Electrophoretic Deposition of Materials

P1: APR/SPD P2: APR/ARY QC: APR

May 15, 1999 14:52 Annual Reviews AR086-11

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

ELECTROPHORETIC DEPOSITIONOF MATERIALS

Omer O. Van der Biest and Luc J. VandeperreDepartement Metaalkunde en Toegepaste Materiaalkunde, Katholieke UniversiteitLeuven, De Croylaan 2, 3001 Heverlee, Belgium;e-mail: [email protected]

KEY WORDS: mechanism, kinetics, applications, suspensions

ABSTRACT

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

INTRODUCTION

Electrophoretic deposition (EPD) is essentially a two-step process. In the firststep, particles suspended in a liquid are forced to move toward an electrodeby applying an electric field to the suspension (electrophoresis). In the secondstep, the particles collect at one of the electrode and form a coherent depositon it. It should be noted that the process yields only a powder compact, andtherefore electrophoretic deposition should be followed by a densification stepsuch 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 ifthey carry a charge. Four mechanisms have been identified by which the chargeon the particles can develop (1): (a) selective adsorption of ions onto the solidparticle from the liquid, (b) dissociation of ions from the solid phase into theliquid, (c) adsorption or orientation of dipolar molecules at the particle surface,

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328 VAN DER BIEST & VANDEPERRE

and (d) electron transfer between the solid and liquid phase due to differencesin work function.

A charged particle in a suspension is surrounded by ions with an oppositecharge in a concentration higher than the bulk concentration of these ions; thisis the so-called double-layer (Figure 1). When an electric field is applied, theseions and the particle should move in opposite directions. However, the ions arealso attracted by the particle, and as a result, a fraction of the ions surroundingthe particle will not move in the opposite direction but move along with the

Figure 1 Schematic of the double layer surrounding a charged particle and evolution of the electricpotential from the surface potential,ψ0, to zero far from the particle. The potential at the surfaceof shear, the limit between the liquid moving with the particle and the liquid, which does notmove with the particle, is termed the zeta-potential,ψζ , and is the main parameter determining theelectrokinetic behaviour of the particle.



ELECTROPHORETIC DEPOSITION 329

particle. Hence, the speed of a particle is not determined by the surface chargebut by the net charge enclosed in the liquid sphere, which moves along withthe particle. The potential at the surface of shear is termed the zeta-potential orelectrokinetic potential. In principle, a particle with a negative surface chargecan show a positive zeta-potential. The latter occurs, for example, when thecharge of specifically adsorbed ions is higher than the surface charge.

In fact, the equilibrium speed of the particle is determined by four forces act-ing on the particle. The first, which accelerates the particle, is the force causedby the interaction of the surface charge with the electric field. All other forcesslow the particle: These are viscous drag from the liquid following Stoke’s law,the force exerted by the electric field on the counter-ions in the double layer (re-tardation) and, when a particle moves, the distortion in the double layer causedby a displacement between the center of the negative and positive charge, (re-laxation). A more complete treatment of these forces, along with informationon calculations of their magnitudes, can be found, for example, in Reference 2.

In this paper, we first present a number of applications of electrophoreticdeposition. Next, the preparation of suspensions for electrophoretic deposi-tion is discussed, followed by a synthesis of the available information on themechanisms of electrophoretic deposition and the kinetics of the process.

APPLICATIONS

Electrophoretic deposition can be applied to any solid that is available in theform of a fine powder (<30µm) or a colloidal suspension. Examples of elec-trophoretic deposition of materials of almost any material class can be found,including metals, polymers, carbides, oxides, nitrides, and glasses (see Table 1).Furthermore, the process can be used for producing coatings, for shaping mono-lithic, laminated and graded free-standing objects, and for infiltration of porousmaterials and woven fiber preforms for composite production.

CoatingsElectrophoretic deposition of coatings has already gained a world-wide accep-tance for automotive, appliance, and general industrial (organic) coatings (3).For example, the construction and operation of an 800 bodies-per-hour cat-aphoretic coating plant by a German car builder was described recently (4).The high level of automation, low levels of pollution, high throw power andhomogeneity of the coating are the advantages that have led to the use of elec-trophoretic deposition (3). Other advantages are the improved adherence ofelectrophoretically deposited coatings and their higher density compared withdipped or sprayed coatings (5).




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

Material class Examples

Acids and hydroxides Calcium hydroxide (91), magnesium hydroxide (91), antimonic acid(96), boric acid (52)

Borides LaB6 (97)

Carbides B4C (36), C (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)

Metals Al (83, 97, 104), Al-Cr (65), Al-Si (65), Al-Ti (65), Au (88, 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)

Organic materials Starch (47), styrene-acrylic copolymer (106), latex (3, 85), vinylcopolymers (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)

aNumbers in parentheses are references

Throw power is defined as the ability to cover recessed portions of complexlyshaped parts. This property, combined with improved thickness homogeneity,is a decided asset for application in the enamel industry (6–11). The claim thatrelatively complex shapes can be coated is clearly shown by some examplesgiven by Ortner (12): These include taps coated with carbides, fasteners coatedwith Ni, and metal cones coated with a ceramic glaze. Platinum grids havebeen coated completely with ferrite (13).

Other examples of coatings made by electrophoretic deposition include thedeposition of phosphor coatings in the manufacture of screens for cathode raytubes (CRTs) for advanced display applications (14–16), deposition of insulat-ing glass for electronic applications (17), and superconductive coatings (18–23).For the latter, an automated process has enabled the coating of wires of kilome-ter length (24). By performing electrophoretic deposition in a magnetic field,some authors were able to create a desirable texture for superconductance inthe c oatings (20, 21).




Shaping Free-Standing ObjectsMONOLITHS For traditional ceramics, such as sanitary ware, the main advan-tage of electrophoretic deposition lies in its higher speed and in the low wearof the moulds compared with slip casting (25). Just as complexly shaped ob-jects can be coated using EPD, so too can it be used for shaping objects. Tiles,closed and open end tubes, hemispheres, tubes with changes in diameter, andconical sections are some of the shapes that have been made, albeit mainly on alaboratory scale, using solid compositions normally used for table and sanitaryware (25–28).

An example of a hybrid process, somewhere between tape casting and elec-trophoretic deposition, is the ELEPHANT process (29). Continuous tapes ofceramic material are made by depositing on two rolling cylinders, which thenpress the two slabs together in one long tape. The process was developedfor the tile industry. After electrophoretic deposition, the tape is cut, punched,dried, and sintered. The economic advantages stem mainly from the lower man-power requirements, low-energy consumption, low wear (of moulds) and lowmaintenance costs (29).

Technical ceramics such as alumina (Al2O3) (30–33), silicon carbide (SiC)(34–36), and aluminium nitride (AlN) (37, 38) can also be shaped by elec-trophoretic deposition.

Beta-alumina tubes, used as electrolyte in sodium sulfur batteries, are a clas-sical example of the use of electrophoretic deposition. Whereas charging formany ceramics is related to adsorption or desorption of hydrogen ions, thepreferential dissolution of sodium ions from the beta-alumina during millingis the main factor leading to charging of such aluminas (39, 40). When thesebeta-alumina tubes were studied, it was discovered that the difference in den-sity between two or more powders used to produce a material was unimportant,and a homogeneous composition could be ensured. Powers (40) showed thisby comparing the composition of the first and the seventh tubes depositedfrom a suspension containing a beta-alumina powder with a low soda con-tent (8% Na2O) with one with a higher soda content (14–25%) also containingvarious additives such as MgO and Li2O.

LAMINATED MATERIALS Layered materials can also be produced via EPD.When the desired thickness of the first layer is reached, the deposition elec-trode can be moved to a second suspension for deposition of a layer of differentcomposition. By changing back and forth, a layered material is readily ob-tained. Nicholson et al (41) have produced ZrO2/Al2O3 laminates with aluminalayers as thin as 12µm and zirconia layers of 2µm from ethanol-based sus-pensions. The boundaries between the layers were straight and well defined,




showing that although EPD can be a fast process, good control of the growth ofthe layers can be obtained. The same group also produced alumina/lanthanumaluminate laminates (42). Ferrari et al (43) and Fischer et al (44) produced alu-mina/zirconia laminates from aqueous suspensions. Vandeperre & Van der Biestmade a range of SiC-based laminates with graphite (45), SiC+graphite (46),and porous SiC interlayers (47). They also produced laminated SiC/graphitecomposite tubes (35), thus combining the ability of EPD to produce laminatedmaterials with the shaping capabilities of the process.

GRADED MATERIALS While layered materials are obtained by immersing thedeposition electrode in different baths, graded materials can also be made bygradually changing the composition of the suspension from which EPD is car-ried out. Sarkar et al demonstrated the ability to form graded materials by slowlyadding an ethanol-based suspension of an alumina powder to an ethanol-basedsuspension of an yttria-stabilized zirconia powder during deposition (48). Agradual increase in the alumina content of the deposit was observed. Later,the same group also produced Al2O3/MoSi2- and Al2O3/Ni-graded materials(49). Chao et al produced alumina/ceria-stabilized zirconia-graded rods froman acetone-based suspension (50).

UNIQUE MICROSTRUCTURES AND SELECTED DEPOSITION The full potential ofEPD in producing unique microstructural features is perhaps not yet realized.Nicholson et al (51) did some exploratory work in making nonplanar laminates:By placing a grid before the deposition electrode, they were able to producelaminates with wavy interlayers. Thus by using auxiliary electrodes, depositioncan be enhanced locally. Scala & Sandor (52) have deposited B2O3 on siliconwafers as a boron diffusion source. The deposition onto the entire silicon waferwas prevented by growing a silica film on the wafer and etching the silica awayonly where deposition of boron was required.

InfiltrationThe throwing power of electrophoretic deposition can also be used to infiltrateobjects with a matrix material or to apply an internal coating. Gal-Or et al (53)infiltrated porous graphite electrodes with silicon carbide and silicon oxideparticles. For a good infiltration, the particle buildup in an external coatingshould be decreased so that the pores are not blocked. Lower applied voltagesand a high-particle concentration in the suspension improved the infiltration.Ishihara et al (54) applied an internal coating of yttria-stabilized zirconia toa porous Ni-CaO stabilized ZrO2 cermet containing 40 wt% Ni for a solidoxide fuel cell. By repetitive infiltration and sintering, a dense film of uniformthickness could be applied.




Another application where electrophoretic deposition is gaining increasinginterest is the infiltration of fiber preforms with matrix material for compos-ite production (55–59). For conducting fibers, the preform is used directlyas the deposition electrode, whereas for nonconducting fibers, the preform ispositioned in front of an electrode. The advantage of using electrophoretic de-position for composite fabrication is that it allows a reduction of the costs com-pared with, for example, chemical vapor infiltration. Moreover, electrophoreticdeposition is much faster, and the composition of the matrix material can becontrolled quite easily, as is shown by the example of infiltration of woven SiCfiber mats by a mixed sol of mullite composition (58).

GUIDELINES FOR SUSPENSION PREPARATION

Suspensions used in other colloidal-shaping techniques, such as slip castingand tape casting, can serve as a starting point for development of suspensionsfor electrophoretic deposition. However, in contrast to many of these colloidalprocesses, suspensions with much lower solids loading can be used for EPD. Theresulting lower viscosity is an advantage in handling the suspensions. Greendensities of about 40–60 vol% are obtained via EPD from suspensions withsolids loadings as low as 1–2 vol% (34).

The key to successful development of suspensions for electrophoretic de-position is to find a systematic approach to making suspensions in which theparticles have a high zeta-potential, while keeping the ionic conductivity ofthe suspensions low. A necessary but not sufficient condition for a high zeta-potential is a high surface charge. As mentioned above, four mechanisms havebeen identified for charging (1): selective adsorption of ions onto the solidparticle from the liquid, dissociation of ions from the solid phase into the liq-uid, adsorption or orientation of dipolar molecules at the particle surface, andelectron transfer between the solid and liquid phase due to differences in workfunction.

For most oxides in the presence of water, the charge will be determinedmainly by adsorption or desorption of protons. A comprehensive model todescribe the charging of oxides in water has been developed by James et al(60). They describe the charging as chemical reactions of surface groups, whichcan be acid, alkaline, or amphoteric in nature. The value of the pH, where theconcentration of negatively and positively charged surface groups is equal, istermed the point of zero charge (pzc) and is a property of the chemical groupson the surface of the powder. The pzc should not be confused with the iso-electric point (iep), which is the pH at which the powder does not move whenan electric field is applied in a given suspension. The pzc and iep can differ




when, for example, multivalent metal ions, which adsorb specifically on thesurface, are added to the suspension (61).

Because of the lower solids loading, simply changing the pH to a value farfrom the pzc using concentrated acids or bases is generally all that is requiredfor obtaining a stable suspension in water. Naturally, the ionic concentrationmust be kept low as the stability decreases with increasing ionic concentration(Figure 2), and the pH range must be adapted so that the oxide does not dissolve.For example, yttria dissolves readily in an acid aqueous environment.

However, the use of water-based suspensions causes a number of problems inelectrophoretic forming (1). Electrolysis of water occurs at low voltages (∼5 V),and therefore gas evolution at the electrodes is inevitable at field strengths highenough to give reasonably short deposition times. If electrophoresis is used toform objects, the inclusion of gas bubbles in the deposit can be prevented bydepositing on a porous membrane placed before the electrode. However, currentdensities are higher compared with non-aqueous media, which leads to Jouleheating of the suspension and sometimes loss of stability of the suspension.

Figure 2 Interaction energy (attraction is negative) between two particles surrounded by a dou-ble layer for a surface potential of 25 mV, a particle radius of 0.35µm, a Hamaker constant of10−20 J, and a relative dielectric constant of 20, for various concentrations of a background elec-trolyte (indicated in the figure). With increasing electrolyte concentration, the height of the repul-sion barrier due to double layer overlap decreases, and the repulsion vanishes completely at evenhigher electrolyte concentration.




Organic liquids generally have a lower dielectric constant, which is a disad-vantage because it limits the charge on the particles: The dielectric constantexpresses the dissociating power of the solvent. However, much higher fieldstrengths (∼100–1000 V/cm) can be used because the problem of electrolyticgas evolution and Joule heating are greatly reduced. Therefore, organic liq-uids are generally preferred over water for electrophoretic deposition, althoughpapers describing successful electrophoretic deposition from aqueous suspen-sions continue to be published (25, 43, 44, 62). The fact that water is much lessexpensive and less harmful for the environment is a driving force toward itsuse.

Within the range of non-aqueous media, alcohols are similar to water, i.e.both can be regarded as neutral amphiprotic solvents. The hydrogen ion concen-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 changedrastically, and consequently many of the principles explained above remainvalid. A stable suspension can be obtained by adjusting the pH value to the sta-ble range. A recent paper by Wang et al (63) describes how the electrophoreticmobility of alumina in ethanol goes from positive at low pH to negative athigh pH, completely analogous to the behavior in water. They also showed thatelectrostatic stabilization in ethanol is possible.

The electrical conductivity rises steeply with the dielectric constant (64),making suspensions in methanol and ethanol relatively susceptible to ohmicheating. Therefore, the current should be limited either by working at constant-applied current or by limiting the voltage that is applied.

Ketones, ethers, and hydrocarbons, on the other hand, are known as apro-tic solvents. Their dielectric constant and electrical conductivity are generallylower. Ketones have a dielectric constant close to 15, whereas for ethers andhydrocarbons the dielectric constant is lower than 10. An empirical study onelectrophoretic deposition of beta-alumina from a series of organic solventsshowed that a dielectric constant within the range of 12 to 25 was required forelectrophoretic deposition (64). The dissociation power of solvents with a lowerdielectric constant is probably not enough to allow for a reasonable charge toform on the particles, whereas deposits from solvents with dielectric constantshigher than 25 were found experimentally to be more fluid and slide off theelectrodes more easily (64). An explanation was not provided, and it shouldbe noted that electrophoretic deposits also have been made from suspensionsin water (43, 44, 65) (dielectric constant of water is 78.5) (66).

Labib & Williams (67, 68) found that electron transfer from the solid tothe liquid phase or vice versa can indeed determine the charge of the powderin non-aqueous media. They measured the zeta-potential of a number of driedoxides in a series of dried solvents with different values for the electron donicity




(69). Electron donicity is defined as a measure for the tendency of a solvent todonate electrons. It is determined by measuring the enthalpy of reaction of thesolvent with a reference acid (SbCl5) in 1,2-dichloroethane. The usefulness ofthe electron donicity for charging of solids can be illustrated using the exampleof the degree of ionic dissociation of a solute in a series of donor solvents.The ionization process can be broken up into two steps. The first involves abase displacement whereby the solvent displaces the basic or anionic portionof 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 solvatedcation and anion. The first step is a function of the electron donicity of thesolvent. 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 con-ductivity, is a function of both. If A:B is thought to be a surface group ofthe powder, it is clear that charging of the surface through electron exchangeshould be possible. Labib & Williams (67, 68) proposed to attribute a donicityvalue to the surface of solids (DS). If the solvent has a higher electron donicitythan the solid, electrons will be transferred from the solvent to the solid, result-ing in a negatively charged particle. If the electron donicity of the solvent islower than the electron donicity of the solid, the particle will acquire a positivecharge. The values of the zeta-potential of dried SiO2, TiO2, Al2O3, and MgOin a series of dried solvents with increasing electron donicity were consistentwith this view. The sign of the zeta-potential in these solvents changed at somepoint within the donicity scale. The interpolated value, where the sign of thezeta-potential changed, was assumed to correspond to the electron donicity ofthe solid. The ranking of the solids based on their electron donicity valuescompared very well with the ranking of the iep in water, i.e. from basic to acidthe ranking was MgO, Al2O3, TiO2, and SiO2. When a limited amount of waterwas introduced in the system by equilibrating the solids in an environment with50% relative humidity, an increase in the donicity of the surfaces was observed.This was attributed to the fact that adsorption of water molecules on the sur-face makes Lewis acids much weaker and in some cases changes their identity.This last result indicates that if water is present, exchange of protons becomesmore important compared with electron exchange as a charging mechanism innon-aqueous solvents.

Okuyama et al (70) dispersed a series of silicon carbide powders in a widerange of non-aqueous media. They found that the powder dispersability, as de-termined from its sedimentation behavior, did not correlate well with dielectricconstant, pKa, hydrogen bond index, and Lewis acid/base interaction. How-ever, in work by Bolger (71) on oxide powders, a good correlation was found




between powder dispersability and the difference of the iep of the powder inwater and the pKa or the pKb value of the solvent.

Given the importance of the pH in determining the charging of powders inwater, it was only logical that parameters describing the acidity/alkalinity ofnon-aqueous media were used in the first attempts to understand charging innon-aqueous media. However, one important difference has not attracted muchattention: The charge on a powder in aqueous suspensions is not determinedby the pH of the water before the suspension is prepared, but by the pH of thesuspension once prepared. This simple fact is not stressed when dealing withaqueous suspensions because the pH is regarded as a tuneable experimentalparameter. Indeed, only limited amounts of concentrated acids or bases arerequired in practice to tune the pH. Hence, changes in pH upon addition of apowder to water are given almost no attention. That the pH can change substan-tially and unpredictably when a powder is added to water becomes apparent ifthe natural pH values of several powders are compared. The term natural pHof a powder is used in this review to describe the pH at which the suspensionequilibrates when a powder is suspended in water. For example, in two batchesof 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 in-vestigation showed that simply washing powders in an acid or alkaline aqueoussuspension for a short time suffices to adsorb acids or bases onto the powder,which results in a remarkable difference in charging and deposition behavior inacetone, ethanol, and iso-propyl alcohol (72, 73). As a result, the charge on thepowder when added to a suspension medium will not only be correlated withthe difference in pzc and the acidity of the medium, but also with the amount ofacids and bases adsorbed on the powder as expressed by the natural pH. Evenfor a clean powder surface, the natural pH of a suspension tends toward thepzc of the powder (72). Figure 3 illustrates these two effects for a number ofcommercially available powders (74).

If it is likely that ions will leach from the solid to be deposited, as e.g. Na+

ions from beta-alumina (75), or barium ions from barium titanate powders (76),the design of the suspension must take into account that the ionic conductivitywill increase, that the solid might be charged through the dissolution of theseions, and that re-adsorption of these ions can also influence the zeta-potential.

MECHANISM OF DEPOSITION

Although electrophoretic deposition is an old process, the exact mechanismsthat allow a deposit to be formed are still not entirely clear. According toHamaker & Verwey (77), the formation of a deposit by electrophoresis is akin tothe formation of a sediment due to gravitation. The pressure exerted by incoming




Figure 3 The natural pH of a range of commercially available powders, at a powder loading of20 g · L−1, as a function of their point of zero charge in water. In general, the natural pH indeedtends to be close to the point of zero charge. Deviations are observed but can be attributed to thepresence of adsorbed acids or bases.

particles enables particles next to the deposit to overcome the interparticlerepulsion.

Koelmans (78) calculated the expected increase in ionic strength next to anelectrode and found that the ionic strength was of the same order as required toflocculate a suspension. In his view, the interparticle repulsion decreases and theparticles collapse in a deposit due to the increase in the electrolyte concentration.Since a finite time is required for the electrolyte concentration to build up nearthe electrode, one expects that deposition would not occur before a certain timehas passed. Indeed, although MgCO3 particles suspended in methanol werefound to collect on the electrode as soon as a potential was applied, only aftersome critical time did these particles remain on the electrode after the voltagewas cut off (78). The magnitude of the critical time is inversely proportionalto the applied voltage to the second power (78). Hence, the critical time can bevery short, which explains why it is frequently not observed.

Grillon suggested that particles would neutralize upon contact with the depo-sition electrode or deposit (79). Sarkar & Nicholson (80) note that the double




layer of a particle in motion is distorted so that the double layer is thinner aheadof the particle. Further, they argue that since counter ions move along with theparticle, their concentration in the deposit is much higher than normally wouldbe in the suspension. Thus they probably recombine with incoming co-ions toreform their original salts, which results in a thinning of the double layers of theparticles in the deposit. Thus an incoming particle with a thinner double layermeets with the particles in the deposit that also have a thinned double layerand as a result the repulsion is decreased substantially, allowing the particles toovercome this repulsion barrier.

A type of suspension probably best documented with respect to the deposi-tion mechanism is one in which the powder is charged positively by adsorptionof metal ions on the powder. These metal ions are introduced in the suspensionthrough addition of salts such as Mg(NO3)2, La(NO3)3, Y(NO3)3, MgCl2, orAlCl3. If a current is passed through solutions of these salts, the formation ofa hydroxide (16, 81, 82) is observed. If the water content of such solutions iniso-propyl alcohol is limited to below approximately 5 vol%, the main elec-trochemical reaction product becomes an alkoxide for Mg(NO3)2 or Al(NO3)3(81). Analysis of deposits made from such suspensions has revealed that thesehydroxides also form during deposition (14). Therefore, it is likely that bind-ing by these hydroxides is the mechanism of deposition. Hydroxides provide abetter adhesion than alkoxides (81), and a limited amount of water is thus an ad-vantage for obtaining well-adhering coatings. Consequently, the deposition ofthe powder is induced by an electrochemical reaction at the electrode, and onewould expect that no deposition would occur away from an electrode. Indeed,Brown & Salt (83) had already observed that an aluminium powder chargedthrough addition of AlCl3 to ethanol did not deposit on a porous membraneplaced away from the electrode, whereas a deposit could be obtained from thissuspension on the cathode.

However, in the absence of such salts, electrophoretic deposition can oc-cur away from the electrode. For example, magnesium oxide suspended inmethanol was found to deposit on a porous membrane placed before the elec-trode (83), and aluminium oxide particles suspended in ethanol with additionof an acid deposited on a dialysis membrane placed before a graphite cathode(84). Thus although the mechanism of hydroxide formation is supported witha substantial amount of experimental evidence, it clearly is not the only mecha-nism for formation of a deposit. When Koelmans (78) proposed his mechanismof flocculation (discussed above), he had already noted that electrolysis couldlead to insoluble or non-ionized products and therefore not to the increase inthe electrolyte concentration near the electrode required for his mechanism.However, he also argued that for oxides, protons and hydroxyl ions are po-tential determining ions. Therefore, if during electrolysis their concentration




changes near the electrode, the surface charge and the interparticle repulsionwill decrease. Thus the deposition could also be induced by a shift in pH atthe electrode. The fact that, in the absence of salts such as AlCl3, deposits canbe formed on membranes placed away from the electrode is not necessarily acontradiction for such a mechanism. Sheppard (85) pointed out in his discus-sion of electrophoretic deposition of rubber on membranes that generally insuch experiments the side of the membrane toward the anode becomes morealkaline, whereas the side toward the cathode becomes more acidic. The con-comitant increase in hydrogen ions is, in his opinion, a considerable factor inthe deposition of rubbers on such membranes. Note that if deposition is causedby a change in pH, a critical time will be required before the pH has changedsufficiently. Pierce (3) discussed the cathodic deposition of resins quite simi-larly. He postulates that in order for a resin with amine groups to deposit, thehydroxyl ion concentration at the electrode should reach a critical value relatedto the solubility product for the ammonium hydroxide:[

RESIN− NH+3][OH−] = Ksp. 2.

Further, in the absence of migration and chemical reactions, the concentrationof hydroxyl ions at the cathode will increase as the square root of time for anunstirred electrolyte at constant current density. Therefore, the product of thesquare root of the critical time and the current density is expected to be constantfor such systems, which is also found experimentally (3).

To summarize, there are many indications that in order for powder to beincorporated in the deposit, the charge on the surface must be reduced. Forpowders charged by adsorption or de-sorption of protons, a different pH inthe suspension, and near the deposition electrode or the membrane on whichdeposition is performed, could reduce the surface charge. For powders chargedby adsorption of metal ions, an electrode reaction converting these adsorbed ionsinto uncharged metal hydroxides is required. Hence, no deposition is observedon membranes in such systems. These hydroxides improve the coherence ofthe deposit and the adherence to the electrode. Therefore using salts to chargepowders is probably better for making coatings than for making free-standingobjects, which have to be released from the electrode. If other ways are used tocharge a powder, for example by adsorption of a poly-electrolyte (33), whetherdeposition has occurred will be determined by the decharging reactions requiredfor the surface groups of these poly-electrolytes.

Although the decharging due to changes in pH as a consequence of thecurrent is very appealing, it should be noted that such changes normally donot extend very far into the liquid. However, it is possible that the change inthe contribution of convection to the transport of ions to the electrode throughthe growth of the deposit causes the changes in pH to extend farther into the




liquid. Moreover, merely an increase in the concentration of the powder nearthe deposit could cause a decrease in the equilibrium surface charge for powderscharged through proton adsorption/de-sorption. Vandeperre (72) has calculatedthe natural pH of a suspension of oxide powders as a function of the surface siteconcentration in the suspension and found that the pH shifted toward the point ofzero charge with increasing solids loading. Furthermore, the fact that the doublelayer of incoming particles is thinner ahead of the particles, as pointed out bySarkar & Nicholson (80), can certainly also contribute to the incorporation in adeposit.

Finally, if anything is clear, it is that almost any material can be laid downby EPD. The fact that in many reports on electrophoretic deposition little at-tention is given to the deposition mechanism clearly shows that even if a fullunderstanding of the exact mechanism is lacking, electrophoretic deposition isalready being used successfully. However, a better understanding is needed todecrease the amount of experimental work required to determine the optimalcomposition of a suspension and the deposition parameters such as appliedvoltage or current .

KINETICS OF DEPOSITION

Single-Powder DepositionThe kinetics of electrophoretic deposition has been the subject of numer-ous investigations. By simply applying the principle of conservation of mass,Hamaker (77) proposed the following equation

dY

dt= f.µ.E.S.c. 3.

with Y, yield of deposition (kg);t , deposition time (s);µ, electrophoreticmobility (m2 · V−1 · s−1); S, surface area of the electrode (m2); c, solids con-centration (kg· m−3); and f a factor taking into account that not all powderbrought to the electrode by electrophoresis is also incorporated in the deposit( f = 1 or less). This equation is now accepted as describing the basis for thekinetics of electrophoretic deposition.

The surface area of the electrodes and the concentration of solids in a sus-pension can easily be determined or calculated during the process. The elec-trophoretic mobility can also be measured. Although a good qualitative notionof the electric field strength can be obtained through measurement (86), theelectric field strength is almost always evaluated from the applied voltage andthe inter-electrode distance. However, such a calculation is subject to condi-tions. The potential drop over an electrophoretic deposition cell with flat equal




Figure 4 Evolution of the potential in an electrophoretic deposition cell: The applied potential isconsumed by a potential drop at each electrode and by an ohmic potential drop over the suspensionand the deposit.

surface area electrodes can in general be described as consisting of four terms(Figure 4):

Va = 1φ1+ I .Rd.d1+ I .Rs.(d − d1)+1φ2, 4.

where Va is the applied potential (V); I is the current (A); Rd and Rs are theresistance per unit length of the deposit and of the suspension, respectively(Ohm/m); d is the electrode separation (m); d1 the thickness of the deposit(m); and1φi is the potential drop at one electrode (V). The resistance perunit length is defined as the specific resistance (Ohm/m) divided by the surfacearea of the electrodes (m2). The conditions required in order for the electricfield strength in a suspension to be close to the applied potential divided by the




inter-electrode distance are thus: The potential drops at each of the electrodesshould be negligible, and the specific resistance of the deposit should be of thesame order as the specific resistance of the suspension.

The validity of the approximation for the electric field strength is renderedeven more doubtful by the fact that at constant applied potential, the currentis frequently found to decrease during deposition. The following explanationshave been proposed: a change in polarization at the electrode (87, 88), a higherspecific resistance of the deposit compared with the suspension (17, 30, 57,89–92), and changes in the conductivity of the suspension (72, 93).

Starting at the electrode, one should realize that in order for a net current toflow through the solid/liquid interface, a potential in excess of the equilibriumpotential is required (activation overvoltage). The relation between the excesspotential over the interface and the resulting net current is exponential (94).Therefore, small variations in activation overvoltage (order of 1 V) result inlarge changes in current. Hence, the activation overvoltage can be expectedto remain more or less constant during the process when depositing from non-aqueous suspensions, where the applied voltage is typically of the order of100–1000 V. A second effect to be taken into account is that according toNernst’s law, the equilibrium potential over a solid/liquid interface depends onthe concentration of the reactants (94):

1φ = 1φ0+ RT

zFln

(aox

ared

)= 1φ0+ ηc, 5.

where1φ0 is the standard equilibrium potential drop over the solid/liquid in-terface (V),1φ is the equilibrium potential drop over the solid/liquid interface(V), aox is the activity of the species being oxidized,ared, is the activity ofthe species being reduced,R is the gas constant (J· mol−1 · K−1), T is thetemperature (K),z is the valence of the reduction reaction, and F is Faraday’sconstant (C· mol−1). Changes in polarization due to a change in concentrationof the reactants for the electrode reaction are often referred to as concentrationovervoltage (ηc). If during electrophoretic deposition the transport of reactantsfor the electrode reaction toward the electrode is slower than the consumptionat the electrode, the concentration at the electrode will drop and the concen-tration overvoltage will increase. Note that changes in the proton or hydroxilconcentration were expected and observed in some of the investigations intothe mechanism of deposition (3). Moreover, the fact that a deposit forms onone of the electrodes can be expected to hinder the transport of reactants towardthat electrode, thus enhancing the probability for an increase in concentrationovervoltage (see Figure 5). Cathodic deposits are especially prone to seriouschanges in concentration overvoltage. Species capable of being reduced musttravel from the solution toward the cathode. At the anode, one of the electrode




Figure 5 Illustration of a possible influence of the growing deposit on polarization at the electrode.The concentration of the reactant for the electrode reaction C as a function of the distanceX fromthe electrode is shown for four conditions: (a) Before any potential is applied, no concentrationgradients exist. (b) As soon as a current starts to flow, the concentration of the species reacting at theelectrode drops by a small amount, and the amount consumed at the electrode is replaced throughdiffusion, convection and migration. (c) The presence of the deposit influences the transport ofreactants to the electrode. If this results in a decreased incoming flux, the concentration at theelectrode will start to drop. (d) Because the diffusion flux is determined by the slope of theconcentration gradient, the diffusion term in the flux decreases with increasing deposit thickness

reactions can be the dissolution of the electrode material. Naturally, in that casethe reactant concentration for the oxidation reaction will not drop. Some typesof electrodes, however, can also suffer from a serious increase of the poten-tial drop at the electrode, even when they are used as an anode. For example,aluminum, when polarized anodically, does not dissolve but forms a protectiveoxide layer. Not only does this layer introduce an extra electrical resistance,the oxidation of new aluminium becomes increasingly more difficult as oxygenhas to travel through the oxide layer. Hence, other oxidation reactions can berequired, and these can require the transport of species from the solution to theanode. Note that even when the deposit forms on the anode, an increase incathodic concentration overvoltage can also occur. For example, during anodicdeposition of SiC from acetone, Vandeperre & Van der Biest (72) found that thepotential drop at the cathode increased during deposition, resulting in a drasticincrease of the total resistance of the deposition cell (Figure 6).




Figure 6 (a) Evolution of the apparent resistance (applied potential divided by the measuredcurrent, Rapp) during deposition of SiC from acetone and of the resistance of the suspension, Rsus,as determined from a measured difference in potential drop between two reference electrodesimmersed in the cell and the measured current. Initially the magnitude of the current is controlledby the resistance of the suspension, but after a few minutes, concentration polarization at thecounter electrode becomes very important, and the linear relation between amount of powder thathas deposited and the apparent resistance is lost. Initially the magnitude of the current is controlledby the resistance of the suspension, but after a few minutes, concentration polarization at thecounter electrode becomes very important, and the linear relation between amount of powder thathas deposited and the apparent resistance is lost. (b) Evolution of the potential drop over thesuspension (Vsusp), at the counter electrode (Vcount) and at the deposition electrode (Vdep) for thesame deposition experiment

In regard to the conductivity of the suspension and the deposit, many authorsclaim that the deposit has a much higher specific electrical resistance than doesthe suspension. In this review the term resistance of the deposit is used onlyto indicate the ohmic potential drop over the deposit as defined in Equation 4and does not include possible effects of deposit formation on the concentrationovervoltage discussed above.

Most authors have simply inferred the higher specific resistance of the depositfrom the decrease in current (17, 30, 57, 89–92). Sarkar et al (86) measured theelectric field strength by measuring the potential drop between two platinumprobes placed between the electrodes and also monitored the potential dropbetween the deposition electrode and one platinum probe in the suspension.During cathodic deposition of alumina from ethanol at constant applied current,these authors found that the electric field strength remained constant and that thepotential drop between the deposition electrode and the probe in the suspensionincreased at the same rate as the voltage required to keep the current constantincreased. Since both the required increase of applied voltage, as well as the




growth of the deposit, were linear with time, they concluded that this resultwas proof for the higher specific resistance of the deposit. However, changesin the concentration overvoltage at the cathode would have given the sameresult. In fact, Sarkar et al also observed a transient effect when the voltage wasdisconnected for 30 s and the experiment continued afterward: The increasein resistance had disappeared while the deposit was still on the electrode. Itwas only after about 30 s that the potential required for the constant appliedcurrent reached the value it had reached before the experiment was interrupted.Such a transient effect is more easily understood if the increase in potentialdrop is caused by concentration overvoltage. During the time the experimentis paused, the formed concentration gradients can be relieved by diffusion, andthe concentration overvoltage needs to be built up once again.

It should be noted that a higher specific resistance of the deposit comparedwith the suspension is indeed expected. In a deposit, the volume fraction ofthe liquid is lower than in the suspension, and therefore the effective area forionic conductance is lower (17). Pierce (3) found a high resistance for denselayers of a cathodically applied polymeric primer. If the deposit was a porousfilm, however, as sometimes observed during the electrodeposition of latex, thevoltage drop over the film remained very low due to the conductance throughthe pores in the film (3). Since during deposition of ceramic or metal powdersthe volume fraction of solids in the deposit is typically between 40 and 60vol%, the resistance of the deposit cannot be many times higher than the ionicresistance of the suspension.

The resistance of the suspension is also not necessarily constant. Vandeperre& Van der Biest (93) modeled the conductivity of a suspension as consist-ing of a contribution of ions outside the double layers of the powder particlesand a contribution by the electric charges associated with the moving particles(Figure 7). The decrease in current during electrophoretic deposition of SiCfrom acetone with addition ofN-butylamine was found to be mainly from a de-crease in suspension conductivity (93) caused by depletion of the suspension.Naturally, the decrease in conductivity of the suspension through this mech-anism is observed only if the amount of powder deposited is large comparedwith the total amount of powder in suspension and if the ionic conductivity isrelatively small. Moreover, another factor contributing to the lower conductiv-ity of the deposit compared with the suspension is that while in the suspensionthe movement of the particles contributes to the conductivity, in the deposit theparticles are immobile and therefore do not contribute to the conductivity of thedeposit. Figure 8 shows the evolution of the current and yield as a function oftime for deposition of alumina (Baikowski-type SM8, 100 g· L−1) from tech-nical ethanol with addition of 1 M acetic acid (72), together with the calculatedcurrent and yield based on the description of the conductivity of suspensions as




Figure 7 Schematic diagram for the resistance of the deposition cell. In the suspension, movingions as well as moving particles contribute to the conductivity, which can be represented by tworesistors (Rpowder& Rion) in parallel. In the deposit, only ions are moving, hence one resistance,Rdep, in series with the resistance of the suspension. Finally, there is also a potential drop,1i , ateach electrode. The value of the resistance related to the contribution of the powder is a functionof the solid concentration.

being composed of a ionic component and a component related to the movementof the powder. The expected yield of deposition, if the decrease in current wasdue to changes in polarization or a very high specific resistance of the deposit,is also indicated.

Co-Deposition of Two or More PowdersIf two or more different powders are deposited simultaneously, and if the volumefraction of solids in the suspension is high, the powders deposit at an equal rate.




Figure 8 Measured evolution of current (filled symbols) and yield (open symbols) during elec-trophoretic deposition of alumina from technical ethanol with 1 M acetic acid. Thesolid lineswere calculated based on the assumption that the decrease in current is caused by the vanishingcontribution of the alumina particles to the conductivity of the suspension, whereas thedotted lineswere calculated assuming that the current decrease was the result of very high specific resistance ofthe deposit and no contribution of the powder to the conductivity. Although the calculated currentscoincide, the evolution of the yield is better described by the assumption that the conductivity ofthe suspension decreases.

If, however, the volume fraction of solids is low, the particles can deposit at arate proportional to their individual electrophoretic mobility (37, 38, 47, 95). Inwork by Moritz on deposition of aluminum nitride with incorporation of yttriaas a sintering additive, an equal deposition rate was observed from solids load-ing higher than 28 vol% (95). These observations are in accordance with thefindings by Massoud (25), who states that in slips for sanitary ware no segre-gation between the components was found. The suspension used in his workcontained a high volume fraction of solids.

CONCLUSIONS

EPD of any material available in the form of a fine powder or colloidal suspen-sion is possible. The technique allows the application of coatings, the shapingof objects, and the infiltration of porous structures and woven fiber preforms.Moreover, laminated and graded materials can also be made. It is a fast andeconomical process that can be automated for use on an industrial scale. The




full potential of EPD in making controlled microstructures is perhaps not yetknown; e.g. by using special electrode configurations, deposition can be en-hanced locally to make wavy multilayers. Suspension preparation for EPD,whether for aqueous or non-aqueous media, can be based on acid/base chem-istry. In water as well as non-aqueous media the charge can be controlled throughaddition of an acid or a base. Simply selecting solvents based on acid or basecharacteristics of the solvents will often not suffice because the effect of acidsor bases adsorbed on the powder surface can alter the acidity of the mediumdrastically. Other charging mechanisms are through adsorption of metal ions,such as Al3+ or Mg2+, or adsorption of poly-electrolytes.

In order to understand the kinetics of deposition, the electric field strengthmust be known. A qualitative notion can be obtained through measurement,and a number of mechanisms have been identified that can lead to a decreasein electric field strength during deposition, i.e. changes in polarization at oneof the electrodes and a high electrical resistance of the deposit. Both effectscan be used advantageously if a self-limiting thickness is required and can beavoided when thick deposits need to be produced.

A homogeneous composition of deposits of two or more powders can beensured, provided the volume fraction of solids in the suspension is sufficientlyhigh. In contrast to almost any other colloidal-forming technique, density dif-ferences between the powders do not result in segregation.

Most investigations regarding the mechanism of deposition propose that thesurface charge of the powder is reduced in the process. A difference in pHnear electrodes or membranes compared with the rest of the suspension isbelieved to play a major role in this reduction of charge. It was suggested thatthe increase in powder concentration in or near the deposit would also changethe equilibrium charge, and it has been pointed out that the double layer isthinner ahead of a traveling particle, which results in a decreased repulsionfor incoming particles. Although the lack of a complete understanding of thedeposition mechanism has not prevented the use of the process on an industrialscale, more detailed investigations, sustained by calculations, will be requiredto control the process better and to decrease the amount of experimental workrequired during development of a deposition system.

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