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