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 oppositely charged electrode and coagulate there to form a stable deposit. Although EPD has been applied on an industrial scale for years as enamelling and lacquering technique (cataphoresis) [2, 3], it is not yet established 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], photovoltaic 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 successful so far. Nevertheless, several ceramic systems were investigated on a laboratory scale, including alumina [9], zirconia [10], SiC [11, 12] and PZT [13]. Comprehensive reviews about EPD as shaping technique for ceramics are given in [14, 15]. In all of these cases organic 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 comparison 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, especially in case of high voltages and/or high current densities. As a consequence the process cost increases. Finally, disposal of hazardous organic waste and treatment 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 necessary 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 aqueous suspensions for EPD, is the electrolysis-induced decomposition 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 deposition 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 successful for the production of alumina microstructures [19]. But due to the low deposition rate only small wall thickness can be achieved. Another possibility is to use sacrificial 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 electrode 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 shaping 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 membranemethod [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 distribution 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 densities 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 electrophoretic impregnation (EPI), where nanosized particles are deposited within the pores or voids of a green body or fibre fabric. The principles of EPI are summarized in [28]. Investigations were made, to characterize the influence of several process parameters, like viscosity, 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 silica glass and advanced ceramics are shown. The aim of this work was to demonstrate that near-shape manufacturing 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 particles (Degussa Aerosil OX50, mean particle size 40 nm) were prepared by dispersing the particles in bidistilled water containing tetramethylammoniumhydroxide (TMAH) by means of a dissolver (LDV1, PC Laborsysteme). Vacuum was applied to avoid incorporation 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 description of the influence of pH on ζ -potential, rheological properties, suspension structure and homogeneity can be found in [33]. The suspensions had a solids content 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 suspensions 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 powder (Degussa, FW200) 13 nm. A small amount of dispersing aid (0.2 wt% correlated to the amount of carbon 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 positive 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 constant applied voltage by the membrane method. A simple 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 measuring the effective electric field strength within the electrophoresis 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 readjusted 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 experimental 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 observed for the SiC coatings for a thickness of up to 200 µm. Sintering of the compacts was carried out either 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 sintered samples were measured by Archimedes method and mercury porosimetry (Porotec Pascal P140, P440), respectively. Microstructural homogeneity was investigated on basis of SEM and high resolution SEM (HRSEM) 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 reference first. The densification of the impregnated green bodies was determined on the basis of SEM pictures with the same magnification, that were taken equidistantly (300 µm) from the surface in contact with the suspension during EPI towards the bulk. A densification 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 absolute values of the density and pore size, achieved from Archimedes method and mercury porosimetry. Furthermore, Raman microscopy was used for a positionsensitive characterization of graded composites. 3. Results and discussion The deposition rate for the electrophoretic deposition of the different powders used was determined first, because only an exact control of this parameter can guarantee a reproducible manufacturing of components with given specifications concerning wall thickness and tolerances. 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 observed, 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 dependency 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 technique, 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 multimodal 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 techniques, 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 microstructure was investigated. Fig. 3 shows the specific cumulative pore volume of the silica green bodies measured by mercury porosimetry, 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 electrophoretically deposited green body. Both, mercury porosimetry and HR-SEM pictures show that the highest 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 aqueous suspension of nanosized fumed silica (OX50). A tubular polymer membrane was used as an ionpermeable deposition surface. As can be seen (Fig. 4, right-hand side) a fully dense and fully transparent silica 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 outstanding 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 shrinkage during drying and sintering of about 15 to 30% (OX50). Thus sophisticated process control is necessary for larger and more complex-shaped components. This makes near-shape manufacturing very complicated. One possible solution is to increase green density significantly. This can be achieved by combining powders 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 promising shaping technique for such powder mixtures, because deposition rate is independent from particle size (cp. Fig. 2). The process was successfully used for shaping 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 nanoparticles. This low green density results in a very high shrinkage during sintering (≈30%) and leads to the formation of micro cracks. Thus the maximum density reached is app. 96%TD at 1400◦C [23]. By combining 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 deposit. Fig. 5 shows a porous polymer mould that was 807