L Besra, M. Liu/ Progress in Materials Science 52(2007)1-61 Ishihara et al. [30] and Chen and Liu [31]used the following equation for the weight(w) of charged particles deposited per unit area of electrode in the initial period, ignoring the charge carried by the free ions w=Ca45·(7) where C is the concentration of the particle, Eo is the permittivity of vacuum, er is the rel- ative permittivity of the solvent, s is the zeta potential of the particles, l is the viscosity of the solvent, E is the applied potential, L is the distance between the electrodes, and t is the deposition time. The above equations, often termed as Hamaker equation, suggests that the deposition weight of the charged particles under ideal electrophoretic deposition depends on the above parameters. However, if the solvent, the particles, and the apparatus for EPD are fixed, the factors $, Er, n and L in the above equation are constant. Conse quently, the weight of the deposited particles (w)in the EPD method is a function of E, t and C. Therefore, the mass of the deposited particles, namely, the thickness of the films can be readily controlled by the concentration of the suspension, applied potential, and deposition time in the EPd method. 3.1. Parameters related to the suspension Regarding the suspension properties, many parameters must be considered, such as the physicochemical nature of both suspended particle and the liquid medium, surface prop erties of the powder, and the influence of the type and concentration of the additives, mainly dispersants 3. Particle size Although there is no general thumb rule to specify particle sizes suitable for electropho etic deposition, good deposition for a variety of ceramic and clay systems have been reported to occur in the range of 1-20 um [2]. But this does not necessarily mean that deposition of particles outside this size range is not feasible. Recently, with increasing thrust on nanostructured materials, the EPd technique is being viewed with more interest for assembly of nanoparticles, and will be discussed in more detail in later section. It is important that the particles remain completely dispersed and stable for homogeneous and smooth deposition. For larger particles, the main problem is that they tend to settle due to gravity. Ideally, the mobility of particles due to electrophoresis must be higher than that due to gravity. It is difficult to get uniform deposition from sedimenting suspension of deposition, i. e, thinner above and thicker deposit at the bottom when the deposi ient in large particles. Electrophoretic deposition from settling suspension will lead to gradient in trode is placed vertical. In addition, for electrophoretic deposition to occur with particles, either a very strong surface charge must be obtained, or the electrical e layer region must increase in size. Particle size has also been found to have a prominent influence on controlling the cracking of the deposit during drying. Sato et al. [4]investi- ated the effect of Y Ba2Cu307-8(YBCO) particle size reduction on crack formation and their results are shown in Fig. 2. Crack in films deposited from a suspension consisting of relatively smaller particle(0.06 um)was much less than that in films deposited from the suspension containing larger particles (3 um). Hence, reduction in particle size improvedIshihara et al. [30] and Chen and Liu [31] used the following equation for the weight (w) of charged particles deposited per unit area of electrode in the initial period, ignoring the charge carried by the free ions w ¼ 2 3 C e0 er n 1 g E L t ð4Þ where C is the concentration of the particle, e0 is the permittivity of vacuum, er is the rel￾ative permittivity of the solvent, n is the zeta potential of the particles, g is the viscosity of the solvent, E is the applied potential, L is the distance between the electrodes, and t is the deposition time. The above equations, often termed as Hamaker equation, suggests that the deposition weight of the charged particles under ideal electrophoretic deposition depends on the above parameters. However, if the solvent, the particles, and the apparatus for EPD are fixed, the factors n, er, g and L in the above equation are constant. Conse￾quently, the weight of the deposited particles (w) in the EPD method is a function of E, t and C. Therefore, the mass of the deposited particles, namely, the thickness of the films can be readily controlled by the concentration of the suspension, applied potential, and deposition time in the EPD method. 3.1. Parameters related to the suspension Regarding the suspension properties, many parameters must be considered, such as the physicochemical nature of both suspended particle and the liquid medium, surface prop￾erties of the powder, and the influence of the type and concentration of the additives, mainly dispersants. 3.1.1. Particle size Although there is no general thumb rule to specify particle sizes suitable for electropho￾retic deposition, good deposition for a variety of ceramic and clay systems have been reported to occur in the range of 1–20 lm [2]. But this does not necessarily mean that deposition of particles outside this size range is not feasible. Recently, with increasing thrust on nanostructured materials, the EPD technique is being viewed with more interest for assembly of nanoparticles, and will be discussed in more detail in later section. It is important that the particles remain completely dispersed and stable for homogeneous and smooth deposition. For larger particles, the main problem is that they tend to settle due to gravity. Ideally, the mobility of particles due to electrophoresis must be higher than that due to gravity. It is difficult to get uniform deposition from sedimenting suspension of large particles. Electrophoretic deposition from settling suspension will lead to gradient in deposition, i.e., thinner above and thicker deposit at the bottom when the deposition elec￾trode is placed vertical. In addition, for electrophoretic deposition to occur with larger particles, either a very strong surface charge must be obtained, or the electrical double layer region must increase in size. Particle size has also been found to have a prominent influence on controlling the cracking of the deposit during drying. Sato et al. [4] investi￾gated the effect of YBa2Cu3O7d (YBCO) particle size reduction on crack formation and their results are shown in Fig. 2. Crack in films deposited from a suspension consisting of relatively smaller particle (0.06 lm) was much less than that in films deposited from the suspension containing larger particles (3 lm). Hence, reduction in particle size improved 6 L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61