Papers by Author: Yoshio Sakka

Abstract: A series of layered rare-earth hydroxides (LRHs) of (Y,Ln)₂(OH)₅NO₃•nH₂O ( Ln=Tb,Eu), have been synthesized via a hydrothermal route. Crystal structures and optical properties of the materials have been investigated in detail by the combined techniques of XRD, FT-IR, FE-SEM, HR-TEM, and PLE/PL spectroscopies. It is shown that Tb³⁺ and Eu³⁺ are successfully incorporated into the Y-LRH host lattice to form solid solutions. Under UV excitation, the binary (Y_0.97Tb_0.03)- and (Y_0.97Eu_0.03)-LRHs exhibit their respective characteristic photoluminescence of the Eu³⁺ and Tb³⁺ activators. The ternary (Y_0.965Eu_0.005Tb_0.03)-LRH simultaneously shows red and green emissions and both the intensity and emission color can be adjusted by changing the excitation wavelength. Compared with (Y_0.995Eu_0.005)-LRH, the ternary (Y_0.965Eu_0.005Tb_0.03)-LRH exhibits appreciably enhanced 615nm red-emission, which indicates the existence of non- radiative energy transfer from Tb³⁺ to Eu³⁺. The ternary (Y_0.965Eu_0.005Tb_0.03)-LRH also readily undergoes anion exchange with a series of inorganic and organic anions at room temperature.

Abstract: The densification mechanism during the park-plasma-sintering (SPS) processing was examined in high purity MgAl2O4 spinel. As the density ρt increases, that is, as the effective stress σeff decreases, stress exponent n evaluated from σeff dependence of densification rate varies from n ≥ 4 in the low ρt region, n ≈ 2 in the intermediate ρt region to n ≈ 1 in the high ρt region. TEM observation shows that significant stacking faults caused by partial dislocations are observed in the low ρt region, but limited in the high ρt region. The ρt dependent densification behavior and microstructure suggest that the predominant densification mechanism during the SPS processing changes with ρt from plastic flow by partial dislocation motion for the low ρt region (n ≥ 4) to diffusion-related creep for the high ρt region (n ≈ 1).