Papers by Author: Hirofumi Kakemoto

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Authors: Takuya Hoshina, Hirofumi Kakemoto, Takaaki Tsurumi, Satoshi Wada, Masatomo Yashima, Kenichi Kato, Masaki Takata
Abstract: The crystal structures of barium titanate (BaTiO3) fine particles with a size around 140 nm were investigated using a synchrotron radiation X-ray diffraction method. The observed diffraction pattern was analyzed by Rietveld method assuming several models. As a result, it is found that the BaTiO3 fine particles have composite structures include (a) a tetragonal region with a constant c/a ratio, (b) a tetragonal region with gradient c/a ratios and (c) a cubic region. In order to estimate these structures, the partial profile relaxation technique was applied in the Rietveld refinement. It is considered that the composite structure is important for the size effect on BaTiO3 fine particles.
Authors: Kayo Takizawa, Takuya Hoshina, Hirofumi Kakemoto, Takaaki Tsurumi, Yoshihiro Kuroiwa, Satoshi Wada
Abstract: Barium titanate (BaTiO3) fine particles were prepared using the 2-step thermal decomposition method of barium titanyl oxalate. At the 2nd step of this method, the intermediate compound (Ba2Ti2O5·CO3) was decomposed into BaTiO3 and CO2 under various degrees of vacuum pressure. As a result, the particle size of prepared BaTiO3 nanoparticles decreased with decreasing pressure. Moreover, the dielectric constants of these BaTiO3 nanoparticles were measured using the powder dielectric measurement method using slurry. The dielectric constant of BaTiO3 nanoparticles increased with decreasing pressure at the same particle size. It is considered that mesoscopic particle structure controlled by vacuum pressure is important for the dielectric properties of BaTiO3 nanoparticles.
Authors: Satoshi Wada, Koichi Yako, Tomomitsu Muraishi, Hirofumi Kakemoto, Takaaki Tsurumi
Abstract: For the [111] oriented barium titanate (BaTiO3) single crystals, the patterning electrode was applied to induce the finer engineered domain configurations with domain size of 3 2m. The poling treatment was performed at 134 °C under electric fields below 6 kV/cm to inhibit the burning of the patterning electrode with photoresist. As the results, the gradient domain sizes from 3 to 8-9 2m were induced into the 31 resonator. The d31 was measured at -243.2 pC/N, and this value was almost 70 % of the expected d31 of –337.7 pC/N for the resonator with domain size of 3 2m. This difference was originated from lower applied electric field below 6 kV/cm. However, this study was revealed that the patterning electrode was very powerful tool to induce much finer domain sizes below 5 2m.
Authors: Michiyasu Nishiyama, Takuya Hoshina, Hirofumi Kakemoto, Takaaki Tsurumi, Satoshi Wada
Abstract: A new method for ultrafine barium titanate (BaTiO3) particles with diameters of around 5 nm is proposed. In this method, barium titanyl oxalate aqua solution with low concentration below 10-3 mol/l was used as the starting material. The droplets with a size below 3 μm were atomized with an ultrasonic vibrator, dried and thermally decomposed at higher temperatures over 300°C. In the preparation of the BaTiO3 particles, there were two parameters such as thermal decomposition temperature and precursor solution concentration. Therefore, various particles were prepared by changing these parameters. Finally, non-aggregated nm-sized BaTiO3 particles with an average diameter of 5.2 nm, despite wide size distribution from 2 to 20 nm, were prepared by using the precursor solution with 10-6 mol/l.
Authors: Takuya Hoshina, Hirofumi Kakemoto, Takaaki Tsurumi, Satoshi Wada
Abstract: Barium titanate (BaTiO3) nanoparticles with various particle sizes from 20 to 430 nm were prepared using a 2-step thermal decomposition method. Powder dielectric measurement clarified that dielectric constant of BaTiO3 particles with 140 nm exhibited a maximum around 5,000. To explain this high dielectric constant, THz-region dielectric properties of BaTiO3 nanoparticles, especially Slater transverse optic (TO) mode frequency, were estimated using the far infrared (FIR) reflection method. As the result, it was found that the Slater TO mode of BaTiO3 particles with 140 nm exhibited a minimum. Therefore, the high dielectric constant around 5,000 at 140 nm can be originated from the softening of the Slater TO mode.
Authors: Takashi Sekine, Takuya Hoshina, Song Min Nam, Hiroaki Yasuno, Hirofumi Kakemoto, Satoshi Wada, Takaaki Tsurumi
Abstract: Dielectric permittivity of BaTiO3 powders was determined by measuring permittivity of slurries prepared from BaTiO3 powder and propylene carbonate as dispersion medium followed by analyzing with a finite element methods (FEM). The permittivity of powders depended on their characters such as tetragonality (c/a ratio), density, particle size and specific surface area. In order to extract the effect of each character, a statistical analysis was carried out to represent the permittivity of powders with an empirical equation where the c/a ratio, density and particle size were used as parameters. A fairly good agreement between observed data and those estimated from the relation was obtained and the contribution of each character to the permittivity could be determined.
Authors: Jianyong Li, Hirofumi Kakemoto, Song Min Nam, Satoshi Wada, Takaaki Tsurumi
Authors: Jianyong Li, Hirofumi Kakemoto, Satoshi Wada, Takaaki Tsurumi
Abstract: A new measuring method and analyzing procedure were proposed to determine the complex dielectric permittivity of materials with relatively high permittivity using an RF-impedance analyzer. Samples used for the measurement were (Ba0.6Sr0.4)TiO3 and Ba(Zr0.25Ti0.75)O3 ceramics. Micro planar electrodes were used for the measurement of complex admittance of these samples. Electromagnetic simulations were carried out for determining the relative dielectric permittivity and dielectric loss. The complex dielectric permittivity vs. frequency curves of Ba(Zr0.25Ti0.75)O3 showed a broad dielectric relaxation, while that of (Ba0.6Sr0.4)TiO3 was almost flat up to 3 GHz.
Authors: Keisuke Yokoh, Song Min Nam, Hirofumi Kakemoto, Takaaki Tsurumi, Hirohiko Kumagai, Satoshi Wada
Abstract: For potassium niobate (KNbO3) single crystal, the 31 resonators with the highest piezoelectric constant d31 were designed using transformation of axis. We confirmed that the engineered domain configurations with maximum d31 of –55.1 pC/N was caused by a combination between two polarization with polar directions along [101]c and [-101]c directions. Moreover, if there are larger piezoelectric constants from domain wall region, we can expect the much higher piezoelectric properties. To induce the above domain configuration, a new poling method using patterning electrode was investigated. In this study, the two methods on the basis of temperature-induced phase transition at 207 °C and electric-field-induced phase transition at room temperature were investigated.
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