L Besra, M. Liu Progress in Materials Science 52(2007)1-61 multifunctional. Polymer binders are used to obtain adherent deposits and prevent cracks Moreover, the adsorbed polymer can provide steric stabilization of suspension of ceramic particles and reduce viscosity of the suspension. In EPD processing, charged polymer par ticles transport adsorbed polymer to the electrode surface, thus allowing the polymer bin der to be included in the deposit. This is in contrast to some other ceramic techniques, where the entire dissolved polymer is included in the green body after solvent evaporation erefore, the control of polymer adsorption is of paramount importance for electropho- retic deposition. The amount of polymer adsorption depends on polymer concentration in uspension and specific polymer-particle, polymer-solvent, particle-solvent and particle dispersant interactions. Good solvents are necessary in order to achieve high polymer con- centration in solution. However, the polymer can be adsorbed on the surface of ceramic particles when its solubility in the dispersion medium is low. Adsorption of polymer on ceramic particles in poor solvent can result in bridging flocculation. In contrast, good sol- vents are important to achieve steric stabilization. Polymer stabilizing moieties, which extend out from the particle surface must be well solvated in a good solvent. Therefore, for electrophoretic deposition, it could be advantageous to use copolymers of a block or graft type. Indeed, soluble polymers serve to anchor copolymer molecules to the parti urface, whereas chains of soluble polymers enable steric stabilization 6. Importance of powder washing before EPD Successful EPD techniques requires a stable suspension wherein well dispersed particles have a controlled surface charge. Thus the preparation of a particulate suspension with a carefully defined chemistry before conducting EPD is essential. The first step in suspension preparation is powder washing to remove any residual impurities incorporated during powder preparation. As an example, during preparation of yttria stabilized zirconia (YSZ) powders(Tz-8YS)by co-precipitation technique using chloride precursors(e.g zir conyl chloride, ZrOCl), the solution contain residual surface chlorides which can be tested by measuring specific conductivity of the supernatant of the dispersion. The conduc tivity of de-ionised water is about 0.04 uS/cm. Presence of Cl ions can also be confirmed by adding a small amount of silver nitrate(AgNO3)salt to the supernatant. If there is for mation of an insoluble precipitate(of AgCl)then there is residual chloride impurities in the tion e, Removal of the CI ions and other impurities is very important because it can affect the suspension stability, deposition characteristics and later, the sintering. Basu et al. [41] found that unwashed powder led to unstable suspension which needed agitation every 5-10 min. When the suspension is settling constantly, during EPD, the unwashed powder led to lower deposition yield, gradient in the EPD film thickness(thinner coating at top, hicker on bottom), a decrease of 15-25% in overall green density of the as deposited ng. Basu et al. [41] accomplished removal of Cl impurities from surface of the particles using successive washing in de-ionised water because water easily participates in an exchange interaction between the impurities on the powder surface and the bulk solvent in accordance with Eq (6) Cl(surface)+ H,O(solvent)-H,O(surface)+CI-(solvent)
multifunctional. Polymer binders are used to obtain adherent deposits and prevent cracks. Moreover, the adsorbed polymer can provide steric stabilization of suspension of ceramic particles and reduce viscosity of the suspension. In EPD processing, charged polymer particles transport adsorbed polymer to the electrode surface, thus allowing the polymer binder to be included in the deposit. This is in contrast to some other ceramic techniques, where the entire dissolved polymer is included in the green body after solvent evaporation. Therefore, the control of polymer adsorption is of paramount importance for electrophoretic deposition. The amount of polymer adsorption depends on polymer concentration in suspension and specific polymer–particle, polymer–solvent, particle–solvent and particle– dispersant interactions. Good solvents are necessary in order to achieve high polymer concentration in solution. However, the polymer can be adsorbed on the surface of ceramic particles when its solubility in the dispersion medium is low. Adsorption of polymer on ceramic particles in poor solvent can result in bridging flocculation. In contrast, good solvents are important to achieve steric stabilization. Polymer stabilizing moieties, which extend out from the particle surface must be well solvated in a good solvent. Therefore, for electrophoretic deposition, it could be advantageous to use copolymers of a block or graft type. Indeed, soluble polymers serve to anchor copolymer molecules to the particle surface, whereas chains of soluble polymers enable steric stabilization. 6. Importance of powder washing before EPD Successful EPD techniques requires a stable suspension wherein well dispersed particles have a controlled surface charge. Thus the preparation of a particulate suspension with a carefully defined chemistry before conducting EPD is essential. The first step in suspension preparation is powder washing to remove any residual impurities incorporated during powder preparation. As an example, during preparation of yttria stabilized zirconia (YSZ) powders (TZ-8YS) by co-precipitation technique using chloride precursors (e.g. zirconyl chloride, ZrOCl2), the solution contain residual surface chlorides which can be tested by measuring specific conductivity of the supernatant of the dispersion. The conductivity of de-ionised water is about 0.04 lS/cm. Presence of Cl ions can also be confirmed by adding a small amount of silver nitrate (AgNO3) salt to the supernatant. If there is formation of an insoluble precipitate (of AgCl) then there is residual chloride impurities in the solution. Removal of the Cl ions and other impurities is very important because it can affect the suspension stability, deposition characteristics and later, the sintering. Basu et al. [41] found that unwashed powder led to unstable suspension which needed agitation every 5–10 min. When the suspension is settling constantly, during EPD, the unwashed powder led to lower deposition yield, gradient in the EPD film thickness (thinner coating at top, thicker on bottom), a decrease of 15–25% in overall green density of the as deposited coating. Basu et al. [41] accomplished removal of Cl impurities from surface of the particles using successive washing in de-ionised water because water easily participates in an ionexchange interaction between the impurities on the powder surface and the bulk solvent in accordance with Eq. (6) Cl (surface) + H2O (solvent) !H2O (surface) + Cl (solvent) ð6Þ 16 L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61
L Besra, M. Liu/ Progress in Materials Science 52(2007)1-61 A significant reduction in conductivity of supernatant was observed on washing In addi tion, washing the Tz-8YS powder eight times in de- ionised water was sufficient to yield reproducible behaviour during EPD 7. Practical considerations In view of the sensitivity of the electrophoretic mobility to factors such as chemical environment and particle surface topography, and the need for suspension of marginal sta bility, it might be thought that a process based on electrophoretic deposition would be inherently difficult to control. This situation is not helped by the shortcomings in funda- mental understanding of electrophoretic deposition, and it is almost impossible to predict whether suspensions will deposit electrophoretically. With any system, it is of course abso- lutely necessary to avoid contamination by any impurity that can adversely influence the electrokinetic properties of the suspension; a stringent requirement, but one that should perhaps be regarded as a strength rather than a weakness of the process. The EPD process can deposit powder uniformly on a complicated shape electrode and, as a result, can pro- duce geometrically complicated shapes. However, in the case of bulk ceramic, after shape forming the substrate( depositing electrode) needs to be removed from the deposit. Com mercial applicability of EPD forming depends on the effective separation of the substrate from the deposit In the case of simple geometry, separation can be done after drying of the deposit For complicated shapes, a combustible substrate that can be removed during the sintering process could be used In the case of coatings, the sample often develops cracking during drying and sintering and the successful application of EPD to this area depends on overcoming this problem Another important area of concern is how to avoid cracking in the ceramic coating dur ing drying and sintering [48]. During drying and sintering, the coating densifies, and as a result shrinks, but the substrate typically does not change dimension. During this process the coating will develop tensile stress in it and these stresses will be relieved by the forma tion of cracks. There are several approaches that can be taken to avoid this cracking. Care ful control of the EPD process together with moderate control on drying may avoid the formation of drying cracks. During sintering the coating typically has about 10-15% linear shrinkage. Traditionally, this sintering cracking is avoided by using a liquid phase during sintering; a good example is glass enamel on a metal substrate. The enamel composition is adjusted in such a way that its thermal expansion is closely matched with the substrate. As a result it does not form cracks during cooling from the sintering temperature. This liquid phase sintering is also equally effective in avoiding cracking in fiber composite synthesis The second approach is to use a substrate that also shrinks during sintering. Recently, this approach has been used, particularly in SOFC fabrication where typically a YSZ electro- lyte is applied to a partially sintered or an unsintered anode substrate. During sintering, both the substrate and coating shrink thereby avoiding cracks 8. Water-based EPD In general, the organic solvents are very popular as dispersing media in electrophoretic deposition. Table 3 presents a summary of some solvents commonly used in electropho retic deposition. But the use of aqueous system has important advantages since they need much lower voltage to be applied and the environmental problems associated with
A significant reduction in conductivity of supernatant was observed on washing. In addition, washing the TZ-8YS powder eight times in de-ionised water was sufficient to yield reproducible behaviour during EPD. 7. Practical considerations In view of the sensitivity of the electrophoretic mobility to factors such as chemical environment and particle surface topography, and the need for suspension of marginal stability, it might be thought that a process based on electrophoretic deposition would be inherently difficult to control. This situation is not helped by the shortcomings in fundamental understanding of electrophoretic deposition, and it is almost impossible to predict whether suspensions will deposit electrophoretically. With any system, it is of course absolutely necessary to avoid contamination by any impurity that can adversely influence the electrokinetic properties of the suspension; a stringent requirement, but one that should perhaps be regarded as a strength rather than a weakness of the process. The EPD process can deposit powder uniformly on a complicated shape electrode and, as a result, can produce geometrically complicated shapes. However, in the case of bulk ceramic, after shape forming the substrate (depositing electrode) needs to be removed from the deposit. Commercial applicability of EPD forming depends on the effective separation of the substrate from the deposit. In the case of simple geometry, separation can be done after drying of the deposit. For complicated shapes, a combustible substrate that can be removed during the sintering process could be used. In the case of coatings, the sample often develops cracking during drying and sintering and the successful application of EPD to this area depends on overcoming this problem. Another important area of concern is how to avoid cracking in the ceramic coating during drying and sintering [48]. During drying and sintering, the coating densifies, and as a result shrinks, but the substrate typically does not change dimension. During this process, the coating will develop tensile stress in it and these stresses will be relieved by the formation of cracks. There are several approaches that can be taken to avoid this cracking. Careful control of the EPD process together with moderate control on drying may avoid the formation of drying cracks. During sintering the coating typically has about 10–15% linear shrinkage. Traditionally, this sintering cracking is avoided by using a liquid phase during sintering; a good example is glass enamel on a metal substrate. The enamel composition is adjusted in such a way that its thermal expansion is closely matched with the substrate. As a result it does not form cracks during cooling from the sintering temperature. This liquid phase sintering is also equally effective in avoiding cracking in fiber composite synthesis. The second approach is to use a substrate that also shrinks during sintering. Recently, this approach has been used, particularly in SOFC fabrication where typically a YSZ electrolyte is applied to a partially sintered or an unsintered anode substrate. During sintering, both the substrate and coating shrink thereby avoiding cracks. 8. Water-based EPD In general, the organic solvents are very popular as dispersing media in electrophoretic deposition. Table 3 presents a summary of some solvents commonly used in electrophoretic deposition. But the use of aqueous system has important advantages since they need much lower voltage to be applied and the environmental problems associated with L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61 17
L Besra, M. Liu Progress in Materials Science 52(2007)1-61 Table 3 Solvents used for electrophoretic deposition SI Solvent Reference yEro LaI- Sr Gai-MMg, O3-r+r YSZ Ethyl alcohol Al2O3, ZrO2 Ethyl alcohol-wate Ceo Ethyl alcohol-acetylacetone MgO Glacial acetic acid PZT AlgO3 Toluene-ethyl alcohol organics are avoided [35]. Obviously, the use of water implies advantages such as higher temperature-control during the process or a faster kinetics, in addition to important health, environmental, and cost benefits. These advantages promoted the interest in some research groups in the 1990s to consider using aqueous EPD to process technical ceramics he water-based suspensions however causes a number of problems in electrophoretic forming [63]. The main problems are related to electrochemical reaction in the electrodes when current is passed through, which seriously affects the efficiency of the process and the uniformity of the deposit. First and foremost, there is a deviation in the deposition kinetics from the linear Hamaker growth due to deviation in current density and powder concen tration [64]. The deviation from linearity could not be prevented even after controlling the current density and powder concentration suggesting that there could be factors other than those already discussed, which affect the aqueous EPD process. Electrolysis of water occurs at low voltages, and gas evolution at the electrodes is inevitable at field strengths high enough to give reasonably short deposit times. This causes bubbles to be trapped within the deposit unless special procedures are adopted, such as the use of absorbing or porous electrode materials, or high speed chamber flows. Current densities are high, leading to Joule heating of the suspension, and electrochemical attack of the deposit. Sec overpassed. This facilitates oxidation of the electrodes and migration of metallic impur sely ondly, when metallic electrodes are used, the normal potential of the electrode is larg towards the slurry in the opposite direction to that of the migrating particles. In cases, these impurities are retained in the deposit as heterogeneities and/or residual poros- ity, thus degrading its expected properties Another electrokinetic phenomena occurring in an aqueous EPD is water electroosmo- sis, which consist of the movement of the liquid phase because of an external electric field This could be helpful in EPD because it would accelerate drying of the deposit surface
organics are avoided [35]. Obviously, the use of water implies advantages such as higher temperature-control during the process or a faster kinetics, in addition to important health, environmental, and cost benefits. These advantages promoted the interest in some research groups in the 1990s to consider using aqueous EPD to process technical ceramics. The water-based suspensions however causes a number of problems in electrophoretic forming [63]. The main problems are related to electrochemical reaction in the electrodes when current is passed through, which seriously affects the efficiency of the process and the uniformity of the deposit. First and foremost, there is a deviation in the deposition kinetics from the linear Hamaker growth due to deviation in current density and powder concentration [64]. The deviation from linearity could not be prevented even after controlling the current density and powder concentration suggesting that there could be factors other than those already discussed, which affect the aqueous EPD process. Electrolysis of water occurs at low voltages, and gas evolution at the electrodes is inevitable at field strengths high enough to give reasonably short deposit times. This causes bubbles to be trapped within the deposit unless special procedures are adopted, such as the use of absorbing or porous electrode materials, or high speed chamber flows. Current densities are high, leading to Joule heating of the suspension, and electrochemical attack of the deposit. Secondly, when metallic electrodes are used, the normal potential of the electrode is largely overpassed. This facilitates oxidation of the electrodes and migration of metallic impurities towards the slurry in the opposite direction to that of the migrating particles. In most cases, these impurities are retained in the deposit as heterogeneities and/or residual porosity, thus degrading its expected properties. Another electrokinetic phenomena occurring in an aqueous EPD is water electroosmosis, which consist of the movement of the liquid phase because of an external electric field. This could be helpful in EPD because it would accelerate drying of the deposit surface Table 3 Solvents used for electrophoretic deposition Sl. no. Solvent Deposited material Reference 1 Water Al2O3 [34,35,49] Al2O3/ZrO2 [50] 2 Acetone YSZ [51] La1xSrxGa1yMgyO3(x+y)/2 [52] 3 Acetone–ethanol YSZ [45] 4 Acetylacetone YSZ [53] [51] 5 Cyclohexane YSZ [51] 6 Isopropylalcohol Hydroxyapatite [54] YBa2Cu3O7x [55] 7 Ethyl alcohol Al2O3, ZrO2 [56] 8 Ethyl alcohol–water CeO2 [57] SnO2 [58] CaSiO3 [59] 9 Ethyl alcohol–acetylacetone MgO, Al2O3 [60] 10 Glacial acetic acid PZT [27] 11 Dichloromethane b-alumina [61] 12 Methyl ethyl ketone (MEK) Al2O3 [62] 13 Toluene-ethyl alcohol Al2O3 [62] 18 L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61
L Besra, M. Liu/Progress in Materials Science 52(2007)1-61 which is in contact with the electrode. So, if the process were well controlled, demoulding of the self-supported deposits would be easier. Contrarily, if the deposits were too thick, or the process were too fast, crack formation would occur as a consequence of the drying gra- dient. Therefore, an adequate control of the colloid chemistry of the particles in the slurr Is necessary 9. Non-aqueous EPD In general, organic liquids are superior to water as a suspension medium for electropho- retic forming. While the generally lower dielectric constant in organic liquids limits the charge on the particles as a result of the lower dissociating power, much higher field strengths can be used since the problems of electrolytic gas evolution, joule heating and electrochemical attack of the electrodes are greatly reduced or non-existent. Moreover, the organic liquids are preferred due to their higher density, good chemical stability and low conductivity. The electrolysis and gas evolution associated with aqueous EPD processing can be avoided by using solvents of extremely high oxidation-reduction potentials like benzene or ketones. However, the electric charge on oxide particles in benzene or ketones will be insufficient for EPD as very small amounts of free ions exist in these solvents. Conse- quently, a few hundreds of volts are required for EPD. It is reported that protons are formed by a reaction between ketone and iodine. The reaction of iodine with acetone can be represented by Eq (7)[52 CH3-CO-CH3+ 212<> ICH2-CO-CH2I+ 2H++21 Similarly, the reaction between acetylacetone and iodine can be represented by the follow ing equation [51] CH3COCH2COCH3台→ICH2 CO-CO-CH2l+2I-+2H Adsorption of the formed protons onto the suspended particles will make them positively harged. Application of a DC field causes the positively charged particles to move toward and deposit on the cathode The major problems associated with the use of organics are that higher voltages are required. Moreover, the cost, toxicity and flammability of organic liquids warrants judi- lous selection and practice of solvent reclamation in order to minimise these problems 10. Charge development on powder surface in suspension 10.1. Aqueous suspension Electrophoretic deposition relies on the capability of the powder to acquire an electric charge in the liquid in which it is dispersed. In general when solid powder is dispersed in a liquid, such as water, usually it results in the buildup of a charge at the solid-liquid interface [65]. The interfacial charge is a result of a range of mechanisms such as adsorp- tion or orientation of dipolar molecules at the particle surface, electron transfer between the solid and the liquid phase due to differences in work function, selective adsorption of ions onto the solid particle and dissociation of ions from the solid phase into the liquid
which is in contact with the electrode. So, if the process were well controlled, demoulding of the self-supported deposits would be easier. Contrarily, if the deposits were too thick, or the process were too fast, crack formation would occur as a consequence of the drying gradient. Therefore, an adequate control of the colloid chemistry of the particles in the slurry is necessary. 9. Non-aqueous EPD In general, organic liquids are superior to water as a suspension medium for electrophoretic forming. While the generally lower dielectric constant in organic liquids limits the charge on the particles as a result of the lower dissociating power, much higher field strengths can be used since the problems of electrolytic gas evolution, joule heating and electrochemical attack of the electrodes are greatly reduced or non-existent. Moreover, the organic liquids are preferred due to their higher density, good chemical stability and low conductivity. The electrolysis and gas evolution associated with aqueous EPD processing can be avoided by using solvents of extremely high oxidation–reduction potentials like benzene or ketones. However, the electric charge on oxide particles in benzene or ketones will be insufficient for EPD as very small amounts of free ions exist in these solvents. Consequently, a few hundreds of volts are required for EPD. It is reported that protons are formed by a reaction between ketone and iodine. The reaction of iodine with acetone can be represented by Eq. (7) [52] CH3–CO–CH3 + 2I2 () ICH2–CO–CH2I + 2Hþ + 2I ð7Þ Similarly, the reaction between acetylacetone and iodine can be represented by the following equation [51]: CH3–CO–CH2–CO–CH3 () I2 ICH2–CO–CO–CH2I þ 2I þ 2Hþ ð8Þ Adsorption of the formed protons onto the suspended particles will make them positively charged. Application of a DC field causes the positively charged particles to move towards and deposit on the cathode. The major problems associated with the use of organics are that higher voltages are required. Moreover, the cost, toxicity and flammability of organic liquids warrants judicious selection and practice of solvent reclamation in order to minimise these problems. 10. Charge development on powder surface in suspension 10.1. Aqueous suspension Electrophoretic deposition relies on the capability of the powder to acquire an electric charge in the liquid in which it is dispersed. In general when solid powder is dispersed in a polar liquid, such as water, usually it results in the buildup of a charge at the solid–liquid interface [65]. The interfacial charge is a result of a range of mechanisms such as adsorption or orientation of dipolar molecules at the particle surface, electron transfer between the solid and the liquid phase due to differences in work function, selective adsorption of ions onto the solid particle and dissociation of ions from the solid phase into the liquid. L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61 19
L Besra, M. Liu /Progress in Materials Science 52(2007)1-61 For aqueous suspensions of ceramic powders, especially oxides, the role of protons as harge determining ions has been clearly established in literature. Yates et al. [66] pro- posed the following description of the interaction of surfaces of oxides with the liquid through simple ionization reactions of surface groups SOH→SO-+H+ (9) SOH→SOH+H+ Later, the description was improved to account for the reaction of major electrolyte ions with ionisable surface sites. The net charge as evident by the above reactions, is controlled by pH and reaction constant for the respective dissociation reaction. The point of zero charge (pzc)is the pH value where the surface concentration of (S-o")and(S-OH;) are equal. The surface charge is negative at pH> pHpzc and positive at ph < pHpzc 10.2. Non-aqueous suspension For non-aqueous media, however, the hydrogen concentration(pH) looses its validity as a general measure for the acidity or alkalinity of a medium, and systematic information on charging in these media is relatively scarce. Nonetheless, the necessity of a global approach towards the behaviour of a solute in a series of solvents, such as its solubility, redox potential or degree of ionization, has led to the development of donor numbers, which express the tendency of a solvent to donate electrons [67]. The relative measure of the alkalinity of a solvent is given by the enthalpy of its reaction with an arbitrarily cho sen reference acid. Ranking the relative degree of acidity of media can be performed by using the acceptor number scale, which is a measure for the tendency of solvents to accept electrons. Indeed, Labib and Williams [68] have shown that in the absence of water, the sign of the charge on the surface of ceramic powders depend on the donor number of the solvent, strongly suggesting that charging through electron exchange with the solvent is possible. However, Wang et al. [39]found the surface charge characteristics on alumina powder dispersed in ethanol to be analogous to the behaviour of oxides in water where a mere adjustment in pH of ethanol by addition of acetic acid or tetra-methyl-ammonium hydroxide allowed control of the sign of the charge on alumina powder. Since in the EPD processing, organic solvents are used more frequently than aqueous suspensions, this observation on control of surface charge on ceramic powder through addition of acid or bases to organic media serves as a valuable general guideline for suspension prepara tion. In reality, many organic solvents of technical quality do contain some residual water. Although the evidence for the charging in non-aqueous media by electron transfer is inter esting from a theoretical point of view, for practical use in ceramic industry the require- ment of working under absolutely dry conditions is far too expensive. To test how far the observation of Wang et al. [39]for alumina can be generalized to ceramic processing, Vandeperre et al. [67] measured the charging of a range of ceramic powders(oxides, car- bides, nitrides and borides) by potentiometric titration and compared them with sign of the electrophoretic mobility in an acidic and alkaline organic reference medium. Their measurement showed that to a large extent the rationale for charging of ceramic powders in water also applies to charging in non-aqueous media Negishi et al. [69], while preparing YSZ suspension in n-propanol, suggested that the small amount of residual water present in commercial n-propanol used for preparation of YSZ suspension for EPD, generated H
For aqueous suspensions of ceramic powders, especially oxides, the role of protons as charge determining ions has been clearly established in literature. Yates et al. [66] proposed the following description of the interaction of surfaces of oxides with the liquid through simple ionization reactions of surface groups: S–OH ! S–O þ Hþ ð9Þ S–OHþ 2 ! S–OH þ Hþ ð10Þ Later, the description was improved to account for the reaction of major electrolyte ions with ionisable surface sites. The net charge as evident by the above reactions, is controlled by pH and reaction constant for the respective dissociation reaction. The point of zero charge (pzc) is the pH value where the surface concentration of (S–O) and ðS–OHþ 2 Þ are equal. The surface charge is negative at pH > pHpzc and positive at pH < pHpzc. 10.2. Non-aqueous suspension For non-aqueous media, however, the hydrogen concentration (pH) looses its validity as a general measure for the acidity or alkalinity of a medium, and systematic information on charging in these media is relatively scarce. Nonetheless, the necessity of a global approach towards the behaviour of a solute in a series of solvents, such as its solubility, redox potential or degree of ionization, has led to the development of donor numbers, which express the tendency of a solvent to donate electrons [67]. The relative measure of the alkalinity of a solvent is given by the enthalpy of its reaction with an arbitrarily chosen reference acid. Ranking the relative degree of acidity of media can be performed by using the acceptor number scale, which is a measure for the tendency of solvents to accept electrons. Indeed, Labib and Williams [68] have shown that in the absence of water, the sign of the charge on the surface of ceramic powders depend on the donor number of the solvent, strongly suggesting that charging through electron exchange with the solvent is possible. However, Wang et al. [39] found the surface charge characteristics on alumina powder dispersed in ethanol to be analogous to the behaviour of oxides in water where a mere adjustment in pH of ethanol by addition of acetic acid or tetra-methyl-ammoniumhydroxide allowed control of the sign of the charge on alumina powder. Since in the EPD processing, organic solvents are used more frequently than aqueous suspensions, this observation on control of surface charge on ceramic powder through addition of acids or bases to organic media serves as a valuable general guideline for suspension preparation. In reality, many organic solvents of technical quality do contain some residual water. Although the evidence for the charging in non-aqueous media by electron transfer is interesting from a theoretical point of view, for practical use in ceramic industry the requirement of working under absolutely dry conditions is far too expensive. To test how far the observation of Wang et al. [39] for alumina can be generalized to ceramic processing, Vandeperre et al. [67] measured the charging of a range of ceramic powders (oxides, carbides, nitrides and borides) by potentiometric titration and compared them with sign of the electrophoretic mobility in an acidic and alkaline organic reference medium. Their measurement showed that to a large extent the rationale for charging of ceramic powders in water also applies to charging in non-aqueous media. Negishi et al. [69], while preparing YSZ suspension in n-propanol, suggested that the small amount of residual water present in commercial n-propanol used for preparation of YSZ suspension for EPD, generated H+ 20 L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61