Papers by Author: Shinzo Yoshikado

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: A low-cost composite electromagnetic wave absorber made of sendust dispersed in resin was developed that does not contain any rare metals. In this study, spherical sendust particles with average particle sizes of approximately 5 and 20 μm were dispersed in polystyrene resin at volume ratios in the range 2040 vol% to broaden the absorption frequency bandwidth at frequencies above 10 GHz. The optimal volume ratios of sendust with average particle sizes of approximately 5 and 20 μm were found to be approximately 30 and 35 vol%, respectively. Electromagnetic wave absorbers can be flexibly designed by controlling the volume ratio of spherical sendust particles in resin.

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: The absence of cracks and a high optical transparency are critical factors for obtaining high performance when TiO₂ thin films are used as cathodes in dye-sensitized solar cells (DSSCs). Synthesized and classified TiO₂ nanoparticles were deposited by constant-current electrophoresis in ethanol. The optical transparency of thin films and the DSSC efficiency increased rapidly with decreasing particle size and increasing film homogeneity. This increase in the DSSC efficiency suggests that the electron conduction path in a thin film consists of connections in the crystal lattice formed between TiO₂ nanoparticles. This formation of connections increases the electron diffusion length.

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: With the goal of fabricating varistors with low breakdown voltages (varistor voltages), the effect of adding Sb to Bi-Co-Mn-Ba-added ZnO varistors on the ZnO grain size was investigated. To obtain a uniform ZnO grain size without reducing the grain size, a small amount (e.g., 0.01 mol%) of Sb was added as an additive. This addition suppresses the variation in the ZnO grain size without reducing the grain size. It also improved the resistance to electrical degradation because compounds of Ba and Mn were not formed.

Abstract: In order to design a ferrite absorber that can be used at frequencies of several GHz, the frequency dependences of the relative complex permeability μ_r^*, the relative complex permittivity ε_r^*, and return loss were investigated for a composite made of Ni-Zn ferrite and SiO₂. When ferrite particles were dispersed and isolated in an SiO₂ medium, the frequency dependence of μ_r^* was different from that for a composite made of SiO₂ particles dispersed and isolated in the ferrite medium. Moreover, when ferrite particles were isolated and a suitable mixture ratio of ferrite and SiO₂ was selected, the return loss was less than −20 dB at frequencies of several GHz. The dispersion states of ferrite and SiO₂ particles are thus important factors to design an absorber, and improvement in the absorption characteristics of the ferrite tile which is used as a practical absorber could be achieved using a composite made of Ni-Zn ferrite particles dispersed and isolated in an SiO₂ medium.

Abstract: Equipment was designed for the application of Al₂O₃ thin films by the aerosol deposition method (ADM) at room temperature. Al₂O₃ film could be deposited on Pyrex-glass, indium tin oxide (ITO), polyvinyl chloride (PVC) plastic, single crystal silicon, and sapphire substrates. The films had the same crystal structure as the raw material particles and were highly transparent. The breakdown electric field of the Al₂O₃ film for the ADM was more than 35 kV/mm.

Abstract: The absence of cracks and a high optical transparency are critical factors for obtaining high performance when TiO₂ thin films are used as photocatalysts and as the cathode material in dye-sensitized solar cells. Synthesized TiO₂ nanoparticles were deposited by constant-current electrophoresis in ethanol. TiO₂ nanoparticle thin films deposited at a low current density had no apparent cracks and a high optical transparency. Small TiO₂ nanoparticles deposited are thought to be transported at low current densities. This enables TiO₂ nanoparticle chains to form by the oriented attachment mechanism and thereby increases the electron diffusion length.

Abstract: With the goal of fabricating low-breakdown-voltage varistors, the effect of adding Ba to ZnO varistors on the ZnO grain size was investigated. Grain growth of ZnO could be markedly promoted by adding both Ba and Bi. The maximum grain size was approximately 150 μm and the minimum varistor voltage was approximately 12 V/mm. However, it had relatively poor tolerance characteristics for electrical degradation. It is speculated that when adding both Ba and Bi to a Mn–Co-added ZnO varistor, it is necessary to form the molten phases of Ba and Bi to promote grain growth of ZnO. It is also conjectured that the growth of ZnO grains is not promoted when Ba and Bi do not coexist in the molten phase because Ba forms compounds with Mn independently with the addition of small amounts of Bi.