Koji Moriguchi

Nano-tube-like surface structure in graphite particles and its formation mechanism: A role in anodes of lithium-ion secondary batteries

Nano-structures on the surface of graphite based carbon particles have been investigated by means of high resolution transmission electron microscopy. The surfaces consist of “closed-edge” structures in a similar manner as carbon nano-tube. That is, they are composed of coaxial carbon tubes consisting of adequate coupling of graphite layer edges. These graphite particles are chemically stable and, therefore, applicable for lithium-ion secondary battery anodes. Molecular dynamics simulations based on the Tersoff potential reveal that the vibrations of the graphite layers at the free edges play an important role in the formation of the closed-edge structures. In lithium-ion secondary batteries, Li ions can intrude into bulk carbon anodes through these closed-edge structures. In order to clarify this intrusion mechanism, we have studied the barrier potentials of Li intrusion through these closed edges using the first-principles cluster calculations. From electrochemical measurements, the carbon anodes compos...

Verification of Tersoff's Potential for Static Structural Analysis of Solids of Group-IV Elements

We have carried out several stringent tests of Tersoffs potential through static structural analysis of solids of group-IV elements and examined the chemical properties of the Tersoffs potential for C, Si and Ge. It is clear that Tersoffs potential has a limited ability to describe the differences in chemical reactivity between elemental Ge and Si, but well describes the chemical differences of valence s and p electron properties between elemental C and Si. Tersoffs potential has higher force constants for angular distortions than those seen in actual systems. We assume that the well-known higher melting points of Tersoffs potential are due to the larger force constants for angular distortions. The calculated elastic constants and Poisson ratios indicate congruity with the experimental values in every element. Tersoffs potential has good transferability and will be an effective tool for evaluating new materials composed of group-IV elements.

Epitaxial Growth of a Low-Density Framework Form of Crystalline Silicon

Crystal growth processes of low-density framework forms of crystalline silicon, named Si clathrates ( Si34 and Si46), during solid phase epitaxy (SPE) have been successfully observed in molecular-dynamics simulations using the Tersoff potential. The activation energy of SPE for Si34 has been found to correspond with the experimental value ( approximately 2.7 eV) for the cubic diamond phase, while the SPE rates of Si46 are much lower than that of Si34. The structural transition from Si46 to Si34 can be also observed during the Si46-[001] SPE. The present results suggest that new wide-gap Si semiconductors with clathrate structures can be prepared using epitaxial growth techniques.

Crystallinity Evaluation of 4H-SiC Single Crystal Grown by Solution Growth Technique Using Si-Ti-C Solution

Crystallinity of 4H-SiC bulk crystal obtained by solution growth technique was characterized mainly by KOH etching of the off-ground and serially ground specimen. Marked reduction of basal plane dislocation, threading edge and screw dislocations during the growth of on-axis crystal was confirmed. Cross-sectional TEM observation revealed the rapid reduction behavior of threading dislocations microscopically. AFM observation of as-grown morphology showed that screw dislocation dipoles is related to the reduction of threading screw dislocations and single domain formation, which is essential for establishing the high crystallinity.

Effect of Close-Packed Plane Boundaries in a Bain Zone on the Crack Path in Simulated Coarse-Grained HAZ of Bainitic Steel

Global effect of Bain-zone boundaries and the local effect of close-packed plane (CP) boundaries on the path of secondary cleavage cracks (observed on a fractured V-notch Charpy specimen) were visualized and discussed in simulated coarse-grained HAZ of bainitic steel. Microstructural unit map (Bain-zone map and CP map) was obtained by electron backscatter diffraction method for a prior austenite grain of a few hundred micrometers. Furthermore, a correlation between CP boundaries and sites that favored formation of the blocky martensite–austenite (M–A) constituent was confirmed. It was clarified that two crack deviation/local changing factors (CP boundaries and blocky M–A) are paired.

Correlation between the thermodynamic stability of austenite and the shear modulus of polycrystalline steel alloy

Elastic moduli in steel alloys with different martensitic start temperatures were measured during the cooling cycle by using the ultrasonic pulse sing-around method. Our findings show that the shear modulus governs austenite stability prior to martensitic transformation in polycrystalline steel alloys. A non-linear increase in shear modulus is also observed during the cooling cycle. The correlation between elastic modulus and austenite stability in polycrystalline steel alloys is discussed, and the compatibility of the results obtained in this study with the stability theory analyzed in a single crystal is also discussed.

Visualization and Analysis of Variant Grouping in Continuously Cooled Low-Carbon Steel Welds

The variant grouping behavior of a simulated heat-affected zone in low-carbon steel was tracked in situ and analyzed by laser scanning confocal microscopy and electron backscatter diffraction. By visualizing the microstructural changes and crystallographic properties under rapid cooling conditions, the transition from Bain zone grouping to close-packed plane grouping was directly studied. The results were compared with those obtained under slow cooling and the link between the variant grouping behavior and the nested microstructure of lower bainite and martensite is clarified.

Top-seeded solution growth of 3 inch diameter 4H-SiC bulk crystal using metal solvents

We performed top-seeded solution growth of 4H-SiC for obtaining longer length crystal. Si-Cr and Si-Ti melts were used as solvents. Meniscus formation technique was applied to the present study. Special attention was paid to improve the process stability during long-term growth. One of major technological problems in the solution growth is that the precipitation of polycrystalline SiC which hiders the stable single crystal growth. Another problem is the fluctuation of supersaturation at the growth interface during the growth. Through the optimization of growth process conditions, we have successfully grown 4H-SiC single crystals up to 14 mm long with three-inch-diameter, and evaluated their crystalline quality.

Nano-tube-like surface structure in graphite anodes for lithium-ion secondary batteries

Abstract We report microstructures on the surface of graphite particles found in practicable carbon anodes by means of high-resolution transmission electron microscopy. The surfaces consist of “closed-edge” structures constructed in a similar manner as carbon nano-tube. We have also investigated the formation mechanism of these nano-structures using molecular dynamics simulations based on the Tersoff potential. From electrochemical measurements, the carbon anodes composed of these “closed-edge” structures show actually high battery performance with a large discharge capacity and a small irreversible capacity.

Visualization of Microstructural Factor Resisting the Cleavage-Crack Propagation in the Simulated Heat-Affected Zone of Bainitic Steel

Cleavage-crack propagation behavior was investigated in the simulated coarse-grained heat-affected zone (CGHAZ) of bainitic steel using electron backscattering diffraction (EBSD) pattern analysis when a low heat input welding was simulated. From viewpoint of crystallographic analysis, it was the condition in which the Bain zone was smaller than the close-packed plane (CP) group. It was clarified that the Bain zone and CP group boundaries provided crack-propagation resistance. The results revealed that when the Bain zone was smaller than the CP group, crack length was about one quarter the size of that measured when the CP group was smaller than the Bain zone because of the increasing Bain-zone boundaries. Furthermore, it was clarified that the plastic work associated with crack opening and resistance at the Bain and CP boundaries could be visualized by the kernel average misorientation maps.