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 improved
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 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 relative 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. Consequently, 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 properties 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 electrophoretic 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 electrode 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] investigated 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
L Besra, M. Liu/Progress in Materials Science 52(2007)1-61 Film B 6007 Fig. 2. SEM images of YBCO film electrophoretically deposited on silver electrode from its suspension in acetone 80s(film A: mean particle size =3 um; film B: mean particle size =0.06 um). The films were sintered h and annealed at 500C for 6 h [4]- the morphology of the YBCO superconducting film fabricated by electrophoretic deposi tion suggesting that it is a useful technique to minimize cracking of deposits 3. 1.2. Dielectric constant of liquid Powers [32] investigated beta-alumina suspensions in numerous organic media and determined the incidence of deposition as a function of the dielectric constant of the liquid and the conductivity of the suspension. A sharp increase in conductivity with dielectric constant was noted; which apparently refers to the liquid in their pure state. It should also be noted that impurities, in particular water, affects the conductivity and that conductivity of milled suspension is very different to that of pure liquid, as a consequence of dissociative or adsorptive charging modes. Powers [32] obtained deposits only with liquid for which the dielectric constant was in the range of 12-25. With too low a dielectric constant, depo- ition fails because of insufficient dissociative power, whilst with a high dielectric constant, the high ionic concentration in the liquid reduces the size of the double layer region and consequently the electrophoretic mobility. Consequently, the ionic concentration in the liquid must remain low, a condition favoured in liquids of low dielectric constant. The
the morphology of the YBCO superconducting film fabricated by electrophoretic deposition suggesting that it is a useful technique to minimize cracking of deposits. 3.1.2. Dielectric constant of liquid Powers [32] investigated beta-alumina suspensions in numerous organic media and determined the incidence of deposition as a function of the dielectric constant of the liquid and the conductivity of the suspension. A sharp increase in conductivity with dielectric constant was noted; which apparently refers to the liquid in their pure state. It should also be noted that impurities, in particular water, affects the conductivity and that conductivity of milled suspension is very different to that of pure liquid, as a consequence of dissociative or adsorptive charging modes. Powers [32] obtained deposits only with liquid for which the dielectric constant was in the range of 12–25. With too low a dielectric constant, deposition fails because of insufficient dissociative power, whilst with a high dielectric constant, the high ionic concentration in the liquid reduces the size of the double layer region and consequently the electrophoretic mobility. Consequently, the ionic concentration in the liquid must remain low, a condition favoured in liquids of low dielectric constant. The Fig. 2. SEM images of YBCO film electrophoretically deposited on silver electrode from its suspension in acetone at 10 V for 180 s (film A: mean particle size = 3 lm; film B: mean particle size = 0.06 lm). The films were sintered at 945 C for 1 h and annealed at 500 C for 6 h [4]. L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61 7
L Besra, M. Liu Progress in Materials Science 52(2007)1-61 Table 2 Physical properties of solvents [33] Solvents Viscosity (cP)=10-'Nsm Relative dielectric constant 0.557 32.63 Ethanol 885 24.55 20.33 Ethylene glycol .265 0.3087 dielectric constant is generally the product of relative dielectric constant and dielectric con- stant in vacuum. Table 2 shows physical properties such as viscosity and relative dielectric constant of some solvents [33]. 3.1.3. Conductivity of suspension Ferrari and Moreno [34], after a careful study proposed that the conductivity of the sus- pension is a key factor and needs to be taken into account in EPD experiments. It has been pointed out that if the suspension is too conductive, particle motion is very low, and if the suspension is too resistive, the particles charge electronically and the stability is lost. They observed increase in conductivity of the suspension with both temperature and with poly electrolyte(dispersant)concentration; but not all conductivity values were found useful for electrophoretic deposition. They found the existence of a narrow band of conductivity range at varying dispersant dosage and temperature, in which the deposit is formed. Con ductivity out of this region are not suitable for EPD, limiting the forming possibilities This suitable region of conductivity is however expected to be different for different sys- tems. The margin of conductivity region suitable for EPD, however can be increased by the applied current assuring the success of the EPd process [35] 3. 1.4. Viscosity of suspension In casting processes, the main controlling parameter is the viscosity. Rheological mea surements on concentrated slips give us a good idea about the optimum dispersing state when adding dispersants. In EPD process, the solid loading is very low and the viscosity cannot be used to evaluate the dispersion state [34, 35]. But the desired properties in the suspension vehicle are low viscosity, high dielectric constant and low conductivity 3.1.5. Zeta potential Fe the zeta potential of particles is a key factor in the electrophoretic deposition process It is imperative to achieve a high and uniform surface charge of the suspended particles. It plays a role in: (i)stabilization of the suspension by determining the intensity of repulsive interaction between particles, (ii) determining the direction and migration velocity of particle during EPD, (ii) determining the green density of the deposit. The overall stability of a system depends on the interaction between individual particles in the suspension. Two mechanisms affect this interaction which are due to electrostatic and van der waals forces The probability of coagulation of a disperse system depends on the interaction energ resulting from this forces and will be dealt with in detail later. a high electrostat
dielectric constant is generally the product of relative dielectric constant and dielectric constant in vacuum. Table 2 shows physical properties such as viscosity and relative dielectric constant of some solvents [33]. 3.1.3. Conductivity of suspension Ferrari and Moreno [34], after a careful study proposed that the conductivity of the suspension is a key factor and needs to be taken into account in EPD experiments. It has been pointed out that if the suspension is too conductive, particle motion is very low, and if the suspension is too resistive, the particles charge electronically and the stability is lost. They observed increase in conductivity of the suspension with both temperature and with polyelectrolyte (dispersant) concentration; but not all conductivity values were found useful for electrophoretic deposition. They found the existence of a narrow band of conductivity range at varying dispersant dosage and temperature, in which the deposit is formed. Conductivity out of this region are not suitable for EPD, limiting the forming possibilities. This suitable region of conductivity is however expected to be different for different systems. The margin of conductivity region suitable for EPD, however can be increased by the applied current assuring the success of the EPD process [35]. 3.1.4. Viscosity of suspension In casting processes, the main controlling parameter is the viscosity. Rheological measurements on concentrated slips give us a good idea about the optimum dispersing state when adding dispersants. In EPD process, the solid loading is very low and the viscosity cannot be used to evaluate the dispersion state [34,35]. But the desired properties in the suspension vehicle are low viscosity, high dielectric constant and low conductivity. 3.1.5. Zeta potential The zeta potential of particles is a key factor in the electrophoretic deposition process. It is imperative to achieve a high and uniform surface charge of the suspended particles. It plays a role in: (i) stabilization of the suspension by determining the intensity of repulsive interaction between particles, (ii) determining the direction and migration velocity of particle during EPD, (iii) determining the green density of the deposit. The overall stability of a system depends on the interaction between individual particles in the suspension. Two mechanisms affect this interaction, which are due to electrostatic and van der Waals forces. The probability of coagulation of a disperse system depends on the interaction energy resulting from this forces, and will be dealt with in detail later. A high electrostatic Table 2 Physical properties of solvents [33] Solvents Viscosity (cP) = 103 Nsm2 Relative dielectric constant Methanol 0.557 32.63 Ethanol 1.0885 24.55 n-Propanol 1.9365 20.33 Iso-propanol 2.0439 19.92 n-Butanol 2.5875 17.51 Ethylene glycol 16.265 37.7 Acetone 0.3087 20.7 Acetylacetone 1.09 25.7 8 L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61
L Besra, M. Liu/ Progress in Materials Science 52(2007)1-61 epulsion due to high particle charge is required to avoid particle agglomeration. The par- ticle charge also affects the green density of the deposit. During formation of the deposit the particles become closer to each other and with increasing attraction force. If the par ticle charge is low, the particles would coagulate even for relative large inter-particle distances, leading to porous, sponge-like deposits. On the contrary, if the particles have a high surface charge during deposition they will repulse each other, occupying positions which will lead to high particle packing density [36]. It is therefore very important to con trol the solids loading and concentration of solvents and additives in the EPd suspension in order to reach the highest possible green density of the deposit. The zeta potential can be controlled by a variety of charging agents such as acids, bases and specifically adsorbed ions or polyelectrolytes, to the suspension [37]. Thus there exists a variety of additives that affect the charge magnitude and its polarity. These additives act by different mechanisms The main criteria for selection of a charging agent are the preferred polarity and deposi- tion rate of the particles Chen et al. [38] found that the stability and deposition rates of alumina from its suspen- on in ethanol was maximum at pH value of 2.2 at which the positive zeta potential of alumina was maximum(Fig 3). However, under higher pH value of ll, the suspensions were less stable. This can be explained based on a charging mechanism recently proposed by Wang et al. [39]on the alumina surface 1OH, AIOH = AlO+H,0 Under basic conditions such as phll, AlOH tends to form alo however, the presence of water is prone to bring the above reaction towards the formation of AlOH,, rather than the formation of AlO, resulting in an absolute value of the zeta potential greater pH 2 than at pH ll. This led to high stability of suspension at lower pH tha higher pH conditions AL,o, powder dispered in ethanol (1.5 vol%6) 0 oelectric point (EP pH value Fig 3. Zeta potential of Al2O3 powder in ethanol [38]
repulsion due to high particle charge is required to avoid particle agglomeration. The particle charge also affects the green density of the deposit. During formation of the deposit, the particles become closer to each other and with increasing attraction force. If the particle charge is low, the particles would coagulate even for relative large inter-particle distances, leading to porous, sponge-like deposits. On the contrary, if the particles have a high surface charge during deposition they will repulse each other, occupying positions which will lead to high particle packing density [36]. It is therefore very important to control the solids loading and concentration of solvents and additives in the EPD suspension in order to reach the highest possible green density of the deposit. The zeta potential can be controlled by a variety of charging agents such as acids, bases and specifically adsorbed ions or polyelectrolytes, to the suspension [37]. Thus there exists a variety of additives that affect the charge magnitude and its polarity. These additives act by different mechanisms. The main criteria for selection of a charging agent are the preferred polarity and deposition rate of the particles. Chen et al. [38] found that the stability and deposition rates of alumina from its suspension in ethanol was maximum at pH value of 2.2 at which the positive zeta potential of alumina was maximum (Fig. 3). However, under higher pH value of 11, the suspensions were less stable. This can be explained based on a charging mechanism recently proposed by Wang et al. [39] on the alumina surface AlOHþ 2 ( Hþ AlOH ) OH AlO þ H2O ð5Þ Under basic conditions such as pH 11, AlOH tends to form AlO; however, the presence of water is prone to bring the above reaction towards the formation of AlOHþ 2 , rather than the formation of AlO, resulting in an absolute value of the zeta potential greater at pH 2 than at pH 11. This led to high stability of suspension at lower pH than at higher pH conditions. Fig. 3. Zeta potential of Al2O3 powder in ethanol [38]. L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61 9
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 time
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 additive is exerted also by its effect on the ionic conductivity of the suspension. The ionic conductivity 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 suspension 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 particles will move relative to the liquid phase when the electric field is applied. Colloidal particles 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 agitation 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 adhering 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 electrode 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 investigators 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