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BaTiO3 is a well-known piezoelectric material with commercial uses. The ferroelectric state of BaTiO3 generally comprises electrically polarized domains separated by domain walls (DWs). The DW alters local polarization vectors by an angle of 90° for 90° DW or 180° for 180° DW. The DW is crucial to piezoelectric properties such as response time and fatigue. Furthermore, the DW structure and its dynamics in BaTiO3 are not well understood. Hence, for the first time, we theoretically obtained the atomistic structure of the 90° DW via molecular dynamics simulations at 300 K with the core–shell interatomic potential, using a large-scale system with a side length of 2.8 × 10 3 Å. The width of the 90° DW thereby obtained was approximately 30 Å, which was 20 Å wider than that of the 180° DW. Under the external electric field E → ext parallel to the DW, we observed an extension of a domain having a polarization vector with a positive component along the E → ext-direction. The migration velocity of the 90° DW was approximately two times that of the 180° DW at the same E ext in the range 7 – 20 MV / m. For E ext ≥ 15 MV / m, the migration velocity of the 90° DW in the direction with a positive component along the polarization vector of the extending domain was substantially higher than that in the opposite direction. The physical causes of the difference in the migration velocities of the 90° DW in the two directions were analyzed.

BaTiO3 is one of the well-known ferroelectric and piezoelectric materials, which has been widely used in various devices. However, the microscopic mechanism of the ferroelectric domain growth is not understood well. We investigated the effects of point defects, mono- and di-vacancies of Ba, Ti, and O, on the domain growth of BaTiO3 using molecular dynamics simulation with the core-shell inter-atomic potential. We found the following: ｓ(1) One kind of monovacancy, VO1, located on the TiO plane perpendicular to the applied electric field direction, acts to hinder the polarization inversion induced by the applied electric field. The monopole electric field produced by VO1 either hinders or assists the local polarization inversion in accordance with the local intensity of the total electric field. (2) The 1st-neighbor divacancies VBa-VO and VTi-VO as compared to the 2nd-neighbor divacancies asymmetrically affect the domain growth with respect to the applied electric field, making the hysteresis behavior of applied electric field vs. polarization relation. The domain grows even at a small electric field when the directions of the applied electric field and the divacancy dipole are mutually the same. (3) The domain growth speed towards the applied electric field direction is about 2 orders of magnitude higher than that towards the perpendicular direction.

In a density of states format of the frequency-extended surface-enhanced Raman spectrum, potential-induced variations of the lifetime of hydrogen bonds and their dynamic behavior were observed at aqueous solution/Au interfaces.

Silane coupling is commonly used to connect organic polymers to inorganic substrates for surface modification and composite material fabrication. It is known that the covalent bonds that form betwe...

The moving components of combustion engines are operated under harsh conditions of high pressures and temperatures. Extreme-pressure anti-wear additives, such as tricresyl phosphate (TCP), are mixed with base oil to prevent wear through the formation of a lubricant film on the substrate. We studied the effect of liquid pressure on the decomposition pathway of TCP in base oil molecules (2,5-dimethylhexane) using hybrid quantum-classical simulations with density functional theory for electrons. At a temperature of 300 K, we found that: (i) bond-breaking barrier energies of both the OC and PO bonds of TCP decrease monotonically as the liquid pressure increases; (ii) the bond-breaking barrier energy of PO is lower than that of OC at pressures of 0 and 2.0 GPa, but is higher at a pressure of 5.0 GPa; and (iii) the applied pressure significantly lowers the bond-breaking barrier energies of both OC and PO when the PO bond of TCP is directed upward from the substrate. These findings are explained by the inhomogeneous distribution of base oil molecules around TCP and the steric repulsion of the PO bond of TCP. These results indicate that the internal structures of the lubricant films are pressure-dependent.

In this study, we investigated the protonation of the amine group in epoxy resins prepared using amine-based curing agents by theoretical methods. Density functional theory (DFT)-based free-energy calculations of the corresponding deprotonation subreactions showed that the amine group of the epoxy resin is protonated at equilibrium depending on the location of the amine group when the epoxy resin is embedded in water under standard conditions. Additional DFT calculations demonstrate that the energetic barrier for breaking the ether bond of the epoxy resin is lowered by about 0.6 eV as a result of the cooperative effect of H2O dissociation and that the transition-state energy for breaking the amine group bond is lowered by about 0.4 eV after the protonation of the amine group. Comparing the transition-state energies, we predict that the bond breakage of the protonated amine groups is the principal process causing the weakening of epoxy resins under wet conditions.

A nonequilibrium molecular dynamics method is proposed to produce nonequilibrium states flexibly. In this method, virtual points are set in a simulation box, and coarse-grained physical quantities at these points are constrained using Gauss's principle of least constraint. The coarse-grained physical quantities are evaluated by averaging microscopic quantities with an appropriate weight. To obtain the weight to evaluate the coarse-grained physical quantities, a shape function matrix is initially constructed from the particle configuration. This matrix expresses an interpolation of the physical quantities at particle positions from the coarse-grained quantities at the virtual points. Then, a matrix form of the weight is calculated as the Moore-Penrose pseudoinverse matrix for the shape function matrix. This method is applied to constrain the coarse-grained kinetic energy and produce a nonuniform temperature field in the system. The temperature profile at a nonequilibrium steady state depends on the method for constructing the shape function matrix. In particular, a local temperature coincides with the coarse-grained temperature when the shape function matrix is constructed based on a higher-order interpolation.

BaTiO 3 is one of the well-known ferroelectric and piezoelectric materials, which has been widely used in various devices. However, the microscopic mechanism of the ferroelectric domain growth is not understood well. We investigated the effects of point defects, mono- and di-vacancies of Ba, Ti, and O, on the domain growth of BaTiO 3 using molecular dynamics simulation with the core-shell inter-atomic potential. We found the following: s (1) One kind of monovacancy, V O1 , located on the TiO plane perpendicular to the applied electric field direction, acts to hinder the polarization inversion induced by the applied electric field. The monopole electric field produced by V O1 either hinders or assists the local polarization inversion in accordance with the local intensity of the total electric field. (2) The 1st-neighbor divacancies V Ba -V O and V Ti -V O as compared to the 2nd-neighbor divacancies asymmetrically affect the domain growth with respect to the applied electric field, making the hysteresis behavior of applied electric field vs. polarization relation. The domain grows even at a small electric field when the directions of the applied electric field and the divacancy dipole are mutually the same. (3) The domain growth speed towards the applied electric field direction is about 2 orders of magnitude higher than that towards the perpendicular direction.

BaTiO3 is a well-known piezoelectric material, which is widely used in various devices. In general, the ferroelectric state of BaTiO3 is composed of polarized domains. The growth of these domains due to an applied electric field or stress is related to the piezoelectric performance. We investigated the effects of various point defects, monovacancies {VBa, VTi, VO}, and first- and second-neighbor divacancies {VBa–VO, VTi–VO} on polarized domain growth in BaTiO3 under an applied electric field by molecular dynamics simulations using the core–shell inter-atomic potential. We found that (i) the first-neighbor divacancy VBa–VO is the most effective in assisting the domain growth under an applied electric field (i.e., a smaller coercive electric field) in an asymmetrical manner with respect to the electric field direction. This is mainly due to the creation of an electric field around VBa–VO by significant Ti shifts toward VBa with the assistance of VO. (ii) Domain growth proceeds in a [Formula: see text] dimensional manner. The domain growth velocity in the direction of the applied electric field is approximately two orders of magnitude higher than that in the perpendicular direction. (iii) Increasing the density of the divacancy VBa–VO further lowers the coercive electric field when the applied electric field is parallel to the divacancy dipoles. The present results will be essential for designing the type, orientation, and density of defects to modify the coercive electric field of BaTiO3 in defect engineering.

The adhesive bonding of metals using epoxy resin is an important technology in manufacturing industries. It is well known that adhesion strength becomes significantly reduced in a moist environment...

The system of quinone molecules encapsulated in the single-wall carbon nanotube (SWCNT) has been proposed as a next-generation cathode electrode material for rechargeable battery. We investigate the complex interaction among the SWCNT, phenanthrene-quinone (PhQ), and Li atoms in the encapsulated system by using our original DFT code. We thereby find that the shape of the SWCNT changes significantly in the relaxed state depending on the extent of Li atoms inserted. The SWCNT shows a circular cylinder shape when no Li exists. With sufficient Li atoms inserted, the SWCNT is flattened. Substantial electron transfer from the PhQs to SWCNT is found. As for the dynamics of Li atoms in insertion or extraction process, we find that the Li atoms can take either of the two paths: one is along the inner wall of the SWCNT and the other is hopping on the PhQs.

A novel and efficient scheme for evaluating the work of adhesion between a liquid and a polymer-grafted surface is proposed. A set of spherically symmetric potentials are gradually inserted at the interface to separate the liquid molecules from the surface according to its shape. This method is applied to the interface between the water and the gold substrate modified by poly(ethylene glycol). We find that the work of adhesion becomes maximum at the intermediate density of grafted poly(ethylene glycol). This is attributed to penetration of the water molecules into grafted poly(ethylene glycol) and hydrophilic interaction between them.

A nanocrystalline metal contains grains that are extremely small, with a high proportion of grain boundary (GB). For understanding of the mechanical properties of nanocrystalline metals, it is indispensable to elucidate the influence of GBs on plastic deformation. In this study, molecular dynamics simulations of uniaxial deformation tests on magnesium bicrystals with [ 1 ¯ 1 0 0 ] symmetric tilt GBs were performed in order to reveal the role of GBs during the plastic deformation of hexagonal close-packed metals. We found that not only the critical resolved shear stress but also the active deformation mode itself is sensitively dependent on both the misorientation angle between the grains and the loading direction. In particular, ease of nucleation of basal dislocations from the GB could be qualitatively explained by the dependence of the GB energy on the misorientation angle, where dislocation nucleation from the GB is regarded as a process by which GBs accommodate changes in misorientation angle caused by elastic deformation due to an applied load.

Dislocation nucleation from free surfaces is one of the important process of plastic deformation of a size reduced pillar, in which pre-existing dislocations is not sufficient to take plastic deformation. In this work, nucleation of a basal dislocation from a free surface of an elliptic cylindrical Mg single crystal is considered. Molecular dynamics simulations of tensile deformation of crystals show that the nucleation site of a dislocation changes when the ratio between axes of the ellipse changes, though the critical resolved shear stress is almost the same. Dependence of the activation free energy of nucleation of a dislocation on the nucleation site is also studied by applying metadynamics method to the atomistic simulation. Then, we found that the activation free energy of nucleation of a dislocation lowers at the site where a dislocation is actually nucleated in molecular dynamics simulations.

A quantitative evaluation of the deformation of metals is usually based on the critical resolved shear stress (CRSS), in which the influence of the normal stresses is not taken into account. Although it is plausible to use the CRSS for a quantitative evaluation of the deformation mode, some previous studies show that the CRSS depends on other stress components. In this work, atomistic simulations are performed to study the influence of the normal stresses on the nucleation of basal dislocations in a magnesium single crystal. We found that the activation free energy of dislocation nucleation decreases as the normal stress applied in the [ 1 1 2 ¯ 0 ] direction decreases, or that in the [ 0 0 0 1 ] direction increases. This tendency is consistent with the dependence of CRSS on the loading direction found in a previous work done by Barrett et al. (2012) on molecular dynamics simulations of tensile deformation tests of a magnesium single crystal.

We propose a method for calculating the work of adhesion between a liquid and solid surface by using molecular simulations. Two ideas are introduced for efficient calculation when the proposed method is applied at the interface between a liquid and a polymer-grafted substrate. First, the liquid molecules are separated from the solid surface based on its shape by placing spherically symmetric potentials around the atoms selected from the substrate and the polymers grafted onto it. Second, to avoid deterioration of accuracy during numerical integration of the work, the parameters that appear in the potential are updated so that variations in the gradient of the work are suppressed. This method is applied to the interface between water and a gold substrate modified by poly(ethylene oxide) (PEO), and it is found that the work of adhesion is greater at intermediate PEO densities.

The long-period stacking-ordered (LPSO) phase discovered in magnesium alloys is deformed upon the generation of a large number of unique deformation zones, which have no distinct orientation relationships at the deformation boundaries. These deformation zones are considered kink bands, but the mechanisms underlying their generation are not well understood. It has been suggested that the kink bands are responsible for the deformation of the LPSO phase, while simultaneously strengthening the material. In this study, the kink deformation process of the LPSO phase under compressive deformation was investigated through molecular dynamics (MD) simulations. The MD simulations showed that numerous prismatic 〈a〉 dislocations were nucleated first, which led to cross-slips towards various basal planes and caused kink deformation. This was followed by the nucleation and motion of a large number of basal dislocations, as well as kink deformations in tandem with the formation of kink bands, which occurred through another process. In addition, the individual dislocations were indistinguishable at kink boundaries. In other words, sharp boundaries were formed. Next, a simulation was performed that applied tensile strain to the model after the compressive deformation described above was implemented on it. This revealed that while kink boundaries with large misorientation angles intermittently migrated because of the tensile strain, the kink bands were not easily removed.

A system of quinone molecules encapsulated in single-walled carbon nanotubes (SWCNTs) has been proposed as a next-generation cathode material for rechargeable batteries [Y. Ishii et al., Phys. Chem. Chem. Phys. 18 (2016) 10411–10418]. We use density functional theory (DFT) to theoretically investigate (i) the electronic and structural states of SWCNT-encapsulated quinone with or without Li and (ii) the Li insertion and extraction dynamics in the system. Substantial electron transfer from the quinone molecules to the SWCNT is thereby observed. This electron transfer stabilizes the positively charged quinone molecules in the negatively charged SWCNT through Coulomb attraction and decreases the electronic band gap for the SWCNT with semiconductor chirality. In the case of 9,10-phenanthrenequinone (PhQ), we observe that the cross-sectional shape of the SWCNT changes substantially in the relaxed state depending on the extent of Li insertion: the SWCNT exhibits a circular cylinder shape when no Li is present, whereas it is flattened upon sufficient Li insertion. These SWCNT shapes well reflect the aggregated shapes of PhQ molecules, which depend on the amount of Li inserted. As for the Li insertion and extraction dynamics, we find that the Li atoms can take either of two paths: one is along the SWCNT wall, and the other involves hopping on the PhQ molecules and/or the sites where the C atoms of the SWCNT and the O atoms of PhQ molecules contact each other. The Li-transfer rate on the SWCNT wall is large; hence, the hopping is the rate-limiting step.

Magnesium alloys containing long-period-stacking ordered (LPSO) phases have attracted considerable attention because they have been reported to exhibit excellent mechanical properties, including high strength and reasonable ductility. It is thought that the LPSO phase plays a critical role in producing these favorable mechanical properties. We analyze the deformation behavior of the LPSO phases with different stacking sequences using molecular dynamics simulations. To highlight the specific deformation behavior of the LPSO phases, we also perform deformation analyses of hexagonal-close-packed and face-centered-cubic (FCC) structures. We focus on the influence of the stacking order rather than the segregated atoms around the FCC-structured layers, and we model an LPSO structure by single element composition where the interatomic interaction is described by a smoothed Lennard-Jones potential. Our simulations indicate that an LPSO structure with a shorter stacking sequence tends to exhibit a higher compressive flow stress, because FCC-structured layers inhibit twinning deformations and non-basal slips. Kinking deformation is observed for an LPSO structure when both compression and shear deformation are present. It is shown that the first-order pyramidal-< c + a> dislocation disarranges the stacking of an LPSO structure and leaves behind many lattice defects. In addition, those lattice defects activate numerous basal slips. Finally, basal dislocations arrange in a line and generate a misorientation angle. Furthermore, this angle originates the compressive deformation. We also observed some prismatic-< a> dislocations and cross slips to the basal plane. These results suggest the importance of non-basal slips for kinking deformation.

Kink deformation is one of the possible principal deformation modes of alloys with a long-period stacking ordered structure under compression parallel to the basal plane. In this deformation, dislocation pairs are massively nucleated, and these dislocations align in a line to form kink bands. In this study, we investigated the nucleation of a dislocation pair in a pure magnesium single crystal by molecular dynamics simulations. We also evaluated the activation free energy of nucleation of a dislocation pair and investigated the dependence of the activation free energy on the applied shear stress and temperature. [doi:10.2320/matertrans.MI201213]

The nucleation of a dislocation loop for basal and prismatic slips of a magnesium single crystal was studied via atomistic simulations. The metadynamics method was applied in order to overcome the time-scale restrictions that occur in conventional molecular dynamics simulations. The activation free energy of a dislocation loop nucleation is also evaluated via the metadynamics method. In particular, from the temperature dependence, we showed that a linear relationship between the enthalpic and entropic contributions to the activation free energy is satisfied. We found that the difference between the dependence of the activation free energy on the temperature for basal and prismatic slips is related to the structure of the $\ensuremath{\Gamma}$ surface.

A simple spin system with self-interaction is proposed and studied where the interaction among spins is taken into account within a mean field model. The self-interaction, which turns out to be a kind of activation energy, gives rise to slow relaxation due to disorder introduced to the self-interaction. In a thermodynamic limit (N-->infinity), a phase diagram is obtained and a spin-spin time correlation function (TCF) is calculated exactly to show that relaxation time diverges at low temperature. Dynamical property of a finite system, such as relaxation of the TCF, is studied by numerically solving a master equation.

In this paper we investigate some statistical mechanical properties of systems consisting of two hard disks in a circular cavity and two hard spheres in a cylindrical pore. Both systems conserve not only the total energy but the total angular momentum, and this conservation affects thermodynamic quantities such as the temperature and pressure of these systems. We show the dependence of the thermodynamic quantities of these systems on conserved quantities with a statistical mechanical treatment and molecular dynamics simulations.

The dynamical behavior of a system consisting of a (heavy) piston and two rectangular boxes, each containing two hard disks and in contact with the piston, is studied based on a projection operator method for a microcanonical ensemble. We derive a coupled set of nonlinear equations for slow variables of the system and solve it to confirm that our theory with no adjustable parameters reproduces experimental results fairly well. Some limitations of the theory are discussed from the viewpoint of the separation of slow and fast time scales and ergodicity.

We investigate some statistical mechanical properties of a system consisting of two hard spheres in a D-dimensional rectangular box (D=1, 2, ...). We give a theoretical method for computing a configurational partition function Zc,D of this system and compare the equation of state obtained from Zc,D with molecular dynamics simulations. Especially in D=3, we give a fully analytic expression for the pressure which turns out to have one or more negative compressibility regions when the box size is small.

Using molecular dynamics simulations in which ions are explicitly treated as charged particles, we examined the electrostatic potential around a charged colloidal particle in an electrolyte solution for various situations including the ion strong coupling regime. The electrostatic potential computed from the simulation was compared with the solution of the Poisson–Boltzmann (PB) equation. We found that the electrostatic potential obtained from the simulation can be fitted to the solution of the PB equation if we adopt the surface potential of the colloidal particle and the normalization constants of the Boltzmann distribution function as fitting parameters.

The generalized Langevin equation (GLE) is considered with emphasis put on a mechanical random force , whose time evolution is not natural due to the presence of a projection operator in a propagator. We first derive the GLE for a set of dynamical variables of interest not by formal manipulation of the operator identity as is usually done, but by expressing the random force in terms of natural trajectory in the phase space. Based on this new derivation we observe two structural characteristics, (1) a linear functional interdependence between and and (2) a self-consistent structure (SCS) inherent in the GLE. The SCS gives an iterative procedure to determine and its time correlation function, i.e. a memory function K(t), once we have numerical data for supplied by computer experiments. This method generates and a memory function K(t), nicely for an exactly solvable many-body system after several iterations. Some merits of using the SCS are further discussed in connection with numerical methods to calculate the autocorrelation function of .