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(atELECTROPHORETIC 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