Papers by Author: Yoshio Sakka

Abstract: Orientation technique based on the magneto-scientific crystal alignment phenomenon combined with electrophoretic deposition (EPD) technique was applied for the seeding process of zeolite L particles. Well-dispersed, ethanol-based zeolite L suspension was prepared and then consolidated on porous zirconia substrates by EPD. Conducting polypyrrole film synthesized on the zirconia substrate was used as an anodic substrate for the EPD process. The EPD was performed in a superconducting magnet with applying 12 T strong magnetic field to the suspension. The degree of orientation was characterized by XRD and compared with that of the zeolite L layer prepared by slip casting in a 12 T strong magnetic field using the same suspension.

Abstract: Beta-sialon:Eu²⁺ phosphor deposits were fabricated by electrophoretic deposition (EPD) process within a strong magnetic field (12 T). The direction of the magnetic field was adjusted to be parallel or perpendicular to that of the electric field, that is, vertical-or horizontal setup. The oriented deposits were fabricated by aligning the β-sialon:Eu²⁺ particles along the higher magnetic-susceptibility c-crystal axis (a, b-crystal plane). For the case of vertically-setup magnetic field, the oriented deposit aligned along the c-axis possessed higher relative deposit density than the randomly fabricated deposit, as a result, varying the intensity ratio of emission and transmitted excitation, and therefore, presenting different chromaticity coordinates; for the case of horizontally-setup magnetic field, photoluminescence (PL) intensities of the deposits oriented along c-axis were significantly improved by comparing with those of the randomly-oriented ones.

Abstract: The metastable garnet lattice of Gd₃Al₅O₁₂ (GdAG) was effectively stabilized via doping with significantly smaller Lu³⁺, and based on which (Gd,Lu)AG:Yb/Ho was developed in this work as a new type of upconversion phosphor. The phosphor particles calcined from the precursors synthesized via carbonate precipitation were observed to have good dispersion and fairly uniform morphologies. Optical spectroscopy found that the [(Gd_1-xLu_x)_0.948Yb_0.05Ho_0.002]₃Al₅O₁₂ (x=0.1-0.5) garnet powders exhibit a green emission centered at ~543 nm (the ⁵F₄,⁵S₂⁵I₈ transition of Ho³⁺) and a red emission centered at ~668 nm (the ⁵F₅⁵I₈ transition of Ho³⁺) under laser excitation at 978 nm. The upconversion emission intensity was found to decrease with increasing Lu³⁺ doping. Meanwhile, the dependence of up-conversion emission intensity on the pumping power was measured and the up-conversion mechanism was discussed in detail. The Yb/Ho codoped (Gd,Lu)AG garnet system developed herein may potentially be used as a new type of luminescent material.

Abstract: In order to attain high-strain-rate superplasticity (HSRS) in ceramics, flow behavior was examined with ZrO₂ reference sample. The results suggest that the enhancement of the accommodation processes of grain boundary sliding (GBS) is important in addition to the careful controlling the microstructural factors, such as stable fine grain structure, reducing residual pores and so on. The spinel particles dispersion can simultaneously provide the following positive factors to ZrO₂: i) suppressed grain growth due to pinning effect of spinel particles, enhanced accommodation due to ii) accelerated relaxation of stress concentrations exerted by GBS through dislocation motion and iii) accelerated lattice diffusion caused by the dissolution of aluminum and magnesium into ZrO₂ from the spinel particles. The positive factors due to spinel dispersion make it possible to attain HSRS in ZrO₂ ceramics.

Abstract: Through strong magnetic field alignment (SMFA) method followed by spark plasma sintering (SPS), dense tailored ZrB₂ ceramic could be prepared using the plate-like grains as initial material. The measured Lotgering orientation factor f_(00l) on the textured top surface (TTS) was high as 0.62 and f_(hk0) on the textured side surface (TSS) was 0.65. Vickers hardness measurement showed the anisotropic mechanical property of textured sample. On the TTS, the hardness was 13.4 ± 0.9 GPa, while on the TSS the hardness was 12.0 ± 0.9 GPa.

Abstract: Fabrication of textured hematite was achieved from paramagnetic goethite by electrophoretic deposition in a strong magnetic field followed by thermal treatment via topotactic phase transformation. The textured microstructure of the hematite was characterized by XRD and SEM observations. It was confirmed that the a,b-axes of the goethite were taken over the c-axis of the hematite through the topotactic phase transformation The magnetization vs magnetic field measurement using a VSM revealed that the textured hematite showed an anisotropic magnetic property depending on the direction of the crystalline orientation.

Abstract: We present a method for fabrication of textured MAX phase ceramics, particularly, Ti₃SiC₂; by EPD in a strong magnetic field (12T). Ti₃SiC₂ was dispersed in cationic polyelectrolyte-Polyethylenimine (PEI). Addition of 0.3-1dwb PEI resulted in high zeta potential values and suspension was found to be stable and of good fluidity. The optimized suspension parameters for EPD were determined as 10vol% Ti₃SiC₂ and 1dwb PEI in 50 % ethanolic water at pH ~ 7. X-ray diffraction analysis of the textured samples revealed that the preferred orientation of Ti₃SiC₂ grains parallel to the magnetic eld direction was along the a,b-axis. The Lotgering orientation factors on the textured top surface and textured side surface were determined as f _(hk0) = 0.35 and f _(00l) = 0.75, respectively.

Abstract: Grain-oriented Barium titanate (BaTiO₃) ceramics were prepared using an electrophoresis deposition (EPD) method under high magnetic field of 12 T and sintering at 1350 °C. It was found that BT03 and BTKK nanoparticles were necessary to synthesize the grain-oriented BT ceramics. The grain orientation was correlated with particles size and c/a ratio of lattice constants. The reason why the grain-oriented BT ceramics using the BT03 and BTKK were prepared was having the large values of the particles size and the c/a ration of lattice constants.

Abstract: Dense bulk ZrB2 ceramic was synthesized by mechanical alloying (MA) and followed spark plasma sintering (SPS) using zirconium and boron as initial materials. It was found that MA process was effective to fragment the coarse metal zirconium particles from 45 m to less than 1 m within 20 hours. In comparison with the commercial ZrB2 powder, the as-obtained zirconium and boron mixture powders showed higher sinterability. When the sintering was carried out at 1800oC, the relative density of synthesized ZrB2 samples using mixture powder was above 95%, higher than that of ZrB2 sample prepared using commercial powder (73%). Vickers hardness of those ZrB2 samples was at the same level of 15 GPa. However, the fracture toughness of ZrB2 samples seemed to depend on the heating rate of the SPS process. Corresponding to the heating rates of 10, 50, and 100oC/min, the fracture toughness of as-prepared ZrB2 samples were 3.83, 3.19, and 2.74 MPa•m1/2, respectively.

Abstract: This paper reviews the most important results by the authors on the processing of textured -Si3N4 and -sialon by slip casting in a strong magnetic filed of 12 T and reaction-sintering. The a, b-axis textured -Si3N4 and -sialon have been obtained using the static magnetic field because of the magnetic susceptibility of ca, b > c c for -Si3N4. However, the c-axis textured -Si3N4 has also been successfully obtained using a rotating magnetic field. The -Si3N4 crystal was found to exhibit substantially stronger orientation ability than the a-Si3N4 crystal regardless of the Si3N4 raw powders. It reveals that the -Si3N4 nuclei play a key role in the texture development in -Si3N4/-sialon.