Adaptive Intermolecular Reactive Bond Order Research Articles

Atomistic insights into the effect of cooling rates on mechanical properties of glassy graphene

Glassy graphene is obtained via molecular dynamics simulation, and we investigated the effect of cooling rates on the forming and mechanical properties of glassy graphene (GG). The system interactions are modeled using the adaptive intermolecular reactive bond order (AIREBO) potential. The results of our simulations are in good agreement with experimental ones in the case of crystalline graphene. We calculated the radial distribution function (RDF) to characterize the structural properties of the forming glass and used the Wendt-Abraham parameter to determine the glass temperature transition (Tg). As a result, we found that as the cooling rate increased, the rigidity of the glassy graphene decreased. The calculated Young's modulus equals 70.33 GPa, 29.76 GPa, 12.42 GPa, and 2.85 GPa for cooling rates equal to 0.1 K/ps, 1 K/ps, 10 K/ps, and 100 K/ps, respectively.

The role of topological defects on the mechanical properties of single-walled carbon nanotubes

ABSTRACT Carbon nanotubes (CNTs) are among the most employed nanomaterials in developing new technologies. Their applicability requires a fundamental understanding of the chirality and defect effects on the mechanical properties. In this study, molecular dynamics (MD) simulations were performed to investigate the mechanical response of defective single-walled carbon nanotubes (SWCNTs) under tension loading. The Stones-Wales, monovacancy, and the divacancy reconstructions (585, 555777, and 555567777) defects were considered. In addition, we investigated the influence of the adaptive intermolecular reactive bond order (AIREBO) potential cut-off radii on the defect formation energy of SWCNTs. Our results reveal that the tensile strength properties are notably dependent on the chirality and defect configurations at strains over 8%. Energetically favourable defects have a high impact on the mechanical response of SWCNTs. A combination of certain defects may lead the control on the fracture pattern of the SWCNTs, which can significantly contribute to the designing of innovative nanostructures with tailored properties.

Molecular dynamics simulations of structural instability of fullerene family under tension force

ABSTRACTFullerene molecules are cage-like nanoscopic structures with pentagonal and hexagonal faces. In practical applications such as fullerene-reinforced nanocomposites (FRNCs), these structures may be subjected to tension force. In this research, we employ molecular dynamics (MD) simulation to compute the behaviour and deformation of different fullerene molecules, ranging from C60 to C2000, under tension force. To model the interactions between carbon atoms in the MD simulations, the adaptive intermolecular reactive bond order (AIREBO) force field is used. The displacement–force and the displacement–strain energy curves are obtained. It is observed that a new type of structural instability occurs in the fullerene molecules when the applied tension force increases. This abnormal structural instability in the fullerenes is investigated for the first time in the literature. The critical tensile forces and the corresponding mode shapes are determined for different fullerenes. The results indicate that the critical forces and deformations strongly depend upon the number of carbon atoms.

Synergistic effect of temperature and point defect on the mechanical properties of single layer and bi-layer graphene

The present study reports a comprehensive molecular dynamics simulation of the effect of a) temperature (300–1073 K at intervals of every 100 K) and b) point defect on the mechanical behaviour of single (armchair and zigzag direction) and bilayer layer graphene (AA and AB stacking). Adaptive intermolecular reactive bond order (AIREBO) potential function was used to describe the many-body short-range interatomic interactions for the single layer graphene sheet. Moreover, Lennard Jones model was considered for bilayer graphene to incorporate the van der Waals interactions among the interlayers of graphene. The effect of temperature on the strain energy of single layer and bilayer graphene was studied in order to understand the difference in mechanical behaviour of the two systems. The strength of the pristine single layer graphene was found to be higher as compared to bilayer AA stacked graphene at all temperatures. It was observed at 1073 K and in the presence of vacancy defect the strength for single layer armchair sheet falls by 30% and for bilayer armchair sheet by 33% as compared to the pristine sheets at 300 K. The AB stacked graphene sheet was found to have a two-step rupture process. The strength of pristine AB sheet was found to decrease by 22% on increase of temperature from 300 K to 1073 K.

Diaphite, a new type of surface with mixed sp2-sp3 hybridization for adsorption and functionalization

We theoretically study a new carbon phase with mixed sp2-sp3 bond hybridization called diaphite. Using ab initio calculations and the adaptive intermolecular reactive bond order (AIREBO) potential approach, we show that the surface of this structure exhibits enhanced adsorption capabilities. Specifically, using hydrogen as a test adsorbate, we calculate that the chemical binding energy, depending on the configuration of hydrogen atoms, varies from 2.08 to 2.9eV. Furthermore, the adsorption sites form a regular matrix; thus, we postulate that this new stable carbon phase may be a universal matrix for functionalization.

High-temperature behavior of monolayer graphyne and graphdiyne

Molecular dynamics simulations with the adaptive intermolecular reactive bond order (AIREBO) potential are performed to study the high-temperature (T) behavior of monolayer graphyne and graphdiyne (MGY and MGDY). During the melting process, MGY and MGDY undergo three continuous phase transitions: (i) transformation into the initial amorphous graphene phase (AGP) starting from 2800 to 2500 K for MGY and MGDY, respectively, where broken bonds and large holes appear; (ii) transformation into AGP through a structural adjustment process until T reaches 3650 K. This AGP will remain relatively stable until T ≈ 5000 K; (iii) the AGP will then gradually transform into a nearly fluid state for T > 5000 K. Even prior to the first phase transition, some defects, such as 5, 7, 8 and 9-membered rings start appearing at 1700 and 1200 K for MGY and MGDY, respectively. In addition to the defects derived from severe thermal vibration, the sharp contraction and accompanying stretch of MGY and MGDY are decisive in the formation of the initial AGP. In contrast to the prediction of the Kosterlitz-Thouless-Halperin-Nelson-Young theory for the melting of two-dimensional materials, we don't observe a hexatic phase but rather an AGP as an intermediate phase.

Mechanical modeling of graphene using the three-layer-mesh bridging domain method

Abstract Recently developed three-layer-mesh bridging domain method (TBDM) enhanced the conventional bridging domain method (BDM) by (1) mitigating the temperature cooling effect on the atoms in the bridging domain, and (2) employing a mesh-independent physics-based discrimination between thermal and mechanical atomic motions. In this paper, we present the new enhancements for the TBDM to achieve an appropriate framework for concurrently coupled atomistic–continuum simulation of graphene. To capture the mechanical behavior of graphene accurately, we employed the adaptive intermolecular reactive bond order (AIREBO) potential in the atomistic model, which is carefully weighted by the atomic energy scaling function in the bridging zone. In the continuum model, a thermodynamically rigorous high-order continuum description, considering the symmetries of graphene, is used which is parameterized using full molecular dynamics (MD) simulations. To accurately capture the bending behavior of graphene, a recently developed explicit finite-deformation solid-shell element is used to discretize the continuum domain, and its formulation is modified to include the continuum energy scaling function. To achieve realistic constant-temperature condition (canonical ensemble), the Nose–Hoover thermostat is used in the full MD domain and also as local thermostats in the bridging domain. 5-value Gear predictor–corrector time integrator is implemented, which is well-suited to be used with the Nose–Hoover thermostat. Accordingly, the TBDM formulation is modified to work with this time integrator. Some modifications are also made in the TBDM formulation to increase the robustness of the multiscale simulations. Finally, the effectiveness of the proposed multiscale method for graphene is demonstrated by running in-plane shear, out-of-plane bending, and nanoindentation simulations and comparing the results with those obtained from full MD and full finite-element simulations.

Continuum Shell Model for Buckling of Single-Walled Carbon Nanotubes with Different Chiral Angles

In this paper, an equivalent thick cylindrical shell model is proposed for the buckling analysis of short single-walled carbon nanotubes (SWCNTs) with allowance for different chiral angles. Extensive, molecular dynamics (MD) simulations are first performed using the adaptive intermolecular reactive bond order potential to determine the critical buckling loads/strains. The MD simulations buckling results are then used as reference solutions to calibrate the properties of the thick cylindrical shell model. Central to this development is the establishment of an empirical expression for the Young's modulus that is a function of both the diameter and the chiral angle of the SWCNT. For the shell model, we have assumed that the Poisson ratio ν = 0.19 and the shell thickness h = 0.066 nm . It will be shown that the proposed shell model furnishes good estimates of the critical buckling loads for SWCNTs with different chiral angles. The critical buckling strains are also evaluated from the critical buckling load with the aid of the stress–strain relation of SWCNTs.

CONTINUUM SHELL MODEL FOR BUCKLING OF ARMCHAIR CARBON NANOTUBES UNDER COMPRESSION OR TORSION

Molecular dynamics (MD) simulations are performed using adaptive intermolecular reactive bond order potential to analyze single-walled and double-walled carbon nanotubes. These carbon nanotubes were analyzed for buckling under compression and under torsion. The MD simulations create a comprehensive database for the critical buckling loads/strains and critical buckling torques/twist angles for armchair SWCNTs and DWCNTs of varying diameters and lengths. Using MD results as a computational benchmark, an equivalent thick shell model of CNT is proposed, which is amenable for analysis using a commercially available software ABAQUS. Based on our MD results, an empirical equation that describes the size-dependent Young's modulus for a single-walled carbon nanotube is established. Buckling analysis of CNT under compression and under torsion are performed with the equivalent shell model using size-dependent Young's modulus, Poisson's ratio = 0.19 and shell thickness h = 0.066 nm. We show that the equivalent shell model gives good estimate of critical buckling load/strain and critical buckling torque with respect to the MD results. Variation of critical twist angle with length of CNT, predicted by the shell model is in good qualitative agreement with MD simulation. However, the equivalent shell model underestimates the critical twist angle by 30% because the continuum shell model overestimates torsional stiffness of CNT compared to an atomistic model of CNT. The equivalent shell model is less computational intensive to implement as compared with MD. Its accuracy for predicting the buckling states for long carbon nanotubes allows it to be used for moderately long CNTs under compression/torsion, in-lieu of MD simulations.

A molecular dynamics study of round and flattened carbon nanotube structures

Large diameter single wall carbon nanotubes exist with both round right cylinder and barbell-like flattened cross sections. Using an adaptive intermolecular reactive bond order potential, we examine the characteristics of both configurations by using molecular dynamics simulations. We then continue to examine the dynamical transition from the flattened to round state by charge injection. Predictions based on these simulations provide a basis for the design of active fluid transport devices, and nanoscale mechanical systems/motors.