L Zhang et al. / Journal of the European Ceramic Society 30(2010)1195-1202 1197 ducted for 450s. The 50 ml vertical EPD cell, presented in Fig 3, 3. Results consisted of two vertically positioned electrodes with a separa- tion distance of 3.5 cm and a surface area of 9 cm. The edges 3.1. Vertical deposition from a stirred suspension of the deposition electrode were shielded by a non-conductive PTFE cover. Two configurations were studied in the vertical cell, The influence of the platelet template can be assessed by i.e., with a stagnant( configuration 5)and a magnetically stirred comparison with the random alumina powder based ceramic 250rpm)(configuration 1)suspension. The texture of the platelets containing material was confirmed Constant voltage anodic electrophoretic deposition was per- by XRD, as shown in Fig. 4. The diffraction spectrum of the formed with freshly prepared suspensions, using a F.U. G(type random ceramic(Fig 4(a)) is consistent with the JCPDS card MCN 1400-50)power supply. The pH* of the suspension before of alumina(card number 43-1484 ), which means that no signif EPD was 11.45 pH* denotes the operational pH for which a icant texture was formed in the sample. The(1 10)and (300) standard ph electrode was used to measure the pH in ethanol diffraction peak intensities are pronounced in the spectrum of suspensions. 2 The conductivity of the suspension at room tem- the sample, cross-sectioned perpendicular to the depositionelec- perature was 24.5 uS/cm, as measured by a conductivity sensor trode(perpendicular section, see Fig. 4(b)). Those peaks are Cond Level 2 type, WTW). After EPD, the deposit was care- stronger than in the random sample as well as in the sample fully removed from the suspension and dried in air. Afterwards, cross-sectioned parallel to the electrode(parallel section, see the deposit was sintered in air(Nabertherm furnace, Germany) Fig 4(c). The(006)and(00 12)peaks are hardly observed in at 1550C for I h with a heating rate of 10C/min. The green the perpendicular section and the random sample, whereas they and sintered density was measured in ethanol by the Archimedes are prominent in the parallel section. The(104)and(1010) method peaks are also stronger in the parallel section than in the random The microstructure of the polished and thermally etched sample and the perpendicular section. These results imply that surface of sintered samples was investigated by scanning elec- a large volume fraction of strong(00 1)textured alumina grains tron microscopy (SEM, XL30-FEG, FEL, Netherlands). Texture are formed in the platelet containing material. The c-axis is ori analysis was performed by X-ray diffraction(type Seifert ented perpendicular to the surface of the deposition electrode 3003), pole figure measurements (Siemens D500 Texture Based on the XRD spectrum, the Lotgering factor is calculated Stress) and Electron Back-Scatter Diffraction(EBSD, EDAX, to be 0.49, which implies a well-textured material. Netherlands) The microstructural anisotropy is confirmed by the SEM The Lotgering factor is widely used in literature to charac crographs shown in Fig. 5. SEM analysis at different loca- terize the texture degree of hexagonal a-alumina, 3, 4 and can tions in the sintered deposit revealed that the platelet particles be obtained from the X-ray diffraction pattern of a sample. The are nearly homogeneously distributed throughout the deposit Lotgering factor is defined as: The seeded platelets acted as templates for the grain growth C∑100/∑/(hA)-(∑P(0O∑P(hkD) during sintering. The basal planes of the grown platelets have f 1-∑(00/∑P(hkD) (1) been aligned parallel to the surface of the deposition electrode During sintering, the aligned platelet seeds grew very fast by with El(00D, the summation of all (00D peak intensities and means of coarsening, i.e by the consumption of the fine matrix 2I(hkl), the summation of all peak intensities in the spectrum alumina particles, resulting in a highly textured ceramic. Grain Superscript 0 corresponds to a random sample. Thef factor growth has been fast by means of grain boundary migration since changes between 0 and 1. A large f value implies a highly tex here is a substantial amount of pores trapped inside the grains tured material, since f=0 for a random sample and f=l for a Intergranular pores are also clearly observed in Fig. 5. The rel fully oriented material. ative density of this ceramic is 97. 9%0. Beside the intergranular In order to give a complete estimation of texture formation. the texture index was calculated depending on the orientation distribution function(ODF). The OdF was obtained from mea- sured pole figures by"Hexagonal ODF software system"(Dept MTM-K.U. Leuven). The texture index was used as an indica- tor of the sharpness of the texture, and is defined as the integral of the square of the ODF,f(g), over the entire orientation space L=∮Uf(g)2dg 010 The higher the value, the sharper the texture. A value close to 1 (c) (116) implies random texture. EbSd was used to examine the microstructure and 20304050607080 the regional gra 2 Theta 180 x 520 um- and the step size was 1 um. The grain orientation Fig. 4. X-ray diffraction pattern of sintered (a)a random alumina sample and maps and inverse pole figures were all generated from the experi- cross-sectioned template containing ceramic(b)perpendicular and(c)parallel mental data using commercial software (TSL OIM analysis 4.5). to the electrode surfaceL. Zhang et al. / Journal of the European Ceramic Society 30 (2010) 1195–1202 1197 ducted for 450 s. The 50 ml vertical EPD cell, presented in Fig. 3, consisted of two vertically positioned electrodes with a separa￾tion distance of 3.5 cm and a surface area of 9 cm2. The edges of the deposition electrode were shielded by a non-conductive PTFE cover. Two configurations were studied in the vertical cell, i.e., with a stagnant (configuration 5) and a magnetically stirred (∼250 rpm) (configuration 1) suspension. Constant voltage anodic electrophoretic deposition was per￾formed with freshly prepared suspensions, using a F.U.G. (type MCN 1400-50) power supply. The pH* of the suspension before EPD was 11.45. pH* denotes the operational pH for which a standard pH electrode was used to measure the pH in ethanol suspensions.12 The conductivity of the suspension at room tem￾perature was 24.5S/cm, as measured by a conductivity sensor (Cond Level 2 type, WTW). After EPD, the deposit was care￾fully removed from the suspension and dried in air. Afterwards, the deposit was sintered in air (Nabertherm furnace, Germany) at 1550 ◦C for 1 h with a heating rate of 10 ◦C/min. The green and sintered density was measured in ethanol by the Archimedes method. The microstructure of the polished and thermally etched surface of sintered samples was investigated by scanning elec￾tron microscopy (SEM, XL30-FEG, FEI, Netherlands). Texture analysis was performed by X-ray diffraction (type Seifert 3003), pole figure measurements (Siemens D500 Texture & Stress) and Electron Back-Scatter Diffraction (EBSD, EDAX, Netherlands). The Lotgering factor is widely used in literature to charac￾terize the texture degree of hexagonal -alumina,13,14 and can be obtained from the X-ray diffraction pattern of a sample. The Lotgering factor is defined as: f = I(00l)/ I(hkl)  − I0(00l)/ I0(hkl)  1 − I0(00l)/ I0(hkl)  (1) with ΣI(0 0 l), the summation of all (0 0 l) peak intensities and ΣI(hkl), the summation of all peak intensities in the spectrum. Superscript 0 corresponds to a random sample. The f factor changes between 0 and 1. A large f value implies a highly tex￾tured material, since f = 0 for a random sample and f = 1 for a fully oriented material. In order to give a complete estimation of texture formation, the texture index was calculated depending on the orientation distribution function (ODF). The ODF was obtained from mea￾sured pole figures by “Hexagonal ODF software system” (Dept MTM—K.U.Leuven). The texture index was used as an indica￾tor of the sharpness of the texture, and is defined as the integral of the square of the ODF, f(g), over the entire orientation space15: T.I. =  [f (g)]2dg (2) The higher the value, the sharper the texture. A value close to 1 implies random texture. EBSD was used to examine the microstructure and the regional grain orientation. The examined area was 180 × 520m2 and the step size was 1 m. The grain orientation maps and inverse pole figures were all generated from the experi￾mental data using commercial software (TSL OIM analysis 4.5). 3. Results 3.1. Vertical deposition from a stirred suspension The influence of the platelet template can be assessed by comparison with the random alumina powder based ceramic. The texture of the platelets containing material was confirmed by XRD, as shown in Fig. 4. The diffraction spectrum of the random ceramic (Fig. 4(a)) is consistent with the JCPDS card of alumina (card number 43-1484), which means that no signif￾icant texture was formed in the sample. The (1 1 0) and (3 0 0) diffraction peak intensities are pronounced in the spectrum of the sample, cross-sectioned perpendicular to the deposition elec￾trode (perpendicular section, see Fig. 4(b)). Those peaks are stronger than in the random sample as well as in the sample cross-sectioned parallel to the electrode (parallel section, see Fig. 4(c)). The (0 0 6) and (0 0 12) peaks are hardly observed in the perpendicular section and the random sample, whereas they are prominent in the parallel section. The (1 0 4) and (1 0 10) peaks are also stronger in the parallel section than in the random sample and the perpendicular section. These results imply that a large volume fraction of strong (0 0 1) textured alumina grains are formed in the platelet containing material. The c-axis is ori￾ented perpendicular to the surface of the deposition electrode. Based on the XRD spectrum, the Lotgering factor is calculated to be 0.49, which implies a well-textured material. The microstructural anisotropy is confirmed by the SEM micrographs shown in Fig. 5. SEM analysis at different loca￾tions in the sintered deposit revealed that the platelet particles are nearly homogeneously distributed throughout the deposits. The seeded platelets acted as templates for the grain growth during sintering. The basal planes of the grown platelets have been aligned parallel to the surface of the deposition electrode. During sintering, the aligned platelet seeds grew very fast by means of coarsening, i.e., by the consumption of the fine matrix alumina particles, resulting in a highly textured ceramic. Grain growth has been fast by means of grain boundary migration since there is a substantial amount of pores trapped inside the grains. Intergranular pores are also clearly observed in Fig. 5. The rel￾ative density of this ceramic is 97.9%. Beside the intergranular Fig. 4. X-ray diffraction pattern of sintered (a) a random alumina sample and cross-sectioned template containing ceramic (b) perpendicular and (c) parallel to the electrode surface.