Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Joumal of the European Ceramic Society 30(2010)1195-1202 www.elsevier.comlocate/jeurceramsoc Fabrication of textured alumina by orienting template particles during electrophoretic deposition Li Zhang, Jef vleugels, Omer Van der biest K ULeunen, Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, B-3001 Heverlee, belgium Available online 16 July 2009 orientation during EPD was examined with respect to the impact of the electric field force, gravity and hydrodynamic force in two different deposition cells with vertically or horizontally positioned deposition electrodes. A sharp(00 1)fbre texture'was obtained after templated grain growth during sintering of a deposit formed from a stirred 5 vol% platelet containing suspension in a vertical deposition cell. The texture was characterized by means of the Lotgering factor, texture index and electron backscattering diffraction(EBSD) 2009 Elsevier Ltd. All rights reserved. Keywords: Suspensions; Platelets; Al2 O3: Texture Introduction This work shows the possibility to align platelet template dur- ing electrophoretic deposition(EPD). EPD is a colloidal proces Textured materials have been investigated extensively in the wherein particles suspended in a stable suspension are deposited past few years due to their improved electronic and structural on one of the electrodes by applying an electric field. The sim- properties. If polycrystalline particles could be aligned, they plicity in experimental set-up leads to low equipment cost. EPD may be able to exhibit the anisotropic characteristics typical is also a fast compacting technique and the deposit thickness of single crystals. For instance, as an important engineer- can be easily controlled. EPD has the capability to form com- ing ceramic, textured a-alumina is widely investigated and the plex shapes and patterns. Moreover, EPD can be applied on a property of this textured a-alumina was reported to be signif- wide range of substrates such as porous ceramics, conductive icantly enhanced 24 Textured materials can be produced by a polymers or metals. 0. I variety of techniques including magnetic orientation and tem- In this study, hexagonal a-alumina platelet templates were growth(TGG). -6Templated grain growth is widely aligned in a fine matrix alumina powder by EPD. After tem- investigated. 4-6 Large and anisotropically shaped template par- plated grain growth, a highly textured material was developed ticles are homogeneously aligned in a fine matrix powder during The mechanism of platelet alignment during deposition was compacting. During sintering, textured material was obtained by investigated considering the impact of orientation forces grain growth in the anisotropic direction of the aligned template particles. The template alignment in the green compact is a crit ical factor in TGG since it determines the final texturisation. 2. Experimental procedure Templates could be aligned by various powder consolidation techniques including uniaxial pressing, slip casting, tape cast- High purity fine a-alumina powder(0.3 um, Baikowski ing, gel casting, centrifugal casting and extrusion. - In those grade SM8, France)was used as matrix powder, whilst hexag- cases, the active forces orientate the template grains during the onal alumina platelets(10-15 um in diameter, -0.5 um thick, powder consolidation process ELF Atochem, France)were used as templates. The morphology of the alumina matrix powder and platelet template is shown in milled for 24 h on a multi-directional mixer(type Turbula, WAB Corresponding author. Tel: +32 16 321777: fax: +32 16 321992 Switzerland) at 70rpm in absolute ethanol(99.9% Merck Bel E-mail address: LiZhang@mtm kuleuven. be(L. Zhang gium) with 0.5 vol% de-ionised water. Zirconia milling balls 0955-2219 front matter@ 2009 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2009.06.026
Available online at www.sciencedirect.com Journal of the European Ceramic Society 30 (2010) 1195–1202 Fabrication of textured alumina by orienting template particles during electrophoretic deposition Li Zhang ∗, Jef Vleugels, Omer Van der Biest K.U.Leuven, Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44, B-3001 Heverlee, Belgium Available online 16 July 2009 Abstract (0 0 1)-Textured -alumina has been processed by electrophoretic deposition (EPD) and templated grain growth. The mechanism of platelet template orientation during EPD was examined with respect to the impact of the electric field force, gravity and hydrodynamic force in two different deposition cells with vertically or horizontally positioned deposition electrodes. A sharp (0 0 1) ‘fibre texture’ was obtained after templated grain growth during sintering of a deposit formed from a stirred 5 vol% platelet containing suspension in a vertical deposition cell. The texture was characterized by means of the Lotgering factor, texture index and electron backscattering diffraction (EBSD). © 2009 Elsevier Ltd. All rights reserved. Keywords: Suspensions; Platelets; Al2O3; Texture 1. Introduction Textured materials have been investigated extensively in the past few years due to their improved electronic and structural properties. If polycrystalline particles could be aligned, they may be able to exhibit the anisotropic characteristics typical of single crystals.1 For instance, as an important engineering ceramic, textured -alumina is widely investigated and the property of this textured -alumina was reported to be significantly enhanced.2–4 Textured materials can be produced by a variety of techniques including magnetic orientation and templated grain growth (TGG).3–6 Templated grain growth is widely investigated.4–6 Large and anisotropically shaped template particles are homogeneously aligned in a fine matrix powder during compacting. During sintering, textured material was obtained by grain growth in the anisotropic direction of the aligned template particles. The template alignment in the green compact is a critical factor in TGG since it determines the final texturisation. Templates could be aligned by various powder consolidation techniques including uniaxial pressing, slip casting, tape casting, gel casting, centrifugal casting and extrusion.5–9 In those cases, the active forces orientate the template grains during the powder consolidation process. ∗ Corresponding author. Tel.: +32 16 321777; fax: +32 16 321992. E-mail address: Li.Zhang@mtm.kuleuven.be (L. Zhang). This work shows the possibility to align platelet template during electrophoretic deposition (EPD). EPD is a colloidal process wherein particles suspended in a stable suspension are deposited on one of the electrodes by applying an electric field. The simplicity in experimental set-up leads to low equipment cost. EPD is also a fast compacting technique and the deposit thickness can be easily controlled. EPD has the capability to form complex shapes and patterns. Moreover, EPD can be applied on a wide range of substrates such as porous ceramics, conductive polymers or metals.10,11 In this study, hexagonal -alumina platelet templates were aligned in a fine matrix alumina powder by EPD. After templated grain growth, a highly textured material was developed. The mechanism of platelet alignment during deposition was investigated considering the impact of orientation forces. 2. Experimental procedure High purity fine -alumina powder (∼0.3m, Baikowski grade SM8, France) was used as matrix powder, whilst hexagonal alumina platelets (10–15m in diameter, ∼0.5m thick, ELF Atochem, France) were used as templates. The morphology of the alumina matrix powder and platelet template is shown in Fig. 1. Fine alumina matrix powder suspensions were initially milled for 24 h on a multi-directional mixer (type Turbula, WAB, Switzerland) at 70 rpm in absolute ethanol (99.9% Merck Belgium) with 0.5 vol% de-ionised water. Zirconia milling balls 0955-2219/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2009.06.026
L Zhang et al. Joumal of the European Ceramic Society 30(2010)1195-1202 clectrode Fig. 2. EPD cell with horizontally positioned deposition electrode counter deposition electrode electrode magnetic stirrer bar To estimate the influence of the electric field force, gravity and hydrodynamic force on the alignment of the platelets, two different set-ups were used, i.e., with a vertical and horizontal EPD cell. Three configurations were studied in a slightly mod- .V Spot Magn Det WD Exp ified horizontal cell as summarised in Table 1. The standard horizontal EPD cell is schematically presented in Fig. 2. In case of an applied suspension flow(configuration 2 in Table 1),a Fig. 1. SEM image of (a) as received alumina grade SM8 and(b) platelets suspension was flown through the deposition cell by means of a suspension circulation system driven by a peristaltic pump Tosoh grade TZ-3Y) with a diameter of 5 mm were added to The distance between the flat disc shaped electrodes was 3. 5 cm the polyethylene container to facilitate breaking the agglomer- and the electrodes had a diameter of 3. 7 cm. The suspension ates. Afterwards, platelets (5 wt% relative to the total amount in the reservoir was stirred to avoid sedimentation. In addition of powder) were added to the ball-milled suspension, whilst the flow-through system also decreases the sedimentation of the n-butylamine (99.5% Acros Belgium, 3. 4 vol% relative to the suspension in the cell with the fluid flow. Homogeneous deposits suspension volume)was added to negatively charge the particles were made by pumping 200 ml suspension at 1 ml/s through the and Dolapix Ce-64(Zschimmer Schwarz, Germany, 1. I wt% deposition cell. The suspension circulation system was not used relative to the powder mass)was added as dispersant. For com- in the other two configurations (3 and 4 of Table 1) where only parison, a suspension without platelets was also prepared. This 50 ml suspension was used to fill the EPD cell. Configuration suspension was magnetically stirred for 60 min, ultrasonicated 4 and 3 form respectively depositions along and in the oppo- in an ultrasonic bath(Branson 2510)for 15 min and magnetically site direction of the gravity force, in which the perforated top stirred for another 15 min electrode( Fig. 2)was replaced with a normal one. EPD was con- Density, Lotgering factor and texture index for different EPD configurations Green density [%] Sintered density [%I ening factor index Vertical Vertical stirred 594 0.49 18.32 Horizontal Bottom Flow through Horizontal 8.12 Horizontal Bottom Stagnant Stagnant
1196 L. Zhang et al. / Journal of the European Ceramic Society 30 (2010) 1195–1202 Fig. 1. SEM image of (a) as-received alumina grade SM8 and (b) platelets. (Tosoh grade TZ-3Y) with a diameter of 5 mm were added to the polyethylene container to facilitate breaking the agglomerates. Afterwards, platelets (5 wt% relative to the total amount of powder) were added to the ball-milled suspension, whilst n-butylamine (99.5% Acros Belgium, 3.4 vol% relative to the suspension volume) was added to negatively charge the particles and Dolapix Ce-64 (Zschimmer & Schwarz, Germany, 1.1 wt% relative to the powder mass) was added as dispersant. For comparison, a suspension without platelets was also prepared. This suspension was magnetically stirred for 60 min, ultrasonicated in an ultrasonic bath (Branson 2510) for 15 min and magnetically stirred for another 15 min. Fig. 2. EPD cell with horizontally positioned deposition electrode. Fig. 3. EPD cell with vertically positioned deposition electrode. To estimate the influence of the electric field force, gravity and hydrodynamic force on the alignment of the platelets, two different set-ups were used, i.e., with a vertical and horizontal EPD cell. Three configurations were studied in a slightly modified horizontal cell, as summarised in Table 1. The standard horizontal EPD cell is schematically presented in Fig. 2. In case of an applied suspension flow (configuration 2 in Table 1), a suspension was flown through the deposition cell by means of a suspension circulation system driven by a peristaltic pump. The distance between the flat disc shaped electrodes was 3.5 cm and the electrodes had a diameter of 3.7 cm. The suspension in the reservoir was stirred to avoid sedimentation. In addition, the flow-through system also decreases the sedimentation of the suspension in the cell with the fluid flow. Homogeneous deposits were made by pumping 200 ml suspension at 1 ml/s through the deposition cell. The suspension circulation system was not used in the other two configurations (3 and 4 of Table 1) where only 50 ml suspension was used to fill the EPD cell. Configuration 4 and 3 form respectively depositions along and in the opposite direction of the gravity force, in which the perforated top electrode (Fig. 2) was replaced with a normal one. EPD was conTable 1 Density, Lotgering factor and texture index for different EPD configurations. Configuration Cell Deposition Suspension Green density [%] Sintered density [%] Lotgering factor Texture index 1 Vertical Vertical stirred 59.4 97.9 0.49 18.32 2 Horizontal Bottom Flow through 58.9 97.5 0.01 1.60 3 Horizontal Top Stagnant 62.7 99.7 0.21 8.12 4 Horizontal Bottom Stagnant 61.1 98.0 0.02 2.52 5 Vertical Vertical Stagnant – – 0.12 8.05
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 surface
L. 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 separation 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 performed 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 temperature was 24.5S/cm, as measured by a conductivity sensor (Cond Level 2 type, WTW). After EPD, the deposit was carefully 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 electron 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 characterize 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 textured 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 measured pole figures by “Hexagonal ODF software system” (Dept MTM—K.U.Leuven). The texture index was used as an indicator 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 experimental 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 significant 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 electrode (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 oriented 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 locations 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 relative 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.
l198 L Zhang et al. Joumal of the European Ceramic Society 30(2010)1195-1202 lated texture index of thi e Is 3.2. Horizontal downward deposition from a fowing suspension the bottom and suspension flowing through. The platelets in the deposit however were not well aligned as shown in the eBsd of the perper (Fig. 6(b)). As shown in Fig. 6(e), the peaks are relatively weak and are not homogenously distributed implying that the pris matic planes are not well aligned. The corresponding X-ray pole 如Em figure confirms a very weak texturing after grain growth with a limited texture index of 1.60 (b) 3.3. Deposition from a stagnant suspension The abov nat the cell ical factor for platelet alignment during EPD. The electric field Eg force, gravity and hydrodynamic force applied on the platelets are believed to be the main factors influencing platelet alignment The influence of these forces is studied using 3 additional con- figurations as summarised in Table 1. in order to investigate the electric field force effect, an upward deposition was performed the horizontal cell without suspension flowing through(con- figuration 3). In order to investigate the gravity effect, EPD was performed from a stagnant suspension in the horizontal cell(con- figuration 4). In order to study the impact of the hydrodynamic force, the suspension is stirred comparing with stagnant suspen sion(configuration 5)or pumped through the cell as described in Sections 3.1 and 3.2 Fig. 5. Textured alumina deposited in a horizontal cell, cross-sectioned (a) Table 1 summarises the Lotgering factor and texture index parallel and (b) perpendicular to the deposition electrode. measured on the parallel cross-sectioned sintered deposits obtained under different EPd cell configurations, as well as the porosity,the residual porosity also results from platelet particle green and sintered density. The green relative density of all the constrained sintering, as discussed in literature. 6, 7 Since th deposits was in the 58.9-62.7% range, which is quite compara template platelets are essential for texture development, all fur. ble. After sintering, quite dense ceramics with some remaining ther investigated grades were prepared from platelet containing porosity due to constrained sintering were obtained. The den- sification of the powder matrix is significantly retarded by the suspensions. presence of large inclusions, i.e., platelets, resulting in a lower EBSD of perpendicularly cross-sectioned grades was investi- densification as reported in literature. 16. I The sintered density gated to characterize the texture. The EBSD pattern and inverse pole figure(IPF) of the above material grade are presented is proportional to the green density of the samples, as shown in ig. 6(a)and (d). The platelets are well aligned parallel to th Table 1. EPD from a stirred suspension in a vertical cell(con- deposition electrode surface, positioned at the bottom of the figuration 1)results in the highest Lotgering factor and texture picture in Fig. 6(a), revealing that the c-axis has been well- index, which indicates that the best texture was formed. The extent of texturisation is quite different for the five investigated aligned perpendicular to the surface of the deposition electrode, configurations. The mechanism of texture formation is discussed in Fig. 6(d)is large and the peak is homogeneously distributed at the edge of the IPF, proving that the prismatic planes are well ori- ented in the normal direction. The a-and b-axes are not aligned. 4. Discussion i.e., the material has no preferred in-plane orientation(a-axis or 4.1. Influence of the electric field force b-axis orientation)as indicated by the fact that the peak intensity is homogeneously distributed at the edge of the IPF. The electric field force is the driving force for powder con- In order to investigate the global sample area, X-ray pole solidation during EPD. In order to investigate the impact of the figures were used to characterize the macro-texture. The calcu- electric field force on platelet alignment, EPD from a stagnant
1198 L. Zhang et al. / Journal of the European Ceramic Society 30 (2010) 1195–1202 Fig. 5. Textured alumina deposited in a horizontal cell, cross-sectioned (a) parallel and (b) perpendicular to the deposition electrode. porosity, the residual porosity also results from platelet particle constrained sintering, as discussed in literature.16,17 Since the template platelets are essential for texture development, all further investigated grades were prepared from platelet containing suspensions. EBSD of perpendicularly cross-sectioned grades was investigated to characterize the texture. The EBSD pattern and inverse pole figure (IPF) of the above material grade are presented in Fig. 6(a) and (d). The platelets are well aligned parallel to the deposition electrode surface, positioned at the bottom of the picture in Fig. 6(a), revealing that the c-axis has been wellaligned perpendicular to the surface of the deposition electrode, as shown in Fig. 6(a). The peak intensity of the [0 0 1] IPF shown in Fig. 6(d) is large and the peak is homogeneously distributed at the edge of the IPF, proving that the prismatic planes are well oriented in the normal direction. The a- and b-axes are not aligned, i.e., the material has no preferred in-plane orientation (a-axis or b-axis orientation) as indicated by the fact that the peak intensity is homogeneously distributed at the edge of the IPF. In order to investigate the global sample area, X-ray pole figures were used to characterize the macro-texture. The calculated texture index of this sample is 18.32, which implies a sharp texture formation. 3.2. Horizontal downward deposition from a flowing suspension The same suspension composition was used for EPD in the horizontal cell, shown in Fig. 2, with the deposition electrode at the bottom and suspension flowing through. The platelets in the deposit however were not well aligned as shown in the EBSD pattern of the perpendicular cross-section of the sintered ceramic (Fig. 6(b)). As shown in Fig. 6(e), the peaks are relatively weak and are not homogenously distributed implying that the prismatic planes are not well aligned. The corresponding X-ray pole figure confirms a very weak texturing after grain growth with a limited texture index of 1.60. 3.3. Deposition from a stagnant suspension The above results clearly show that the cell geometry is a critical factor for platelet alignment during EPD. The electric field force, gravity and hydrodynamic force applied on the platelets are believed to be the main factors influencing platelet alignment. The influence of these forces is studied using 3 additional con- figurations as summarised in Table 1. In order to investigate the electric field force effect, an upward deposition was performed in the horizontal cell without suspension flowing through (con- figuration 3). In order to investigate the gravity effect, EPD was performed from a stagnant suspension in the horizontal cell (con- figuration 4). In order to study the impact of the hydrodynamic force, the suspension is stirred comparing with stagnant suspension (configuration 5) or pumped through the cell as described in Sections 3.1 and 3.2. Table 1 summarises the Lotgering factor and texture index measured on the parallel cross-sectioned sintered deposits obtained under different EPD cell configurations, as well as the green and sintered density. The green relative density of all the deposits was in the 58.9–62.7% range, which is quite comparable. After sintering, quite dense ceramics with some remaining porosity due to constrained sintering were obtained. The densification of the powder matrix is significantly retarded by the presence of large inclusions, i.e., platelets, resulting in a lower densification as reported in literature.16,17 The sintered density is proportional to the green density of the samples, as shown in Table 1. EPD from a stirred suspension in a vertical cell (con- figuration 1) results in the highest Lotgering factor and texture index, which indicates that the best texture was formed. The extent of texturisation is quite different for the five investigated configurations. The mechanism of texture formation is discussed below, based on these experimental findings. 4. Discussion 4.1. Influence of the electric field force The electric field force is the driving force for powder consolidation during EPD. In order to investigate the impact of the electric field force on platelet alignment, EPD from a stagnant
L Zhang et al. / Journal of the European Ceramic Society 30(2010)1195-1202 喝() 0001 1100 1210 0110 (d)0001 2110 max=2.5 2213 1100 12001101210 2110 max=1.638 0001 100 1 01101210 min=0.191 70 um Fig. 6. EBSD data on a textured alumina. EBSD pattern of the perpendicular cross-section of the sample formed(a)in vertical cell with stirred suspension; (b)in orizontal cell with suspension fowing through; (c) corresponding colour coded map and (d)[001] IPF for(a), and(e)100 1]IPF for(b) suspension in a horizontal cell with a deposition electrode on the top(configuration 3)was investigated. In this case, the down- ward gravity force on the platelets is counteracted by the upward electric field force and the platelet alignment in the deposit is mainly induced by the electric field force. As shown in Fig. 7, the basal planes of the platelets are well aligned parallel to the depo- sition electrode surface. The electric field force can orientate platelets in two possible ways, i.e., during electrophoresis or One possible mechanism is that the electric field force aligns the platelets during electrophoresis due to the charge distribution on the platelet surfaces. The electrical charge on the basal plane of the platelets is different from that on the side plane due to t large difference in surface area. Under the present experimental conditions at pH 1l. 4, all platelet surfaces are negatively charged although the natural charge density could be different between the basal plane and prismatic plane. 8-20 The electric field force 10 am applied on the basal plane is therefore larger than on the pris- matic plane. The platelet alignment mechanism may depend on Fig. 7. SEM micrograph of a perpendicularly cross-sectioned ceramic obtained the polarisation of the electrical double layer in the electric field by upward EPD in a horizontal cell without suspension flowing through
L. Zhang et al. / Journal of the European Ceramic Society 30 (2010) 1195–1202 1199 Fig. 6. EBSD data on a textured alumina. EBSD pattern of the perpendicular cross-section of the sample formed (a) in vertical cell with stirred suspension; (b) in horizontal cell with suspension flowing through; (c) corresponding colour coded map and (d) [0 0 1] IPF for (a), and (e) [0 0 1] IPF for (b). suspension in a horizontal cell with a deposition electrode on the top (configuration 3) was investigated. In this case, the downward gravity force on the platelets is counteracted by the upward electric field force and the platelet alignment in the deposit is mainly induced by the electric field force. As shown in Fig. 7, the basal planes of the platelets are well aligned parallel to the deposition electrode surface. The electric field force can orientate the platelets in two possible ways, i.e., during electrophoresis or upon deposition. One possible mechanism is that the electric field force aligns the platelets during electrophoresis due to the charge distribution on the platelet surfaces. The electrical charge on the basal plane of the platelets is different from that on the side plane due to the large difference in surface area. Under the present experimental conditions at pH 11.4, all platelet surfaces are negatively charged although the natural charge density could be different between the basal plane and prismatic plane.18–20 The electric field force applied on the basal plane is therefore larger than on the prismatic plane. The platelet alignment mechanism may depend on the polarisation of the electrical double layer in the electric field, Fig. 7. SEM micrograph of a perpendicularly cross-sectioned ceramic obtained by upward EPD in a horizontal cell without suspension flowing through
L Zhang et al. Joumal of the European Ceramic Society 30(2010)1195-1202 +++++++++++++ E + 30b Elf+SM8 bot. EPD b Fig. 8. Schematic illustration of the platelet orientation in an applied electric field during epd mainly from the basal plane. This might be explained by the difference in charge density on platelets oriented parallel or per pendicular to the electric field. The negatively charged alumina platelets gather a diffuse double layer of counter ions around) them, which becomes asymmetrically polarised in the electric field. If the platelet is initially oriented parallel to the electric field(case I in Fig 8), the positive ions above the basal plane in- etween the platelet and the anode will try to counteract the elec trical force on the platelet, resulting in a reduced platelet mobility in the electric field. However, if the platelet is initially positione parallel with respect to the electrodes(case Il in Fig. 8), polar isation of the diffuse ion cloud is easier resulting in preferred deposition. This would actually imply that the platelets are ori-500 w 30 200x SE 99 1 30 EI-SMS botEPDe ented perpendicularly to the electric field during electrophoresis. It should also be noted that when particles are moving towards Fig 9. SEM micrograph of the ceramic obtained (a) after downward deposition the electrode during EPD, there is a hydrodynamic force in the in a horizontal cell without suspension flowing through and(b)detail of the opposite direction due to the fact that the solvent has to move electrode contact side revealing sedimentation prior to EPD in the opposite direction of particle movement. That could also influence the alignment of platelets. Mody et al. observed fip- anisotropic platelets can be orientated with their basal plane ping over of platelets in a parallel-plate flow cell. 2 Under the perpendicular to the gravity direction in the sediment although impact of the hydrodynamic force during electrophoresis, the their basal plane could be parallel to the gravity direction during platelets may also move in a rotational flip-over way. sedimentation in the suspensions. 23.24 Another mechanism for platelet alignment might be When depositing in the opposite direction of the gravity force attributed to the reorientation of platelets once they make (configuration 3 in Table 1), the platelets were found to be contact with the deposit or the deposition electrode. As mod- oriented with their basal plane parallel to the surface of the depo- elled by Hanau et al., based on density functional theory sition electrode, as discussed above. When depositing along the plate-like particles in the vicinity of a hard wall will adopt direction of the gravity force(configuration 4 in Table 1), the nearly a fully parallel alignment due to interactions with the platelets were not aligned, as shown in Fig 9(a). This could be wall. When applying this model to the electrode surface attributed to the interference of platelet sedimentation. Gravity during EPD, the platelet templates will align parallel to the causes the sedimentation of larger particles, especially platelets, deposition electrode irrespective of their orientation in the sus- due to their relatively larger mass. Prior to EPD, i.e., before pension. However, the surface charge influence is ignored in this applying the voltage, suspension sedimentation was already ini- mode tiated, as shown in Fig. 9(b). It should also be noted that a downward electric field force speeds up powder settling. The 4.2. Influence of gravity platelet orientation in the deposit is quite weak when EPD along the gravity force, as indicated by the Lotgering factor of 0.02 The effect of gravity on platelet orientation during sedimen- and texture index of 2.52 in Table I, whereas texturing is more tation depends on many parameters. As reported in literature, pronounced when depositing in the opposite direction of gravity
1200 L. Zhang et al. / Journal of the European Ceramic Society 30 (2010) 1195–1202 Fig. 8. Schematic illustration of the platelet orientation in an applied electric field during EPD. mainly from the basal plane. This might be explained by the difference in charge density on platelets oriented parallel or perpendicular to the electric field. The negatively charged alumina platelets gather a diffuse double layer of counter ions around them, which becomes asymmetrically polarised in the electric field. If the platelet is initially oriented parallel to the electric field (case I in Fig. 8), the positive ions above the basal plane inbetween the platelet and the anode will try to counteract the electrical force on the platelet, resulting in a reduced platelet mobility in the electric field. However, if the platelet is initially positioned parallel with respect to the electrodes (case II in Fig. 8), polarisation of the diffuse ion cloud is easier resulting in preferred deposition. This would actually imply that the platelets are oriented perpendicularly to the electric field during electrophoresis. It should also be noted that when particles are moving towards the electrode during EPD, there is a hydrodynamic force in the opposite direction due to the fact that the solvent has to move in the opposite direction of particle movement. That could also influence the alignment of platelets. Mody et al. observed flipping over of platelets in a parallel-plate flow cell.21 Under the impact of the hydrodynamic force during electrophoresis, the platelets may also move in a rotational flip-over way. Another mechanism for platelet alignment might be attributed to the reorientation of platelets once they make contact with the deposit or the deposition electrode. As modelled by Harnau et al., based on density functional theory plate-like particles in the vicinity of a hard wall will adopt nearly a fully parallel alignment due to interactions with the wall.22 When applying this model to the electrode surface during EPD, the platelet templates will align parallel to the deposition electrode irrespective of their orientation in the suspension. However, the surface charge influence is ignored in this model. 4.2. Influence of gravity The effect of gravity on platelet orientation during sedimentation depends on many parameters. As reported in literature, Fig. 9. SEM micrograph of the ceramic obtained (a) after downward deposition in a horizontal cell without suspension flowing through and (b) detail of the electrode contact side revealing sedimentation prior to EPD. anisotropic platelets can be orientated with their basal plane perpendicular to the gravity direction in the sediment although their basal plane could be parallel to the gravity direction during sedimentation in the suspensions.23,24 When depositing in the opposite direction of the gravity force (configuration 3 in Table 1), the platelets were found to be oriented with their basal plane parallel to the surface of the deposition electrode, as discussed above. When depositing along the direction of the gravity force (configuration 4 in Table 1), the platelets were not aligned, as shown in Fig. 9(a). This could be attributed to the interference of platelet sedimentation. Gravity causes the sedimentation of larger particles, especially platelets, due to their relatively larger mass. Prior to EPD, i.e., before applying the voltage, suspension sedimentation was already initiated, as shown in Fig. 9(b). It should also be noted that a downward electric field force speeds up powder settling. The platelet orientation in the deposit is quite weak when EPD along the gravity force, as indicated by the Lotgering factor of 0.02 and texture index of 2.52 in Table 1, whereas texturing is more pronounced when depositing in the opposite direction of gravity
L Zhang et al. / Journal of the European Ceramic Society 30(2010)1195-1202 Fig. 11. SEM micrograph revealing the microstructure of a perpendicularly cross-sectioned sintered ceramic after EPD from a stagnant suspension in a face. Platelet alignment is substantially better when stirring the over, stirring completely inhibited suspension sedimentation. As a result, the magnetically stirred vertical EPD cell(configura tion 1)gives the best texture, as shown in Table 1. It should also be noted that the magnitude of the hydrodynamic force in the two EPD cells is different. The suspension flows through the horizontal cell with a volume of about 50ml at 1 ml/s. In the vertical cell, the magnetic stirrer stirred 50 ml of suspension at c250rpm. The hydrodynamic force is therefore substantially lower in the horizontal cell. the influence of a fluid fow or platelet orientation in the horizontal cell is limited as shown by the comparable Lotgering factors(0.01 versus 0.02)and tex ture indexes(1.60 versus 2. 52)in Table 1. In the vertical cell, the shear force applied by the fluid is an important factor for platelet Fig. 10. Flow pattern during EPD(a)in a cross-sectioned horizontal cell with orientation, as indicated by the significant increase in Lotgering suspension flowing through and(b) top view of the flow pattern during EPD in factor from 0. 12 to 0.49 and texture index from 8.05 to 18.32, a vertical cell with magnetic stirring. The deposition electrode is marked by the as induced by the hydrodynamic force 5. Conclusions 43. Influence of the hydrodynamic force Platelet template particles used for templated grain growth When stirring the suspension or generating a suspension d be aligned during the electrophoretic deposition process flow in the EPD cell, the generated hydrodynamic force in the The influence of the electric field force, gravity and hydrody- neighbourhood of the deposition electrode will influence the namic force are studied in a vertical and horizontal deposition deposition process. The hydrodynamic force in the horizontal cell. The electric field force orients the c-axis of the platelets par- cell is different than in the vertical cell, as schematically pre- allel to the electric field force direction. Powder sedimentation sented in Fig. 10. The basal plane of the platelets would be induced by gravity however is detrimental for platelet alignment aligned along the fluid direction in order to minimize the drag- in the deposit. The hydrodynamic force aligns the basal plane ging force. As shown in Fig. 10(a), the hydrodynamic force is parallel to the suspension flow direction. The ceramics processed not parallel to the electrode surface in the horizontal cell, which under different EPD configurations result in a difterent degree of may interfere with the alignment of the platelets. The platelets texture, implying that the EPD deposition cell configuration is are indeed not well aligned as shown in Fig. 6(b). In the vertical a critical factor for the platelet template alignment. The highest cell,however, the shear force applied by the fluid flow is almost (00 1)alumina texture, with a Lotgering factor of 0. 49 and tex- parallel to the electrode surface, as illustrated in Fig. 10(b). In ture index of 18.32, was obtained after templated grain growth this case, the hydrodynamic force assists in aligning the basal of the deposits obtained from a vertical EPD cell configuration plane of the platelets parallel to the deposition electrode suI with a stirred suspension
L. Zhang et al. / Journal of the European Ceramic Society 30 (2010) 1195–1202 1201 Fig. 10. Flow pattern during EPD (a) in a cross-sectioned horizontal cell with suspension flowing through and (b) top view of the flow pattern during EPD in a vertical cell with magnetic stirring. The deposition electrode is marked by the arrow at the bottom. 4.3. Influence of the hydrodynamic force When stirring the suspension or generating a suspension flow in the EPD cell, the generated hydrodynamic force in the neighbourhood of the deposition electrode will influence the deposition process. The hydrodynamic force in the horizontal cell is different than in the vertical cell, as schematically presented in Fig. 10. The basal plane of the platelets would be aligned along the fluid direction in order to minimize the dragging force. As shown in Fig. 10(a), the hydrodynamic force is not parallel to the electrode surface in the horizontal cell, which may interfere with the alignment of the platelets. The platelets are indeed not well aligned as shown in Fig. 6(b). In the vertical cell, however, the shear force applied by the fluid flow is almost parallel to the electrode surface, as illustrated in Fig. 10(b). In this case, the hydrodynamic force assists in aligning the basal plane of the platelets parallel to the deposition electrode surFig. 11. SEM micrograph revealing the microstructure of a perpendicularly cross-sectioned sintered ceramic after EPD from a stagnant suspension in a vertical cell. face. Platelet alignment is substantially better when stirring the suspension (Fig. 5 (b)) than without stirring (Fig. 11). Moreover, stirring completely inhibited suspension sedimentation. As a result, the magnetically stirred vertical EPD cell (configuration 1) gives the best texture, as shown in Table 1. It should also be noted that the magnitude of the hydrodynamic force in the two EPD cells is different. The suspension flows through the horizontal cell with a volume of about 50 ml at 1 ml/s. In the vertical cell, the magnetic stirrer stirred 50 ml of suspension at ∼250 rpm. The hydrodynamic force is therefore substantially lower in the horizontal cell. The influence of a fluid flow on platelet orientation in the horizontal cell is limited, as shown by the comparable Lotgering factors (0.01 versus 0.02) and texture indexes (1.60 versus 2.52) in Table 1. In the vertical cell, the shear force applied by the fluid is an important factor for platelet orientation, as indicated by the significant increase in Lotgering factor from 0.12 to 0.49 and texture index from 8.05 to 18.32, as induced by the hydrodynamic force. 5. Conclusions Platelet template particles used for templated grain growth could be aligned during the electrophoretic deposition process. The influence of the electric field force, gravity and hydrodynamic force are studied in a vertical and horizontal deposition cell. The electric field force orients the c-axis of the platelets parallel to the electric field force direction. Powder sedimentation induced by gravity however is detrimental for platelet alignment in the deposit. The hydrodynamic force aligns the basal plane parallel to the suspension flow direction. The ceramics processed under different EPD configurations result in a different degree of texture, implying that the EPD deposition cell configuration is a critical factor for the platelet template alignment. The highest (0 0 1) alumina texture, with a Lotgering factor of 0.49 and texture index of 18.32, was obtained after templated grain growth of the deposits obtained from a vertical EPD cell configuration with a stirred suspension
L Zhang et al. Joumal of the European Ceramic Society 30(2010)1195-1202 Acknowledgements 11. Boccaccini. A. R. Roether. J.A. Thomas. B.J. C. Shaffer. M.S. P Ch E, Stoll, E. et aL, The electrophoretic deposition of inorganic nanos This work was performed within the framework of the materials. J Ceram Soc Jpn, 2006, 114, 1-14. Research Fund of K U. Leuven under project GOA/2005/ 12. Menon, M, Decourcelle, S, Ramousse,Sand Larsen,PH, Stabilization 08/TBA and GOA/2008/007. The authors also acknowledge the of ethanol-based alumina suspensions. J Am Ceram Soc, 2006, 89, 457- support of the Flemish Institute for the Promotion of Scientific 13. Lotgering, F. K. Topotactical reactions with ferromagnetic oxides hav echnological Research in Industry (TwT)under contract ing hexagonal crystal structures. J Inorg Nucl Chem, 1959,9, 113- SBO-PROMAG(60056 14. Gonenli, L. E. and Messing. G. L, Texturing of mullite by templated References grain growth with aluminum borate whiskers. J Eur Ceram Soc, 2001, 21 2495-2501. Bunge H. J. analysis in materials science. mathematical models 1. Randle, V. and Engler, O, Introduction to texture analysis. Gordon and Butterworths London. 1982,pp.l19152 Breach Science Publishers, 2000, Pp 3-11 16. Belmonte, M, Moya, J. S. and Miranzo, P, Bimodal sintering of 2. Zhou, Y, Vleugels, J, Laoui, T. and Van der Biest, O, Toughening of X Al2O3/Al203 platelet ceramic composites. J Am Ceram Soc, 1995, 78 Sialon with AlO3 Platelets. J Eur Ceram Soc, 1995, 15, 297-305 1661-1667 3. Uchikoshi, T, Suzuki, T.S. and Sakka, Y, Crystalline orientation of alumina 17. Suder, O. and Lange, F. F. Effect of inclusions on densification: I ceramics prepared by electrophoretic deposition under a high magnetic field. microstructural development in an Al2O3 matrix containing a high volume J Mater Sci,2006,41,80748078 fraction of ZrO, inclusions. J Am Ceram Soc. 1992. 75. 519-524. 4. Carisey, T, Levin, I and Brandon, D. G, Microstructure and mechanical 18. Eng, P J, Trainor, T P, Brown, G E, Waychunas, G.A., Newville, M, Sut- properties of textured Al203. J Eur Ceram Soc, 1995, 15, 283-289. ton, S.R. et al, Structure of the hydrated a-Al2O3(0001)surface. Science, 5. Seabaugh, M. M, Kerscht, L H. and Messing, G. L, Texture developmen 2000,288,1029-1033. by templated grain growth in liquid-phase sintered a-alumina. J Am Cera 19. Franks. G. V and Gan, Y. Charging behavior at the alumina-water inter- Soc,1997,80,1181-1188 face and implications for ceramic processing. J Am Ceram Soc, 2007, 90 6. Suvaci, E. and Messing, G. L, Critical factors in the templated grain 3373-3388 owth of textured reaction-bonded alumina. J Am Ceram Soc, 2000, 83, 20. Hunter, R.J., Foundations of colloid science. Oxford University Press, New 2041-2048 York,1986,pp.25-32 7. Seabaugh, M. M, Messing, G L and Vaudin, M. D, Texture development 21. Mody, N. A, Lomakin, O, Doggett, T.A., Diacovo, T. G and King, M. and microstructure evolution in liquid-phase-sintered a-alumina ceramics R. Mechanics of transient platelet adhesion to von willebrand factor under prepared by templated grain growth. J Am Ceram Soc, 2000, 83, 3109-3116 fow. Biophys J,2005,88.1432-1443. 8. Wei, M., Zhi, D and Brandon, D. G, Microstructure and texture evolution 22. Hanau, L. and Dietrich, S, Fluids of platelike particles near a hard wall. in gel-cast a-alumina/alumina platelet ceramic composites. Scripta Mater, Phys Rev e,2002,65,021505 2005,53,1327-1332. 23. Dimasi, E, Fossum, J O, Gog. T and Venkataraman, C, Orientational of 9. Ozer, I.O Suvaci, E, Karademir, B, Missiaen, J. M, Carry, C P and Bou in gravity dispersed clay colloids: a synchrotron x-ray scattering study of vard, D, Anisotropic sintering shrinkage in alumina ceramics containing Na fluorohectorite suspensions. Phys Rev E, 2001, 64, 061704 platelets. J Am Ceram Soc, 2006, 89, 1972-1976 24. Van der Kooij, F. M. and Lekkerkerker, H. N. w, Formation of nematic Biest, O. and Vandeperre, L.J., Electrophoretic deposition of mate- liquid crystals in suspensions of hard colloidal platelets. J Phys Chem B, nn Rey Mater Sci. 1999. 29.327-352 1998,102,78297832
1202 L. Zhang et al. / Journal of the European Ceramic Society 30 (2010) 1195–1202 Acknowledgements This work was performed within the framework of the Research Fund of K.U.Leuven under project GOA/2005/ 08/TBA and GOA/2008/007. The authors also acknowledge the support of the Flemish Institute for the Promotion of Scientific Technological Research in Industry (IWT) under contract SBO-PROMAG (60056). References 1. Randle, V. and Engler, O., Introduction to texture analysis. Gordon and Breach Science Publishers, 2000, pp. 3–11. 2. Zhou, Y., Vleugels, J., Laoui, T. and Van der Biest, O., Toughening of XSialon with Al2O3 Platelets. J Eur Ceram Soc, 1995, 15, 297–305. 3. Uchikoshi, T., Suzuki, T. S. and Sakka, Y., Crystalline orientation of alumina ceramics prepared by electrophoretic deposition under a high magnetic field. J Mater Sci, 2006, 41, 8074–8078. 4. Carisey, T., Levin, I. and Brandon, D. G., Microstructure and mechanical properties of textured Al2O3. J Eur Ceram Soc, 1995, 15, 283–289. 5. Seabaugh, M. M., Kerscht, I. H. and Messing, G. L., Texture development by templated grain growth in liquid-phase sintered -alumina. J Am Ceram Soc, 1997, 80, 1181–1188. 6. Suvaci, E. and Messing, G. L., Critical factors in the templated grain growth of textured reaction-bonded alumina. J Am Ceram Soc, 2000, 83, 2041–2048. 7. Seabaugh, M. M., Messing, G. L. and Vaudin, M. D., Texture development and microstructure evolution in liquid-phase-sintered -alumina ceramics prepared by templated grain growth. J Am Ceram Soc, 2000, 83, 3109–3116. 8. Wei, M., Zhi, D. and Brandon, D. G., Microstructure and texture evolution in gel-cast -alumina/alumina platelet ceramic composites. Scripta Mater, 2005, 53, 1327–1332. 9. Ozer, I. O., Suvaci, E., Karademir, B., Missiaen, J. M., Carry, C. P. and Bouvard, D., Anisotropic sintering shrinkage in alumina ceramics containing oriented platelets. J Am Ceram Soc, 2006, 89, 1972–1976. 10. Van der Biest, O. and Vandeperre, L. J., Electrophoretic deposition of materials. Ann Rev Mater Sci, 1999, 29, 327–352. 11. Boccaccini, A. R., Roether, J. A., Thomas, B. J. C., Shaffer, M. S. P., Chavez, E., Stoll, E. et al., The electrophoretic deposition of inorganic nanoscaled materials. J Ceram Soc Jpn, 2006, 114, 1–14. 12. Menon, M., Decourcelle, S., Ramousse, S. and Larsen, P. H., Stabilization of ethanol-based alumina suspensions. J Am Ceram Soc, 2006, 89, 457– 464. 13. Lotgering, F. K., Topotactical reactions with ferromagnetic oxides having hexagonal crystal structures. J Inorg Nucl Chem, 1959, 9, 113– 123. 14. Gönenli, I. E. and Messing, G. L., Texturing of mullite by templated grain growth with aluminum borate whiskers. J Eur Ceram Soc, 2001, 21, 2495–2501. 15. Bunge, H. J., Texture analysis in materials science: mathematical models. Butterworths, London, 1982, pp. 119–152. 16. Belmonte, M., Moya, J. S. and Miranzo, P., Bimodal sintering of Al2O3/Al2O3 platelet ceramic composites. J Am Ceram Soc, 1995, 78, 1661–1667. 17. Suder, O. and Lange, F. F., Effect of inclusions on densification: I, microstructural development in an Al2O3 matrix containing a high volume fraction of ZrO2 inclusions. J Am Ceram Soc, 1992, 75, 519–524. 18. Eng, P. J., Trainor, T. P., Brown, G. E., Waychunas, G. A., Newville, M., Sutton, S. R. et al., Structure of the hydrated -Al2O3 (0001) surface. Science, 2000, 288, 1029–1033. 19. Franks, G. V. and Gan, Y., Charging behavior at the alumina-water interface and implications for ceramic processing. J Am Ceram Soc, 2007, 90, 3373–3388. 20. Hunter, R. J., Foundations of colloid science. Oxford University Press, New York, 1986, pp. 25–32. 21. Mody, N. A., Lomakin, O., Doggett, T. A., Diacovo, T. G. and King, M. R., Mechanics of transient platelet adhesion to von willebrand factor under flow. Biophys J, 2005, 88, 1432–1443. 22. Harnau, L. and Dietrich, S., Fluids of platelike particles near a hard wall. Phys Rev E, 2002, 65, 021505. 23. Dimasi, E., Fossum, J. O., Gog, T. and Venkataraman, C., Orientational of in gravity dispersed clay colloidas: a synchrotron x-ray scattering study of Na fluorohectorite suspensions. Phys Rev E, 2001, 64, 061704. 24. Van der Kooij, F. M. and Lekkerkerker, H. N. W., Formation of nematic liquid crystals in suspensions of hard colloidal platelets. J Phys Chem B, 1998, 102, 7829–7832.