Papers by Author: Nobuhiro Kumada

Abstract: Barium titanate (BaTiO₃, BT) grain-oriented ceramics were prepared by electrophoresis deposition (EPD) method under high magnetic field (HM-EPD) of 12 T. For this objective, the BT single-domain nanoparticles with high c/a ratio of 1.008 and size of 103 nm were prepared by two-step thermal decomposition method. Using the BT nanoparticle slurry, BT nanoparticle accumulations were prepared by EPD or slipcasting (SC) methods with/without high magnetic field. After binder burnout, these accumulations were sintered at 1350 °C and it was revealed that only the BT ceramics prepared by the HM-EPD method was assigned to grain-oriented ceramics with weak preferential crystallographic orientation along [11 direction.

Abstract: Barium titanate (BaTiO₃) nanoparticles were prepared by a 2-step thermal decomposition method using nanooxalate particles. BaTiO₃ nanoparticles were characterized using various methods. Accumulations were prepared by electrophoresis deposition method using the obtained BaTiO₃ nanoparticles. They were performed necking structure by a solvothermal method. Polymer was infiltrated into the accumulations in vacuum to prepare a polymer film capacitor.

Abstract: 1-x-y)BaTiO₃-xBi (Mg_1/2Ti_1/2)O₃-yBiFeO₃ ceramics were prepared by a conventional solidstate synthesis, and crystal structure and electric properties were investigated. Synchrotron X-ray diffraction pattern revealed that a single perovskite phase was obtained for samples at x=0.05-0.15, y=0.46.5-73.5. Splitting of the (111) peak was observed when (x, y) = (0.050, 0.685) and (0.050, 0.735) indicating that crystal structure of samples changed from a pseudo-cubic phase to a rhombohedral phase with increasing BF. As the BF content increased, the Curie temperature increased. The largest maximum strain (S_max) over the applied maximum electric field (E_max) value (S_max / E_max) of 300 pm/V was observed for 0.365BT-0.050BMT-0.585BF at 25 °C and 1Hz with the Curie temperature of 390 °C.

Abstract: Bismuth and niobium based perovskite oxides with non-integer A and B site valences were prepared by a solid-state route. For the A^1+xB^5xO₃ type (Bi_x/2K_1-x/2)(Ti_xNb_1-x)O₃ (x=0.1 0.9) ceramics (the examples of samples studied), the Curie temperature increased from approximately 250 to 350 °C with increasing x from 0.5 to 0.9. These results were discussed with the Abrahams relations.

Abstract: Highly dispersed barium titanate (BaTiO₃, BT) nanoparticles were prepared by the new 2-step thermal decomposition method of barium titanyl oxalate of 30 nm in size. The nanoparticles were heated at 450 °C for 5 hours in air to yield intermediate product: Ba₂Ti₂O₅CO₃. Highly dispersed BaTiO₃ nanoparticles were prepared by rotationally stirring it at the reduced pressure of 0.2 Pa at various temperatures between 800 °C and 900 °C. The particle size and morphology of the BaTiO₃ nanoparticles were investigated by X-ray diffraction and scanning electron microscopy. These measurements showed that the BT nanoparticles were highly dispersed and well-crystallized.

Abstract: Barium titanate (BaTiO₃, BT) - potassium niobate (KNbO₃, KN) nanostructured ceramics with artificial morphotropic phase boundary (MPB) structure were successfully prepared at temperatures below 230 °C by solvothermal method. Various characterizations suggested that the BT-KN nanostructured ceramics exhibited a porosity of around 30 % and heteroepitaxial interface between BT and KN. Their dielectric and piezoelectric properties were measured at room temperature, and the dielectric constant and apparent piezoelectric constant estimated from slope of strain and electric field curve was 370 and 136 pm/V, respectively.

Abstract: Barium titanate (BaTiO₃, BT)-bismuth magnesium titanate (Bi (Mg_1/2Ti_1/2)O₃, BMT)-bismuth ferrite (BiFeO₃, BF) solid solution ceramics were prepared using a conventional solidstate synthesis, and their piezoelectric properties and microstructure were investigated. Strain electric field curves of the 0.3BT-0.1BMT-0.6BF ceramics with a single perovskite phase were ferroelectric butterfly-like curves. A strain maximum / electric field maximum (S_max/E_max) was 330 pm/V. Transmission electron microscopy revealed ferroelectric-like domain structure in the 0.3BT-0.1BMT-0.6BF ceramics.

Abstract: Barium titanate (BaTiO₃, BT) nanoparticle accumulations were prepared by an electrophoresis deposition (EPD) method, and their dielectric properties were measured. The BTKK accumulation exhibited high dielectric constant of 100. To prepare the BT ceramic/polymer film capacitors, the polymer was injected into the accumulations in vacuum. Finally, dielectric properties were measured, and it was clarified that the dielectric constant of the ceramic/polymer film capacitors became smaller than those of the accumulation because of surfactant used as particle dispersion.

Abstract: In this study, 0.8 KNbO₃ (KN) -0.2 BaTiO₃ (BT) ceramics were prepared using KN powder with the diameter of 100 nm and BT powders with the diameter of 100, 300, or 500 nm. The relative densities were more than 96 % of the theoretical densities of the samples. The dielectric constant of the samples was temperature-stable at temperatures between-50 and 300 °C and it was found that the dielectric constant of the 0.8KN-0.2BT ceramics increased as the BT diameter increased.

Abstract: Grain-oriented Barium titanate (BaTiO₃) ceramics were prepared using an electrophoresis deposition (EPD) method under high magnetic field of 12 T and sintering at 1350 °C. It was found that BT03 and BTKK nanoparticles were necessary to synthesize the grain-oriented BT ceramics. The grain orientation was correlated with particles size and c/a ratio of lattice constants. The reason why the grain-oriented BT ceramics using the BT03 and BTKK were prepared was having the large values of the particles size and the c/a ration of lattice constants.