Shinji Munetoh

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...

Solution Growth of Self-Standing 6H-SiC Single Crystal Using Metal Solvent

Silicon carbide (SiC) crystal growth from ternary solutions Si-C-X where X is a transition metal was studied. In order to select the desirable transition element and to determine the solution composition, we have conducted the calculations of ternary phase diagrams by means of CALPHAD (CALculation of PHase Diagrams) method. Preliminary growth experiments without a seed crystal were also performed. Among various Si-based solutions, Si-C-Ti was one of the most effective solutions to increase crystal growth rate compared with Si-C. Optical microscopic observation of the obtained SiC etched by molten KOH showed no micropipe defects in the crystals. We have also performed the growth experiments with 6H-SiC seed crystal under temperature gradient. As a result, we have successfully obtained a 12mm×12mm self standing SiC crystal. Introduction At present, commercially available silicon carbide (SiC) wafers are produced by the Physical Vapor Transport (PVT) technique. Although a large number of attempts have been made on the reduction of structural defects, PVT crystals still exhibit remaining defects, such as micropipes and dislocations. Especially, micropipe is a primary defect degrading electronic devise performance and thought to be inevitable using PVT technique. Thus it is very important to establish new growth method of the low defect SiC bulk crystal. Solution growth technique is a promising answer to this problem. Growth from liquid phase has expected to improve the quality of the crystals because the growth proceeds under thermal equilibrium. In the case of SiC, although the congruent melt cannot be obtained, SiC can be precipitated from Si-C based solutions[1,2,3]. Si solvent (self flux) has only a small C solubility at moderate temperature, which reduces the SiC growth rate two orders of magnitude lower than that of PVT technique. On the other hand, ternary Si-C-X solution, where X is a carefully selected additive metal might exhibit a large C solubility at relatively low temperature. However lack of well-established ternary phase diagram, Si-C-X makes it difficult to select the desirable X element and to design the solution. In this study we report the solution growth of 6H-SiC from ternary Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 123-126 doi:10.4028/www.scientific.net/MSF.457-460.123

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.

Surface nucleation of the (111) plane of excimer laser annealed Si on SiO2 substrates: A molecular dynamics study

We have investigated the nucleation and crystallization processes of molten silicon (Si) on SiO2 substrates by performing molecular dynamics (MD) simulations based on the modified Tersoff potential parameters. A heat flow that leads to a steady fall of the local temperature in the molten Si is achieved by determining the atomic movements with the combination of Langevin and Newton equations. Good agreement is reached between the predictions of temperatures based on the kinetic energies and the velocity distributions of atoms at local regions. The results of simulations revealed that the (111) plane of the Si nuclei formed at the surface was predominantly parallel to the substrate of MD cell. The surface energies of the (100), (110), and (111) planes of Si at 77 K were calculated to be 2.27, 1.52, and 1.20 J∕m2, respectively, and they were in good agreement with the experimental results. The lowest value of surface energy, 1.20 J∕m2, for the (111) plane at 1700 K was obtained under the condition of elastic...

Thermoelectric properties of single crystalline Ba8AlxSi46-x clathrate by using flux Czochralski method

Single crystalline Ba8AlxSi46−x clathrates for use as thermoelectric materials were synthesized by the flux Czochralski method. Their aluminum content increased with that of the solution. The Seebeck coeffcients and electric specifc resistances of the Ba8AlxSi46−x clathrates increased with aluminum content, whereas the carrier concentration decreased. The chemical compositions of the single crystalline samples from solutions of Ba8Al30Si30, Ba8Al40Si30 and Ba8Al50Si30 were Ba7.8Al14.1Si31.9, and Ba7.8Al14.6Si31.4, and Ba7.8Al14.9Si31.1, respectively. The power factors of the single crystalline Ba7.8Al14.1Si31.9, Ba7.8Al14.6Si31.4 and Ba7.8Al14.9Si31.1 at 773 K were 6.86×10−4 7.75×10−4 and 8.89×10−4 V2/K2Ωm. The results indicate that the carrier concentration of the Ba8AlxSi46−x clathrates can be controlled by the composition of the solution, and that the flux Czochralski method is effective for synthesizing high performance thermoelectric materials.

Molecular-dynamics simulations of nucleation and crystallization processes of laser crystallized poly-Si

The nucleation and crystallization processes of excimer-laser annealed Si on a SiO2 substrate for complete melting conditions have been investigated by using molecular-dynamics simulations. In the early stage of nucleation, the preferential growth of nuclei with a {111} face normal to the surface was originated from the {111} twin boundaries with a low surface energy. The partial rotation of the dimer leads to the growth of {111}-oriented nuclei along twins that have different stacking sequences. The recombination of vacancies and dimers at the solidification front is directly related to {111} growth from the twin boundaries.

Crystal Growth of Silicon Thin Films on Glass by Excimer Laser Annealing: A Molecular-Dynamics Study

We have investigated crystallization processes during excimer laser annealing of silicon (Si) thin films on glass by molecular-dynamics simulations and laser power dependence of the polycrystalline Si grain size was discussed. The temperature range for the highest growth rate was found to be approximately 500 degrees higher than that for the highest nucleation rate. It was also found that a steady state temperature gradient was obtained in the direction of the surface normal during laser irradiation. These results suggest that nucleation occurs in the Si/glass interface region and then crystallization proceed toward the high temperature region during laser irradiation in the near-complete melting condition.

Crystal Quality Evaluation of 6H-SiC Layers Grown by Liquid Phase Epitaxy around Micropipes using Micro-Raman Scattering Spectroscopy

We performed homoepitaxial growth of 6H-SiC layers on substrates including micropipes by the LPE method and evaluated the crystal quality by Raman scattering spectroscopy. In particular, we focused on the crystal quality of layers covering micropipes. It was made clear that there is no stress due to morphological macroscopic defects in the crystal over micropipes. Moreover, LPE growth not only closes a micropipe but also reduces the inhomogeneity of carrier density which exists in the area of the micropipe before growth.

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.

New method to obtain (001) surface-oriented polycrystalline silicon films by intensity-modulated excimer laser annealing: Molecular dynamics study

We have investigated the dependence of the melting and crystal growth rates on the crystal orientation at solid/liquid (s/l) silicon (Si) interfaces by molecular dynamics (MD) simulations. It was found that there was no appreciable difference in the melting rates, but that the growth rates substantially depend on the crystal orientation at the s/l interface. The growth rate at the (001) interface was found to be more than twice that at the (111) interface. We have also performed MD simulations of an intensity-modulated excimer laser annealing (IMELA) of Si thin films, and these results suggest that (001) surface-oriented Si without {111} stacking faults can be obtained by repetitions of melting and crystallization of amorphous Si on glass by IMELA owing to the preferential growth in the direction.