Papers by Author: Kotaro Kuroda

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Authors: Tsuyoshi Sasaki, Yoshio Ukyo, Kotaro Kuroda, Shigeo Arai, Hiroyasu Saka
Abstract: The crystal structures of reduced-reoxidized ceria-zirconia solid solutions, Ce2Zr2O7+x, with an ordered arrangement of Ce and Zr ions were studied by transmission electron microscopy (TEM), X-ray powder diffraction and thermogravimetric analysis. The oxygen absorption of Ce2Zr2O7 with pyrochlore structure (space group m Fd 3 ) was revealed to proceed through two-step reaction; reaction from Ce2Zr2O7 to b-Ce2Zr2O7.5 around 400K and reaction from b-Ce2Zr2O7.5 to Ce2Zr2O8 about 600K. b-Ce2Zr2O7.5 phase with space group m F 3 4 was thought to be formed by occupation of the half ordered oxygen vacancies with oxygen ions periodically. The oxidation from Ce2Zr2O7 to b-Ce2Zr2O7.5 was also occurred at room temperature by keeping for about 1 year or by irradiation of an electron beam in TEM. On the other hand, the crystal structure of Ce2Zr2O8, completely oxidized state of Ce2Zr2O7, belongs to the space group of m P 3 4 or 23 P from the rule for possible diffractions and the symmetry of the ordering cations.
Authors: Takayuki Oshima, Yasuhiro Habara, Kotaro Kuroda
Authors: Masazumi Okido, Ryoichi Ichino, Kotaro Kuroda
Abstract: Hydroxyapatite (Ca10(PO4)6(OH)2, HAp), carbonated HAp and titanium oxide are of interest for bone-interfacing implant applications, because of their demonstrated osteoconductive properties. They were coated on the titanium implants and investigated the in vitro and in vivo performance. HAp coatings were performed by the thermal substrate method in aqueous solutions. Titanium oxide film was also formed on the titanium implants by gaseous oxidation, or by anodizing in the acidic solution. All the specimens covered with HAp, carbonated HAp or TiO2 (rutile or anatase). were characterized by XRD, EDX, FT-IR and SEM. In the in vitro testing, the mouse osteoblast-like cells (MC3T3-E1) were cultured on the coated and non-coated specimens for up to 30 days. Moreover, the osseointegration was evaluated from the rod specimens implanted in rats femoral for up to 8 weeks. In in vivo evaluations two weeks postimplantation, new bone formed on the coated and non-coated titanium rods in the cancellous bone and cortical bone, respectively. Bone-implant contact ratio, in order to evaluate of new bone formation, was significantly depended on the compound formed on the titanium implant.
Authors: Shinichi Shibata, H. Unoki, Kotaro Kuroda, N. Watanabe, N. Koshizuka
Authors: K. Tanaka, T. Miwa, K. Morishita, Katsuhiro Sasaki, Kotaro Kuroda
Abstract: The structure of melt-spun and crystallized Mg-10%Ni and Mg-10%Ni-5%La alloys is studied using HRTEM, coupled with ED and EELS techniques, for specimens subjected to hydrogenation and dehydrogenation. The presence of nano-sized (5-10nm) Mg2Ni grains dispersed in the matrix of Mg nano-grains is observed before hydrogenation. This structure is almost preserved after hydrogenation and dehydrogenation at 300°C. In the hydrogenated specimen, nanoboundaries lying between MgH2 and Mg2NiH4 nano-grains are observed. They appear to provide main routes for the hydrogen transport in these nanostructured materials.
Authors: Kotaro Kuroda, Yukio Miyashita, Ryoichi Ichino, Masazumi Okido
Authors: Masatoshi Nakanishi, Katsuhiro Sasaki, Hiroyasu Saka, Kotaro Kuroda
Abstract: The process of phase transformation in individual Fe-Pt and Fe-Pt-Cu nanoparticles synthesized by the reverse micelle method with chemical homogeneity and mono-dispersion has been investigated by in-situ high-resolution electron microscopy (HREM) observation and in-situ nano-beam diffraction (NBD). The Fe-Pt particles, initially polycrystalline with the chemically disordered fcc (A1) phase, were reconstructed into A1 single crystals between 550 and 650°C, followed by a phase transformation from A1 to the chemically ordered fct (L10) phase between 650 and 680°C. The coalescence began almost concurrently with the phase transformation. They were transformed into round-shaped single-crystalline particles between 680 and 720°C. Similar processes were also observed in the Fe-Pt-Cu nanoparticles. The temperatures at which these processes occurred were substantially lower than those required for the Fe-Pt nanoparticles. We investigated the magnetic-field distribution of a submicron-size island comprising isolated L10 Fe-Pt nanoparticles magnetized along one direction by using in-situ electron holography at elevated temperatures. Although the magnetization decreased between 212 and 412°C to 25% of the strength at 25°C, it recovered 67% of the initial strength during cooling. However, when an island was heated to 512°C, the magnetization diminished and did not recover during cooling. The Curie temperature (Tc) was determined to be 350°C and was in good agreement with the Tc determined by bulk measurements, which was approximately 100°C lower than the Tc for bulk Fe55Pt45.
Authors: Kotaro Kuroda, Ryoichi Ichino, Masazumi Okido
Abstract: Hydroxyapatite (HAp) coatings were formed on cp titanium plates and rods by the thermal substrate method in an aqueous solution that included 0.3 mM Ca(H2PO4)2 and 0.7 mM CaCl2. The coating experiments were conducted at 40-140 oC and pH = 8 for 15 or 30 min. The properties for the coated samples were studied using XRD, EDX, FT-IR, and SEM. All the specimens were covered with HAp, which had different surface morphologies such as net-like, plate-like and needle-like. After cleaning and sterilization, all the coated specimens were subjected to in vivo and vitro testing. In the in vitro testing, the mouse osteoblast-like cells (MC3T3-E1) were cultured on the coated and non-coated specimens for up to 30 days. Moreover, the specimens (φ2 x 5 mm) were implanted in rats femoral for up to 8 weeks, the osseoinductivity on them were evaluated. In in vitro evaluations, there were not significant differences between the different surface morphologies. In in vivo evaluations, however, two weeks postimplantation, new bone formed on both the HAp coated and non-coated titanium rods in the cancellous and cortical bone. The bone-implant contact ratio, which was used for the evaluation of new bone formation, was significantly dependent on the surface morphology of the HAp, and the results demonstrated that the needle-like coating appears to promote rapid bone formation.
Authors: Bralee Chayasombat, Y. Kimata, T. Kato, Tomoharu Tokunaga, Katsuhiro Sasaki, Kotaro Kuroda
Abstract: Microstructures of switch-back epitaxy cubic silicon carbide (3C-SiC) before and after Al ion implantation were investigated by transmission electron microscopy (TEM). Stacking faults aligned along the {111} were observed in 3C-SiC. A surface bulge was observed in some regions and planar defects were observed under the bulge region. After ion implantation of 3C-SiC, defects were observed to be distributed up to a depth approximately 500 nm from the surface.
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