Monte Carlo simulation of metal deposition on foreign substrates

The deposition of a metal on a foreign substrate is studied by means of grand canonical Monte Carlo simulations and a lattice-gas model with pair potential interactions between nearest neighbors. The influence of temperature and surface defects on adsorption isotherms and differential heat of adsorption is considered. The general trends can be explained in terms of the relative interactions between adsorbate atoms and substrate atoms. The systems Ag/Au(1 0 0), Ag/Pt(1 0 0), Au/Ag(1 0 0) and Pt/Ag(1 0 0) are analyzed as examples.     

Monte Carlo simulation of growth of porous SiOx by vapor deposition

A random network model containing defects has been developed and applied to the deposition of amorphous SiOx films on a flat substrate. A new Monte Carlo procedure enables dangling bonds to migrate and annihilate. The degree of porosity in the films is found to increase with oxygen content. As the oxygen content increases a larger fraction of pore surfaces is covered with oxygen, and the density of dangling bonds on pore surfaces decreases. Oxygen plays the role of a surfactant, lowering the energies of pore surfaces and enhancing the porosity of amorphous SiO2 compared to amorphous Si.     

Investigation of optical emission spectroscopy in diamond chemical vapor deposition by Monte Carlo simulation

The optical emission spectra(atomic hydrogen(Hα,Hβ,Hγ),atomic carbon C(2p3s→2p2:λ=165.7 nm) and radical CH(A2△→X2П:λ=420-440 nm))in the gas phase process of the diamond film growth from a gas mixture of CH4 and H2 by the technology of electron-assisted chemical vapor deposition (EACVD)have been investigated by using Monte Carlo simulation.The results show that the growth rate may be enhanced by the substrate bias due to the increase of atomic hydrogen concentration and the mean temperature of electrons.And a method of determining the mean temperature of electrons in the plasma in-situ iS given.The strong dependence on substrate temperature of the quality of diamond film mainly attributes to the change of gas phase process near the substrate surface.     

电子助进化学气相沉积金刚石中发射光谱的蒙特卡罗模拟

采用蒙特卡罗方法，对源料气体为CH4/H2混合气的电子助进化学气相沉积（EACVD）中 的氢原子(H)、碳原子(C)以及CH基团的发射过程进行了模拟.研究了CH4浓度、反应室气压 和衬底偏压等工艺参数对发射光谱及成膜的影响.研究发现，CH基团可能是有利于金刚石薄 膜生长的活性基团，而碳原子不是；偏压的升高可提高电子平均温度及衬底表面附近氢原子 的相对浓度；通过氢原子谱线可测定电子平均温度并找到最佳成膜实验条件.该结果对EACVD 生长金刚石薄膜过程中实时监测电子平均温度，有效控制工艺条件，生长出高质量的金刚石 薄膜具有重要的意义. 关键词： 蒙特卡罗模拟 金刚石薄膜 发射光谱     

Numerical simulation of metal evaporation based on the kinetic model equation and the direct simulation Monte Carlo method

Metal evaporation on the basis of the kinetic model equations(BGK and S-model) and the direct simulation Monte Carlo(DSMC) method was investigated computationally under the circumstances of collimators existing or not. Numerical data of distributions of number density, bulk velocity and temperature were reported over a wide range of evaporation rate.It was shown that these results reached a good agreement for the case of small evaporation rate, while the deviations became increasingly obvious with the increase of evaporation rate, especially when the collimators existed. Moreover, the deposition thickness over substrate obtained from the kinetic model equations were inaccurate even though the evaporation rate was small. All of the comparisons showed the reliability of the kinetic model equations, which require less computational cost at small evaporation rate and simple structure.     

Monte Carlo simulation of the spontaneous oscillation in electrochemical deposition

We report a new Monte Carlo model to simulate the spontaneous oscillating growth in Cu(II)-acid electrochemical deposition system, and present the one-to-one correspondence between the oscillation of pH and the layered structure. Two mechanisms essential for the oscillation behavior, i.e. the adsorption and buffering, are incorporated into the deposition sequences. The simulation results are qualitatively in agreement with the experimental observations. Some system parameters essential for the oscillation generation are discussed. And through adjusting the adsorption ratio, the spatially columnar deposited structure is generated and the columnar alternative growth of copper and cuprous oxide phases is achieved.     

Direct Monte Carlo simulation of high-temperature chemical reactions in air

A novel approach to modeling high-temperature nonequilibrium dissociation in air at a level of molecular collisions is proposed. Information on the energy dependence of the specific reaction cross sections, which is necessary for such modeling, is determined numerically from available macroscopic information on the dependence of the reaction rate constant on translational and vibrational temperatures. The results of Direct Simulation Monte Carlo (DSMC) computations show that the proposed model yields a correct reaction rate in vibrational-translational nonequilibrium. The use of the new model in DSMC computations of high-altitude aerothermodynamics results in obtaining a noticeably different flow structure and a higher heat flux, as compared to that predicted by standard DSMC models (such as the total collision energy model).     

Monte Carlo simulation of mictomagnets

Monte Carlo simulations of a binary alloy with impurity concentrations between 20 and 45 at.% have been carried out. The proportion of large clusters relative to that of small clusters increases with the number of MC diffusion steps as well as impurity concentration. Magnetic susceptibility peaks become more prominent and occur at higher temperatures with increasing impurity concentration. The different peaks in the susceptibility and specific heat curves seem to correspond to different sized clusters. A freezing model would explain the observed behaviour with the large clusters freezing first and the small clusters contributing to susceptibility (specific heat) peaks at lower temperatures.Contribution No. 153 from the Solid State and Structural Chemistry Unit     

A Monte Carlo simulation study of associated liquid crystals

We have performed a Monte Carlo simulation study of a system of ellipsoidal particles with donor—acceptor sites modelling complementary hydrogen-bonding groups in real molecules. We have considered elongated Gay—Berne particles with terminal interaction sites allowing particles to associate and form dimers. The changes in the phase transitions and in the molecular organization and the interplay between orientational ordering and dimer formation are discussed. Particle flip and dimer moves have been used to increase the convergency rate of the Monte Carlo (MC) Markov chain.     

制冷剂R410a的气液相平衡的吉布斯蒙特卡罗模拟

制冷剂的气液相平衡性能,对制冷循环的研究至关重要。文中基于UFF(universal force field)力场,运用吉布斯蒙特卡罗方法模拟了混合制冷剂R410a的气液相平衡性质,模拟结果同Refprop8.0提供的数据有很好的一致性。并根据模拟结果得到了R410a的临界状态点的温度和密度,与Refprop8.0的相对误差分别在0.4%和3.3%。表明采用合理的分子力场,运用吉布斯蒙特卡罗模拟方法模拟混合制冷剂的气液相平衡是可行的。     

超薄膜生长的Monte Carlo模拟研究

利用Monte Carlo(MC)方法模拟研究了薄膜生长的初始阶段岛的形貌和岛的尺寸与基底温度和入射粒子剩余能量之间的关系.模型中考虑了粒子的沉积、吸附粒子的扩散和蒸发等过程.结果表明当基底温度从200K变化到260K时,岛的形貌经历了一个从分散生长逐渐过渡到分形生长的过程,并且在较低温度(200K)下,随入射粒子剩余能量的增加,岛的形貌也经历了同样的变化过程.进一步研究证明,随着基底温度的升高或入射粒子剩余能量的增加,沉积粒子的扩散能力显著增强,从而使岛的形貌发生了改变.     

Synthesis of nanoclusters by nanosecond laser ablation: Direct simulation Monte Carlo modelling

Collisional processes leading to the formation of nanoparticles in a laser-ablated plume are numerically simulated with the aid of an atomistic-level model based on direct simulation Monte Carlo (DSMC) method. The formation of nanoparticles in nanosecond laser ablation of a mono-atomic target is investigated in the presence of an inert background gas. The DSMC procedure is modified in order to account for numerous plume species and to describe several reactions (i.e., recombination/dissociation, sticking, evaporation) taking place in the plume and affecting the size and spatial distribution of the produced nanoclusters. Calculation results allow us to visualize the nanoparticles and to correlate their space distributions with plume dynamics. In addition, cluster size distributions are investigated at different pressures. The effects of the background gas on cluster formation within the plume are furthermore shown.     

Monte Carlo simulation of nanometric cutting

Nanometric cutting of single-crystal materials at conventional cutting speeds (5?m?s^?1) is simulated for the first time using a new Monte Carlo method that is applicable to systems that are neither canonical nor microcanonical. This is accomplished by defining a local temperature in the cutting zone using the thermal analysis developed by Komanduri and Hou for conventional machining. Extension of this method to the nanometric regime permits an accurate estimate of the local temperature in cutting. This temperature is then employed in the Boltzmann probability distribution function that is used to determine the acceptance–rejection of Monte Carlo moves in the simulation. Since cutting speed is closely related to cutting temperature, the cutting speed enters the calculation via the thermal analysis equations. The method is applied to nanometric cutting of single-crystal aluminium with the crystal oriented in the (001) plane and cut in the [100] direction. Three positive rake cutting tools, namely 10°, 30° and 45°, are employed to investigate the effect of the rake angle on the forces, the specific energy and the nature of the chip formation. The method is evaluated by direct comparison with corresponding molecular dynamics simulations conducted under the same conditions.     

Monte Carlo simulation of exposure factor

In this paper, we simulate the exposure factor by a simple model of a free-air ionization chamber with the Monte Carlo programme Geant4. Special emphasis is placed on the discussion of the exposure factor related to parameters of the chamber model. The reason for the variation in exposure factor with incident ray energy is also analysed in terms of reaction cross section for different types of reactions. The obtained results indicate that our simulation is accurate in the calculation of the exposure factor and can serve as a reference in designing air ionization chambers.     

Monte Carlo simulation of liquid neopentane

Monte Carlo simulations are reported for liquid neopentane at 25°C and 0·592 g cm^-3 using three different potential models. These results are compared with X-ray diffraction data and simulations for other tetrahedral molecules.     

A Monte Carlo simulation of coagulation

A Monte Carlo simulation technique is described for the study of the coagulation of suspended particles. The method is computationally efficient since the particle trajectories are not used to determine coagulations. Instead, pairs of particles are assigned probabilities to coagulate and the evolution is computed as a stochastic Markov game. We also describe a simple analytic method to obtain the stationary distribution of sizes for the various mechanisms of relative particle motion. It is demonstrated that the simulation yields the correct stationary size distribution independent of initial condition.     

Monte Carlo simulation of aorta autofluorescence

Results of numerical simulation of autofluorescence of the aorta by the method of Monte Carlo are reported. Two states of the aorta, normal and with atherosclerotic lesions, are studied. A model of the studied tissue is developed on the basis of information about optical, morphological, and physico-chemical properties. It is shown that the data obtained by numerical Monte Carlo simulation are in good agreement with experimental results indicating adequacy of the developed model of the aorta autofluorescence.