《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-4

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

. Corni et al /Journal of the European Ceramic Society 28(2008)1353-136 100ym 。Hn111175 6. Microstructure of of Al2O3 and Y-TZP, prepared by electrophoretic deposition of suspensions of Fig. 5. SEM picture of a uniform and crack-free HA tube produced by EPD each material by Ferrari et al. 104(Published with permission of Elsevier. after burning out the carbon rod substrate according to Wang et al. &4(Published with permission of Elsevier. which EPD is carried out. Developments achieved in this field up to year 2000 are reviewed by Van der Biest and heat treatment.The biocompatibility of these materials was eval- Vandeperre' and more recent progresses in FGM fabrication uated and demonstrated in vitro using a simulated body fiuid have been reported by Put et al. 95 showing the manufac (SBF). Yabutsuka et al 8s deposited wollastonite particles on ture of graded wC-Co composites using a suspension of c porous ultrahigh molecular weight polyethylene (UHMWPE) powder in acetone with variable Co powder content. Other substrates producing a composite. When this composite was significant developments achieved by the same group con- soaked in SBF for 14 days an apatite film grew from the wol- sisted of anodic co-deposition of Al2O3 and CeO2-stabilized lastonite inside the pores to the top surface of the composite. zirconia powders to produce cylindrical and tubular-shaped The apatite presented high adhesive strength to the composite Al2O3/zirconia FGM components% and the production of func probably due to an interlocking effect. This material might be tionally graded Si3N4-TiCo s Nos composites using Si3N4 as a employed for hard and soft tissue implants for its mechanical matrix and TiCo. s No s as the hard phase. Other studies on the properties and high bioactivity. EPD has been also applied to EPD production of nickel-alumina, 98 alumina-zirconia, 99-101 fabricate biodegradable polymer foams coated with bioactive Al2O3-ZrO2-Ti(C, N) 02 and hydroxyapatite- bioactive glass 103 mina membranes with uniform porous structure and suitable size Further efforts have been devoted to the devel lopment of EPD for application in microfiltration. Huang and Yang produced fabrication approaches for laminated ceramic composites uniform and dense zeolite membranes on an alumina support and particular in the system zirconia/alumina, due to the high frac Negishi et al.deposited lanthanum cobaltite on a porous alu- ture resistance of these structures. 104 As an example, a general mina tube by EPD and they were able to obtain dense membranes overview of the microstructure of Al203/Y-TZP layered com- after a sintering step. Sun et al.deposited by EPD a well- posites with 10 layers obtained by Moreno and Ferrari is adherent Y-AlO3 washcoat on metallic wire-mesh monoliths shown in fig. 6. The studies of moreno and ferrari. 104 fis- for applications in environmental catalysis and other industrial cher et al. 105 and Uchikoshi et al. 106 have been focused on catalysis process(e.g, air pollution abatement devices and auto- the optimization of EPD from aqueous suspensions for the motive emissions controls). In related applications, Yanagida et production of laminated ceramics. Water should always be pre- al. prepared a TiO2 coating on stainless steel mesh by EPD ferred over organic solvents due to environmental and economic examine the synergy effect on photocatalysis of both 1, 4-dioxane considerations. 107 In some cases however. other solvents and ethylene glycol diformate acetone, ethanol, must be used. For example You et al. reported the fabrication of SiC/TiC laminated structures by elec 3.2.3. Laminated, functionally graded and composite trophoretic deposition from acetone-based suspensions. Based materials on the promising results achieved so far, a significant growth of EPD has been employed successfully to fabricate ceramic R&D work related to the electrophoretic deposition of laminated laminates, fibre reinforced ceramic composites and functionally and functionally graded coatings is anticipated graded materials(FGMs). Laminated materials can be pro duced via EPD moving the deposition electrode to a second 3.2.4. Ceramic layers for solid oxide fuel cells suspension for deposition of a layer of different composi Numerous investigations have recently concentrated on solid on when the desired thickness of the first layer is reached. oxide fuel cells(SOFCs)as new electric power generation By changing back and forth, a layered material is readily systems. 09 The increased interest in SOFCs is due to their high obtained. Moreover FGM can also be produced using EPD by energy conversion efficiency, clean power generation, reliabil- gradually changing the composition of the suspension from ity, modularity, fuel adaptability, the fact that they are noise-free

1358 I. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 Fig. 5. SEM picture of a uniform and crack-free HA tube produced by EPD after burning out the carbon rod substrate, according to Wang et al.84 (Published with permission of Elsevier.) heat treatment. The biocompatibility of these materials was eval￾uated and demonstrated in vitro using a simulated body fluid (SBF). Yabutsuka et al.88 deposited wollastonite particles on porous ultrahigh molecular weight polyethylene (UHMWPE) substrates producing a composite. When this composite was soaked in SBF for 14 days an apatite film grew from the wol￾lastonite inside the pores to the top surface of the composite. The apatite presented high adhesive strength to the composite probably due to an interlocking effect. This material might be employed for hard and soft tissue implants for its mechanical properties and high bioactivity. EPD has been also applied to fabricate biodegradable polymer foams coated with bioactive glass particles.89 Moreover Chen et al.90 fabricated porous alu￾mina membranes with uniform porous structure and suitable size for application in microfiltration. Huang and Yang91 produced uniform and dense zeolite membranes on an alumina support and Negishi et al.92 deposited lanthanum cobaltite on a porous alu￾mina tube by EPD and they were able to obtain dense membranes after a sintering step. Sun et al.93 deposited by EPD a well￾adherent -Al2O3 washcoat on metallic wire-mesh monoliths for applications in environmental catalysis and other industrial catalysis process (e.g., air pollution abatement devices and auto￾motive emissions controls). In related applications, Yanagida et al.94 prepared a TiO2 coating on stainless steel mesh by EPD to examine the synergy effect on photocatalysis of both 1,4-dioxane and ethylene glycol diformate. 3.2.3. Laminated, functionally graded and composite materials EPD has been employed successfully to fabricate ceramic laminates, fibre reinforced ceramic composites and functionally graded materials (FGMs).1–3 Laminated materials can be pro￾duced via EPD moving the deposition electrode to a second suspension for deposition of a layer of different composi￾tion when the desired thickness of the first layer is reached. By changing back and forth, a layered material is readily obtained. Moreover FGM can also be produced using EPD by gradually changing the composition of the suspension from Fig. 6. Microstructure of a laminar material consisting of 10 alternating layers of Al2O3 and Y-TZP, prepared by electrophoretic deposition of suspensions of each material by Ferrari et al.104 (Published with permission of Elsevier.) which EPD is carried out. Developments achieved in this field up to year 2000 are reviewed by Van der Biest and Vandeperre3 and more recent progresses in FGM fabrication have been reported by Put et al.,95 showing the manufac￾ture of graded WC–Co composites using a suspension of WC powder in acetone with variable Co powder content. Other significant developments achieved by the same group con￾sisted of anodic co-deposition of Al2O3 and CeO2-stabilized zirconia powders to produce cylindrical and tubular-shaped Al2O3/zirconia FGM components96 and the production of func￾tionally graded Si3N4–TiC0.5N0.5 composites using Si3N4 as a matrix and TiC0.5N0.5 as the hard phase.97 Other studies on the EPD production of nickel–alumina,98 alumina–zirconia,99–101 Al2O3–ZrO2–Ti(C,N)102 and hydroxyapatite-bioactive glass103 FGM coatings have been successfully carried out. Further efforts have been devoted to the development of EPD fabrication approaches for laminated ceramic composites, in particular in the system zirconia/alumina, due to the high frac￾ture resistance of these structures.104 As an example, a general overview of the microstructure of Al2O3/Y-TZP layered com￾posites with 10 layers obtained by Moreno and Ferrari104 is shown in Fig. 6. The studies of Moreno and Ferrari,104 Fis￾cher et al.105 and Uchikoshi et al.106 have been focused on the optimization of EPD from aqueous suspensions for the production of laminated ceramics. Water should always be pre￾ferred over organic solvents due to environmental and economic considerations.107 In some cases, however, other solvents, e.g. acetone, ethanol, must be used. For example You et al.108 reported the fabrication of SiC/TiC laminated structures by elec￾trophoretic deposition from acetone-based suspensions. Based on the promising results achieved so far, a significant growth of R&D work related to the electrophoretic deposition of laminated and functionally graded coatings is anticipated. 3.2.4. Ceramic layers for solid oxide fuel cells Numerous investigations have recently concentrated on solid oxide fuel cells (SOFCs) as new electric power generation systems.109 The increased interest in SOFCs is due to their high energy conversion efficiency, clean power generation, reliabil￾ity, modularity, fuel adaptability, the fact that they are noise-free

. Corni et al Jounal of the European Ceramic Sociery 28(2008)1353-1367 1359 have excellent long term stability and also due to the versatile low bodies and objects of complex 3D shape using EPd of nature of the technology for direct conversion of chemical energy silica nanoparticles in aqueous suspensions. However, numerous to electrical energy materials can be deposited by EPD starting from water-based or In general, a solid oxide fuel cell device consists of a repeat- non-aqueous suspensions of nanoparticles. For example nanos- ing stack of multiple single fuel cells. In order to render SOFCs tructured BaTiO3 and Eu-doped BaTiO3 thin films have been economically competitive, however, it is fundamental to sig- recently reported, which were produced by EPD of nanoparti nificantly reduce the cost of materials and fabrication. For this cles from acetylacetone suspensions, 24 aqu reason EPD has been progressively considered for the fabrica- and ethanol suspensions. 26, 27 Zinc oxide nanoparticles have tion of cathode-and anode-supported solid oxide fuel cells of been deposited from aqueous suspensions, isopropyl alcohol both planar and tubular geometry. Several recent papers describe suspensions and 2-propanol suspensions. 30 The produc the use of EPD in this field2-12 and a complete review of the tion of nanostructured zirconia coatings by EPD has been developments achieved so far has been written by Besra and reported both from aqueous suspensions 31, 32 to pro uce den- tal crowns and from ethanol suspensions to produce thermal From these studies it has emerged that the relative advan- barrier coatings tages of EPD in the production of SoFCs are the ability to: Moreover a wide range of dense, nanostructured fund (a) deposit coatings on substrates of any shape,(b)control the tional films have been produced by electrophoretic deposition deposition conditions thus being able to prepare porous coat- of nanoparticles. Mahajan et al uce ing as electrode and dense coating as electrolyte, (c)obtain europium oxide(Eu2O3) thin films and noticed that the films laminate structures of electrodes and electrolyte and (d)pro- optical properties varied with deposit morphology: translucent, duce Ni-yttria stabilized zirconia (YSZ) cermets(anodes) by when there was a uniform size and distribution of the microstruc electrophoretic co-deposition. Yttria-stabilized zirconia (YSz) ture, and opaque, when there was a marked anisotropy of the by far the most popular material used as electrolyte in SOFCs size and distribution of the constituents of the microstruc- anks to its exceptional combination of properties such as high ture. The deposition of nanostructured titania films by EPD chemical and thermal stability and pure ionic conductivity over has been carried out by several groups. 35-14 For example a wide range of conditions. Also Lao. 3 Sro. 17Gao. 3Mgo 1702.83 Dittrich et al. 35, 36 produced TiO2 coatings with different (LSGM) has been employed to produce intermediate tempera- porosity, systematically changed by pressing, in order to opti- ture solid oxide fuel cells (IT-SOFCs). EPD has been also mize the electron diffusion. These coatings present potential shown to be effective to deposit glass-ceramic layers, which are applications in many fields, such as batteries, displays, photo- used as sealant material in some SOFCs designs. 22 Moreover catalysis and solar energy conversion systems. Manriquez and because of the short formation times and the simple equipment Godinez u deposited Ti(m)-doped nanocrystalline TiO2 films needed the use of EPD should simplify the fabrication process on optically transparent electrodes. Valatka and Kulesius41 of SoFC stacks with complex design architecture achieving fur- deposited nanosized titania films on stainless steel and then ther cost reductions. In spite of the progress achieved recently they used the decoloration of methylene blue dye to evaluate in this area, many problems remain unsolved as described ear- the photoelectrocatalytic activity of the coatings. Hydroxyap er by Zhitomirsky and Petric, 2 who underlined that major atite(HA)nanoparticles have been extensively deposited by difficulties are linked to the selection of adequate solvents and means of EPD 42-146 Other nanoparticles were deposited by of the components of the binder-dispersant-solvent system, the an ethanol suspension 69 silicon carbide. 141 ceric<48 powder additives, in particular concerning the chemical compatibility EPD such as: nanosized lead zirconate titanate(Pzt) powder in solubility of the binder, the viscosity and the electrical resistin- pentoxide, 49 hydrous ruthenium oxide, 50, 5I gamma ferric broaden the range of applications of EPD in SOFC technology. and yttria-stabilized zirconia anotubes 159 1 0ve-155nickel ity of the suspensions. Therefore more studies are needed and a oxide, 52 nickel, 53, 54 iron, 53 aluminium, 53 better understanding of the process has to be achieved in order to ferrite, 56 silica, 57, 58 titania polyimide161 Recently, research has been carried out on the deposition of 3.3. Nanotechnology polymeric nanocomposite coatings. For example, polythiophene and metal oxide (alumina, titania and silica) nanocompos 3.3.1. EPD of nanoparticle ites have been electrophoretically deposited from an ethanol The electrophoretic deposition of ceramic nanoparticles(size suspension by Vu et al. 63 They obtained thin films of con- 100 nm)is a special colloidal processing method employed to ducting polymer/metal oxide with a core-shell structure that produce a variety of materials, including monolithic ceramics, can be employed in electronic devices. Kim et al. 6- deposited well as ceramic laminates and ceramic matrix composites of poly(methyl methacrylate)-BaTiO3 nanocomposite coatings ceramic coatings and films, functionally graded materials, as from an isopropyl alcohol/acetone solvent mixture on copper high microstructural homogeneity. Previous work on EPD of foils. The composite films presented a uniform microstructure nanoparticles has been reviewed comprehensively elsewhere& without particle agglomeration. Furthermore, the deposition of and only selected recent papers published in the last 2 years are metal-ceramic nanocomposite coatings has also been inves therefore covered in this section tigated. For example, nanostructured Ni-wC-Co composite Tabellion and Clasen07 have discussed previous work on coatings have been produced by means of EPD on nickel the fabrication of large components, free standing objects, hol- plated stainless steel substrates. These composite coatings

I. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 1359 have excellent long term stability and also due to the versatile nature of the technology for direct conversion of chemical energy to electrical energy.109–111 In general, a solid oxide fuel cell device consists of a repeat￾ing stack of multiple single fuel cells. In order to render SOFCs economically competitive, however, it is fundamental to sig￾nificantly reduce the cost of materials and fabrication. For this reason EPD has been progressively considered for the fabrica￾tion of cathode- and anode-supported solid oxide fuel cells of both planar and tubular geometry. Several recent papers describe the use of EPD in this field112–121 and a complete review of the developments achieved so far has been written by Besra and Liu.5 From these studies it has emerged that the relative advan￾tages of EPD in the production of SOFCs are the ability to: (a) deposit coatings on substrates of any shape, (b) control the deposition conditions thus being able to prepare porous coat￾ing as electrode and dense coating as electrolyte, (c) obtain laminate structures of electrodes and electrolyte and (d) pro￾duce Ni-yttria stabilized zirconia (YSZ) cermets (anodes) by electrophoretic co-deposition. Yttria-stabilized zirconia (YSZ) is by far the most popular material used as electrolyte in SOFCs thanks to its exceptional combination of properties such as high chemical and thermal stability and pure ionic conductivity over a wide range of conditions. Also La0.83Sr0.17Ga0.83Mg0.17O2.83 (LSGM) has been employed to produce intermediate tempera￾ture solid oxide fuel cells (IT-SOFCs).118 EPD has been also shown to be effective to deposit glass-ceramic layers, which are used as sealant material in some SOFCs designs.122 Moreover because of the short formation times and the simple equipment needed the use of EPD should simplify the fabrication process of SOFC stacks with complex design architecture achieving fur￾ther cost reductions. In spite of the progress achieved recently in this area,5 many problems remain unsolved as described ear￾lier by Zhitomirsky and Petric,123 who underlined that major difficulties are linked to the selection of adequate solvents and additives, in particular concerning the chemical compatibility of the components of the binder-dispersant-solvent system, the solubility of the binder, the viscosity and the electrical resistiv￾ity of the suspensions. Therefore more studies are needed and a better understanding of the process has to be achieved in order to broaden the range of applications of EPD in SOFC technology. 3.3. Nanotechnology 3.3.1. EPD of nanoparticles The electrophoretic deposition of ceramic nanoparticles (size <100 nm) is a special colloidal processing method employed to produce a variety of materials, including monolithic ceramics, ceramic coatings and films, functionally graded materials, as well as ceramic laminates and ceramic matrix composites of high microstructural homogeneity. Previous work on EPD of nanoparticles has been reviewed comprehensively elsewhere8 and only selected recent papers published in the last 2 years are therefore covered in this section. Tabellion and Clasen107 have discussed previous work on the fabrication of large components, free standing objects, hol￾low bodies and objects of complex 3D shape using EPD of silica nanoparticles in aqueous suspensions. However, numerous materials can be deposited by EPD starting from water-based or non-aqueous suspensions of nanoparticles. For example nanos￾tructured BaTiO3 and Eu-doped BaTiO3 thin films have been recently reported, which were produced by EPD of nanoparti￾cles from acetylacetone suspensions,124 aqueous suspensions125 and ethanol suspensions.126,127 Zinc oxide nanoparticles have been deposited from aqueous suspensions,128 isopropyl alcohol suspensions129 and 2-propanol suspensions.130 The produc￾tion of nanostructured zirconia coatings by EPD has been reported both from aqueous suspensions131,132 to produce den￾tal crowns132 and from ethanol suspensions to produce thermal barrier coatings.133 Moreover a wide range of dense, nanostructured func￾tional films have been produced by electrophoretic deposition of nanoparticles. Mahajan et al.134 produced nanocrystalline europium oxide (Eu2O3) thin films and noticed that the films optical properties varied with deposit morphology: translucent, when there was a uniform size and distribution of the microstruc￾ture, and opaque, when there was a marked anisotropy of the size and distribution of the constituents of the microstruc￾ture. The deposition of nanostructured titania films by EPD has been carried out by several groups.135–141 For example Dittrich et al.135,136 produced TiO2 coatings with different porosity, systematically changed by pressing, in order to opti￾mize the electron diffusion. These coatings present potential applications in many fields, such as batteries, displays, photo￾catalysis and solar energy conversion systems. Manr´ıquez and God´ınez140 deposited Ti(III)-doped nanocrystalline TiO2 films on optically transparent electrodes. Valatka and Kulesius141 deposited nanosized titania films on stainless steel and then they used the decoloration of methylene blue dye to evaluate the photoelectrocatalytic activity of the coatings. Hydroxyap￾atite (HA) nanoparticles have been extensively deposited by means of EPD.142–146 Other nanoparticles were deposited by EPD such as: nanosized lead zirconate titanate (PZT) powder in an ethanol suspension,69 silicon carbide,147 ceria,148 vanadium pentoxide,149 hydrous ruthenium oxide,150,151 gamma ferric oxide,152 nickel,153,154 iron,153 aluminium,153 silver,155 nickel ferrite,156 silica,157,158 titania nanotubes,159,160 polyimide161 and yttria-stabilized zirconia.162 Recently, research has been carried out on the deposition of polymeric nanocomposite coatings. For example, polythiophene and metal oxide (alumina, titania and silica) nanocompos￾ites have been electrophoretically deposited from an ethanol suspension by Vu et al.163 They obtained thin films of con￾ducting polymer/metal oxide with a core-shell structure that can be employed in electronic devices. Kim et al.164 deposited poly(methyl methacrylate)-BaTiO3 nanocomposite coatings from an isopropyl alcohol/acetone solvent mixture on copper foils. The composite films presented a uniform microstructure without particle agglomeration. Furthermore, the deposition of metal–ceramic nanocomposite coatings has also been inves￾tigated. For example, nanostructured Ni–WC–Co composite coatings have been produced by means of EPD on nickel￾plated stainless steel substrates.165 These composite coatings

1360 . Corni et al /Journal of the European Ceramic Society 28(2008)1353-136 10 ur b Fig. 7. SEM micrographs of a Nextel M 720 fibre mat infiltrated with alumina nano particles, (a)shows the complete and homogeneous infiltration of the NextelTM 720 fibre mat with alumina and(b)shows the high "green"density of the alumina matrix by Stoll et al. 169(Published with permission of Elsevier. presented a significant increase in hardness and wear resis- fibres employed can be both electrically conductive(e.g. Sic tance compared with pure nickel coatings or stainless steel. Nicalon, carbon)and non-conductive(e.g. alumina or mullite). Wang et al. 46 deposited Co-YSZ/HA nanocomposite coatings When the fibres are non-conductive, the fibre weave is placed on Ti substrate using the combination of electrocodeposition in front of the deposition electrode, the ceramic deposit devel and electrophoretic deposition. They demonstrated that the ops on the electrode and grows around and through the fibre Co-YSZ/HA composite coatings exhibited better mechanical mat.8, 168 Stoll et al. 69 fabricated alumina-alumina CMCs by properties than nano-HA single coatings. Moreover the Co-YSZ means of EPD from a suspension of Al2 O3 nanoparticles using interlayer reduced the mismatch of the thermal expansion coef- Nextel-type(alumina) fibres. The same composite(alumina ficients between HA and Ti and the adhesive strength of the particles and Nextel-type fibres)was investigated recently by composite coating and Ti substrate was higher than that of nano- Bao and Nicholson. In this particular system it was shown that HA single coatings on Ti substrate. Pang and Zhitomirsky in order to be able to infiltrate fibre mats by EPD it is fundam deposited by EPD composite hydroxyapatite-chitosan coatings tal that both the suspended particles and the fibres have the same on stainless steel substrates. They observed that the addition polarity. In these conditions, when the particles approach the of chitosan to the HA suspension promoted the deposition of fibre mat repulsion forces take place between them, the mag- HA. Moreover, from the analysis of the coatings it was demon- nitude of these forces depends on the relative distance between strated that changing the concentration of the two chemicals particles and fibres. Due to the applied external electrical field, in the suspension resulted in a variation in the coating com- each charged particle is attracted to the fibre mat, which is fixed position. These composite coatings were adherent and uniform to the electrode, but, at the same time, the particles are repelled with increased corrosion protection properties. Louh et al. 67 before they can reach the fibre surfaces(coagulation point)due deposited by EPD nanosized platinum/carbon nanocatalysts and to the charge on the fibres. It has been hypothesised that under Nafion solution on carbon-based substrates for applications in the effect of the repulsive forces due to the surrounding fibres, proton exchange membrane fuel cells(PEMFCs). They obtained the particles follow the path with the fewest possible obstacles a uniform distribution of catalyst and Nafionon the electrodes, until reaching the next interstice between adjacent fibres. There in fact the Pt/C nanopowders covered the whole carbon fibre fore, when the particles reach the electrode or the surface of surface and also infiltrated into the gaps and voids between previously deposited particles, they cannot move further and consequently the electrophoretic ceramic deposit grows with a EPD has been also employed for the production of fibre high particle packing density. Fig. 7(a)and(b) shows the unifor reinforced ceramic and glass matrix composites with a variety mity of the infiltration of alumina particles in the Nextel -type of ceramic matrices and fibres. 168 This method is particu-(alumina)fibre mat and the high level of packing achieved by the larly attractive for the production of CMCs with complex fibre electrophoretically deposited alumina particles. 69 Conversely, structures(e. g. 2D fibre fabrics)as reinforcement because it if the fibre and particles exhibit opposite surface charge, coa allows adequate infiltration of ceramic(nano) particles into the lation is expected on the first layer of fibres encountered by the inter-fibre spaces, which would be difficult to be achieved by travelling particles. In this case a deposit on the outer fibre layer other routes. 68 Additionally, it should be emphasised that the will block the movement of the particles towards the interior

1360 I. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 Fig. 7. SEM micrographs of a NextelTM 720 fibre mat infiltrated with alumina nano particles, (a) shows the complete and homogeneous infiltration of the NextelTM 720 fibre mat with alumina and (b) shows the high “green” density of the alumina matrix by Stoll et al.169 (Published with permission of Elsevier.) presented a significant increase in hardness and wear resis￾tance compared with pure nickel coatings or stainless steel.165 Wang et al.146 deposited Co-YSZ/HA nanocomposite coatings on Ti substrate using the combination of electrocodeposition and electrophoretic deposition. They demonstrated that the Co-YSZ/HA composite coatings exhibited better mechanical properties than nano-HA single coatings. Moreover the Co-YSZ interlayer reduced the mismatch of the thermal expansion coef- ficients between HA and Ti and the adhesive strength of the composite coating and Ti substrate was higher than that of nano￾HA single coatings on Ti substrate. Pang and Zhitomirsky166 deposited by EPD composite hydroxyapatite-chitosan coatings on stainless steel substrates. They observed that the addition of chitosan to the HA suspension promoted the deposition of HA. Moreover, from the analysis of the coatings it was demon￾strated that changing the concentration of the two chemicals in the suspension resulted in a variation in the coating com￾position. These composite coatings were adherent and uniform with increased corrosion protection properties. Louh et al.167 deposited by EPD nanosized platinum/carbon nanocatalysts and Nafion® solution on carbon-based substrates for applications in proton exchange membrane fuel cells (PEMFCs). They obtained a uniform distribution of catalyst and Nafion® on the electrodes, in fact the Pt/C nanopowders covered the whole carbon fibre surface and also infiltrated into the gaps and voids between them. EPD has been also employed for the production of fibre reinforced ceramic and glass matrix composites with a variety of ceramic matrices and fibres.8,168 This method is particu￾larly attractive for the production of CMCs with complex fibre structures (e.g. 2D fibre fabrics) as reinforcement because it allows adequate infiltration of ceramic (nano) particles into the inter-fibre spaces, which would be difficult to be achieved by other routes.168 Additionally, it should be emphasised that the fibres employed can be both electrically conductive (e.g. SiC Nicalon®, carbon) and non-conductive (e.g. alumina or mullite). When the fibres are non-conductive, the fibre weave is placed in front of the deposition electrode, the ceramic deposit devel￾ops on the electrode and grows around and through the fibre mat.8,168 Stoll et al.169 fabricated alumina–alumina CMCs by means of EPD from a suspension of Al2O3 nanoparticles using Nextel®-type (alumina) fibres. The same composite (alumina particles and Nextel®-type fibres) was investigated recently by Bao and Nicholson.170 In this particular system it was shown that in order to be able to infiltrate fibre mats by EPD it is fundamen￾tal that both the suspended particles and the fibres have the same polarity.169 In these conditions, when the particles approach the fibre mat repulsion forces take place between them, the mag￾nitude of these forces depends on the relative distance between particles and fibres. Due to the applied external electrical field, each charged particle is attracted to the fibre mat, which is fixed to the electrode, but, at the same time, the particles are repelled before they can reach the fibre surfaces (coagulation point) due to the charge on the fibres. It has been hypothesised that under the effect of the repulsive forces due to the surrounding fibres, the particles follow the path with the fewest possible obstacles until reaching the next interstice between adjacent fibres. There￾fore, when the particles reach the electrode or the surface of previously deposited particles, they cannot move further and consequently the electrophoretic ceramic deposit grows with a high particle packing density. Fig. 7(a) and (b) shows the unifor￾mity of the infiltration of alumina particles in the Nextel®-type (alumina) fibre mat and the high level of packing achieved by the electrophoretically deposited alumina particles.169 Conversely, if the fibre and particles exhibit opposite surface charge, coagu￾lation is expected on the first layer of fibres encountered by the travelling particles. In this case a deposit on the outer fibre layer will block the movement of the particles towards the interior

. Corni et al. Journal of the European Ceramic Society 28(2008)1353-1367 nm (b) Inm 212nm Fig8. Schematic of the EPD nanocell employed by Iwata et al. 73 to deposit Au dots: (a)topographical(AFM) image of Au dots deposited by EPD and(b) cross-section of the Au dot shown in image(a). (Reproduced with permission of IoP Publishing Ltd, UK) of the fibre mat, resulting in poor infiltration and low quality CNTs are now well-known and their use in a wide range of nIcI rostructure applications is spreading, with current research and develop- A further development of EPD is reactive electrophoretic ment efforts focused on expanding the application potential of deposition,that has been described by Clasen et al. 71. 72 CNTs.76-178However, it is recognized that to produce partic produce doped functional glasses. These glasses are normally ular arrangements of CNTs, individually or collectively, for a created by melting silica at 2100.C, but at this high temperature given application and to combine CNTs with other materials most of the suitable dopants evaporate. An alternative process for to form composite materials and devices, it is fundamental to that consists of adding soluble salts into a silica suspension, ceramic or metallic matrix. It is now well known that EPD is a these salts dissociate and the ions are adsorbed on the surface very convenient technique to manipulate CNT in order to form of the particles, which are then deposited by EPD producing ordered, oriented nanotubes arrays a homogeneously doped green body Shaping of a green bod To prepare a stable CNT suspension for EPD, several solvents and doping are achieved in a single step by means of reactive have been employed, including distilled water, mixtures of ace- EPD, whereas more than one step and very high temperatures tone and ethanol, and pure organic solvents such as ethanol are needed by the conventional route. isopropyl alcohol, n-pentanol, ethyl alcohol, tetrahydrofuran, Iwataetal. demonstrated that EPD can also be employed to dimethylformamide and deionised water with pyrrole. The fabricate nanostructures, such as nanomachines and components preparation of a stable suspension of CNTs, in which CNTs for nanoelectronics. This group utilized local EPD to deposit have a high s-potential and the suspension has a low ionic con- gold nanoparticles( dots)on Si surfaces from a nanopipette probe ductivity, is necessary for successful EPD. The stability of CNt filled with the deposition suspension. The nanopipette probe was suspensions, determined by s-potential measurements, has been the Epd cell and the two electrodes were a thin metal wire posi- studied mainly in aqueous and ethanol-based suspensions. Fur- tioned inside the nanopipette and a conductive surface that was thermore, it has also been shown that the presence of chargersalts practically in contact with the edge of the nanopipette(Fig 8). can play an important role in improving the adhesion of CNTs When a difference of potential was applied between them, the to substrates and in increasing the deposition rate. The salts can colloidal particles migrated toward the edge of the probe and also contribute to the stability of the suspensions by associating a were deposited on the surface. They also demonstrated that it charge to the CNT surface. This charge determines the migration was possible to modify the size of the Au dots by changing the direction of CNTs in suspension during EPD and therefore the deposition time and the voltage deposition electrode. Using different types of salts: quaternary ammonium salts, benzalkonium chloride, NiCl, Mg(NO3) 3.3.2. EPD of carbon nanotubes(CNTs) MgCl2 and NaoH a finer control can be achieved. Additionally, In the last few years the interest of the scientific commu- it is worth noting that the high aspect ratio and surface charge nity in carbon nanotubes(CNTs), both single-walled(SWCNTs) of acid-treated CNTs makes them suitable scaffolds or tem- and multi-walled (MWCNTs), has increased dramatically as plates for deposition of other nanoparticles, such as metallic and reflected by the huge number of papers published and patents oxide nanoparticles, via adsorption or nucleation at the acidic filed related to CNTs 7.8, 74 175 Many of the properties of sites

I. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 1361 Fig. 8. Schematic of the EPD nanocell employed by Iwata et al.173 to deposit Au dots: (a) topographical (AFM) image of Au dots deposited by EPD and (b) cross-section of the Au dot shown in image (a). (Reproduced with permission of IOP Publishing Ltd., UK.) of the fibre mat, resulting in poor infiltration and low quality microstructure. A further development of EPD is reactive electrophoretic deposition, that has been described by Clasen et al.171,172 to produce doped functional glasses. These glasses are normally created by melting silica at 2100 ◦C, but at this high temperature most of the suitable dopants evaporate. An alternative process for the fabrication of doped glasses is the reactive EPD technique that consists of adding soluble salts into a silica suspension, these salts dissociate and the ions are adsorbed on the surface of the particles, which are then deposited by EPD producing a homogeneously doped green body. Shaping of a green body and doping are achieved in a single step by means of reactive EPD, whereas more than one step and very high temperatures are needed by the conventional route. Iwata et al.173 demonstrated that EPD can also be employed to fabricate nanostructures, such as nanomachines and components for nanoelectronics. This group utilized local EPD to deposit gold nanoparticles (dots) on Si surfaces from a nanopipette probe filled with the deposition suspension. The nanopipette probe was the EPD cell and the two electrodes were a thin metal wire posi￾tioned inside the nanopipette and a conductive surface that was practically in contact with the edge of the nanopipette (Fig. 8). When a difference of potential was applied between them, the colloidal particles migrated toward the edge of the probe and were deposited on the surface. They also demonstrated that it was possible to modify the size of the Au dots by changing the deposition time and the voltage. 3.3.2. EPD of carbon nanotubes (CNTs) In the last few years the interest of the scientific commu￾nity in carbon nanotubes (CNTs), both single-walled (SWCNTs) and multi-walled (MWCNTs), has increased dramatically as reflected by the huge number of papers published and patents filed related to CNTs.7,8,174,175 Many of the properties of CNTs are now well-known and their use in a wide range of applications is spreading, with current research and develop￾ment efforts focused on expanding the application potential of CNTs.176–178 However, it is recognized that to produce partic￾ular arrangements of CNTs, individually or collectively, for a given application and to combine CNTs with other materials to form composite materials and devices, it is fundamental to disperse the CNTs homogeneously in the appropriate polymer, ceramic or metallic matrix. It is now well known that EPD is a very convenient technique to manipulate CNT in order to form ordered, oriented nanotubes arrays.7 To prepare a stable CNT suspension for EPD, several solvents have been employed, including distilled water, mixtures of ace￾tone and ethanol, and pure organic solvents such as ethanol, isopropyl alcohol, n-pentanol, ethyl alcohol, tetrahydrofuran, dimethylformamide and deionised water with pyrrole.7 The preparation of a stable suspension of CNTs, in which CNTs have a high -potential and the suspension has a low ionic con￾ductivity, is necessary for successful EPD. The stability of CNT suspensions, determined by -potential measurements, has been studied mainly in aqueous and ethanol-based suspensions. Fur￾thermore, it has also been shown that the presence of charger salts can play an important role in improving the adhesion of CNTs to substrates and in increasing the deposition rate. The salts can also contribute to the stability of the suspensions by associating a charge to the CNT surface. This charge determines the migration direction of CNTs in suspension during EPD and therefore the deposition electrode. Using different types of salts: quaternary ammonium salts, benzalkonium chloride, NiCl2, Mg(NO3)2, MgCl2 and NaOH a finer control can be achieved. Additionally, it is worth noting that the high aspect ratio and surface charge of acid-treated CNTs makes them suitable scaffolds or tem￾plates for deposition of other nanoparticles, such as metallic and oxide nanoparticles, via adsorption or nucleation at the acidic sites.

1362 . Corni et al /Journal of the European Ceramic Society 28(2008)1353-136 over Wang et al. 8 have deposited uniformly CNT coatings on glass plates with a silver or a ITo film layer and they observed that the emission properties of these CNT films were as good as those of the coatings produced by other methods such as screen printing. EPD seems thus to be a low cost and favourable method to produce CNT films for field emitters. Moreover composites 彐 consisting of ceramic nanoparticles and MWCNT have been produced recently by sequential EPD and by electrophoretic co-deposition The results presented in the literature demonstrate that EPD is a very versatile method to manipulate CNTs, thus the tech- nique is likely to be a focus of research efforts in the near future. EPD is a potentially powerful method to produce CNt Mag- 59.02Kx based devices, particularly because few alternative techniques exist to deposit and align CNTs on the required surfaces. CNT Fig 9. SEM image of the electrophoretically deposited MWCNT coating on the films may be suitable for a wide range of applications: sugges- surface of a bioactive glass foam, which was placed in front of the anode durin tions to date include field emission devices. 183, 184 biomedical EPD(MWCNT were negatively charged ). 186 scaffolds, 86 catalyst supports, structural composites and coat ings, large surface area electrodes for fuel cells, photochemical The most effective method for dispersion of carbon nan- solar cells. 85 electrodes for electronic devices(si supercapacitors otubes in a liquid medium is to add a surfactant and to apply and battery manufacturing), 81, 82 capacitors and gas sensors ultrasonication 7A different method based on the application of Similarly, EPD of CNTs can be seen as a very effective process an electric field to the suspension has been recently developed by to create CNT membranes and nanofilters which are commonly Zhang et al. 79, 180 They applied an AC electric field to suspen- made by slow and tedious filtration of CNT suspensions sions of carbon nanotubes and compared the results with those obtained without applying an electric field; they noticed that the 3.3.3. Nanorods, nanowires, nanotubes and nanosheets dispersion status of carbon nanotubes in liquid media was con- There has been an increasing interest in the use of elec siderably improved by applying an electric field. Moreover they trophoretic deposition techniques for the fabrication of a variety observed that the dispersion results were strongly dependent on of nano-architectures, including nanorods, nanowires, nan- the magnitude and frequency of the applied electric field. Du tubes and nanosheets. This section presents the most recent and Pan81, 182 deposited MWCNT thin films by EPD to pro- research carried out in this field. duce supercapacitors with high specific power density and a Zinc oxide nanorods, nanowires and nanotubes have been very small equivalent series resistance. These thin films could synthesized andelectrophoretically deposited by Routet al 9to be applied as coating layers over ordinary current collectors to produce sensors for hydrogen and ethanol. The sensing charac considerably enhance the electrode performance teristics of these nanostructures were examined before and after Junget al. 83, 184 deposited a thin film of horizontally aligned impregnating them with 1% Pt The nanowires exhibited excel SWCNTs from an aqueous mixture of CNT and detergent. The lent hydrogen sensing characteristics at temperatures below coating was naturally dried and then pressed on the surface to 150C especially when impregnated with Pt. The nanorods make it smooth. The films strongly adhered to the substrate showed a satisfactory dependence of sensitivity on hydrogen and showed good field emission properties. Recently, Hasobe et concentration. Moreover nanorods and nanowires impregnated al.8 deposited SWCNTs and protonated porphyrin on nanos- with Pt demonstrated high sensitivity for ethanol at temperatures tructured SnOz electrodes for applications in photochemical below 150C, with short recovery and response times. Kim et solar cells al.converted commercial titania nanoparticles to nanotubes As mentioned above, beyond the fabrication of uniform, pla- using an hydrothermal method and then deposited nar, CNT-based coatings, EPD can be applied to deposit CNTs nanotube film on a Si substrate by EPD Porous nanosheet-stacked NiCo2O4/Ni composite electrode strates and fibrous bodies or textiles. Boccaccini et al. 86 have have been produced by a novel electrophoretic deposition deposited MWCNTs onto highly porous bioactive glass scaf- calcination method. These electrode films present a crys- folds with the intention of imparting a monotopography to the talline structure and they are composed of regular hexagonal pore wall surfaces. Fig 9 shows the surface of MWCnT depo- NiCo2O4 nanosheets with an average diameter of 200 nm. sition on the 3D pore network of the Bioglass foam, which Moreover these films exhibited good electrocatalytic proper- was placed in front of the anode (MWCNT were negatively ties for water electrolysis. 93, 94 Sugimoto et al. 9> prepared charged). There has been also recent development on the fabri- stable colloids of ruthenic acid nanosheets in acetonitrile or cation by EPD of CNT reinforced HA coatings for biomedical N, N-dimethylformamide. The nanosheets were subsequently applications. 87 The fabrication of more complex patterns of deposited by EPD on gold, indium-tin oxide coated glass and CNT deposits can be realized by using masks or by designing indium-tin oxide coated polyethylene terephthalate)electrodes combinations of conductive and non-conductive surfaces. More- which presented high energy density at room temperature

1362 I. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 Fig. 9. SEM image of the electrophoretically deposited MWCNT coating on the surface of a bioactive glass foam, which was placed in front of the anode during EPD (MWCNT were negatively charged).186 The most effective method for dispersion of carbon nan￾otubes in a liquid medium is to add a surfactant and to apply ultrasonication.7 A different method based on the application of an electric field to the suspension has been recently developed by Zhang et al.179,180 They applied an AC electric field to suspen￾sions of carbon nanotubes and compared the results with those obtained without applying an electric field; they noticed that the dispersion status of carbon nanotubes in liquid media was con￾siderably improved by applying an electric field. Moreover they observed that the dispersion results were strongly dependent on the magnitude and frequency of the applied electric field. Du and Pan181,182 deposited MWCNT thin films by EPD to pro￾duce supercapacitors with high specific power density and a very small equivalent series resistance. These thin films could be applied as coating layers over ordinary current collectors to considerably enhance the electrode performance. Jung et al.183,184 deposited a thin film of horizontally aligned SWCNTs from an aqueous mixture of CNT and detergent. The coating was naturally dried and then pressed on the surface to make it smooth. The films strongly adhered to the substrate and showed good field emission properties. Recently, Hasobe et al.185 deposited SWCNTs and protonated porphyrin on nanos￾tructured SnO2 electrodes for applications in photochemical solar cells. As mentioned above, beyond the fabrication of uniform, pla￾nar, CNT-based coatings, EPD can be applied to deposit CNTs onto complicated structures, including microwires, porous sub￾strates and fibrous bodies or textiles.7 Boccaccini et al.186 have deposited MWCNTs onto highly porous bioactive glass scaf￾folds with the intention of imparting a monotopography to the pore wall surfaces. Fig. 9 shows the surface of MWCNT depo￾sition on the 3D pore network of the Bioglass® foam, which was placed in front of the anode (MWCNT were negatively charged). There has been also recent development on the fabri￾cation by EPD of CNT reinforced HA coatings for biomedical applications.187 The fabrication of more complex patterns of CNT deposits can be realized by using masks or by designing combinations of conductive and non-conductive surfaces. More￾over Wang et al.188 have deposited uniformly CNT coatings on glass plates with a silver or a ITO film layer and they observed that the emission properties of these CNT films were as good as those of the coatings produced by other methods such as screen printing. EPD seems thus to be a low cost and favourable method to produce CNT films for field emitters. Moreover composites consisting of ceramic nanoparticles and MWCNT have been produced recently by sequential EPD and by electrophoretic co-deposition.189,190 The results presented in the literature demonstrate that EPD is a very versatile method to manipulate CNTs, thus the tech￾nique is likely to be a focus of research efforts in the near future. EPD is a potentially powerful method to produce CNT￾based devices, particularly because few alternative techniques exist to deposit and align CNTs on the required surfaces. CNT films may be suitable for a wide range of applications; sugges￾tions to date include field emission devices,183,184 biomedical scaffolds,186 catalyst supports, structural composites and coat￾ings, large surface area electrodes for fuel cells, photochemical solar cells,185 electrodes for electronic devices (supercapacitors and battery manufacturing),181,182 capacitors and gas sensors. Similarly, EPD of CNTs can be seen as a very effective process to create CNT membranes and nanofilters which are commonly made by slow and tedious filtration of CNT suspensions. 3.3.3. Nanorods, nanowires, nanotubes and nanosheets There has been an increasing interest in the use of elec￾trophoretic deposition techniques for the fabrication of a variety of nano-architectures, including nanorods, nanowires, nan￾otubes and nanosheets. This section presents the most recent research carried out in this field. Zinc oxide nanorods, nanowires and nanotubes have been synthesized and electrophoretically deposited by Rout et al.191 to produce sensors for hydrogen and ethanol. The sensing charac￾teristics of these nanostructures were examined before and after impregnating them with 1% Pt. The nanowires exhibited excel￾lent hydrogen sensing characteristics at temperatures below 150 ◦C especially when impregnated with Pt. The nanorods showed a satisfactory dependence of sensitivity on hydrogen concentration. Moreover nanorods and nanowires impregnated with Pt demonstrated high sensitivity for ethanol at temperatures below 150 ◦C, with short recovery and response times. Kim et al.192 converted commercial titania nanoparticles to nanotubes using an hydrothermal method and then deposited a titanate nanotube film on a Si substrate by EPD. Porous nanosheet-stacked NiCo2O4/Ni composite electrodes have been produced by a novel electrophoretic deposition calcination method. These electrode films present a crys￾talline structure and they are composed of regular hexagonal NiCo2O4 nanosheets with an average diameter of 200 nm. Moreover these films exhibited good electrocatalytic proper￾ties for water electrolysis.193,194 Sugimoto et al.195 prepared stable colloids of ruthenic acid nanosheets in acetonitrile or N,N-dimethylformamide. The nanosheets were subsequently deposited by EPD on gold, indium-tin oxide coated glass and indium-tin oxide coated poly(ethylene terephthalate) electrodes which presented high energy density at room temperature