L Besra, M. Liu/ Progress in Materials Science 52(2007)1-61 From the current density profile in Fig. 7, it is reasonable to suggest that the applied volt age should be less than 100 V in the case of n-propanol. It was observed that amount of YSZ deposition from the n-propanol bath increased with increasing applied voltage. How ever, the deposit surface morphologies were found to be flat at low voltages and it became more rough with increasing applied voltage 3.2.3. Concentration of solid in suspension The volume fraction of solid in the suspension play an important role, particularly for multi-component EPD. In some cases, although each of the particle species have same sign of surface charge, they could deposit at different rates depending on the volume fraction of solids in the suspension. If the volume fraction of solids is high, the powders deposit at an equal rate. If however, the volume fraction of solids is low, the particles can deposit at rates proportional to their individual electrophoretic mobility [44] 3. 2.4. Conductivity of substrate The uniformity and conductivity of substrate electrode is an important parameter crit- ical to the quality of the deposition of green film by EPD. Peng and Liu [45]observed that low conductivity of the Lao. Sro. MnO3 (LSM) substrate leads to non-uniform green film and slow deposition. Chen and Liu [31] noticed that when as pressed LSM or LSM-YSZ composite pellets were used as substrate for EPD, the deposition rate of YSz was slow and obtained film was non-uniform. This was attributed to be due to the high resistance of the substrates resulting from the binder added. When the pellets were fired at 700C for 0.5 h to remove the binder, the conductivity of the substrates increased substantially. Conse- quently, the green YSZ film obtained was of high quality It is quite evident from the above discussion that the kinetics of electrophoretic depo- sition and the quality of deposit formed is dependent on a large number of parameters. It is required to have a careful control of these individual parameters during electrophoretic deposition. However, many of the parameters are inter-related to one another. It is noted that the quality of electrophoretic deposition heavily depends on the suspension conditions 46]. In general, a well-dispersed stable suspension will provide a better deposition during EPD compared to an unstable or agglomerated powder suspension. Zeta potential is an important parameter that relates to suspension stability and mobility. It measures the potential difference between the particle surface and the shear layer plane formed by the adsorbed ions. As zeta potential is closely related to the particle's double layer thickness it hence provides information on the agglomeration of the particles in the suspension. In general, the higher the absolute value of the measured zeta potential, the better is the dis- persion of the particles in the suspension. Besides the stability criteria, it is also noted that it is the ions in the suspension that are carrying most of the current when an electric field is generated during EPD [4, 5] as a result, the electrical conductivity of the suspension also plays an important role in the process. Fig. 8 shows the zeta potential and electrical con ductivity of lead zirconate titanate(PZT) suspension as a function of pH [46]. It can be seen that as the pH value decreases from 7.5 (isoelectric point) to 4.5, the value of the zeta potential increases. This is attributed to the adsorption of the ht ions onto the particle surfaces which enhances the electrostatic repulsion force. However, as more H ions are added to the suspension, i. e, when pH value decreases from 4.5 to 2.0, the large amount of positive ions results in the reduction of the double layer thickness and, hence, a decrease in repulsive force between the particles. This will promote particle agglomeration andFrom the current density profile in Fig. 7, it is reasonable to suggest that the applied volt￾age should be less than 100 V in the case of n-propanol. It was observed that amount of YSZ deposition from the n-propanol bath increased with increasing applied voltage. How￾ever, the deposit surface morphologies were found to be flat at low voltages and it became more rough with increasing applied voltage. 3.2.3. Concentration of solid in suspension The volume fraction of solid in the suspension play an important role, particularly for multi-component EPD. In some cases, although each of the particle species have same sign of surface charge, they could deposit at different rates depending on the volume fraction of solids in the suspension. If the volume fraction of solids is high, the powders deposit at an equal rate. If however, the volume fraction of solids is low, the particles can deposit at rates proportional to their individual electrophoretic mobility [44]. 3.2.4. Conductivity of substrate The uniformity and conductivity of substrate electrode is an important parameter crit￾ical to the quality of the deposition of green film by EPD. Peng and Liu [45] observed that low conductivity of the La0.9Sr0.1MnO3 (LSM) substrate leads to non-uniform green film and slow deposition. Chen and Liu [31] noticed that when as pressed LSM or LSM–YSZ composite pellets were used as substrate for EPD, the deposition rate of YSZ was slow and obtained film was non-uniform. This was attributed to be due to the high resistance of the substrates resulting from the binder added. When the pellets were fired at 700 C for 0.5 h to remove the binder, the conductivity of the substrates increased substantially. Conse￾quently, the green YSZ film obtained was of high quality. It is quite evident from the above discussion that the kinetics of electrophoretic depo￾sition and the quality of deposit formed is dependent on a large number of parameters. It is required to have a careful control of these individual parameters during electrophoretic deposition. However, many of the parameters are inter-related to one another. It is noted that the quality of electrophoretic deposition heavily depends on the suspension conditions [46]. In general, a well-dispersed stable suspension will provide a better deposition during EPD compared to an unstable or agglomerated powder suspension. Zeta potential is an important parameter that relates to suspension stability and mobility. It measures the potential difference between the particle surface and the shear layer plane formed by the adsorbed ions. As zeta potential is closely related to the particle’s double layer thickness, it hence provides information on the agglomeration of the particles in the suspension. In general, the higher the absolute value of the measured zeta potential, the better is the dis￾persion of the particles in the suspension. Besides the stability criteria, it is also noted that it is the ions in the suspension that are carrying most of the current when an electric field is generated during EPD [4,5], as a result, the electrical conductivity of the suspension also plays an important role in the process. Fig. 8 shows the zeta potential and electrical con￾ductivity of lead zirconate titanate (PZT) suspension as a function of pH [46]. It can be seen that as the pH value decreases from 7.5 (isoelectric point) to 4.5, the value of the zeta potential increases. This is attributed to the adsorption of the H+ ions onto the particle surfaces which enhances the electrostatic repulsion force. However, as more H+ ions are added to the suspension, i.e., when pH value decreases from 4.5 to 2.0, the large amount of positive ions results in the reduction of the double layer thickness and, hence, a decrease in repulsive force between the particles. This will promote particle agglomeration and, L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61 13