Papers by Author: 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.

Abstract: 6H-SiC hetero-epitaxially grown on a (111) 3C-SiC was observed with TEM. High-density stacking faults were formed around the hetero-interface, and the density of stacking faults decreased with increasing distance from interface. On the other hand, when 3C-SiC was homo-epitaxially grown on a 3C-SiC, any stacking faults did not exist at the interface between the grown crystal and the seed crystal. Thus, the stacking faults formation started from the 6H/3C hetero-interface. Considering the lattice-mismatch strain between 3C-SiC and 6H-SiC, the strain energy is equivalent to the stacking fault energy of 6H-SiC. This similarity suggests that the stacking faults formation could be caused by the relaxation of the lattice-mismatch strain.

Abstract: A solution growth of 3C-SiC was performed on (111)Si-face or )111(C-face of 3C-SiC seed crystal at around 1700 °C by dipping method. The polytype of the crystal grown on the Si-face immediately changed to 6H-SiC. On the other hand, 3C-SiC stably grew on the C-face except for a small number of 6H-SiC precipitates. The polytype transition phenomenon can be explained by the difference of the chemical potential and the solution-crystal interfacial energy between 3C-SiC and 6H-SiC. To grow a larger 3C-SiC crystal, we carried out a long-term growth for 30 hours on the C-face. In the first 10 hours, the polytype of the grown crystal was 3C-SiC. In the next 10 hours, however, the polytype changed from 3C-SiC to 6H-SiC. According to our studies, 6H-SiC tends to grow on 6H-SiC at around 1700 °C, while both of 3C-SiC and 6H-SiC can grow on 3C-SiC at around the same temperature. In this case, 6H-SiC grows on 6H-SiC precipitates and then the dominant polytype changes to 6H-SiC after several 6H-SiC precipitations. To grow 3C-SiC crystal stably, it is necessary to surpress completely the polytype transition by the growth on C-face at lower growth temperatures.

Abstract: La2O3 and/or Ga2O3 doped, and the double dopant γ-Al2O3 composite powder samples were prepared by the sol-gel method. Nanostructural characterizations of the powder samples were performed by high resolution transmission electron microscopy (HRTEM) in order to clarify the role of the metal oxide dopants for the improved hydrothermal stability of the La2O3-doped Ga2O3-Al2O3 composite material. The results of HRTEM study and selected area electron diffraction (SAED) analysis revealed the formation of the uniform solid solution of Ga2O3-Al2O3. In addition, the results of X-ray elemental mapping analysis revealed the existence of well-dispersed La2O3 on the grain surface of Ga2O3-Al2O3 solid solution. These structural features could act an important role for the improved hydrothermal stability of the La2O3-doped Ga2O3-Al2O3 composite material.

Abstract: The microstructure of oxide scale formed in the early stage of oxidation of cold-rolled sheets of 17%Cr-Mn-Ni austenitic stainless steel was examined using SEM and TEM. Samples were oxidized at 973 to1373K for 1 to 15min. Nodules were observed on the surfaces of specimens oxidized at 1173K. The nodules were composed of two layers, which comprised Fe oxides and Cr-rich Fe-Cr-Mn oxides, respectively. The other scale was composed of two layers of Fe-Cr-Mn oxides with different compositions. Nodules were not observed on the surfaces of the specimens oxidized at 973 and 1373K. After oxidation at 1373K for 1min, the scale was composed of three layers. The first layer consisted of Fe-Mn oxides with a spinel structure, the second consisted mainly of Cr oxides, and the third consisted of Cr-Mn oxides containing a small amount of Fe.

Abstract: The microstructures of high-temperature oxide scales on the Si-terminated surface and C-terminated surface of 6H-SiC were investigated by transmission electron microscopy (TEM). We found that mechanical polishing caused surface strains, about 100 nm in depth, on both sides of specimens. Mechanically polished specimens were oxidized at 1473 K for 20 h in air. Oxide scales of about 250 nm in thickness were formed on the Si-terminated surface and of about 400 nm on the C-terminated surface. Since the strain regions caused by mechanical polishing were oxidized, strains were no longer observed. As a result, this oxidation condition effectively removed the strains. The oxide scales were identified as amorphous silica on the Si-terminated face, while crystalline oxides and amorphous silica were observed on the C-terminated face.

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.

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.

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.