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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...

Diffusion of Li-ions in graphite is an essential elementary process in the current lithium-ion battery. The C-layers of graphite deform with Li due to relatively large size of Li-ion, act to confine the Li-ions, and thereby creates correlation between them. We address theoretically the thermal diffusivity of such correlated Li-ions in graphite by the hybrid quantum–classical simulation method. In this method, the quantum-region composed of the Li-ions and surrounding C atoms is treated by the density-functional theory, while it is embedded dynamically in the total system described with an empirical inter-atomic interaction potential. We thereby take into account the long-ranged deformation field in graphite in simulating the Li-ion dynamics. Two kinds of settings of Li-ions are considered for the simulation runs at temperature 443 K : (i) seven Li-ions are inserted in the same inter-layer space of the C-layers to study their intra-plane correlation, and (ii) additional seven Li-ions are inserted in the neighboring space (i.e., fourteen Li-ions totally, 7 Li-ions in upper and 7 Li-ions in lower spaces) to study their inter-plane correlation. As for (i), the Li-ions, concentrated initially with inter-ion distances of 2.5 – 4.2 A , scatter due to their mutual Coulomb repulsion. After about 1 ps , the Li-ions and surrounding C atoms thermalize well with deformed C-layers creating a cage of radius about 13.5 A for 7 Li-ions. Diffusivity of Li-ions inside the cage is much higher than that of the cage itself. The long-time diffusion constant of the cage is the same order as that of an isolated Li-ion in graphite. As for (ii), the Li-ions, concentrated initially in the upper and lower inter-layer spaces of the C-layer, firstly form domains, and then the domains repel each other horizontally. The result is in accord with the experimental finding that the Li-rich and Li-poor planes stack in an alternating sequence in graphite.

The composite of epoxy polymers and α-alumina fillers is used as a heat dissipation material. The fillers often agglomerate with nanometer-depth polymers sandwiched in between. We address theoretically the effective thermal conductivity of such a filler-polymer-filler system. The non-equilibrium molecular dynamics simulation is performed to obtain the effective thermal conductivity of the system, in which bisphenol-A (bisA) epoxy polymer sub-system with depth 14–70 A is inserted between two α-alumina slabs. Effects of surface-coupling (SC) agent are also investigated by adding model molecules to the polymer sub-system. For smaller polymer-depth cases, the effective thermal conductivity is determined essentially by the interfacial thermal conductance that relates to the temperature-gaps at the interfaces. We find for the interfacial thermal conductance that: (i) it is decreased by decreasing the polymer depth toward the chain length of a single bisA molecule, and (ii) it is increased by adding the SC molecules to the polymer sub-system. Combining separate simulation analyses, we show that the (i) results from effectively weakened interaction between a bisA molecule and two α-alumina slabs due to the orientation constraint on the bisA molecule by the slabs. Reasons of the (ii) are enhancement of the following three quantities by addition of the SC molecules: the phonon population of the bisA molecules at those frequencies corresponding to that of acoustic phonons of α-alumina, the phonon transmission coefficient from the α-alumina slab to the polymer sub-system for the transverse acoustic phonon, and the group velocity of the transverse acoustic phonon in the polymer sub-system.

Molecular dynamics simulation has been performed to investigate the effects of irradiated helium atoms in tungsten on the bubble nucleation and the dislocation loop formation. Simulation results clearly show that helium atoms in tungsten tend to migrate as isolated interstitials at high temperatures and to be absorbed to existing tungsten-vacancies or defects such as bubbles or dislocations. Tungsten self-interstitial atoms pushed out from the helium bubble tend to stay in the vicinity of the bubble and, then form a dislocation loop when the number of the atoms exceed the threshold. Since the bubbles and dislocation loops cause further nucleation of bubbles, there appears a helium bubble array along 〈 1 1 1 〉 direction. The bubble growth rate within this self induced bubble growth mechanism will be much faster than that of existing growth model. The growth model needs to be reformulated by taking the self-induced effects into account.

The generation of tungsten fuzzy nanostructure by exposure to helium plasma is one of the important problems for the use of tungsten material as divertor plates in nuclear fusion reactors. In the present paper, the formation mechanisms of the helium bubble and the tungsten fuzzy nanostructure were investigated by using several simulation methods. We proposed the four-step process which is composed of penetration step, diffusion and agglomeration step, helium bubble growth step, and fuzzy nanostructure formation step. As the fourth step, the formation of the tungsten fuzzy nanostructure was successfully reproduced by newly developed hybrid simulation combining between molecular dynamics and Monte-Carlo method. The formation mechanism of tungsten fuzzy nanostructure observed by the hybrid simulation is that concavity and convexity of the surface are enhanced by the bursting of helium bubbles in the region around the concavity.

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.

In hybrid electronic-density-functional/molecular-dynamics schemes, a total system is partitioned in real space into the quantum-mechanical (QM) region treated by the electronic-density-functional theory and the molecular dynamics (MD) region in which atoms are interacting through the empirical inter-atomic potential. In the former hybrid scheme [Ogata et al. Comput. Phys. Commun. 149 (2002) 30], appropriate selection of QM atoms for seamless coupling between the QM and MD regions is limited in Si systems, and applications of the scheme to other materials are difficult. Novel hybrid scheme that is free from the limitation and applicable to both Si and alumina systems, is presented.

Multimillion atom molecular-dynamics (MD) simulations are performed to investigate dynamic frac- ture in glasses and nanostructured ceramics. Using multiresolution algorithms, simulations are carried out for up to 70 ps on massively parallel computers. MD results in amorphous silica (a-SiO2) reveal the formation of nanoscale cavities ahead of the crack tip. With an increase in applied strain, these cavities grow and coalesce and their coalescence with the advancing crack causes fracture in the system. Recent AFM studies of glasses confirm this behavior. The MD value for the critical stress intensity factor of a-SiO2 is in good agreement with experiments. Molecular dynamics simulations are also performed for nanostructured silicon nitride (n-Si3N4). Structural correlations in n-Si3N4 reveal that interfacial regions between nanoparticles are amorphous. Under an external strain, nanoscale cavities nucleate and grow in interfacial regions while the crack meanders through these regions. The fracture toughness of n-Si3N4 is found to be six times larger than that of crystalline α-Si3N4. We also investigate the morphology of fracture surfaces. MD results reveal that fracture surfaces of n-Si3N4 are characterized by roughness exponents 0.58 below and 0.84 above a certain crossover length, which is of the order of the size of Si3N4 nanoparticles. Experiments on a variety of materials reveal this behavior. The final set of simulations deals with the interaction of water with a crack in strained silicon. These simulations couple MD with a quantum-mechanical (QM) method based on the density functional theory (DFT) so that chemical processes are included. For stress intensity factor K = 0. 4M Pa m 1/2 , we find that a decomposed water molecule becomes attached to dangling bonds at the crack or forms a Si-O-Si structure. At K = 0. 5M Pa m 1/2 , water molecules

The authors describe simulation approaches that seamlessly combine continuum mechanics with atomistic simulations and quantum mechanics. They also discuss computational and visualization issues associated with these simulations on massively parallel computers. Scientists are combining continuum mechanics and atomistic simulations through integrated multidisciplinary efforts so that a single simulation couples diverse length scales. However, the complexity of these hybrid schemes poses an unprecedented challenge, and developments in scalable parallel algorithms as well as interactive and immersive visualization are crucial for their success. This article describes such multiscale simulation approaches and associated computational issues using recent work as an example.

Large-scale molecular-dynamics simulations are performed on parallel computers to study critical issues on ultrathin dielectric films and device reliability in next-decade semiconductor devices. New interatomic-potential models based on many-body, reactive, and quantum-mechanical schemes are used to study various atomic-scale effects: growth of oxide layers; dielectric properties of high-permittivity oxides; dislocation activities at semiconductor/dielectric interfaces; effects of amorphous layers and pixellation on atomic-level stresses in lattice-mismatched nanopixels; and nanoindentation testing of thin films. Enabling technologies for 10 to 100 million-atom simulations of nanoelectronic structures are discussed, which include multiresolution algorithms for molecular dynamics, load balancing, and data management. In ten years, this scalable software infrastructure will enable trillion-atom simulations of realistic device structures with sizes well beyond /spl mu/m on petaflop computers.

The rate of nuclear fusion reaction, which is proportional to the tunnel-contact probability of a reacting pair of nuclei, is enhanced significantly in dense stellar matter by the many-body correlations of the surrounding nuclei. Path-integral Monte Carlo calculations for the contact probabilities of a tunnelling pair in a dense Coulomb liquid of nuclei, an accurate model for the matter in the interiors and on the surfaces of white dwarfs and neutron stars, are performed to investigate quantum effects of the surrounding nuclei. We show that the probabilities are enhanced significantly by the wave-mechanical spreading of the surrounding nuclei. Exchange of the Bose nuclei alters the probability by negligible amounts for conditions of astrophysical interest.

The oxidation of an Al nanocluster with radius 100 ? placed in oxygen gas at room temperature is investigated by performing molecular dynamics simulations on parallel computers. The simulations take into account the effects of dynamic charge transfer between Al and O on the basis of electronegativity-equalization principles. We thereby find that molten surface oxides are saturated to depths of 28-33 ?; this saturation is accompanied by depletion of oxygen near the cluster in the environment. Upon quenching the cluster further in oxygen gas, amorphous surface oxides with depths are formed. The microscopic structures of the amorphous oxides are characterized.

Molecular dynamic simulation of a faceted ice-Ih crystal with the largest dimension of 0.06 µm in a vacuum is performed by employing the TIP4P intermolecular potential at temperatures Tm − 23, Tm − 13, and Tm − 1 K, where Tm is the melting point of the TIP4P bulk ice. We thereby observe at all the temperatures that the quasi-liquid layers (QLLs) formed on the basal (0001) surfaces are bumpy and that the liquid bumps repeatedly form and break at various places in a random manner. At Tm − 1 K, a liquid sheet appears under the liquid bumps. At an intermediate temperature for the bilayer-by-bilayer surface melting, the molecules under the local areas of either thin or thick QLLs have respectively the tendency to melt or recrystallize.

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.

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)10.1143/JPSJ.80.114002] 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.

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.