Calcium Calcium hydrox hydroxide ide (91), (91), magnes magnesium ium hydroxi hydroxide de (91), (91), antimon antimonic ic acid acid (96), boric acid (52)

Borides

LaB6 (97)

Carbides

B4C (36), C (34–36, (34–36, 98), Cr3C2 (99), NpC (88), diamond (100, 101), PuC (88), ThC (88), UC (88), SiC (34–36, 57, 98, 102), UWC2 (88), WC (97)

Carbonates

(87, 91, 97, 103)

Metal tals

Al (83, (83, 97, 97, 104 104), Al-C Al-Crr (65) (65),, Al-S Al-Sii (65) (65),, Al-T Al-Tii (65) (65),, Au (88, (88, 105 105), B (88), Cu (83), Dy (88), Fe (83), Mo (88, 97), MoSi2 (51), Nb (88), Nb2Sn (88), Ni (83, 97), Sn (83), Re (88), Ru (97), W (88, 97), Zn (83, 97), Zr (88)

Nitrides

Si3N4 (56), AlN (37)

Organ Organic ic materi materials als

Starch Starch (47), (47), styren styrene-a e-acry cryli licc copoly copolyme merr (106), (106), latex latex (3, 85), 85), vinyl vinyl copolymers (3), epoxy resins (3), polyamides (3), poly-urethanes (3)

Oxides

Clay (25, 26), Al2O3 (30, 31, 48, 87, 97, 98), ß-Al2O3 (39, 64), BaTiO3 (107), Cr2O3 (108), Fe2O3 (108), glass (17, 109), In2O3 (97), La2O3 (51), LiAlO2 (110), mica (111), MgO (97, 112), NiO (88, 108), ReBaCuO (24), SiO2 (53, 58, 59, 97), TiO2 (97, 108), UO2 (88), YBaCuO (18–20), ZnO (97), ZrO2 (41, 44, 48, 51, 54, 85)

Phosphors

(14–16, 81, 97, 113, 114)

a

Numbers in parentheses are references

Thro Throw w powe powerr is defin defined ed as the abili ability ty to cove coverr rece recess ssed ed porti portions ons of comp comple lexl xly y shaped parts. This property, combined with improved thickness homogeneity, is a decided asset for application in the enamel industry (6–11). The claim that relatively complex shapes can be coated is clearly shown by some examples given by Ortner (12): These include taps coated with carbides, fasteners coated with with Ni, and metal metal cones coated coated with a cerami ceramicc glaze. glaze. Platinu Platinum m grids grids have been coated completely with ferrite (13). Other examples examples of coatings coatings made by electrophore electrophoretic tic deposition include the deposition deposition of phosphor coatings coatings in the manufacture manufacture of screens screens for cathode ray














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Shaping Free-Standing Objects For traditional ceramics, such as sanitary ware, the main advantage of electrophoretic deposition lies in its higher speed and in the low wear of the moulds compared with slip casting (25). Just as complexly shaped ob jects jects can be coated using EPD, so too can it be used for shaping objects. objects. Tiles, Tiles, closed and open end tubes, hemispheres, tubes with changes in diameter, and conical sections are some of the shapes that have been made, albeit mainly on a laboratory scale, using solid compositions normally used for table and sanitary ware (25–28). An example of a hybrid process, somewhere between tape casting and electrophoretic trophoretic deposition, deposition, is the ELEPHANT ELEPHANT process (29). Continuous Continuous tapes of ceramic material are made by depositing on two rolling cylinders, which then press press the two slabs slabs together together in one long tape. tape. The process process was deve develope loped d for the tile industry. industry. After electrophoret electrophoretic ic deposition, the tape is cut, punched, drie dried, d, and and sinte sintere red. d. The econ econom omic ic adva advanta ntage gess stem stem mainl mainly y from from the lowe lowerr manmanpower requirements, low-energy consumption, low wear (of moulds) and low maintenance costs (29). Technical ceramics such as alumina (Al 2O3) (30–33), silicon carbide (SiC) (34–36) (34–36),, and aluminium aluminium nitride nitride (AlN) (37, 38) can also also be shaped shaped by elecelectrophoretic deposition. Beta-alumina tubes, used as electrolyte in sodium sulfur batteries, are a classical example example of the use of electrophore electrophoretic tic deposition. deposition. Whereas Whereas charging charging for many ceramics is related to adsorption or desorption of hydrogen ions, the preferential dissolution of sodium ions from the beta-alumina during milling is the main factor factor leading leading to charging charging of such aluminas (39, (39, 40). When these beta-alumina tubes were studied, it was discovered that the difference in density between two or more powders used to produce a material was unimportant, and a homogeneous composition composition could be ensured. Powers Powers (40) showed this by comparing the composition of the first and the seventh tubes deposited from a suspension containing a beta-alumina powder with a low soda content tent (8% Na2O) with one with a higher soda content (14–25%) also containing containing various additives such as MgO and Li 2O. MONOLITHS

LAMINATED MATERIALS

Layered materials can also be produced via EPD.













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showing that although EPD can be a fast process, good control of the growth of the layers can be obtained. The same group also produced produced alumina/lanthanum alumina/lanthanum aluminate laminates (42). Ferrari et al (43) and Fischer et al (44) produced alumina/z mina/zirc irconia onia lamina laminates tes from from aqueou aqueouss suspen suspension sions. s. Vandeper andeperre re & Van der Biest Biest made a range of SiC-based laminates with graphite (45), SiC +graphite (46), and porous SiC interlayers (47). They also produced laminated SiC/graphite composite tubes (35), thus combining the ability of EPD to produce laminated materials with the shaping capabilities of the process. While layered materials are obtained by immersing the deposition electrode in different baths, graded materials can also be made by gradually changing the composition of the suspension from which EPD is carried ried out. out. Sark Sarkar ar et al demo demons nstr trat ated ed the abil abilit ity y to form form grade graded d mate materi rial alss by slow slowly ly adding an ethanol-based suspension of an alumina powder to an ethanol-based suspension suspension of an yttria-stabil yttria-stabilized ized zirconia zirconia powder during deposition deposition (48). A gradua graduall increa increase se in the alumina alumina conten contentt of the deposit deposit was observe observed. d. Later Later,, the same group also produced Al 2O3 /MoSi2- and Al2O3 /Ni-graded materials (49). Chao et al produced alumina/ceriaalumina/ceria-stabi stabilized lized zirconia-grade zirconia-graded d rods from an acetone-based suspension (50). GRADED MATERIALS

The full potential of EPD in producing unique microstructural features is perhaps not yet realized. Nich Nichols olson on et al (51) (51) did did some some explo explora rator tory y work work in maki making ng nonpl nonplan anar ar lami lamina nate tes: s: By placing a grid before the deposition electrode, they were able to produce lamina laminates tes with with wavy wavy interla interlayer yers. s. Thus Thus by using using auxilia auxiliary ry electr electrode odes, s, deposi deposition tion can be enhanced locally. locally. Scala & Sandor (52) have have deposited B 2O3 on silicon silicon wafers as a boron diffusion source. The deposition onto the entire silicon wafer was prevented by growing a silica film on the wafer and etching the silica away only where deposition of boron was required. required. UNIQUE MICROSTRUCTURES AND SELECTED DEPOSITION

Infiltration The throwing power of electrophoretic deposition can also be used to infiltrate objects objects with a matrix matrix material or to apply an internal coating. coating. Gal-Or Gal-Or et al (53) infiltrated porous graphite electrodes with silicon carbide and silicon oxide partic particles les.. For a good infiltrat infiltration ion,, the particle particle buildup buildup in an external external coating coating
















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Another application where electrophoretic deposition is gaining increasing interest is the infiltration of fiber preforms with matrix material for composite producti production on (55–59). (55–59). For conduct conducting ing fibers, fibers, the preform preform is used used direct directly ly as the deposition electrode, whereas for nonconducting fibers, the preform is positioned positioned in front of an electrode. electrode. The advantage advantage of using electrophoretic electrophoretic deposition for composite fabrication is that it allows a reduction of the costs compared with, for example, example, chemical chemical vapor vapor infiltration. infiltration. Moreover Moreover,, electro electrophoreti phoreticc deposition is much faster, and the composition of the matrix material can be controlled quite easily, as is shown by the example of infiltration of woven SiC fiber mats by a mixed sol of mullite composition (58).

GUIDELINES FOR SUSPENSION PREPARATION Suspensions used in other colloidal-shaping techniques, such as slip casting and tape casting, can serve as a starting point for development of suspensions for electrophoretic deposition. However, in contrast to many of these colloidal proc proces esse ses, s, susp suspen ensi sion onss with with much much lowe lowerr solid solidss load loading ing can can be used used for for EPD. EPD. The The resulting resulting lower viscosity viscosity is an advantage advantage in handling handling the suspensions. suspensions. Green densities of about 40–60 vol% are obtained via EPD from suspensions with solids loadings as low as 1–2 vol% (34). The key to successful development of suspensions for electrophoretic deposition is to find a systematic approach to making suspensions in which the particles have a high zeta-potential, while keeping the ionic conductivity of the suspensions suspensions low. low. A necessary necessary but not sufficient sufficient condition for a high zetapotential is a high surface charge. As mentioned above, four mechanisms have been been identifi identified ed for charging charging (1): select selectiv ivee adsorpt adsorption ion of ions ions onto the solid solid particle from the liquid, dissociation of ions from the solid phase into the liq-













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when, for example, multivalent metal ions, which adsorb specifically on the surface, are added to the suspension (61). Because of the lower solids loading, simply changing the pH to a value far from the pzc using concentrated acids or bases is generally all that is required for obtaining a stable suspension suspension in water water. Naturally Naturally,, the ionic concentration concentration must be kept low as the stability decreases with increasing ionic concentration (Fig (Figur uree 2), 2), and and the the pH rang rangee must must be adap adapte ted d so that that the the oxid oxidee does does not not diss dissol olve ve.. For example, yttria dissolves readily in an acid aqueous environment. Howe Howeve verr, the use use of wate waterr-ba base sed d susp suspen ensi sions ons caus causes es a numb number er of proble problems ms in electr electropho ophoret retic ic formin forming g (1). (1). Electr Electrolys olysis is of water water occurs occurs at low low voltag voltages es ( ∼5V), and therefore gas evolution at the electrodes is inevitable at field strengths high enough to give reasonably reasonably short deposition times. If electrophoresi electrophoresiss is used to form objects, the inclusion of gas bubbles in the deposit can be prevented by deposi depositing ting on a porous porous membra membrane ne placed placed before before the electr electrode ode.. Howe However ver,, curren currentt densities are higher compared with non-aqueous media, which leads to Joule heating heating of the suspension suspension and sometimes sometimes loss of stability of the suspension. suspension.














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Organic liquids generally have a lower dielectric constant, which is a disadvantage vantage because it limits the charge charge on the particles: particles: The dielectric dielectric constant expresses expresses the dissociating dissociating power of the solvent. solvent. Howev However er,, much higher field strengths (∼100–1000 V/cm) can be used because the problem of electrolytic gas evolution evolution and Joule heating heating are greatly reduced. reduced. Therefore, Therefore, organic organic liquids are generally preferred over water for electrophoretic deposition, although papers describing successful electrophoretic deposition from aqueous suspensions continue continue to be published published (25, (25, 43, 44, 62). The fact fact that water water is much much less expensive and less harmful for the environment is a driving force toward its use. Within the range of non-aqueous media, alcohols are similar to water, i.e. both both can can be rega regard rded ed as neut neutra rall amphi amphipr proti oticc solv solven ents ts.. The hydro hydroge gen n ion ion conc concen en-tration continues to be a measure of the acidity of the medium (63). Therefore, the acid/base chemistry upon changing from water to alcohols does not change drastically, and consequently many of the principles explained above remain valid. A stable suspension can be obtained by adjusting the pH value to the stable range. A recent paper by Wang et al (63) describes how the electrophoretic mobility of alumina in ethanol goes from positive at low pH to negative at high pH, completely analogous to the behavior in water. They also showed that electrostatic stabilization in ethanol is possible. The electrical conductivity rises steeply with the dielectric constant (64), making suspensions in methanol and ethanol relatively susceptible to ohmic heat heating ing.. There Therefor fore, e, the the curr curren entt shoul should d be limit limited ed eithe eitherr by worki working ng at cons consta tant nt-applied applied current or by limiting the voltage voltage that is applied. Ketones, ethers, and hydrocarbons, on the other hand, are known as aprotic solvents. Their dielectric constant and electrical conductivity are generally low Ketones Ketones have a dielectric dielectric constant close to 15, whereas whereas for ethers and













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(69). Electron Electron donicity is defined as a measure for the tendency tendency of a solvent to donate electrons. electrons. It is determined by measuring measuring the enthalpy enthalpy of reaction of the solvent with a reference acid (SbCl 5) in 1,2-dichloroe 1,2-dichloroethane. thane. The usefulness usefulness of the electron donicity for charging of solids can be illustrated using the example of the degree of ionic dissociation of a solute in a series of donor solvents. The ionizati ionization on process process can be broken up into into two steps. steps. The first involv involves es a base displacement whereby the solvent displaces the basic or anionic portion of the solute, giving a solvated ion pair: D : + A : B ⇔ ( D : A)+ (: B )− .

1.

The second step involves the separation of the ion pair to give a free solvated cation cation and anion. anion. The first first step is a function function of the electr electron on donicity donicity of the solvent. The second step is a function of the solvent’s local dielectric constant, and the resulting degree of dissociation, as measured by the solution’s conduct ductiv ivit ity y, is a func functi tion on of both. both. If A:B is thought thought to be a surf surfac acee group group of the powder, it is clear that charging of the surface through electron exchange should be possible. possible. Labib & Williams Williams (67, 68) proposed to attribute attribute a donicity value to the surface of solids (D S). If the solvent has a higher electron electron donicity donicity than the solid, electrons will be transferred from the solvent to the solid, resulting in a negativ negatively ely charged charged particle. particle. If the electron donicity donicity of the solvent is lower than the electron donicity of the solid, the particle will acquire a positive charge. charge. The values values of the zeta-potentia zeta-potentiall of dried SiO 2, TiO2, Al2O3, and MgO in a series of dried solvents with increasing electron donicity were consistent with this view. The sign of the zeta-potential in these solvents changed at some point within the donicity donicity scale. The interpolated interpolated value, value, where the sign of the zeta-potent zeta-potential ial changed, was assumed assumed to correspond correspond to the electron donicity of the solid. solid. The ranking ranking of the solids solids based on their electron electron donicit donicit lue














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between powder dispersability and the difference of the iep of the powder in water and the pK a or the pKb value of the solvent. Given the importance of the pH in determining the charging of powders in water, it was only logical that parameters describing the acidity/alkalinity of non-aqueous media were used in the first attempts to understand charging in non-aqu non-aqueou eouss media. media. Howev However er,, one import important ant differ differenc encee has not attrac attracted ted much much attention: attention: The charge on a powder in aqueous suspensions suspensions is not determined determined by the pH of the water before the suspension is prepared, but by the pH of the suspension suspension once prepared. prepared. This simple fact is not stressed stressed when dealing with aqueous suspensions because the pH is regarded as a tuneable experimental parameter parameter.. Indeed, Indeed, only limited amounts amounts of concentrated concentrated acids or bases are required in practice to tune the pH. Hence, changes in pH upon addition of a powd powder er to water ater are are give given n almo almost st no atte attent ntio ion. n. That That the the pH can can chan change ge subs substa tanntially and unpredictably unpredictably when a powder is added to water water becomes apparent if the natural pH values of several several powders powders are compared. The term natural pH of a powder is used in this review to describe the pH at which the suspension equilibrates when a powder is suspended in water. For example, in two batches of a nominally identical SiC powder, one had a natural pH of 3.2 at 20 g · L−1, whereas the other had a natural pH of 8.2 at 20 g · L−1. A more systematic investigation showed that simply washing powders in an acid or alkaline aqueous suspension for a short time suffices to adsorb acids or bases onto the powder, which results in a remarkable difference in charging and deposition behavior in acetone, acetone, ethanol, and iso-propyl iso-propyl alcohol alcohol (72, 73). As a result, the charge on the powder when added to a suspension medium will not only be correlated with the difference in pzc and the acidity of the medium, but also with the amount of acids and bases adsorbed on the powder as expressed by the natural pH. Even for a clean powder surface, the natural pH of a suspension tends toward the















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laye layerr of a part partic icle le in motio otion n is dis distort torteed so tha that the the doub double le laye layerr is thin thinne nerr ahead head of the particle. Further, they argue that since counter ions move along with the particle, their concentration in the deposit is much higher than normally would be in the suspension. suspension. Thus they probably recombine recombine with incoming co-ions to refo reform rm thei theirr orig origin inal al salt salts, s, whic which h resu result ltss in a thin thinni ning ng of the the doub double le laye layers rs of the the particles particles in the deposit. deposit. Thus an incoming particle particle with a thinner double layer layer meets with the particles in the deposit that also have a thinned double layer and as a result the repulsion is decreased substantially, allowing the particles to overcome this repulsion barrier. A type of suspension probably best documented with respect to the deposition mechanism is one in which the powder is charged positively by adsorption of metal ions on the powder. These metal ions are introduced in the suspension through addition of salts such as Mg(NO 3)2, La(NO3)3, Y(NO3)3, MgCl2, or AlCl3. If a current is passed through solutions of these salts, the formation of a hydroxide hydroxide (16, 81, 82) is observe observed. d. If the water water content content of such such solutions solutions in iso-propyl alcohol is limited to below approximately 5 vol%, the main electrochemical reaction product becomes an alkoxide for Mg(NO 3)2 or Al(NO3)3 (81). Analysis of deposits made from such suspensions has revealed that these hydroxides hydroxides also form during deposition deposition (14). Therefore, Therefore, it is likely that bind-













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changes near the electrode, the surface charge and the interparticle repulsion will will decrease decrease.. Thus the deposit deposition ion could could also also be induced induced by a shift in pH at the electrode. electrode. The fact that, in the absence of salts such as AlCl 3, deposits deposits can be formed on membranes placed away from the electrode is not necessarily a contradiction for such a mechanism. Sheppard (85) pointed out in his discussion of electrophoretic deposition of rubber on membranes that generally in such experiments the side of the membrane toward the anode becomes more alkaline, alkaline, whereas the side toward the cathode becomes more acidic. acidic. The concomitant increase in hydrogen ions is, in his opinion, a considerable factor in the deposition of rubbers on such membranes. Note that if deposition is caused by a change in pH, a critical time will be required before the pH has changed sufficient sufficiently ly.. Pierce Pierce (3) discussed discussed the cathodic deposition of resins quite similarly. He postulates that in order for a resin with amine groups to deposit, the hydroxyl ion concentration at the electrode should reach a critical value related to the solubility solubility product for the ammonium hydroxide: hydroxide: − RESIN − RESIN − NH + sp . 3 [OH ] = K sp





2.

Further, in the absence of migration and chemical reactions, the concentration of hydroxyl ions at the cathode will increase as the square root of time for an














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liquid. Moreover Moreover,, merely an increase increase in the concentration concentration of the powder near the the depos deposit it coul could d cause cause a decr decrea ease se in the the equil equilibr ibrium ium surf surfac acee char charge ge for for powd powder erss charge charged d through through proton proton adsorpt adsorption/ ion/dede-sor sorptio ption. n. Vandeper andeperre re (72) has calcul calculate ated d the the natu natura rall pH of a susp suspen ensi sion on of oxid oxidee powd powder erss as a func functi tion on of the the surf surfaace site site conc concen entr trat atio ion n in the the susp suspen ensi sion on and and foun found d that that the pH shif shifte ted d towa toward rd the point point of zero zero char charge ge with with incre increas asin ing g solid solidss load loading ing.. Furt Furthe herm rmor ore, e, the fact fact that that the double double layer of incoming particles is thinner ahead of the particles, as pointed out by Sarkar & Nicholson (80), can certainly also contribute to the incorporation in a deposit. Finally, if anything is clear, it is that almost any material can be laid down by EPD. The fact that in many reports on electrophoretic deposition little attention is given to the deposition mechanism clearly shows that even if a full understanding of the exact mechanism is lacking, electrophoretic deposition is already being used successfully. However, a better understanding is needed to decrease the amount of experimental work required to determine the optimal composition of a suspension and the deposition parameters such as applied voltage voltage or current current .













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inter-elec inter-electrode trode distance are thus: The potential drops at each of the electrodes electrodes should be negligible, negligible, and the specific resistance resistance of the deposit deposit should be of the same order as the specific resistance of the suspension. The validity of the approximation for the electric field strength is rendered even more doubtful by the fact that at constant applied potential, the current is frequently found to decrease decrease during deposition. The following following explanations explanations have have been proposed: proposed: a change in polarization polarization at the electrode electrode (87, 88), a higher specifi specificc resist resistanc ancee of the deposit deposit compar compared ed with with the suspensi suspension on (17, 30, 57, 89–92), and changes changes in the conductivi conductivity ty of the suspension suspension (72, 93). Starting at the electrode, one should realize that in order for a net current to flow through the solid/liquid solid/liquid interface, interface, a potential potential in excess excess of the equilibrium potential is required (activation overvoltage). The relation between the excess













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growth of the deposit, were linear with time, they concluded that this result was proof for the higher specific specific resistance resistance of the deposit. Howev However er,, changes changes in the concentration overvoltage at the cathode would have given the same resu result lt.. In fact fact,, Sark Sarkar ar et al also also obse observ rved ed a tran transi sien entt effe effect ct when when the the volta oltage ge was disconnecte disconnected d for 30 s and the experiment experiment continued continued afterward: afterward: The increase increase in resistance resistance had disappeared disappeared while the deposit deposit was still on the electrode. electrode. It was only after about 30 s that the potential required for the constant applied current reached the value it had reached before the experiment was interrupted. Such a transient effect is more easily understood if the increase in potential drop is caused by concentratio concentration n overvoltage. overvoltage. During the time the experiment experiment is paused, the formed concentration gradients can be relieved by diffusion, and the concentration overvoltage needs to be built up once again.














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full potential of EPD in making controlled microstructures is perhaps not yet known; known; e.g. by using special electrode electrode configuratio configurations, ns, deposition can be enhanced hanced locally to make wavy multilayer multilayers. s. Suspension Suspension preparation preparation for EPD, whether for aqueous or non-aqueous media, can be based on acid/base chem-













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Electrophoretic Deposition of Materials

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