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 in252 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 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 [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 sus￾They 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 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. 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 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,**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