J ournal J. Am. Ceram. Soc., 89 [1] 381–384 (2006) DOI: 10.1111/j.1551-2916.2005.00694.x r 2005 The American Ceramic Society Preparation of Grain-Oriented Sr 0.5 Ba 0.5 Nb 2 O 6 Ferroelectric Ceramics by Magnetic Alignment Weiwu Chen, w Yoshiaki Kinemuchi, and Koji Watari* Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan Takuya Tamura and Kenji Miwa Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan The properties of polycrystalline ceramics are strongly influ- enced by their crystallographic texture. In this study, highly grain-oriented tungsten bronze structure ferroelectric ceramics, Sr 0.5 Ba 0.5 Nb 2 O 6 , were successfully fabricated by magnetic align- ment and gelcasting techniques using only the conventional sol- id-state-synthesized starting powder. Spherical Sr 0.5 Ba 0.5 Nb 2 O 6 particles were aligned according to their anisotropic magnetic property in 40 vol% slurry in a 10 T magnetic field, and then in situ locked by polymerization via a gelcasting technique for 30 min. A /00lS-axis orientation perpendicular to the magnetic field direction (B) was obviously observed in the green compact and sintered sample. The sintered Sr 0.5 Ba 0.5 Nb 2 O 6 sample con- tained equiaxial grains and reached 98% theoretical density. Compared with the sample with randomly oriented grains, the magnetically aligned sample showed an enhanced with dielectric constant in the > B direction (1100 versus 750 at room tem- perature and 4300 versus 2800 at Curie temperature). This new method is readily applicable to other ceramics with tungsten bronze structure, and is expected to facilitate mass preparation of large and dense grain-oriented ceramic materials. I. Introduction G RAIN orientation of polycrystalline ferroelectric ceramics offers a means of achieving single crystal-like properties. 1 A mechanical force is normally applied to prepare grain-orient- ed ceramics using hot-forging, hot-pressing, and tape-casting processes. 2–9 In these processes, grain orientation was controlled by a high pressure at a high temperature, or by shear force dur- ing green compact formation. In general, crystalline materials with non-cubic structure have more or less anisotropic magnetic properties, offering an alter- native particle alignment method to use the magnetic force. In a magnetic field, a particle with an anisotropic magnetic suscep- tibility tends to rotate at an angle in order to minimize the system energy, DE 5 DwVB 2 /2m 0 , where Dw is the anisotropic magnetic susceptibility, V is the particle volume, B is the applied magnetic field, and m 0 is the permeability in vacuum. 10 This is the driving force for magnetic alignment. For materials with high anisotropic magnetic properties, the particles are easily magnetically aligned. Some grain-oriented ferromagnetic and superconductor materials have been prepared successfully by magnetic alignment even in high viscosity resin. 11,12 But for most of the existing structural and electronic ceramics with weak anisotropic magnetic properties, the magnetic driving force is very low and the particle rotation is greatly inhibited by gravity, steric hindrance, thermal motion, and so on, even in a strong magnetic field. According to a previous report, the anisotropic energy of a single-crystalline Al 2 O 3 particle with 0.2 mm diam- eter in a 10 T magnetic field was estimated to be around 8.78  10 21 J. This value is only two times its thermal motion energy KT at 300 K (4.14  10 21 J). 13 Dispersion of particles in a low-viscosity slurry can facilitate their rotation under the magnetic force. But obtaining a dense ceramic compact without damaging the grain orientation is also a challenge. Slip casting has been examined in the preparation of grain-oriented Al 2 O 3 , ZnO, Si 3 N 4 , and AlN. 14–17 But this proc- ess introduces extra force, i.e., suction force or capillary pressure force, which influences the particle rotation and degrades the uniformity of the resulting sample. 18 Gelcasting is a novel tech- nique to prepare large and complex-shaped ceramic compo- nents. 19 A typical gelcasting process consists of dispersing the ceramic powders in a solution containing organic monomers, casting the slurry in a mold, and initiating a polymerization re- action to form a gelled body. Temperature and catalyst are used to adjust the polymerization rate. Gelcasting provides an approach to in situ lock the rotated particles after magnetic alignment. 20 Sr x Ba 1x Nb 2 O 6 (SBN, where x 5 0.25–0.75) ferroelectric ma- terials with a tungsten bronze structure have excellent pyroelec- tric, linear electric–optic coefficients, photo-refractive effects, and interesting piezoelectric properties. 21,22 Because SBN crys- tals have obvious anisotropic properties, researchers have tried to prepare grain-oriented SBN ceramics by hot pressing 23 and TGG. 5 However, magnetic alignment has never been examined for this material. In this study, grain-oriented Sr 0.5 Ba 0.5 Nb 2 O 6 (SBN50) was explored by magnetic alignment via a gelcasting forming tech- nique. To simplify the process, only conventionally solid-state- synthesized SBN50 powder was used. The grain orientation and anisotropic dielectric property of the resulting samples were characterized. II. Experimental Procedure (1) Starting Powders Stoichiometric amounts of chemically pure BaCO 3 (Wako Ltd., Osaka, Japan), SrCO 3 (Wako Ltd.), and Nb 2 O 5 (Wako Ltd.) were mixed by a planetary mill for 1 h and then calcined at 12501C for 4 h at a heating rate of 41C/min. The obtained SBN50 powder was ball- milled for 1 h. 381 R. Cloots—contributing editor This work was financially supported by Japan Society for the Promotion of Science (JSPS). *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: Weiwu.chen@aist.go.jp Manuscript No. 20538. Received May 8, 2005; approved July 11, 2005.