L Besra, M. Liu/Progress in Materials Science 52(2007)1-61 330 270 240 (c)100V 180 120 b)60V 60 (a)20V Fig. 4. Relationship between deposit thickness and time of deposition for Zno coatings on copper electrode at different applied potential [42]. 0.20 0.15 0.10 100150200250300 Fig. 5. Current density versus deposition time for deposition of hydroxyapatite at different applied voltages:(a) 50v;(b)100v;(c)200V[43y 3. 2.2. Applied volt Normally the amount of deposit increases with increase in applied potential. Fig. 6 shows the weight of deposited hydroxyapatite on Ti6Al4V substrate from its suspension in isopropyl alcohol. Although powders can be deposited more quickly if greater applie fields are used, the quality of the deposit can suffer. Basu et al. [41] found that more uni- form films are deposited at moderate applied fields(25-100 V/cm), whereas the film qual- ity deteriorates if relatively higher applied fields(>100 V/cm)are used. because the formation of particulate film on the electrode is a kinetic phenomenon, the accumulation rate of the particles influences their packing behaviour in coating. For a higher applied field, which may cause turbulence in the suspension, the coating may be disturbed by flows in the surrounding medium, even during its deposition. In addition, particles can move so
3.2.2. Applied voltage Normally the amount of deposit increases with increase in applied potential. Fig. 6 shows the weight of deposited hydroxyapatite on Ti6Al4V substrate from its suspension in isopropyl alcohol. Although powders can be deposited more quickly if greater applied fields are used, the quality of the deposit can suffer. Basu et al. [41] found that more uniform films are deposited at moderate applied fields (25–100 V/cm), whereas the film quality deteriorates if relatively higher applied fields (>100 V/cm) are used. Because the formation of particulate film on the electrode is a kinetic phenomenon, the accumulation rate of the particles influences their packing behaviour in coating. For a higher applied field, which may cause turbulence in the suspension, the coating may be disturbed by flows in the surrounding medium, even during its deposition. In addition, particles can move so Fig. 4. Relationship between deposit thickness and time of deposition for ZnO coatings on copper electrode at different applied potential [42]. Fig. 5. Current density versus deposition time for deposition of hydroxyapatite at different applied voltages: (a) 50 V; (b) 100 V; (c) 200 V [43]. L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61 11
L Besra, M. Liu Progress in Materials Science 52(2007)1-61 10 Applied voltage Fig. 6. Weight of deposited hydroxyapatite on Ti6Al4V substrate versus applied voltage for different deposition durations:(a)30 s and (b)120 s[431 fast that they cannot find enough time to sit in their best positions to form a close-packed structure. Finally, in high field situations, lateral motion of the particles once deposited also are restricted on the surface of the already deposited layer, because higher applied potential exerts more pressure on particle flux and movement, the applied field affects the deposition rate and the structure of the deposit. Negishi et al. [33]observed that the current density of n-propanol solvent in absence of any powder, were proportional to applied voltage and it tend to unstable with increasing applied voltages(Fig. 7). Such stability data serves as a good guideline for deciding the deposition parameters and consequently the quality of deposit formed by EPD. It is con idered that the unstable current density influences the quality of deposition morphology 300V 100V Time(min) Fig. 7. Stability of current density of n-propanol for different applied voltages [33]
fast that they cannot find enough time to sit in their best positions to form a close-packed structure. Finally, in high field situations, lateral motion of the particles once deposited, also are restricted on the surface of the already deposited layer, because higher applied potential exerts more pressure on particle flux and movement, the applied field affects the deposition rate and the structure of the deposit. Negishi et al. [33] observed that the current density of n-propanol solvent in absence of any powder, were proportional to applied voltage and it tend to unstable with increasing applied voltages (Fig. 7). Such stability data serves as a good guideline for deciding the deposition parameters and consequently the quality of deposit formed by EPD. It is considered that the unstable current density influences the quality of deposition morphology. Fig. 6. Weight of deposited hydroxyapatite on Ti6Al4V substrate versus applied voltage for different deposition durations: (a) 30 s and (b) 120 s [43]. Fig. 7. Stability of current density of n-propanol for different applied voltages [33]. 12 L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61
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 and
From the current density profile in Fig. 7, it is reasonable to suggest that the applied voltage 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. However, 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 critical 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. Consequently, the green YSZ film obtained was of high quality. It is quite evident from the above discussion that the kinetics of electrophoretic deposition 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 dispersion 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 conductivity 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
L Besra, M. Liu Progress in Materials Science 52(2007)1-61 -Zeta Potential114 Qg Fig. 8. Zeta potential and conductivity of the PZT suspension at varying pH in aqueous media [4 hence, give rise to poorer deposition results. At the alkaline range, similar phenomenon is observed but with the adsorption of oH ions in the suspension. It is, however, noted that at the alkaline range, the zeta potential measured appeared to be much lower in absolute magnitude compared to that at the acidic range. The ionic concentration not only affects the zeta potential, but is also closely related to the suspension electrical conductivity. This can be seen from Fig. 8 which shows that the electrical conductivity is low when the ionic concentration in the suspension is low. However, as the ionic concentration in the suspen sion increases, the conductivity of the suspension increases rapidly. It is found that at high ionic concentration, not only the rate of agglomeration will increase and form larger agglomerates that have lower mobility, but also the large amount of free ions in the suspen- sion may become the main current carrier and, hence, reduce the electrophoretic mobility of the particles. The conductivity of suspension is also directly related to dielectric constant of the suspending medium and it increases with increase in dielectric constant [32]. Hence the choice of the suspension parameters needs to be made judiciously for preparation of a suit- able EPD suspension. Once the parameters related to the suspension are fixed, the process parameters can be altered conveniently for attaining desired deposition. Obviously, the most dominant parameters influencing the electrophoretic deposition are the process parameters such as applied voltage, deposition time and particle concentration in the sus- pension. Invariably, high applied potential leads to higher deposition rate but care has to be taken to ensure stable current density to obtain uniform deposit. Similarly, higher deposi- tion rate is expected with increasing particle concentration and deposition time [44] 4. Kinetics of electrophoretic deposition To make EPD process commercially more viable, a knowledge of the kinetics of EPD process is necessary in order to(a)control and manipulate deposition rate, and(b) achieve flexibility in microstructural manipulation. Hamaker [1]observed a linear dependence of the deposited weight or yield of the EPD with the amount of charge passed, and proposed that the amount deposited is proportional to the concentration of the suspension, time of
hence, give rise to poorer deposition results. At the alkaline range, similar phenomenon is observed but with the adsorption of OH ions in the suspension. It is, however, noted that at the alkaline range, the zeta potential measured appeared to be much lower in absolute magnitude compared to that at the acidic range. The ionic concentration not only affects the zeta potential, but is also closely related to the suspension electrical conductivity. This can be seen from Fig. 8 which shows that the electrical conductivity is low when the ionic concentration in the suspension is low. However, as the ionic concentration in the suspension increases, the conductivity of the suspension increases rapidly. It is found that at high ionic concentration, not only the rate of agglomeration will increase and form larger agglomerates that have lower mobility, but also the large amount of free ions in the suspension may become the main current carrier and, hence, reduce the electrophoretic mobility of the particles. The conductivity of suspension is also directly related to dielectric constant of the suspending medium and it increases with increase in dielectric constant [32]. Hence the choice of the suspension parameters needs to be made judiciously for preparation of a suitable EPD suspension. Once the parameters related to the suspension are fixed, the process parameters can be altered conveniently for attaining desired deposition. Obviously, the most dominant parameters influencing the electrophoretic deposition are the process parameters such as applied voltage, deposition time and particle concentration in the suspension. Invariably, high applied potential leads to higher deposition rate but care has to be taken to ensure stable current density to obtain uniform deposit. Similarly, higher deposition rate is expected with increasing particle concentration and deposition time [44]. 4. Kinetics of electrophoretic deposition To make EPD process commercially more viable, a knowledge of the kinetics of EPD process is necessary in order to (a) control and manipulate deposition rate, and (b) achieve flexibility in microstructural manipulation. Hamaker [1] observed a linear dependence of the deposited weight or yield of the EPD with the amount of charge passed, and proposed that the amount deposited is proportional to the concentration of the suspension, time of Fig. 8. Zeta potential and conductivity of the PZT suspension at varying pH in aqueous media [46]. 14 L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61
L Besra, M. Liu/Progress in Materials Science 52(2007)1-61 2000 4000 6000 Time(sec) Fig 9. Schematic of EPD kinetics [47] deposition, surface area of deposit, and the electric field. EPD can be conducted under constant current or constant voltage mode with either constant or changing concentration ( decreasing concentration of suspension) with deposition time. Sarkar et al. [47] demon strated the kinetic aspects of EPD through schematic plots(Fig. 9) of deposit weight against time of deposition for four possible deposition conditions: curve A(constant-cur- rent and constant-suspension concentration), curve B(constant-current but decreasing suspension concentration ), curve C(constant-voltage and constant-suspension concentra tion)and curve D(constant-voltage but decreasing suspension concentration). Except curve a where the rate of deposition is constant with time, the rate of deposition decreases asymptotically with deposition time in either curve B, C, or D. After allowing for sufficient deposition time, the final yield and rate of deposition are highest in curve A, followed by curve B, C, and D, respectively. The effect of decreasing suspension concentration on the reduction of the final yield and rate of deposition is obvious during either constant-current (curves A and B)or constant-voltage curve C and D) EPD. Comparison of curve A(con stant-current)and curve C(constant-voltage) clearly reveals that even if the suspension concentration is kept constant during deposition in both of them, (a) the rate of deposition was constant in curve a while it decreased asymptotically with time in curve C and(b) final yield was considerably higher in curve A than that in curve C. Thus the deviation of curve A from curve C is not due to decreasing suspension concentration but is due to a decrease of particle velocity as a function of deposition time. Such decrease in particle velocity during constant-voltage EPD is due to the fact that deposited mass acts as shielding effect and has higher electrical resistance than the suspension from which depo- electrical driving force or voltage per unit length of suspension decreases with ti able sition takes place. Consequently, as the deposit grows with deposition time, the avail 5. Role of polymer binders in EPD Polymer binders are common additives in ceramic processing. The EPD processing employs binder only seldom or minimal. The role of binders in EPD processing is
deposition, surface area of deposit, and the electric field. EPD can be conducted under constant current or constant voltage mode with either constant or changing concentration (decreasing concentration of suspension) with deposition time. Sarkar et al. [47] demonstrated the kinetic aspects of EPD through schematic plots (Fig. 9) of deposit weight against time of deposition for four possible deposition conditions: curve A (constant-current and constant-suspension concentration), curve B (constant-current but decreasing suspension concentration), curve C (constant-voltage and constant-suspension concentration) and curve D (constant-voltage but decreasing suspension concentration). Except in curve A where the rate of deposition is constant with time, the rate of deposition decreases asymptotically with deposition time in either curve B, C, or D. After allowing for sufficient deposition time, the final yield and rate of deposition are highest in curve A, followed by curve B, C, and D, respectively. The effect of decreasing suspension concentration on the reduction of the final yield and rate of deposition is obvious during either constant-current (curves A and B) or constant-voltage curve C and D) EPD. Comparison of curve A (constant-current) and curve C (constant-voltage) clearly reveals that even if the suspension concentration is kept constant during deposition in both of them, (a) the rate of deposition was constant in curve A while it decreased asymptotically with time in curve C and (b) final yield was considerably higher in curve A than that in curve C. Thus the deviation of curve A from curve C is not due to decreasing suspension concentration but is due to a decrease of particle velocity as a function of deposition time. Such decrease in particle velocity during constant-voltage EPD is due to the fact that deposited mass acts as a shielding effect and has higher electrical resistance than the suspension from which deposition takes place. Consequently, as the deposit grows with deposition time, the available electrical driving force or voltage per unit length of suspension decreases with time. 5. Role of polymer binders in EPD Polymer binders are common additives in ceramic processing. The EPD processing employs binder only seldom or minimal. The role of binders in EPD processing is Fig. 9. Schematic of EPD kinetics [47]. L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61 15