《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-18

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS JOURNAL OF MATERIALS SCIENCE 39(2004)803-81I Electrophoretic deposition from aqueous suspensions for near-shape manufacturing of advanced ceramics and glasses-applications J. TABELLION R CLASEN Department of Powder Technology, Saarland University, Bd. 43, D-66123 Saarbrucken, Germany E-mail: j. tabellion@matsci uni sb.de Due to high deposition rates and the avoidance of inflammable, often hazardous organic solvents EPD from aqueous suspensions is a fast and low-cost shaping technique for ceramics and glasses. Since the deposition rate is independent of particle size EPd has an outstanding ability for the shaping of nano-particles. In this paper the shaping of complex silica glass and zirconia components, like tubes or structured parts by means of the membrane method is shown. Three-dimensional shaped porous polymer moulds were used as ion-permeable deposition surface. To enable near-shape manufacturing, mixtures inosized and microsized particles were electrophoretically deposited. No size-dependent separation was observed. Due to the very high green density of these green bodies (up to 84% of the theoretical value) shrinkage could be reduced to 4.7%. Not only oxide ceramics but also silicon carbide was deposited from aqueous suspensions. Apart from bulk SiC, protective coatings with a thickness of app. 60 um were applied on top of CFC substrates by EPD. Good adhesion was observed and no cracking occurred. Furthermore, electrophoretic impregnation was used for the modification of porous green bodies. Thus silica glasses with graded density and pore size as well as functionally graded composites were prepared. C 2004 Kluwer Academic Publishers 1. Introduction have to be applied to achieve acceptable shaping times Electrophoretic deposition(EPD)is a process known Furthermore, severe safety precautions have to be met, for decades [l], where charged particles move under if organic, often inflammable dispersants are used, es- the influence of a direct electric field towards an op- pecially in case of high voltages and/or high current positely charged electrode and coagulate there to form densities. As a consequence the process cost increases a stable deposit. Although EPD has been applied on Finally, disposal of hazardous organic waste and treat an industrial scale for years as enamelling and lacquer- ment of the suspensions for recovery of the dispersant ing technique( cataphoresis)[2, 3], it is not yet estab- as well as the absorption of volatile cracking produ lished as a shaping process for functional ceramics and during heat treatment can prevent the introduction of glasses. Among the ceramic coatings applied by Epd an EPD facility into an existing production line are e.g., phosphors [4], coatings for SOFCs [5], pho- As an alternative aqueous susp ons can be used tovoltaic applications [6] or insulating layers [7]. In for the EPD of ceramics. Since only water is used as some cases even manufacturing on an industrial scale dispersant, no sophisticated safety precautions are nec- has become likely [8] essary and treatment of the suspensions after EPD is But in case of bulk ceramic or glass components no easy. Most important, very high deposition rates can industrial manufacturing process has proved success- be reached for comparably low applied electric field ful so far. Nevertheless, several ceramic systems were strengths [16]. Thus process times can be reduced to investigated on a laboratory scale, including alumina several seconds to minutes depending on the material [9], zirconia [101, SiC [ll, 12] and PZT [13]. Compre- used. Clay components with a thickness of 10 mm could hensive reviews about EPD as shaping technique for be shaped from aqueous sanitary slips within 15s[17] ceramics are given in [14, 15]. In all of these cases or- But the only industrial attempt to produce tiles by EPD ganic solvents were used as dispersant, which seems to from aqueous suspensions [ 18] failed be one of the problems why EPD has never established The main problem associated with the use of aque yet for large-scale production of ceramics. First of all, ous suspensions for EPD, is the electrolysis-induced de- the deposition rates are significantly lower in compar- composition of water above a DC voltage of about 1. 4V ison with aqueous suspensions because of the much The electrolysis results in a movement of ions towards lower permittivity, thus very high electric field strengths the oppositely charged electrode, where recombination 0022-2461 2004 Kluwer Academic Publishers

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS JOURNAL OF MATERIALS SCIENCE 3 9 (2 0 0 4 ) 803 – 811 Electrophoretic deposition from aqueous suspensions for near-shape manufacturing of advanced ceramics and glasses—applications J. TABELLION, R. CLASEN Department of Powder Technology, Saarland University, Bd. 43, D-66123 Saarbrucken, Germany E-mail: j.tabellion@matsci.uni.sb.de Due to high deposition rates and the avoidance of inflammable, often hazardous organic solvents EPD from aqueous suspensions is a fast and low-cost shaping technique for ceramics and glasses. Since the deposition rate is independent of particle size EPD has an outstanding ability for the shaping of nano-particles. In this paper the shaping of complex silica glass and zirconia components, like tubes or structured parts by means of the membrane method is shown. Three-dimensional shaped porous polymer moulds were used as ion-permeable deposition surface. To enable near-shape manufacturing, mixtures of nanosized and microsized particles were electrophoretically deposited. No size-dependent separation was observed. Due to the very high green density of these green bodies (up to 84% of the theoretical value) shrinkage could be reduced to 4.7%. Not only oxide ceramics but also silicon carbide was deposited from aqueous suspensions. Apart from bulk SiC, protective coatings with a thickness of app. 60 µm were applied on top of CFC substrates by EPD. Good adhesion was observed and no cracking occurred. Furthermore, electrophoretic impregnation was used for the modification of porous green bodies. Thus silica glasses with graded density and pore size as well as functionally graded composites were prepared.
C 2004 Kluwer Academic Publishers 1. Introduction Electrophoretic deposition (EPD) is a process known for decades [1], where charged particles move under the influence of a direct electric field towards an op￾positely charged electrode and coagulate there to form a stable deposit. Although EPD has been applied on an industrial scale for years as enamelling and lacquer￾ing technique (cataphoresis) [2, 3], it is not yet estab￾lished as a shaping process for functional ceramics and glasses. Among the ceramic coatings applied by EPD are e.g., phosphors [4], coatings for SOFCs [5], pho￾tovoltaic applications [6] or insulating layers [7]. In some cases even manufacturing on an industrial scale has become likely [8]. But in case of bulk ceramic or glass components no industrial manufacturing process has proved success￾ful so far. Nevertheless, several ceramic systems were investigated on a laboratory scale, including alumina [9], zirconia [10], SiC [11, 12] and PZT [13]. Compre￾hensive reviews about EPD as shaping technique for ceramics are given in [14, 15]. In all of these cases or￾ganic solvents were used as dispersant, which seems to be one of the problems why EPD has never established yet for large-scale production of ceramics. First of all, the deposition rates are significantly lower in compar￾ison with aqueous suspensions because of the much lower permittivity, thus very high electric field strengths have to be applied to achieve acceptable shaping times. Furthermore, severe safety precautions have to be met, if organic, often inflammable dispersants are used, es￾pecially in case of high voltages and/or high current densities. As a consequence the process cost increases. Finally, disposal of hazardous organic waste and treat￾ment of the suspensions for recovery of the dispersant as well as the absorption of volatile cracking products during heat treatment can prevent the introduction of an EPD facility into an existing production line. As an alternative aqueous suspensions can be used for the EPD of ceramics. Since only water is used as dispersant, no sophisticated safety precautions are nec￾essary and treatment of the suspensions after EPD is easy. Most important, very high deposition rates can be reached for comparably low applied electric field strengths [16]. Thus process times can be reduced to several seconds to minutes depending on the material used. Clay components with a thickness of 10 mm could be shaped from aqueous sanitary slips within 15 s [17]. But the only industrial attempt to produce tiles by EPD from aqueous suspensions [18] failed. The main problem associated with the use of aque￾ous suspensions for EPD, is the electrolysis-induced de￾composition of water above a DC voltage of about 1.4 V. The electrolysis results in a movement of ions towards the oppositely charged electrode, where recombination 0022–2461
C 2004 Kluwer Academic Publishers 803

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS of the ions results in generation of gases. If the deposi- of this work was to demonstrate that near-shape manu tion takes place at one of the electrodes, these gases can facturing of complex structures and large components be incorporated into the deposit, resulting in large and is possible by EPD from aqueous suspensions irregularly distributed faults. Several approaches have been made to avoid the formation of gas bubbles at the 2. Experimental deposition surface First, the applied voltage can be kept lower than the 2.1. Materials and suspensions Aqueous suspensions of nanosized fumed silica parti- decomposition voltage of water, which proved success- cles(Degussa Aerosil OX50, mean particle size 40 nm) ful for the production of alumina microstructures [ 19] But due to the low deposition rate only small wall thick- were prepared by dispersing the particles in bidis- tilled water containing tetramethylammoniumhydrox ness can be achieved. Another possibility is to use sac- ide(TMah) by means of a dissolver (LDvl, PC rificial electrodes In [18] a process is described where zinc rollers are used as deposition electrodes. But the aborsysteme) Vacuum was applied to avoid incorpo problem is that the deposition rate is limited due to elec ration of air bubbles. TMAH was used to adjust the pH trode reactions and zinc ions are dissolved because of and thus the s-potential of the silica particles as well the electrolysis reaction. These contaminations prevent as the visco sity of the sion both of which are the application for pure materials very important factors for the EPD. a detailed descrip Using cathode materials that can store hydrogen tion of the influence of pH on s-potential, rheologic within their structure, like palladium, enables EPD from properties, suspension structure and homogeneity can be found in [33]. The suspensions had a solids con- aqueous suspensions at the cathode. A process for shap- tent of 30 wt%. In case of electrophoretic impregnation disadvantage is the small amount of hydrogen that can aqueous suspensions of oX50 or alumina(Al2O3-C, be incorporated into the structure of these materials, Degussa)with 5 wt% were used. Similarly, suspensions which limits this process to components with low wall (Solid loading 75 wt%)containing a mixture of fumed thickness. Furthermore, a positive s-potential of the (OX50) and fused silica(Dso=15 um)were prepared particles is a necessary demand. The ratio of nanosized to microsized (fused) silica was A much more promising way is the membrane- sions of nanosized zirconia(Degussa Zirconia-3YSZ method [21]. In this case the electrophoresis chamber is subdivided by a porous, 10n-permeable membrane into and mixtures of nano- and microsized ZrO2(D50 3 um, doped with 15.5 mol% ceria)were prepared two chambers that contain the suspension and another In case of silicon carbide, aqueous suspensions with fluid, respectively. Deposition occurs at the membrane whereas the ions can pass the pores of the membrane above. The silicon carbide powder used was siC sMi5 so that recombination of the ions and generation of gas from ESK with a dso value of 0.77 um. To enable bubbles occurs at the electrodes. No gas bubbles can be incorporated into the deposit. This process was used solid state sintering boron carbide and carbon black for different materials like e.g., zirconia [22, 23] ar were added to the silica glass [24]. Green density and pore size distribu- The mean particle size of the B4C used(Tetrabor F1500 tion of silica green bodies could be tailored (25). After from ESK) was I5 um, that of the carbon black pow optimization the silica green bodies could be sintered der( Degussa, FW200)13 nm. A small amount of dis- to full density at 1320C, which is about 100%C lower persing aid(0.2 wt% correlated to the amount of car- than for gel-cast green bodies of the same powder [26] black particles in water. To achieve a co-deposition of A further improvement in the manufacturing of silica Sic and the sintering additives B.C and carbon black glasses was achieved by using mixtures of nanosized two different approaches were made. First of all, EPD and microsized silica particles By means of EPD very was carried out from suspensions with a pH value of homogeneous green bodies with very high green den sities of up to 84%TD could be shaped Shrinkage was I1, where all particles have a s-potential of the same reduced to 4 to 7%(linear)[271 sign. Alternatively, suspensions with pH 7 were used A modification of the EPD process is the elec for EPD, where the carbides have a highly negative s-potential whereas the carbon black particles have pos- trophoretic impregnation(EPD), where nanosized par- itive surface charge(cp. Fig. 6). pH was adjusted by ticles are deposited within the pores or voids of a green adding different amounts of TMAH to the bidistilled body or fibre fabric. The principles of EPI are summa- water prior to dispersing the particles rized in [28]. Investigations were made, to characterize cosity, solids content, surface charge of both particles 2. 2. Electrophoretic deposition/ and porous structure and ratio of pore to particles size, electrophoretic impregnation on the efficiency of the EPI process [29]. Applications Electrophoretic deposition was carried out under con of the EPI are the manufacturing of fibre-reinforced stant applied voltage by the membrane method. A sim- composites[30, 31]and the incorporation of functional ple experimental set-up for the EPD of plates is shown secondary phases into glasses [32] in Fig. 1. An electrophoresis cell, with a cross section o In this paper several applications of EPD by means of 40 x 40 mm, was subdivided by an ion-permeable, the membrane method as shaping technique for porous polymer mould, so that deposition of particles ica glass and advanced ceramics are shown. The (onto the porous mould) and recombination of ions(at

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS of the ions results in generation of gases. If the deposi￾tion takes place at one of the electrodes, these gases can be incorporated into the deposit, resulting in large and irregularly distributed faults. Several approaches have been made to avoid the formation of gas bubbles at the deposition surface. First, the applied voltage can be kept lower than the decomposition voltage of water, which proved success￾ful for the production of alumina microstructures [19]. But due to the low deposition rate only small wall thick￾ness can be achieved. Another possibility is to use sac￾rificial electrodes. In [18] a process is described where zinc rollers are used as deposition electrodes. But the problem is that the deposition rate is limited due to elec￾trode reactions and zinc ions are dissolved because of the electrolysis reaction. These contaminations prevent the application for pure materials. Using cathode materials that can store hydrogen within their structure, like palladium, enables EPD from aqueous suspensions at the cathode. A process for shap￾ing alumina ferrules is described in [20]. One severe disadvantage is the small amount of hydrogen that can be incorporated into the structure of these materials, which limits this process to components with low wall thickness. Furthermore, a positive ζ -potential of the particles is a necessary demand. A much more promising way is the membrane￾method [21]. In this case the electrophoresis chamber is subdivided by a porous, ion-permeable membrane into two chambers that contain the suspension and another fluid, respectively. Deposition occurs at the membrane whereas the ions can pass the pores of the membrane so that recombination of the ions and generation of gas bubbles occurs at the electrodes. No gas bubbles can be incorporated into the deposit. This process was used for different materials like e.g., zirconia [22, 23] and silica glass [24]. Green density and pore size distribu￾tion of silica green bodies could be tailored [25]. After optimization the silica green bodies could be sintered to full density at 1320◦C, which is about 100◦C lower than for gel-cast green bodies of the same powder [26]. A further improvement in the manufacturing of silica glasses was achieved by using mixtures of nanosized and microsized silica particles. By means of EPD very homogeneous green bodies with very high green den￾sities of up to 84%TD could be shaped. Shrinkage was reduced to 4 to 7% (linear) [27]. A modification of the EPD process is the elec￾trophoretic impregnation (EPI), where nanosized par￾ticles are deposited within the pores or voids of a green body or fibre fabric. The principles of EPI are summa￾rized in [28]. Investigations were made, to characterize the influence of several process parameters, like vis￾cosity, solids content, surface charge of both particles and porous structure and ratio of pore to particles size, on the efficiency of the EPI process [29]. Applications of the EPI are the manufacturing of fibre-reinforced composites [30, 31] and the incorporation of functional secondary phases into glasses [32]. In this paper several applications of EPD by means of the membrane method as shaping technique for sil￾ica glass and advanced ceramics are shown. The aim of this work was to demonstrate that near-shape manu￾facturing of complex structures and large components is possible by EPD from aqueous suspensions. 2. Experimental 2.1. Materials and suspensions Aqueous suspensions of nanosized fumed silica parti￾cles (Degussa Aerosil OX50, mean particle size 40 nm) were prepared by dispersing the particles in bidis￾tilled water containing tetramethylammoniumhydrox￾ide (TMAH) by means of a dissolver (LDV1, PC Laborsysteme). Vacuum was applied to avoid incorpo￾ration of air bubbles. TMAH was used to adjust the pH and thus the ζ -potential of the silica particles as well as the viscosity of the suspension, both of which are very important factors for the EPD. A detailed descrip￾tion of the influence of pH on ζ -potential, rheological properties, suspension structure and homogeneity can be found in [33]. The suspensions had a solids con￾tent of 30 wt%. In case of electrophoretic impregnation aqueous suspensions of OX50 or alumina (Al2O3-C, Degussa) with 5 wt% were used. Similarly, suspensions (solid loading 75 wt%) containing a mixture of fumed (OX50) and fused silica (D50 = 15 µm) were prepared. The ratio of nanosized to microsized (fused) silica was 10:90 (per weight) In the same manner, aqueous suspen￾sions of nanosized zirconia (Degussa Zirconia-3YSZ) and mixtures of nano- and microsized ZrO2 (D50 = 3 µm, doped with 15.5 mol% ceria) were prepared. In case of silicon carbide, aqueous suspensions with 45 wt% solids content were prepared as described above. The silicon carbide powder used was SiC SM15 from ESK with a D50 value of 0.77 µm. To enable solid state sintering boron carbide and carbon black were added to the suspension as sintering additives. The mean particle size of the B4C used (Tetrabor F1500 from ESK) was 1.5 µm, that of the carbon black pow￾der (Degussa, FW200) 13 nm. A small amount of dis￾persing aid (0.2 wt% correlated to the amount of car￾bon black) was added to allow dispersion of the carbon black particles in water. To achieve a co-deposition of SiC and the sintering additives B4C and carbon black two different approaches were made. First of all, EPD was carried out from suspensions with a pH value of 11, where all particles have a ζ -potential of the same sign. Alternatively, suspensions with pH 7 were used for EPD, where the carbides have a highly negative ζ -potential whereas the carbon black particles have pos￾itive surface charge (cp. Fig. 6). pH was adjusted by adding different amounts of TMAH to the bidistilled water prior to dispersing the particles. 2.2. Electrophoretic deposition/ electrophoretic impregnation Electrophoretic deposition was carried out under con￾stant applied voltage by the membrane method. A sim￾ple experimental set-up for the EPD of plates is shown in Fig. 1. An electrophoresis cell, with a cross section of 40 × 40 mm2, was subdivided by an ion-permeable, porous polymer mould, so that deposition of particles (onto the porous mould) and recombination of ions (at 804

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS 4 1-porous mould(EPD)or porous green body(EPl) 2-cathode(platinum hite 68 3-anode(platinum or graphite) 4-DC power supply 5-suspension 6-bidistillied water(+ electrolyte 7-pH meter with PC interface 8- interface for data acquisition and conditioning 9-measuring gages 5 measurement 10-PC for process control and data analyzing 25 mm 10 mm 10 m Figure I Experimental set-up for the electrophoretic deposition/impregnation process from aqueous suspensions the electrodes) were separated. Thus no gas bubbles on basis of SEM images by means of image analysis were found within the deposit. One chamber was filled software(Image C). For this purpose, SEM pictures of with the suspension, the other with bidistilled water not impregnated silica green bodies were taken as ref- containing different amounts of TMAH. The width of erence first. The densification of the impregnated green the different chambers was optimized after measur- bodies was determined on the basis of SEM pictures ing the effective electric field strength within the elec- with the same magnification, that were taken equidis trophoresis cell [34]. This set-up was used to determine tantly(300 um) from the surface in contact with the deposition rate, green density and pore size. For more suspension during EPI towards the bulk. A densifica complex shaped components the set-up had to be re- tion of 0%o means that the same porosity is visible as adjusted concerning the shape of the porous mould and for the not impregnated references. a densification of the electrodes. The applied voltage was varied between 100%o means, that at the given magnification no porosity I and 15 V/cm. Deposition time was 3 min in case of is visible. These values were correlated finally to abse silica and zirconia and 30 s in case of the SiC coatings. lute values of the density and pore size, achieved from In case of electrophoretic impregnation, the same ex- Archimedes method and mercury porosimetry. Fur- perimental set-up was used, but instead of the porous thermore, Raman microscopy was used for a position polymer mould a cast silica green body with open sensitive characterization of graded composites pores(26% porosity, mean pore size 1. 8 um)was used. The electric field strength was varied between 1.5 and 6 V/cm and deposition time was 30 and 60 s, 3. Results and discussion respectively The deposition rate for the electrophoretic deposition After shaping, the green bodies and coated substrates of the different powders used was determined first, be were dried in air under ambient humidity. No cracking cause only an exact control of this parameter can guar occurred for the green bodies and no cracking was ob- antee a reproducible manufacturing of components with served for the Sic coatings for a thickness of up to given specifications concerning wall thickness and tol- 200 um Sintering of the compacts was carried out ei- trances. The deposition rate for an aqueous suspension ther in vacuum(SiO2 powder mixtures, 1500-1600 C), containing 30 wt% of nanosized fumed silica(OX50) argon atmosphere(SiC, 1700-2100 C), air(zirconia, is shown in Fig. 2(dark line, circles) as function of the 1300-1700C)or in a zone-sintering furnace(OX50, applied electric field strength for a deposition time of 1320°C 3 min A linear dependency was observed. The green density of the corresponding silica green bodies was not influenced by the electric field strength and was 2. 3. Characterization determined by Archimedes method to be 39.4%TD. Density and size distribution of green and sin- This means that e.g., a green body with a thickness of tered samples were measured by Archimedes method 12 mm was deposited within 3 min for a field strength and mercury porosimetry(Porotec Pascal P140, P440), of 10 V/cm respectively. Microstructural homogeneity was investi- For nanosized zirconia a similar dependency of the gated on basis of SEM and high resolution SEM deposition rate from the electric field strength was ob SEM)images. Densification due to EPi was determined served but the absolute values are lower than for silica 805

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS Figure 1 Experimental set-up for the electrophoretic deposition/impregnation process from aqueous suspensions. the electrodes) were separated. Thus no gas bubbles were found within the deposit. One chamber was filled with the suspension, the other with bidistilled water containing different amounts of TMAH. The width of the different chambers was optimized after measur￾ing the effective electric field strength within the elec￾trophoresis cell [34]. This set-up was used to determine deposition rate, green density and pore size. For more complex shaped components the set-up had to be re￾adjusted concerning the shape of the porous mould and the electrodes. The applied voltage was varied between 1 and 15 V/cm. Deposition time was 3 min in case of silica and zirconia and 30 s in case of the SiC coatings. In case of electrophoretic impregnation, the same ex￾perimental set-up was used, but instead of the porous polymer mould a cast silica green body with open pores (26% porosity, mean pore size 1.8 µm) was used. The electric field strength was varied between 1.5 and 6 V/cm and deposition time was 30 and 60 s, respectively. After shaping, the green bodies and coated substrates were dried in air under ambient humidity. No cracking occurred for the green bodies and no cracking was ob￾served for the SiC coatings for a thickness of up to 200 µm. Sintering of the compacts was carried out ei￾ther in vacuum (SiO2 powder mixtures, 1500–1600◦C), argon atmosphere (SiC, 1700–2100◦C), air (zirconia, 1300–1700◦C) or in a zone-sintering furnace (OX50, 1320◦C). 2.3. Characterization Density and pore size distribution of green and sin￾tered samples were measured by Archimedes method and mercury porosimetry (Porotec Pascal P140, P440), respectively. Microstructural homogeneity was investi￾gated on basis of SEM and high resolution SEM (HR￾SEM) images. Densification due to EPI was determined on basis of SEM images by means of image analysis software (ImageC). For this purpose, SEM pictures of not impregnated silica green bodies were taken as ref￾erence first. The densification of the impregnated green bodies was determined on the basis of SEM pictures with the same magnification, that were taken equidis￾tantly (300 µm) from the surface in contact with the suspension during EPI towards the bulk. A densifica￾tion of 0% means that the same porosity is visible as for the not impregnated references. A densification of 100% means, that at the given magnification no porosity is visible. These values were correlated finally to abso￾lute values of the density and pore size, achieved from Archimedes method and mercury porosimetry. Fur￾thermore, Raman microscopy was used for a position￾sensitive characterization of graded composites. 3. Results and discussion The deposition rate for the electrophoretic deposition of the different powders used was determined first, be￾cause only an exact control of this parameter can guar￾antee a reproducible manufacturing of components with given specifications concerning wall thickness and tol￾erances. The deposition rate for an aqueous suspension containing 30 wt% of nanosized fumed silica (OX50) is shown in Fig. 2 (dark line, circles) as function of the applied electric field strength for a deposition time of 3 min. A linear dependency was observed. The green density of the corresponding silica green bodies was not influenced by the electric field strength and was determined by Archimedes method to be 39.4%TD. This means that e.g., a green body with a thickness of 12 mm was deposited within 3 min for a field strength of 10 V/cm. For nanosized zirconia a similar dependency of the deposition rate from the electric field strength was ob￾served, but the absolute values are lower than for silica. 805

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS Nanosized fumed silica(OX50 green density of 49%TD and a thickness of 3 mm was Mixture fumed/fused silica: 10:90 achieved after 3 min epd with 10 V/cm Nanosized zirconia(ZrOr-VP These results show that electrophoretic deposition 04◆sc(7m+ C+carbon black from aqueous suspensions is a very fast shaping tech- E巴E8 ique, even though the deposition rate strongly depends on the material, the solids content, viscosity and the s-potential of the particles. Especially in the case of nano-particles and powder mixtures with bi- or multi- modal particle size distribution, fast and homogeneous shaping is possible by means of EPD. Furthermore, these results show that wall or coating thickness can be E(V/cm) controlled accurately and reproducibly in case of EPD by adjusting the applied electric field stre Figure 2 Deposition rate as function of externally applied electric fiel To compare EPD with competing shaping tech strength for different materials and different particle size (deposition niques, silica green bodies(plates)were shaped from an aqueous suspension with 30 wt% OX50 by means of EPD, slip casting and pressure casting, respectively This is related to the lower value of the s-potential of After drying, porosity was measured and microstruc the zirconia particles. For an electric field strength of ture was investigated 10 V/cm a green body with a green density of 32%TD Fig 3 shows the specific cumulative pore volume of and a thickness of 4 mm was achieved was deposited the silica green bodies measured by mercury porosime- within 3 min try, which corresponds directly to the open porosity Furthermore, mixtures of nanosized silica(OX50) of the samples On the right hand side of Fig 3 two and fused silica were deposited(Fig. 2, dotted line). HR-SEM images are shown. The upper one shows No significant difference was observed compared to a fracture surface of the silica green body shaped pure OX50. The slightly lower values can be explained by pressure casting, the lower image shows the elec- by the higher viscosity(240 mPa. s at a shear rate of trophoretically deposited green body. Both, mercury 300 s" compared to 60 mPa. s in case of 30 wt% porosimetry and HR-SEM pictures show that the high OX50. But what is most important, no influence of est density and the best microstructural homogeneity is particle size on deposition rate was found. A green achieved if EPD is used as shaping technique, at least for body with a green density of 74%TD and a thickness of nano-particles 7 mm was achieved after 3 min deposition with a field Electrophoretically deposited silica glass tubes strength of 10 V/cm. Finally, the deposition rate for the (green state, left-hand side) of different diameter and o-deposition of SiC, B4C and carbon black at pH 7 with different wall thickness are shown in Fig 4. These (Fig. 2, light line)was investigated. Again a linear de- tubes were electrophoretically deposited from an aque pendency of the deposition rate from field strength was ous suspension of nanosized fumed silica(OX50) found. But the values are much lower than for silica or A tubular polymer membrane was used as an ion- zirconia. This is obviously related to the much higher permeable deposition surface. As can be seen(Fig 4, viscosity (460 mPa. s at 300 s" ) and to the fact that right-hand side)a fully dense and fully transparent sil the net-charge of the agglomerated Sic/carbon black ica glass tube was achieved after sintering at 1320oC and B.C/carbon black particles at pH 7(cp. Fig. 6) is No gas bubbles were found. This shows that by means lower than for the pure carbides. A green body with a of electrophoretic deposition from aqueous suspensions pressure casting EPD(3V/cm) 41 um pore radius (nm) Figure 3 Comparison of different shaping techniques by means of mercury porosimetry and HR-SEM images of fracture surfaces: Porosity and homogeneity of silica green bodies prepared from aqueous suspension of nanosized fumed silica(OX50)by EPD, slip casting and pressure casting

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS Figure 2 Deposition rate as function of externally applied electric field strength for different materials and different particle size (deposition time: 3 min). This is related to the lower value of the ζ -potential of the zirconia particles. For an electric field strength of 10 V/cm a green body with a green density of 32%TD and a thickness of 4 mm was achieved was deposited within 3 min. Furthermore, mixtures of nanosized silica (OX50) and fused silica were deposited (Fig. 2, dotted line). No significant difference was observed compared to pure OX50. The slightly lower values can be explained by the higher viscosity (240 mPa ·s at a shear rate of 300 s−1 compared to 60 mPa ·s in case of 30 wt% OX50). But what is most important, no influence of particle size on deposition rate was found. A green body with a green density of 74%TD and a thickness of 7 mm was achieved after 3 min deposition with a field strength of 10 V/cm. Finally, the deposition rate for the co-deposition of SiC, B4C and carbon black at pH 7 (Fig. 2, light line) was investigated. Again a linear de￾pendency of the deposition rate from field strength was found. But the values are much lower than for silica or zirconia. This is obviously related to the much higher viscosity (460 mPa ·s at 300 s−1) and to the fact that the net-charge of the agglomerated SiC/carbon black and B4C/carbon black particles at pH 7 (cp. Fig. 6) is lower than for the pure carbides. A green body with a Figure 3 Comparison of different shaping techniques by means of mercury porosimetry and HR-SEM images of fracture surfaces: Porosity and homogeneity of silica green bodies prepared from aqueous suspension of nanosized fumed silica (OX50) by EPD, slip casting and pressure casting, respectively. green density of 49%TD and a thickness of 3 mm was achieved after 3 min EPD with 10 V/cm. These results show that electrophoretic deposition from aqueous suspensions is a very fast shaping tech￾nique, even though the deposition rate strongly depends on the material, the solids content, viscosity and the ζ -potential of the particles. Especially in the case of nano-particles and powder mixtures with bi- or multi￾modal particle size distribution, fast and homogeneous shaping is possible by means of EPD. Furthermore, these results show that wall or coating thickness can be controlled accurately and reproducibly in case of EPD by adjusting the applied electric field strength. To compare EPD with competing shaping tech￾niques, silica green bodies (plates) were shaped from an aqueous suspension with 30 wt% OX50 by means of EPD, slip casting and pressure casting, respectively. After drying, porosity was measured and microstruc￾ture was investigated. Fig. 3 shows the specific cumulative pore volume of the silica green bodies measured by mercury porosime￾try, which corresponds directly to the open porosity of the samples. On the right hand side of Fig. 3 two HR-SEM images are shown. The upper one shows a fracture surface of the silica green body shaped by pressure casting, the lower image shows the elec￾trophoretically deposited green body. Both, mercury porosimetry and HR-SEM pictures show that the high￾est density and the best microstructural homogeneity is achieved if EPD is used as shaping technique, at least for nano-particles. Electrophoretically deposited silica glass tubes (green state, left-hand side) of different diameter and with different wall thickness are shown in Fig. 4. These tubes were electrophoretically deposited from an aque￾ous suspension of nanosized fumed silica (OX50). A tubular polymer membrane was used as an ion￾permeable deposition surface. As can be seen (Fig. 4, right-hand side) a fully dense and fully transparent sil￾ica glass tube was achieved after sintering at 1320◦C. No gas bubbles were found. This shows that by means of electrophoretic deposition from aqueous suspensions 806

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS NIVERSITAT Lahrturl fu 40 mm Figure 4 Silica tubes(green state) shaped by EPD from an aqueous suspension of nanosized fumed silica(OX50)and silica glass tube after sintering complex components can be shaped fast and with out- particles occurs during shaping because this can result standing quality in inhomogeneous sintering behaviour and distortion of the component. Therefore, EPD seems to be a pror ing shaping technique for such powder mixtures, be- 3.1. Near shape manufacturing cause deposition rate is independent from particle size of complex-shaped glasses and (cp Fig. 2). The process was successfully used for shap. ceramics by EPD of powder mixtures ing of silica glass components[27] and within the scope Due to the high surface area of nano-particles the solids of this work adapted to ceramic materials. As shown in content of suspensions is limited(about 60 wt% in case Fig. 2 nanosized zirconia can be deposited very fast by of OX50) and so is the green density (up to 50%TD) EPD. But again only low green densities(<35%TD) [25]. The low green density results in a high shrink- can be reached due to the high surface area of the nano- age during drying and sintering of about 15 to 30% particles. This low green density results in a very high (OX50). Thus sophisticated process control is neces- shrinkage during sintering(M30%)and leads to the fo sary for larger and more complex-shaped components. mation of micro cracks. Thus the maximum densit This makes near-shape manufacturing very compli- reached is app. 96%TD at 1400oC [23]. By combin- cated. One possible solution is to increase green density ing nanosized powders with micrometer powders green significantly. This can be achieved by combining pow- density could be increased significantly and shrinkage ders with distinctly different particle size distribution, could be minimized(<12%). No particle separation like mixtures of nanosized and microsized particles. It is due to the different sizes was observed within the de most important that no size dependent separation of the posit. Fig. 5 shows a porous polymer mould that was mm 2 40 mm Figure 5 Porous polymer mould and sintered zirconia component shaped by EPD(membrane method) from an aqueous suspension of

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS Figure 4 Silica tubes (green state) shaped by EPD from an aqueous suspension of nanosized fumed silica (OX50) and silica glass tube after sintering (1320◦C). complex components can be shaped fast and with out￾standing quality. 3.1. Near shape manufacturing of complex-shaped glasses and ceramics by EPD of powder mixtures Due to the high surface area of nano-particles the solids content of suspensions is limited (about 60 wt% in case of OX50) and so is the green density (up to 50%TD) [25]. The low green density results in a high shrink￾age during drying and sintering of about 15 to 30% (OX50). Thus sophisticated process control is neces￾sary for larger and more complex-shaped components. This makes near-shape manufacturing very compli￾cated. One possible solution is to increase green density significantly. This can be achieved by combining pow￾ders with distinctly different particle size distribution, like mixtures of nanosized and microsized particles. It is most important that no size dependent separation of the Figure 5 Porous polymer mould and sintered zirconia component shaped by EPD (membrane method) from an aqueous suspension of nano- and microsized zirconia. particles occurs during shaping because this can result in inhomogeneous sintering behaviour and distortion of the component. Therefore, EPD seems to be a promis￾ing shaping technique for such powder mixtures, be￾cause deposition rate is independent from particle size (cp. Fig. 2). The process was successfully used for shap￾ing of silica glass components [27] and within the scope of this work adapted to ceramic materials. As shown in Fig. 2 nanosized zirconia can be deposited very fast by EPD. But again only low green densities (<35%TD) can be reached due to the high surface area of the nano￾particles. This low green density results in a very high shrinkage during sintering (≈30%) and leads to the for￾mation of micro cracks. Thus the maximum density reached is app. 96%TD at 1400◦C [23]. By combin￾ing nanosized powders with micrometer powders green density could be increased significantly and shrinkage could be minimized (<12%). No particle separation due to the different sizes was observed within the de￾posit. Fig. 5 shows a porous polymer mould that was 807

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS used as ion-permeable deposition surface for the EPd by the lower viscosity. Nevertheless, a higher density (left-hand side)and a sintered zirconia green body, elec- was found for the sample deposited at pH 7 after sin- trophoretically deposited from an aqueous suspension tering. First of all, this shows that co-deposition of Sic of a mixture of nanosized (ZrO2-VP)and microsized and the sintering additives B4 C and carbon black from zirconia. First of all, it can be seen, that complex-shaped an aqueous suspension is possible at both pH values be suspensions and as a result complex glass or ceramic additives were deposited. Furthermore, the distribution components can be manufactured. Furthermore, only of carbon black seems to be more homogeneous for the low shrinkage (10% linear) was observed because of deposition at pH 7, which results in a better sintering the bimodal powder mixture use behaviour. This corroborates the theory that a collective motion of carbide particles, coated with nanosized car bon black particles, occurs, if an electric field is applied 3.2. Electrophoretic deposition of silicon for EPD. A quantitative analysis of the distribution of carbide from aqueous suspensIons B4C and carbon black within the green bodies has yet Another application is the shaping of silicon carbide to be made bulk components and coatings by EPD from aqueous Fig. 7 shows one further application of EPD of sili- suspensions. As described above deposition was con carbide from aqueous suspensions apart from bulk ried out at pH Il, where SiC as well as the sintering components. A protective Sic coating was deposited additives, B4C and carbon black, had the same sign onto the surface of a CFC substrate to fulfil oxidation of the s-potential, and at pH 7 where the carbides protection at elevated temperatures. SiC was chosen show a negative s-potential while the carbon black has because of its thermal expansion coefficient, which fits a positive surface charge(see Fig. 6, left-hand side). comparably well to CFC and lies between the thermal The deposition rate for pH 7 is shown in Fig. 2. For expansion coefficient of carbon fibres parallel and per- pH Il a slightly higher deposition rate was observed, pendicular to the fibre axis. A good adhesion of the because of the lower viscosity. On the right-hand side SiC coating to the substrate was found after sintering of Fig. 6 green and sintered density of SiC green bodies at 19500C. The coating showed no cracks and had a are shown, electrophoretically deposited at pH 1l and thickness of app. 60 um. To achieve better oxidation pH7, respectively. A significantly higher green density protection first experiments were carried out to apply was observed for pH ll. Again, this can be explained a graded coating system with gradual change of 8≥0 Figure 6 5-potential of SiC, B. C and carbon black as function of pH value(left-hand side)and density of electrophoretically deposited (ph 7. pHIl) SiC samples before and after sintering (right-hand side 50 um 10m igure 7 CFC substrate with protective silicon carbide coating applied by electrophoretic co-deposition of SiC, B. C and carbon black from an aqueous suspension(SEM, fracture surface)after sintering

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS used as ion-permeable deposition surface for the EPD (left-hand side) and a sintered zirconia green body, elec￾trophoretically deposited from an aqueous suspension of a mixture of nanosized (ZrO2-VP) and microsized zirconia. First of all, it can be seen, that complex-shaped porous membranes can be used for EPD from aqueous suspensions and as a result complex glass or ceramic components can be manufactured. Furthermore, only low shrinkage (10% linear) was observed because of the bimodal powder mixture used. 3.2. Electrophoretic deposition of silicon carbide from aqueous suspensions Another application is the shaping of silicon carbide bulk components and coatings by EPD from aqueous suspensions. As described above deposition was car￾ried out at pH 11, where SiC as well as the sintering additives, B4C and carbon black, had the same sign of the ζ -potential, and at pH 7 where the carbides show a negative ζ -potential while the carbon black has a positive surface charge (see Fig. 6, left-hand side). The deposition rate for pH 7 is shown in Fig. 2. For pH 11 a slightly higher deposition rate was observed, because of the lower viscosity. On the right-hand side of Fig. 6 green and sintered density of SiC green bodies are shown, electrophoretically deposited at pH 11 and pH 7, respectively. A significantly higher green density was observed for pH 11. Again, this can be explained Figure 6 ζ -potential of SiC, B4C and carbon black as function of pH value (left-hand side) and density of electrophoretically deposited (ph 7, pH 11) SiC samples before and after sintering (right-hand side). Figure 7 CFC substrate with protective silicon carbide coating applied by electrophoretic co-deposition of SiC, B4C and carbon black from an aqueous suspension (SEM, fracture surface) after sintering. by the lower viscosity. Nevertheless, a higher density was found for the sample deposited at pH 7 after sin￾tering. First of all, this shows that co-deposition of SiC and the sintering additives B4C and carbon black from an aqueous suspension is possible at both pH values be￾cause no sintering could have occurred if no sintering additives were deposited. Furthermore, the distribution of carbon black seems to be more homogeneous for the deposition at pH 7, which results in a better sintering behaviour. This corroborates the theory that a collective motion of carbide particles, coated with nanosized car￾bon black particles, occurs, if an electric field is applied for EPD. A quantitative analysis of the distribution of B4C and carbon black within the green bodies has yet to be made. Fig. 7 shows one further application of EPD of sili￾con carbide from aqueous suspensions apart from bulk components. A protective SiC coating was deposited onto the surface of a CFC substrate to fulfil oxidation protection at elevated temperatures. SiC was chosen because of its thermal expansion coefficient, which fits comparably well to CFC and lies between the thermal expansion coefficient of carbon fibres parallel and per￾pendicular to the fibre axis. A good adhesion of the SiC coating to the substrate was found after sintering at 1950◦C. The coating showed no cracks and had a thickness of app. 60 µm. To achieve better oxidation protection first experiments were carried out to apply a graded coating system with gradual change of 808

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS the chemical composition from SiC at the substrate SEM images with a constant magnification, was den- interface to mullite on top. sified with nanosized particles. A relative densification of 100% corresponds to a density of the silica green body of 86%TD 3.3. Modification of porous green bodies by As can be seen the EPi process is sensitive to small electrophoretic impregnation variations of the process parameters but can be con Fig. 8 shows the fracture surface of a silica green body trolled reproducibly. The highest relative densification (SEM) after electrophoretic impregnation with nano- at the suspension surface was found for an electric field zed fumed silica particles (OX50). The surface on strength of3. 0 V/cm, whereas for 1.5 and 6.0 V/cm sig the right-hand side was in contact with the suspen- nificantly lower densification were observed. A similar sion during EPI (suspension surface). As can be seen result was found for the depth of impregnation, which a graded densification was achieved with an impregna- is much higher for 3.0 V/cm(28 mm)compared to 1.5 tion depth of about 1 mm. Prior to EPI the silica green and 6.0 V/cm(I and 4 mm, respectively). On the right- dy showed a narrow monomodal pore size distribu- hand side of Fig. 9 the three corresponding sintered sil- tion with a mean pore radius of 1. 8 um, investigated by ica glass samples are shown. Sintering temperature was mercury porosimetry. The density of the green bodies 1525.C. At this temperature the electrophoretically im- was determined by Archimedes method to be 74%TD. pregnated parts are already transparent whereas the not SEM pictures of not impregnated silica green bodies densified parts are white because full density was not were taken and the porosity was determined by image reached there. This can be explained by the higher den- analysis as reference. After EPI the pore channels at and sity of the impregnated parts and the higher sinterin near the suspension surface were completely filled with activity of the nanosized silica particles incorporated nanosized particles and the density was increased from by EPI 74 to 86%TD. As a consequence thereof the mean pore Finally, not only a densification can be achieved by size was diminished from 1. 8 um to 30 nm. With in- means of EPI but also graded composites can be man- creasing distance from the impregnation surface a grad- factured. In this case a second material is deposited ual decrease in densification and increase in pore size within the pore channels of a green body. An example could be found(see Fig 8, lower left-hand corner). At is shown in Fig. 10, where a silica green body was elec- a distance of 0. 8 mm from impregnation surface no trophoretically impregnated with nanosized alumina nanosized particles could be found particles The influence of process parameters on depth and de- The alumina particles act as crystallization nuclei gree of densification was investigated by image analy- during sintering and stimulate the formation of cristo- sis on basis of SEM pictures as described above. Fig 9 balite. After sintering(1600oC)the polished cross sec- shows the relative densification of silica green bodies as tion of the sample was characterized by means of function of distance from the suspension surface for dif- Raman-microscopy. A Raman spectrum was measured ferent appliedelectric field strengths. A relative densifi- every 300 um from surface towards the bulk of the cation of e. g, 70%means that 70% of the open porosity composite. At the surface(Pl) the characteristic spec- of the not impregnated silica green body, visible on the trum for cristobalite was found, whereas at a distance 0 um g!100m 10m一 Figure Silica green body with graded density and pore size distribution(SEM, fracture surface)after electrophoretic impregnation(surface on the right-hand side in contact with suspension) with nanosized silica particles(OX50

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS the chemical composition from SiC at the substrate interface to mullite on top. 3.3. Modification of porous green bodies by electrophoretic impregnation Fig. 8 shows the fracture surface of a silica green body (SEM) after electrophoretic impregnation with nano￾sized fumed silica particles (OX50). The surface on the right-hand side was in contact with the suspen￾sion during EPI (suspension surface). As can be seen a graded densification was achieved with an impregna￾tion depth of about 1 mm. Prior to EPI the silica green body showed a narrow monomodal pore size distribu￾tion with a mean pore radius of 1.8 µm, investigated by mercury porosimetry. The density of the green bodies was determined by Archimedes method to be 74%TD. SEM pictures of not impregnated silica green bodies were taken and the porosity was determined by image analysis as reference. After EPI the pore channels at and near the suspension surface were completely filled with nanosized particles and the density was increased from 74 to 86%TD. As a consequence thereof the mean pore size was diminished from 1.8 µm to 30 nm. With in￾creasing distance from the impregnation surface a grad￾ual decrease in densification and increase in pore size could be found (see Fig. 8, lower left-hand corner). At a distance of 0.8 mm from impregnation surface no nanosized particles could be found. The influence of process parameters on depth and de￾gree of densification was investigated by image analy￾sis on basis of SEM pictures as described above. Fig. 9 shows the relative densification of silica green bodies as function of distance from the suspension surface for dif￾ferent applied electric field strengths. A relative densifi- cation of e.g., 70% means that 70% of the open porosity of the not impregnated silica green body, visible on the Figure 8 Silica green body with graded density and pore size distribution (SEM, fracture surface) after electrophoretic impregnation (surface on the right-hand side in contact with suspension) with nanosized silica particles (OX50). SEM images with a constant magnification, was den￾sified with nanosized particles. A relative densification of 100% corresponds to a density of the silica green body of 86%TD. As can be seen the EPI process is sensitive to small variations of the process parameters but can be con￾trolled reproducibly. The highest relative densification at the suspension surface was found for an electric field strength of 3.0 V/cm, whereas for 1.5 and 6.0 V/cm sig￾nificantly lower densifications were observed. A similar result was found for the depth of impregnation, which is much higher for 3.0 V/cm (≈8 mm) compared to 1.5 and 6.0 V/cm (1 and 4 mm, respectively). On the right￾hand side of Fig. 9 the three corresponding sintered sil￾ica glass samples are shown. Sintering temperature was 1525◦C. At this temperature the electrophoretically im￾pregnated parts are already transparent whereas the not densified parts are white because full density was not reached there. This can be explained by the higher den￾sity of the impregnated parts and the higher sintering activity of the nanosized silica particles incorporated by EPI. Finally, not only a densification can be achieved by means of EPI but also graded composites can be man￾ufactured. In this case a second material is deposited within the pore channels of a green body. An example is shown in Fig. 10, where a silica green body was elec￾trophoretically impregnated with nanosized alumina particles. The alumina particles act as crystallization nuclei during sintering and stimulate the formation of cristo￾balite. After sintering (1600◦C) the polished cross sec￾tion of the sample was characterized by means of Raman-microscopy. A Raman spectrum was measured every 300 µm from surface towards the bulk of the composite. At the surface (P1) the characteristic spec￾trum for cristobalite was found, whereas at a distance 809

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS 100 after sintering (1525oC) 1 cm 合E=1.5Vcm 一E=3.0Vcr 10 0 300 600900120015001800 distance from impregnation surface(um) Figure 9 Densification of silica green bodies as function of distance from impregnation surface for different electric field strength during EPI and silica glass samples with graded density after sintering(vacuum, 1525.C) 0,18 0,15 ≥0,12 0,09 silica glass P5 0,03 wave number(cm) 10 Silica glass/cristobalite composite with gradual decrease of cristobalite content from surface to bulk: Position sensitive characterization of 3 mm from the surface(P5) only the silica glass and subsequently sintered to fully dense and transparent spectrum was found At points P2 to P4 a superposi- silica glass tubes tion of both spectra was found with decreasing inten- Using three-dimensional shaped porous polymer sity of the cristobalite peaks with increasing distance moulds as ion-permeable deposition surface complex from surface. This shows that a graded impregnation ceramic and glass components can be produced. But was achieved, although this measurement was only a due to the high specific surface area the density of green semI-quantitative one bodies shaped of nano-particles is comparably low and high shrinkage(up to 30%)occurs during drying and sintering. To enable near-shape manufacturing shrink 4. Conclusions age had to be reduced. This was achieved by EPD of Electrophoretic deposition from aqueous suspensions mixtures of nanosized and microsized particles by the membrane method is a fast, low-cost and flexible Furthermore, Sic was electrophoretically co- shaping technique for glasses and ceramics. High depo- deposited with B4C and carbon black, as sintering ad- sition rates are reached that allow shaping of large com- ditives. Co-deposition was successfully carried out at ponents with thick walls within several seconds to min- pHIl, where all particles have a s-potential of the same utes even for nano-particles. Thus a silica green body sign, and at a pH of 7, where the carbides show a nega- with a thickness of 12 mm was deposited within 3 min tive surface charge whereas the carbon black particles (E=10 V/cm). Silica tubes of different diameter and have a positive s-potential. In both cases solid state sin- wall thickness were shaped of nanosized fumed silica tering was observed, with a better sintering behaviour 810

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS Figure 9 Densification of silica green bodies as function of distance from impregnation surface for different electric field strength during EPI and silica glass samples with graded density after sintering (vacuum, 1525◦C). Figure 10 Silica glass/cristobalite composite with gradual decrease of cristobalite content from surface to bulk: Position sensitive characterization by Raman microscopy. of 3 mm from the surface (P5) only the silica glass spectrum was found. At points P2 to P4 a superposi￾tion of both spectra was found with decreasing inten￾sity of the cristobalite peaks with increasing distance from surface. This shows that a graded impregnation was achieved, although this measurement was only a semi-quantitative one. 4. Conclusions Electrophoretic deposition from aqueous suspensions by the membrane method is a fast, low-cost and flexible shaping technique for glasses and ceramics. High depo￾sition rates are reached that allow shaping of large com￾ponents with thick walls within several seconds to min￾utes even for nano-particles. Thus a silica green body with a thickness of 12 mm was deposited within 3 min (E = 10 V/cm). Silica tubes of different diameter and wall thickness were shaped of nanosized fumed silica and subsequently sintered to fully dense and transparent silica glass tubes. Using three-dimensional shaped porous polymer moulds as ion-permeable deposition surface complex ceramic and glass components can be produced. But due to the high specific surface area the density of green bodies shaped of nano-particles is comparably low and high shrinkage (up to 30%) occurs during drying and sintering. To enable near-shape manufacturing shrink￾age had to be reduced. This was achieved by EPD of mixtures of nanosized and microsized particles. Furthermore, SiC was electrophoretically co￾deposited with B4C and carbon black, as sintering ad￾ditives. Co-deposition was successfully carried out at pH 11, where all particles have a ζ -potential of the same sign, and at a pH of 7, where the carbides show a nega￾tive surface charge whereas the carbon black particles have a positive ζ -potential. In both cases solid state sin￾tering was observed, with a better sintering behaviour 810

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS means of EPD on top of CFC substrates Ceram. Trans. 51(1995) A modification of porous green bodies could be 17. 1. HECTOR and R. CLASEN, Ceram. Eng. Sci. Proc. 18(1997) achieved by electrophoretic impregnation, where nand 18 M. S. CHRONBERG and F. HANDLE, Interceram 27(1978) sized particles were deposited within the pore channels 1 of meso-porous green bodies. Thus, a homogeneous or 19. H. V. BOTH andJ. HAUBELT, Electrochem. Soc. Proc. 2002- graded densification can be achieved and composites 21(2002) with graded composition can be produced. The gradi 20.A.V. KERKAR.R. W. RICE andR. M. SPOTNItz. US ent can be tailored by adjusting the process parameters. 21. R. CLASEN. in Proceedings of the 2nd Int. Conf on Powder Processing Science. Berchtesgaden. 12-14. 10. 1988. edited by H. Hausner. G. L. Messing and S. Hirano(Deutsche Keramische Acknowledgement Gesellschaft, Koln, 1988)P 633 The authors gratefully acknowledge the financial sup- 22. K. MORITZ, R. THAUER andE. MULLER, cfi/Ber DKG 77 (2000)8 port of the German Science Foundation(DFG) 23. 1. M. AKIMOVICH, Inorg. Mater(Russia)12(1971)7 24. R. Clasen. in "Science, Technology and Applications of Colloidal Suspensions"(Amer. Ceram Soc., USA, 1995)P 169. 25. J. TABELLION and R. CLASEN. in"Innovative Processing and erences Synthesis of Ceramics, Glasses and Composites IV"(Amer. Ceram. 1.H. C. HAMAKER and e. J. W. VERWEY. Trans. Farad Soc.36(1940 26. J. TABELLION and R. CLASEN. in Proceedings of the 26th 2. M. BENJAMIN and A. B. OSBORN, ibid. 36(1940) 3. H. HOFFMANN. Ceram. Bull. 57(1978)6 Annual Conference on Composites, Advanced Ceramics, Materials and Structures, Cocoa Beach, Florida, USA, 2002, edited by H -T. ANE, J. B. TALBOT, B. G. KINNEY, E. SLUZKY and K. R. HESSE, J. Coll. Interf. Sci. 165(1994). 27. J. TABELLION and R. CLASEN, to be published in Proceed- ngs of the 27th Annual Conference on Composites, Advanced Ce- (2000)12 ramics, Materials tructures. Cocoa Beach. USA. 2003 6. D. MATTHEWS, A. KAY and M. GRATZEL, Aust. J. Chem. 28. S. HABER, J. Coll. Inter. Sci. 179(1996)2. 47(1994). 29. J. TABELLION, C. OETZEL andR. CLASEN, Electrochem. 7. T. NARISAWA, T. ARATO, N. KOGANEZAWA, M SHIBATA and Y. NONAKA, J. Ceram. Soc. Jpn. 103(1995) Soc.Pmoc.2002-21(2002) 30. A.R. BOCCACCINI, C. KAYA and H -G. KRUGER Chem.Ing. Tech. 73(2001)5 8.G.J.VERHOECKX and N. J. M. V. LETH, Electrochem. 31. K. MORITZ and E. MULLER, Key Engin. Mater: 206-213 Soc.Pmoc.,2002-21(2002) (2002 9. C -Y. CHEN, S.-Y. CHEN and D -M. LIU, Acta Mater. 47 32. K. SMEETS,J. TABELLION and R. CLASEN, ibid. 206- 213(2002) 10. F. HARBACH and H. NIENBURG, J Etrop. Cera. Soc. 18 33. K. SMEETS, J. TABELLION and R. CLASEN, ibid. 206- (1998) 213(2002) 11. F. BOUYER andA. FoISSY, J. Amer. Ceram. Soc. 82(1999)8 2. L. vANDEPERRE. O. V. D. BIEST. F. BOUYE ABELLION and R. CLaSEN, Innovative Processing and Synthesis of Ceramics, Glasses and Composites IV(Amer. Ceram. J. PERSELLO and A. FOISSY, J Europ Ceram Soc. 17(1997) Soc…UsA,2000p.19 3. J. LAUBERSHEIMER, H.-J. RITZHAUPT-KLEISSL J. HAUBELT and G. EMIG. ibid. 18(1998) 14. P. SARKAR and P. s. NICholSON /. Amer. Ceram Soc. 79 Received 5 May and accepted 17 August 2003

ELECTROPHORETIC DEPOSITION: FUNDAMENTALS AND APPLICATIONS for the green bodies deposited at pH 7. Apart from bulk ceramics, protective SiC coatings were applied by means of EPD on top of CFC substrates. A modification of porous green bodies could be achieved by electrophoretic impregnation, where nano￾sized particles were deposited within the pore channels of meso-porous green bodies. Thus, a homogeneous or graded densification can be achieved and composites with graded composition can be produced. The gradi￾ent can be tailored by adjusting the process parameters. Acknowledgement The authors gratefully acknowledge the financial sup￾port of the German Science Foundation (DFG). References 1. H. C. HAMAKER and E. J. W. VERWEY, Trans. Faraday Soc. 36 (1940). 2. M. BENJAMIN and A. B. OSBORN, ibid. 36 (1940). 3. H. HOFFMANN, Ceram. Bull. 57 (1978) 6. 4. M. J. SHANE, J. B. TALBOT, B. G. KINNEY, E. SLUZKY and K. R. HESSE, J. Coll. Interf. Sci. 165 (1994). 5. I. ZHITOMIRSKY and A. PETRIC, J. Europ. Ceram. Soc. 20 (2000) 12. 6. D. MATTHEWS , A. KAY and M. G RA¨ TZEL, Aust. J. Chem. 47 (1994). 7. T. NARISAWA, T. ARATO, N. KOGANEZAWA, M. SHIBATA and Y. NONAKA, J. Ceram. Soc. Jpn. 103 (1995) 1. 8. G. J. VERHOECKX and N. J. M. V. LETH, Electrochem. Soc. Proc., 2002–21 (2002). 9. C.-Y. CHEN, S .-Y. CHEN and D.-M. LIU, Acta Mater. 47 (1999) 9. 10. F . HARBACH and H. NIENBURG, J. Europ. Ceram. Soc. 18 (1998). 11. F . BOUYER and A. FOISSY, J. Amer. Ceram. Soc. 82 (1999) 8. 12. L. VANDEPERRE, O. V. D. BIEST, F . BOUYER, J. PERSELLO and A. FOISSY, J. Europ. Ceram Soc. 17 (1997). 13. J. LAUBERSHEIMER, H.-J. RITZHAUPT-KLEISSL, J. HAUßELT and G. EMIG, ibid. 18 (1998). 14. P . SARKAR and P . S . NICHOLSON, J. Amer. Ceram. Soc. 79 (1996) 8. 15. M. S . J. GANI, Indust. Ceram. 14 (1994) 4. 16. R. CLASEN, S . JANES , C. OSWALD and D. RANKER, Ceram. Trans. 51 (1995). 17. I. HECTOR and R. CLASEN, Ceram. Eng. Sci. Proc. 18 (1997) 2. 18. M. S . CHRONBERG and F . HA¨ NDLE, Interceram 27 (1978) 1. 19. H. V. BOTH and J. HAUßELT, Electrochem. Soc. Proc. 2002– 21 (2002). 20. A. V. KERKAR, R. W. RICE and R. M. SPOTNITZ, US Patient no. 5,194,129 (1993). 21. R. CLASEN, in Proceedings of the 2nd Int. Conf. on Powder Processing Science, Berchtesgaden, 12–14. 10. 1988, edited by H. Hausner, G. L. Messing and S. Hirano (Deutsche Keramische Gesellschaft, K¨oln,1988) p. 633. 22. K. MORITZ, R. THAUER and E. MU¨ LLER, cfi/Ber. DKG 77 (2000) 8. 23. I. M. AKIMOVICH, Inorg. Mater. (Russia) 12 (1971) 7. 24. R. Clasen, in “Science, Technology and Applications of Colloidal Suspensions” (Amer. Ceram. Soc., USA, 1995) p. 169. 25. J. TABELLION and R. CLASEN, in “Innovative Processing and Synthesis of Ceramics, Glasses and Composites IV” (Amer. Ceram. Soc., USA, 2000) p. 185. 26. J. TABELLION and R. CLASEN, in Proceedings of the 26th Annual Conference on Composites, Advanced Ceramics, Materials and Structures, Cocoa Beach, Florida, USA, 2002, edited by H.-T. Lin and M. Singh (Amer. Ceram. Soc., USA, 2002) p. 617. 27. J. TABELLION and R. CLASEN, to be published in Proceed￾ings of the 27th Annual Conference on Composites, Advanced Ce￾ramics, Materials and Structures, Cocoa Beach, USA, 2003. 28. S . HABER, J. Coll. Inter. Sci. 179 (1996) 2. 29. J. TABELLION, C. OETZEL and R. CLASEN, Electrochem. Soc. Proc. 2002–21 (2002). 30. A. R. BOCCACCINI, C. KAYA and H.-G. K RU¨ GER, Chem.-Ing. Tech. 73 (2001) 5. 31. K. MORITZ and E. MUL L E R ¨ , Key Engin. Mater. 206–213 (2002). 32. K. SMEETS , J. TABELLION and R. CLASEN, ibid. 206– 213 (2002). 33. K. SMEETS , J. TABELLION and R. CLASEN, ibid. 206– 213 (2002). 34. J. TABELLION and R. CLASEN, Innovative Processing and Synthesis of Ceramics, Glasses and Composites IV (Amer. Ceram. Soc., USA, 2000) p. 197. Received 5 May and accepted 17 August 2003 811

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