Monte Carlo Simulation of Crystal Growth

A simple Monte Carlo simulation has been used for the study of crystal growth from solutions. The effect of concentration of the solution, its supersaturation, agitation and presence of admixtures has been investigated. The results of simulation qualitatively correspond to the phenomenon observed experimentally.     

Monte Carlo Simulation Algorithms of Grain Growth in Polycrystalline Materials

In the paper we present a variety of Monte Carlo algorithms, that can be employed to simulate the grain growth in polycrystalline materials during sintering. The simulation algorithms of monophase or two‐phase structure, on both the square and triangular distribution of lattice points, possibly with the second phase particles being either of static or dynamic nature, are described. The paper deals with oriented and anisotropic grain growth as well. A considerable number of input parameters in the simulation procedure makes it possible to set up a large amount of combinations of conditions under which we want to simulate the sintering process.     

Cu薄膜生长过程的Monte Carlo模拟

用动力学晶格蒙特卡洛模型(Kinetic Lattice Monte Carlo,KLMC)模拟Cu薄膜的生长过程,讨论了基底温度、沉积原子数、单原子最大迁移步数和原子相互作用范围等参数对薄膜表面形貌的影响,并与实验结果进行了比较.结果表明:基底温度升高或沉积原子数增加时,沉积在基底上的原子逐步由众多各自独立的离散型分布向聚集状态过渡形成团簇,并且温度越低,团簇越趋于分散生长.当最大迁移步数减小或相互作用范围增大时,团簇亦趋于分散生长.     

Monte Carlo Simulation of the Liquid-Crystalline Ordering of Semiflexible Polymers

We present results from Monte Carlo simulations of the liquid-crystalline ordering in the athermal systems of partially flexible polymers in a two- and three-dimensional continuum. Any two non-bonded chain monomers interact via a hard spher potential. The chains are placed in a periodic box consistent with the number density C. It is observed that the mean dimensions of chains and the orientational order parameters increase sharply with increasing C. In other words, we observe elongation and stiffening of the chains. In contrast to semiflexible polymers, the mean dimensions of flexible chains are reduced as the density is increased.     

半导体薄膜材料外延生长的蒙特卡罗模拟

半导体自组织生长量子点是一种新型半导体材料,它在纳米电子学、光电子学和生命科学中有广泛的应用前景,本文讨论了计算机模拟量子点的生长在材料设计中的重要意义,建立动力学蒙特卡罗二维模型, 通过获得的原子沉积的表面图样和对原子簇大小分布的数学统计,得到提高生长温度或者降低沉积速率等量子点外延生长的优化条件.     

晶体生长机制和生长动力学的蒙特卡罗模拟研究

采用动力学蒙特卡罗方法,对在完整光滑界面上低过饱和度溶液中的晶体生长机制和动态过程进行计算机模拟,得到了晶体生长速率与溶液过饱和度之间的关系以及晶体生长的表面形态.对以二维成核为主要生长机制的动力学生长规律进行分析,发现了二维成核生长的生长死区以及单核生长转变为多核生长时的过饱和度临界值,讨论了热粗糙度、表面扩散、台阶平均高度以及表面尺寸对晶体平均生长速率的影响.     

Surface Roughening and Surface Self-diffusion. A Monte Carlo Simulation

A model for atom diffusion on surface is proposed, the energy barrier for surface self-diffusion as well as the barrier for diffusion along surface steps being considered. Using a Monte Carlo method, the equilibrium structure of crystal surface is obtained and its effect on the surface self-diffusion is studied. The mass transfer surface self-diffusivity D_s and the mean diffusion distance λ are calculated for different temperatures T. A nonlinear Arrhenius plot (log D vs l/T) is obtained. The maximal deviation from linearity occurs at roughening temperature.     

动力学蒙特卡洛(KMC)模拟薄膜生长

本文以扩散理论为基础,利用KMC模拟方法,考察了温度对薄膜生长速率和表面形貌的影响以及生长表面的粗糙化相变过程.模拟表明,温度升高有利于提高薄膜生长速率,薄膜生长以“成核-岛数增长-岛的长大融合”的方式进行.模拟发现薄膜生长初期存在粗糙化相变过程,当温度低于相变温度时,薄膜分层生长,生长速率较慢;当温度高于相变温度时,薄膜表面粗糙度骤然升高,生长速率加快.     

Surface Roughening and Second Neighbour Interaction. A Monte Carlo Simulation

The structure of (100) surface of simple cubic crystal is studied by means of Monte Carlo method, the first and the second neighbour interactions being considered. For different energies of second neighbour interaction the roughening temperature T_R is obtained. The ratio kT_R/L (L is the latent heat) remains constant and equal to 0.19. In order to calculate the roughening temperature T_R of a (100) crystal face with attractive interatomic forces it is not necessary to know separately the bond energies of first, second, etc. neighbour interactions. For this purpose it is sufficient to know only the latent heat.     

Monte Carlo Simulation of Transient CSD in a Continuous Crystallizer under Stochastic Dispersion Effects

Monte Carlo simulation is a very powerful tool for simulation of transient and steady state crystal size distribution (CSD) in a continuous crystallizer under stochastic dispersion effects. In the present work, transient CSD in a continuous crystallizer has been reported when shape factor and growth rate dispersions conform to normal distribution. For the steady state run, the algorithm reported by Sen Gupta and Dutta elsewhere has been used to validate the results obtained in the present work when the steady state is reached.     

Monte Carlo Simulation of Transient Crystal Size Distribution in a Continuous Crystallizer Using the ASL Model

A Monte Carlo simulation scheme is proposed for transient crystal size distribution in a continuous crystallizer to account for size-dependent growth rate. Crystal growth rates are described by Abegg , Stevens and Larson (ASL) model. The proposed model is used to predict the transient and steady state crystal size distribution from potassium carbonate crystallizer. The agreement between theory and available data confirms the validity of the model.     

蒙特卡罗模拟金刚石薄膜化学气相沉积过程中甲烷浓度的影响

本工作采用蒙特卡罗模拟,研究了以CH4/H2气体混合物作为源料气体的电子辅助热丝化学气相沉积中甲烷浓度对气相沉积过程的影响.计算了典型实验条件下电子能量分布,研究了电子平均能量、碎片H及CH3数目的空间分布、H与CH3的比值及CH3携带的能量随甲烷浓度的变化.结果表明:当电子能量为2-3eV时,电子的数密度达到一峰值;平均电子能量随着甲烷浓度的增加而增加;电子与气体分子碰撞产生的碎片H、CH3和CH2的数密度随距热丝的距离而变化;随着甲烷浓度的增加,原子氢H的数目缓慢下降,然而,官能团CH3和CH2的数目缓慢上升;H与CH3的数量比随甲烷浓度的增加而减少;碎片CH3携带的能量处于1～5eV范围之内,且当甲烷浓度为1.3;时,该能量达到一最值.     

Properties of polymer chains in confinement. Monte Carlo simulations

The main goal of this study was to investigate the dynamic properties of polymer chains located in a slit. The idealized models of polymers were built of united atoms (segments) restricted to vertices of a simple cubic lattice. The macromolecules were linear, cyclic and star-branched with f = 3 and f = 4 arms. The chains were at good solvent conditions and the only interaction between the segments was the excluded volume. The polymers were located in a slit formed by a pair of two parallel and impenetrable surfaces which were attractive for polymer segments. The properties of the model chains were determined by means of dynamic Monte Carlo method using a sampling algorithm based on chain’s local changes of conformation. It was shown, for all polymer architectures under consideration, that at a certain temperature and for a certain size of the slit the chains were adsorbed at one of the confining surfaces and after a certain period of time they detached from this surface and approached the opposite one. The conditions necessary for these jumps were determined. The influence of the internal chain architecture on the frequency of jumps was shown and discussed.     

Monte Carlo Study of Slot-waveguide Liquid Crystal Phase Shifters

Abstract

We present detailed Monte Carlo simulations of a simple model of a nematic liquid crystal slot waveguide shifter, investigating the effect of an applied electric external field. The simulations are based on the Lebwohl-Lasher lattice spin model with boundary conditions chosen to mimic the planar alignment as in Silicon Organic Hybrid waveguides and the homeotropic anchoring appropriate for Polydimethylsiloxane polymer walls. The external field is modeled by adding a term to the Hamiltonian which describes its coupling to the mesogenic molecules. We have investigated the effect of the external field on the optical transmission and the ordering across the cell.     

Disorder in ice polymorphs: A Monte Carlo simulation study

Two water molecules connected by hydrogen bond in hexagonal ice can have four possible configurations. These configurations are distinguished by the relative orientation of the two molecules and termed for obvious reasons as c-cis, h-cis, c-trans, and h-trans. The occurrence of symmetry permitted dimer orientations is a characteristic feature of each ice polymorph. In the proton-ordered structures the occurrence of orientations is strictly determined, while in the proton-disordered structures it can vary within certain limits. We performed Monte Carlo simulations using the so-called TIP5P-EW, TIP4P-EW and TIP4P-2005 interaction models to study this isomerism for the polymorphs of ice. We found that the variation of energy with the frequency of different dimer orientations in the proton-disordered phases is large enough to influence the results of phase stability studies. Knowing the distributions of dimer orientations of the ice IX-ice III ordered-disordered polymorph pairs, we could estimate the internal energy of ice IX using dimer energies assigned to certain orientations in the disordered phase of ice III. In agreement with experimental evidences at low temperatures the TIP4P-EW and TIP4P-2005 potentials predicted lower energy for ice VIII than for ice VII.     

Monte Carlo simulations of the dissolution of borosilicate glasses in near-equilibrium conditions

Monte Carlo simulations were performed to investigate the mechanisms of glass dissolution as equilibrium conditions are approached in both static and flow-through conditions. The glasses studied are borosilicate glasses in the compositional range (80 ? x)% SiO₂ (10 + x / 2)% B₂O₃ (10 + x / 2)% Na₂O, where 5 < x < 30%. In static conditions, dissolution/condensation reactions lead to the formation, for all compositions studied, of a blocking layer composed of polymerized Si sites with principally 4 connections to nearest Si sites. This layer forms atop the altered glass layer and shows similar composition and density for all glass compositions considered. In flow-through conditions, three main dissolution regimes are observed: at high flow rates, the dissolving glass exhibits a thin alteration layer and congruent dissolution; at low flow rates, a blocking layer is formed as in static conditions but the simulations show that water can occasionally break through the blocking layer causing the corrosion process to resume; and, at intermediate flow rates, the glasses dissolve incongruently with an increasingly deepening altered layer. The simulation results suggest that, in geological disposal environments, small perturbations or slow flows could be enough to prevent the formation of a permanent blocking layer. Finally, a comparison between predictions of the linear rate law and the Monte Carlo simulation results indicates that, in flow-through conditions, the linear rate law is applicable at high flow rates and deviations from the linear rate law occur under low flow rates (e.g., at near-saturated conditions with respect to amorphous silica). This effect is associated with the complex dynamics of Si dissolution/condensation processes at the glass–water interface.     

The structure of star-branched copolymers. A Monte Carlo study

A coarse-grained model of a many-arm star-branched polymer chain has been studied. The chains were flexible heteropolymers built of two kinds of united atoms (segments) – hydrophobic and hydrophilic. The positions of segments were restricted to the vertices of a [310] hybrid lattice. The force field consisted of the softened excluded volume and the long-distance potential between a pair of non-bonded segments. The properties of these macromolecular models were studied in a wide range of temperatures from random coil to dense globule states. Monte Carlo simulations were performed using an algorithm based on the chain’s local changes of conformation and employing the Replica Exchange technique. The influence of the polymer length, the sequence of segments in the chain and the temperature on the dimension and the structure of chains have been studied. It has been shown that the process of the heteropolymer chain collapse to low-temperature structures is a complicated phenomenon and a possible explanation of this behavior has been discussed.     

Magnetocaloric effects in magnetic nanoparticle systems: A Monte Carlo study

A superparamagnetic nanoparticle system is a good candidate for implementing the magnetocaloric effect, due to its suitable properties as a frozen ferrofluid able to follow a thermodynamic cycle. For such aim, we have concentrated on the study of the entropy dependence on both the particle size and sample concentration using a Monte Carlo simulation. We have found that for a given sample concentration there exists a particle size for the larger entropy increase, and reciprocally for a given particle volume there exists a sample concentration able to produce the larger entropy change.     

用改进的MonteCarlo算法模拟多孔Al2O3陶瓷烧结过程中的晶粒生长

基于经典的晶粒生长机理,采用改进的蒙特卡罗( Monte Carlo)模拟方法,对在不同的烧结温度和保温时间下的晶粒生长演化过程进行计算机模拟,并使用多孔Al2 O3陶瓷烧结实验过程中的晶粒生长对模型进行了验证.结果表明,当晶粒生长指数为2.95时,模拟与实验中的晶粒生长趋势相一致,说明改进的MC方法可真实的反映晶粒长大的物理过程.