Pt(111)表面低能溅射现象的分子动力学模拟

利用嵌入原子方法的原子间相互作用势，通过分子动力学模拟，详细研究了贵金属原子在Pt (111)表面的低能溅射现象.模拟结果显示：对于垂直入射情况，入射原子的质量对Pt (11 1)表面的溅射阈值影响不大.当入射原子的能量小于溅射阈值时，入射原子基本以沉积为主 ；当入射原子的能量大于溅射阈值时，溅射产额随入射原子能量的增加而线性增大；当入射 原子能量达到200 eV时，各种入射原子的溅射产额都达到或接近1，此时入射原子主要起溅 射作用.溅射原子发射的角分布概率和溅射花样与高能溅射相类似.研究表明：与基于二体碰 撞近似的线性级联溅射理论不同，当入射原子能量大于溅射阈值时，低能入射原子的溅射产 额正比于入射原子的约化能量和入射原子与基体原子的质量比.通过对低能入射原子的钉扎 能力分析，提出了支配低能溅射的入射原子反射物理机理. 关键词： 分子动力学模拟、低能溅射     

表面粗糙度对面向等离子体钨材料溅射的影响

依据钨材料表面溅射的实验现象,建立钨材料表面粗糙模型,模拟了高能H⁺、He⁺粒子辐照下的钨材料表面的溅射行为过程,并与基于离子输运的双群模型计算得到的结果作了比较。结果表明,随着钨材料表面粗糙程度的增加,溅射率降低;对一定的粗糙表面,相同能量的不同入射粒子,质量越大粒子溅射率越高,这些结果为分析聚变装置中心等离子体杂质水平和评价偏滤器寿命等提供了一定的理论支撑。     

Ni原子倾斜轰击Pt(111)表面低能溅射现象的分子动力学模拟

采用嵌入原子方法的原子间相互作用势,通过分子动力学模拟详细研究了以不同角度入射的低能Ni原子与Pt (111)基体表面相互作用过程中的低能溅射行为.结果表明:随着入射角度从0°增加到80°,溅射产额Y_s和入射原子钉扎系数S的变化均可以根据入射角θ近似地分为以下三个区域:当θ ≤ 20°时,Y_s和S几乎保持不变,其值与垂直入射时接近,溅射原子的发射角分布和能量分布也与垂直入射时的情 关键词： 分子动力学模拟 入射角 低能溅射     

EAST边缘等离子体中钨表面电弧诱导实验研究

在EAST装置中利用材料与等离子体测试平台MAPES对不同表面性质和结构的钨样品进行了边缘等离子体条件下的起弧诱导实验研究。结果显示只有纳米丝结构的钨表面成功产生了电弧,表明了钨表面的纳米丝结构更有利于电弧的触发,甚至在装置远离刮削层区域的地方也能诱导电弧触发。在纳米丝钨表面起弧过程中会产生两种不同的电弧,其中较强电弧能产生熔坑,并溅射出液滴,是等离子体中尘埃及杂质的重要来源。弧坑直径约为3μm,深度约为0.7μm,在电弧熔坑中心区域形成了一些小孔和纳米结构,其周围的熔化液滴也存在类似的结构。     

表面黏附导致的接触行为转变

应用大规模分子动力学方法, 模拟了具有不同原子级粗糙形貌的两种刚性球形探头与弹性平面基体的黏附接触行为. 研究了载荷与真实接触面积、接触界面排斥力与真实接触面积, 以及黏附力与真实接触面积之间的关系. 分子模拟得到的载荷与真实接触面积的关系, 与连续力学接触理论预测很好地定性一致. 无论是原子级光滑探头还是粗糙探头, 黏附接触下的排斥力与真实接触面积的关系, 都与无黏附接触时的规律相一致, 即黏附力对接触行为的影响作用, 可以等效为附加在真实外载荷基础上的虚拟载荷, 将对黏附接触行为的分析转变为无黏附接触分析. 两种探头的黏附力随真实接触面积都呈幂函数形式的增长, 但是, 原子级光滑探头的幂指数大于1, 而原子级粗糙探头的幂指数小于1. 关键词： 接触行为 表面黏附 分子动力学模拟     

金属Zn液态结构变化的研究

利用ＴＢ模型给出的原子间相互作用势详细计算了不同温度下Ｚｎ的双体分布函数ｇ（ｒ），结果发现随着温度的不断降低，液态金属Ｚｎ的ｇ（ｒ）第一峰变得高而尖，第二峰由弱变强，说明了液态金属Ｚｎ的有序度随温度降低而不断增强；利用键对分析技术统计了液态金属Ｚｎ在不同温度下的键取向序参数、键对数。键取向序参数及键对数随温度的变化，进一步证明了低温液态的有序度高于高温液态，从而充分说明液态金属在不同温度下有不同的结构形式，而不像人们想象得那样杂乱无章。     

辐照损伤对钨晶格热导率影响的分子动力学研究

钨是最具应用前景的面向等离子体候选材料,但核聚变堆内强烈的辐照环境会使钨的近表面区域产生辐照损伤,进而影响其关键的导热性能.本文构建了包含辐照损伤相关缺陷的晶体钨模型,并采用非平衡分子动力学的方法定量研究了这些缺陷对钨导热性能的影响.结果表明,随中子辐射能量的增加,晶体内部留下的Frenkel缺陷数目增多进而导致钨的晶格热导率降低;间隙原子比空位更易于向晶界偏聚,且钨中的间隙钨原子与空位相比,使晶格热导率下降程度更大.纳米级氦气泡导致晶格热导率的显著降低,气孔率为2.1%时晶格热导率降至完美晶体的约25%.这些不同的缺陷造成不同程度的周围晶格扭曲,增加了声子散射几率,是导致晶格热导率下降的根源.     

溅射原子角分布与靶点表面形貌的关联研究

27keV的Ar⁺离子垂直轰击处于不同温度的Cd靶,用捕获膜技术和Rutherford背散射谱仪(RBS)测定溅射原子角分布,并对所有样品的靶点形貌进行扫描电子显微镜(SEM)观察。结果发现,所有的角分布都呈over-cosine形状,但是在极角为0°处实验值与cosine值的偏离△,是不同的,在靶温度为150℃时的偏离△_T小于在室温时的偏离△_R,亦即△_T<△_R,提出一个简单的模型对这种溅射角分布 关键词：     

单轴应力场下钨中氦扩散行为的分子动力学模拟研究

本文采用分子动力学方法研究了沿<100>及<111>晶向的单轴应变对钨中单个氦原子扩散的影响。结果表明,应变会使得金属钨材料发生相变,且引起相变的临界应变随温度升高而减小。相变起始的应变在达到抗拉强度的应变附近。计算结果表明,拉应变使得单个氦原子在钨中的扩散系数发生骤降,在不同应变下扩散系数变化平缓。沿<100>晶向氦扩散系数随应变的增大而线性减小,而<111>晶向则出现了震荡变化趋势。研究结果表明,沿<100>晶向应变达到+0.15%时阿纽列斯方程不再适用,而沿<111>晶向应变大+5%阿纽列斯方程仍然适用;沿<111>晶向随应变增加氦扩散激活能减小,说明应变使得单个氦原子在钨中迁移性增强。     

纳米尺度下毛细流动的分子动力学模拟

采用分子动力学模拟研究纳米尺度下壁面润湿性对毛细流动的影响,主要考虑纳米通道两侧壁面润湿性相同与不同两种情况。研究表明：两侧壁面润湿性相同条件下,毛细流动随着壁面润湿性增强而加快, 毛细高度随时间的变化早期偏离Lucas-Washburn理论,但后期与其预测符合。在纳米通道两侧壁面润湿性不同的情况下,液面会发生振荡,两侧壁面毛细高度也不相等,且液面振荡的幅度和两侧壁面毛细高度差都随着两侧壁面润湿性差异的增大而增大。基于能量转化分析,提出两侧壁面湿润性不同情况下纳米通道中毛细流动发生的条件以及毛细流动快慢的判别依据。研究结果加深了对纳米尺度下毛细流动机理的认识,并为相关工程应用提供理论参考。     

Simulations of grazing-incidence pumped Ni-like Sn X-ray laser at 11.9 nm

Grazing-incidence pumped Ni-like Sn X-ray laser media at 11.9 nm (4d-4p, J = 0-1) is modelled using code EHYBRID and a post-processor code. The required atomic data are obtained using the Cowan code. In this study the pre-formed plasma is pumped on longitudinal direction with a grazing angle. Detailed simulations were performed to optimize the driving laser configurations. Relatively high gain is produced for the Ni-like Sn X-ray laser at 11.9 nm with long pre-pulse and short main pulse drive energy of only 100 mJ on 4 mm slab targets. Using low intensity pre-pulse prior to long pulse decreases the electron density gradient. X-ray resonance lines between 13 and 25 Å emitted from tin plasma have been simulated using post-processor coupled with EHYBRID. The ratio of these resonance lines can be used to measure electron temperature of the laser produced Sn plasma.     

Atomistic Simulations of Integranular Fracture in Symmetric-Tilt Grain Boundaries

Fracture experiments on symmetric-tilt grain boundaries in Cu are interpreted using the Peierls-Nabarro continuum model of dislocation nucleation as a starting point. Good agreement is found only when the continuum model is modified according to the results of atomistic simulations. The same experiments are also reproduced by direct Molecular Dynamics simulations of fracture propagation and dislocation emission from a microcrack placed in the interface plane of the symmetric-tilt (221)(221) grain boundary in fcc Cu. Direction-dependent fracture response is observed, namely the microcrack advancing by brittle fracture along the [11 ] direction and being blunted by dislocation emission along the opposite [ 4] direction. Moreover, the simulations allow us to establish important differences with respect to the continuum-model predictions due to the shielding of the stress field at the crack-tip and to the presence of the residual stress at the interface.     

Surface diffusion of Si,Ge and C adatoms on Si （001） substrate studied the molecular dynamics simulation

Depositions of Si, Ge and C atoms onto a preliminary Si (001) substrate at different temperatures are investigated by using the molecular dynamics method. The mechanism of atomic self-assembling occurring locally on the flat terraces between steps is suggested. Diffusion and arrangement patterns of adatoms at different temperatures are observed. At 900 K, the deposited atoms are more likely to form dimers in the perpendicular [110] direction due to the more favourable movement along the perpendicular [110] direction. C adatoms are more likely to break or reconstruct the dimers on the substrate surface and have larger diffusion distances than Ge and Si adatoms. Exchange between C adatoms and substrate atoms are obvious and the epitaxial thickness is small. Total potential energies of adatoms and substrate atoms involved in the simulation cell are computed. When a newly arrived adatom reaches the stable position, the potential energy of the system will decrease and the curves turns into a ladder-like shape. It is found that C adatoms can lead to more reduction of the system energy and the potential energy of the system will increase as temperature increases.     

Interaction of plasma facing materials for fusion devices with low energy hydrogen and helium particles

Abstract

Radiation damage formed in metal specimens exposed to long pulse tokamak plasmas of TRIAM-1M was examined by transmission electron microscopy. By comparing these results with those of low energy hydrogen ion irradiation it was concluded that the charge exchange energetic neutrals of hydrogen emitted from the core plasma caused remarkable displacement damage. The flux of the neutrals in the energy range of 0.5–3 keV which were responsible for displacement damage, was estimated to be about 1.5–3 × 10¹⁸ H/m²/s. These energetic neutrals cause not only material degradation at the sub-surface region but also change bulk properties of plasma facing components in a plasma confinement device. Effect of helium plasma was also discussed with emphasis on very strong effects on damage accumulation. Damage by He is a serious issue of plasma facing materials.     

钠原子团簇相变的多极形变特征

运用距离相关紧密结合的分子动力学模型,通过无量纲的形变参数S1对简单金属原子团簇Na4、Na8、Na14和Na20相变时的多极形变特征进行了分析,发现S1可以用作表征团簇形变的灵敏探针.     

Molecular Dynamics Simulations of Warm Dense Carbon

We present classical and DFT‐based molecular dynamics (MD) simulations of carbon in the warm dense matter regime (? = 3.7 g/cc, 0.86 eV < T < 8.62 eV [T < 100 eV for classical MD]). Two different classical interatomic potentials are used: 1. LCBOP, designed to simulate condensed (e.g. solid) phases of C, and 2. linearly screened Coulomb (Yukawa) potentials. It is shown that LCBOP over‐predicts minima and maxima in the pair correlation functions of liquid‐C in this regime when compared to the DFT‐MD results. The screened Coulomb model, while under‐correlating at low‐T, seems to produce the correct qualitative features in the static ionic pair distributions at the highest‐T. However, both approaches predict the decay in the ionic contribution of the specific heat as T → ∞ to be much slower than that predicted by a model based on DFT‐MD. These differences in the MD‐derived equations of state in warm dense regimes could have important consequences when using classical inter‐ionic forces such as these in large‐scale MD simulations aimed at studying, for instance, processes of relevance to inertial confinement fusion when C is used as an ablator material. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Molecular Dynamics Simulations of Liquid Phosphorus at High Temperature and Pressure

By performing ab initio molecular dynamics simulations, we have investigated the microstructure, dynamical and electronic properties of liquid phosphorus （P） under high temperature and pressure. In our simulations, the calculated coordination number （CN） changes discontinuously with density, and seems to increase rapidly after liquid P is compressed to 2.5 g/cm3. Under compression, liquid P shows the first-order liquid-liquid phase transition from the molecular liquid composed of the tetrahedral P4 molecules to complex polymeric form with three-dimensional network structure, accompanied by the nonmetal to metal transition of the electronic structure. The order parameters Q6 and Q4 are sensitive to the microstructural change of liquid P. By calculating diffusion coefficients, we show the dynamical anomaly of liquid P by compression. At lower temperatures, a maximum exists at the diffusion coefficients as a function of density; at higher temperatures, the anomalous behavior is weakened. The excess entropy shows the same phenomena as the diffusion coefficients. By analysis of the angle distribution functions and angular limited triplet correlation functions, we can clearly find that the Peierls distortion in polymeric form of liquid P is reduced by further compression.     

钨离子双电子复合过程的理论研究

摘要: 利用全相对论组态相互作用理论方法, 研究了W28+离子由基态俘获一个电子形成双激发态(3d104s24p64d10 )-1 nln''l''(n = 4~6, n'' = 4~15)的双电子复合(DR)过程. 比较分析了3s、3p、3d、4s、4p和4d电子激发对DR速率系数的贡献, 分析了3d、4s、4p和4d电子激发的DR速率系数随轨道量子数l'' 的变化. 考虑和已有的计算完全相同的初态, 中间双激发态以及辐射和俄歇末态的情况下, 得到了和已有的计算符合很好的结果. 在综合了分析得到的对W28+离子DR过程有明显贡献的各种因素后,进一步得到了总DR速率系数. 其中, 考虑DAC效应对总DR速率系数有不可忽略的影响. 对DR速率系数进行了参数拟合, 拟合值与计算值的偏差小于1％.     

钨离子双电子复合过程的理论研究

利用全相对论组态相互作用理论方法, 研究了W28+离子由基态俘获一个电子形成双激发态(3d104s24p64d10 )-1 nln'l'(n = 4~6, n' = 4~15)的双电子复合(DR)过程. 比较分析了3s、3p、3d、4s、4p和4d电子激发对DR速率系数的贡献, 分析了3d、4s、4p和4d电子激发的DR速率系数随轨道量子数l' 的变化. 考虑和已有的计算完全相同的初态, 中间双激发态以及辐射和俄歇末态的情况下, 得到了和已有的计算符合很好的结果. 在综合了分析得到的对W28+离子DR过程有明显贡献的各种因素后,进一步得到了总DR速率系数. 其中, 考虑DAC效应对总DR速率系数有不可忽略的影响. 对DR速率系数进行了参数拟合, 拟合值与计算值的偏差小于1％.