Microwave dielectric properties of oriented BaLa 4 Ti 4 O 15 ceramics fabricated by templated grain growth
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Microwave dielectric properties of oriented BaLa4Ti4O15 ceramics fabricated by templated grain growth
Yuko Fukami, Kensuke Wada, Ken-ichi Kakimoto and Hitoshi Ohsato Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
ABSTRACT
Dense and highly textured BaLa4Ti4O15 (BLT) ceramics was fabricated by templated grain growth method. Plate-like BLT particles prepared via a molten salt synthesis using NaCl flux were mixed with BLT powders obtained by the conventional solid state method. X-ray diffraction and scanning electron microscope studies of the textured ceramics showed plate-like BLT grains aligned parallel to the casting direction. Large anisotropy was found in the dielectric properties.
INTRODUCTION
BanLa4Ti3+nO12+3n homologous series have been investigated [1-3] as a one of candidate materials for base station of telecommunications. The composition with n=1, i.e., BaLa4Ti4O15 (BLT) has excellent microwave dielectric properties [4] with high dielectric constant εr=45.22 and high quality factor Q·f=42,862 GHz. However the temperature dependence of resonant frequency (τf) is –15.68 ppm/oC [5] which is relatively high and hence precludes its immediate use in practical applications. Usually, τf is improved by changing the composition such as formation of solid solution phases or additives with opposite τf for the compensation. In most cases, it can cause severe degradation to Q·f value and/or εr even though τf is improved [6]. Templated Grain Growth (TGG) is a novel approach to obtain single crystal-like properties. In TGG, texturing is realized from an anisotropic grain growth of an anisotropic particle, called as “template”, with an alignment in the equiaxed matrix phase. Materials with large anisotropy in the crystal structure can exhibit single crystal-like properties by TGG. Many investigations have previously shown that ferroelectric ceramics with a high degree of grain orientation possess directionally dependent dielectric, pyroelectric, and piezoelectric properties like those of a single crystal [7,8]. Therefore, TGG method has been tried in many materials to improve their properties. We have confirmed that sintering of Ba6-3xR8+2xTi18O54 (R: rare earth) with the columnar particles lead to anisotropic grain growth, and it improved their microwave dielectric properties. In the case of R=Sm with x=2/3, Ba4R9.33Ti18O54 obtained 0 ppm/oC of τf [9] by templated growth method. BLT has a layered perovskite-type trigonal crystal system with lattice parameters a=5.572 Å
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and c=22.48 Å. Hence BLT crystals tend to grow plate-like in shape. Fukami et al [5] prepared hexagonal plate-like anisotropic BLT crystals with an average diameter of approximately 13 µm and a thickness of 1 µm via a molten salt synthesis. Uniaxially pressed ceramics with the anisotropic crystal demonstrated the preferred (00l) orientation and anisotropic grain growth [5]. In this paper we report the fabrication of highly grain-oriented BLT ceramics by TGG
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