Papers by Author: Koichiro Fukuda

Abstract: We have been investigating the series of P doped Ca₂SiO₄ (C₂S) using Eu²⁺ or Eu³⁺ as activator with various colors. The crystal structure of C₂S is particularly easily controlled by heating because the established polymorphs of C₂S are, in the order of increasing temperature, γ, β, α’_L, α’_H, and α. In order to control the crystal structure, the phosphors were synthesized and then annealed at temperatures 1473 K-1773 K. The crystal structures and PL properties were compared between slow cooling and quenching (cooled in water). We found unique phenomena when the phosphors were treated by quenching process. In the case of (Ba_1-xCa_x)₂(Si_0.94P_0.06)O₄:Eu³⁺ ( 0.25 ≤ x ≤ 1), color emission changed from red to blue-white for the phosphor with a high concentration of Ba and quenched at 1773 K. In general, Eu³⁺ doped phosphors showed the red emission color in any host materials. However, Ba-included and quenched-treatment phosphors emitted a bright white color. The mechanism and relationship between the PL property and crystal structure were characterized carefully using X-ray diffraction, electron microscope and X-ray absorption fine structure.

Abstract: Mechanical destruction of acicular asbestos has been carried out as a process for decontamination. The dehydration reaction of asbestos occurred at 600°C and has been completed for 2 h at that temperature. This dehydration reaction of asbestos led to weakening of the mechanical properties of asbestos. After grinding, the microstructure of asbestos appeared no acicular shape. Further, this technique could be applied to asbestos-containing cement board. The spent cement board has about 10-20 mass% of asbestos (chrysotile: Mg3Si2O5(OH)4) and 5 mm thickness was used. After heating at 600°C for 2 h and grinding, the microstructure of the sample also appeared no acicular shape. Thus it was cleared that heating at 600°C and then grinding treatment for asbestos (chrysotile) has a grate important effect of mechanical and chemical destruction of asbestos in both cases of free asbestos and asbestos-containing cement board.

Abstract: Crystal structures of SrAl2O4, BaAl2O4 and their solid solutions have been reviewed in terms of the linkage pattern of [AlO4] tetrahedra. With SrAl2O4 the hexagonal-to-monoclinic phase transformation occurs at 950K during cooling. The space group change from P63 to its subgroup P21 eliminates the triad axis of the former phase, which involves a reduction in the symmetry of the trigonally distorted rings. The hexagonal structures of SrAl2O4 and BaAl2O4 differ distinctly in the linkage pattern of the [AlO4] tetrahedra. In the former structure, all of the tetrahedral rings are equivalent. In the latter, there are two types of tetrahedral rings; trigonal rings and asymmetrical ones. The trigonal rings, comprising 25% of the total number of rings, contain in their centers the Ba atoms with the special position. This implies that the triad axes exist in the centers of the rings, and hence they are distorted trigonally as in the hexagonal SrAl2O4. On the other hand, the Ba atoms in the asymmetrical rings are located at the general position site. The structural disorder in Ba0.6Sr0.4Al2O4 (space group P6322) was investigated by the combined use of Rietveld method and maximum-entropy method. The electron density distribution was satisfactorily expressed by the split-atom model, in which the strontium/barium and oxygen atoms were split to occupy the lower symmetry sites.