Large-scale Reactive Molecular Dynamics Simulations Research Articles

Simultaneous improvement in the elastic and fracture properties of graphene-epoxy nanocomposites–a computational perspective

The incorporation of stiff nano-additives (such as graphene) into a relatively soft polymer material (such as epoxy) usually leads to an improvement in elastic properties (i.e., Young's modulus) at the expense of fracture properties (i.e., tensile strength and toughness). Despite over a decade of research in polymer nanocomposites, we still lack a clear understanding of their structure-property relationships, which limits us from enhancing both elastic and fracture properties concurrently. Here, we performed large-scale reactive molecular dynamics simulations to study the deformation and fracture of model graphene/epoxy systems under uniaxial tension. A computationally efficient reactive force field was developed for graphene-epoxy system, allowing covalent bond formation, and breaking, which is crucial to model cross-linking in model epoxy as well as fracture. It was found the mechanical properties of the nanocomposite are very sensitive to the strength of the graphene-epoxy interface. As expected, elastic modulus increases with the interfacial strength. However, there appears to be an optimal interfacial strength to enhance the tensile strength and toughness. This is due to stress concentration occurring near the graphene edges at high interfacial strength, which leads to premature fracture. We show that by appropriately selecting an intermediate interface strength, one can simultaneously improve the ultimate tensile strength, toughness and the Young's modulus of the nanocomposite epoxy at ultra-low (∼0.2 % by weight) loading fraction of graphene additives. Our findings highlight the critical importance of properly engineered additive-matrix interfacial strength to develop high-performing polymer nanocomposites that are concurrently stiff, strong as well as tough.

Relationship between Surface States of Carbon Support and Electrode Reaction Activity in Cathode Catalyst Layer Toward High Power Output of Polymer Electrolyte Fuel Cells by Large-Scale Reactive Molecular Dynamics Simulations

Higher output of polymer electrolyte fuel cells (PEFC) is required for use in many industrial fields such as the transport industry. PEFCs consist of a gas diffusion layer, a catalyst layer (CL), and a polymer electrolyte membrane. The output of PEFCs depends on the electrode reaction activity of the CL consisting of the carbon support, ionomer, water, and Pt nanoparticles (NPs). For high electrode reaction activity, the optimization of the CL structures such as Pt alloy, pore structures of the carbon support, and I/C (Ionomer carbon ratio), are key factors. To optimize the CL structures, computational simulation, which can propose theoretical design principles, is a promising method. Thus, many researchers have investigated the CL structures with high electrode reaction activity by computational simulation to propose design principles. For example, first-principles calculation can optimize highly active Pt catalysts by considering activation barriers, molecular adsorption structures and surface electronic states, which involves the behavior of atoms and molecules. However, this method cannot consider the effects of carbon support, ionomer, and water because of the limitation in computational costs. On the other hand, macro-scale approach using differential equations can discuss the highly active CL structures since this method can consider multiple structures such as carbon support, ionomer distribution, water content, and Pt loading. However, this approach cannot consider the behavior of atoms and molecules. Therefore, large-scale molecular simulations which, can simultaneously consider the behavior of atoms and molecules, and the CL structures are required. Here, reactive molecular dynamics (RMD) simulation is a practical approach to deal with the behavior of atoms and the chemical reactions in the CL. By using RMD method, we successfully constructed large-scale CL structures and established the CL design methodology both including atomic and molecular effects and clarifying the effect of the pore size of carbon support on the electrode reaction activity [1]. In order to design the higher-performance CL structures, we focused on the distribution of water which mediates proton transfer process. Improving the wettability of carbon support by termination of hydroxyl groups to the carbon support surface can alter the water distribution. Therefore, by large-scale RMD simulations, we investigated the effect of the ratio of the hydroxyl groups on the carbon support surface on water distribution to propose design principles for higher-performance cathode CL structures.A CL structures model consisting of 1 million atoms was constructed by modeling carbon support, Pt NPs, ionomer, and water. We selected the carbon support, where there were six meso pores on an amorphous carbon sphere. The ratios of hydroxyl groups to the carbon surface atoms were set to 10 and 32% to control the water distribution on the carbon surface. In addition, Pt NPs were put on the carbon support outer surface and interior of the pores. The water film whose thickness is 0.7 nm and ionomer are coated on Pt-supported carbon. Hereafter, we call this structure catalyst particle (CP) model (Fig. 1).To clarify the effect of the ratio of hydroxyl groups on the carbons support surface, the morphology of the water film was compared in the CP models with different hydroxyl ratios. At the exterior of the pores, the water film was peeled off from the carbon support surface regardless of the ratio of hydroxyl groups. At the interior of the pores, when the ratio of hydroxyl groups was 10%, the water film was peeling from the carbon support surface（Fig. 2(a), (b)). On the other hand, when the ratio of hydroxyl groups was 32%, the water film did not peel off from the carbon support surface, and the structure of the water film was maintained (Fig. 2(c), (d)). Next, to clarify the effect of the water film morphology on the proton conduction to the Pt NPs, the water clusters as proton conduction paths were analyzed. Water clusters were defined as a group of water molecules whose inter-molecular distance is less than 3.5Å. The amount of proton conduction paths to the Pt NPs at the exterior of the pores did not change regardless of the ratio of hydroxyl groups. On the other hand, the amount of proton conduction path to the Pt NPs at the interior of the pores was increased by increasing the ratio of hydroxyl groups (Fig. 3). These results indicate that the increasing the ratio of hydroxyl groups on the carbon support surface to improve the proton conduction path is an effective approach for the design of the higher-performance cathode CL.[1] T. NAKAMURA, M. KUBO et al. J. Comput. Chem. Jpn., 20,150-154, (2021). Figure 1

Gradient evolution in graphene reinforced carbon/carbon composites

Graphene-guided carbonization is an effective route to achieve carbon-reinforcement/carbon matrix (C/C) composites with high mechanical performance at lower energy consumption, due to the reduced anneal temperatures required. However, the underlying microstructure evolution mechanisms associated with this phenomenon remain mysterious. Here, through large-scale reactive molecular dynamics simulations, we revealed a “gradient evolution” effect occurred in carbon matrix, which gives rise to the formation of additional stacked graphene layers in the vicinity of the original introduced graphene sheet. Two microstructure evolution pathways correlated with such effect are identified: (1) near the introduced graphene surface, one or two graphene-like layers rapidly developed due to the growth of curved carbon nanosheets via edge-crosslinking, (2) away from the graphene surface, graphene-like layers progressively promoted by reconstruction of dissociated disordered carbon nanosheets. As a result, the graphene reinforced C/C composites exhibit significantly improved tensile modulus and strength in comparison to those without graphene. Our study not only provides fundamental insights into the graphene-induced carbonization phenomenon, but also emphasizes the unprecedented potential of graphene as an excellent reinforcement for C/C composites.

Insight into sodium penetration with mechanical behaviors of carbon electrodes by large-scale reactive molecular dynamics simulations

Sodium penetration of carbon electrodes leads to large deformation and concurrent changes in their mechanical properties. Taking the industrial carbon cathode for aluminum smelting as an example, this work constructed a complex molecular model of the carbon cathode (over 5,0000 atoms). Then, the large-scale ReaxFFsimulations were applied to correlate the microstructural changes with the associated macroscopic response, revealing the atomic-scale mechanisms of sodium penetration and its relation to mechanical behaviors including sodium expansion and creep deformation. In particular, we provided a detailed knowledge involving the microstructural origin of mechanical deterioration and the molecular basis of cathode strength. This study might be beneficial to improve the understanding of the structure-property relationship, and provide theoretical support for the optimal design of carbon electrode materials against failure.

Overall mechanism of JP-10 pyrolysis unraveled by large-scale reactive molecular dynamics simulation

This paper reports the overall reaction mechanism of JP-10 pyrolysis obtained in large-scale reactive molecular dynamics simulations employing the force field of ReaxFF CHO-2008. The C<5 products obtained from the simulations are basically consistent with what detected using varied experimental techniques reported in literature. Particularly, the product evolution tendency of methane, ethane, ethylene, propylene, acetylene, allene,1−butene, propyne, 1,3-butadiene, cyclopentadiene with temperature is in good agreement with the results of single-pulse shock tube experiment performed in this work. The simulation results indicate that JP-10 pyrolysis reactions can be divided into three stages on basis of the evolution of total radicals, namely, the initial ring-openning through CC bond homolysis in stage I, the initiation and growth of chain radicals through β-scission reactions as well as some of the chain propagation via CH bond scission in stage II, the chain propagation through CH bond cleavage and the chain termination reactions in stage III. The C5H7• and C3H5• radicals were found playing dominant role in the reaction propagation. The obtained initial reaction sites of JP-10 pyrolysis are ranked as: B3>B4>B2>B5>B1>B6>B7 that will not be significantly affected by temperature. The obtained aromatic hydrocarbons in simulations and the increasing C/H ratio with temperature and time during the three pyrolysis stages exhibit the coking tendency during JP-10 pyrolysis.

Correction to “Selective Wear Behaviors of a Water-Lubricating SiC Surface under Rotating-Contact Conditions Revealed by Large-Scale Reactive Molecular Dynamics Simulations”

Correction to “Selective Wear Behaviors of a Water-Lubricating SiC Surface under Rotating-Contact Conditions Revealed by Large-Scale Reactive Molecular Dynamics Simulations”

Selective Wear Behaviors of a Water-Lubricating SiC Surface under Rotating-Contact Conditions Revealed by Large-Scale Reactive Molecular Dynamics Simulations

Selective Wear Behaviors of a Water-Lubricating SiC Surface under Rotating-Contact Conditions Revealed by Large-Scale Reactive Molecular Dynamics Simulations

Development of ReaxFF Reactive Force Field for Aqueous Iron-Sulfur Clusters with Applications to Stability and Reactivity in Water.

Iron–sulfur clusters serve unique roles in biochemistry, geochemistry, and renewable energy technologies. However, a full theoretical understanding of their structures and properties is still lacking. To facilitate large-scale reactive molecular dynamics simulations of iron–sulfur clusters in aqueous environments, a ReaxFF reactive force field is developed, based on an extensive set of quantum chemical calculations. This force field compares favorably with the reference calculations on gas-phase species and significantly improves on a previous ReaxFF parametrization. We employ the new potential to study the stability and reactivity of iron–sulfur clusters in explicit water with constant-temperature reactive molecular dynamics. The aqueous species exhibit a dynamic, temperature-dependent behavior, in good agreement with previous much more costly ab initio simulations.

Fast reaction of aluminum nanoparticles promoted by oxide shell

Combustion of aluminum nanoparticles (AlNPs) has long been investigated experimentally because of their use in various energetic formulations for propellants and explosives. But the limited spatiotemporal resolution in experiments, in particular, makes it challenging to explore the microstructural evolution of AlNP oxidation and associated mechanisms. Here, we perform large-scale reactive molecular dynamics simulations to study the structural evolution of AlNPs with a 2–4 nm thick oxide shell in an oxygen environment. We find the temporal hollowing processes of AlNPs for both symmetrical and asymmetrical oxidations, in which the morphological evolution can be understood by a discrepant electric field and temperature distributions for different systems. In the early time, core aluminum atoms experience a fast reaction with an oxide shell. Environmental oxygen does not react with AlNPs until the surface O/Al ratio decreases to ∼1.2. Moreover, based on our simulation results, previous experimental data agree well with the proposed model, which can well describe the relationship between combustion efficiency and oxide shell thickness, confirming that the oxide shell promotes rather than hinders the combustion of AlNPs. The molecular insights obtained here would be significant for understanding the underlying mechanism and further modeling of AlNP combustion.

Generation and characterization of carbon fiber microstructures by atomistic simulations

A clear understanding of carbon fiber (CF) microstructure is necessary for the development of high strength CFs. Here, we present an atomistic approach for generating and characterizing realistic microstructures of CFs. Large-scale reactive molecular dynamics simulations are used to generate a set of distinct CF microstructures. Comprehensive characterization of the simulated microstructures is enabled by the development of a suite of computational structural analysis tools capable of evaluation of hybridization states of carbon atoms, populations and orientations of individual carbon rings, degree of graphitization, and pore size distribution. The calculation of X-ray diffraction profiles provides a direct link between the structural features of simulated samples and experimental data available for CFs. The CF generation algorithm is shown to produce microstructures with experimental densities and with structural characteristics matching those of PAN-based CFs. The key structural features affecting the properties of CFs, such as the relative fractions of graphitic, turbostratic and amorphous microconstituents, degree of alignment, pore size distributions, and chemical cross-linking can be effectively controlled in simulations, thus enabling efficient exploration of structure–properties relationships in CFs. The capabilities of the developed approach are illustrated by performing computational analysis of the mechanical deformation and fracture of CFs under axial tensile loading.

Thermal Decomposition Mechanism of 2,2′,4,4′,6,6′-Hexanitrostilbene by ReaxFF Reactive Molecular Dynamics Simulations

2,2′,4,4′,6,6′-Hexanitrostilbene (HNS) is an explosive with increased heat resistance, and its mechanism of thermal decomposition is of interest. In this paper, the decomposition processes of HNS at various temperatures (2500, 2750, 3000, 3250, and 3500 K) are calculated by large-scale reactive molecular dynamics simulations. The initial reactions and the evolution of clusters (whose molecular weight is larger than HNS) are analyzed. The reaction kinetics parameters are fitted. The results show that the main initial decomposition mechanisms of HNS are C–NO2 bond dissociation and nitro-nitrite (NO2–ONO) isomerization. During decomposition, O atoms are less likely to be released from the cluster than H and N atoms. Low temperatures tend to produce larger clusters, and clusters at higher temperatures tend to decompose. The thermal decomposition of HNS is a combination of single-molecule and bimolecular decomposition mechanisms. The dimerization reaction is clearly weakened, and the C–N bond cleavage is still...

Interlocking Friction Governs the Mechanical Fracture of Bilayer MoS2.

A molybdenum disulfide (MoS2) layered system is a two-dimensional (2D) material, which is expected to provide the next generation of electronic devices together with graphene and other 2D materials. Due to its significance for future electronics applications, gaining a deep insight into the fundamental mechanisms upon MoS2 fracture is crucial to prevent mechanical failure toward reliable applications. Here, we report direct experimental observation and atomic modeling of the complex failure behaviors of bilayer MoS2 originating from highly variable interlayer frictions, elucidated with in situ transmission electron microscopy and large-scale reactive molecular dynamics simulations. Our results provide a systematic understanding of the effects that different stacking and loading conditions have on the failure mechanisms and crack-tip behaviors in the bilayer MoS2 systems. Our findings unveil essential properties in fracture of this 2D material and provide mechanistic insight into its mechanical failure.

Quantitative 3D evolution of colloidal nanoparticle oxidation in solution.

Real-time tracking of the three-dimensional (3D) evolution of colloidal nanoparticles in solution is essential for understanding complex mechanisms involved in nanoparticle growth and transformation. We used time-resolved small-angle and wide-angle x-ray scattering simultaneously to monitor oxidation of highly uniform colloidal iron nanoparticles, enabling the reconstruction of intermediate 3D morphologies of the nanoparticles with a spatial resolution of ~5 angstroms. The in situ observations, combined with large-scale reactive molecular dynamics simulations, reveal the details of the transformation from solid metal nanoparticles to hollow metal oxide nanoshells via a nanoscale Kirkendall process-for example, coalescence of voids as they grow and reversal of mass diffusion direction depending on crystallinity. Our results highlight the complex interplay between defect chemistry and defect dynamics in determining nanoparticle transformation and formation.

Argonne Discovery Yields Self-Healing Diamond-Like Carbon

Large-scale reactive molecular dynamics simulations carried out on the US Department of Energy's IBM Blue Gene/Q Mira supercomputer at the Argonne Leadership Computing Facility, along with experiments conducted by researchers in Argonne's Energy Systems Division, enabled the design of a self-healing anti-wear coating that drastically reduces friction and related degradation in engines and moving machinery. Now, the computational work advanced for this purpose is being used to identify the friction-fighting potential of other catalysts.

Prediction of structures and properties of 2,4,6-triamino-1,3,5-triazine-1,3,5-trioxide (MTO) and 2,4,6-trinitro-1,3,5-triazine-1,3,5-trioxide (MTO3N) green energetic materials from DFT and ReaxFF molecular modeling

Structures and properties of new green energetic materials: I. MTO (P21): ρ=1.92 g cm-3, ΔHrxn = 1036 kcal kg-1 II. MTO3N (P21/c): ρ=2.1 g cm-3, ΔHrxn =1412 kcal kg-1.