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Control of crystalline texture in polycrystalline alumina ceramics by electrophoretic deposition in a strong magnetic field T. Uchikoshi,a) T.S. Suzuki, H. Okuyama, and Y. Sakka National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0047, Japan (Received 2 September 2003; accepted 9 February 2004) Highly crystalline-textured pure dense alumina ceramics were fabricated from spherical alumina powder without any seed particles and sintering additives by electrophoretic deposition (EPD) in a strong magnetic field of 10 T. The crystalline texture was confirmed by x-ray diffraction (XRD) for alumina ceramics deposited at 10 T followed by sintering at 1873 K. The angle between the directions of the magnetic and electric fields (B-E) was altered to control the dominant crystal faces of the ␣-alumina monoliths. The average orientation angles estimated from the XRD diagram of the samples prepared at B-E ⳱ 0°, 45°, and 90° were 16.52°, 45.15°, and 84.90°, respectively. Alumina/alumina laminar composites with different crystalline-oriented layers were also fabricated by alternately changing the B-E layer by layer during EPD in a 10 T magnetic field. It was demonstrated that by using this technique, it is possible to control the crystalline orientation by changing the angle of E versus B during the EPD. I. INTRODUCTION Recently, there have been a number of studies concerning the fabrication of textured ceramics, as they have anisotropic mechanical, thermal, and electrical properties, which are similar to single crystals.1 Textured ceramics have been produced by a variety of techniques; such as tape casting,2 hot forging or deformation,3–5 eutectic solidification,6 and templated or seeded grain growth.7–9 Ceramic platelets, fibers, or whiskers are used as seed particles along with some additives to promote the anisotropic grain growth during sintering.9 Although it is important to make a distinction between the microstructural texture and crystalline texture,10 they have often been confused. There has been increased interest in fabricating crystalline-textured materials using the influence of an external magnetic field against the magnetic anisotropy of the materials. Many materials in asymmetric (noncubic) crystalline structures have anisotropic magnetic susceptibilities, ⌬ ⳱ || −⊥, associated with their crystal structures, where || and ⊥ are the susceptibilities parallel and perpendicular to the magnetic principal axis, respectively. When a single crystal of these materials is placed in a magnetic field, the crystal is rotated and the a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0198 J. Mater. Res., Vol. 19, No. 5, May 2004 crystallographic axis of high is aligned in the direction of the magnetic field. The driving force of the magnetic alignment is the energy of the crystal anisotropy and is given as11 ⌬E = ⌬VB2 Ⲑ 20 , (1) where V is the volume of the material, B is the applied magnetic field, and 0 is the permeability in a vacuum. This alignment occurs when the energy of anisotropy is higher than the energy of thermal motion, that is ⌬E ⬎ kT , (2) where k is Boltzmann’s constant. Generally, the magnetic susceptibilities of feeble magnetic materials (|| ⳱ 10−3∼10−6) are quite low in comparison with those of ferromagnetic materials (|| ⳱ 102∼104), and the ⌬E of feeble magnetic materials is much lower than kT in a conventional magnetic field generated by a permanent magnet (B ⳱ ∼0.1 T). Therefore, the influence of a magnetic field on feeble magnetic materials has not yet been taken into consideration. The recent development of superconducting magnet technologies has provided academic laboratories with magnetic fields as high as approximately 10 T. Under such strong magnetic fields, the magnetization force acting on feeble magnetic materials is not negligible. Recently, it has been reported that crystalline-textured ceramics such as alumina,12 titania,13-15 zinc oxide,15,16 © 2004 Materials Research Society 1487 T. Uchikoshi et al.: Control of crystalline texture in polycrystalline alumina ceramics by electrophoretic deposition in a strong magnetic field and SiC whisker-dispersed alumina17 have been fabricated by slip casting under 10 T. To extend the use of a strong magnetic field for the colloidal processing of ceramics, we have proposed the use of both magnetic and electric fields, that is, electrophoretic deposition (EPD) in a strong magnetic field.18–21 EPD is a colloidal process wherein ceramic bodies are directly shaped from a stable colloid suspension by a dc electric field.22–24 A schematic illustration of the concept is shown in Fig. 1. Ceramic particles dispersed in a solvent are rotated due to their magnetic anisotropy and then deposited on a substrate. It is essential for each particle to be single crystalline and be deflocculated for their rotation in a solvent.18 We have reported that crystalline-textured alumina18–21 and titania25,26 are fabricated by EPD in a strong magnetic field. This paper reports the significant advantages of this facile and precise technique to synthesize crystallinetextured alumina monoliths and alumina/alumina laminate composites by controlling the directions between the magnetic and electric fields during EPD in a superconducting magnetic environment. II. EXPERIMENTAL Spherical, single crystalline ␣-alumina particles (Sumitomo Chem. Co., Ltd., Tokyo, Japan, AKP-50, average particle size of 0.2 m, high purity of >99.99%) were dispersed at pH 4 in distilled water by ultrasound, and then a deflocculated aqueous suspension with a 10 vol% solid content was prepared. The zeta-potential of alumina measured by the laser Doppler velocimetry method was +40 mV at pH 4. The suspension was placed in a superconducting magnet (Japan Magnet Technology, Kobe, Japan, JMTD-10T100 with a room temperature bore of 100 mm), and then a strong 10 T magnetic field was applied to the suspension to rotate each particle. A pair of electrodes, with an area of 25 × 35 mm2 and 20 mm spacing and held on a phenol resin support, was put in the suspension, and then an electric current was applied. The center of the magnetic field was at the center of the two electrodes. The magnetic field was maintained in the suspension during the EPD at a constant voltage of 30 V at room temperature. A palladium sheet was used as the cathodic substrate to absorb hydrogen produced by electrolysis of the solvent.27 A schematic illustration of the apparatus is shown in Fig. 2. The direction of the electric field relative to the magnetic field (the angle between the vectors E and B) was altered (0°, 45°, 90°) to control the dominant crystal faces. The sintering was conducted at a fixed temperature of 1873 K for 2 h in air out of the magnetic field. The density of the sintered compacts was measured by Archimedes’ method using kerosene. The degree of crystalline orientation of the specimen was characterized by x-ray diffraction (XRD) analysis. The microstructure was observed using a violet laser color 3D probe microscope (Keyence, Osaka, Japan, VK-9500). III. RESULTS AND DISCUSSION The use of a palladium substrate effectively suppressed the bubble formation at the cathode, and dense, bubble-free deposits were obtained. The green density of the deposits was approximately 60% of the theoretical density (TD) regardless of the deposition in and out of the magnetic field. The sintered density of the deposits at 1873 K for 2 h was approximately 97% of TD. The degree of crystalline orientation of the asdeposited specimen characterized by XRD analysis was highly improved with the grain growth during sintering.20 Figure 3 shows the XRD patterns of the ␣-alumina deposited at 10 T, followed by sintering at 1873 K for 2 h. The angle of E versus B was fixed at 0° (B-E ⳱ 0°) during the deposition. The XRD analysis was carried out for the cross-sectional planes that were parallel and perpendicular to the substrate. Hereafter, these planes were designated as the TOP and SIDE planes, respectively. To characterize the XRD peaks, the interplanar angles hkl between the planes (hkl) and the basal plane (00l) were calculated for a hexagonal unit cell of ␣-alumina (a ⳱ FIG. 1. Schematic diagram of the concept of the electrophoretic deposition in a strong magnetic field: (a) deflocculated suspension, (b) alignment of the particles in a strong magnetic field, and (c) electrophoretic deposition in the magnetic field. 1488 J. Mater. Res., Vol. 19, No. 5, May 2004 T. Uchikoshi et al.: Control of crystalline texture in polycrystalline alumina ceramics by electrophoretic deposition in a strong magnetic field FIG. 2. Schematic illustration of the EPD apparatus in a superconducting magnet. FIG. 3. XRD patterns of the ␣-alumina deposited at 10 T followed by sintering at 1873 K. The angle of E versus B was fixed at 0° (B-E ⳱ 0°) during the deposition. FIG. 4. XRD patterns of the ␣-alumina deposited at 0 T (external to the magnetic field) followed by sintering at 1873 K. 0.4758 nm, c ⳱ 1.2991 nm).28 The standard XRD data of ␣-alumina from ICDD29 with the hkl have been described in a previous paper.18 The diffraction peaks of the planes at low interplanar angles such as (006)(006 ⳱ 0°), (0012)(0012 ⳱ 0°), and (1010)(1010 ⳱ 17.5°) are characteristic of the TOP. In contrast, the diffraction peaks of the planes at high interplanar angles such as (110)(110 ⳱ 90°), (030)(030 ⳱ 90°), (220)(220 ⳱ 90°), and (211)(211 ⳱ 83.16°) are characteristic of the SIDE. The XRD data clearly show the crystallite orientation of the ␣-alumina prepared in the strong magnetic field of 10 T. It is also shown that the c axis is easily aligned along the magnetic field. Similar XRD results were observed when the angle of E versus B was fixed at 180° (B-E ⳱ 180°). Figure 4 shows the XRD patterns of the ␣-alumina deposited at 0 T (external to the magnetic field) followed by sintering at 1873 K for 2 h. No difference is observed between the XRD patterns of the TOP and the SIDE. It is obvious that the specimen prepared with no magnetic field has a randomly oriented polycrystalline structure. Figure 5 shows the variation in the XRD patterns of J. Mater. Res., Vol. 19, No. 5, May 2004 1489 T. Uchikoshi et al.: Control of crystalline texture in polycrystalline alumina ceramics by electrophoretic deposition in a strong magnetic field fixes the orientation of each particle; the c axis of ␣-alumina is aligned parallel to B in the suspension. When an electrical field is then applied to the oriented particles, they move along with the electric field lines retaining their orientation relative to the magnetic field lines and then deposit on the substrate. The degree of crystalline orientation was estimated from the intensity of the XRD peaks using the following equation30: FIG. 5. Changes in the XRD patterns of the TOP planes with the angle between the directions of B and E. The interplanar angles hkl between the planes (hkl) and the basal plane (00l) are also noted in the figure. the TOP planes with the angle between the directions of B and E (B-E). The hkl of the appeared peaks are also shown in the figure. When E is parallel to B (B-E ⳱ 0°), the diffraction peaks of the planes at low interplanar angles (hkl is close to 0°) are dominant. When B-E is changed to 45°, the dominant diffraction peaks change to the planes of the intermediate interplanar angles (hkl is close to 45°). Finally, when E is perpendicular to B (B-E ⳱ 90°), the dominant diffraction peaks change to the planes of high interplanar angles (hkl is close to 90°). This result is explained as follows. The magnetic field = 兺共Ihkl ⳯ hkl兲 Ⲑ 兺Ihkl , (3) where is the average orientation angle of the crystals of the TOP, and Ihkl is the intensity of the (hkl) reflection. The average orientation angles of the samples prepared at B-E ⳱ 0°, 45°, and 90° were 16.52°, 45.15°, and 84.90°, respectively. This result clearly shows that the dominant crystal faces on the TOP surface can be controlled by varying the angle of E versus B. When a charged particle moves across a magnetic field line (E × B ⫽ 0), the particle should be affected by the Lorentz force. However, its effect on the alignment of the particles seems to be not that strong. Figure 6 shows a schematic illustration and the crosssectional microstructure of a crystalline-textured alumina/alumina laminate composite prepared by alternately changing B-E ⳱ ±45° layer by layer during EPD in a magnetic field of 10 T. The crystalline orientation is changed at the interface of the two layers. Figure 7 shows the cross-sectional microstructure of a laminar composite prepared by alternately changing B-E ⳱ 0° and 45° layer by layer. Laminar composites with oriented and randomly oriented layers have been fabricated by the alternate EPD of the suspensions placed in and out of a superconducting magnet.21 FIG. 6. Microstructures of the cross-sectional plane of a crystalline-textured alumina/alumina laminate composite prepared by alternately changing B-E ⳱ ±45° during EPD in a magnetic field of 10 T. 1490 J. Mater. Res., Vol. 19, No. 5, May 2004 T. Uchikoshi et al.: Control of crystalline texture in polycrystalline alumina ceramics by electrophoretic deposition in a strong magnetic field the Ministry of Education, Culture, Sports, Science and Technology, Japan. REFERENCES FIG. 7. Microstructures of the cross-sectional plane of a crystallinetextured alumina/alumina laminate composite prepared by alternately changing B-E ⳱ 0° and 45° during EPD in a magnetic field of 10 T. Electrophoretic deposition in a strong magnetic field is an excellent method to fabricate crystalline textured ceramic thick bodies. This method can also be applied to prepare crystalline-oriented, or specified crystal face, thin films for functional applications. IV. CONCLUSIONS The magnetic field fixes the orientation of each particle; the c axis of ␣-alumina is aligned parallel to B in the suspension. When an electrical field is then applied to the oriented particles, they move along with the electric field lines while retaining their orientation against the magnetic field lines, and then deposit on the substrate. By varying the angle between the vectors E and B, the crystalline orientation in the bulk and dominant crystal faces at the surface can be controlled. ACKNOWLEDGMENTS The authors wish to thank Mr. Koji Kuramoto of the Keyence Co. for his help with the microscopic observations and Prof. Patrick S. Nicholson of McMaster University and Dr. Partho Sarkar of the Alberta Research Council for their valuable comments. This research was financially supported by the Hosokawa Powder Technology Foundation and the Budget for Nuclear Research of 1. E. Suvaci and G.L. Messing, J. Am. Ceram. Soc. 83, 2041 (2000). 2. K. Hirao, M. Ohashi, M.E. 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