Yoshitaka Umeno

First-principles approaches to intrinsic strength and deformation of materials: perfect crystals, nano-structures, surfaces and interfaces

First-principles studies on the intrinsic mechanical properties of various materials and systems through ab initio tensile and shear testing simulations based on density-functional theory are reviewed. For various materials, ideal tensile and shear strength and features of the deformation of bulk crystals without any defects have been examined, and the relation with the bonding nature has been analyzed. The surfaces or low-dimensional nano-structures revealpeculiarstrengthanddeformationbehaviorduetolocaldifferentbonding nature. For grain boundaries and metal/ceramic interfaces, tensile and shear behaviors depend on the interface bonding, which impacts on the research of real engineering materials. Remaining problems and future directions in this research field are discussed. (Some figures in this article are in colour only in the electronic version)

Ab initio study of surface stress response to charging

We explore an efficient way to numerically evaluate the response of the surface stress of a metal to changes in its superficial charge density by analysis of the strain-dependence of the work function of the uncharged surface. As an application we consider Au(111), (110) and (100) surfaces, employing density functional theory (DFT) calculations. The sign of the calculated response parameter can be rationalized with the dependence of the surface dipole and the Fermi energy on strain. The numerical value falls within the range indicated by experiment. The magnitude can explain the experimentally observed volume changes of nanoporous materials upon charging.

Shell model potential for PbTiO3 and its applicability to surfaces and domain walls

We have developed an efficient interatomic potential for PbTiO3 in the framework of the shell model by fitting its parameters to reproduce both the mechanical and ferroelectric properties derived from ab initio density functional theory calculations. The optimized potential successfully yields the crystal structures, elastic properties and phonon dispersion curves, whereas the spontaneous polarization and effective charges are slightly underestimated. It reproduces well characteristic ferroelectric (FE) and antiferrodistortive (AFD) instabilities closely associated with the structural phase transition in PbTiO3, and is reliable under high tension and compression along the [001] direction. Furthermore, the potential is effective enough to describe 180? and 90? domain walls as well as the PbO-terminated surface with c(2 ? 2) reconstruction where the FE and AFD distortions coexist. This significant success widely extends the applicable range of atomic-level simulations of ferroelectrics based on the shell model potential.

Ab initio DFT simulation of ideal shear deformation of SiC polytypes

We perform ab initio density functional calculations to investigate the ideal shear deformation of SiC polytypes (3C, 2H, 4H and 6H). The deformation of the cubic and hexagonal polytypes in the small-strain region can be well represented by the elastic property of component Si4C-tetrahedrons. The stacking pattern in the polytypes affects strain localization, which is correlated with the GSF energy profile of each shuffle-set plane and the ideal shear strength. Compressive hydrostatic stress decreases the ideal shear strength but does not much affect the shear elastic coefficient. Tersoff classical interatomic potential can represent the deformation behaviour of SiC crystals in the small-strain region but cannot be applied to largely sheared and compressed situations.

Optimization of interatomic potential for Si/SiO2 system based on force matching

Abstract In order to analyze the behavior of Si and SiO2, which are basic materials for the large-scale-integrated circuit technology, the molecular dynamics simulation has been widely used. For the simulation, it is necessary to select accurate interatomic potential function, which reproduces the force acting on each atom. However, the validity of potential functions proposed has not been discussed in terms of the force. In this study, the force calculated by the original Tersoff potential is compared with that obtained by an ab initio calculation in some snapshots of Si/SiO2 systems at the temperature of 300 K. It clarifies that the potential does not properly reproduce the force. Then, optimizing the parameters in the Tersoff potential on the basis of the force acting on each atom obtained by the ab initio calculation (force matching method), two modified functions are proposed. One is “Potential A” obtained by optimizing the parameters for the Si crystal and the β-cristobalite SiO2 crystal. The “Potential A” correctly reproduces not only the forces but also the lattice constants. However, it is not effective for Si/SiO2 interface. The “Potential B” obtained by optimizing the parameters for a Si/SiO2 interface model as well as the Si and the β-cristobalite SiO2 crystals. The forces and lattice constants are successfully reproduced for the interface as well. Moreover, the “Potential B” gives a good result for the β-quartz SiO2, which is not used for the optimization. This implies the versatility of the proposed function.

Reversible relaxation at charged metal surfaces: An ab initio study

Results of an ab initio density functional theory study of atomic and electronic relaxation at electrically charged surfaces of Au suggest that the outward relaxation of the top layer at negative excess charge is driven by electrostatic forces on the surface atoms due to the incomplete screening of the external electric field. The relaxation amplitude agrees well with experiments on Au(111) in electrolyte. Electron redistribution between bonding and antibonding states in the plane containing the surface atoms may contribute to the charge response of the surface stress.

Validity of effective medium theory for aluminium under tension

Reliability of the potential functions under the condition far from equilibrium states, which is called transferability, is an important factor in the simulations of materials with nanoscopic complex structure under high stress condition. However, it has not been sufficiently investigated because it is difficult to get precise experimental data in such conditions. In this paper, simulations are conducted for aluminium bulk, grain boundary of aluminium and atomic chain under high strain using the potential function of the effective medium theory (EMT) as well as ab initio calculations in order to clarify the validity of EMT. In the cases of single crystal and the grain boundary under tensile strain, the results obtained from the EMT potential agree well with those obtained by ab initio analysis. However, the EMT cannot be applied to the atomic chain because the distribution of charge density differs significantly from that in the bulk.

Ideal shear strength under compression and tension in C, Si, Ge, and cubic SiC: an ab initio density functional theory study

Ideal shear strength under superimposed normal stress of cubic covalent crystals (C, Si, Ge, and SiC) is evaluated by ab initio density functional theory calculation. Shear directions in [112] and [110] on the (111) plane are examined. The critical shear stress along the former direction is lower than that along the latter in all the crystals unless the hydrostatic tension is extremely high. In both the [112]-shear and [110]-shear, critical shear stress is increased by compression in C but is decreased in the other crystals. The different response of the critical shear stress to normal stress is due to the strength of the bond-order term, i.e., dependence of the short-range interatomic attraction on the bond-angle.

Ideal strength of nano-components

The ideal (theoretical) strength was originally defined as the stress or strain at which perfect crystal lattice became mechanically unstable with respect to arbitrary homogeneous infinitesimal deformation. This has been intensely investigated because the ultimate strength without defects is a fundamental mechanical characteristic of materials. In the analyses, the instability criteria have been studied on the basis of elastic constants. Recent developments in computational technology make it possible to analyze the ideal strength on the basis of quantum mechanics. On the other hand, it is well known that the mechanical strength of components is dependent not only on (1) material (atom species), but also on (2) loading condition and (3) structure. Because most studies on the strength in terms of atomic mechanics have focused on the factor (1) (materials), analysis has mainly been conducted on simple crystal consisting of perfect lattices (e.g. fcc and bcc) under simple loading conditions (e.g. tension), though some have explored the properties of bulk materials with defects (e.g. vacancy and grain boundary). Small atomic components (nano-structured components) such as nano-films, nano-wires (tubes) and nano-dots (clusters) possess their own beautiful, defect-free structures, namely ideal structure. Thus, they show characteristic high strength. Moreover, utilizing the structure at the nanometer or micron level is a key technology in the development of electronic devices and elements of micro (nano) electro-mechanical systems (MEMS/NEMS). Therefore, it is important to understand the mechanical properties not only for the sake of scientific interest, but also for engineering usefulness such as design of fabrication/assembly processes and reliability in service. In the other words, the effects of structure (factor (3); e.g. film/wire/dot) have to be understood as the basic properties of atomic components. Thus, the definition of ideal strength should be expanded to include the strength at instability of components with ideal structures under various external loads (factor (2)), which provides fundamental knowledge of nano-structured materials. In this paper, we review works on the strength of ideal nano-structured components in terms of factor (3), mainly under tension.

Synchronization in flickering of three-coupled candle flames

When two or more candle flames are fused by approaching them together, the resulting large flame often exhibits flickering, i.e., prolonged high-frequency oscillation in its size and luminance. In the present work, we investigate the collective behaviour of three-coupled candle flame oscillators in a triangular arrangement. The system showed four distinct types of syncronised modes as a consequence of spontaneous symmetry breaking. The modes obtained include the in-phase mode, the partial in-phase mode, the rotation mode, and an anomalous one called the “death” mode that causes a sudden stop of the flame oscillation followed by self-sustained stable combustion. We also clarified the correlation between the inter-flame distance and the frequency with which the modes occur.