Atomic-scale Simulations Research Articles

Recent Advancements in Understanding of Growth and Properties of Antiwear Tribofilms Derived from Zinc Dialkyl Dithiophosphate Additives under Nanoscale Sliding Contacts.

Zinc dialkyl dithiophosphate (ZDDP) is a key antiwear additive in lubricants that forms robust phosphate glass-based tribofilms to mitigate wear on rubbing surfaces. The quest to unravel the enigma of these antiwear film formations on sliding surfaces has persisted as an enduring mystery, despite nearly a century of fervent research. This paper presents a comprehensive review of nanotribological investigations, centering on the tribochemical decomposition of ZDDP antiwear additives. The core of the Review explores investigations conducted through the in situ AFM-based technique, which has been used to unveil the underlying stress-assisted thermal activation (SATA) mechanism behind the formation of antiwear tribofilms on diverse surfaces. A thorough analysis is presented, encompassing governing factors, such as compression, shear, and temperature, that wield influence over the intricate process of tribofilm formation. This is substantiated by a spectrum of structural and chemical characterization-based inferences. Furthermore, atomic-scale computer simulation studies are discussed that provide profound insights into tribochemical reaction mechanisms and elucidate the details of chemical processes at atomic level.

A multiscale FEM-MD coupling method for investigation into atomistic-scale deformation mechanisms of nanocrystalline metals under continuum-scale deformation

This study aims to develop a multiscale bridging method for investigating nanocrystalline metals based on macro-scale deformation. For this purpose, we propose a hierarchical multiscale computational method that can focus on some of the elements in a finite element model for scale bridging to atomistic-scale models. This method assumes that atomistic-scale nanocrystalline models are related to the integration points in a finite element and deform based on the macro-scale deformation. Nanocrystalline aluminum was chosen for the validation of the multiscale method. The finite element method (FEM) and the molecular dynamics (MD) method were used for continuum-scale and atomistic-scale simulations, respectively. We utilized the notion of the CauchyBorn rule (CBR) for communicating deformation information from the continuum scale to the atomistic scale. We studied three different cases with two nanocrystalline models and two loading cases to compare differences resulting from crystal structures and loading. Based on the crystal structure change during relaxation, nonequilibrium grain boundaries (NEGBs) were shown to play a role as deformation mechanisms in the plastic regime and induce the onset and migration of crystal defects, including deformation twins, as reported in the experiment. Furthermore, the crystal orientation dependence of the onset of crystal defects was confirmed by the comparison of the results from the two different nanocrystalline models. The qualitative agreement of the results with experimental observations is also confirmed. The proposed ‘FEM-MD’ method can bridge a large-scale gap, for example, from a nano-scale to a continuum-scale such that an MD model can be coupled to a millimeter or centimeter scale compared to other embedding methods. The present method is ideal for investigating the dislocation behavior of nanocrystalline materials, which contain multi-grained nanostructure at finite temperature, undergoing various loading scenarios at the macro-scale.

Atomistic Scale Modeling of Anode/Electrolyte Interfaces in Li-Ion Batteries.

A key issue in lithium-ion batteries is understanding the solid electrolyte interphase (SEI) resulting from a reductive reaction on the anode/electrolyte interface. The presence of the SEI layer affects the transport behavior of the ions and electrons between the anode and electrolyte. Despite the influence on interfacial properties, the formation and evolution mechanism of the SEI layer are unclear owing to their complexity and dynamic nature. Atomistic-scale simulations have promoted the understanding of the reaction mechanism on the anode/electrolyte interface, the formation and evolution of the SEI layer, and their fundamental properties. This Perspective discusses the modeling and interpretations of anode/SEI/electrolyte interfaces through computational methods at the atomic-scale and highlights interfacial modeling techniques for a realistic interface design, which can overcome the limited time and length scale with high accuracy.

Multiscale coupling simulation of surface catalytic effect on hypersonic aerothermodynamic environment

The complicated heat and mass transfer at the high-temperature gas-solid interface of a thermal protection material under highly non-equilibrium conditions is of high importance to hypersonic flows. Surface catalysis, an exothermic reaction that refers to the recombination of dissociated atoms at the surface, and its effect on the aerothermodynamic environment has been investigated by researchers, especially focusing on oxygen recombination process. To further identify the nitrogen recombination mechanism, a hybrid CFD-RMD multiscale simulation method is employed in this work to qualitatively investigate its effect on aerothermodynamic environment prediction under hyperthermal non-equilibrium condition. Taking carbon-based phenolic resin (PR) composite as a typical material with 300 K, the nitrogen recombination coefficient obtained from atomistic-scale RMD simulation is coupled with the continuum two-temperature Navier-Stokes model to capture the catalytic effects and the heat / mass transfer at high temperature: A catalytic benchmark simulation is conducted under a Mach number of 9.7, and the heat flux distribution along the surface coupled with the RMD-derived nitrogen recombination coefficient of 0.0704 is obtained, which result is comparable with the published wind tunnel experimental data and the finite-rate catalytic result is 10 %∼30 % lower than the fully catalytic assumption result. The nitrogen recombination coefficient of PR is theoretically found to increase with thermal dissociation fraction of the free stream and decrease with the wall temperature due to the catalysis-ablation competitive mechanism. The influence of the recombination coefficient on the heat transfer at the surface is also quantitatively analyzed for evaluating the surface catalysis effect on aerothermodynamic heating.

Assessment of the classical theory validity through Cu50Zr50 nucleation and growth molecular dynamics simulations

One of the most promising candidates for novel glass-forming alloy design is atomic-scale simulations, primarily due to their capability to compute properties that are usually experimentally difficult to measure. This work aims to use classical molecular dynamics (MD) simulations to obtain the key nucleation and growth variables for the glass-forming Cu50Zr50 alloy while verifying the validity of the Classical Nucleation Theory (CNT). Through CNT equations and simulation data, nucleation and growth rates could be determined, revealing good agreement between CNT expressions and simulation results. The data were compared with other simulation and experimental data, providing reasonable evidence that supports the validity of CNT for describing the solidification behavior of glassy alloys. Simulation data seems to overestimate the experimental growth rate by one order of magnitude, a trend that requires further investigation and correlation with the system/potential to enhance the method’s robustness and reliability for other alloy compositions.

Unravelling the jerky glide of dislocations in body-centred cubic crystals.

In situ straining tests in high purity α-Fe thin foils at low temperatures1 have demonstrated that crystal line defects, called dislocations, have a jerky type of motion made of intermittent long jumps of several nanometres. This observation conflicts with the standard Peierls mechanism for plastic deformation in body-centred cubic crystals, where the screw dislocation jumps are limited by inter-reticular distances, that is, distances of a few angstroms. Employing atomic-scale simulations, we show that although the short jumps are initially more favourable, their realization requires the propagation of a kinked profile along the dislocation line, which yields coherent atomic vibrations acting as travelling thermal spikes. Such local heat bursts favour the thermally assisted nucleation of new kinks in the wake of primary ones. The accumulation of new kinks leads to long dislocation jumps like those observed experimentally. Our study constitutes an important step towards predictive atomic-scale theory for materials deformation.

Overcoming the limitations of atomic-scale simulations on semiconductor catalysis with changing Fermi level and surface treatment

A new groundbreaking method for efficient optimization of doping concentration and cocatalyst materials for Fermi level engineering of wide band gap semiconductors.

Atomistic-Scale Simulations Toward the Understanding of the Conversion–Alloying Mechanism in Li-Ion Battery Anodes

The increasing demands for energy storage call for improved capacity and cycling stability of modern Li-ion batteries (LIBs). However, the application of the state-of-the-art LIB anode material – graphite is limited as graphite underperforms as a high-rate/high-storage-capacity anode. Alternatively, materials capable of alloying with Li-ions have attracted attention as a promising pathway to solve high-rate and high-capacity challenges. Furthermore, when alloying reaction in a material is coupled with irreversible conversion reaction, the materials operating under such mechanism exhibit a high Li-ion storage capacity and long-term stability during electrochemical cycling. However, despite the optimistic performance of these materials, a number of parameters need to be optimized and fundamentally understood.Among the unknowns for the conversion/alloying materials are the atomistic details of the reaction mechanism - understanding the atomistic structure during transformations is a pre-requisite for the future rational optimization of the materials. Furthermore, amorphization, which is prevalent in these materials, severely limits the conventional characterization methods essentially impeding further development and understanding the degradation mechanisms. Hence, to investigate the atomistic structure changes, we proposed a computational methodology for these conversion-alloying materials and their reaction mechanism by taking amorphous substoichiometric silicon nitride as a showcase system [1]. The molecular dynamics (MD) simulations utilized the ReaxFF parameter set which was specifically developed for the investigation. Subsequently, the experimental pair distribution functions (PDF) were used to verify the modeling results.While the reactive molecular dynamics was coupled with the PDF simulation to complement the experimental measurements, the effective and practical approach guided us to the atomistic structure evolution understanding for LIBs in a conversion–alloying process, revealing the bond coordinations around the Si atoms in the active material. Consequently, the analysis opened an in-depth understanding of the cycling stability of substoichiometric a-SiNx anode materials by revealing the atomistic features relevant to the changes this material undergoes during lithiation and delithiation processes in a LIB. Furthermore, the proposed approach allows modeling of the changes at the atomistic scale for the amorphous materials and predicting the experimental PDF at different cycling stages, verifying the modeling outcome. The topic also addresses the applications of the approach for other anode materials.

Role of and dislocations on the room-temperature grain boundary migration in a deformed Mg alloy

During deformation, dislocation movements at grain boundaries (GBs) directly affect GB plastic behaviors, thus affecting the mechanical properties of metal materials. As common defects in deformed Mg alloys, the specific role of <a> and <c+a> dislocations on room-temperature GB migration is still unclear. This work systematically investigates and answers this scientific question via experimental observations and atomic simulations. High-density serrated GBs of the AZ80 Mg alloy are achieved by multi-axial compression at room temperature. At the atomic scale, the overall slip of interfacial <a> dislocations leads to GB migration, while the GB can remain flat. Different local migration rates caused by slip and interlock of adjacent <a> and <c+a> dislocations result in GB steps, thus forming the serrated GB as the deformation proceeds. The basal-pyramidal lock at the GB prevents the continuous migration of the local GB. A new basal-pyramidal lock model at the GB is established to give a criterion for stable interlock of partial <a> and <c+a> dislocations. The experimental results and the atomic simulations show good agreement at the atomic scale and atomic simulations can explain the GB structure changes observed in the experiments. This work contributes to understanding the GB migration mechanisms of Mg alloys, which helps design deformed Mg alloys through GB engineering.

First-principles investigation on twin-related lattice reorientation in hexagonal metals and alloys

High-throughput ab initio calculations were employed to study the lattice reorientation process accompanying twinning under plastic deformation in hexagonal metals, as well as in alloyed Mg and Ti. The results indicate that, in unalloyed hexagonal metals, reorientation consumes the highest and lowest energies in Be and Mg, respectively, with intermediate energies in Sc, Co, Ti, Zr and Dy. The shuffle component always contributes a significant part of the reorientation energy in Mg, whereas in Ti with sufficient shear strain, subsequent reorientation process becomes energy downhill. In alloyed Mg and Ti, appropriate alloying significantly reduces the reorientation energy, and especially in alloyed Ti, there is significant negative correlation between the reorientation energy and certain properties of the alloying elements. The results are consistent with the fact that reorientation has been experimentally observed in compressed Mg nanopillars, but only in atomic scale simulations under high strain rate in Ti.

Atomic-scale simulations of constant-load scratching of gallium nitride under monolayer graphene lubrication

Reducing the friction and wear of nanodevices are the key to improve the performance of micro/nanoelectromechanical systems (M/NMES). In this paper, a gallium nitride (GaN) substrate coated with a single layer of graphene was constructed using molecular dynamics (MD) simulation, and then scratched with a diamond tip under a constant load. The results showed that graphene can improve the abrasion resistance and hardness of the substrate, and reduce surface wear, temperature, potential energy, and subsurface damage. However, the effect of graphene to inhibit the generation of dislocations within the substrate was not obvious. In addition, the substrate surface exhibited super-lubrication at low load scratching. As the load increased, the friction and wear on the substrate surface increased. The application of graphene can extend the load range over which the substrate undergoes full elastic deformation, which means a greater range of super-lubrication. The protective and lubricating effects of graphene provide guidance for the design of GaN-based nanodevices.

Tuning the Through-Plane Lattice Thermal Conductivity in van der Waals Structures through Rotational (Dis)ordering.

It has recently been demonstrated that MoS2 with irregular interlayer rotations can achieve an extreme anisotropy in the lattice thermal conductivity (LTC), which is, for example, of interest for applications in waste heat management in integrated circuits. Here, we show by atomic-scale simulations based on machine-learned potentials that this principle extends to other two-dimensional materials, including C and BN. In all three materials, introducing rotational disorder drives the through-plane LTC to the glass limit, while the in-plane LTC remains almost unchanged compared to those of the ideal bulk materials. We demonstrate that the ultralow through-plane LTC is connected to the collapse of their transverse acoustic modes in the through-plane direction. Furthermore, we find that the twist angle in periodic moiré structures representing rotational order provides an efficient means for tuning the through-plane LTC that operates for all chemistries considered here. The minimal through-plane LTC is obtained for angles between 1 and 4° depending on the material, with the biggest effect in MoS2. The angular dependence is correlated with the degree of stacking disorder in the materials, which in turn is connected to the slip surface. This provides a simple descriptor for predicting the optimal conditions at which the LTC is expected to become minimal.

Unlocking the mysterious polytypic features within vaterite CaCO3

Calcium carbonate (CaCO3), the most abundant biogenic mineral on earth, plays a crucial role in various fields such as hydrosphere, biosphere, and climate regulation. Of the four polymorphs, calcite, aragonite, vaterite, and amorphous CaCO3, vaterite is the most enigmatic one due to an ongoing debate regarding its structure that has persisted for nearly a century. In this work, based on systematic transmission electron microscopy characterizations, crystallographic analysis and machine learning aided molecular dynamics simulations with ab initio accuracy, we reveal that vaterite can be regarded as a polytypic structure. The basic phase has a monoclinic lattice possessing pseudohexagonal symmetry. Direct imaging and atomic-scale simulations provide evidence that a single grain of vaterite can contain three orientation variants. Additionally, we find that vaterite undergoes a second-order phase transition with a critical point of ~190 K. These atomic scale insights provide a comprehensive understanding of the structure of vaterite and offer advanced perspectives on the biomineralization process of calcium carbonate.

Electrochemical C-N Bond Formation within Boron Imidazolate Cages Featuring Single Copper Sites.

Electrocatalysis expands the ability to generate industrially relevant chemicals locally and on-demand with intermittent renewable energy, thereby improving grid resiliency and reducing supply logistics. Herein, we report the feasibility of using molecular copper boron-imidazolate cages, BIF-29(Cu), to enable coupling between the electroreduction reaction of CO2 (CO2RR) with NO3- reduction (NO3RR) to produce urea with high selectivity of 68.5% and activity of 424 μA cm-2. Remarkably, BIF-29(Cu) is among the most selective systems for this multistep C-N coupling to-date, despite possessing isolated single-metal sites. The mechanism for C-N bond formation was probed with a combination of electrochemical analysis, in situ spectroscopy, and atomic-scale simulations. We found that NO3RR and CO2RR occur in tandem at separate copper sites with the most favorable C-N coupling pathway following the condensation between *CO and NH2OH to produce urea. This work highlights the utility of supramolecular metal-organic cages with atomically discrete active sites to enable highly efficient coupling reactions.

Chirality reversal of resonant modes in GdFe ferrimagnets

Chirality of antiferromagnetic spin waves as an intrinsic degree of freedom has been attracting considerable attention due to its potential applications for magnonic devices. In this paper, atomistic-scale dynamics simulations were conducted to investigate the chirality of spin wave resonant modes in ferrimagnetic alloy GdxFe1−x (0 &amp;lt; x &amp;lt;1) under different proportion x and external magnetic fields near the angular momentum compensation point. Simulation results reveal that as the proportion of Gd increases, the resonance mode of spin waves undergoes two distinct handedness flipping at magnetization compensation point and angular momentum compensation point. When the proportion x deviates from the magnetization compensation point, a frequency degeneracy point emerges at a non-zero magnetic field, indicating that the chirality of spin waves can also be switched by an external magnetic field. A theoretical analysis is developed to explain the observed phenomena. These findings provide valuable insights into the control and manipulation of spin wave chirality in ferrimagnetic alloys, with potential implications for the development of spin-based devices and technologies.

Insights on grain boundary effects on crack formation and propagation in Nb3Sn coatings at low temperature and high strain rates: a molecular dynamics simulation study

Mechanical behavior of Nb3Sn coatings is inherently correlated with the anti-interference ability of the superconducting cavity system and the amplitude and phase stability of the cavity field. We report on atomic-scale simulation and analysis of grain boundary effects on the crack formation and propagation in Nb3Sn coatings at low temperature and high strain rates. For tensile-deformed single crystal Nb3Sn, accompanied by the occurrence of slips, micro voids propagate along the () and (210) planes (stretching the crystal along the [100] direction), and subsequently coalesce, leading to the cracks formation. For tensile-deformed single-crystal Nb3Sn coating on Nb substrate, it fails by crack propagation in the coating before showing significant yielding plateaus. In the Nb3Sn coating, the slip bands on the (201) and () planes meet and merge, inducing the breaking of atomic bonds and the formation of voids at the slip band intersections, and the subsequent micro-crack initiation. The misfit dislocations of Nb3Sn/Nb heterostructure interface and the dislocations nucleated on slip systems control the plastic deformation of the Nb substrate, causing the BCC-to-FCC and FCC-to-HCP phase transformation in the substrate, for which, ductile fracture occurs after necking. For polycrystalline Nb3Sn coating on Nb substrate, the simulations demonstrate that cracks propagate entirely along grain boundaries, exhibiting the intergranular fracture characteristics. The composite exhibits a significant strain rate effect. At high strain rates, crack propagation across grain boundaries in Nb3Sn is hindered, dislocations are emitted into the both adjacent grains from the grain boundary facets, they glide across the grain, and voids are nucleated at the intersections of dislocations inside the grains, which leads to the formation of voids and ultimately the occurrence of trans-crystalline fracture. The findings provide important information about the crack evolution in Nb3Sn coatings, which are critical to the fabrication and performance analysis of Nb3Sn superconducting radio frequency cavities.

Direct pen writing and atomic-scale molecular dynamics simulation study of a novel silver nano-ink

Direct pen writing and atomic-scale molecular dynamics simulation study of a novel silver nano-ink

Transferable Force Field for Gallium Nitride Crystal Growth from the Melt Using On-The-Fly Active Learning.

Atomic-scale simulations of reactive processes have been stymied by two factors: the lack of a suitable semiempirical force field on one hand and the impractically large computational burden of using ab initio molecular dynamics on the other hand. In this paper, we use an "on-the-fly" active learning technique to develop a nonparameterized force field that, in essence, exhibits the accuracy of density functional theory and the speed of a classical molecular dynamics simulation. We developed a force field capable of capturing the crystallization of gallium nitride (GaN) during a novel additive manufacturing process featuring the reaction of liquid Ga and gaseous nitrogen precursors to grow crystalline GaN thin films. We show that this machine learning model is capable of producing a single force field that can model solid, liquid, and gas phases involved in the process. We verified our computational predictions against a range of experimental measurements relevant to each phase and against ab initio calculations, showing that this nonparametric force field produces properties with excellent accuracy as well as exhibits computationally tractable efficiency. The force field is capable of allowing us to simulate the solid-liquid coexistence interface and the crystallization of GaN from the melt. The development of this transferable force field opens the opportunity to simulate the liquid-phase epitaxial growth more accurately than before to analyze reaction and diffusion processes and ultimately to establish a growth model of the additive manufacturing process to create the gallium nitride thin films.

Metal organic frameworks with carbon black for the enhanced electrochemical detection of 2,4,6-trinitrotoluene

The sensing of explosives such as 2,4,6-trinitrotoluene (TNT) directly at an explosion site requires a fast, simple and sensitive detection method, to which electrochemical techniques are well suited. Herein, we report an electrochemical sensor material for TNT based on an ammonium hydroxide (NH4OH) sensitized zinc-1,4–benzenedicarboxylate Zn(BDC) metal organic framework (MOF) mixed with carbon black on a glassy carbon electrode. In the solvent modulation mechanism, by merely changing the concentration of NH4OH during synthesis, two Zn(BDC) MOFs with novel morphologies were fabricated via a hydrothermal approach. The as-prepared MOFs were characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and high-resolution field emission electron microscopy (FESEM) equipped with energy dispersive X-ray spectroscopy (EDS). The different morphologies of the MOFs, and their impact on the performance of the modified electrodes towards the electrochemical detection of TNT was investigated. Under optimum conditions, 0.7–Zn(BDC) demonstrated the best electrochemical response for TNT detection using square wave voltammetry (SWV) with a linear calibration response in the range of 0.3–1.0 μM, a limit of detection (LOD) of 0.042 μM, a limit of quantification (LOQ) of 0.14 μM and a high rate of repeatability. Atomic-scale simulations based on density functional theory authenticated the efficient sensing properties of Zn(BDC) MOF towards TNT. Furthermore, the promising response of the sensors in real sample matrices (tap water and wastewater) was demonstrated, opening new avenues towards the real-time detection of TNT in real environmental samples.

ColabFit exchange: Open-access datasets for data-driven interatomic potentials.

Data-driven interatomic potentials (IPs) trained on large collections of first principles calculations are rapidly becoming essential tools in the fields of computational materials science and chemistry for performing atomic-scale simulations. Despite this, apart from a few notable exceptions, there is a distinct lack of well-organized, public datasets in common formats available for use with IP development. This deficiency precludes the research community from implementing widespread benchmarking, which is essential for gaining insight into model performance and transferability, and also limits the development of more general, or even universal, IPs. To address this issue, we introduce the ColabFit Exchange, the first database providing open access to a large collection of systematically organized datasets from multiple domains that is especially designed for IP development. The ColabFit Exchange is publicly available at https://colabfit.org, providing a web-based interface for exploring, downloading, and contributing datasets. Composed of data collected from the literature or provided by community researchers, the ColabFit Exchange currently (September 2023) consists of 139 datasets spanning nearly 70 000 unique chemistries, and is intended to continuously grow. In addition to outlining the software framework used for constructing and accessing the ColabFit Exchange, we also provide analyses of the data, quantifying the diversity of the database and proposing metrics for assessing the relative diversity of multiple datasets. Finally, we demonstrate an end-to-end IP development pipeline, utilizing datasets from the ColabFit Exchange, fitting tools from the KLIFF software package, and validation tests provided by the OpenKIM framework.