自由分子区内纳米颗粒的热泳力计算

基于非平衡态分子动力学模拟方法,研究了自由分子区内纳米颗粒的热泳特性.理论研究表明,纳米颗粒与周围气体分子之间的非刚体碰撞效应会明显地改变其热泳特性,经典的Waldmann热泳理论并不适用,但尚未有定量的直接验证.模拟计算结果表明:对于纳米颗粒而言,当气-固相互作用势能较弱或气体温度较高时,气体分子与纳米颗粒之间的非刚体碰撞效应可以忽略,Waldmann热泳理论与分子动力学模拟结果吻合较好;当气-固相互作用势能较强或气体温度较低时,非刚体碰撞效应较为明显,Waldmann热泳理论与模拟结果存在较大误差.基于分子动力学模拟结果,对纳米颗粒的等效粒径进行了修正,并考虑了气体分子与纳米颗粒之间的非刚体碰撞效应,理论计算结果与分子动力学模拟结果吻合较好.     

Analysis the complex interaction among flexible nanoparticles and materials surface in the mechanical polishing process

Mechanical polishing (MP), being the important technique of realizing the surface planarization, has already been widely applied in the area of microelectronic manufacturing and computer manufacturing technology. The surface planarization in the MP is mainly realized by mechanical process which depended on the microdynamic behavior of nanoparticle. The complex multibody interaction among nanoparticles and materials surface is different from interaction in the macroscopic multibody system which makes the traditional classical materials machining theory cannot accurately uncover the mystery of the surface generation in the MP. Large-scale classical molecular dynamic (MD) simulation of interaction among nanoparticles and solid surface has been carried out to investigate the physical essence of surface planarization. The particles with small impact angle can generate more uniform global planarization surface but the materials removal rate is lower. The shear interaction between particle and substrate may induce large friction torque and lead to the rotation of particle. The translation plus rotation makes the nanoparticle behaved like micro-milling tool. The results show that the nanoparticles may aggregrate together and form larger cluster thus deteriorate surface the quality. This MD simulation results illuminate that the final planarized surface can only be acquired by synergic behavior of all particles using various means such as cutting, impacting, scratching, indentation and so on.     

Irreversible energy flow in forced Vlasov dynamics

Molecular dynamics (MD) simulations are used to investigate the thermodynamic properties and structural changes of KCl spherical nanoparticles at various sizes (1064, 1736, 2800, 3648, 4224 and 5832 ions) upon heating. The melting temperature is dependent on both the size and shape of KCl models, and the behaviour of the first order phase transition is also found in the present work. The surface melting found here is different from the melting phenomena of KCl models or other alkali halides studied in the past. In the premelting stage, a mixed phase containing liquid and solid ions covers the surface of nanoparticles. The only peak of heat capacity spreads out a significant segment of temperature, probably exhibiting both heterogeneous melting on the surface and homogeneous melting in the core. The coexistence of two melting mechanisms, homogeneous and heterogeneous ones, in our model is unlike those considered previously. We also found that the critical Lindemann ratio of the KCl nanoparticle becomes much more stable when the size of the nanoparticle is of the order of thousands of ions. A picture of the structural evolution upon heating is studied in more detail via the radial distribution function (RDF) and coordination numbers. Our results are in a good agreement with previous MD simulations and experimental observations.     

Soot primary particle formation from multiscale coarse-grained molecular dynamics simulation

A new multiscale coarse-graining procedure is used to study carbonaceous nanoparticle agglomeration in combustion environments. The computational methodology is applied to an ensemble of 10,000 nanoparticles (or effectively 2 million total carbon atoms) to simulate, for the first time, the agglomeration of carbonaceous nanoparticles using coarse-grained atomistic-scale information. In particular, with the coarse-graining approach we are able to assess the influence of nanoparticle morphology and temperature on the agglomeration process. The coarse-graining of the interparticle force field is accomplished applying a force-matching procedure to data obtained from trajectories and forces from all-atom MD simulations. The coarse-grained MD results show rich and varied clustering behaviors for different particle morphology and, in some cases, the formation of primary particles with a diameter around 15 nm are observed for the first time by molecular simulation techniques.     

T2 relaxation induced by clusters of superparamagnetic nanoparticles: Monte Carlo simulations

Clustering strongly affects the transverse (T2) relaxation induced by superparamagnetic nanoparticles in magnetic resonance experiments. In this study, we used Monte Carlo simulations to investigate systematically the relationship between T2 values and the geometric parameters of nanoparticle clusters. We computed relaxation as a function of particle size, number of particles per cluster, interparticle distance, and cluster shape (compact vs. linear). We found that compact clusters induced relaxation equivalent to similarly sized single particles. For small particles, the shape and density of clusters had a significant effect on T2. In contrast, for larger particles, T2 relaxation was relatively independent of cluster geometry until interparticle distances within a cluster exceeded ten times the particle diameter. Results from our simulations suggest principles for the design of nanoparticle aggregation-based sensors for MRI.     

Molecular dynamics study of the role of material properties on nanoparticles formed by rapid expansion of a heated target

Rapid expansion of a heated target and its decomposition into fragments is investigated by using molecular dynamics simulations. Particular attention is focused on the void formation and nucleation that governs the target disintegration. The cluster formation process is investigated as a function of material properties (initial temperature, interaction potential and composition). Calculation results demonstrate the influence of these properties on void nucleation and growth and on the characteristic parameters of nanoparticles to be formed. In particular, larger initial temperature and expansion rate lead to the formation of smaller fragments. These effects are found to be similar for three different materials (silicon, nickel and metal alloy). In addition, the stoichiometrical cluster composition obtained in the expansion of a binary alloy is found to be fairly well preserved. The calculation results can be used for the interpretation of the experimental findings showing the formation of nanoparticles by short and ultra-short pulse laser ablation of both simple and more complex materials.     

新的相互作用势对MD模拟钙钛矿结构MgSiO3热力学性质的影响

通过分析势能曲线解释了钙钛矿结构MgSiO3熔化模拟过程中模拟熔化温度存在较大差异的原因，并进一步研究了对势参数在分子动力学模拟中的影响. 通过调整已有的经验势得到了一组新的势参数，以此来进行分子动力学研究，得到的常温常压下摩尔体积与Belonoshko和Dubrovinsky的结果符合较好，并且其状态方程、常压下热容和常压下热膨胀系数与他人的实验值都较好地吻合. 另外，所得到的熔化温度也与以前的研究进行了比较.     

Phase transition,formation and fragmentation of fullerenes

We present a statistical mechanics model treating the formation and the fragmentation of fullerenes as a phase transition. Based on this model, we investigate the formation and fragmentation of C₆₀ and C₂₄₀ fullerenes from and to a gas of carbon dimers by means of molecular dynamics (MD) simulations. These simulations were conducted for 500 ns using a topologically-constrained forcefield. At the phase transition temperature, both the cage and gaseous phases were found to coexist and the system continuously oscillates between the two phases. Combining the results of the MD simulations and the statistical mechanics approach, we obtain the dependence of the phase transition temperature on pressure and compare the results of our model with arc-discharge experiments.     

Molecular dynamics computation of clusters in liquid Fe–Al alloy

By means of constant pressure molecular dynamics (MD) simulation technique, a series of simulations of the Fe₅₀Al₅₀ alloy have been carried out. The atoms interact via semi-empirical n-body noncentral potential. The pair correlation functions and the pair analysis technique is applied to reveal the cluster evolution in the process of quick solidification. By using the bond orientation order parameters, we have measured both local and extended orientation symmetries for computer-generated models of dense liquid and glass. A lot of polyhedra in liquid system, for example, icosahedra, are also obtained. In order to test the reliance of the computation results, corresponding X-ray diffraction experiments have been performed on the material.     

Molecular dynamics simulation of thermal accommodation coefficients for laser-induced incandescence sizing of nickel particles

Extending time-resolved laser-induced incandescence (TiRe-LII), a diagnostic traditionally used to characterize soot and other carbonaceous particles, into a tool for measuring metal nanoparticles requires knowledge of the thermal accommodation coefficient for those systems. This parameter can be calculated using molecular dynamics (MD) simulations provided the interatomic potential is known between the gas molecule and surface atoms, but this is not often the case for many gas/surface combinations. In this instance, researchers often resort to the Lorentz–Berthelot combination rules to estimate the gas/surface potential using parameters derived for homogeneous systems. This paper compares this methodology with a more accurate approach based on ab initio derived potentials to estimate the thermal accommodation coefficient for laser-energized nickel nanoparticles in argon. Results show that the Lorentz–Berthelot combining rules overestimate the true potential well depth by an order of magnitude, resulting in perfect thermal accommodation, whereas the more accurate ab initio derived potential predicts an accommodation coefficient in excellent agreement with experimentally-determined values for other metal nanoparticle aerosols. This result highlights the importance of accurately characterizing the gas/surface potential when using MD to estimate thermal accommodation coefficients for TiRe-LII.     

纳米通道滑移流动的分子动力学模拟研究

本文采用非平衡分子动力学方法对平板纳米通道滑移流动进行了非平衡分子动力学模拟,获得了不同壁面势能和不同温度时流体的速度分布及密度分布。研究结果表明滑移速度在很大程度上决定于流体温度和壁面吸引力作用强度的大小。由于不同壁面吸引力时流体的密度分布受温度的影响规律不同,使得不同壁面吸引力时流体的滑移速度受温度影响规律也不一致。而且,流体结构受壁面流速的影响要受到壁面势能的制约。     

Numerical analysis of TOF measurements in pulsed laser ablation

The results of numerical simulation of pulsed laser ablation both in vacuum and into a background gas are presented. The influences of different processes, such as time evolution of the surface temperature, interspecies interactions (elastic collisions, recombination-dissociation reaction), interaction with an ambient gas, and excitations-relaxation processes on time-of-flight (TOF) distributions are examined. Experimentally obtained time-of-flight distributions are further analyzed, based on the results of numerical simulation. It is found that with the aid of numerical results one can explain not only the shape of the TOF distribution, but also the distance dependency of its maximum position (mean delay time). In addition, the mechanisms leading to the appearance of bimodal time-of-flight distribution are revealed. The study presents particular interest for the analysis of experimental results obtained during pulsed laser ablation.     

Molecular dynamics simulation of iron nanoparticle sintering during flame synthesis

The sintering process of iron nanoparticles produced in a flame environment is investigated by molecular dynamic (MD) simulations. The thermodynamic characteristics and restructuring pathways are studied for two-body and three-body sintering processes. The melting point, energy, and structures are computed for nanoparticles before and after sintering. A simplified model is proposed to predict the equilibrium temperature of nanoparticles upon sintering. The MD results are used to explain the formation mechanisms of two size ranges of nanoparticles during the flame synthesis. The role of sintering during nanoparticle growth is analyzed.     

Molecular dynamics simulation of the structural evolution of misfit dislocation networks at γ/γ′ phase interfaces in Ni-based superalloys

The structural evolution of misfit dislocation networks at γ/γ′ phase interfaces in Ni-based single crystal superalloys under tensile loading and temperatures is simulated by molecular dynamics. From the simulation, we find that, with increasing load or temperature, the patterns of dislocation networks on the (100), (110) and (111) phase interfaces change from regular to irregular or disappear. Under the same load and temperature, the dislocation networks of the different phase interfaces show different degrees and patterns of damage. The density and stability of the dislocation networks decrease with increasing temperature. When the interfacial dislocation networks become more regular, the γ/γ′ interfaces become more stable. The simulated results are supported by related experimental findings. Moreover, based on MD simulations, the averaged stress–strain responses for different phase interfaces under loading are presented. The results indicate that the combined influences of temperature and load play an important role for the structure evolution of misfit dislocation networks at γ/γ′ phase interfaces of Ni-based superalloys.     

Numerical simulations for description of UV laser interaction with gold nanoparticles embedded in silica

We have performed simulations of laser energy deposition in an engineered absorbing defect (i.e. metal nanoparticle) and the surrounding fused silica taking into account various mechanisms for the defect-induced absorption of laser energy by SiO₂. Then, to simulate the damage process in its entirety, we have interfaced these calculations of the energy absorption with a 2-D Lagrange–Euler hydrodynamics code, which can simulate crack formation and propagation leading to craters. The validation of numerical simulations requires detailed knowledge of the different parameters involved in the interaction. To concentrate on a simple situation, we have made and tested a thin-film system based on calibrated gold nanoparticles (600-nm diameter) inserted between two silica layers. Some aspects of our simulations are then compared with our experimental results. We find reasonable agreement between the observed and simulated crater sizes.     

MD simulation study of the sputtering process by high-energy gas cluster impact

We performed molecular dynamics (MD) simulations to study the characteristic sputtering process with large cluster ion impact. The statistical properties of incident Ar and sputtered Si atoms were examined using 100 different MD simulations with Ar₁₀₀₀ cluster impacting on a Si(0 0 1) target at a total acceleration energy of 50 keV. The results show that the kinetic energy distribution of Ar atoms after impact obeys the high-temperature Boltzmann distribution due to thermalization through high-density multiple collisions on the target. On the other hand, the kinetic energy distribution of sputtered target atoms demonstrates a hybrid model of thermalization and collision-cascade desorption processes.     

Calculation of thermodynamic properties of Ni nanoclusters via selected equations of state based on molecular dynamics simulations

We present an approach for constant-pressure molecular dynamics simulations. This approach is especially designed for finite systems, for which no periodic boundary condition applies. A molecular dynamics (MD) simulation for Ni nanoclusters is used to calculate their pressure–volume–temperature (p–v–T) data for the temperature range 200 K≤T≤400 K, and pressures up to 600 kbar. Isothermal sets of p–v–T data were generated by the simulation; each set was fitted by three equations of state (EoSs): Linear Isotherm Regularity-II (LIRII), Birch–Murnaghan (BM), and EOS III. It is found that the MD data are satisfactorily reproduced by the EoSs with reasonable precision. Some features of the EoSs criteria, such as the temperature dependences of the coefficients, the isothermal bulk modulus and its pressure derivative at the zero-pressure limit, and isobaric thermal expansion for Ni nanoclusters, are investigated. We have found that same EoSs are valid for both bulk Ni and Ni nanoclusters, but with different values of the parameters, which depend on the cluster size and temperature. An increase in bulk modulus with decrease of cluster size can be observed. Also, an increase in isobaric expansion coefficient with decrease of cluster size has been found.     

Molecular dynamics investigation of shape effects on thermal characteristics of platinum nanoparticles

Using molecular dynamics simulations with the quantum corrected Sutton-Chen type many-body potential, we investigate the thermal characteristics and structural evolution of Pt nanoparticles with spherical and polyhedral shapes under the heating process. The main focus of this work is the shape effects on the thermal characteristics of Pt nanoparticles. The simulation results show that all types of nanoparticles present the same overall melting temperature in spite of their different shapes. These nanoparticles can hold their initial shapes and structures at low temperature. However, polyhedral nanoparticles undergo a remarkable shape transformation before their overall melting. The critical temperature of shape transformation depends on their shapes and associated Miller index of the surface. Our study indicates that octahedron-truncated nanoparticle displays a better thermal stability than other polyhedral nanoparticles.     

Numerical Simulation of the Formation of Granular Shock Wave Over Cylindrical Obstacle

A two dimensional (2‐D) stream of granular flow with zero initial granular temperature passing over a cylindrical obstacle is simulated by means of both molecular dynamics (MD) simulation and finite volume method (FVM). In experiments, a bow‐shaped shock wave with higher area fraction forms in front of the obstacle that was reproduced in our simulations. Due to the different circumstances to which particles are subjected, the granular flow is divided in two zones. One is undisturbed where quantities, such as space fraction (volume fraction for 3‐D and area fraction for 2‐D geometries), velocity and granular temperature are uniformly distributed and the other is called the shock wave zone. In this region, the values of the space fraction increases and the velocity of particles changes. From the MD simulation, it is found that the area fraction of the shock wave depends on surface roughness, coefficient of restitution (COR) of particles, the obstacle diameter as well as velocity of the granular stream, and a triangular region forms with almost zero velocity, and granular temperature forms in front of the cylindrical obstacle. The bigger is the size of the obstacle, the more stable this region is. In FVM simulations solid phase velocity and area fraction distributions similar to the MD simulation results are obtained for proper parameters.