Papers by Author: Shinzo Yoshikado

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: In order to fabricate varistors with low varistor voltage, the effects of thermal annealing of CoMnBaSi-added Bi based ZnO varistors on electrical properties and the grain boundary structure were investigated. The varistor voltage for the BiCoMnBaSi-added ZnO varistor decreased in half by thermal annealing for a short time. The resistance to electrical degradation was most improved by the addition of SiO₂ and thermal annealing for 1020 min. It is suggested that the composition of Bi and Si in the sheet-like deposit is changed by varying the annealing time and the resistance to electrical degradation is improved by both addition of SiO₂ as well as thermal annealing for short time.

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: X-ray emission using pyroelectric crystals is intermittent, and has low intensity and stability. One of the factors for low stability is related to creeping discharge, due to the accumulation of surface electric charges that change in response to the temperature. The time dependence of the net amount of electric charge was investigated by changing the cycle period of the crystal temperature. The stability of the X-ray emission is demonstrated to be strongly dependent on the temperature cycle period.

Abstract: The temperature dependence of resistivity for thin films of a composite of molybdenum disilicide (MoSi₂) and silicon (Si) fabricated by radio frequency magnetron sputtering using a target made of a powder mixture of MoSi₂ and Si with molar ratio of Si to Mo of 1:X (2.02X2.55) was analyzed. The temperature dependence could be explained by the multiplicative model consisting of a conduction model similar to the Grüneisen-Bloch model, a modified Anderson localization model for 2.02X2.21 and the modified Anderson localization model for 2.39X2.55 over a wide temperature range.

Abstract: Titanium dioxide nanotube (TNT) thin films were fabricated, for application as the negative electrode of dye-sensitized solar cells (DSC), by anodic oxidation of titanium thin films deposited by RF magnetron sputtering on indium tin oxide glass substrates. Anodic oxidation was carried out by a constant voltage or constant current method in an electrolyte in order to determine the best method to increase TNT film thickness. Thick titanium dioxide films were fabricated by the constant voltage method at higher voltages. Thicker titanium dioxide films were obtained by the constant current method compared to the constant voltage method. However, the constant current method with optimal current yielded TNT films with higher quality (high transparency). The power conversion efficiency of DSC was improved using films fabricated by the constant current method.

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.