Control of crystalline texture in polycrystalline alumina ceramics by electrophoretic deposition in a strong magnetic fi
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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
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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 i
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