《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_电泳沉积

Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Joumal of the European Ceramic Society 28(2008)1353-1367 www.elsevier.comlocate/jeurceramsoc Electrophoretic deposition: From traditional ceramics to nanotechnology Ilaria Corni, Mary P. Ryan, Aldo r boccaccini Department of Materials, Imperial College London, Prince Consort Road, London Sw7 2BP UK Available online 25 January 2008 Electrophoretic deposition(EPD)is attracting increasing interest as a materials processing technique for a wide range of technical applications This technique enables the production of unique microstructures and nanostructures as well as novel and complex material combinations in a variety of macroscopic shapes, dimensions and arrangements starting from micron-sized or nanosized particles. This review presents a comprehensive summary of relevant recent work on EPd describing the application of the technique in the processing of several traditional and advanced materials (functional and structural ceramic coatings, composite and porous materials, laminated ceramics, functionally graded materials, thin films and nanostructured materials), with the intention to highlight how EPD evolved from being a technique restricted only to traditional ceramics to become an important tool in advanced materials processing and nanotechnology. Moreover the fundamental EPD mechanisms and novel theories proposed C 2007 Elsevier Ltd. All rights reserved Keywords: Electrophoretic deposition; Films: Composites; Suspension; Fuel cells 1. Introduction EPD was discovered in 1808 by the Russian scientist Ruess and it was first used in a practical application in 1933 to deposit Electrophoretic deposition(EPD) is an electrochemical thoria particles on a platinum cathode as an emitter for elec method attracting increasing interest as a material processing tron tube applications. In the following years, e.g. until the technique -EPD is usually carried out in a two electrode early 1990s, EPD was mainly used for the processing of tra cell, as schematically shown in Fig. 1. The mechanism of elec- ditional ceramics, including enamels and porcelain, and very trophoretic deposition involves two steps. In the first step an limited work was carried on the EPD of engineering ceramics. 6 electric field is applied between two electrodes and charged par- However, in the last 15 years the interest in electrophoretic depo- ticles suspended in a suitable liquid move toward the oppositely sition as a technique to produce advanced materials has widely charged electrode(electrophoresis). In the second step the parti- increased, both in academia and in the industrial sector, and since cles accumulate at the deposition electrode and create a relatively then a wide range of new applications of EPd for processing a compact and homogeneous film(deposition ). In order to effec- variety of bulk materials and coatings has been reported.-6EPD tively apply this technique to process materials, it is essential has demonstrated the possibility to realize unique microstruc- to produce a stable suspension containing charged particles free tures and nanostructures as well as novel and complex materials to move when an electric field is applied. Therefore EPD can combinations in a variety of macroscopic shapes, dimensions <30 um particle size) or as a colloidal suspension, including the number of published scientific papers, identified searching metals, polymers, ceramics and glasses. After the deposition, the tool Web of Science by the keyword "electrophoretic depo- a heat-treatment step is normally needed to further densify the sition, from only a few papers per year in the 1970s to just under deposit and to eliminate porosity two hundred papers published in 2006 Recently, EPD has been employed for the processing of func- tional and composite ceramics, layered and functionally graded materials, thin films, high performance ceramic and composite Corresponding author. TeL. +44 2075946731: fax: +44 2075946757 coatings and biomaterials and also for the deposition of nanopar E-mail address: a boccaccini@ic ac uk(AR. Boccaccini) ticles and carbon nanotubes to produce advanced nanostructured 0955-2219/S-see front matter o 2007 Elsevier Ltd. All rights reserved

Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 1353–1367 Electrophoretic deposition: From traditional ceramics to nanotechnology Ilaria Corni, Mary P. Ryan, Aldo R. Boccaccini ∗ Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK Available online 25 January 2008 Abstract Electrophoretic deposition (EPD) is attracting increasing interest as a materials processing technique for a wide range of technical applications. This technique enables the production of unique microstructures and nanostructures as well as novel and complex material combinations in a variety of macroscopic shapes, dimensions and arrangements starting from micron-sized or nanosized particles. This review presents a comprehensive summary of relevant recent work on EPD describing the application of the technique in the processing of several traditional and advanced materials (functional and structural ceramic coatings, composite and porous materials, laminated ceramics, functionally graded materials, thin films and nanostructured materials), with the intention to highlight how EPD evolved from being a technique restricted only to traditional ceramics to become an important tool in advanced materials processing and nanotechnology. Moreover the fundamental EPD mechanisms and novel theories proposed to clarify the processes involved are explained. © 2007 Elsevier Ltd. All rights reserved. Keywords: Electrophoretic deposition; Films; Composites; Suspension; Fuel cells 1. Introduction Electrophoretic deposition (EPD) is an electrochemical method attracting increasing interest as a material processing technique.1–3 EPD is usually carried out in a two electrode cell, as schematically shown in Fig. 1. The mechanism of elec￾trophoretic deposition involves two steps. In the first step an electric field is applied between two electrodes and charged par￾ticles suspended in a suitable liquid move toward the oppositely charged electrode (electrophoresis). In the second step the parti￾cles accumulate at the deposition electrode and create a relatively compact and homogeneous film (deposition). In order to effec￾tively apply this technique to process materials, it is essential to produce a stable suspension containing charged particles free to move when an electric field is applied. Therefore EPD can be applied to any solid that is available as a fine powder (e.g. <∼30m particle size) or as a colloidal suspension, including metals, polymers, ceramics and glasses. After the deposition, a heat-treatment step is normally needed to further densify the deposit and to eliminate porosity.1–5 ∗ Corresponding author. Tel.: +44 2075946731; fax: +44 2075946757. E-mail address: a.boccaccini@ic.ac.uk (A.R. Boccaccini). EPD was discovered in 1808 by the Russian scientist Ruess and it was first used in a practical application in 1933 to deposit thoria particles on a platinum cathode as an emitter for elec￾tron tube applications.1 In the following years, e.g. until the early 1990s, EPD was mainly used for the processing of tra￾ditional ceramics, including enamels and porcelain, and very limited work was carried on the EPD of engineering ceramics.6 However, in the last 15 years the interest in electrophoretic depo￾sition as a technique to produce advanced materials has widely increased, both in academia and in the industrial sector, and since then a wide range of new applications of EPD for processing a variety of bulk materials and coatings has been reported.1–6 EPD has demonstrated the possibility to realize unique microstruc￾tures and nanostructures as well as novel and complex materials combinations in a variety of macroscopic shapes, dimensions and arrangements. Fig. 2 shows the extraordinary increase of the number of published scientific papers, identified searching the tool Web of Science® by the keyword “electrophoretic depo￾sition”, from only a few papers per year in the 1970s to just under two hundred papers published in 2006. Recently, EPD has been employed for the processing of func￾tional and composite ceramics, layered and functionally graded materials, thin films, high performance ceramic and composite coatings and biomaterials and also for the deposition of nanopar￾ticles and carbon nanotubes to produce advanced nanostructured 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.12.011

. Corni et al /Journal of the European Ceramic Society 28(2008)1353-136 tory and time-consuming trial-and-error approaches, due to the lack of available relationships linking the parameters of the EPD process to the final deposit properties. The intention of this review is to present a comprehensive summary of relevant previous work on EPD describing the appli cation of the technique in the processing of a range of traditional ⊕←由 and advanced materials, with the intention to highlight how EPD ← evolved from being a technique restricted only to traditional ceramics to become an important tool in advanced materials pro- cessing, including nanomaterials. The review is divided into two main parts. One is dedicated to briefly revise the mechanisms proposed to explain the phenomena involved in EPD(Section 2) and the other section presents an overview of EPD applications dividing them into traditional ceramics, advanced materials and nanotechnology(Section 3). Due to the availability of previ ous comprehensive review articles covering different aspects of Fig. 1. Two electrodes cell for electrophoretic deposition showing positively the theory and applications of EPD. the focus of the present harged particles in suspension migrating towards the negative electrode. article is to highlight the most recent published research in this materials.7-9 The increasing significance of this electrochemical technique in materials processing follows from its high versa- 2. Mechanisms of EPD tility for application with different materials and combinations of materials. its cost-effectiveness, simplicity, the requirement 2.1. Traditional approaches of only basic equipment and the ability to be scaled-up to large product volumes and sizes. -Moreover, compared with other The fundamental mechanisms of EPD have been largely processing methods based on the packing of particles, epd described in the literature mainly in the framework of the is able to produce uniform deposits with high microstructural Derjaguin-Landau-Verwey-Overbeek (DLvO) theory and in homogeneity, to provide adequate control of deposit thickness relation to the distortion of the particle double layer under the and to deposit coatings on a wide range of shapes and 3D com- application of a DC electric field, as discussed by Sarkar and olex and porous structures. -3.5,7,.8 he success of EPD as processing method for advanced mate- other theories(flocculation by particle accumulation, parti rials and the increasing opportunities being explored for its cle charge neutralization, electrochemical particle coagulation, application in a wide range of materials have been confirmed by electrical double layer(EDL) distortion and thinning mecha- the establishment of an international conference series dedicated nism) have been proposed to explain the particle interactions exclusively to EPD, the proceedings of the first two conferences and the kinetics Additional (held in 2002 and 2005)have been published. 0, II and modelling studies are being carried out in order to clarify Despite the numerous improvements of the EpD technique the mechanisms of deposition and the role of electrochemical nd the large range of applications achieved, there is need fo arameters on the complex interactions between solvent, part further theoretical and modelling work to gain a full and quan cles and electric field titative understanding of the mechanisms of EPD. In fact many 2.1 .1. Flocculation by particle accumulation experimental studies are presently carried out using unsatisfac- Hamaker and Verwey 3.14 observed similarities between for- mation of deposits by electrophoresis and gravitation. In fact, in both processes, the exerted by the arriving parti cles enables the particles close to the deposit to prevail over the inter-particle repulsion. Therefore the primary function of the applied electric field in EPD is to move the particles towards the a5100 electrode to accumulate. This mechanism can also explain the deposition of coatings onto membranes that are not serving as 2.1.2. Particle charge neutralization mechanism 1990 2000 Grillon et al. suggested that the charged particles are neu- tralized when they touch the electrode. This mechanism explains the deposition of single particles and monolayers electrophoretic deposition"in the open literature(Web-of-Science research- tion of powders charged by the addition of salts to the suspension Jul-2007), from 1960 until 2006 (e. g. the experiments described by Brown and Salt).However

1354 I. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 Fig. 1. Two electrodes cell for electrophoretic deposition showing positively charged particles in suspension migrating towards the negative electrode. materials.7–9 The increasing significance of this electrochemical technique in materials processing follows from its high versa￾tility for application with different materials and combinations of materials, its cost-effectiveness, simplicity, the requirement of only basic equipment and the ability to be scaled-up to large product volumes and sizes.1–5 Moreover, compared with other processing methods based on the packing of particles, EPD is able to produce uniform deposits with high microstructural homogeneity, to provide adequate control of deposit thickness and to deposit coatings on a wide range of shapes and 3D com￾plex and porous structures.1–3,5,7,8 The success of EPD as processing method for advanced mate￾rials and the increasing opportunities being explored for its application in a wide range of materials have been confirmed by the establishment of an international conference series dedicated exclusively to EPD, the proceedings of the first two conferences (held in 2002 and 2005) have been published.10,11 Despite the numerous improvements of the EPD technique and the large range of applications achieved, there is need for further theoretical and modelling work to gain a full and quan￾titative understanding of the mechanisms of EPD. In fact many experimental studies are presently carried out using unsatisfac￾Fig. 2. Increasing number of publications featuring as the keyword “electrophoretic deposition” in the open literature (Web-of-Science® research￾Jul-2007), from 1960 until 2006. tory and time-consuming trial-and-error approaches, due to the lack of available relationships linking the parameters of the EPD process to the final deposit properties.10,11 The intention of this review is to present a comprehensive summary of relevant previous work on EPD describing the appli￾cation of the technique in the processing of a range of traditional and advanced materials, with the intention to highlight how EPD evolved from being a technique restricted only to traditional ceramics to become an important tool in advanced materials pro￾cessing, including nanomaterials. The review is divided into two main parts. One is dedicated to briefly revise the mechanisms proposed to explain the phenomena involved in EPD (Section 2) and the other section presents an overview of EPD applications, dividing them into traditional ceramics, advanced materials and nanotechnology (Section 3). Due to the availability of previ￾ous comprehensive review articles covering different aspects of the theory and applications of EPD,1–8 the focus of the present article is to highlight the most recent published research in this rapidly expanding field. 2. Mechanisms of EPD 2.1. Traditional approaches The fundamental mechanisms of EPD have been largely described in the literature mainly in the framework of the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory and in relation to the distortion of the particle double layer under the application of a DC electric field, as discussed by Sarkar and Nicholson in their key reference in EPD.1 However, numerous other theories (flocculation by particle accumulation, parti￾cle charge neutralization, electrochemical particle coagulation, electrical double layer (EDL) distortion and thinning mecha￾nism) have been proposed to explain the particle interactions and the kinetics of deposition1–3,5,12. Additional theoretical and modelling studies are being carried out in order to clarify the mechanisms of deposition and the role of electrochemical parameters on the complex interactions between solvent, parti￾cles and electric field. 2.1.1. Flocculation by particle accumulation Hamaker and Verwey13,14 observed similarities between for￾mation of deposits by electrophoresis and gravitation. In fact, in both processes, the pressure exerted by the arriving parti￾cles enables the particles close to the deposit to prevail over the inter-particle repulsion. Therefore the primary function of the applied electric field in EPD is to move the particles towards the electrode to accumulate. This mechanism can also explain the deposition of coatings onto membranes that are not serving as electrodes. 2.1.2. Particle charge neutralization mechanism Grillon et al.15 suggested that the charged particles are neu￾tralized when they touch the electrode. This mechanism explains the deposition of single particles and monolayers and the deposi￾tion of powders charged by the addition of salts to the suspension (e.g. the experiments described by Brown and Salt16). However

. Corni et al Jounal of the European Ceramic Sociery 28(2008)1353-1367 1355 this mechanism cannot clarify depositions carried out for longer LYOSPHERE DISTORTION BY EPD times, or for processes in which the particle-electrode contact is not permitted, for example when the deposition occurs on a semi-permeable membrane placed between the electrodes 2.1.3. Electrochemical particle coagulation mechanism This mechanism implies the reduction of the repulsive forces between the particles in suspension. Koelmans calculated the rise of the ionic strength close to the electrode when a difference LOCAL LYOSPHERE THINNING of potential was applied. This behaviour was due to an increase of the electrolyte concentration around the particles. He discov ered that the value of ionic strength was similar to that required to flocculate a suspension. Therefore, Koelmans' proposed a mechanism based on the fact that an increase of the electrolyte oncentration produces a decrease of the repulsion between the 澜一: particles close to the electrode(lower -potential) and conse- quently the particles coagulate. Considering that a finite time is needed for the increase of the electrolyte concentration next to the electrode, it can be concluded that a certain time has to pass COAGULATION in order to have deposition. This time is inversely proportional to the square of the applied voltage(to 1/E), i. e. the higher the applied potential the shorter the time required for deposi tion. This mechanism is plausible when the electrode reactions OH ions, e.g., suspensions containing water, but it when there is no increase of electrolyte concentration Fig. 3. Schematic representation of the deposition mechanism due to elec trical double layer distortion and thinning. I(Reproduced with permission of 2.1.4. Electrical double layer(edl) distortion and Blackwell Publishing. Sarkar and Nicholson proposed a model mainly based on the distortion of the particle double layer to explain the invalida- 2.2. Novel theories and models tion of the electrochemical coagulation mechanism when there is no increase of electrolyte concentration near the electrode. Studies of electrodynamic particle tion during They noted that when a positive particle and its shell are moving EPD have been carried out under steady and alternating towards the cathode, the double layer is distorted( thinner ahead electric fields. These models produced equations for the time and wider behind), as shown in Fig 3, due to fluid dynamics and evolution of the probability of separation between deposited to the effect of the applied electric field. As a result the counter particles in different conditions. These equations are able to ions(negative)in the extended tail experience a smaller coulom- explain the experimentally observed clustering of colloidal bic attraction to the positively charged particle and can more particles deposited near an electrode in a DC electric field easily react with other cations moving towards the cathode. This by considering convection by electro-osmotic flow about the process reduces the thickness of the double layer and therefore, particles. Numerical simulations have been also employe hen another particle with a thin double layer is approaching, to a limited extent to model the accumulation of charged the two particles come close enough to interact through London particles on an electrode during EPD. 21,22 These studies are Van der Waal attractive forces and coagulate. This mechanism of fundamental and practical interest to describe the local is plausible considering a high concentration of particles close variations of particle interaction during deposition, which can to the electrode (or high collision frequency). This mechanism be used to optimize the EPd technique works also for incoming particles with thin double layer heads, Regarding the growth of colloidal films during EPD, Sarkar coagulating with particles already in the deposit. et al. provided another fundamental study observing the depo- Subsequently Nicholson et al. showed that the model pre- sition of silica particles on silicon wafers as a function of viously proposed by Sarkar and Nicholson was not complete deposition time. They compared the nucleation and growth and proposed a new theory based on a decrease of the concen- of the silica particle layer with that of atomic film growth tration of H at the cathode due to particle discharge or other via molecular-beam epitaxy and noticed a prominent similar- hemical reactions. Therefore the local pH increases towards ity between the two processes. From this observation a new the isoelectric point (iep), s-potential decreases and the parti- direction for further research could follow in order to optimize cles coagulate. This mechanism is general for all suspensions the microstructure of EPD films. Theoretical work was also car containing hydrogen ions ried out by Van der Biest et al.24-27 who produced a model

I. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 1355 this mechanism cannot clarify depositions carried out for longer times, or for processes in which the particle-electrode contact is not permitted, for example when the deposition occurs on a semi-permeable membrane placed between the electrodes. 2.1.3. Electrochemical particle coagulation mechanism This mechanism implies the reduction of the repulsive forces between the particles in suspension. Koelmans17 calculated the rise of the ionic strength close to the electrode when a difference of potential was applied. This behaviour was due to an increase of the electrolyte concentration around the particles. He discov￾ered that the value of ionic strength was similar to that required to flocculate a suspension. Therefore, Koelmans17 proposed a mechanism based on the fact that an increase of the electrolyte concentration produces a decrease of the repulsion between the particles close to the electrode (lower -potential) and conse￾quently the particles coagulate. Considering that a finite time is needed for the increase of the electrolyte concentration next to the electrode, it can be concluded that a certain time has to pass in order to have deposition. This time is inversely proportional to the square of the applied voltage (t ∝ 1/E2), i.e. the higher the applied potential the shorter the time required for deposi￾tion. This mechanism is plausible when the electrode reactions generate OH− ions, e.g., suspensions containing water, but it is invalid when there is no increase of electrolyte concentration near the electrode. 2.1.4. Electrical double layer (EDL) distortion and thinning mechanism Sarkar and Nicholson1 proposed a model mainly based on the distortion of the particle double layer to explain the invalida￾tion of the electrochemical coagulation mechanism when there is no increase of electrolyte concentration near the electrode. They noted that when a positive particle and its shell are moving towards the cathode, the double layer is distorted (thinner ahead and wider behind), as shown in Fig. 3, due to fluid dynamics and to the effect of the applied electric field. As a result the counter ions (negative) in the extended tail experience a smaller coulom￾bic attraction to the positively charged particle and can more easily react with other cations moving towards the cathode. This process reduces the thickness of the double layer and therefore, when another particle with a thin double layer is approaching, the two particles come close enough to interact through London Van der Waal attractive forces and coagulate. This mechanism is plausible considering a high concentration of particles close to the electrode (or high collision frequency). This mechanism works also for incoming particles with thin double layer heads, coagulating with particles already in the deposit. Subsequently Nicholson et al.12 showed that the model pre￾viously proposed by Sarkar and Nicholson1 was not complete and proposed a new theory based on a decrease of the concen￾tration of H+ at the cathode due to particle discharge or other chemical reactions. Therefore the local pH increases towards the isoelectric point (iep), -potential decreases and the parti￾cles coagulate. This mechanism is general for all suspensions containing hydrogen ions. Fig. 3. Schematic representation of the deposition mechanism due to elec￾trical double layer distortion and thinning.1 (Reproduced with permission of Blackwell Publishing.) 2.2. Novel theories and models Studies of electrodynamic particle aggregation during EPD have been carried out under steady18 and alternating electric fields.19 These models produced equations for the time evolution of the probability of separation between deposited particles in different conditions. These equations are able to explain the experimentally observed clustering of colloidal particles deposited near an electrode in a DC electric field by considering convection by electro-osmotic flow about the particles.20 Numerical simulations have been also employed to a limited extent to model the accumulation of charged particles on an electrode during EPD.21,22 These studies are of fundamental and practical interest to describe the local variations of particle interaction during deposition, which can be used to optimize the EPD technique. Regarding the growth of colloidal films during EPD, Sarkar et al.23 provided another fundamental study observing the depo￾sition of silica particles on silicon wafers as a function of deposition time. They compared the nucleation and growth of the silica particle layer with that of atomic film growth via molecular-beam epitaxy and noticed a prominent similar￾ity between the two processes. From this observation a new direction for further research could follow in order to optimize the microstructure of EPD films. Theoretical work was also car￾ried out by Van der Biest et al.24–27 who produced a model to

. Corni et al. Journal of the European Ceramic Society 28(2008)1353-136 predict the yield of the electrophoretic deposition process taking atures above 425C. It was found that EPD coatings presented into account the changes of the electric field over the suspension superior smoothness and uniformity compared to those obtained due to the potential drop over the growing deposit. This model by conventional dipping or spraying processes. These coating was validated for Al2O3 suspensions in ethanol with different have found several applications in the industrial production of More recently Van Tassel and Randall28 electrophoretically have summarised that earlier wotk Os and previous reviews concentrations and with addition of HNO3 deposited alumina powder from an acidic suspension obtainin a very uniform, dense alumina layer and observed an anoma- 3. 2. Advanced materials lous voltage rise across the deposited particulate layer. They showed that these two effects can be explained by the forma- In this section the EPD applications of conventional powders tion of an ion depleted conduction layer in the solvent at the e.g. um-sized or submicrometric (d> 100 nm), are reviewed, deposition electrode, which presents an extremely high voltage whereas the applications of nanopowders in EPD are considered gradient. Therefore the electrophoretic force on the particles in in Section 3.3 this layer is considerably higher than the force on particles in the rest of the system and this high voltage gradient layer also 3. 2.1. Coatings and films produces a large self-levelling effect for deposition thickness. First reports on the use of EPD to prepare advanced ceramic inally, Ristenpart et al. ,S have recently studied, both theo- coatings were published in the late 1980s. For example hydrated retically and experimentally, the flow around a charged spherical alumina prepared by the sol-gel method was deposited by EPD colloid next to an electrode in order to understand the nature of on aluminium alloy substrates and it was demonstrated that long-range particle-particle attraction near the electrodes. From these coatings were thicker, denser and more adherent than their studies it was clear that the direction of flow of a particle those produced by conventional dip-coating techniques. In the depends on the sign of the dipole coefficient and that the flow last two decades EPd has been increasingly employed to pro- consists of two components: the electro-osmotic flow (EOF)and duce advanced ceramic coatings on solid substrates in order to the electrohydrodynamic(EHD)flow. The electro-osmotic flow enhance the substrate properties. For example EPD has been is proportional to the current density and the particle s-potent utilized to deposit materials with improved wear and oxidation while the electrodynamic flow derives from the product of the resistance, to deposit bioactive coatings for biomedical implants current density and the applied potential. Comparing these two and to produce functional coatings for electronic, magnetic and components, Ristenpart et al. 29,30 found that the attractive EHD related applications, and key early references are given in pre flow predominated far from the particle, whereas the attractive vious review articles. -, In order to improve the wear and OF predominated over the repulsive EHD flow close to the par- abrasion resistance of materials, research has been also focused ticle. Moreover they also observed that under certain conditions, on the development of metal/ceramic and ceramic/ceramic com- the two flows are both directed toward the particle producing posite coatings For the production of metal/ceramic composite aggregation coatings EPD is usually employed in combination with electro- The novel theoretical and modelling approaches summarised plating or galvanic deposition of metals. 32-37Moreover yttria in the literature to investigate basic phenomena occurring dur- produced on Fecralloys by EPo- Composite coatings have been in this section represent examples of the few efforts available stabilized zirconia (YSZ/alumina ing EPD. We highlight here the necessity for further theoretical quent reaction bonding processes. and densified by a subse nd modelling work in the field of EPD and the need for estab- It is clear that the most difficult task in the production of lishing reliable correlations between model variables and the ceramic coatings on a metal substrates is related to the limited experimental processing EPD conditions temperature capability of the metals and the high temnes'g tures required for sintering the ceramic layer. Wang et al 3. Applications of EPD partially resolved this problem by demonstrating that reaction- bonding is an excellent alternative to conventional sintering 3.. Traditional ceramics Electrophoretic deposition has also been used in ceramic joining applications. Mixtures of SiC or Si3 N4 and reactive carbon were Electrophoretic deposition initially found commercial inter- deposited onto SiC or Si3 N4 parts to provide intermediate layers est and industrial applications for the deposition of uniform for reaction bonding with molten silicon. *U The results obtained coatings made of clay based material, vitreous enamel or by Lessing et al. are significant because they showed for the alumina, on electrically conductive surfaces from aqueous first time how the combination of EPD and reaction bonding suspensions. The use of EPD for the production of clay based allows for the fabrication of large complex ceramic structures bodies, e.g. sanitary ware, tiles, table ware, etc, on an industrial manufactured from smaller components made of SiC or Si3N4 scale has been extensively investigated because of the order of De Riccardis et al. electrophoretically deposited alumina and magnitude improvement in formation rates achieved compared alumina-zirconia coatings with uniform thickness and homo- to slip casting 4,6 EPD has also been employed to produce vitre- geneous composition on stainless steel substrates starting from ous(or porcelain) enamel coatings on metals. After deposition ethanolic suspensions. They extensively studied the suspension of a layer of glass particles, inorganic coatings were obtained properties(conductivity, stability, particle size, transmittance by fusing the powder deposited on the metal surface at temper- and s-potential) to optimize the composition and the amount of

1356 I. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 predict the yield of the electrophoretic deposition process taking into account the changes of the electric field over the suspension due to the potential drop over the growing deposit. This model was validated for Al2O3 suspensions in ethanol with different concentrations and with addition of HNO3. More recently Van Tassel and Randall28 electrophoretically deposited alumina powder from an acidic suspension obtaining a very uniform, dense alumina layer and observed an anoma￾lous voltage rise across the deposited particulate layer. They showed that these two effects can be explained by the forma￾tion of an ion depleted conduction layer in the solvent at the deposition electrode, which presents an extremely high voltage gradient. Therefore the electrophoretic force on the particles in this layer is considerably higher than the force on particles in the rest of the system and this high voltage gradient layer also produces a large self-levelling effect for deposition thickness. Finally, Ristenpart et al.29,30 have recently studied, both theo￾retically and experimentally, the flow around a charged spherical colloid next to an electrode in order to understand the nature of long-range particle–particle attraction near the electrodes. From their studies it was clear that the direction of flow of a particle depends on the sign of the dipole coefficient and that the flow consists of two components: the electro-osmotic flow (EOF) and the electrohydrodynamic (EHD) flow. The electro-osmotic flow is proportional to the current density and the particle -potential, while the electrodynamic flow derives from the product of the current density and the applied potential. Comparing these two components, Ristenpart et al.29,30 found that the attractive EHD flow predominated far from the particle, whereas the attractive EOF predominated over the repulsive EHD flow close to the par￾ticle. Moreover they also observed that under certain conditions, the two flows are both directed toward the particle producing aggregation. The novel theoretical and modelling approaches summarised in this section represent examples of the few efforts available in the literature to investigate basic phenomena occurring dur￾ing EPD. We highlight here the necessity for further theoretical and modelling work in the field of EPD and the need for estab￾lishing reliable correlations between model variables and the experimental processing EPD conditions. 3. Applications of EPD 3.1. Traditional ceramics Electrophoretic deposition initially found commercial inter￾est and industrial applications for the deposition of uniform coatings made of clay based material, vitreous enamel or alumina, on electrically conductive surfaces from aqueous suspensions.6 The use of EPD for the production of clay based bodies, e.g. sanitary ware, tiles, table ware, etc., on an industrial scale has been extensively investigated because of the order of magnitude improvement in formation rates achieved compared to slip casting.4,6 EPD has also been employed to produce vitre￾ous (or porcelain) enamel coatings on metals. After deposition of a layer of glass particles, inorganic coatings were obtained by fusing the powder deposited on the metal surface at temper￾atures above 425 ◦C. It was found that EPD coatings presented superior smoothness and uniformity compared to those obtained by conventional dipping or spraying processes.6 These coatings have found several applications in the industrial production of domestic whiteware in the early 1970s and previous reviews have summarised that earlier work.1,6 3.2. Advanced materials In this section the EPD applications of conventional powders, e.g. m-sized or submicrometric (d > 100 nm), are reviewed, whereas the applications of nanopowders in EPD are considered in Section 3.3. 3.2.1. Coatings and films First reports on the use of EPD to prepare advanced ceramic coatings were published in the late 1980s. For example hydrated alumina prepared by the sol–gel method was deposited by EPD on aluminium alloy substrates and it was demonstrated that these coatings were thicker, denser and more adherent than those produced by conventional dip-coating techniques.31 In the last two decades EPD has been increasingly employed to pro￾duce advanced ceramic coatings on solid substrates in order to enhance the substrate properties. For example EPD has been utilized to deposit materials with improved wear and oxidation resistance, to deposit bioactive coatings for biomedical implants and to produce functional coatings for electronic, magnetic and related applications, and key early references are given in pre￾vious review articles.1–3,5 In order to improve the wear and abrasion resistance of materials, research has been also focused on the development of metal/ceramic and ceramic/ceramic com￾posite coatings. For the production of metal/ceramic composite coatings EPD is usually employed in combination with electro￾plating or galvanic deposition of metals.32–37 Moreover yttria stabilized zirconia (YSZ)/alumina composite coatings have been produced on Fecralloys by EPD38,39 and densified by a subse￾quent reaction bonding processes.38 It is clear that the most difficult task in the production of ceramic coatings on a metal substrates is related to the limited temperature capability of the metals and the high tempera￾tures required for sintering the ceramic layer. Wang et al.38 partially resolved this problem by demonstrating that reaction￾bonding is an excellent alternative to conventional sintering. Electrophoretic deposition has also been used in ceramic joining applications. Mixtures of SiC or Si3N4 and reactive carbon were deposited onto SiC or Si3N4 parts to provide intermediate layers for reaction bonding with molten silicon.40 The results obtained by Lessing et al.40 are significant because they showed for the first time how the combination of EPD and reaction bonding allows for the fabrication of large complex ceramic structures manufactured from smaller components made of SiC or Si3N4. De Riccardis et al.41 electrophoretically deposited alumina and alumina–zirconia coatings with uniform thickness and homo￾geneous composition on stainless steel substrates starting from ethanolic suspensions. They extensively studied the suspension properties (conductivity, stability, particle size, transmittance and -potential) to optimize the composition and the amount of

. Corni et al Jounal of the European Ceramic Sociery 28(2008)1353-1367 Fig. 4. SEM micrographs of diamond/borosilicate glass composite coatings after sintering. These coatings have been electrophoretically co-deposited on stainless steel substrates from(a)1.5g/100 ml diamond and 1.0g/100 ml borosilicate glass in ethanol and(b)3.0 g/100 ml diamond and 1.0g/100 ml borosilicate glass in ethanol.(Published with permission of Elsevier. dispersants(citric acid and triethylamine)to be added in order to for use in gas separations, b ordered mesoporous silicate(MPS) obtain the required particle dispersion and high suspension sta- for energy-efficient adsorption systems (e.g. desiccant and bility. Novaket al +produced a firm and pore-free SiC fibre-Sic cooling systems),67 PZT coatings for embedded components or particle composite by EPD. They studied the effect of suspension for optical switches, 68-73 TaO N, on Ti for its catalytic activity composition on the deposition results and they observed that the in oxygen reduction reactions, yttrium silicate(YSI)coatings suspension ph, the solid loading and the particle size(micro or on C/C-Si-Sic composites for protection against oxidation at nano)have all a strong influence on the process and on the prop- high temperatures, 7 boron films and MgB2 films obtained by erties of the fresh Sic deposits. In a recent investigation, Wang heat treatment of Mg/B coatings for diffusion of the Mg into the et al. deposited smooth, uniform, dense diamond/borosilicate boron film, 76, 77 Pb-Zr-Ti-Nb-Si-O ferroelectric thick films78 glass composite coatings onto stainless steel by electrophoretic and aluminium coatings on Fe CrAl substrates. 79 Moreover, the co-deposition. They also demonstrated that is possible to control electrophoretic deposition of high-temperature superconducting the coating microstructure and composition by tailoring the EPd films with controlled thickness on substrates of various sha Ispension, in fact, for this system, the concentration ratio of the and dimensions has gained increasing interest. 8 two materials in the coating(borosilicate glass and diamond) was found to be in direct correlation with the diphasic susper 3. 2. 2. Porous materials concentration. The coatings were sintered to spread the glass EPD has been increasingly used to coat textile and porous over the diamond particle surface and to protect the diamond substrates with ceramic particles to produce a range of poro particles from oxidization or graphitization. The microstructure materials that can be applied for filters, porous carriers, bioactive of the sintered EPD coatings is shown in Fig 4 scaffolds, photocatalysis and hollow fibre fabrication. Zhito- EPD has also found successful applications in the pro- mirsky and Gal-Or 2 electrophoretically deposited submicron duction of bioactive coatings for biomedical implants and alumina and zirconia powders on carbon fibres and were able to devices. For example, the improvement of EPD for deposi- obtain hollow ceramic fibres after burning out the inner carbon tion of bioactive hydroxyapatite and related calcium phosphate core. Moreover Zhitomirsky demonstrated that hydroxyap- films on biocompatible metallic substrates(e.g. TiAl4V alloys atite(HA)coated carbon fibres can produce, after burning out and Fecralloys)u and the deposition of zirconia layers the fibrous carbon substrates, hollow HA fibres of various diam- on dental crowns and bridges have been recently reported. eters. a similar study was carried out by Wang et al.84 wh The deposition of bioactive glass and polyetheretherketone performed repeated HA deposition on carbon rod in order to (PEEK)/bioactive glass composite coatings on shape memory obtain a thick, uniform and crack-free HA film. It was observed alloy substrates has also been successfully achieved. that the uniformity of the coatings and the avoidance of cracking Some other significant recent developments include: the were the result of the repeated deposition process which fills up fabrication of BaTiO3 thick films for sensor and actuator cracks and hinders crack propagation. After burning out the car- applications, 5.36 ZnO thick films for gas sensors, MgO- bon rod a uniform and crack-free HA ceramic tube is produced, devices, S8 LiCoO2 electrodes for rechargeable lithium as sh modified Bao.6 Sro4TiO3 thick films for tunable microwave as shown in Fig. 5.84 EPD has also been applied by Maet al. >to prepare bioactive batteries, 9,0 LiNio. Mn1sO4 thick-film electrodes for use in porous hydroxyapatite(HA)scaffolds. They demonstrated that high voltage lithium-ion batteries, V2O5 microparticles for the pores were interconnected and that pore size was between cathodes for Li-secondary batteries, 62 MgO thick films for several microns and hundreds of microns. Moreover these scaf- electronics,63carbon-polytetrafluoroethylene thin films for gas folds exhibited excellent mechanical properties. Hamagami et diffusion electrodes, BaNd2 Tis O14 thick films for microwave al.86 87 studied the fabrication of highly ordered macroporous communication devices,S zeolites for supported membranes bioactive ceramic coating onto titanium by EpD followed by a

I. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 1357 Fig. 4. SEM micrographs of diamond/borosilicate glass composite coatings after sintering. These coatings have been electrophoretically co-deposited on stainless steel substrates from (a) 1.5 g/100 ml diamond and 1.0 g/100 ml borosilicate glass in ethanol and (b) 3.0 g/100 ml diamond and 1.0 g/100 ml borosilicate glass in ethanol.43 (Published with permission of Elsevier.) dispersants (citric acid and triethylamine) to be added in order to obtain the required particle dispersion and high suspension sta￾bility. Novak et al.42 produced a firm and pore-free SiC fibre–SiC particle composite by EPD. They studied the effect of suspension composition on the deposition results and they observed that the suspension pH, the solid loading and the particle size (micro or nano) have all a strong influence on the process and on the prop￾erties of the fresh SiC deposits. In a recent investigation, Wang et al.43 deposited smooth, uniform, dense diamond/borosilicate￾glass composite coatings onto stainless steel by electrophoretic co-deposition. They also demonstrated that is possible to control the coating microstructure and composition by tailoring the EPD suspension, in fact, for this system, the concentration ratio of the two materials in the coating (borosilicate glass and diamond) was found to be in direct correlation with the diphasic suspension concentration. The coatings were sintered to spread the glass over the diamond particle surface and to protect the diamond particles from oxidization or graphitization. The microstructure of the sintered EPD coatings is shown in Fig. 4. EPD has also found successful applications in the pro￾duction of bioactive coatings for biomedical implants and devices. For example, the improvement of EPD for deposi￾tion of bioactive hydroxyapatite and related calcium phosphate films on biocompatible metallic substrates (e.g. TiAl4V alloys and Fecralloys)44–50 and the deposition of zirconia layers on dental crowns and bridges51 have been recently reported. The deposition of bioactive glass and polyetheretherketone (PEEK)/bioactive glass composite coatings on shape memory alloy substrates has also been successfully achieved.52–54 Some other significant recent developments include: the fabrication of BaTiO3 thick films for sensor and actuator applications,55,56 ZnO thick films for gas sensors,57 MgO￾modified Ba0.6Sr0.4TiO3 thick films for tunable microwave devices,58 LiCoO2 electrodes for rechargeable lithium batteries,59,60 LiNi0.5Mn1.5O4 thick-film electrodes for use in high voltage lithium-ion batteries,61 V2O5 microparticles for cathodes for Li-secondary batteries,62 MgO thick films for electronics,63 carbon-polytetrafluoroethylene thin films for gas diffusion electrodes,64 BaNd2Ti5O14 thick films for microwave communication devices,65 zeolites for supported membranes for use in gas separations,66 ordered mesoporous silicate (MPS) for energy-efficient adsorption systems (e.g. desiccant and cooling systems),67 PZT coatings for embedded components or for optical switches,68–73 TaOxNy on Ti for its catalytic activity in oxygen reduction reactions,74 yttrium silicate (YSI) coatings on C/C–Si–SiC composites for protection against oxidation at high temperatures,75 boron films and MgB2 films obtained by heat treatment of Mg/B coatings for diffusion of the Mg into the boron film,76,77 Pb–Zr–Ti–Nb–Si–O ferroelectric thick films78 and aluminium coatings on FeCrAl substrates.79 Moreover, the electrophoretic deposition of high-temperature superconducting films with controlled thickness on substrates of various shapes and dimensions has gained increasing interest.80,81 3.2.2. Porous materials EPD has been increasingly used to coat textile and porous substrates with ceramic particles to produce a range of porous materials that can be applied for filters, porous carriers, bioactive scaffolds, photocatalysis and hollow fibre fabrication. Zhito￾mirsky and Gal-Or82 electrophoretically deposited submicron alumina and zirconia powders on carbon fibres and were able to obtain hollow ceramic fibres after burning out the inner carbon core. Moreover Zhitomirsky83 demonstrated that hydroxyap￾atite (HA) coated carbon fibres can produce, after burning out the fibrous carbon substrates, hollow HA fibres of various diam￾eters. A similar study was carried out by Wang et al.84 who performed repeated HA deposition on carbon rod in order to obtain a thick, uniform and crack-free HA film. It was observed that the uniformity of the coatings and the avoidance of cracking were the result of the repeated deposition process which fills up cracks and hinders crack propagation. After burning out the car￾bon rod a uniform and crack-free HA ceramic tube is produced, as shown in Fig. 5. 84 EPD has also been applied by Ma et al.85 to prepare bioactive porous hydroxyapatite (HA) scaffolds. They demonstrated that the pores were interconnected and that pore size was between several microns and hundreds of microns. Moreover these scaf￾folds exhibited excellent mechanical properties. Hamagami et al.86,87 studied the fabrication of highly ordered macroporous bioactive ceramic coating onto titanium by EPD followed by a