Molecular dynamics simulation of graphite melting

Questions on the behavior of the graphite melting curve have remained open during the last fifty years. The process of graphite melting in the pressure range of 2–14 GPa is investigated by the method of molecular dynamics using the model of reactive interatomic potential; the dynamics of melting-front propagation upon crystal superheating is considered, and the melting curve is plotted. The self-diffusion coefficient in the liquid phase is determined for the aforementioned pressure range, and the question of the existence of the liquid-liquid phase transition in carbon is considered.     

石墨/树状大分子复合材料的分子动力学模拟

在COMPASS(Condensed-phase optimized molecular potentials for atomistic simulation studies)力场下, 对以氨(Amine)、 丁二胺(Butanediamine)为核的1代~3代(1G~3G)石墨/树状大分子纳米复合材料进行了分子动力学模拟(Molecular dynamics simulation)。介绍了复合体系的构建过程及分子动力学模拟细节, 从微观构形、 能量变化研究了正则系综(恒定的NVT)中6种插层复合物的稳定性及其机理, 最后利用径向分布函数(Radial distribution function)对能量变化结果进行了分析。结果表明, 当树状大分子体积较小时, 石墨层容易弯曲, 体系能量较高, 导致复合体系不稳定; 随着树状大分子代数的增加, 石墨层形变减小, 体系能量降低, 3代时树状大分子体系最稳定。      

Characterizing mechanical properties of graphite using molecular dynamics simulation

The mechanical properties of graphite in the forms of single graphene layer and graphite flakes (containing several graphene layers) were investigated using molecular dynamics (MD) simulation. The in-plane properties, Young’s modulus, Poisson’s ratio, and shear modulus, were measured, respectively, by applying axial tensile stress and in-plane shear stress on the simulation box through the modified NPT ensemble. In order to validate the results, the conventional NVT ensemble with the applied uniform strain filed in the simulation box was adopted in the MD simulation. Results indicated that the modified NPT ensemble is capable of characterizing the material properties of atomistic structures with accuracy. In addition, it was found the graphene layers exhibit higher moduli than the graphite flakes; thus, it was suggested that the graphite flakes have to be expanded and exfoliated into numbers of single graphene layers in order to provide better reinforcement effect in nanocomposites.     

Molecular dynamics simulation of silicon oxidization

We compare here the different oxidization protocols that can be used to generate an SiO₂/Si interface. All these protocols are based on molecular dynamics at high temperature but differ by the way the oxygen atoms are incorporated one-by-one. When they are inserted between two neighbouring Si atoms, forming one of the Si-Si pair closest to the surface, the silicon oxide grows layer-by-layer and it is structured as a random network of SiO₄ entities connected by vertices with only a small amount of SiO₃ and SiO₅ defects. On the contrary, when they are incorporated into the longest Si-Si bond instead of the highest one, a dendritic-like SiO₂ oxide spreads into the substrate in contradiction with experimental observations, which definitely rules out such protocols as realistic ones.     

Molecular dynamics simulation of mechanical behaviour

A molecular dynamics (MD) simulation of strain and failure of a crystal, the effect of environment on these processes, the interaction between the adsorption-active atoms and the environment walls, the effect of stress on mobility of interstitial admixtures, the formation and the failure of a contact between two crystals has been performed using a two-dimensional system consisting of Lennard-Jones atoms. The basic features of strain observed by means of MD included generation and motion of dislocations, various mechanisms of shear and brittle-to-ductile transition at low temperature. Environmentsensitive mechanical behaviour has been studied for the first time on an atomic scale. It is shown that rapid local processes whose unit act takes about 10^–10 sec and involves several tens or a few hundred atoms may provide for environment-induced embrittlement. The features common to these microscopic processes are (1) pronounced interaction between the foreign atoms and the atoms of the solid, i.e. a sharp decrease in the surface energy of the solid in contact with the environment, and (2) direct participation of thermal fluctuations in the failure and rearrangement of interatomic bonds. By interacting with the crack walls, the environment atoms create a force compatible with the interatomic bond strength, which promotes crack propagation. Tensile stress causes appreciable acceleration of diffusion of interstitial admixtures in the direction normal to the strain axis and hinders diffusion along the axis. Under constant load the failure of interatomic bonds and sintering involve a thermal fluctuation mechanism.     

Molecular dynamics simulation of polarizable carbon nanotubes

This paper is aimed to develop a modified force field for molecular dynamics (MD) simulations of polarizable carbon nanotubes (CNTs). The effects of electrical polarization and the associated electronic degrees of freedom are represented by a network of negative charged shell particles which move relative to the surrounding positively charged carbon atoms in response to an applied electric field. In this setting, the negative and positive charges are exactly balanced so that the total system remains electrically neutral, and the motion of the shell particles relative to their equilibrium positions leads to polarization within the nanotube. Potential applications of the proposed model include simulations of controlled translocation of ions, water and polymers through solid-state CNT membranes.     

Molecular dynamics simulation of boundary lubricated interfaces

A molecular dynamics simulation study of friction in boundary lubrication was conducted in order to investigate the atomic-scale behavior of lubricant molecules during sliding motion. The simulated system consisted of two silicon (001) semi-infinite substrates lubricated by a three-layer film of dodecane. Silicon was modeled using the Stillinger–Weber potential, and the dodecane with the Consistent Force Field function; a novel scheme was used to generate the silicon–dodecane interaction potentials. The simulations show that dodecane molecules strongly prefer to adsorb into the ledges on the silicon surface. The orientation of the adsorbed molecules depends, however, on the concentration of the lubricant at the surface, showing a tendency to stand up at high lubricant concentrations. In sliding, the dodecane layers adsorbed on the surfaces behave as a solid, whereas the middle layer exhibits liquid-like characteristics. The friction coefficient of this well-lubricated case was calculated to be 0.08.     

Molecular dynamics simulation in investigating mechanical properties

Conclusions The use of the molecular dynamics method, advantageously combining features of experimental and theoretical approaches, makes it possible to reconstruct without application of any a priori schematicizing hypotheses the actual picture of behavior of atoms in a crystal, their behavior in thermal movement, the elementary acts of reconstruction and rupture of the bonds between them as a result of mechanical actions, the influence of the atoms of the medium, etc. A single approach proved to cover the processes of elastic and plastic deformation of the lattice, the formation of defects, local fracture, adsorption, and migration. Together with new quantitative determinations resulting from the micromechanism itself, the use of the molecular dynamics method led to a series of interesting conclusions of a qualitative character, both those expected a priori and those by no means obvious (which is related, in particular, to the variability of the observed micropicturea predetermined by the rules of random events, elements of fluctuations, in such small systems where not the averaged picture but individual elementary acts of the investigated phenomena are observed).Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 17, No. 4, pp. 46–60, July–August, 1981.     

DMA水溶液的分子动力学模拟

采用OptimizedPotentialsforLiquidSimulations-AllAtom(OPLSAA)模型对298.15KN,N-二甲基乙酰胺(DMA)的水溶液进行了分子动力学模拟,确定了溶液的径向分布函数,统计了不同浓度的DMA水溶液中各形态氢的比例分数并对该温度下的1HNMR数据进行了拟合.模拟结果表明:选用的OPLSAA模型是可靠的,它能反映DMA水溶液体系本质的势能;并与前期N,N-二甲基甲酰胺(DMF)水溶液的研究结果进行了比较,研究发现:酰胺自身结构和酰胺浓度是影响酰胺水溶液性质的主要因素.     

Molecular dynamics of stage 3 cesium intercalated graphite C36Cs

Stage 3 casium intercalated graphite is simulated. The dependence of the structural properties of the simulated system on the barrier height of the graphite periodic potential is shown. The periodic component of the radial distribution function g(r) due to the substrate modulation potential is clearly visible. It appears as a result of the delicate balance between the in-plane cesium-cesium interactions and the substrate modulation potential.     

Molecular dynamics simulation of microcrack healing in copper

The molecular dynamics method is used to simulate microcrack healing during heating or/and under compressive stress. A center microcrack in Cu crystal could be sealed under a compressive stress or by heating. The role of compressive stress and heating in crack healing was additive. During microcrack healing, dislocation generation and motion occurred. If there were pre-existed dislocations around the microcrack, the critical temperature or compressive stress necessary for microcrack healing would decrease, and the higher the number of dislocations, the lower the critical temperature or compressive stress. The critical temperature necessary for microcrack healing depended upon the orientation of crack plane. For example, the critical temperature of the crack along (0 0 1) plane was the lowest, i.e., 770 K.     

石墨纳米片/聚乙烯复合物分子动力学模拟

通过分子动力学模拟对石墨纳米片（GNP）/聚乙烯（PE）复合物的结构、力学和气体输运性质进行计算研究,分析其随模拟温度和GNP填充量的变化规律,探讨纳米界面形成、复合机制及结构与特性的关系。GNP/PE复合物呈现二维结构,GNP趋向于平面取向排列并通过范德华力和纳米石墨片层表面上的碳氢-π键使周围几个原子尺度内的PE分子固化为有序原子层,而PE基体仍然为各向同性的无定形结构。GNP/PE界面上纳米复合作用使体系能量降低,与PE体系相比,GNP/PE的杨氏模量和泊松比分别显著增高和降低。GNP平面取向导致GNP/PE的力学特性表现出二维各向异性的弹性常数张量,在石墨纳米片层平面方向上的杨氏模量明显增高,并且随温度的降低和GNP填充量的提高而增大,填充GNP有效改善了GNP/PE的力学性质。GNP/PE复合物的气体输运性质明显受到填充GNP的气体阻隔和取向的影响并且对3种气体渗透没有明显的选择性。GNP与基体的纳米复合导致N₂、O₂和CO₂的分子输运呈现二维各向异性,随着石墨纳米颗粒填充量的增加,取向GNP层面方向的扩散系数比垂直方向高5~8倍,可用于气体分子屏障与渗流控制。     

Molecular dynamics simulation of elastic properties of CuPd nanowire

In this study, the molecular dynamics (MD) simulation technique is employed to investigate the influence of nanowire diameter on the elastic properties of pure Cu and Pd as well as Cu–20%Pd, Cu–40%Pd, Cu–60%Pd and Cu–80%Pd (at%) nanowires. The interaction between atoms Cu–Cu, Pd–Pd as well as Cu–Pd were calculated by Quantum Sutton–Chen (Q–SC) many body potential. The temperature and pressure of the system were controlled by Nose–Hoover thermostat and Berendsen barostat, respectively. The effect of the nanowire thickness on the cohesive energy, Poisson ratio, elastic stiffness constants (C₁₁, C₁₂ and C₄₄) as well as the bulk modulus (B) was studied in NPT ensemble at 10 K. The obtained results show that there is an initial change in the mechanical properties of B, C₁₁ and C₁₂ with the thickness of the nanowire, following which the properties remained unchanged as the nanowire grows in size. However the C₄₄ value was found to be almost independent of nanowire thickness. Moreover, The MD results have shown the cohesive energy per atom and Poisson ratio depends to diameter of nanowires.     

Molecular dynamics simulation of crack growth under cyclic loading

The mechanical behaviors around a crack tip for a system including both a crack and two tilt grain boundaries under cyclic loading are examined using a molecular dynamics simulation. Not only a phase transition but also the emission of edge dislocations is observed in order to relax stress concentration around a crack tip during the first loading. Then, a dislocation pile-up is formed near the grain boundary after the edge dislocations reach the grain boundary, because they cannot move beyond the grain boundary. During the first unloading, the edge dislocations emitted from the crack tip return to the crack tip and disappear in the system. We observe several vacancies generated around the crack tip and crack growth corresponding to an atomic scale during cyclic loading. Conclusively, we propose the fatigue crack growth mechanism for the initial phase of the fatigue fracture. That is, a fatigue crack propagates due to coalescence of the crack and the vacancies caused by the emission and absorption of dislocations.     

纳米尺度通道中气体粘度的分子动力学模拟

利用非平衡分子动力学方法模拟了气体在纳米尺度通道中的运动特性,统计获得通道中流动的速度剖面和剪切应力分布,并利用牛顿粘性定律首次获得了纳米尺度通道中的等效粘度分布。结果表明,纳米尺度通道中的粘度不是一个常数。在壁面附近,由于壁面原子和气体分子的相互作用,存在壁面效应,气体的粘度较小;而通道中心区域的粘度与实验结果符合较好,壁面对粘度的影响范围为20 nm左右。通道高度对中心区域粘度的影响很小,而温度对其的影响较大,粘度值随温度的增加而增大。不同通道高度下,壁面附近粘度的分布几乎一致;不同温度时,壁面附近粘度的分布随温度的增加而增加。     

超临界L-J流体粘度的分子动力学模拟

用分子动力学模拟了超临界Lennard-Jones(L-J)流体在微通道内的剪切应力与速度分布.利用经典流体力学理论中剪切应力和速度分布之间的关系式,得到了超-临界氩与超临界氦在不同温度和密度条件下的动力粘度.结果说明,L-J流体模型得到的超临界氩的粘度与美国国家标准局(NIST)公布的数据十分接近,超临界氦的粘度与NIST中的粘度数据具有相同的数量级,但是数值稍有偏差,说明氦这种量子流体,单纯以L.J流体势能模型为基础是不完全的,需要在此基础上进行量子效应的修正.     

金刚石膜的计算机虚拟制备技术中的分子动力学模拟

综述了近年来金刚石薄膜形成过程的分子动力学（Molecular Dynamics，简称MD）模拟研究，详细地阐述了原子间相互作用势的选取，总结了不同沉积条件下MD的计算模型和几种典型情况下的模拟结果。研究表明：在原子尺度上，MD方法能较全面地提供有关膜生长的信息，对进一步了解金刚石膜形成的微观机制以及为细观层次仿真提供基本信息均具有重要意义。     

Volumetric relaxation in simple liquid: Molecular dynamics simulation

The example of a mixture of simple liquids with Lennard-Jones potentials and soft spheres is used for considering the general principles of relaxation of energy in dense media. The processes of relaxation to uniform velocity distribution are studied, and the main stages of relaxation from nonisothermal state are identified. The dependences of characteristic relaxation times on parameters of the medium such as the density, initial temperature, and the mass ratio of components of the mixture are obtained. The results are compared with the data on nonideal plasma for determining the importance of the concrete interaction potential.     

Molecular dynamics simulation of the laser disintegration of aerosol particles

The mechanisms of disintegration of submicrometer particles irradiated by short laser pulses are studied by a molecular dynamics simulation technique. Simulations at different laser fluences are performed for particles with homogeneous composition and particles with transparent inclusions. Spatially nonuniform deposition of laser energy is found to play a major role in defining the character and the extent of disintegration. The processes that contribute to the disintegration include overheating and explosive decomposition of the illuminated side of the particle, spallation of the backside of large particles, and disruption of the transparent inclusion caused by the relaxation of the laser-induced pressure. The observed mechanisms are related to the nature of the disintegration products and implications of the simulation results for aerosol time-of-flight mass spectrometry are discussed. Application of multiple laser pulses is predicted to be advantageous for efficient mass spectrometry sampling of aerosols with a large size to laser penetration depth ratio.