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We develop an automated high-throughput workflow for calculating lattice dynamical properties from first principles including those dictated by anharmonicity. The pipeline automatically computes interatomic force constants (IFCs) up to 4th order from perturbed training supercells, and uses the IFCs to calculate lattice thermal conductivity, coefficient of thermal expansion, and vibrational free energy and entropy. It performs phonon renormalization for dynamically unstable compounds to obtain real effective phonon spectra at finite temperatures and calculates the associated free energy corrections. The methods and parameters are chosen to balance computational efficiency and result accuracy, assessed through convergence testing and comparisons with experimental measurements. Deployment of this workflow at a large scale would facilitate materials discovery efforts toward functionalities including thermoelectrics, contact materials, ferroelectrics, aerospace components, as well as general phase diagram construction.

Large-scale molecular dynamics (MD) simulations enabled by computationally efficient semiempirical potentials are an invaluable tool for materials modeling. In the case of metallic alloys, embedded atom method (EAM) and Finnis–Sinclair (FS) potentials are a reasonable choice based on their good balance of quality and computational cost. However, these semiempirical potentials are not suitable for simulating ionic systems, which prevents their use in studying many technologically relevant metal–oxide systems. The charge transfer ionic potential (CTIP), which can utilize EAM/FS potentials available in the literature together with a variable charge representation of electrostatic interactions, should be a reasonable choice for performing reliable and computationally efficient MD simulations of such systems. However, only a few such potentials are available in the literature, and their computational cost is much higher compared to EAM/FS potentials. In the present work, we have attempted to remedy these deficiencies by combining several modifications to the CTIP model proposed in the literature and efficiently implementing them into the widely used Large-scale Atomic/Molecular Massively Parallel Simulator MD code. Using these modifications, we have developed a new Ni–O CTIP parameterization, which has been tested in several different scenarios of interest. First, the early stages of Ni surface oxidation were simulated, demonstrating the nucleation and growth of a crystalline NiO film across the surface. Second, solidification and vitrification in the Ni–O system were investigated, demonstrating that the new CTIP parameterization provides reasonable agreement with the experimentally determined equilibrium phase diagram. Finally, we studied the interaction of dislocations in a Ni matrix with a NiO inclusion using a simulation cell with an unprecedented number of atoms for a variable charge MD simulation. Thus, the approach utilized in the present study is an efficient method to simulate large scale atomic mechanisms in metal–oxide systems. [ABSTRACT FROM AUTHOR]

Climbing the ladder of density functional approximations has long been proposed to systematically improve the accuracy of first-principles calculations employing the density functional theory (DFT); however, up until now, the Perdew–Burke–Ernzerhof (PBE) functional at the second rung of the ladder, has dominated. Here, we present a study of the martensitic phase transition in NiTi based on ab initio molecular dynamics simulations and thermodynamic integration using the third-rung approximation of the strongly constrained and appropriately normalized (SCAN) meta-generalized gradient approximation (GGA). Although the predicted equilibrium lattice constants and formation enthalpy agree well with experimental data, the martensitic transition temperature (MTT) is overestimated by 94% (or 324 K too high), compared with only 22% (77 K) overestimation by PBE. The latent heat (q) is severely overestimated by SCAN as well. This deteriorated performance originates from the enlarged energy difference (ΔE) between the austenite and martensite phases, compared with the PBE result. Furthermore, a large variation (over 50 meV/atom) in ΔE using different meta-GGAs indicates large variations in computed MTTs (∼ 400 − 500 K) and q, i.e., the predicted thermodynamic properties depend sensitively on the choice of meta-GGA. This would pose a serious problem when upgrading DFT calculations to the third rung. One possible solution is to add NiTi as a norm system so that the revised SCAN meta-GGA could reproduce the PBE results of the relevant energy difference. [ABSTRACT FROM AUTHOR]

The structure of interfacial water near suspended graphene electrodes in contact with aqueous solutions of Na2SO4, NH4Cl, and (NH4)2SO4 has been studied using confocal Raman spectroscopy, sum frequency vibrational spectroscopy, and Kelvin probe force microscopy. SO42- anions were found to preferentially accumulate near the interface at an open circuit potential (OCP), creating an electrical field that orients water molecules below the interface, as revealed by the increased intensity of the O-H stretching peak of H-bonded water. No such increase is observed with NH4Cl at the OCP. The intensity of the dangling O-H bond stretching peak however remains largely unchanged. The degree of orientation of the water molecules as well as the electrical double layer strength increased further when positive voltages are applied. Negative voltages on the other hand produced only small changes in the intensity of the H-bonded water peaks but affected the intensity and frequency of dangling O-H bond peaks. The TOC figure is an oversimplified representation of the system in this work.

Interface free energy is a fundamental material parameter needed to predict the nucleation and growth of new phases. The high cost of experimentally determining this parameter makes it an ideal target for calculation through a physically informed simulation. Direct determination of interface free energy has many challenges, especially for solid–solid transformations. Indirect determination of the interface free energy from the nucleation data has been done in the case of solidification. However, a slow on molecular dynamics (MD) simulation time scale atomic diffusion makes this method not applicable to the case of nucleation from the solid phase when precipitate composition is different from that in matrix. To address this challenge, we outline the development of a new technique for determining the critical nucleus size from an MD simulation using a recently developed method to accelerate solid-state diffusion. The accuracy of our approach for the Ni–Al system for Ni3Al (γ′) precipitates in a Ni–Al (γ) matrix is demonstrated well within experimental accuracy and greatly improves upon previous computational methods [Herrnring et al., Acta Mater. 215(8), 117053 (2021)]. [ABSTRACT FROM AUTHOR]

Commercialization of lithium-oxygen batteries faces many challenges, such as electrolyte decomposition, short cycle life, low energy and power density, and high cell mass. However, the commercialization of LiO2 batteries for aeronautics is more challenging due to additional safety constraints on cyclability and performance (high specific-power and specific-energy). In this paper, we perform experiments and use the results to calibrate a 1-D finite element model to simulate discharge profiles up to 5 mA cm-2. The calibrated model is used to perform parametric studies of geometrical, microstructural, transport, and material properties for different discharge times, and discharge current densities. Next, a simulation-based optimization study shows optimal cell designs for various electrolytes for high specific-power. Results show that a high specific-power LiO2 cell needs to have a cathode with a thickness equal to oxygen diffusion length, thin separator, optimized anode, and other components with low mass. The specific-power is sensitive to discharge time, discharge current density, and microstructural and geometrical properties. Also, in some cases, electrolytes with high oxygen solubility or high external partial pressure can compensate for electrolytes with low oxygen diffusivity. However, reducing the cell mass is the most straight forward path to improving specific-power.

The material behavior of the API-60 epoxy resin was investigated using atomistic molecular dynamics simulations. In this study, a hybrid, reactive force field (M-GAFF2), which combined Morse and harmonic bond potentials from GAFF2, was developed to understand the elastic/plastic response of the cured epoxy polymer. We found that elastic deformation was caused by epoxy network rearrangement at low strain while plastic deformation was observed by stretching and breaking covalent bonds after the yield point. Ab initio calculations at the CASPT2(2,2)/6-311+G** level of theory were performed to compute parameters for M-GAFF2. These are necessary for breaking covalent bonds in the epoxy network backbone when they are deformed. The M-GAFF2 was effective in revealing the ductile-like deformation behavior of the crosslinked epoxy polymer at the microscopic level, including elastic/plastic deformations and progressive failure. The effect of testing temperature at 300 and 400 K and degree of cure (DoC) on tensile properties such as Young's modulus, ultimate strength, and failure strain was evaluated to verify the feasibility of the M-GAFF2 in different thermodynamic constraints. This new approach is easy to use for nonequilibrium MD simulations and computationally efficient for studying the microscopic deformation and failure behavior of thermosetting polymers at the atomistic scale. KEYWORDS epoxy polymer, fracture mechanism, hybrid force field, molecular dynamics simulations, 1 | INTRODUCTION Molecular dynamics (MD) simulations are widely used to understand the complex phenomena of epoxy polymers associated with their atomistic- and molecular-level structures. Both quantitative and qualitative equilibrium [...]