Current opinion in Solid state Materials Science ELSEVIER Current Opinion in Solid State and Materials Science 6(2002)251-260 Application of electrophoretic and electrolytic deposition techniques in ceramIcs processing Aldo R. Boccaccini,, Igor Zhitomirsky Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Rd, London SW7 2BP, UK Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L&s 4L7 Abstract Electrodeposition is gaining increasing interest as a ceramic processing technique for a variety of technical applications. Major advances in the areas of electrophoretic deposition(EPD) and electrolytic deposition(ELD) achieved in the last 24 months include the well as a variety of advanced films and coatings for electronic, biomedical, optical, catalytic and electrochemical applicatlolde erials as fabrication of: electrodes and films for solid oxide fuel cells, fibre-reinforced and graded ceramic composites, nanostructured mate c 2002 Elsevier Science Ltd. All rights reserved 1. Introduction published in the last 2 years. During this period, key advances have been made towards understanding basic The two most prominent ceramic electrodeposition mechanisms of EPD and ELD, expanding traditional techniques, i.e. electrophoretic deposition(EPD)and elec- applications and exploring new application areas. For trolytic deposition(ELD), are gaining increasing interest reviews of developments before 2000 on EPD and ELD, both in academia and in the industrial sector, and a wide see Refs. [1, 2, 3 and [*], respectively range of novel applications in the processing of advanced ceramic materials and ceramic coatings is emerging. The interest in these processes is based not only on their high 2. Electrophoretic deposition(EPD) versatility to be used with different materials and combina- tions of materials but also because these are cost-effective The phenomenon of electrophoresis has been known techniques usually requiring simple equipment. Moreover since the beginning of the 19th century and it has found they have a high potential for scaling up to large product application in the past 40 years mainly in traditional volumes and variety of product shapes ceramic technology [1]. EPD is essentially a two-step EPD is achieved via motion of charged particles dis- process. In the first step, charged particles suspended in a persed in a liquid towards an electrode under an applied liquid migrate towards an electrode under the effect of an electric field. Deposit formation on the electrode is electric field (electrophoresis). In the second step, the achieved via particle coagulation particles deposit on the electrode forming a relatively ELD leads to thin ceramic films from solutions of metal dense and homogeneous compact or film. A post-EPD y production of colloidal particles in electrode processing step is usually required, which includes a reactions. Thus, electrode reactions in ELD and electro- suitable heat-treatment (firing or sintering) in order to phoretic motion of charged particles in EPD result in the further densify the deposits and to eliminate porosity accumulation of ceramic particles and formation of In general, EPD can be applied to any solid that is ceramic films at the relevant electrodes available in the form of a fine powder(<30 um)or a The present review covers the most recent and signifi- colloidal suspension. Indeed, examples of EPD of any cant developments in the areas of EPD and ELD material class can be found, including metals, polymers carbides, oxides, nitrides and glasses [1, **2, **31 Corresponding author. Tel: +44-20-7594-6731; fax: +44-20-7584- The potential of the EPD technique for the realization of unique microstructures and novel(and complex) materials E-mail address: aboccaccini@ ic ac uk(.R. Boccaccini) combinations in a variety of sha nd dimensions is 359-0286/02/S-see front matter 2002 Elsevier Science Ltd. All rights reserved PIl:S1359-0286(02)00080-3
Current Opinion in Solid State and Materials Science 6 (2002) 251–260 A pplication of electrophoretic and electrolytic deposition techniques in ceramics processing a, b Aldo R. Boccaccini , Igor Zhitomirsky * a Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Rd., London SW7 2BP, UK b Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7 Abstract Electrodeposition is gaining increasing interest as a ceramic processing technique for a variety of technical applications. Major advances in the areas of electrophoretic deposition (EPD) and electrolytic deposition (ELD) achieved in the last 24 months include the fabrication of: electrodes and films for solid oxide fuel cells, fibre-reinforced and graded ceramic composites, nanostructured materials as well as a variety of advanced films and coatings for electronic, biomedical, optical, catalytic and electrochemical applications. 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction published in the last 2 years. During this period, key advances have been made towards understanding basic The two most prominent ceramic electrodeposition mechanisms of EPD and ELD, expanding traditional techniques, i.e. electrophoretic deposition (EPD) and elec- applications and exploring new application areas. For trolytic deposition (ELD), are gaining increasing interest reviews of developments before 2000 on EPD and ELD, both in academia and in the industrial sector, and a wide see Refs. [1,**2,**3] and [**4], respectively. range of novel applications in the processing of advanced ceramic materials and ceramic coatings is emerging. The interest in these processes is based not only on their high 2. Electrophoretic deposition (EPD) versatility to be used with different materials and combinations of materials but also because these are cost-effective The phenomenon of electrophoresis has been known techniques usually requiring simple equipment. Moreover since the beginning of the 19th century and it has found they have a high potential for scaling up to large product application in the past 40 years mainly in traditional volumes and variety of product shapes. ceramic technology [1]. EPD is essentially a two-step EPD is achieved via motion of charged particles dis- process. In the first step, charged particles suspended in a persed in a liquid towards an electrode under an applied liquid migrate towards an electrode under the effect of an electric field. Deposit formation on the electrode is electric field (electrophoresis). In the second step, the achieved via particle coagulation. particles deposit on the electrode forming a relatively ELD leads to thin ceramic films from solutions of metal dense and homogeneous compact or film. A post-EPD salts by production of colloidal particles in electrode processing step is usually required, which includes a reactions. Thus, electrode reactions in ELD and electro- suitable heat-treatment (firing or sintering) in order to phoretic motion of charged particles in EPD result in the further densify the deposits and to eliminate porosity. accumulation of ceramic particles and formation of In general, EPD can be applied to any solid that is ceramic films at the relevant electrodes. available in the form of a fine powder (,30 mm) or a The present review covers the most recent and signifi- colloidal suspension. Indeed, examples of EPD of any cant developments in the areas of EPD and ELD, as material class can be found, including metals, polymers, carbides, oxides, nitrides and glasses [1,**2,**3]. The potential of the EPD technique for the realization of *Corresponding author. Tel.: 144-20-7594-6731; fax: 144-20-7584- 3194. unique microstructures and novel (and complex) materials E-mail address: a.boccaccini@ic.ac.uk (A.R. Boccaccini). combinations in a variety of shapes and dimensions is 1359-0286/02/$ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S1359-0286(02)00080-3
252 A.R. Boccaccini, 1. Zhitomirsky/ Current Opinion in Solid State and Materials Science 6(2002)251-260 being increasingly appreciated by materials scientists and simulation of the epd process in particular seems to be a technologists. This growing interest in EPD both in the promising area where major R&D efforts are required academic and industrial communities has prompted the organization of the very first international conference focused entirely on the application of EPD in material 2. 2. Films for solid oxide fuel cells processing, sponsored by the United Engineering Founda- tion, which is being held in 2002 EPD is being increasingly considered for the fabrication of cathode- and anode-supported solid oxide fuel cells 2. 1. Fundamental principles of the EPD process (SOFC) of both planar and tubular geometry. Several recent papers describe the use of EPD in this area [12-18] The basic mechanisms of EPd have been extensively The relative advantages of EPD in SOFC manufacture considered in the literature mainly in the framework of the have been summarized recently by Negishi et al. [**13 Derjaguin-Landau-Verwey-Overbeek (DLvO)theory and They include: (i)coatings can be made in any shape, (ii)it particle double layer distortion on application of d. c. is possible to prepare porous coating as electrode and electric fields [**2]. There are however multiple theories dense coating as electrolyte by controlling deposition put forward to explain particle interactions and kinetics of conditions, (iii) laminate structures of electrodes and deposition [#2, **3], and further theoretical and modeling electrolyte can be readily obtained, and (iv) Ni-yttria work is being carried out. For example, studies of electro- stabilized zirconia(YSZ)cermets(anodes)can be obtained dynamic particle aggregation during EPd both under by electrophoretic co-deposition. In addition EPD has steady [5] and alternating [6] electric fields have been further advantages such as mass production possibility conducted recently, which have led to equations for the short formation times and simple equipment. Thus EPD time evolution of the probability of separation between makes it possible to simplify the fabrication process of deposited particles under different conditions. The models SoFC stacks with complex design architecture and there- are useful to explain the experimentally observed cluster- fore to achieve further cost reduction ing of colloidal particles deposited near an electrode in a In spite of the progress achieved recently in this area, DC electric field by considering convection by electro- many problems remain, which have been discussed by osmotic flow about the particles [7] Zhitomirsky and Petric [12]. Major difficulties are linked Numerical simulation has been used for the first time to the selection of adequate solvents and additives, in recently to model the buildup of a deposit of charged particular regarding the chemical compatibility of the particles on an electrode during EPD[8, 9]. These studies components of the binder-dispersant-solvent system are both of fundamental and practical interest as they solubility of the binder, as well as issues of viscosity and provide insight into local variations of particle interaction electrical resistivity of the suspension [191 processes during deposition, which can be used for optimi- The potential of the EPD technique for the fabrication of zation of EPD techniques. Another very important recent SOFCs with porous anode substrates and thin zirconia fundamental study on the formation of colloidal films electrolytes has been demonstrated by Will et al. [16], who during EPD has been provided by Sarkar et al. [**10]. investigated the deposition of zirconia from ethanol sus- They observed the deposition of silica particles on silicon pensions on porous NiO/CeO,/ZrO, substrates. Another ers as a function of deposition time, and compared advancement was reported recently by Basu et al.[171 nucleation and growth of the silica particle layer with that who investigated the fabrication of dense zirconia elec- of atomic film growth via molecular-beam epitaxy. a trolyte films for tubular SOFCs by EPD Summarizing, the striking similarity was found between the two growth current research and development efforts in this area are processes. This indicates possible new directions for promising and encourage a very optimistic view for the further research as the equivalence between the two future use of EPD in SOFC manufacture processes provides insight into the growth kinetics of EPD films and can be used for their microstructural optimi- zation 23. Coatings on solids substrates Another fundamental study with practical relevance was conducted by Ferrari et al., who investigated the galvanic EPD has been the processing technique of choice for reactions occurring at the electrodes during EPd fro production of ceramic coatings on a variety of substrates aqueous suspensions [ Ill for numerous applications, which include wear and oxida- The analysis of the literature reveals however that there tion resistance, bioactive coatings for biomedical implants is need for further theoretical and modeling work. Most and devices as well as functional coatings for electronic experimental research is currently carried out following magnetic and related applications [ **2, *31 unsatisfactory and time-consuming trial-and-error ap- Current interest in the fabrication of wear and abrasion proaches, due to the lack of predictive models linking EPD resistant coatings is focused on developing metal/ceramic process parameters and deposit properties. The numerical and ceramic/ceramic composite coatings. EPD, usually in
252 A.R. Boccaccini, I. Zhitomirsky / Current Opinion in Solid State and Materials Science 6 (2002) 251–260 being increasingly appreciated by materials scientists and simulation of the EPD process in particular seems to be a technologists. This growing interest in EPD both in the promising area where major R&D efforts are required. academic and industrial communities has prompted the organization of the very first international conference focused entirely on the application of EPD in materials 2 .2. Films for solid oxide fuel cells processing, sponsored by the United Engineering Foundation, which is being held in 2002. EPD is being increasingly considered for the fabrication of cathode- and anode-supported solid oxide fuel cells 2 .1. Fundamental principles of the EPD process (SOFC) of both planar and tubular geometry. Several recent papers describe the use of EPD in this area [12–18]. The basic mechanisms of EPD have been extensively The relative advantages of EPD in SOFC manufacture considered in the literature mainly in the framework of the have been summarized recently by Negishi et al. [**13]. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and They include: (i) coatings can be made in any shape, (ii) it particle double layer distortion on application of d.c. is possible to prepare porous coating as electrode and electric fields [**2]. There are however multiple theories dense coating as electrolyte by controlling deposition put forward to explain particle interactions and kinetics of conditions, (iii) laminate structures of electrodes and deposition [**2,**3], and further theoretical and modeling electrolyte can be readily obtained, and (iv) Ni-yttria work is being carried out. For example, studies of electro- stabilized zirconia (YSZ) cermets (anodes) can be obtained dynamic particle aggregation during EPD both under by electrophoretic co-deposition. In addition EPD has steady [5] and alternating [6] electric fields have been further advantages such as mass production possibility, conducted recently, which have led to equations for the short formation times and simple equipment. Thus EPD time evolution of the probability of separation between makes it possible to simplify the fabrication process of deposited particles under different conditions. The models SOFC stacks with complex design architecture and thereare useful to explain the experimentally observed cluster- fore to achieve further cost reduction. ing of colloidal particles deposited near an electrode in a In spite of the progress achieved recently in this area, DC electric field by considering convection by electro- many problems remain, which have been discussed by osmotic flow about the particles [7]. Zhitomirsky and Petric [12]. Major difficulties are linked Numerical simulation has been used for the first time to the selection of adequate solvents and additives, in recently to model the buildup of a deposit of charged particular regarding the chemical compatibility of the particles on an electrode during EPD [8,*9]. These studies components of the binder–dispersant–solvent system, are both of fundamental and practical interest as they solubility of the binder, as well as issues of viscosity and provide insight into local variations of particle interaction electrical resistivity of the suspension [19]. processes during deposition, which can be used for optimi- The potential of the EPD technique for the fabrication of zation of EPD techniques. Another very important recent SOFCs with porous anode substrates and thin zirconia fundamental study on the formation of colloidal films electrolytes has been demonstrated by Will et al. [16], who during EPD has been provided by Sarkar et al. [**10]. investigated the deposition of zirconia from ethanol susThey observed the deposition of silica particles on silicon pensions on porous NiO/CeO /ZrO substrates. Another 2 2 wafers as a function of deposition time, and compared the advancement was reported recently by Basu et al. [*17], nucleation and growth of the silica particle layer with that who investigated the fabrication of dense zirconia elecof atomic film growth via molecular-beam epitaxy. A trolyte films for tubular SOFCs by EPD. Summarizing, the striking similarity was found between the two growth current research and development efforts in this area are processes. This indicates possible new directions for promising and encourage a very optimistic view for the further research as the equivalence between the two future use of EPD in SOFC manufacture. processes provides insight into the growth kinetics of EPD films and can be used for their microstructural optimization. 2 .3. Coatings on solids substrates Another fundamental study with practical relevance was conducted by Ferrari et al., who investigated the galvanic EPD has been the processing technique of choice for the reactions occurring at the electrodes during EPD from production of ceramic coatings on a variety of substrates aqueous suspensions [*11]. for numerous applications, which include wear and oxidaThe analysis of the literature reveals however that there tion resistance, bioactive coatings for biomedical implants is need for further theoretical and modeling work. Most and devices as well as functional coatings for electronic, experimental research is currently carried out following magnetic and related applications [**2,**3]. unsatisfactory and time-consuming trial-and-error ap- Current interest in the fabrication of wear and abrasion proaches, due to the lack of predictive models linking EPD resistant coatings is focused on developing metal/ceramic process parameters and deposit properties. The numerical and ceramic/ceramic composite coatings. EPD, usually in
A.R. Boccaccini, 1. Zhitomirsky/ Current Opinion in Solid State and Materials Science 6(2002)251-260 combination with electroplating or galvanic deposition of 2. 4. Coatings on fibres, porous substrates and composite coatings (20, "217. production of metal/ ceramic membranes In recent developments, yttria stabilized zirconia/alumina composite coatings were pro- EPD is being used increasingly to coat fibres and porous duced on Fecralloys by Wang et al. [22 by using EPI ubstrates with ceramic materials for applications ranging and a reaction bonding processes. Certainly, the main from filters and porous carriers to bioactive coatings and difficulty of producing ceramic coatings on metal hollow fibre fabrication The successful use of epd for strates using Epd is that the metal substrates cannot coating carbon and metallic fibrous substrates with alumina withstand the high temperature required for sintering the and titania nanopowders was shown by Boccaccini et al deposited ceramic coating. Therefore optimized densifica- [37]. The coated porous fibrous structures are intended for tion techniques requiring lower temperatures must be high-temperature filtration of fluids, where the titania or developed. Wang et al. showed that reaction bonding is an alumina coatings act as oxidation and corrosion protective excellent alternative [22]. Further R&D efforts are how- layers. A recent paper by Zhitomirsky [38] demonstrates ever required in this important technological area the coating of carbon fibres with hydroxyapatite(HA) EPD has been recently used in ceramic joining applica- After burning out the fibrous carbon substrates hollow HA tions Mixtures of SiC or Si3, and reactive carbon were fibres of various diameters were obtained. EPD has been deposited onto SiC or Sis, parts in preparation for used by Su et al. [39 to deposit lead zirconate titanate reaction bonding with molten silicon [23]. The results of (PZT)films(<5 um)on Pt wires. In this case, EPD allows Lessing et al. are significant as they show for the first time for the use of PzT nanoparticles directly from hydrother how the combination of EPD and reaction bonding allow mal suspensions, thus avoiding agglomeration of particles for the fabrication of large complex structures manufac and resulting in lower sintering temperatures tured from smaller components made of Sic or Si,N4 Membranes and porous materials have also been pre *23] pared recently by EPD techniques, including zeolite [40] The availability of ceramic nanopowders in numerous and alumina [41] membranes. The electrophoretic assem- compositions enables the use of EPD to prepare dielectric, bly of nanozeolites has been investigated by Ke et al. [42] magnetic, semiconducting and superconducting ceramic They were able to prepare hollow zeolite fibres by coating thick films for a variety of applications in electronics. nanozeolites onto carbon fibres and subsequent burn-out of Moreover EPD allows for the fabrication of engineered the carbon core. Significant related research focusing on non-planar structures made of functional ceramics which EPD of zeolites was carried out by Ahlers et al. [43], who find application developed a method for fabricating optimized zeolite- Current work in these areas is leading to encouraging modified electrodes for applications in electroanalysis and results and therefore merit further R&D efforts. Some electrocatalysis recent significant developments include the fabrication of BaTiO, thick films for sensor and actuator applications 2.5. Fibre reinforced ceramic matrix composites 4, Zno thick films for gas sensors [25], zirconia films on porous Lao. Sro. MnO, substrates [26], MgO-modified EPD is a simple and cost-effective method for Bao Sro. 4TiO, thick films for tunable microwave devices ing high-quality fibre reinforced ceramic matrix [27, LiCoo, electrodes for rechargeable lithium batteries ites. In this application, EPD is used to infiltrate preforms [28], phosphor screens for plasma display panel applica- with tight two- or three-dimensional fibre architectures tions [29], photocatalytic titania coatings [30] and Mgo using nanosized ceramic particles. A recent comprehensive hick films for electronics [31, 32 review article reveals the great variety of conducting and Furthermore, the fabrication of high-temperature non-conducting fibre and matrix combinations that have conducting films of controlled thickness on substrates of been explored, including SiC, carbon, and oxide ceramic various shapes and dimensions by EPD is gaining increas- fibre architectures and silica, borosilicate glass, alumina, ing interest [33, *34]. The significant advantages of EPD zirconia, mullite, hydroxyapatite, SiC and Si3 N, matrices over other coating techniques for continuous fabrication of [**44]. Most recent work has been devoted to Ni-coated superconducting coatings include the suitability of EPD to carbon fibre reinforced alumina [45], borosilicate glass be scaled up and adapted to the coating of large areas as matrix composites [46], C-fibre reinforced Sic matrix well as the high deposition rate that can be achieved composites [47] and silica/silica composites [48]. More- over EPD has been recently shown to be an excellent A final area of successful application of EPD in coating pre-infiltration step for decreasing processing time of chnology is in the biomedical materials field. In par- chemical vapour infiltration of SiC-fibre reinforced Sic ticular, the improvement of the EPD technique for deposi- matrix composites [491 tion of bioactive hydroxyapatite and related calci Both aqueous and non-aqueous suspensions have been phosphate films on biocompatible metallic substrates(e.g. used and the different factors affecting the EPD behavior TiAl4V alloys) has been recently reported [35, 36 of ceramic sols and their optimization to obtain high
A.R. Boccaccini, I. Zhitomirsky / Current Opinion in Solid State and Materials Science 6 (2002) 251–260 253 combination with electroplating or galvanic deposition of 2 .4. Coatings on fibres, porous substrates and metals, is being used for the production of metal/ceramic membranes composite coatings [20,*21]. In recent developments, yttria stabilized zirconia/alumina composite coatings were pro- EPD is being used increasingly to coat fibres and porous duced on Fecralloys by Wang et al. [*22] by using EPD substrates with ceramic materials for applications ranging and a reaction bonding processes. Certainly, the main from filters and porous carriers to bioactive coatings and difficulty of producing ceramic coatings on metal sub- hollow fibre fabrication. The successful use of EPD for strates using EPD is that the metal substrates cannot coating carbon and metallic fibrous substrates with alumina withstand the high temperature required for sintering the and titania nanopowders was shown by Boccaccini et al. deposited ceramic coating. Therefore optimized densifica- [37]. The coated porous fibrous structures are intended for tion techniques requiring lower temperatures must be high-temperature filtration of fluids, where the titania or developed. Wang et al. showed that reaction bonding is an alumina coatings act as oxidation and corrosion protective excellent alternative [*22]. Further R&D efforts are how- layers. A recent paper by Zhitomirsky [*38] demonstrates ever required in this important technological area. the coating of carbon fibres with hydroxyapatite (HA). EPD has been recently used in ceramic joining applica- After burning out the fibrous carbon substrates hollow HA tions. Mixtures of SiC or Si N and reactive carbon were fibres of various diameters were obtained. EPD has been 3 4 deposited onto SiC or Si N parts in preparation for used by Su et al. [39] to deposit lead zirconate titanate 3 4 reaction bonding with molten silicon [*23]. The results of (PZT) films (,5 mm) on Pt wires. In this case, EPD allows Lessing et al. are significant as they show for the first time for the use of PZT nanoparticles directly from hydrotherhow the combination of EPD and reaction bonding allow mal suspensions, thus avoiding agglomeration of particles for the fabrication of large complex structures manufac- and resulting in lower sintering temperatures. tured from smaller components made of SiC or Si N Membranes and porous materials have also been pre- 3 4 [*23]. pared recently by EPD techniques, including zeolite [40] The availability of ceramic nanopowders in numerous and alumina [41] membranes. The electrophoretic assemcompositions enables the use of EPD to prepare dielectric, bly of nanozeolites has been investigated by Ke et al. [42]. magnetic, semiconducting and superconducting ceramic They were able to prepare hollow zeolite fibres by coating thick films for a variety of applications in electronics. nanozeolites onto carbon fibres and subsequent burn-out of Moreover EPD allows for the fabrication of engineered the carbon core. Significant related research focusing on non-planar structures made of functional ceramics which EPD of zeolites was carried out by Ahlers et al. [*43], who find applications in microsystems technologies. developed a method for fabricating optimized zeoliteCurrent work in these areas is leading to encouraging modified electrodes for applications in electroanalysis and results and therefore merit further R&D efforts. Some electrocatalysis. recent significant developments include the fabrication of BaTiO thick films for sensor and actuator applications 2 .5. Fibre reinforced ceramic matrix composites 3 [24], ZnO thick films for gas sensors [25], zirconia films on porous La Sr MnO substrates [26], MgO-modified EPD is a simple and cost-effective method for fabricat- 0.9 0.1 3 Ba Sr TiO thick films for tunable microwave devices ing high-quality fibre reinforced ceramic matrix compos- 0.6 0.4 3 [27], LiCoO electrodes for rechargeable lithium batteries ites. In this application, EPD is used to infiltrate preforms 2 [28], phosphor screens for plasma display panel applica- with tight two- or three-dimensional fibre architectures tions [29], photocatalytic titania coatings [30] and MgO using nanosized ceramic particles. A recent comprehensive thick films for electronics [31,32]. review article reveals the great variety of conducting and Furthermore, the fabrication of high-temperature super- non-conducting fibre and matrix combinations that have conducting films of controlled thickness on substrates of been explored, including SiC, carbon, and oxide ceramic various shapes and dimensions by EPD is gaining increas- fibre architectures and silica, borosilicate glass, alumina, ing interest [33,**34]. The significant advantages of EPD zirconia, mullite, hydroxyapatite, SiC and Si N matrices 3 4 over other coating techniques for continuous fabrication of [**44]. Most recent work has been devoted to Ni-coated superconducting coatings include the suitability of EPD to carbon fibre reinforced alumina [45], borosilicate glass be scaled up and adapted to the coating of large areas as matrix composites [46], C-fibre reinforced SiC matrix well as the high deposition rate that can be achieved composites [47] and silica/silica composites [48]. More- [**34]. over EPD has been recently shown to be an excellent A final area of successful application of EPD in coating pre-infiltration step for decreasing processing time of technology is in the biomedical materials field. In par- chemical vapour infiltration of SiC-fibre reinforced SiC ticular, the improvement of the EPD technique for deposi- matrix composites [49]. tion of bioactive hydroxyapatite and related calcium Both aqueous and non-aqueous suspensions have been phosphate films on biocompatible metallic substrates (e.g. used and the different factors affecting the EPD behavior TiAl4V alloys) has been recently reported [35,36]. of ceramic sols and their optimization to obtain high
A.R. Boccaccini, 1. Zhitomirsky/ Current Opinion in Solid State and Materials Science 6(2002)251-260 presented in Ref [50] where the fabrication of compos- ites of tubular shape is demonstrated. The EPD cell used is shown schematically in Fig. 2[50]. Major advances in this area are expected, in particular related to the furthe development of the EPD technique for the fabrication of oxide fibre-oxide matrix ceramic composites with high systems are those by Kooner et al. [51] and Manocha et al [48]. Efforts should however concentrate on the production of complex shape components, for which the EPD tech- nique may represent the most technically viable and cost effective option. 2. 6. Laminated and graded composites Fig. 1. SEM micrograph of EPD-infiltrated SiC (Nicalon")fibre mat using a mixed colloidal suspension of mullite composition. A high-level EPD has been used successfully to fabricate ceramic infiltration is seen which leads to porous-f laminated composites and graded materials. Developments opposites [44 up to 2000 are covered in Ref [**3]. Recent advances in functionally graded materials(FGM) have been reported by Put et al. [52, 53. They manufactured graded wC-Co fibre preforms are now quite well composites using a suspension of wc powder in acetone understood The pH of the solution, the applied with variable Co powder content. In another significant voltage and deposition time are shown to have a strong development by the same group, anodic co-deposition of ence on the quality of the infiltration. Good particle Al,O3 and CeO, -stabilised zirconia powders was used to packing and a high solids-loading can be achieved, produc- obtain cylindrical and tubular-shaped Al, O, /zirconia ing firm ceramic deposits which adhered to the fibres, thus FGM components [*54] leading to pore-free composites after a post-EPD heat- Current efforts are devoted to the development of EPD treatment process. The typical microstructure of a mullite fabrication approaches for laminated ceramic composites, matrix composite reinforced by SiC (Nicalon ) fibres in particular in the system zirconia/alumina, due to the fabricated by EPD is shown in Fig. I high fracture resistance of these structures [55-57. The Most previous research summarized in Ref. [**44] has significance of the papers by Moreno and Ferrari [56] and been focused on components of simple planar shape. The by Uchikoshi et al. [571, is that they focus on optimized use of EPD for near-net shape fabrication of 3-D compo- aqueous suspensions, which should be always preferred ite components of complex shapes is starting to be over organic suspensions due to environmental and econ- investigated. A pioneering development in this area is omic considerations. Based on the promising results balance woven fibre mat in tube form as deposition electrode(-)d. c. central electrode+dc outer electrode(+)d.c boehmite(r-AlOOHD) sol glass container Fig. 2. Schema of the EPD cell recently introduced for the fabrication of tubular metal fibre reinforced alumina matrix composites [ 50](diagram courtesy Dr C Kaya, University of Birmingham, UK)
254 A.R. Boccaccini, I. Zhitomirsky / Current Opinion in Solid State and Materials Science 6 (2002) 251–260 presented in Ref. [*50], where the fabrication of composites of tubular shape is demonstrated. The EPD cell used is shown schematically in Fig. 2 [*50]. Major advances in this area are expected, in particular related to the further development of the EPD technique for the fabrication of oxide fibre-oxide matrix ceramic composites with high oxidation resistance. Recent promising results in these systems are those by Kooner et al. [51] and Manocha et al. [48]. Efforts should however concentrate on the production of complex shape components, for which the EPD technique may represent the most technically viable and costeffective option. 2 .6. Laminated and graded composites Fig. 1. SEM micrograph of EPD-infiltrated SiC (Nicalon ) fibre mat EPD has been used successfully to fabricate ceramic using a mixed colloidal suspension of mullite composition. A high-level of ceramic infiltration is seen which leads to porous-free ceramic laminated composites and graded materials. Developments composites [**44]. up to 2000 are covered in Ref. [**3]. Recent advances in functionally graded materials (FGM) have been reported by Put et al. [52,53]. They manufactured graded WC–Co infiltration of the fibre preforms are now quite well composites using a suspension of WC powder in acetone understood [**44]. The pH of the solution, the applied with variable Co powder content. In another significant voltage and deposition time are shown to have a strong development by the same group, anodic co-deposition of influence on the quality of the infiltration. Good particle Al O and CeO -stabilised zirconia powders was used to 23 2 packing and a high solids-loading can be achieved, produc- obtain cylindrical and tubular-shaped Al O /zirconia 2 3 ing firm ceramic deposits which adhered to the fibres, thus FGM components [*54]. leading to pore-free composites after a post-EPD heat- Current efforts are devoted to the development of EPD treatment process. The typical microstructure of a mullite fabrication approaches for laminated ceramic composites, matrix composite reinforced by SiC (Nicalon ) fibres in particular in the system zirconia/alumina, due to the fabricated by EPD is shown in Fig. 1. high fracture resistance of these structures [55–57]. The Most previous research summarized in Ref. [**44] has significance of the papers by Moreno and Ferrari [56] and been focused on components of simple planar shape. The by Uchikoshi et al. [57], is that they focus on optimized use of EPD for near-net shape fabrication of 3-D compo- aqueous suspensions, which should be always preferred site components of complex shapes is starting to be over organic suspensions due to environmental and econinvestigated. A pioneering development in this area is omic considerations. Based on the promising results Fig. 2. Schema of the EPD cell recently introduced for the fabrication of tubular metal fibre reinforced alumina matrix composites [*50] (diagram courtesy Dr C. Kaya, University of Birmingham, UK)
A.R. Boccaccini, 1. Zhitomirsky/ Current Opinion in Solid State and Materials Science 6(2002)251-260 achieved so far, a significant growth of r&D work in the 3. Electrolytic deposition(ELD) area of EPD processing of FGM and laminated ceramic structures is anticipated Electrolytic deposition produces thin ceramic films from solutions of metal salts and it is a relatively new technique in ceramic processing. There are several basic mechanism 2.7. Nanomaterials and nanostructures of ELD of ceramic films. Cathodic electrolytic deposition has important advantages and can be used for deposition of The synthesis and characterisation of nanostructured various oxide materials. In the cathodic electrodeposition materials and nanostructures are areas of active research. method, cathodic reactions are used to generate OH-groups The main focus of the research on nanostructured material and increase the pH at the electrode. Metal ions o is to gain basic understanding of their intriguing physical complexes, which are stable in the bulk of solutions at low and chemical properties, and EPD, with its high versatility pH, are hydrolyzed by electrogenerated base at the elec and ease of application, is revealing itself as one of the trode surface to form colloidal particles. These particles processing techniques of choice in this increasingly popu- coagulate to form cathodic ceramic deposits lar research area. For example, EPD has been used for the A comprehensive review paper covering developments growth of ceramic nanotubes and nanorods and for the in ELD of ceramic materials before 2000 has been efficient deposit of nanotubes and nanosized ceramic published by Therese and Kamath [**4] particles on different substrates [*58, 59, 60,611 Several studies have recently contributed to both the A major advantage of EpD for the fabrication of fundamental understanding of the mechanisms of ELD and nanorods and nanowires is the ability to grow large areas the practical use of ELD for various applications. The most of uniformly sized and nearly unidirectionally aligned significant advances are summarized in this section nanorods of various oxides, as described by Limmer et al [* 58]. Fig 3 shows the typical structure of TiO, nanorods 3.1. Fundamental principles of the eld process grown in a polycarbonate membrane with 200-nm diameter pores by sol-gel EPD[**581 Recent progress [**64 in the understanding of the Smeets et al. [62] have recently used EPD to incorporate mechanism of cathodic electrodeposition has come from functional nanosized particles into nanoporous glass the application of the classical DLvo theory of colloidal bodies. The main advantage of the EPD technique here is stability. Electrolyte concentration in the solutions used for that it involves lower temperatures than traditional glass ELD exceeds the flocculation values for corresponding melting, and thus components with relatively low thermal ions. Therefore colloidal particles obtained by cathodic capability can be incorporated into glass hosts, as for electrosynthesis are unstable and coagulate to form a example Cds/Se nanoparticles. A related study was con- cathodic deposit. The important recent finding is that the ducted by Subramanian et al.[63], who deposited noble formation of a ceramic deposit during ELD is caused by metal particles of Au, Pt and Ir on nanostructured titania flocculation introduced by the electrolyte **64]. The films using epd study also highlighted the importance of the electric field Although the development of EPD techniques in this electrode reactions and other factors that influence the novel area(nanomaterials and nanostructures) is in the coagulation of particles near the electrode surface. In a initial stage, results so far are encouraging and indicate recent review, novel electrochemical strategies and de- great potential for future R&D efforts velopments of cathodic electrodeposition, focusing on the Fig 3. Typical structure of TiO, nanorods grown in a membrane with 200-nm diameter pores by sol-gel EPD, as produced by Limmer et al. [58], at(A) w and(B) high magnification(micrograph courtesy of Professor G Cao, published with permission of Wiley-VCH, Weinheim
A.R. Boccaccini, I. Zhitomirsky / Current Opinion in Solid State and Materials Science 6 (2002) 251–260 255 achieved so far, a significant growth of R&D work in the 3. Electrolytic deposition (ELD) area of EPD processing of FGM and laminated ceramic structures is anticipated. Electrolytic deposition produces thin ceramic films from solutions of metal salts and it is a relatively new technique in ceramic processing. There are several basic mechanisms 2 .7. Nanomaterials and nanostructures of ELD of ceramic films. Cathodic electrolytic deposition has important advantages and can be used for deposition of The synthesis and characterisation of nanostructured various oxide materials. In the cathodic electrodeposition materials and nanostructures are areas of active research. method, cathodic reactions are used to generate OH-groups The main focus of the research on nanostructured materials and increase the pH at the electrode. Metal ions or is to gain basic understanding of their intriguing physical complexes, which are stable in the bulk of solutions at low and chemical properties, and EPD, with its high versatility pH, are hydrolyzed by electrogenerated base at the elecand ease of application, is revealing itself as one of the trode surface to form colloidal particles. These particles processing techniques of choice in this increasingly popu- coagulate to form cathodic ceramic deposits. lar research area. For example, EPD has been used for the A comprehensive review paper covering developments growth of ceramic nanotubes and nanorods and for the in ELD of ceramic materials before 2000 has been efficient deposit of nanotubes and nanosized ceramic published by Therese and Kamath [**4]. particles on different substrates [**58,*59,*60,61]. Several studies have recently contributed to both the A major advantage of EPD for the fabrication of fundamental understanding of the mechanisms of ELD and nanorods and nanowires is the ability to grow large areas the practical use of ELD for various applications. The most of uniformly sized and nearly unidirectionally aligned significant advances are summarized in this section. nanorods of various oxides, as described by Limmer et al. [**58]. Fig. 3 shows the typical structure of TiO nanorods 3 .1. Fundamental principles of the ELD process 2 grown in a polycarbonate membrane with 200-nm diameter pores by sol–gel EPD [**58]. Recent progress [**64] in the understanding of the Smeets et al. [62] have recently used EPD to incorporate mechanism of cathodic electrodeposition has come from functional nanosized particles into nanoporous glass the application of the classical DLVO theory of colloidal bodies. The main advantage of the EPD technique here is stability. Electrolyte concentration in the solutions used for that it involves lower temperatures than traditional glass ELD exceeds the flocculation values for corresponding melting, and thus components with relatively low thermal ions. Therefore colloidal particles obtained by cathodic capability can be incorporated into glass hosts, as for electrosynthesis are unstable and coagulate to form a example CdS/Se nanoparticles. A related study was con- cathodic deposit. The important recent finding is that the ducted by Subramanian et al. [63], who deposited noble formation of a ceramic deposit during ELD is caused by metal particles of Au, Pt and Ir on nanostructured titania flocculation introduced by the electrolyte [**64]. The films using EPD. study also highlighted the importance of the electric field, Although the development of EPD techniques in this electrode reactions and other factors that influence the novel area (nanomaterials and nanostructures) is in the coagulation of particles near the electrode surface. In a initial stage, results so far are encouraging and indicate recent review, novel electrochemical strategies and degreat potential for future R&D efforts. velopments of cathodic electrodeposition, focusing on the Fig. 3. Typical structure of TiO nanorods grown in a membrane with 200-nm diameter pores by sol–gel EPD, as produced by Limmer et al. [**58], at (A) 2 low and (B) high magnification (micrograph courtesy of Professor G. Cao, published with permission of Wiley–VCH, Weinheim, Germany)