Papers by Author: Toshio Torikai

Abstract: Ho³⁺/Yb³⁺ co-doped ZnO-TiO₂ composite system were synthesized by powder-solution mixing method and their upconversion (UC) luminescence characteristics were investigated under the 980 nm laser excitation. The effect of various ZnO/TiO₂ mixing ratios, and Ho³⁺ and Yb³⁺ concentrations were also studied. The XRD patterns showed that the product fired at 1300 °C consisted of Zn₂TiO₄, TiO₂, RE₂Ti₂O₇, and RE₂TiO₅ (RE = Ho³⁺ and/or Yb³⁺) phases. The green emission centered at 538 nm wavelength was detected as the strongest emission intensity which it was in accordance with the ⁵F_4, ⁵S₂ → ⁵I₈ transition of Ho³⁺ ion. The emission intensity of the product changed by varying ZnO/TiO₂ mixing ratios, and Ho³⁺ and Yb³⁺ concentrations. Brightest UC emission was observed in the sample of 1ZnO:1TiO₂ (in mole) doped with 0.03 mol% Ho³⁺, 9 mol% Yb³⁺ fired at 1300 °C for 1 h. Besides, the dependence of the UC emission intensity on the excitation power indicated that the two-photon process was responsible for this UC system.

Abstract: Calcium silicate phosphors, Ca2-xSiO4(CS):Eu3+x, CS:Eu2+x and Ca2-y-zMgSi2O7 (CMS):Eu2+y,Dy3+z were prepared by the solid state reaction. The phases in CS:Eu3+ system were β- and αL’ -types. The fluorescent color under a black-light irradiation was red and the emission spectrum consisted of 590nm(αL’), 615nm(β) and 625nm(αL’) peaks. The emission intensity took a maximum value at x=0.2. The addition of B3+ accelerated the solid solution of Eu3+. The phase in CS:Eu2+ system was β-type only. The fluorescent color was yellow-green(520nm). The emission intensity took a maximum value at x=0.01. The CMS product showed the akermanite phase. The lattice constants of CMS:Eu2+ increased with increasing Eu content, but those became constant at y>0.05. The fluorescent color of CMS:Eu2+ was yellow-green and the emission intensity took a maximum value at y=0.03. In the case of CMS:Eu2+0.03,Dy3+z, the fluorescent color and the afterglow color were same, yellow-green. The emission intensity took a maximum value at z=0.06. The longest afterglow time, 23min., was obtained at z=0.09. The trap depth were 0.64-0.69 eV.

Abstract: Porous plate with bimodal size distribution was prepared using the molten slag from the incinerator. The slag was pulverized to 1-10μm using a planetary ball mill. On heating the pellet formed with this powder, its relative density at 750°C and 800°C were 70% and 85%, respectively. At 750°C, each particle partially attached together at the contacted point. At 800°C, however, all particles melted, coagulated together and increased their size. The slag powder was granulated using a tumbling granulator, resulting granules of 0.5-5mm size. The compressive strength of the granule sintered at 750°C was 0.39 kgf. The density of the granule decreased by ~60%. The sintered granules were mixed with a proper amount of glass powder and starch water, and the slurry was cast to a mold (φ80mmx10mm). After drying the cast plate, it was heated at 630°C. The density of the plate was 1.2 gcm-3, ~43% of original slag value. The sound absorption coefficients of the plate were 0.02 and 0.4 at 200Hz and 1500Hz, respectively.

Abstract: Sr4Al14O25:Eu2+, Dy3+ long persistent phosphors with different B, Eu and Dy contents were prepared by solid phase reaction at various temperatures in H2/N2=1/9 atmosphere. X- ray diffraction and scanning electron microscopy observations showed that, when the phosphor was doped with 40 at% B, single dense Sr4Al14O25 phase was formed but for the samples with less than 40 at% B, mixed phases containing SrAl12O19 and SrAl2O4 were observed while for higher B content (100 at%) SrAl2B2O7 phases appeared. The phosphor showed emission peak centered at 500 nm with blue green color. When 40 at% of H3BO3 was added and doped with 4 at% of Eu and 8 at% of Dy, it showed the maximum initial intensity of 3170 mcd.m-2 and the longest persistency which is greater than 20 h over value of 5 mcd.m-2.

Abstract: Porous carbon having surface area of 792 m2g-1 was prepared from barley straw by carbonization at 800 oC at a heating rate of 5 oCmin-1 to test the chromium (VI) adsorption from aqueous solution. The effects of pH, contact time and equilibrium Cr (VI) concentration on the adsorption were studied in detail. The optimum pH for Cr (VI) removal was found to be 2. The adsorption of Cr (VI) was very rapid and equilibrium was reached within 1 h. It was interpreted in terms of adsorption-coupled reduction mechanism. Accompanied by the decrease of Cr (VI) ion, Cr (III) ion was slightly released in the solution. Maximum loading capacity of total chromium was found to be 2.35 molkg-1 at pH 2.