Dielectric Constant of Barium Titanate Synthesized by Containerless Processing
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Dielectric Constant of Barium Titanate Synthesized by Containerless Processing
Jianding Yu, Paul-François Paradis, Takehiko Ishikawa, and Shinichi Yoda Japan Aerospace Exploration Agency, ISS Science Project Office, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan
ABSTRACT
Containerless processing is an attractive synthesis technique that permits deep undercooling and provides the possibility to solidify the undercooled liquid into a selected phase, and to synthesize materials with novel properties. Spheroidal BaTiO3 samples with a diameter of approximately 2mm were solidified by containerless processing, using an electrostatic levitation apparatus. Single crystal hexagonal BaTiO3 and polycrystalline perovskite BaTiO3 were successfully synthesized at different undercoolings levels. An oxygen-deficient single crystal of hexagonal BaTiO3 obtained with this method, exhibited a giant permittivity higher than 100000, with a loss component tanδ of about 0.1 at room temperature. The permittivity showed weak temperature dependence in the 70 K to 300 K range, and a dramatic drop by 2 orders of magnitude below 70 K. In comparison, the polycrystalline perovskite BaTiO3 showed a permittivity of 4000 at room temperature.
INTRODUCTION
Since the discovery of the ferroelectric perovskite BaTiO3 (p-BaTiO3), considerable efforts have been devoted to improving the synthesis technique employed to obtain this material with optimized dielectric properties. BaTiO3 exhibits two structural groups: the extensively investigated ferroelectric p-BaTiO3 and the hexagonal modification (h-BaTiO3) [1, 2]. The h-BaTiO3 structure normally exists above 1703 K and can be stabilized to room temperature as a metastable phase. Below room temperature, it undergoes successive structural phase transitions, first into an orthorhombic structure at T0 = 222 K, followed at Tc =
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74 K [3, 4] by a monoclinic phase exhibiting ferroelectric properties. Compared to the wellknown p-BaTiO3, only a few physical properties of h-BaTiO3 were reported in literature [3 5]. Until recently, the h-BaTiO3 ceramics were typically synthesized by re-sintering the perovskite at above 1703 K. However, the resulting material exhibits high porosity. In the present study, we report the containerless solidification of BaTiO3 using a pressurized electrostatic levitation furnace (PELF). Containerless processing is an attractive synthesis technique: it permits deep sample undercooling of molten materials, and has the advantage that the catalytic sites for heterogeneous nucleation are minimized. Thus, it provides the possibility to solidify the undercooled liquid into a selected phase, resulting into materials with higher density and novel transport properties. The technique also offers a new approach to growing single crystals from undercooled liquid, following a report on single crystal growth using containerless processing [6].
EXPERIMENTAL DETAILS
High purity commercial BaTiO3 powder was compressed into rods at 200 MPa, and sintered at 1527 K. Spheroidal samples
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