In this work, we have developed a multiscale shear-transformation-zone (STZ) model to simulate inelastic deformations in amorphous materials at macroscale without using phenomenological constitutive ...modeling. The main novelties of the multiscale method are two: (1) the multiscale method employs an atomistic-based representative sampling cell (RS-cell) that is embedded in every quadrature points of macroscale finite elements, and it generically contains potential shear-transformation-zones. Thus the evolution of RS-cells can naturally allow molecular clusters having irreversible microstructure rearrangements at microscale in response to applied loads, and subsequently translate the effects of the irreversible rearrangements of molecular clusters in each RS-cell to inelastic responses at the continuum scale, i.e. delineating the defect-mediated amorphous plasticity, and (2) the method uses a Parrinello-Rahman molecular mechanics based the Cauchy-Born rule to construct an atomistically-informed constitutive model at continuum level, which is able to quantitatively measure amorphous plasticity behaviors, including plastic flow stress, inelastic hysteresis loops under cyclic loading, and strain localizations at macroscale. By doing so, we have successfully simulated plastic deformation in the Lennard-Jones binary glass (LJBG) at macroscale level. Moreover, we have shown that the multiscale shear-transformation-zone can capture cyclic plasticity and the shear band formations in LJBG material.
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Further attenuation of Rayleigh scattering of silica glass is required to extend the capacity of long‐distance optical fiber communication. To theoretically examine the effect of aluminum and ...fluorine co‐doping on the density fluctuations of silica glass, which is related to Rayleigh scattering, a set of force‐matching potentials (FMP) for simulating F‐doped aluminosilicates was developed using Bayesian optimization based on density functional theory calculations. Molecular dynamics (MD) simulations with the new FMP could evaluate the densities of silica glasses to which a small amount of fluorine was doped and those of aluminosilicate glasses with a wide range of aluminum contents within reasonable accuracy. The FMP successfully represents the changing role of aluminum from a network former without a compensating cation (threefold coordination) to that with a compensating cation and a charge compensator in the aluminosilicates. Indeed, relative concentrations of four, five, and sixfold‐coordinated aluminum observed by NMR measurements were reproduced better than the original Teter potential at a high aluminum content. At an aluminum content lower than 1 mol%, threefold‐coordinated aluminum was observed, which is consistent with ESR measurements. After careful validations of the FMP, the effect of the co‐doping of alumina and fluorine on the density fluctuations of silica glass was computationally examined. Consequently, it was expected that the co‐doping might not sufficiently attenuate the Rayleigh scattering, even though 1 wt% fluorine would be able to reduce the density fluctuations of aluminosilicate glasses for some extent. This is because more alumina‐doping increases density fluctuations of silica glass if the drawing temperature and procedure are the same with those for silica glass fiber. Thereby, a possible fabrication process to sufficiently attenuate the Rayleigh scattering of the F‐doped aluminosilicate glass was proposed, according to the density fluctuation analysis at high temperature.
Energy release rate and fracture toughness of amorphous aluminum nanoparticles reinforced soda‐lime silica glass (SLSG) were measured by performing fracture simulations of a single‐notched specimen ...via molecular dynamics simulations. The simulation procedure was first applied to conventional oxide glasses and the accuracy was verified with comparing to experimental data. According to the fracture simulations on three models of SLSG/‐Al2O3 composite, it was found that the crack propagation in the composites is prevented through following remarkable phenomena; one is that a‐Al2O3 nanoparticles increase fracture surface area by disturbing crack propagation. The other is that the deformation of a‐Al2O3 nanoparticle dissipates energy through cracking. Moreover, one of the models shows us that the crack cannot propagate if the initial notch is generated inside a‐Al2O3 nanoparticle. Such strengthening is partly due to the fact that the strength of the interface between nanoparticle and SLSG matrix is comparable to that of SLSG matrix, implying that their interface does not reduce crack resistance of the oxide glass.
To study the connection between atomistic molecular dynamics and macroscale continuum mechanics, we partition the Lagrangian of first-principle molecular dynamics according to its length scales. By ...doing so, we discover a universal three-scale structure that is embedded in the conventional molecular dynamics formulation, which provides an intrinsic and seamless transition from microscale to macroscale. The multiscale micromorphic molecular dynamics (MMMD) is built on a novel micromorphic multiplicative decomposition that couples a fine scale atomistic dynamics, a mesoscale micromorphic dynamics, and a macroscale particle dynamics of continuum mechanics together in concurrent fashion.
In this work, we discuss the relationship between MMMD and nonlinear continuum mechanics, its computational algorithm, and how to use it to simulate phase transformation under non-equilibrium conditions.
In this study on a novel glass‐ceramic containing hexagonal CaAl2Si2O8 crystals embedded in a SiO2–Al2O3–CaO glass, we used molecular dynamics simulations to unravel the toughening mechanism of the ...partially crystallized composite material. The crystalline phase is composed of alternate layers of SiO4/AlO4 tetrahedra and calcium ions. After careful modeling of crystals embedded in the glass matrix, we conducted crack propagation simulations using single‐notched models. We found that: (a) when a crack propagates parallel to the cleavable calcium layer, the glass‐ceramic breaks in a brittle way since the crack passes through the fragile interlayer promptly, (b) the stiffer SiO4/AlO4 oxide layer can inhibit crack propagation, and the crack is thus deflected to the interface between the crystal and the glass matrix; and (c) a calcium layer present between the glass matrix and the edge of the CaAl2Si2O8 crystal is more fragile than those inside the crystal, indicating that cracks prefer to travel along the glass‐crystal interface. These theoretical simulations successfully demonstrated that the anisotropy and the fragile feature of the crystals lead to microcrack toughening of the glass‐ceramic. In addition, we discuss deformation anisotropy in the microscale by constructing a larger model that includes randomly orientated multiple CaAl2Si2O8 crystals.
The comprehension of the nonlinear effects provided by mixed alkali effect (MAE) in oxide glasses is useful to optimize glass compositions to achieve specific properties that depend on the mobility ...of ions, such as the chemical durability, glass transition temperature, viscosity and ionic conductivity. Although molecular dynamics (MD) simulations have already been applied to investigate the MAE on silicates, less effort has been devoted to study such phenomenon in mixed alkali aluminosilicate glasses where alkali cations can act both as modifiers, forming non-bridging oxygens and percolation channels, and as charge compensator of the AlO
units present in the network. Moreover, the ionic conductivity has not been computed yet; thus, the accuracy of the atomistic simulations in reproducing the MAE on the property is still open to question. In this work, we have validated five major interatomic potentials for the classical MD simulations by modelling the structure, density, glass transition temperature and ionic conductivity for three aluminosilicate glasses, (25 - x)Na
O - x(K
O) - 10(Al
O
) - 65(SiO
) (x = 0, 12.5, 25). It was observed that only the core-shell (CS) polarizable force field well reproduces the experimentally measured MAE on T
and the ionic conductivity as well as the higher conductivity of single sodium aluminosilicate glass at low temperature and the higher conductivity of single potassium aluminosilicate glass at high temperature. The MAE is related to the suppression of jump events of the alkaline ions between dissimilar sites in the percolation channels consisting of both sodium and potassium ions as in the case of alkaline silicates. The superior reproducibility of the CS potential is originated from the larger and the flexible ring structures due to the smaller Si-O-Si inter-tetrahedra angle, creating appropriate percolation channels for ion conductivity. We also report detailed assessments for using the potential models including the CS potential for investigating MAE on aluminosilicates.
Ceria (CeO2) is a well-known catalytic oxide with many environmental, energy production, and industrial applications, most of them involving water as a reactant, byproduct, solvent, or simple ...spectator. In this work, we parameterized a Ce/O/H ReaxFF for the study of ceria and ceria/water interfaces. The parameters were fitted to an ab initio training set obtained at the DFT/PBE0 level, including the structures, cohesive energies, and elastic properties of the crystalline phases Ce, CeO2, and Ce2O3; the O-defective structures and energies of vacancy formation on CeO2 bulk and CeO2 (111) surface, as well as the absorption and reaction energies of H2 and H2O molecules on CeO2 (111). The new potential reproduced reasonably well all the fitted properties as well as the relative stabilities of the different ceria surfaces, the oxygen vacancies formation, and the energies and structures of associative and dissociative water molecules on them. Molecular dynamics simulations of the liquid water on the CeO2 (111) and CeO2 (100) surfaces were carried out to study the coverage and the mechanism of water dissociation. After equilibration, on average, 35% of surface sites of CeO2 (111) are hydroxylated whereas 15% of them are saturated with molecular water associatively adsorbed. As for the CeO2 (100) surface, we observed that water preferentially dissociates covering 90% of the available surface sites in excellent agreement with recent experimental findings.
Transparent silica glass is an indispensable material for today's information technology society as an ultra-low loss optical fiber. Recently, further low-loss optical fibers are demanded due to ...rapidly expanding information. A small amount of fluorine doping is known to reduce the Rayleigh scattering further by reducing the density fluctuations of silica glass, even though the fluorine is an impurity for the pristine silica glass. This study devoted to answer the riddle of F-doping and an accurate force-matching potential optimized by machine-learning enables us to understand how fluorine remedies the disorders in silica glass. It was found that fluorine mainly replaces an oxygen in tetrahedral SiO4 unit, but also forms five-fold Si, such as SiO4F and SiO3F2 units. The former disrupts silica network, while the latter attenuates the network rigidity by loosening the five Si-O(F) bonds. Since the two local effects of fluorine spreads through the glass network, F-doping prompts silica glass to relax further even at temperature lower than the glass transition (fictive) temperature. Consequently, fluorine minimizes the density fluctuation (thus, Rayleigh scattering) of silica glass at around 1 wt%, although fluorine itself is an impurity for silica glass.
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•Accurate force-matching potential for F-doped silica was developed based on DFT calculations using Bayesian optimization.•The new potential successfully reproduced anomaly of density change of silica and effect of F-doping to anomaly attenuation.•Large-scale MD simulations unraveled how fluorine minimizes density fluctuation of silica glass.
We investigated CaO–Al2O3–SiO2 glass partially crystallized with molybdenum particles as nucleating agents. Microstructure of the material was characterized as a house‐of‐cards structure composed of ...plate‐like crystals. Microcracks propagated along the crystal plane parallel to the double layer of SiO4/AlO4 tetrahedrons separated by layers of calcium atoms. To investigate the fracture behavior of the hexagonal CaAl2Si2O8 crystals, molecular dynamics simulations were performed, which demonstrated that a crack can be easily triggered by shear deformation along the calcium layer. Additionally, once a crack was generated in the calcium layer, it propagated rapidly, whereas the crack perpendicular to the calcium layer hardly propagated. This simulated behavior is consistent with the experimentally observed cleavage behavior of the hexagonal CaAl2Si2O8 crystal. The experimental and simulation results effectively explained the non‐elastic fracture behavior of the material.
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