超临界流体分子动力学模拟的概述

通过从多个方面来阐述超临界流体的分子动力学模拟,介绍超临界流体在分子动力学模拟中用到的势能模型,阐述在模拟的过程中我们应该从哪些方面入手来分析数据,确立有用的研究价值。     

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。     

超临界态二氧化碳流体热力学性质的分子动力学模拟北大核心CSCD

采用恒温恒压(NPT)系综分子动力学模拟方法,模拟预测了超临界态区间内(温度600—900 K,压力30—100 MPa)CO_(2)流体的热力学特性(密度和比定压热容),同时对比研究了4种典型的半经验型力场的预测性能。结果表明:4种力场在超临界态热力区间内对CO_(2)流体的密度和比定压热容特性均具有较好的预测精度,其中密度的预测偏差在3%以内,而比定压热容的预测偏差在1%以内。相比单点粗粒化SAFT-γ力场,具有更多参数自由度的全原子力场对超临界区间内流体热物性的预测并不具有显著优势,这表明其力场参数对于超临界态物性的预测而言并非最优解。EPM2和Zhang力场对CO_(2)流体密度和比定压热容的预测偏差均随温度的升高而减小,表明此两种力场可用于更高温度工况下热力学特性的模拟预测。     

绿色超临界流体的应用研究

超临界流体具有与气体相似的密度、粘度、扩散系数等特性 ,应用领域日益广泛 ,主要有 :萃取、催化反应、结晶、吸附、材料制备、环境保护、食品工业、医药工业、染色工业、色谱分析、生物工程等。论述了超临界流体在这些领域的最新研究成果 ,重点介绍超临界流体在环境保护、染色工业、吸附过程、陶瓷脱脂、材料制备过程中的应用     

超临界流体在聚合物中的应用

本文主要介绍超临界流体在聚合物材料中的应用进展，包括废PET瓶的化学再循环；用超临界CO2，N2作超微细发泡塑料的发泡剂；以超临界CO2为单体和聚合溶剂共聚合成聚碳酸酯。     

聚苯乙烯的超临界流体脱挥

在对聚苯乙烯在超临界二氧化碳中的溶解度研究基础上 ,确定了聚苯乙烯超临界脱挥的操作条件是 ,温度为343K,压力为 1 8MPa。实验发现 ,CO2 流量与 PS投料比为 2 .0～ 3.0标 LCO2 /h.g PS较合适 ,在以上脱挥条件下 ,初始浓度为 1 %的 PS/St的聚合物体系脱挥 3h后 ,则可将 PS中的 St含量降低到约 2 60 ppm。     

超临界流体制备超微粉体的研究进展

分析了国内外超微粉的研究现状及主要研究成果,着重研究了超临界流体溶液快速膨胀法制备聚合物超细粉,探索了超临界流体制备超微粉的原理及特点,论证了该技术的可行性、应用前景以及目前达到的水平与存在的问题,提出了今后发展需要解决的关键问题。     

超临界流体萃取技术研究新进展

综述了超临界流体萃取技术的发展概况、基本原理及工艺流程、特点、性质、应用领域等,并对这一新兴技术的发展前景做出了展望。     

Molecular simulation and electron paramagnetic resonance (EPR) studies of rapid bimolecular reactions in supercritical fluids

We present comparisons among Brownian dynamics simulations, molecular dynamics simulations, and electron paramagnetic resonance spectroscopic studies of the Heisenberg spin-exchange reaction between nitroxide free radicals at near-infinite dilution in near-critical and supercritical ethane. We discuss the effects of correlations in the solute-solute and solvent-solute radial distribution functions on the rate constants for collision and reaction. We find that the enhancements in the local density of solvents around solutes strongly affect the rate constant for solvent-solute encounters. This result holds implications for those reactions where collisional-energy transfer from solvent to solute is the rate-limiting step. While the rate of collisions between solutes is strongly affected by solute-solute correlations for all densities, the reaction rate constant is affected by such local density augmentations only for certain combinations of density and collision lenght scale. Rate constants estimated computationally and experimentally show the same qualitative trend as a function of density. Collision lifetimes estimated from the simulations show a strong density dependence. These lifetimes reflect the competing effects of the intermolecular force and the potential of mean force and are distinctly bimodal at the higher densities.     

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 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.     

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.     

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。     

Revisiting Stokes first and second problems for fluids with pressure-dependent viscosities

We revisit the numerical study carried out by Srinivasan and Rajagopal [1] and we show that the long time solutions to the first and second Stokes problem for fluids with pressure dependent viscosities can be given by exact formulae. Moreover we show that the transient solutions – thus for example the solutions studied by numerical means by Srinivasan and Rajagopal [1] – must rapidly converge to these long time solutions.     

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 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.