《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_electrophoretic deposition review.pdf_大学文库

3.1. Parameters related to the suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.1. Particle size. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1.2. Dielectric constant of liquid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1.3. Conductivity of suspension. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.4. Viscosity of suspension. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.5. Zeta potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.6. Stability of suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2. Parameters related to the process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2.1. Effect of deposition time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2.2. Applied voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2.3. Concentration of solid in suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2.4. Conductivity of substrate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4. Kinetics of electrophoretic deposition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5. Role of polymer binders in EPD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6. Importance of powder washing before EPD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 7. Practical considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 8. Water-based EPD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 9. Non-aqueous EPD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 10. Charge development on powder surface in suspension . . . . . . . . . . . . . . . . . . . . . . . . . . 19 10.1. Aqueous suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 10.2. Non-aqueous suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 11. Properties of suspension for EPD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 11.1. The electrical double layer and electrophoretic mobility. . . . . . . . . . . . . . . . . . . 21 11.2. DLVO theory and suspension stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 12. Mechanism of EPD process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 12.1. Flocculation by particle accumulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 12.2. Particle charge neutralization mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 12.3. Electrochemical particle coagulation mechanism. . . . . . . . . . . . . . . . . . . . . . . . 28 12.4. Electrical double layer (EDL) distortion and thinning mechanism . . . . . . . . . . . 29 13. Multi-component deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 14. Drying of deposits produced by EPD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 15. Design of electrophoretic apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 16. Deposition on non-conducting substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 17. Application of EPD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 17.1. Assembly of nanoscale particles into nanostructures and micropatterned thin films. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 17.2. Near shape manufacturing of complex-shaped glasses and ceramics . . . . . . . . . . 41 17.3. Solid oxide fuel cell (SOFC) fabrication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 17.4. Laminated materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 17.5. Functionally graded materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 17.6. Hybrid materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 17.7. Fibre reinforced ceramic matrix composites . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 18. Modeling of EPD process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 19. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 2 L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61

L Besra, M. Liu/ Progress in Materials Science 52(2007)1-61 1. Introduction The electrophoretic deposition(EPD)technique with a wide range of novel applications in the processing of advanced ceramic materials and coatings, has recently gained increas- ing interest both in academia and industrial sector not only because of the high versatility of its use with different materials and their combinations but also because of its cost -effec. tiveness requiring simple apparatus. Electrophoretic deposition(EPD) has been known since 1808 when the russian scientist ruess observed an electric field induced movement of clay particles in water. But the first practical use of the techniques occurred in 1933 when the deposition of thoria particles on a platinum cathode as an emitter for electron tube application was patented in USA. Although the basic phenomena involved in EPD are well known and have been the subject of extensive theoretical and experimental research, the EPD of ceramics was first studied by Hamaker [1] and only in the 1980s did the process receive attention in the field of advanced ceramics. There is general agree- ment in the scientific community that further r&d work needs to be done to develop a full, quantitative understanding of the fundamental mechanisms of EPD to optimise the working parameters for a broader use of EPD in materials processing. This paper presents a review of electrophoretic deposition and its application in various fields of processing 2. Electrophoretic deposition-definition Electrophoretic deposition(EPD) is one of the colloidal processes in ceramic produc tion and has advantages of short formation time, needs simple apparatus, little restriction of the shape of substrate, no requirement for binder burnout as the green coating contains few or no organics Compared to other advanced shaping techniques, the epd process is very versatile since it can be modified easily for a specific application. For example, depo- sition can be made on flat, cylindrical or any other shaped substrate with only minor change in electrode design and positioning. In particular, despite being a wet process, EPD offers easy control of the thickness and morphology of a deposited film through simple adjustment of the deposition time and applied potential. In EPD, charged powder particles, dispersed or suspended in a liquid medium are attracted and deposited onto a conductive substrate of opposite charge on application of a DC electric field. The term electrodeposition is often used somewhat ambiguously to refer to either electroplating or electrophoretic deposition, although it more usually refers to the former. Table 1 presents the distinction between the two processes [2] The basic difference between an electrophoretic deposition process(EPD)and an electro- lytic deposition process(ELD) is that the former is based on the suspension of particles in a solvent whereas the later is based on solution of salts, i.e., ionic species [3]. There can be two types of electrophoretic deposition depending on which electrode the deposition occurs When the particles are positively charged, the deposition happens on the cathode and the pro- cess is called cathodic electrophoretic deposition. The deposition of negatively charged par- ticles on positive electrode(anode)is termed as anodicelectrophoretic deposition. By suitable modification of the surface charge on the particles, any of the two mode of deposition is pos- ble. Fig. I presents a schematic illustration of the two electrophoretic deposition process. With regard to technological application the potential of electrophoretic deposition (EPD) as a materials processing technique is being increasingly recognised by scientists and technologists. In addition to its conventional applications in fabrication of wear

1. Introduction The electrophoretic deposition (EPD) technique with a wide range of novel applications in the processing of advanced ceramic materials and coatings, has recently gained increasing interest both in academia and industrial sector not only because of the high versatility of its use with different materials and their combinations but also because of its cost-effectiveness requiring simple apparatus. Electrophoretic deposition (EPD) has been known since 1808 when the Russian scientist Ruess observed an electric field induced movement of clay particles in water. But the first practical use of the techniques occurred in 1933 when the deposition of thoria particles on a platinum cathode as an emitter for electron tube application was patented in USA. Although the basic phenomena involved in EPD are well known and have been the subject of extensive theoretical and experimental research, the EPD of ceramics was first studied by Hamaker [1], and only in the 1980s did the process receive attention in the field of advanced ceramics. There is general agreement in the scientific community that further R&D work needs to be done to develop a full, quantitative understanding of the fundamental mechanisms of EPD to optimise the working parameters for a broader use of EPD in materials processing. This paper presents a review of electrophoretic deposition and its application in various fields of processing. 2. Electrophoretic deposition – definition Electrophoretic deposition (EPD) is one of the colloidal processes in ceramic production and has advantages of short formation time, needs simple apparatus, little restriction of the shape of substrate, no requirement for binder burnout as the green coating contains few or no organics. Compared to other advanced shaping techniques, the EPD process is very versatile since it can be modified easily for a specific application. For example, deposition can be made on flat, cylindrical or any other shaped substrate with only minor change in electrode design and positioning. In particular, despite being a wet process, EPD offers easy control of the thickness and morphology of a deposited film through simple adjustment of the deposition time and applied potential. In EPD, charged powder particles, dispersed or suspended in a liquid medium are attracted and deposited onto a conductive substrate of opposite charge on application of a DC electric field. The term ‘electrodeposition’ is often used somewhat ambiguously to refer to either electroplating or electrophoretic deposition, although it more usually refers to the former. Table 1 presents the distinction between the two processes [2]. The basic difference between an electrophoretic deposition process (EPD) and an electrolytic deposition process (ELD) is that the former is based on the suspension of particles in a solvent whereas the later is based on solution of salts, i.e., ionic species [3]. There can be two types of electrophoretic deposition depending on which electrode the deposition occurs. When the particles are positively charged, the deposition happens on the cathode and the process is called cathodic electrophoretic deposition. The deposition of negatively charged particles on positive electrode (anode) is termed as anodic electrophoretic deposition. By suitable modification of the surface charge on the particles, any of the two mode of deposition is possible. Fig. 1 presents a schematic illustration of the two electrophoretic deposition process. With regard to technological application the potential of electrophoretic deposition (EPD) as a materials processing technique is being increasingly recognised by scientists and technologists. In addition to its conventional applications in fabrication of wear L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61 3

L Besra, M. Liu Progress in Materials Science 52(2007)1-61 Table I Characteristics of electrodeposition techniques [2] Electroplating trophoretic depositis oving specie Solid particles Charge transfer on deposition Ion reduction Required conductance of liquid medium Preferred liquid b + Fig. 1. Schematic illustration of electrophoretic deposition process.(a) Cathodic EPD and (b) anodic EPD resistant and anti-oxidant ceramic coatings, fabrication of functional films for advanced microelectronic devices and solid oxide fuel cells as well as in the development of novel composites or bioactive coatings for medical implants, there has been increased interest for its application in nanoscale assembly for advanced functional materials [4]. Electropho- retic deposition also offers important advantages in the deposition of complex compounds nd ceramic laminates. The degree of stoichiometry in the electrophoretic deposit is con- trolled by the degree of stoichiometry in the powder used. According to Sarkar and Nich olson [5], particle/electrode reactions are not involved in EPD, and ceramic particles do not lose their charge on being deposited which can be shown from the observation that reversal of the electric field will strip of the deposited layer [6]. Therefore, it is important to use sim- ilarly charged particles and similar solvent-binder-dispersant systems for gaining better control of layer thickness. The principal driving force for electrophoretic deposition (EPD) is the charge on the particle and the electrophoretic mobility of the particles in the solvent under the influence of an applied electric field. The EPd technique has been used successfully for thick film of silica [7]. nanosize zeolite membrane [8] hydroxyapatite coating on metal substrate for biomedical applications [9, 10] luminescent materials [11- 13), high-Tc superconducting films [14, 15), gas diffusion electrodes and sensors [16, 17]. multi-layer composites [18] glass and ceramic matrix composites by infiltration of ceramic particles onto fibre fabrics [19], oxide nanorods[20], carbon nanotube film [21], functionally graded ceramics [22, 23], layered ceramics [24], superconductors [25, 26], piezoelectric mate- rials [27], etc. Indeed, the only intrinsic disadvantages of EPD, compared with other colloi dal processes(e.g. dip and slurry coating), is that it cannot use water as the liquid med because the application of a voltage to water causes the evolution of hydrogen and oxygen gases at the electrodes which could adversely affect the quality of the deposits formed. How- ever, given the numerous non-aqueous solvents that are available, this limitation is minor

resistant and anti-oxidant ceramic coatings, fabrication of functional films for advanced microelectronic devices and solid oxide fuel cells as well as in the development of novel composites or bioactive coatings for medical implants, there has been increased interest for its application in nanoscale assembly for advanced functional materials [4]. Electrophoretic deposition also offers important advantages in the deposition of complex compounds and ceramic laminates. The degree of stoichiometry in the electrophoretic deposit is controlled by the degree of stoichiometry in the powder used. According to Sarkar and Nicholson [5], particle/electrode reactions are not involved in EPD, and ceramic particles do not lose their charge on being deposited which can be shown from the observation that reversal of the electric field will strip of the deposited layer [6]. Therefore, it is important to use similarly charged particles and similar solvent–binder–dispersant systems for gaining better control of layer thickness. The principal driving force for electrophoretic deposition (EPD) is the charge on the particle and the electrophoretic mobility of the particles in the solvent under the influence of an applied electric field. The EPD technique has been used successfully for thick film of silica [7], nanosize zeolite membrane [8], hydroxyapatite coating on metal substrate for biomedical applications [9,10], luminescent materials [11– 13], high-Tc superconducting films [14,15], gas diffusion electrodes and sensors [16,17], multi-layer composites [18], glass and ceramic matrix composites by infiltration of ceramic particles onto fibre fabrics [19], oxide nanorods [20], carbon nanotube film [21], functionally graded ceramics [22,23], layered ceramics [24], superconductors [25,26], piezoelectric materials [27], etc. Indeed, the only intrinsic disadvantages of EPD, compared with other colloidal processes (e.g. dip and slurry coating), is that it cannot use water as the liquid medium, because the application of a voltage to water causes the evolution of hydrogen and oxygen gases at the electrodes which could adversely affect the quality of the deposits formed. However, given the numerous non-aqueous solvents that are available, this limitation is minor. Table 1 Characteristics of electrodeposition techniques [2] Property Electroplating Electrophoretic deposition Moving species Ions Solid particles Charge transfer on deposition Ion reduction None Required conductance of liquid medium High Low Preferred liquid Water Organic Fig. 1. Schematic illustration of electrophoretic deposition process. (a) Cathodic EPD and (b) anodic EPD. 4 L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61

L Besra, M. Liu/ Progress in Materials Science 52(2007)1-61 3. Factors influencing EPD The mechanism of EPD involve charged particles in a suspension being deposited onto n electrode under the influence of an applied electric field. Two groups of parameters determine the characteristics of this process; (i)those related to the suspension, and (ii) those related to the process including the physical parameters such as the electrical nature of the electrodes, the electrical conditions( voltage/intensity relationship, deposition time etc. ) For the EPd of particles, part of the current should be carried not only by the particles but by free ions co-existing in the suspension. Therefore the amount of deposited particle is not simply related to the current. However, the current carried by the free ions could be ignored when the amount of free ions is negligible. Indeed the amount of free ions is generally small in organic suspensions such as ketones. On the other hand, it is believed that the accumulation of anionic and cationic charge at the electrodes during electropho resis suppresses the subsequent deposition rate. However the effect of accumulated ions are negligible in the initial period The first attempt to correlate the amount of particles deposited during EPD with differ nt influencing parameters was described by Hamaker [l] and Avgustnik et al. [28] Hamakers law relates the deposit yield (w)to the electric field strength(E), the electropho retic m y (u), the surface area of the electrode(A), and the particle mass concentration in the sion(C) through the following equation w=/μ·E·A·C.dr Avgustinik's law is based upon cylindrical, coaxial, electrodes and the electrophoretic mobility has been expanded and is represented in terms of permittivity (a), the zeta poten tial ($), and the viscosity of the suspension(n) l·E·E·5 where /and a are the length and radius of the deposition electrode, respectively, b is the radius of the coaxial counter electrode(b>a) e Biesheuval and Verweij [29] improved upon these classical equations and developed ore complex model of the deposition process by considering the presence of three distinct phases namely (i) a solid phase(the deposit), (ii)a suspension phase, and (iii)a phase con- taining little or no solid particles. The deposit phase and the particle-free liquid phase both grow at the expense of the suspension phase. By considering the movement of the bound ary between the deposit and the suspension phase with time along with the continuity equation and expression for velocity of particles in the suspension, Biesheuval and Verweij [29]derived the following equation based on that of Avgustinik et al. [28]: 2·兀·H·l·E·Cdφ where s and d are the volumetric concentration of particles in suspension and deposit respectively, Cd is the mass concentration of particles in the deposit, u is the electropho- retic mobility (=sc/6n)

3. Factors influencing EPD The mechanism of EPD involve charged particles in a suspension being deposited onto an electrode under the influence of an applied electric field. Two groups of parameters determine the characteristics of this process; (i) those related to the suspension, and (ii) those related to the process including the physical parameters such as the electrical nature of the electrodes, the electrical conditions (voltage/intensity relationship, deposition time, etc.). For the EPD of particles, part of the current should be carried not only by the charged particles but by free ions co-existing in the suspension. Therefore the amount of deposited particle is not simply related to the current. However, the current carried by the free ions could be ignored when the amount of free ions is negligible. Indeed the amount of free ions is generally small in organic suspensions such as ketones. On the other hand, it is believed that the accumulation of anionic and cationic charge at the electrodes during electrophoresis suppresses the subsequent deposition rate. However the effect of accumulated ions are negligible in the initial period. The first attempt to correlate the amount of particles deposited during EPD with different influencing parameters was described by Hamaker [1] and Avgustnik et al. [28] Hamakers law relates the deposit yield (w) to the electric field strength (E), the electrophoretic mobility (l), the surface area of the electrode (A), and the particle mass concentration in the suspension (C) through the following equation: w ¼ Z t2 t1 l E A C dt ð1Þ Avgustinik’s law is based upon cylindrical, coaxial, electrodes and the electrophoretic mobility has been expanded and is represented in terms of permittivity (e), the zeta potential (n), and the viscosity of the suspension (g) w ¼ l E e n C t 3 lnða=bÞ g ð2Þ where l and a are the length and radius of the deposition electrode, respectively, b is the radius of the coaxial counter electrode (b > a). Biesheuval and Verweij [29] improved upon these classical equations and developed more complex model of the deposition process by considering the presence of three distinct phases namely (i) a solid phase (the deposit), (ii) a suspension phase, and (iii) a phase containing little or no solid particles. The deposit phase and the particle-free liquid phase both grow at the expense of the suspension phase. By considering the movement of the boundary between the deposit and the suspension phase with time along with the continuity equation and expression for velocity of particles in the suspension, Biesheuval and Verweij [29] derived the following equation based on that of Avgustinik et al. [28]: w ¼ 2 p l l E Cd lnða=bÞ /s /d /s t ð3Þ where /s and /d are the volumetric concentration of particles in suspension and deposit, respectively, Cd is the mass concentration of particles in the deposit, l is the electrophoretic mobility (=en/6pg). L. Besra, M. Liu / Progress in Materials Science 52 (2007) 1–61 5

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 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

《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_electrophoretic deposition review

《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_electrophoretic deposition review