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(OX50ELECTROPHORETIC 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