Hydrothermal synthesis of dy-doped BaTiO 3 powders
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NTRODUCTION
CERAMIC compositions (pure and doped) of BaTiO3 have been one of the main constituents of the “type II” dielectric materials and of multilayer ceramic capacitors. It has been a widely acclaimed conception since the early 1960s that the dielectric properties of these ceramics are closely related to the grain size in their microstructures. Yamaji and co-workers[1,2,3] have demonstrated the strong effect of dysprosium (Dy) in reducing the sintered grain size of BaTiO3; however, they used conventional solid-state reactive firing routes in their synthesis experiments. On the other hand, pure BaTiO3, depending on the synthesis method used and temperature, may exhibit four different polymorphic forms.[4] Among these four polymorphs of BaTiO3, the pseudocubic form (space group: Pm-3m) is “paraelectric,” and the other three (tetragonal: P4mm, orthorhombic: Pmm2, and rhombohedral: R3m) are “ferroelectric.” BaTiO3 is of the tetragonal symmetry from room temperature up to its Curie temperature (TC: ,128 8C), and, above TC, it adopts the cubic symmetry. When the ambient temperature is below TC, BaTiO3 is ferroelectric, and when the temperature is above TC, it becomes paraelectric.[5] The change observed in the crystal structure of barium titanate at its TC could also be observed by the significant change (from about 1000 to 11,000) which simultaneously occurs in its dielectric constant. It has also been shown that the average grain size in the microstructure of BaTiO3 turns out to be quite influential on its dielectric constant.[1–3,6] It is now a wellestablished fact that the decrease to be achieved in the average grain size (from 50 to ,1 mm) of the sintered ceramic microstructure of pure BaTiO3 would show itself up in the form of an increase in its dielectric constant (room temperature ) from 1000 to about 5000,[7] whereas, for Dy-doped BaTiO3 samples, it would increase to about 10,000.[1] For this reason, it has been an important concern in the synthesis of BaTiO3 ceramics that any precautions which would be ERSIN E. OREN, Research Assistant, is with the Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara 06531, Turkey. A. CUNEYT TAS, Associate Professor, formerly with the Department of Metallurgical and Materials Engineering, Middle East Technical University, is with the Max-Planck-Institute, Stuttgart, Germany D-70569. Manuscript submitted December 1, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
exercised toward the precise control to be gained over the particle (presintering) and grain (postsintering) sizes would directly influence the electronic properties of the final product. The preparation of monosize and crystalline BaTiO3 powders by using hydrothermal synthesis has been known for a long time. This process commonly uses the starting materials of water-soluble inorganic Ba(OH)2?8H2O salts and insoluble TiO2 powders, to be mixed in an aqueous solution kept at a temperature near its boiling point for prolonged times. Hydrothermal synthesis involves the format
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