从常温常压到超临界乙醇的分子动力学模拟

采用分子动力学方法系统地研究了从常温常压到超临界状态乙醇的热力学性质、结构性质和动力学性质.模拟发现随着温度的升高,体系焓值增大,乙醇分子间的氢键作用减弱,自扩散系数增大;随着压强的增大,乙醇分子间的氢键作用增强,自扩散系数减小;乙醇自扩散系数在液相区随温度变化不明显,在气相区随压强增大很快减小,超临界区乙醇的自扩散系数比液相区大十几倍.温度和压强对乙醇自扩散系数的影响可通过密度来体现.与常温常压相比,超临界条件下的乙醇体系因密度涨落存在分子聚集现象,且在低密度区域更显著;乙醇分子间的氢键作用明显减弱,结     

分子动力学模拟壁面附近流体特性及降温过程

采用三相模型计算结果表明:近壁区内流体密度呈现出"层状化"的特点,切向应力张量呈"振荡"分布;固液界面张力随温度增加而降低;进一步增强壁面浸润性,固液界面张力变化不大;同温壁面对近壁区流体特性的影响较大,近壁区之外的流体特性与两相模型吻合较好.降低壁面温度的模拟表明,当温差减小时,传热量减小,界面接触热阻占主导地位.     

超临界流体分离与NMR联用技术及其应用

本文重点描述了SFC－NMR和SFE－NMR联用装置的特点及其探头设计,举例说明了它们的几种典型应用,分析比较了超临界流体分离和NMR联用系统与HPLC－NMR联用系统的特性,并讨论了超临界流体分离与NMR联用技术的发展前景。     

纳米流体分散稳定性的分子动力学研究初探

纳米流体的制备和分散稳定性是纳米流体研究工作的前提。本文利用平衡分子动力学模拟方法,尝试从微观角度研究纳米流体的分散稳定性。以纳米颗粒Cu在液态CO_2中的分散稳定为算例,通过跟踪每个固体分子和液体分子的位置,分别观察了颗粒周围的液体吸附现象和颗粒的团聚现象。通过改变固液分子间的引力和斥力,观察其对颗粒周围液体分子的密...     

LJ流体非牛顿现象的分子动力学模拟

使用非平衡分子动力学模拟方法研究了单原子LJ流体的非牛顿流变行为,并在系统中分别施加稳态Couette流场和振荡剪切流场.在Couette流场的模拟中,流体出现剪切变稀和法向应力差效应,不同剪切率下的径向分布函数反映了流体分子由于剪切所导致的微观结构变化,通过分析势能函数发现当剪切率增大时,分子间排斥作用增强,吸引作用减弱.在振荡剪切流场的模拟中,发现剪切应力和剪切率之间的相位差随频率增加而增加,随频率增加复数粘度的实部先增大再减小,虚部单调增加,导致虚部粘度相对实部粘度比例增大,弹性模量和粘性模量之比也随频率增加而增加.这三点现象表明LJ流体出现粘弹性行为,且在高频率下,弹性所占比重增大.     

超临界流体黏度的拉曼光谱测量方法研究

超临界流体黏度的测量对于工业应用具有重要意义。本文利用流体的拉曼光谱随压力的变化关系,基于泊肃叶流动规律,提出超临界流体黏度的非接触式光学测量方法。首先,对甲烷及其混合物、二氧化碳及其混合物的拉曼光谱随压力的变化规律作综述分析,分析表明可以利用甲烷及其混合物的拉曼光谱较为精确地获取体系压力,但利用拉曼光谱精确测量二氧化碳及其混合物压力的方法有待进一步研究。接着,对测量系统作理论设计和灵敏度分析。测量系统由储气罐、可控压力的高压注射泵、温控平台、毛细管阵列和拉曼光谱仪等部件组成,毛细管阵列进出口压力变化显著,用于测量甲烷及其混合物的黏度具有较高灵敏度。     

多环芳烃在超临界环己烷中的溶解：分子动力学模拟研究

重油中大量存在的多环芳烃会经过缩合反应会形成稠环芳烃,而溶解扩散的限制会导致稠环芳烃之间的聚合从形成残焦,使得对重油的利用率大大降低,所以多环芳烃热解过程的溶解扩散是重油改质的关键因素之一.因此,基于环己烷对烃类良好的溶解性,本文采用分子动力学模拟的方法研究了多环芳烃及其混合物在超临界环己烷中的溶解行为,结果表明在不同温度与密度下超临界环己烷对于具有NAP均具有良好的溶解性,而温度对于Bghip溶解的影响较小.在多环芳烃混合物的油滴溶解过程中,NAP优先溶解到环己烷相中,而Bghip则集中在粒径减小的油滴中.通过计算体系中多环芳烃的径向分配函数与溶剂化自由能发现环己烷与多环芳烃间之间能相互吸引,环己烷在多环芳烃分子周围形成的溶剂壳能抑制多环芳烃分子间的聚合.温度升高以及密度的将降低会导致多环芳烃的内聚能密度降低,增加了多环芳烃与超临界环己烷之间的相互作用.环己烷密度越小以及温度的升高可以促进多环芳烃或多环芳烃混合物在超临界环己烷中的溶解.     

超临界水中硫酸钠结晶动力学的分子动力学模拟

在超临界水反应器中，硫酸钠是易造成堵塞的一种常见无机盐，研究其结晶动力学对于防盐沉积反应器的设计具有重要意义.本文采用LAMMPS分子动力学模拟软件研究硫酸钠在超临界水中的微观结晶过程，其中水分子采用SPC/E模型，离子-离子、离子-水分子相互作用采用Coulumb和Lennard-Jones联合势能函数.结果表明：水对离子的静电屏蔽作用随温度升高而增强、随密度减小而减弱；增大超临界水的温度和密度有利于离子扩散，进而促进离子相互碰撞、成核；在模拟的超临界水参数范围内，其成核速率的数量级为10²⁹cm^-3·s^-1，生长速率为(19.8~25.8) m·s^-1.     

用微扰理论推算超临界流体比定容热容

本文利用统计热力学中的BH微扰理论建立数学模型,推算了Ar超临界区域比定容热容,与文献值比较平均偏差为9.54%。引入了修正函数,重新对比热容公式进行推导,再次对Ar超临界区域部分比定容热容进行计算,计算平均偏差0.39%。对CO、CO₂、CH₄等其余11种简单气体的超临界比定容热容进行计算,平均偏差在1%左右。最后给出了计算中各物质势能参数值。     

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.     

Molecular dynamics simulations of one-dimensional Lennard-Jones systems

One-dimensional Lennard-Jones systems are investigated by molecular dynamics simulations. The full Lennard-Jones potential is compared to the repulsive Lennard-Jones potential. It is found that the pair correlation function and the normalized velocity autocorrelation function agree at high densities and high temperature. However, the diffusion coefficient indicates that the attractive potential introduces additional correlations into particle dynamics which are not reflected in the statics. These results are in agreement with three-dimensional studies.     

二元Lennard-Jones液体的相分离过程及其扩散性质的分子动力学研究

采用分子动力学(MD)模拟方法,研究了二元体系中相分离过程、粒子的扩散系数以及相分离域尺寸大小随温度的变化规律.发现,相分离域随温度的生长过程可以分为两个阶段,分别是温度比较高的快速生长阶段和低温时的稳定生长阶段;相分离体系中系统的扩散激活能不是常数,而是一个随温度变化的函数,并且当温度高于60 K时,满足关系式E(T)=a+bT^c.讨论了组元尺寸的变化对相分离过程的影响.结果表明,随两组元中某一组元 关键词： 相分离 扩散 分子动力学模拟     

A classical molecular dynamics study of a Diels Alder cycloaddition in supercritical water

Molecular dynamics simulations were performed to investigate the Diels Alder cycloaddition of cyclopentadiene and methyl vinyl ketone in high pressure, high temperature water. It was found that the reaction was favoured by high temperatures at 1000?atm due to increasing entropy. Similarly, at 400?atm, the entropy caused both the equilibrium and rate constant to increase to a peak at 698?K before rapidly falling once more with increasing temperature. At a constant temperature of either 598?K or 898?K, increasing pressure resulted in a lowering of the equilibrium constant. This effect was significantly more pronounced for 898?K, caused by less favourable solvation of the products and an increasing amount of work required for reaction.     

C36团簇自组装的分子动力学研究

提出了利用C36团簇在气相条件下自组装制备新纳米团簇的设想，并利用分子动力学方法模 拟了包括真实氦气氛作用的碳团簇生长过程，发现环境气体温度是影响最后所生成的团簇结 构的关键因素：C36团簇在1000?K到2000?K的温度范围内，自组装形成保持C36线径特征的 蚕茧状新纳米团簇；在高于2000?K的温度下，最后形成的团簇趋于球状. 关键词： 分子动力学模拟 纳米碳团簇     

Molecular dynamics study of ice structural evolution

Molecular dynamics simulation is employed to study the structural evolution of low density amorphous ice during its compression from one atmosphere to 2.5 GPa. Calculated results show that high density amorphous ice is formed at an intermediate pressure of -1.0 GPa; the O-O-O bond angle ranges from 83° to 113°, and the O-H… O bond is bent from 112° to 160°. Very high density amorphous ice is obtained by quenching to 80 K and decompressing the ice to ambient pressure from 160 K/1.3 GPa or 160 K/1.7 GPa; and the next-nearest O-O length is found to be 0.310 nm, just 0.035 nm beyond the nearest O-O distance of 0.275 nm.     

Molecular dynamics study of mosaic structure in the Ni-based single-crystal superalloy

The mosaic structure in a Ni-based single-crystal superalloy is simulated by molecular dynamics using a potential employed in a modified analytic embedded atom method. From the calculated results we find that a closed three-dimensional misfit dislocation network, with index of $\langle 011\rangle${\{}100{\}} and the side length of the mesh 89.6\,{\AA}, is formed around a cuboidal $\gamma '$ precipitate. Comparing the simulation results of the different mosaic models, we find that the side length of the mesh only depends on the lattice parameters of the $\gamma $ and $\gamma '$ phases as well as the $\gamma $/$\gamma '$ interface direction, but is independent of the size and number of the cuboidal $\gamma '$ precipitate. The density of dislocations is inversely proportional to the size of the cuboidal $\gamma '$ precipitate, i.e.~the amount of the dislocation is proportional to the total area of the $\gamma $/$\gamma '$ interface, which may be used to explain the relation between the amount of the fine $\gamma '$ particles and the creep rupture life of the superalloy. In addition, the closed three-dimensional networks assembled with the misfit dislocations can play a significant role in improving the mechanical properties of superalloys.     

Structure and dynamics of a dense dipolar system in an electric field and their relevance to electrorheological fluids

Results of computer simulations of a dense system of dipolar spheres in an electric field are summarized. Dissipative and Hamiltonian dynamics algorithms have been used to find energy minima of the system for varying particle densities. The structures obtained by these simulations are in reasonable agreement with experimentally observed structures in electrorheological (ER) fluids. Qualitative agreement is also obtained with the limited available experimental observations on the dynamics of ER fluids.     

Molecular dynamics study of the mechanical characteristics of Ni/Cu bilayer using nanoindentation

<正>In the present work,a three-dimensional molecular dynamics simulation is carried out to perform the nanoindentation experiment on Ni single crystal.The substrate indenter system is modeled using hybrid interatomic potentials including the many-body potential embedded atom method(EAM),and two-body morse potential.To simulate the indentation process,a spherical indenter(diameter = 80 A,1 A=0.1 nm) is chosen.The results show that the mechanical behaviour of a monolithic Ni is not affected by crystalline orientation.To elucidate the effect of a heterogeneous interface, three bilayer interface systems are constructed,namely Ni(100)/Cu(111),Ni(110)/Cu(111),and Ni(111)/Cu(111).The simulations along these systems clearly describe that mechanical behaviour directly depends on the lattice mismatch. The interface with the smaller mismatch between the specified crystal planes is proved to be harder and vice versa.To describe the relationship between film thickness and interface effect,we choose various values of film thickness ranging from 20 A to 50 A to perform the nanoindentation experiment.It is observed that the interface is significant only for the relatively small thickness of film and the separation between interface and the indenter tip.It is shown that with the increase in film thickness,the mechanical behaviour of the film shifts more toward that of monolithic material.