| Research Abstract |
A new preparation method was proposed to prepare impurity-free, defect-free barium titanate (BaTiO_3) nanoparticles using barium titanyl oxalate. This method was called a 2-step thermal decomposition method, and composed of the following two steps ; i.e., (1)1^<st> step : a preparation of intermediate compound of Ba_2Ti_2O_5CO_3 by thermal decomposition of barium titanyl oxalate in air and (2) 2^<nd> step : a preparation of BaTiO_3 nanoparticles via thermal decomposition of Ba_2Ti_2O_5CO_3 in vacuum. As the results of characterization using many kinds of measurements such as density, impurity, TG-DTA, FT-IR, Raman, TEM observation, Raman and so on, it was cleared that these BaTiO_3 nanoparticles were defect-free, impurity-free, and dense. However, there was no way to measure dielectric constant of particles. Thus, a new measurement method for powder dielectric constants was newly developed. The powder dielectric measurement of BaTiO_3 fine particles with sizes from 17 to 1,000 nm revea
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led the maximum dielectric constant at a certain particle size. Moreover, the sizes with maximum dielectric constants were strongly dependent on the preparation methods. When the BaTiO_3 fine particles were prepared using the original 2-step thermal decomposition method, a dielectric constant maximum of 15,000 was observed at 70 nm particle size. On the other hand, when the BaTiO_3 fine particles were prepared using the modified 3-step thermal decomposition method, a dielectric constant maximum of 5,000 was observed at 140 nm. To confirm the validity of these high dielectric constants, particle/polymer composite using these particles was prepared and these dielectric properties were measured. As the result, the particle/polymer composite using the high dielectric constant showed always much higher dielectric constants than those prepared using the normal dielectric constant of 3,000. The former BaTiO_3 was prepared in vacuum of 10^<-2> torr while the latter BaTiO_3 was prepared in air. Structure refinement of BaTiO_3 particles using a Rietveld method revealed that all of BaTiO_3 particles were always composed of two parts ; (a)surface cubic layer and (b)bulk tetragonal layer. Moreover, a thickness of surface cubic layer for BaTiO_3 nanoparticles prepared in vacuum of 10^<-2> torr was much thinner than that for BaTiO_3 nanoparticles prepared in air. Thus, to explain these differences, a new model on the basis of "surface relaxation" was proposed. Moreover, to measure phonon state of these particles, 3D colloidal crystals with a smooth surface were prepared, and their far infrared reflection spectra were measured. To date, there was no way to measure phonon state of nanoparticles except for Raman scattering, but using the 3D colloidal crystals, the clear far infrared reflection spectra were obtained. By the analysis of these spectra, soft mode frequency was determined for various BaTiO_3 nanoparticles. As the result, it was cleared that the dielectric constant of BaTiO_3 particles increased with decreasing soft mode frequency. Moreover, the decrease of soft mode frequency was strongly related to the thickness of surface cubic layer. Less
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