Papers by Author: Yuuki Sato

Abstract: Electrophoretic deposition (EPD) is one of useful methods for the preparation of the thin film with homogeneous microstructure on a conductive substrate. In the EPD method, the structure of the particle thin film could be controlled by adjusting the electrical operating conditions. Titania nanoparticle (NP) films, which are used for the electrode of dye-sensitized solar cells (DSSCs), require not only the homogeneous microstructure but also controlled pore size distribution, contributing to high-rate transport of electrons for the high conversion efficiency of DSSCs. In this study, titania NP films were prepared by EPD under DC constant-current conditions using available NPs dispersed in ethanol. The thickness as well as the weight of the titania NP film appeared to be increased almost linearly with EPD operation time, while the porosity of the film calculated from those values was not always constant but increased slightly with the operation time and asymptotically reached about 60%. We confirmed that the forces on the particles depositing onto the substrate became weaker as the EPD operation time increased, due to the electrostatic charges gradually building up on the thin NP film with particle deposition. The deposition behavior of titania NPs was drastically changed upon varying the water content in ethanol. We detected many pinholes on the surface of thin NP films when the water content in ethanol increased. The amount of particles accumulated on the substrate via EPD could be calculated based on the electrical conductivity of the suspension and the mobility of particles in the suspension, but was found to be underestimated when the water content increased.

Abstract: The effects of the addition of tin oxide (SnO₂) and yttrium oxide (Y₂O₃) to bismuth (Bi)-manganese (Mn)-cobalt (Co)-silicon (Si)-chromium (Cr)-nickel (Ni)-added zinc oxide (ZnO) varistors (a basic varistor) on the varistor voltage, resistance to electrical degradation, and leakage current were investigated. The addition of SnO₂ increased both the varistor voltage and the resistance to electrical degradation. However, simultaneous addition of both SnO₂ and Y₂O₃ increased the varistor voltage but the resistance to electrical degradation deteriorated. ZnO varistors with varistor voltage over approximately 520 V/mm, excellent resistance to electrical degradation, and low leakage current could be obtained by adding SnO₂ with SnO₂-to-ZnO molar ratio of approximately 1:10 to the basic varistor.

Abstract: Fabrication of nanoporous titanium dioxide (TiO₂) films was examined for their application as the negative electrode of a dye-sensitized solar cell (DSC). Composite films were fabricated by aerosol deposition using a powder mixture of TiO₂ and aluminum nitride (AlN). A nanoporous structure was subsequently formed in the film by dissolving the AIN in hot water. Remarkable differences in the surface morphology of the films were observed for different mixing ratios of TiO₂ and AlN particles. AlN particles remained in the films, but not at the surface. The power conversion efficiency of a DSC was improved by incorporating these nanoporous TiO₂ films.

Abstract: Films of titanium dioxide (TiO₂) were deposited by aerosol deposition at room temperature. Anatase TiO₂ powders were used. The crystal structure of these films was a mixture of the anatase and brookite TiO₂ phases, while the rutile TiO₂ phase was observed at high carrier gas flow rates. The TiO₂ film thickness increased in proportion with the scan time and the carrier gas flow rate. Film thickness was constant at constant total deposition times. Flake-like and mesh-like surface morphologies were observed on the films. The direct-current resistivity was estimated from the dielectric constant and the loss tangent; it was found to be much smaller than that of bulk TiO₂. We speculate that this low resistivity is due to reduction of TiO₂ particles during room-temperature impact consolidation in a N₂ atmosphere.

Abstract: The mechanism of pryoelectric-induced X-ray emission in LaTiO₃ crystals is discussed. It is suggested that electrons which contribute to the X-ray emission were generated around and close to the crystal and were emitted from the z surface. The poor reproducibility was found to be due to a creeping electrostatic discharge. It is speculated that one of the factors of the discharge is the accumulation of positive charges on the z surface.

Abstract: The effects of adding Sb to a BiMnCoSiCrNiYZr-added ZnO varistor (with the same composition as a commercial varistor) on the varistor voltage, leakage current, and resistance to electrical degradation were investigated. Bi is incorporated in spinel particles, and δ-Bi₂O₃ eventually disappears with the addition of small amounts of Bi, especially as the amount of Sb₂O₃ added increased. Reduction in both the nonlinearity index and the amount of δ-Bi₂O₃ for small amounts of added Bi with the addition of more than approximately 1.25 mol% Sb₂O₃ demonstrates that Sb inhibits Bi₂O₃ from forming deep interfacial impurity levels at the grain boundaries. The sample containing 1.2 mol% Bi₂O₃, 1.0 mol% ZrO₂, 1.0 mol% Y₂O₃, and 1.5 mol% Sb₂O₃ added exhibits a high varistor voltage (approximately 630 V/mm), high resistance to electrical degradation and low leakage current.

Abstract: With the goal of fabricating varistors with low varistor voltages, we investigated the effects of adding Ba and Si to BiCoMn-added ZnO varistors on the varistor voltage and the resistance to electrical degradation. Ba₂Mn₃O₈, which reduces the resistance to electrical degradation, was not formed at the grain boundary when Si was added. The resistance to electrical degradation was considerably improved by adding 0.10.15 mol% Si relative to samples to which small amounts of Sb had been added. The varistor voltage increased monotonically with increasing amount of added Si; it was approximately 36 V/mm for 0.1 mol% Si.

Abstract: Titanium dioxide nanotubes (TNTs) were grown by anodic oxidation of a titanium thin film deposited on an indium tin oxide (ITO) glass substrate. The TNTs were arranged densely and formed a thin film on the ITO substrate. Anodic oxidation was carried out at 550oC in an electrolyte. The inner diameter and tube length of a grown TNT were approximately 15 nm and 0.5 μm, respectively. Several of the TNT tube openings were closed by lids. These lids could be removed by sputter etching for a short time. The crystal structure was non-crystalline. The power conversion efficiency of a dye sensitized solar cell fabricated using the TNT thin film as a negative electrode is much smaller than that fabricated using conventional TiO₂ nanoparticle thin films, at present.

Abstract: Thin films of a composite of molybdenum disilicide (MoSi₂) and silicon (Si) were fabricated by radio frequency magnetron sputtering using a target made of a powder mixture of MoSi₂ and Si with a Si-to-Mo molar ratio of 1:X (2.0 X 2.5). The Hall coefficients were measured to identify the conduction mechanisms in the thin films. The sign and magnitude of the Hall coefficients revealed that thin films with X = 2.02.2 having a hexagonal crystal structure showed p-type conduction, while the mechanism for the n-type film with X = 2.33 was unknown and that for a composite of hexagonal and an unknown structure with X = 2.3, 2.4 and 2.5 showed mixed conduction.

Abstract: Three and six LiTaO₃ single crystals are used to achieve continuous emission of high-intensity X-rays. Furthermore, the interaction between X-rays and the case of the X-ray source is used to generate electrons. X-rays were emitted continuously and fluctuations in the count rate were reduced when three or six crystals were used. Moreover, the X-ray intensity increased and the number of electrons generated by the above-mentioned interaction increased with increasing number of crystals.