Tomohisa Kumagai

Development of empirical bond-order-type interatomic potential for amorphous carbon structures

A bond-order-type interatomic potential has been developed for reproducing amorphous carbon (a-C) structures. Several improvements have been incorporated into the conventional Brenner potential so that the material properties of carbon crystals remain unchanged. The main characteristics of the potential function developed in the present research are the use of a screening function instead of a cutoff function and the introduction of a dihedral angle potential around the bond between two threefold coordinated atoms. By using the developed interatomic potential, we can reproduce the material properties of a-C structures, such as the fraction of sp3-bonded atoms, radial distribution function, and ring statistics. It is found that the correction term enhances the formation of cluster structures in a-C, which is confirmed in the first-principles calculation.

Nanostructural interpretation for elastic softening of amorphous carbon induced by the incorporation of silicon and hydrogen atoms

First-principles molecular dynamics simulation is used to investigate the elastic softening of amorphous carbon on the incorporation of silicon and hydrogen atoms, and the mechanisms responsible for this phenomenon are discussed from the viewpoint of atomic structure. With increasing silicon incorporation, it is found that the bulk moduli of silicon-incorporated amorphous carbon (a-C:Si) and silicon-incorporated hydrogenated amorphous carbon (a-C:Si:H) decrease, whereas the total number of sp3-bonded atoms increases. This is explained on the basis of interatomic bond structures such as: increasing silicon incorporation reduces the number of interatomic (both single and double) bonds between carbon atoms while increasing the number of interatomic bonds between silicon and carbon atoms. Furthermore, for a given density and silicon content, it is found that the bulk modulus of the a-C:Si structure is greater than that of the a-C:Si:H structure, though their interatomic bond structures are similar. The reduce...

Development of a bond-order type interatomic potential for Si–B systems

A bond-order type interatomic potential for Si–B systems is developed. We employed the bond-order type potential function proposed by Tersoff. Properties of Si–B crystals which involve a wide range of local atomic environments are used for fitting. The formation energies of various atomic structures that are thought to appear during B diffusion or Si–B clustering are also fitted. A genetic algorithm is used to find the optimized parameters. The resulting potential reproduced well the Si-interstitial-assisted diffusion of B as well as the physical properties used for fitting.

Development of a Method for Making Interatomic Potential: An Application to Metallic Systems

A method for making interatomic potentials is proposed and is applied to Cu-Zr-Hf-Ni- Al bulk-metallic-glass systems. The method consists of three steps. Firstly, potential function form is determined so that bonding nature can be described. Secondly, materials properties used for fitting are selected so that the potential has enough robustness. Here, it is noted that materials properties must be added in accordance with the purpose of the study. Finally, potential parameters have been optimized using global-search procedure. Developed potential well reproduces material properties of them.

Effects of guest atomic species on the lattice thermal conductivity of type-I silicon clathrate studied via classical molecular dynamics

The effects of guest atomic species in Si clathrates on the lattice thermal conductivity were studied using classical molecular dynamics calculations. The interaction between a host atom and a guest atom was described by the Morse potential function while that between host atoms was described by the Tersoff potential. The parameters of the potentials were newly determined for this study such that the potential curves obtained from first-principles calculations for the insertion of a guest atom into a Si cage were successfully reproduced. The lattice thermal conductivities were calculated by using the Green-Kubo method. The experimental lattice thermal conductivity of Ba8Ga16Si30 can be successfully reproduced using the method. As a result, the lattice thermal conductivities of type-I Si clathrates, M8Si46 (M = Na, Mg, K, Ca Rb, Sr, Cs, or Ba), were obtained. It is found that the lattice thermal conductivities of M8Si46, where M is IIA elements (i.e., M = Mg, Ca, Sr, or Ba) tend to be lower than those of M...

Structures and phonon properties of nanoscale fractional graphitic structures in amorphous carbon determined by molecular simulations

Although amorphous carbon (a-C) materials are being widely used, relaxed atomic structures of a-C have not yet been investigated in detail. In this study, a-C structures were relaxed in molecular simulations, and their structural properties and phonon properties were investigated. As a result, several nanoscale fractional graphitic structures were observed in the annealed a-C structures. Further, it was found that the fractional graphitic structures caused a peak in the fractional phonon density of states of the annealed a-C structures, which corresponded to the D peak. The main phonon mode in the fractional graphitic structure with phonon frequencies of the D peak position and that of the G peak position were the distortion mode of six-membered rings, and the stretching mode of the bonds between threefold coordinated atoms, respectively. Both the distortion mode of six-membered rings and the bond-stretching mode were observed in phonon frequencies between the D peak position and the G peak position.

Simple bond-order-type interatomic potential for an intermixed Fe-Cr-C system of metallic and covalent bondings in heat-resistant ferritic steels

It is known that M23C6(M = Cr/Fe) behavior in heat-resistant ferritic steels affects the strength of the material at high temperature. The ability to garner direct information regarding the atomic motion using classical molecular dynamics simulations is useful for investigating the M23C6 behavior in heat-resistant ferritic steels. For such classical molecular dynamics calculations, a suitable interatomic potential is needed. To satisfy this requirement, an empirical bond-order-type interatomic potential for Fe-Cr-C systems was developed because the three main elements to simulate the M23C6 behavior in heat-resistant ferritic steels are Fe, Cr, and C. The angular-dependent term, which applies only in non-metallic systems, was determined based on the similarity between a Finnis-Sinclair-type embedded-atom-method interatomic potential and a Tersoff-type bond-order potential. The potential parameters were determined such that the material properties of Fe-Cr-C systems were reproduced. These properties include...

Elastic Properties of the Surfaces and Interfaces of Crystal and Amorphous Silicon

We propose a method to calculate well-relaxed interface/surface properties by using order-parameters to evaluate interface/surface microstructures. Structural relaxation greatly influences the energies of the a/c interface and the surface. It is found that the interface energies are smaller than the surface energy and do not depend significantly on the crystal orientation. These findings agree very well with the experiments. The interface stress is also smaller than the surface stress and involves the scattering across a broad range.