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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. [ABSTRACT FROM AUTHOR]

Polarization switching in ferroelectric hysteresis of BaTiO3 proceeds by localized nucleation and subsequent growth of domains of reverse polarization. While this process is driven by the applied electric field, thermal activation can play an important role in domain nucleation. As a consequence of the stochastic nature of thermal activation, coercive fields in small systems exhibit a significant scatter. It is demonstrated that the statistics of coercive fields observed in molecular dynamics simulations is consistent with the statistical distribution expected for field-assisted thermally activated nucleation of reverse polarization domains. The excellent quantitative agreement between the simulation data and the theory of thermally activated processes provides strong evidence that polarization switching occurs by nucleation-and-growth rather than loss of the local minimum of the Gibbs free energy function. In a pristine crystal, switching is controlled by the field at which thermal fluctuations can create a critical nucleus in the bulk (homogeneous nucleation). The analysis of crystals with various vacancy-type defects demonstrates that such defects may enable heterogeneous nucleation at reduced fields. In both cases, the statistical analysis gives access to the respective activation energies, their field dependence, and the sizes of the critical nuclei. [ABSTRACT FROM AUTHOR]

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 1 + 2 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. [ABSTRACT FROM AUTHOR]

Developing a composite material of polymers and micrometer-sized fillers with higher heat conductance is crucial to realize modular packaging of electronic components at higher densities. Enhancement mechanisms of the heat conductance of the polymer-filler interfaces by adding the surface-coupling agent in such a polymer composite material are investigated through the non-equilibrium molecular dynamics (MD) simulation. A simulation system is composed of α-alumina as the filler, bisphenol-A epoxy molecules as the polymers, and model molecules for the surface-coupling agent. The inter-atomic potential between the α-alumina and surface-coupling molecule, which is essential in the present MD simulation, is constructed to reproduce the calculated energies with the electronic density-functional theory. Through the non-equilibrium MD simulation runs, we find that the thermal resistance at the interface decreases significantly by increasing either number or lengths of the surface-coupling molecules and that the effective thermal conductivity of the system approaches to the theoretical value corresponding to zero thermal-resistance at the interface. Detailed analyses about the atomic configurations and local temperatures around the interface are performed to identify heat-transfer routes through the interface. [ABSTRACT FROM AUTHOR]

In this paper, we present time-reversible simulation algorithms for rigid bodies in the quaternion representation. By advancing a time-reversible algorithm [Y. Kajima, M. Hiyama, S. Ogata, and T. Tamura, J. Phys. Soc. Jpn. 80, 114002 (2011)] that requires iterations in calculating the angular velocity at each time step, we propose two kinds of iteration-free fast time-reversible algorithms. They are easily implemented in codes. The codes are compared with that of existing algorithms through demonstrative simulation of a nanometer-sized water droplet to find their stability of the total energy and computation speeds. [ABSTRACT FROM AUTHOR]

This paper reports a numerical simulation study about the chemical reactions of a nanosized water droplet inserted between H-terminated Si(001) surface and a nanosized, H-terminated diamond-tip when the tip is either slid on or pushed to the surface. The hybrid quantum-classical simulation method, in which the quantum region described with the density-functional theory is embedded in the total system of classical atoms, is used to perform the simulation runs in realistic settings. A feature to select the quantum region adaptively during the run is added to trace the time evolution of the contact area of the tip and surface. When the tip pushes the water droplet, while the Si surface interacts weakly with the water molecule, the tip draws a water molecule from the droplet into a unique metastable state in close proximity to the end of the tip. When the tip is further slid on or pushed to the Si surface, the water molecule in the metastable state decomposes due to high stresses concentrated at the contact area and oxidizes the surface if the molecule is trapped in a dimple structure of the surface. On the other hand, if the water molecule finds enough space between the tip and surface, it runs away without changing the bonding characteristics of both tip and surface. [ABSTRACT FROM AUTHOR]

A hybrid quantum-mechanical/molecular-dynamics simulation is performed to study the effects of environmental molecules on fracture initiation in silicon. A (110) crack under tension (mode-I opening) is simulated with multiple H 2 O molecules around the crack front. To accurately model the long-range stress field, the quantum-mechanical description is embedded in a large classical molecular-dynamics simulation. The hybrid simulation results show that the reaction of H 2 O molecules at a silicon crack tip is sensitive to the stress intensity factor K. For K 0.4 MPa. √ m, an H 2 O molecule either decomposes and adheres to dangling-bond sites on the crack surface or oxidizes Si, resulting in the formation of a Si-O-Si structure. For a higher K value of 0.5 MPa· m, an H 2 O molecule either oxidizes or breaks a Si-Si bond. [ABSTRACT FROM AUTHOR]

Presents information on a study which focused on an interatomic potential model for molecular-dynamic simulations of titanium dioxide atoms. Information on a variable-charge interatomic potential for bulk rutile titanium dioxide; Comparison of various quantities with experimental data; Discussion of results.

The stability of surface structure and space-charge distribution in free TiO[sub 2] nanoparticles are investigated for both rutile and anatase phases using a variable-charge interaction potential in which atomic charges vary dynamically depending on their environment. We find the dynamic charge transfer: (i) enhances atomic diffusion at surfaces of the spherical nanoparticles at high temperatures; and (ii) creates additional repulsive force between the two nanospheres through formation of a double-charge surface layer in each nanosphere. The surface diffusion due to the charge transfer clearly distinguishes the two nanospheres with different underlying crystalline structures; the surface diffusion constant of the anatase sphere is almost three times as great as that of the rutile sphere. Variable-charge molecular dynamics simulations are then applied to sintering of two TiO[sub 2] nanospheres. It turns out that the enhanced surface diffusion in the anatase nanosphere at high temperatures significantly promotes neck formation between the two spheres. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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.