A molecular dynamics study of the mechanical properties of h-BCN monolayer using a modified Tersoff interatomic potential

Using molecular dynamics (MD) simulations, we investigate the mechanical properties of hexagonal BCN monolayer, a newly synthesized two-dimensional material with an atom ratio of B/C/N = 1:1:1. The Tersoff potential is modified to get good agreement between predicted and measured fracture strengths of graphene. With this modified Tersoff potential, we perform extensive MD simulations to study the effect of temperature, strain rate and vacancy defect on the mechanical properties of h-BCN. It is found that h-BCN is a strong material with fracture strength of 81.4–93.5 GPa, albeit ∼35% lower than that of graphene. Similar to graphene, temperature has strong effect on the mechanical properties of h-BCN. As the temperature increases from 10 K to 1300 K, the fracture strength and strain of h-BCN drops by 55% and 62%, respectively. The strain rate is found to have a moderate effect. When the strain rate increases from 0.00002 to 0.0125 ps⁻¹, the fracture strength and strain of h-BCN increases 6.1% and 12%, respectively. As for the atomic defect, a very small concentration (0.028%) of vacancy in h-BCN is able to cause a 28% reduction in fracture strength and a 35.5% reduction in fracture strain. These findings have significance for its future applications in nanodevices.     

Tensile properties of microtubules: A study by nonlinear molecular structural mechanics modelling

A nonlinear molecular structural mechanics (MSM) model is proposed in this paper for studying the tensile properties of microtubules (MTs). In the nonlinear MSM models, the interactions between tubulin monomers in MTs are treated as nonlinear axial and torsional springs, whose stiffness coefficients are extracted from all-atom molecular dynamics simulations. The Young's modulus and fracture properties of MTs under tension extracted from the present nonlinear MSM models are found to agree well with the existing simulation and experiment results, which shows the efficiency and accuracy of the proposed nonlinear MSM models. In addition, the nonlinear MSM models are also extended to investigate the tensile properties including Young's modulus and fracture strain of MTs possessing lattice defects. The results obtained from nonlinear MSM models are utilized to develop a predictive equation for quickly predicting the tensile properties of MTs with different lattice defect levels.     

Void effect on mechanical properties of copper nanosheets under biaxial tension by molecular dynamics method

The relationship between void size/location and mechanical behavior under biaxial loading of copper nanosheets containing voids are investigated by molecular dynamics method. The void location and the void radius on the model are discussed in the paper. The main reason of break is discovered by the congruent relationship between the shear stress and its dislocations. Dislocations are nucleated at the corner of system and approached to the center of void with increased deformation. Here, a higher stress is required to fail the voided sheets when smaller voids are utilized. The void radius influences the time of destruction. The larger the void radius is, the lower the shear stress and the earlier the model breaks. The void location impacts the dislocation distribution.     

Influence of line defects on relaxation properties of graphene: A molecular dynamics study

The relaxation properties of single layer graphene sheets containing line defects were investigated using molecular dynamics simulation with AIROBE bond-order interatomic potential. The dynamic evolution of graphene sheets during relaxation condition was analyzed. The simulation results show that the single layer graphene sheets are not perfectly flat in an ideal state, and the graphene sheet shows a significant corrugations at the verge of sheet. The graphene sheet is bent with the line defects at the end of the sheet, and the extent of this bend also increases with the increase of the defect number. Furthemore, the graphene sheet transforms into a paraboloid with the line defects at the middle of the sheet.     

Mechanical properties of kirigami phosphorene via molecular dynamics simulation

The newly discovered two-dimensional phosphorene suffers low stretchability which limits its application in flexible devices. Herein we employ kirigami technique to overcome this limitation. Molecular dynamics simulation is employed to investigate the mechanical properties of kirigami-phosphorene under shear and tensile loadings. Our simulation results show that loading type, intrinsic structural anisotropy, and the height of middle cuts are three key factors that govern the mechanical response of kirigami-phosphorene. Under the tensile loading along the armchair direction, phosphorene exhibits a considerable increase in its tensile strain. By contrast, phosphorene is too weak to stand any structural modification induced by kirigami in the zigzag direction. Under shear loading, there is merely no improvement in the shear properties of kirigami-phosphorene. Our results demonstrate the prospective applications of kirigami-phosphorene along the armchair direction in modern wearable, and stretchable electronics and optoelectronics devices.     

A molecular dynamics study of uranyl hydration

Molecular dynamics calculations were carried out in order to investigate the hydration structure of uranyl in aqueous solution. The CF1 model of flexible water molecules is used. This model allows one to investigate a hydrolysis reaction for water molecules in the first uranyl hydration shell. Charge redistribution effects on hydrolysis products are also taken into account. We found five ligands in uranyl hydration shell, which is of bipyramidal pentacoordinated structure. The charge redistribution effects resulted in ligands of four water molecules and one hydroxyl, which was found closer to uranium than the other ligands.     

Mechanical properties of nickel-coated single-walled carbon nanotubes and their embedded gold matrix composites

The effects of nickel coating on the mechanical behaviors of armchair single-walled carbon nanotubes (SWCNTs) and their embedded gold matrix composites under axial tension are investigated using molecular dynamics (MD) simulation method. The results show that the Young's moduli and tensile strength of SWCNTs obviously decrease after nickel coating. For armchair SWCNTs, the decreased ratio of the Young's moduli of SWCNTs with smaller radius is larger than that of SWCNTs with larger radius. A comparison is made between the response to Young's modulus of a composite with parallel embedded nanotube and the response of a composite with vertically embedded nanotube. The results show that the uncoated SWCNT can enhance the Young's modulus of composite under the condition of parallel embedment, but such improvement disappears under the condition of vertical embedment because the interaction between SWCNT and gold matrix is too weak for effective load transfer. However, the nickel-coated SWCNT can indeed significantly improve the composite behavior.     

Energetic and structural evolution of gold nanowire under heating process: A molecular dynamics study

The energetic and structural evolution of a squared gold nanowire under heating process is investigated via molecular dynamics with many-body potential. The simulations reveal that the nanowire undergoes distinct energetic and structural developments during the following four heating processes: low temperature, melting, breaking and high temperature. The cross-section of nanowire is found to change from a square to a circle shape with rising temperature at first. A neck is then found to be initiated above the overall melting point, followed by the formation of a two- to five-atom-thick chain structure before the breaking of neck. The nanowire transforms to a spherical cluster after the final breaking.     

Molecular dynamics study on the evolution of interfacial dislocation network and mechanical properties of Ni-based single crystal superalloys

The evolution of misfit dislocation network at $\gamma /{\gamma }^{\prime }$ phase interface and tensile mechanical properties of Ni-based single crystal superalloys at various temperatures and strain rates are studied by using molecular dynamics (MD) simulations. From the simulations, it is found that with the increase of loading, the dislocation network effectively inhibits dislocations emitted in the γ matrix cutting into the ${\gamma }^{\prime }$ phase and absorbs the matrix dislocations to strengthen itself which increases the stability of structure. Under the influence of the temperature, the initial mosaic structure of dislocation network gradually becomes irregular, and the initial misfit stress and the elastic modulus slowly decline as temperature increasing. On the other hand, with the increase of the strain rate, it almost has no effect on the elastic modulus and the way of evolution of dislocation network, but contributes to the increases of the yield stress and tensile strength. Moreover, tension–compression asymmetry of Ni-based single crystal superalloys is also presented based on MD simulations.     

功能化石墨烯改性聚乙烯热力学特性的分子动力学模拟

聚乙烯绝缘材料在我国高压电缆中有着广泛的应用,为了提高其耐热稳定性和力学性能,利用石墨烯对聚乙烯进行掺杂改性,并基于分子动力学模拟的研究方法分别建立了低密度聚乙烯（LDPE）、石墨烯和3种官能团接枝石墨烯掺杂聚乙烯的复合模型。研究表明,相比石墨烯直接掺杂聚乙烯,羧基（-COOH）、氨基(-NH2)和羟基(-OH)接枝石墨烯能够更加有效地提高聚乙烯的玻璃化温度（分别提高了16k、7k、5k）、减弱聚乙烯分子链的移动和降低聚乙烯的热膨胀系数、均方位移(MSD),从而使得聚乙烯复合体系的热学性能得到了有效增强;此外发现石墨烯的掺杂能够提高复合模型的力学模量,其中官能团接枝石墨烯改进效果更明显,室温下弹性模量和剪切模量的提升幅度由不接枝的33.98%、36.18%,提升到了44%和42.89%（羧基功能化体系）。研究结果可为聚乙烯绝缘材料的热老化抑制和力学性能的改善提供有益的参考。     

Pressure effect of structural and vibrational properties of solid pentaerythritol by molecular dynamics simulations

The molecular dynamics simulations (MD) are used to calculate the structural, vibrational and thermodynamic properties of pentaerythritol (PE) crystal up to 4 GPa. The pressure effect for the cell volume, lattice constants, and molecular geometry of solid PE are presented and discussed. It is observed that the C–C bonds has maximum variation, followed by C–H and C–O bonds, which means decomposition of the initial explosion may begin with the C–C bonds. The vibrational frequencies at ambient conditions slightly more than experimental results, and the pressure-induced frequency shifts of these modes are discussed.     

A molecular dynamics study on iridium

In this study, molecular dynamics simulations are performed by using a modified form of Morse potential function in the framework of the Embedded Atom Method (EAM). Temperature-and pressure-dependent behaviours of bulk modulus, second-order elastic constants (SOEC), and the linear-thermal expansion coefficient is calculated and compared with the available experimental data. The melting temperature is estimated from 3 different plots. The obtained results are in agreement with the available experimental findings for iridium.      

Molecular dynamics simulation of mechanical properties of nanocrystalline platinum: Grain-size and temperature effects

A series of molecular dynamics simulations has been carried out to study the mechanical properties of nanocrystalline platinum. The effects of average grain size and temperature on mechanical behaviors are discussed. The simulated uniaxial tensile results indicate the presence of a critical average grain size about 14.1 nm, for which there is an inversion of the conventional Hall-Petch relation at temperature of 300 K. The transition can be explained by a change of dominant deformation mechanism from dislocation motion for average grain size above 14.1 nm to grain boundary sliding for smaller grain size. The Young's modulus shows a linear relationship with the reciprocal of grain size, and the modulus of the grain boundary is about 42% of that of the grain core at 300 K. The parameters of mechanical properties, including Young's modulus, ultimate strength, yield stress and flow stress, decrease with the increase of temperature. It is noteworthy that the critical average grain size for the inversion of the Hall-Petch relation is sensitive to temperature and the Young's modulus has an approximate linear relation with the temperature. The results will accelerate its functional applications of nanocrystalline materials.     

Mechanical properties of defective single-layered graphene sheets via molecular dynamics simulation

In this paper, the effects of two main types of structural defects, i.e. Stone–Wales and single vacancy, on the mechanical properties of single-layered graphene sheets (SLGSs) are investigated. To this end, molecular dynamics simulations based on the Tersoff–Brenner potential function and Nose–Hoover thermostat technique are implemented. The results obtained have revealed that the presence of defects significantly reduces the failure strain and the intrinsic strength of SLGSs, while it has a slight effect on Young’s modulus. Furthermore, the examination of loading in both armchair and zigzag directions demonstrated that SLGSs are slightly stronger in the armchair direction and defects have lower effect in this direction. Considering the fracture mechanism, the failure process of defective and perfect graphene sheets is also presented.     

A molecular dynamics study of effective parameters on nano-droplet surface tension

The main purpose of this paper is to numerically study the effect of droplet radius, temperature, and surface wettability on droplet surface tension. Moreover, the validity of Young-Laplace equation (Y-L) for nano-droplet is examined. Simulations of droplet surrounded by its vapor and droplet on solid surface are carried out and the results are compared to each other in order to comprehend the role of surface wettability on droplet surface tension. The pair potential for the liquid-liquid and liquid-solid interaction is considered using Lennard-Jones model. Different numbers of atoms and surface wettabilities are employed to generate droplet of different radiuses. In addition, contact angle of droplet on solid surface is computed. Pressure tensor and density profile is locally calculated. Furthermore, liquid pressure is evaluated far from the interface using the virial theorem and gas pressure is obtained using an equation of state. In order to calculate the surface tension, two different approaches are employed; Young-Laplace equation and direct molecular dynamics (MD) simulation. The surface tension increases with increase in droplet radius and it is seen that the surface wettability does not directly influence the surface tension.     

Lattice thermal conductivity of δ-graphyne — A molecular dynamics study

Thermal conductivity of δ-graphyne was investigated using reverse non-equilibrium molecular dynamics simulations. The dependence of the thermal conductivities with the temperature, acetylenic linkages, and external strain were explained by the phonon density of states. Our simulations revealed that as the temperature increased, the thermal conductivity of graphene first increased and then decreased, whereas that of δ-graphyne monotonically decreased. Owing to the presence of the acetylenic linkages, a significant reduction was found in the thermal conductivity of δ-graphyne, which resulted in a phonon vibration mismatch or weakened coupling. Moreover, the temperature profile changed from mono linear to the ladder the number of acetylenic linkages increased. These results play a guidance role in the design and application of thermoelectrics devices using 2D carbon materials.     

Canonical ensemble and nonequilibrium states by molecular dynamics

We present a new technique to simulate the contact of a molecular dynamics system with a thermal wall. A canonical ensemble is obtained, and its statistical and thermodynamic fluctuations are studied. The values of the specific heat found by simulation agree with the experimental data. By means of thermal walls at different temperatures, thermal gradients are obtained. The values of the thermal conductivity are consistent with the experimental data.This work was supported in part by the Commissariat à l'Énergie Atomique (France).     

温度对吡啶离子液体性质影响的分子动力学模拟研究

运用分子动力学模拟方法研究了温度对三种吡啶离子液体[BPy][BF4]、[HPy][BF4]、[OPy][BF4]热力学性质的影响, 得到了每个体系的密度、自扩散系数、电导率和黏度等. 研究结果表明: 随着温度升高, 同一种离子液体的密度减小, 阴阳离子的自扩散系数明显增大, 电导率升高, 而黏度降低. 在同一温度下, 随着阳离子上烷基链的增长, 离子液体的密度减小, 但热力学性质的变化规律并不完全同步. 烷基链长最短的[BPy][BF4]的自扩散系数和电导率在每个温度下均为最大, 而黏度最小; 但烷基碳链更长的[OPy][BF4]和[HPy][BF4]的各种性质相差不大,甚至当温度大于323K时, 烷基链较长的[OPy][BF4]的自扩散系数比[HPy][BF4]的大.     

Size and temperature dependence of the tensile mechanical properties of zinc blende CdSe nanowires

The effect of size and temperature on the tensile mechanical properties of zinc blende CdSe nanowires is investigated by all atoms molecular dynamic simulation. We found the ultimate tensile strength and Young?s modulus will decrease as the temperature and size of the nanowire increase. The size and temperature dependence are mainly attributed to surface effect and thermally elongation effect. High reversibility of tensile behavior will make zinc blende CdSe nanowires suitable for building efficient nanodevices.     

Lennard-Jones binary fluids: A comparative study between the molecular dynamics and Monte Carlo descriptions of their structural properties

Molecular dynamics and Monte Carlo techniques are employed for the study of binary Lennard-Jones fluids. Systematic comparisons between the predictions of both techniques are discussed, with particular emphasis on the dependency of the structural properties with respect to temperature and Lennard-Jones potential parameters.