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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]

Metal–semiconductor contacts in silicon carbide (SiC) diodes endure damages at the interface when exposed to harsh radiation environments. Due to the rapid rise in temperature and ultrafast cooling that follows the radiation impact, the structural properties of the materials can be altered through melting, recrystallization, and amorphization. A detailed understanding of the material failure modes at the interface is lacking, specifically at the nanoscale. We use molecular simulations to investigate the ultrafast melting at tungsten (W)–SiC interfaces following radiation damage and apply deep learning techniques to track the transient evolution of the local molecular structures. We show that W near the radiation track undergoes melting and, eventually, most of it recrystallizes with a noticeable degree of undercooling, while SiC is rendered permanently amorphous. The observation of local undercooling in W films is important as it can affect the device performance even before the bulk melting temperature of the material is reached. We also show that at high temperatures, the interface undergoes a fracture-like failure. The results presented here are significant in understating the different failure modes of SiC diode materials. [ABSTRACT FROM AUTHOR]

Recent experiments have suggested that melt flow plays a critical role in the ablation of meteoroids during atmospheric entry. Thus, modeling ablation requires knowledge of the melt properties of meteoritic constituents. These properties, however, are poorly understood and difficult to obtain with experimental techniques at entry conditions. An alternative means of obtaining high-temperature melt properties is through ab initio molecular dynamics (AIMD) simulations. Such simulations are performed here to characterize the melt properties of enstatite ( MgSiO 3), which is prevalent in certain types of chondrites, and its constitutive oxides ( SiO 2 and MgO). The structure, thermodynamic properties (density, bulk modulus, heat capacity, and coefficient of thermal expansion), and transport properties (diffusion and viscosity) are computed across the entire liquid phase and agree well with the limited number of available experiments. The high fidelity AIMD results are compared against less accurate models for melt property determination, which include classical molecular dynamics simulations and empirical mixture rules. Properties obtained from these alternative models generally show large relative errors compared to experiment, with viscosity, in particular, having errors of up to 98%. The present results highlight the potential of AIMD simulations to provide "quantitatively accurate" properties for melts of complex silicates found in meteorites and terrestrial rocks. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present a procedure to determine temperature-dependent thermodynamic properties of crystalline materials from density functional theory molecular dynamics (DFT-MD). Finite temperature properties (structural, thermal, and mechanical properties) of the phases (ground state monoclinic B33, martensitic B19', and austenitic B2) of the shape memory alloy NiTi are investigated. Fluctuation formulas and numerical derivatives are used to evaluate mechanical and thermal properties. A modified version of thermodynamic upsampling is used to predict properties in the high DFT convergence threshold limit from low threshold simulations. DFT convergence thresholds. In addition, a thermodynamic integration expression is developed to compute free energies from isobaric DFT-MD simulations that accounts for volume changes. Structural parameters, elastic constants, volume expansion, and specific heats as a function of temperature are evaluated. Phase transitions between B2 and B19' and between B19' and B33 are characterized according to their thermal energy, entropy, and free energy differences as well as their latent heats. Anharmonic effects are shown to play a large role in both stabilizing the austenite B2 phase and suppressing the martensitic phase transition. The quasiharmonic approximation to the free energy results in large errors in estimating the martensitic transition temperature by neglecting these large anharmonic components. [ABSTRACT FROM AUTHOR]

We investigate how systematically increasing the accuracy of various molecular dynamics modeling techniques influences the structure and capacitance of ionic liquid electric double layers (EDLs). The techniques probed concern long-range electrostatic interactions, electrode charging (constant charge versus constant potential conditions), and electrolyte polarizability. Our simulations are performed on a quasi-two-dimensional, or slab-like, model capacitor, which is composed of a polarizable ionic liquid electrolyte, [EMIM][BF4], interfaced between two graphite electrodes. To ensure an accurate representation of EDL differential capacitance, we derive new fluctuation formulas that resolve the differential capacitance as a function of electrode charge or electrode potential. The magnitude of differential capacitance shows sensitivity to different long-range electrostatic summation techniques, while the shape of differential capacitance is affected by charging technique and the polarizability of the electrolyte. For long-range summation techniques, errors in magnitude can be mitigated by employing two-dimensional or corrected three dimensional electrostatic summations, which led to electric fields that conform to those of a classical electrostatic parallel plate capacitor. With respect to charging, the changes in shape are a result of ions in the Stern layer (i.e., ions at the electrode surface) having a higher electrostatic affinity to constant potential electrodes than to constant charge electrodes. For electrolyte polarizability, shape changes originate from induced dipoles that soften the interaction of Stern layer ions with the electrode. The softening is traced to ion correlations vertical to the electrode surface that induce dipoles that oppose double layer formation. In general, our analysis indicates an accuracy dependent differential capacitance profile that transitions from the characteristic camel shape with coarser representations to a more diffuse profile with finer representations. [ABSTRACT FROM AUTHOR]

A combination of ab initio, atomistic, and finite element methods ( FEM) was used to investigate fundamental properties of grain boundaries and grain boundary networks and their impact on lattice thermal conductivity in the ultra high-temperature ceramic ZrB2. The structure, energetics, and lattice thermal conductance of certain low energy grain boundaries were studied. Atomic models of these boundaries were relaxed using density functional theory. Information about bonding across the interfaces was determined from the electron localization function. Interfacial thermal conductances were computed using nonequilibrium molecular dynamics. Microstructural models were used to determine the reduction in lattice thermal conductivity due grain boundary networks where FEM meshes were constructed on top of microstructural images. [ABSTRACT FROM AUTHOR]

Abstract: We report on the first interatomic potentials for Zirconium Diboride and Hafnium Diboride. The potentials are of the Tersoff form, and are obtained by fitting to a first-principles database of basic properties of elemental Zr, Hf, B, and the compounds ZrB 2 and HfB 2. Two variants of the Zr–B potentials have been obtained, and one for Hf–B. The potentials have been tested against a variety of properties of the compound, with the conclusion that they are stable and provide a reasonable representation of the desired properties of the two diborides. [Copyright &y& Elsevier]

Abstract: Reactive molecular dynamics simulations are used to study the initial stage of pyrolysis of phenolic polymers with carbon nanotube and carbon fiber. The products formed are characterized and water is found to be the primary product in all cases. The water formation mechanisms are analyzed and the value of the activation energy for water formation is estimated. A detailed study of graphitic precursor formation reveals the presence of two temperature zones. In the lower temperature zone (<2000 K) polymerization occurs resulting in the formation of large, stable graphitic precursors, while in the high temperature zone (>2000 K) polymer scission results in formation of short polymer chains/molecules. Simulations performed in the high temperature zone of the phenolic resin (with carbon nanotubes and carbon fibers) show that the presence of interfaces does not have a substantial effect on the chain scission rate or the activation energy value for water formation. [ABSTRACT FROM AUTHOR]

Current–voltage characteristics of gold-benzene-1,4-dithiol-gold junctions are calculated using a combined density functional theory/non-equilibrium Green''s functions approach with local atomic basis sets. Improving the basis set with the addition of polarization or diffuse functions has some effect on the computed I–V curves, but even the small valence double zeta sets yield reasonable results. The results obtained using the hybrid B3LYP functional give slightly smaller conductance than those obtained using the pure BPW91 approach. Substituting Cl for H on the bridging molecule has only a small effect on the I–V curves. [Copyright &y& Elsevier]

Ultrahigh temperature shape memory alloys (UHT-SMAs) have transition temperatures above 600 °C. They are needed for sensing and actuating devices in aerospace applications. However, very few such UHT-SMAs have been found. Among them are Ru-based alloys such as RuNb and RuTa, whose martensite structures and phase transitions are totally different from those of NiTi-based SMAs and were poorly understood. In this work, we carried out a systematical study of RuNb using first-principles total energy calculations and molecular dynamics (MD) simulations. We revealed the transition paths and mechanisms in cubic → tetragonal → monoclinic transitions. We determined the transition sequence and martensitic transition temperatures (MTTs) by evaluating the Gibbs free energies using thermodynamic integration. The calculated MTTs are in very good agreement with the experimental data. We also found that the monoclinic phase at the second transition has the P 2 1 / m symmetry instead of experimentally identified P 2 / m. The insights gained by this study and the verified ab initio methods for accurate MTT calculations can be applied to fast screen and quantitatively design novel UHT-SMAs having similar properties with desirable MTTs and much reduced cost. [Display omitted] [ABSTRACT FROM AUTHOR]

• Hybrid molecular dynamics – kinetic Monte Carlo based algorithm. • Enabling accelerated diffusion rates in molecular dynamics simulations. • User-configurable rate for nearest-neighbor atom swapping. • Investigate diffusion dependent processes with Molecular Dynamics. Molecular dynamics (MD) simulations have long been unable to provide significant insights into large-scale processes involving diffusion. Here, we implement a kinetic Monte Carlo (kMC) algorithm to circumvent such issues. We have implemented this approach in the widely used MD simulation package, LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). Validation of the MD/kMC algorithm is achieved by replicating results predicted by diffusion equations. Comparisons are made between a traditional MC approach and the kMC algorithm for the case of L1 2 phase growth in Ni-Al alloys. These examples highlight the unique advantages provided by the kMC approach to unlock new capabilities for MD simulations. [ABSTRACT FROM AUTHOR]

The stretching of a single chain of phenolic polymer is studied using density functional theory (DFT) and the reactive force field (ReaxFF) potential. The most detailed studies are performed at 0 K, which shows the breaking of the C(ring)-CH 2 bridge bond can be delayed to longer polymer length by a ring opening mechanism that relieves the strain. Molecular dynamics (MD) simulations, using the DFT approach, show that C–CH 2 bridge bond breaking occurs much more frequently than the ring opening. The stretching potentials determined using ReaxFF compare favorably with the DFT results. Using ReaxFF, stretching of cross-linked systems were considered and compared to single chains. The effect of changing the linking group on the bond stretching portion of the potential is found to be relatively small, but the effect on the bond energies can be significant. [ABSTRACT FROM AUTHOR]

A systematiccomparison of atomistic modeling methods including density functionaltheory (DFT), the self-consistent charge density-functional tight-binding(SCC-DFTB), and ReaxFF is presented for simulating the initial stagesof phenolic polymer pyrolysis. A phenolic polymer system is simulatedfor several hundred picoseconds within a temperature range of 2500to 3500 K. The time evolution of major pyrolysis products includingsmall-molecule species and char is examined. Two temperature zonesare observed which demark cross-linking versus fragmentation. Thedominant chemical products for all methods are similar, but the yieldsfor each product differ. At 3500 K, DFTB overestimates CO production(300–400%) and underestimates free H (∼30%) and smallCmHnO molecules(∼70%) compared with DFT. At 3500 K, ReaxFF underestimatesfree H (∼60%) and fused carbon rings (∼70%) relativeto DFT. Heterocyclic oxygen-containing five- and six-membered carbonrings are observed at 2500 K. Formation mechanisms for H2O, CO, and char are discussed. Additional calculations using a semiclassicalmethod for incorporating quantum nuclear energies of molecules werealso performed. These results suggest that chemical equilibrium canbe affected by quantum nuclear effects at temperatures of 2500 K andbelow. Pyrolysis reaction mechanisms and energetics are examined indetail in a companion manuscript. [ABSTRACT FROM AUTHOR]

Martensitic transition temperatures (MTTs) can be tuned by alloying binaries with other elements to create multi-component shape memory alloys (SMAs). However, it is inefficient to use the trial-and-error approach to find compositions with desirable operating temperatures because of the large number of combinations of metals possible to form ternaries, quaternaries, etc. Thus it is crucial to develop the theoretical capability of accurately predicting MTTs as a function of composition, in order to provide experimentalists with necessary and reliable guidance. Previous work has focused on developing and applying first-principles methods to compute phase transitions and MTTs in binary SMAs such as NiTi (nitinol), but certain technical problems associated with the multi-component SMAs remain unsolved. In this work, we employed ab initio molecular dynamics (MD) and thermodynamics integration to study the NiTiHf-based high-temperature ternary SMAs. We overcome the technical challenges to accurately obtain the Gibbs free energy in cubic ternaries where the reference structures are unknown. Specifically, we examined the cubic, monoclinic and orthorhombic structures of Ni 0.5 Ti 0. 5 − x Hf x for x ∈ [ 0 , 0. 5 ] , and our results suggest that the cubic-to-monoclinic martensitic transition occurs when x < 0. 08 , for x > 0. 17 the martensitic transition is between the cubic and orthorhombic phases, whereas in between our calculations cannot distinguish these two martensite structures near the MTT. The computed MTTs vs Hf content x are in good agreement with measured data. Thus our current work paves the way for computational design of multi-component SMAs with desired properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Micro-twinning is one of the dominant creep deformation mechanisms in Ni-based superalloys at intermediate temperatures. The Kolbe micro-twinning mechanism, based on thermally activated reordering, has been confirmed experimentally and analyzed theoretically. However, many aspects of the reordering process are still under debate. This work presents the first molecular dynamics (MD) simulation study of the Kolbe mechanism. We demonstrate that atomic diffusion-mediated reordering takes place in the immediate vicinity of the cores of twinning partials and proceeds in a highly concerted manner, directly influenced by propagation of the dislocations. [Display omitted] [ABSTRACT FROM AUTHOR]

• New mechanism for nucleation and growth of twins in Ni-based superalloys is proposed. • It is qualitatively different from the Kolbe mechanism for micro-twinning. • The first step of our mechanism is related to the Decamps mechanism for SISF formation. Micro-twinning is the major creep deformation mechanism in most Ni-based superalloys at temperatures above 700 °C. Nevertheless, many aspects of twin nucleation and growth remain unexplored. The Kolbe mechanism for micro-twinning is currently widely accepted in the community to explain these processes, based on post mortem TEM characterization and indirect theoretical estimations. However, this does not mean that other mechanisms cannot contribute to creep. Molecular dynamics (MD) simulations offer an alternative possibility to probe different creep mechanisms. In this work, we use MD simulations to demonstrate that a qualitatively different mechanism for nucleation and growth of twins can be competitive with the Kolbe mechanism in the intermediate temperature regime of 600 °C–800 °C. The proposed mechanism is closely related to the formation mechanism of super intrinsic stacking faults (SISFs) originally introduced by Condat and Decamps in 1987. [ABSTRACT FROM AUTHOR]

Algorithms to generate atomistic models of cross-linked phenolic resins suitable for molecular dynamics simulations were investigated. The influence of five parameters (initial volume of uncross-linked material, cross-linking approach, relaxation time, equilibration temperature) on generating cross-linked structures was studied quantitatively using a full factorial sensitivity analysis. The parameters were found to be dependent on the degree of cross linking (D). For low cross-linking, only the equilibration temperature has a significant impact on the final energetics and densities. However, for higher cross-linking (D > 70%), the equilibration temperature, initial volume and cross-linking approach were shown to influence the phenolic structures. Iterative, rather than single step, methods were shown to produce better structures. The initial volume of the uncross-linked material was identified as having the most influence on the final volume of fully cross-linked systems. By optimizing all five parameters, highly cross-linked samples with low energetics and consistent densities could be generated. To validate the models, thermo-mechanical properties of cross-linked phenolic samples were characterized as a function of density and degree of cross-linking. Good agreement with experimental values was obtained for properties such as the glass transition temperature, coefficient of thermal expansion (CTE), elastic moduli, and thermal conductivity. [ABSTRACT FROM AUTHOR]

We employ molecular dynamics (MD)simulation and experiment toinvestigate the structure, thermodynamics, and transport of N-methyl-N-butylpyrrolidinium bis(trifluoromethylsufonyl)imide([pyr14][TFSI]), N-methyl-N-propylpyrrolidiniumbis(fluorosufonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazoliumboron tetrafluoride ([EMIM][BF4]), as a function of Li-saltmole fraction (0.05 ≤ xLi+≤ 0.33) and temperature (298 K ≤ T≤ 393 K). Structurally, Li+is shown to be solvatedby three anion neighbors in [pyr14][TFSI] and four anion neighborsin both [pyr13][FSI] and [EMIM][BF4], and at all levelsof xLi+we find the presenceof lithium aggregates. Pulsed field gradient spin-echo NMR measurementsof diffusion and electrochemical impedance spectroscopy measurementsof ionic conductivity are made for the neat ionic liquids as wellas 0.5 molal solutions of Li-salt in the ionic liquids. Bulk ionicliquid properties (density, diffusion, viscosity, and ionic conductivity)are obtained with MD simulations and show excellent agreement withexperiment. While the diffusion exhibits a systematic decrease withincreasing xLi+, the contributionof Li+to ionic conductivity increases until reaching asaturation doping level of xLi+= 0.10. Comparatively, the Li+conductivity of [pyr14][TFSI]is an order of magnitude lower than that of the other liquids, whichrange between 0.1 and 0.3 mS/cm. Our transport results also demonstratethe necessity of long MD simulation runs (∼200 ns) to convergetransport properties at room temperature. The differences in Li+transport are reflected in the residence times of Li+with the anions (τLi/–), which arerevealed to be much larger for [pyr14][TFSI] (up to 100 ns at thehighest doping levels) than in either [EMIM][BF4] or [pyr13][FSI].Finally, to comment on the relative kinetics of Li+transportin each liquid, we find that while the net motion of Li+with its solvation shell (vehicular) significantly contributes tonet diffusion in all liquids, the importance of transport throughanion exchange increases at high xLi+and in liquids with large anions. [ABSTRACT FROM AUTHOR]

[Display omitted] Recent studies on the constituent binaries of common NiTi-based shape memory alloys (NiTi, NiZr, NiHf, PdTi, PtTi) have led to questions regarding the accuracy of current phase diagrams, including the existence of theoretical phases, and the atomistic nature of stability in observed martensitic and austenitic phases. These issues are investigated here with ab initio molecular dynamics. Theoretically identified ground state structures are found to become mechanically unstable well below room temperature, while the experimentally determined martensite structures are found to be stabilized by anharmonic effects at elevated temperatures. The martensite phases of all binaries are found to transition to a common austenite structure (cubic B2), with transition temperatures in agreement with available experiments. Interestingly, the B2 structures are unstable at low temperature but are stabilized by large anharmonic effects that also govern the martensitic transition. An observed signature of this anharmonicity is the transitory symmetry breaking of B2 to a distorted, glassy structure known as B2'. The energy of the observed B2' structure is quantified at low temperatures and, across the binaries, is found to be linearly correlated to the anharmonic energy and entropy of transition from the martensite phases to high-temperature B2. The implication of this finding is the properties of the low-temperature B2' structure govern anharmonicity and ultimately the high-temperature transitions to B2. Means of utilizing these results to simplify the characterization of the austenite thermodynamics and associated transitions important to the shape memory effect are discussed. A reduced order model is presented that captures the competition between crystal lattice energy differences (B2 vs martensite) and the degree of anharmonicity characterized by the B2-B2' energy differences for these sytems. [ABSTRACT FROM AUTHOR]