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Size Dependence of Dielectric Properties for nm-sized Barium Titanate Crystallites and Its Origin
S. Wada,H. Yasuno,H. Kakemoto,M. Yashima,S.-M. Nam,T. Tsurumi,T. Hoshina 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.46 No.1
Barium titanate (BaTiO3) crystallites with various particle sizes from 17 to 1,000 nm were prepared by the modiffed 2-step thermal decomposition method. Investigation of impurities in these particles by using both TG-DTA and FT-IR measurements revealed that no impurity was detected in the BaTiO3 lattice, while hydroxyl and carbonate groups were detected only on the surface. Moreover, their relative densities were always above 99 %.The dielectric constants of these powders were measured by using slurries by a modied powder dielectric-measurement method. As a result, the dielectric constant of BaTiO3 particles with a size of around 140 nm exhibited a maximum of around 5,000. By using a synchrotron-radiation X-ray powder experiment, the crystal structure of the BaTiO3 particles with sizes below 100 nm was investigated. As a result, in the BaTiO3 particles with sizes over 40 nm, it was conrmed that their crystal structure was assigned to 4 mm. In the particle size with a maximum dielectric constant of 5,000, its c=a ratio was smaller than 1.011. Finally, to explain the origin of the high dielectric constant, a model related to \superparaelectric behavior" was proposed.
Satoshi Wada,H. Kakemoto,K Yokoh,T Tsurumi,T Muraishi 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.2I
The phase transition behaviors of [111]-oriented barium titanate (BaTiO3) crystals were investigated as functions of temperature, uniaxial stress, and electric field. For the phase transition caused by temperature, with decreasing temperature above Tc, the paraelectric phase changed to an intermediate phase with a superparaelectric state and finally changed to a ferroelectric phase with randomly oriented spontaneous polarizations. Moreover, the phase transition caused by the uniaxial stress field above Tc was found to be almost similar to the one caused by temperature. On the other hand, for the phase transition caused by an electric field above Tc, with increasing electric field, the paraelectric phase changed to an intermediate phase and finally to a ferroelectric phase with an oriented polar direction. These results suggest that above Tc, a combination of uniaxial stress with an electric field may be effective as a poling treatment for BaTiO3 crystals. Thus, in this study, a new poling method for the BaTiO3 crystals by using control of the temperature, the uniaxial stress, and the electric field is proposed.t
Piezoelectric Properties of Potassium Niobate Single Crystals by Domain Engineering
S. Wada,H. Kumagai,H. Kakemoto,K. Muraoka,T. Tsurumi 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.46 No.1
Potassium niobate (KNbO3) single crystals were grown by a TSSG method. At rst, the electric eld was applied along the [001]c (cubic notation system) direction of KNbO3 crystals to induce the engineered domain congurations into KNbO3 crystals. Prior to domain engineering, the piezoelectric properties of [001]c-oriented KNbO3 single-domain crystals were measured. These measurement values were completely consistent with the calculated apparent d31 and d32. Finally, the engineered domain congurations were induced into KNbO3 crystals. As a result, piezoelectric properties increased with decreasing domain sizes of the engineered domain conguration. However, the symmetry of the KNbO3 crystals was mm2, and there were four kinds of domain structures such as 90,180, 60 and 120 domains. Thus, the engineered domain structure induced in this study was very complicated, and the piezoelectric properties were also ependent on domain pattern and kinds of domain walls.
Origin of Ultrahigh Dielectric Constants for Barium Titanate Nanoparticles
Satoshi Wada,C Moriyoshi,H. Yasuno,K Kakemoto,K Takizawa,M Ohishi,T Hoshina,T Tsurumi,Y Kuroiwa 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.2I
Barium titanate (BaTiO3) nanoparticles with various particle sizes from 17 to 1,000 nm were prepared by using the 2-step thermal decomposition method of barium titanyl oxalate under various degree of vacuum. Various characterizations revealed that these particles were impurity-free, defectfree, dense BaTiO3 nanoparticles. When the degree of vacuum was high (pressure of 150 Pa at 650 C), the dielectric constant of BaTiO3 particles with a size of around 60 nm exhibited a maximum of around 15,000. On the other hand, when the degree of vacuum was low (pressure of 400 Pa at 650 C), no dielectric maximum was observed. To explain this size dependence, we precisely investigated a particle structure by using synchrotron radiation. As a result, the particles were always composed of two layers, i.e., a surface cubic layer and a bulk tetragonal layer, and the thickness of the surface cubic layer decreased with increasing degree of vacuum during the preparation of BaTiO3 nanoparticles. Thus, we confirmed that the surface structure was an important factor in determining the dielectric properties of BaTiO3 nanoparticles.