Papers by Author: Yoshihiro Kuroiwa

Abstract: Domain structures and dynamics of BaTiO₃ single crystals under in-situ electric fields along <110>_c were investigated by using synchrotron radiation single-crystal X-ray diffractions. Diffraction patterns clearly show the presence of a 90 ^° domain structure in the crystals poled along <110>_c. The diffraction analysis provides direct evidence of a reversible change in the volume fractions of two kinds of the 90 ^o domains under unipolar in-situ electric fields. This reversible change in the domain structures under unipolar fields is suggested to originate from the interaction between spontaneous polarization and defect dipoles composed of acceptor and oxygen vacancy.

Abstract: Ferroelectric (Bi_0.5Na_0.5)TiO₃ single crystals were grown by a top-seeded solution growth method at a high oxygen pressure of 0.9 MPa and their polarization switching dynamics were investigated at ex-situ electric fields along <100>_c. Synchrotron radiation single-crystal X-ray diffraction analyses showed that a splitting of each reflection into four spots were clearly observed during polarization reversal. This splitting is associated with the polarization states with the spontaneous polarization vectors along four directions of equivalent <111>_c It is concluded that the polarization reversal is achieved through non-180 ° (71 ° and 109 °) domain switching in the BNT crystals along <100>_c.

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: 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: The structural, and dielectric and piezoelectric properties of 0.3BaTiO₃ - 0.1x (x= Bi (Mg_1/2Ti_1/2)O₃, Bi (Cu_1/2Ti_1/2)O₃, Bi (Cu_2/3Nb_1/3)O₃, (Bi_1/2K_1/2)(Cu_1/3Nb_2/3)O₃) 0.6BiFeO₃ ceramics were studied. The relative density of all the samples was more than 94 %. A single perovskite phase was obtained for the samples, and the crystal structure was pseudo-cubic. The Curie temperature of x=Bi (Mg_1/2Ti_1/2)O₃ and Bi (Cu_1/2Ti_1/2)O₃ system ceramics was higher than 400 °C. The apparent piezoelectric constant d₃₃* of x=Bi (Mg_1/2Ti_1/2)O₃, Bi (Cu_1/2Ti_1/2)O₃, Bi (Cu_2/3Nb_1/3)O₃ and (Bi_1/2K_1/2)(Cu_1/3Nb_2/3)O₃ system ceramics was 311, 212, 75 and 70 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: 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: Nucleation and particle growth conditions of barium titanate (BaTiO₃, BT) were investigated for preparation of the BT/strontium titanate (SrTiO₃, ST) multilayered nanoparticles. The conditions with and without BT nucleation were clarified. Epitaxial growth of the BT layer on the ST substrate particles was studied using both conditions. The formation of the BT layer on the ST substrate particles was confirmed using the condition with BT nucleation.

Abstract: Effect of the microstructural homogeneity of 0.5 BaTiO₃ - 0.5 KNbO₃ (0.5BT-0.5KN) solid solution ceramics on the dielectric and piezoelectric properties was investigated. Microstructure of a sample prepared by a conventional sintering method was homogenous, and the room temperature crystal structure was assigned to cubic Pm3m symmetry and therefore the sample was paraelectric. On the other hand, microstructure of samples prepared by a two-step sintering method was inhomogeneous, that is, it was made of BT and KN grains. The large electric field piezoelectric constant d₃₃* increased with increasing interface area.