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 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. Therefore, the control of polymer adsorption is of paramount importance for electropho￾retic 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 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 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. zir￾conyl chloride, ZrOCl2), 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 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 for￾mation 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 ion￾exchange 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