Papers by Author: Shinzo Yoshikado

Abstract: The effects of the addition of Y or both Y and Zr to Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistors on the varistor voltage and the tolerance characteristics of electrical degradation were investigated. The deterioration of the tolerance characteristics of electrical degradation by the addition of Y was probably caused by an increase in the number of willemite (Zn2SiO4)-type particles or a decrease in the number of spinel (Zn2.33Sb0.67O4)-type particles, but this deterioration was reduced by adding Zr. Moreover, the reduction in the average ZnO grain size due to the addition of Y was a major factor in the increased varistor voltage, and the ZnO grain growth was inhibited by the formation of an un-known compound after adding Y. The varistor voltage of a varistor with 2 mol% added Y increased by approximately 50% compared to a varistor with no Y added.

Abstract: To design an electromagnetic wave absorber with good absorption properties at frequencies above 1 GHz, the frequency dependences of the relative complex permeability μr*, the relative complex permittivity εr*, and return loss were investigated for the composite made of both sendust (an alloy of Al 5%, Si 10%, and Fe 85%) and aluminum particles dispersed in polystyrene resin. It was found that the frequency dependence of μr* for this composite can be changed by adjusting the particle size of aluminum and the volume mixture ratio of sendust and aluminum. Therefore, a flexible design of an absorber with good absorption characteristics was proposed based on the ability to control μr*. The composite made of both sendust and aluminum was found to exhibit a return loss of less than −20 dB in the frequency range of not only several GHz but also around 20 GHz if appropriate volume mixture ratio and particle size were selected.

Abstract: Thin films of mixtures of molybdenum silicate (MoSi2) and silicon (Si) (MoSiX, where the Mo to Si molar ratio = 1:X) were deposited on silicon nitride (Si3N4) polycrystalline substrates by radio-frequency magnetron sputtering using a target made of a mixture of MoSi2 and Si powders. The crystal structure of MoSiX thin films deposited on the Si3N4 substrate consisted of a mixture of a hexagonal phase and an unknown phase when X > 2.05. A thin film consisting almost entirely of the unknown phase could be deposited when X = 2.1−2.15. Molybdenum silicate can exist in the forms Mo3Si, Mo5Si3, or MoSi2, but to date there has been no report of molybdenum silicate having a Si to Mo molar composition ratio of larger than 2. It was found that the surfaces of thin films of the hexagonal phase or the unknown phase were readily oxidized, whereas the surfaces of thin films of a mixture of the hexagonal phase and the unknown phase exhibit excellent oxidation resistance in air at temperatures up to 700 °C.

Abstract: The transparency and uniformity of thin films of titanium oxide (TiO2) are important factors in determining the characteristics of photocatalysts and dye-sensitized solar cells. Synthesized TiO2 nanoparticles were deposited by constant-current electrophoresis deposition in ion-free water and ethanol. The number of cracks was reduced when ethanol was used instead of water. TiO2 nanoparticle thin films produced using a short deposition time had no apparent cracks and a high optical transparency. Moreover, thick TiO2 nanoparticle thin films, which have high optical trans-parency and no apparent cracks, could be deposited by repeating short-time deposition in ethanol.

Abstract: X-ray radiation using pyroelectric crystal is intermittent and the X-ray intensity is low and unstable compared with a conventional X-ray radiation method, such as X-ray tube. It is expected that the X-ray intensity becomes stable if electric field intensity and supply of electron are stable. In this study, to use X-ray radiation equipment as an electron source, tandem-type X-ray radiation equipment which is composed of two LiNbO3 single crystals polarized in a z-axis is proposed. When the temperature gradient for each crystal was the same, the X-ray intensity became approximately 6 times higher at a maximum. When the temperature gradient for each crystal was reversed, the period of X-ray radiation became approximately two times longer and the X-ray intensity became approximately 20 times higher at a maximum. Moreover, the stability of X-ray radiation for the repetition of temperature could be improved.

Abstract: The effects of thermal annealing on the electrical degradation of Sb2O3-Bi-Mn-Co-doped ZnO varistors were investigated. For samples with 0.04 mol% Sb2O3 or more, the nonlinearity index  of the voltage-current (V-I) characteristics after electrical degradation increased upon annealing. More-over, the value of  after electrical degradation was proportional to the full width at half maximum (FWHM) of the X-ray diffraction peak for Zn2.33Sb0.67O4-type spinel particles under various annealing conditions. The added Sb2O3 did not dissolve in the ZnO grains but became segregated at grain boundaries. Therefore, it is speculated that the increase in the FWHM of the spinel particles is due to an increase in the numbers of fine spinel particles at grain boundaries and triple points during annealing. Furthermore, it is suggested that the improvement in the electrical degradation upon annealing is due to a decrease in the mobility of oxide ions or Zn2+ ions owing to their being blocked by uniformly distributed fine spinel particles at grain boundaries.

Abstract: The effects of the particle size of sendust, which is an alloy of Al 5%, Si 10%, and Fe 85%, on the absorption characteristics of composite electromagnetic wave absorbers made of polystyrene resin and sendust were investigated in the frequency range from 1 to 40 GHz. The size of sendust particles was varied between approximately 5 and 20 m. A metal-backed single-layer absorber made of 20 m sendust particles absorbed more than 99% of electromagnetic wave power at frequencies above 20 GHz. Meanwhile, a composite made of 5 m particles exhibited a return loss of less than −20 dB in the frequency range of not only several GHz but also above 30 GHz. In addition, the relative complex permeability r* was shown to be controlled by adjusting the particle size of sendust, and an electromagnetic wave absorber with a flexible design was proposed.

Abstract: ZnO varistors of the excellent tolerance characteristics for electrical degradation were made by adding Bi2O3-MnO2-Co3O4-Cr2O3-SiO2-Sb2O3-NiO in ZnO. The tolerance characteristics for electrical degradation were evaluated by changing amount of ZrO2-additive. The evaluation methods are voltage-current characteristics, X-ray diffraction, scanning electron microscope, and energy dispersion X-ray spectroscopy. Monoclinic and tetragonal ZrO2 and the compounds originated in Zr were observed at both grain boundaries and triple points. Moreover, the compounds originated in both Zr and Sb improved the tolerance characteristics for electrical degradation. On the other hand, especially monoclinic ZrO2 deteriorated the tolerance characteristics for electrical degradation. It is one key factor of the improvements of the tolerance characteristics for electrical degradation that the mobility of oxide ions or interstitial Zn2+ ions was hindered by forming the compounds contained Zr, Sb, Si, and, Bi atoms.

Abstract: The gas pressure and the types of ambient gas dependence of X-ray intensity were investigated for LiNbO3 single crystals polarized in the c-axis direction at pressures of approximately 1 to 30 Pa. The integrated X-ray intensity showed a local maximum value at the pressure Pmax. Pmax moved to the high-pressure side in the ambient with a large first ionization energy. Pmax was proportional to the Boltzmann factor using the first ionization energy of each ambient gas molecule. The X-ray intensity was approximated using the quadratic function, which was convex upward for the pressure. It was found that one of the causes of the decrease in X-ray intensity on the pressure side higher than Pmax was the adsorption of positive ions on the crystal electric surface.

Abstract: TiO2-nanoparticles of anatase-type were synthesized and deposited using the electrophoresis depo-sition method. All of TiO2-nanoparticles were positively charged in colloidal aqueous solution and were attracted toward ITO glass as the negative electrode as same as nanoparticles (P25) whose diameter is larger than that of TiO2-nanoparticles. Many of the TiO2-nanoparticles were connected with each other and formed clusters, although single TiO2-nanoparticles were also observed. The cracks generated in the thin film with increasing the deposition time and thin film was flaked off from substrate. High optical transparency TiO2-nanoparticles-thin film without cracks was deposited when the deposition time was short. Moreover, the thickness of TiO2-thin film, which has high optical transparency and no cracks, could be increased using mixture of TiO2-nanoparticles and P25.