L Besra, M. Liu Progress in Materials Science 52(2007)1-61 Zarbov et al. [37]established that while the deposition rate is directly dependent on the zeta potential, which is determined by the charging additive, the influence of such an addi- tive is exerted also by its effect on the ionic conductivity of the suspension. The ionic con- ductivity determines the potential drop in the bulk of the suspension, which constitute the driving force for the transfer of the particles to the electrode 3.1.6. Stability of suspension Electrophoresis is the phenomenon of motion of particles in a colloidal solution or sus- . nsion in an electric field, and generally occurs when the distance over which the double layer charge falls to zero is large compared to the particle size. In this condition, the par- ticles will move relative to the liquid phase when the electric field is applied. Colloidal pa ticles which are I um or less in diameter, tend to remain in suspension for long periods due to Brownian motion. Particles larger than I um require continuous hydrodynamic agita- tion to remain in suspension. The suspension stability is characterized by settling rate and tendency to undergo or avoid flocculation Stable suspensions show no tendency to flocculate, settle slowly and form dense and strongly adhering deposits at the bottom of the container Flocculating suspensions settle rapidly and form low density, weakly adher ing deposits. If the suspension is too stable, the repulsive forces between the particles will not be overcome by the electric field, and deposition will not occur. According to some models for electrophoretic deposition the suspension should be unstable in the vicinity of the electrodes [5]. This local instability could be caused by the formation of ions from electrolysis or discharge of the particles; these ions then cause flocculation close to the elec trode surface. It is desirable to find suitable physical/chemical parameters that characterize a suspension sufficiently in order that its ability to deposit can be predicted. Most inves- tigators use zeta potential or electrophoretic mobility, but these do not uniquely determine he ability of a suspension to deposit. For example, in suspension of aluminium in alcohol the addition of electrolyte causes no significant change to the zeta potential, but deposit can only be obtained in the presence of the electrolyte [40]. The stability of the suspension is evidently its most significant property, but this is a somewhat empirical property not closely related to fundamental parameters 3. 2. Parameters related to the process 3. 1. Effect of Basu et al. [41] found that deposition rate for a fixed applied field decreases with increased or prolonged deposition time. Similar observation was made by Chen and Liu din g. 4 shows a typical deposition characteristics of ZnO coating on Copper electrode at different applied potentials, with increasing time of deposition [42]. It is clearly evident that the deposition is linear during the initial time of deposition. But as more and more time is allowed, the deposition rate decreases and attains a plateau at very high deposition In a constant voltage EPD, this is expected because: while the potential difference between the electrodes is maintained constant, the electric field influencing electrophoresis decreases(Fig. 5) with deposition time because of the formation of an insulating layer of ceramic particles on the electrode surface [43]. But during the initial period of EPD, there is generally a linear relationship between deposition mass and timeZarbov et al. [37] established that while the deposition rate is directly dependent on the zeta potential, which is determined by the charging additive, the influence of such an addi￾tive is exerted also by its effect on the ionic conductivity of the suspension. The ionic con￾ductivity determines the potential drop in the bulk of the suspension, which constitute the driving force for the transfer of the particles to the electrode. 3.1.6. Stability of suspension Electrophoresis is the phenomenon of motion of particles in a colloidal solution or sus￾pension in an electric field, and generally occurs when the distance over which the double layer charge falls to zero is large compared to the particle size. In this condition, the par￾ticles will move relative to the liquid phase when the electric field is applied. Colloidal par￾ticles which are 1 lm or less in diameter, tend to remain in suspension for long periods due to Brownian motion. Particles larger than 1 lm require continuous hydrodynamic agita￾tion to remain in suspension. The suspension stability is characterized by settling rate and tendency to undergo or avoid flocculation. Stable suspensions show no tendency to flocculate, settle slowly and form dense and strongly adhering deposits at the bottom of the container. Flocculating suspensions settle rapidly and form low density, weakly adher￾ing deposits. If the suspension is too stable, the repulsive forces between the particles will not be overcome by the electric field, and deposition will not occur. According to some models for electrophoretic deposition the suspension should be unstable in the vicinity of the electrodes [5]. This local instability could be caused by the formation of ions from electrolysis or discharge of the particles; these ions then cause flocculation close to the elec￾trode surface. It is desirable to find suitable physical/chemical parameters that characterize a suspension sufficiently in order that its ability to deposit can be predicted. Most inves￾tigators use zeta potential or electrophoretic mobility, but these do not uniquely determine the ability of a suspension to deposit. For example, in suspension of aluminium in alcohol the addition of electrolyte causes no significant change to the zeta potential, but deposits can only be obtained in the presence of the electrolyte [40]. The stability of the suspension is evidently its most significant property, but this is a somewhat empirical property not closely related to fundamental parameters. 3.2. Parameters related to the process 3.2.1. Effect of deposition time Basu et al. [41] found that deposition rate for a fixed applied field decreases with increased or prolonged deposition time. Similar observation was made by Chen and Liu [31]. Fig. 4 shows a typical deposition characteristics of ZnO coating on copper electrode at different applied potentials, with increasing time of deposition [42]. It is clearly evident that the deposition is linear during the initial time of deposition. But as more and more time is allowed, the deposition rate decreases and attains a plateau at very high deposition times. In a constant voltage EPD, this is expected because: while the potential difference between the electrodes is maintained constant, the electric field influencing electrophoresis decreases (Fig. 5) with deposition time because of the formation of an insulating layer of ceramic particles on the electrode surface [43]. But during the initial period of EPD, there is generally a linear relationship between deposition mass and time. 10 L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61