纳米晶体微观畸变与弹性模量的模拟研究

采用分子动力学方法模拟纳米晶体铜原子的结构,又对纳米晶体铜原子进行了X射线衍射模拟．计算了晶粒尺寸和点阵畸变,还计算了能量分布和弹性模量等．结果表明不但晶界产生很大的应力场,而且晶粒内部的畸变也起着与晶界相似的重要作用．由于原子半径的增加,导致弹性模量的减少. 关键词：     

纳米晶体结构与性能的模拟研究

通过分子动力学方法模拟纳米晶体（1—3nm）的结构．利用模拟的结果，进行了X射线衍射点阵常数、晶粒尺寸及点阵畸变的模拟计算，还计算了结合能及弹性模量等．结果表明纳米晶体无论是晶界和晶粒都与传统的粗晶粒晶体材料没有本质的区别，只是由于晶粒尺寸变小，以及晶界的体积分数等的作用，导致一系列的性能差异 关键词：     

Grain Growth Kinetics of BaTiO3 Nanocrystals During Calcining Process

以醋酸钡和钛酸四丁酯为原料,采用溶胶-凝胶法合成了纳米BaTiO3粉体;运用差示/热重、X射线衍射及透射电镜对前驱体凝胶和产物进行了表征,并根据XRD结果,研究了纳米BaTiO3的晶格常数、晶格畸变度和晶粒尺寸随焙烧温度及时间的变化。结果表明,焙烧温度与时间对纳米BaTiO3晶格常数的影响不明显;随焙烧温度或时间的延长,纳米BaTiO3的晶格畸变度减小,晶粒尺寸增大,但晶格畸变度和晶粒尺寸更敏感于焙烧温度. 基于扩散控制机理的传统模型探讨了焙烧过程纳米BaTiO3晶粒生长动力学,得出其晶粒生长指数为7,晶粒生长活化能为75.49 kJ/mol. 将基于扩散与反应共同控制机理的新型等温模型应用于本研究中,结果表明,新型等温模型更能真实地反映纳米BaTiO3焙烧过程中的晶粒生长行为,说明纳米BaTiO3晶粒生长过程同时受溶质扩散和表面反应控制,其藕合晶粒生长活化能为27.23 kJ/mol.     

分子动力学模拟纳米晶体铜的结构与性能

通过分子动力学方法模拟纳米面心立方晶体铜的结构，并对模拟的结果进行了不同晶粒尺寸的纳米晶体的密度、能量分布以及弛豫前后的X射线衍射、径向分布函数等计算.结果表明，大体积分数的晶界和畸变的晶粒是纳米晶体有别于传统的粗晶粒晶体材料结构的重要方面，由此导致纳米晶体一系列不同性能. 关键词：     

金属Cu晶化与玻璃化行为的分子动力学模拟

采用分子动力学模拟的方法,运用Mishin镶嵌原子模型,研究了液态Cu在不同冷却条件下的晶化与玻璃化行为.记录了系统能量、体积、径向分布函数及公共近邻的变化,并在0K分析了弛豫后系统原子内能、原子Voronoi体积及原子应力等热力学性质的分布情况.模拟结果表明,在较慢的冷却条件下,液态Cu形成晶态;在较快的冷却条件下,液态Cu形成非晶态.与晶态Cu比较,金属玻璃Cu有较高的能量和较大的体积,其内部晶格畸变导致了本征应力的存在.     

A model for the propagation of nonlinear dispersive strain waves in a solid with defect clusters

A model for the propagation of nonlinear dispersive one-dimensional longitudinal strain waves in an isotropic solid with quadratic nonlinearity of elastic continuum is developed with taking into account the interaction with atomic defect clusters. The governing nonlinear dispersive-dissipative equation describing the evolution of longitudinal strain waves is derived. An approximate solution of the model equation was derived which describes asymmetrical distortion of geometry of the solitary strain wave due to the interaction between the strain field and the field of clusters. The contributions of the finiteness of the relaxation times of cluster-induced atomic defects to the linear elastic modulus and the lattice dissipation and dispersion parameters are determined. The amplitudes and width of the nonlinear waves depend on the elastic constants and on the properties of the defect subsystem (atomic defects, clusters) in the medium. The explicit expression is received for the sound velocity dependence upon the fractional cluster volume, which is in good agreement with experiment. The critical value of cluster volume fraction for the influence on the strain wave propagation is determined.     

模拟纳米晶体原子分布及X射线散射理论图案

通过分子动力学方法模拟纳米晶体（1—3nm）的结构．利用模拟的结果，进行了X射线衍射及径向分布函数的模拟计算．结果表明：纳米晶体晶界呈短程有序，界面原子间距分布很宽；随着晶粒尺寸的减小，晶粒的畸变越大，原子体积常数也明显增大 关键词：     

Elastic properties and electronic structures of lanthanide hexaborides

The structural, elastic, and electronic properties of a series of lanthanide hexaborides(Ln B6) have been investigated by performing ab initio calculations based on the density functional theory using the Vienna ab initio simulation package.The calculated lattice and elastic constants of Ln B6 are in good agreement with the available experimental data and other theoretical results. The polycrystalline Young's modulus, shear modulus, the ratio of bulk to shear modulus B/G, Poisson's ratios, Zener anisotropy factors, as well as the Debye temperature are calculated, and all of the properties display some regularity with increasing atomic number of lanthanide atoms, whereas anomalies are observed for Eu B6 and Yb B6. In addition, detailed electronic structure calculations are carried out to shed light on the peculiar elastic properties of Ln B6.The total density of states demonstrates the existence of a pseudogap and indicates lower structure stability of Eu B6 and Yb B6 compared with others.     

Temperature effect on elastic modulus of thin films and nanocrystals

The stability of nanoscale devices is directly related to elasticity and the effect of temperature on the elasticity of thin films and nanocrystals. The elastic instability induced by rising temperature will cause the failure of integrated circuits and other microelectronic devices in service. The temperature effect on the elastic modulus of thin films and nanocrystals is unclear although the temperature dependence of the modulus of bulk materials has been studied for over half a century. In this paper, a theoretical model of the temperature-dependent elastic modulus of thin films and nanocrystals is developed based on the physical definition of the modulus by considering the size effect of the related cohesive energy and the thermal expansion coefficient. Moreover, the temperature effect on the modulus of Cu thin films is simulated by the molecular dynamics method. The results indicate that the elastic modulus decreases with increasing temperature and the rate of the modulus decrease increases with reducing thickness of thin films. The theoretical predictions based on the model are consistent with the results of computational simulations, semi-continuum calculations and the experimental measurements for Cu, Si thin films and Pd nanocrystals.     

First-principles study of structural,elastic, lattice dynamical and thermodynamical properties of GdX (X = Bi,Sb)

The results are presented of first-principles calculations of the structural, elastic and lattice dynamical properties of GdX (X = Bi, Sb). In particular, the lattice parameters, bulk modulus, phonon dispersion curves, elastic constants and their related quantities, such as Young's modulus, Shear modulus, Zener anisotropy factor, Poisson's ratio, Kleinman parameter, and longitudinal, transverse and average sound velocities, were calculated and compared with available experimental and other theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacities, Grüneisen parameter and Debye temperatures were predicted in wide pressure (0?50 GPa) and temperature ranges (0–500 K). The plane-wave pseudopotential approach to the density-functional theory within the GGA approximation implemented in VASP (Vienna ab initio simulation package) was used in all computations.     

Multiple grains in nanocrystals: effect of initial shape and size on transformed structures under pressure

Pressure-induced structural transformations in spherical and faceted gallium arsenide nanocrystals of various shapes and sizes are investigated with a parallel molecular-dynamics approach. The results show that the pressure for zinc blende to rocksalt structural transformation depends on the nanocrystal size, and all nanocrystals undergo nonuniform deformation during the transformation. Spherical nanocrystals above a critical diameter >/=44 A transform with grain boundaries. Faceted nanocrystals of comparable size have grain boundaries in 60% of the cases, whereas the other 40% are free of grain boundaries. The structure of transformed nanocrystals shows that domain orientation and strain relative to the initial zinc blende lattice are not equivalent. These observations may have implications in interpreting the experimental x-ray line shapes from transformed nanocrystals.     

单轴拉伸下氧化锆纳米柱相变机理的分子动力学研究

通过分子动力学模拟，观察到[001]取向的四方氧化锆纳米柱在拉伸载荷下具有两个线弹性变形的应力-应变关系.这一现象是四方结构向单斜结构相变的结果 .为了进一步阐明应力-应变曲线，进行了包括晶体结构分析和原子应变计算在内的详细研究.晶格取向强烈影响塑性变形机制，即[001]和[111]取向的纳米柱在拉伸载荷下经历相变，而沿[110]取向的纳米柱导致脆性断裂.观察到显著的温度效应，随着温度从300K升高到1500K，弹性模量从573.45GPa线性降低到482.65GPa.此外，还用轻推弹性带(NEB)理论估算了相变能垒，观察到相变能垒随温度的升高而降低.这一工作将有助于加深对氧化锆的四方相到单斜相转变和纳米尺度力学行为的理解.     

单晶钨纳米线拉伸变形机理的分子动力学研究

利用分子动力学方法, 对本课题组率先采用金属催化的气相合成法制备出的高纯度单晶钨纳米线进行拉伸变形数值模拟, 通过分析拉伸应力-应变全曲线及其微观变形结构, 揭示出单晶钨纳米线的拉伸变形特征及微观破坏机理. 结果表明: 单晶钨纳米线的应力-应变全曲线可分为弹性阶段、损伤阶段、相变阶段、强化阶段、 破坏阶段等五个阶段, 其中相变是单晶钨纳米线材料强化的重要原因; 首次应力突降是由于局部原子产生了位错、孪生等不可逆变化所致; 第二次应力突降是发生相变的材料得到强化后, 当局部原子再次产生位错导致原子晶格结构彻底破坏而形成裂口、且裂口不断发展成颈缩区时, 材料最终失去承载能力而断裂. 计算模拟得到的单晶钨纳米线弹性模量值与实测值符合较好. 关键词： 分子动力学 应力应变曲线 微观机理 单晶钨纳米线     

Determination of viral capsid elastic properties from equilibrium thermal fluctuations

We apply two-dimensional elasticity theory to viral capsids to develop a framework for calculating elastic properties of viruses from equilibrium thermal fluctuations of the capsid surface in molecular dynamics and elastic network model trajectories. We show that the magnitudes of the long wavelength modes of motion available in a simulation with all atomic degrees of freedom are recapitulated by an elastic network model. For the mode spectra to match, the elastic network model must be scaled appropriately by a factor which can be determined from an icosahedrally constrained all-atom simulation. With this method we calculate the two-dimensional Young's modulus Y, bending modulus κ, and F?ppl-von Kármán number γ, for the T=1 mutant of the Sesbania mosaic virus. The values determined are in the range of previous theoretical estimates.     

Melting of nanocrystals embedded in a crystal matrix heated by nanosecond laser pulses

The kinetics of phase transformations of nanocrystals in a crystal matrix is considered upon non-stationary heating by laser pulses. The melting and crystallization kinetics of nanocrystals is described taking into account their size, shape, elemental composition, and elastic deformations appearing due to the mismatch of the lattice constants for nanocrystals and the matrix. The possibility of decreasing the dispersion of nanocrystals over their size in heterostructures with quantum dots is predicted. As an example, melting of Ge nanocrystals in a Si matrix is considered.     

Al_xCrFeNiTi系高熵合金成分和弹性性质关系

为了探索Al_xCrFeNiTi系高熵合金组成成分和弹性性质的关系,结合固溶体特征参数和第一性原理计算,研究Al元素含量对Al_xCrFeNiTi (x=0, 0.5, 1, 2, 3, 4)合金结构和弹性性质的影响,并分析合金固溶体特征参数与弹性性质之间的关系.结果表明:Al_xCrFeNiTi系合金的价电子浓度随着Al含量的增加逐渐减小,合金在体心立方结构下的形成焓均低于面心立方结构,说明研究的Al_xCrFeNiTi系合金会形成单一的体心立方结构固溶体;合金的晶格常数和形成能力强弱随着Al含量的增加而增大,但合金的结构稳定性略有下降;当合金元素按照等原子比进行成分配比时,合金的原子尺寸差异最大; Al_xCrFeNiTi系合金中不同原子之间除了金属键结合外,还表现出一定的共价和离子键结合特征;对于Al_xCrFeNiTi系合金而言,随着热力学熵焓比的增大,合金体弹模量和韧性随之增大;随着合金混合焓的增加,合金在压缩方向的各向异性程度明显降低.热力学熵焓比和混合焓可作为Al_xCrFeNiTi系高熵合金成分设计的重要参数.     

Ca改性钛酸铅Pb_(1-x)Ca_xTiO_3纳米晶的拉曼光谱研究

本文在２８８Ｋ和３７８Ｋ温度下测定了不同Ｃａ含量Ｐｂ１－ｘＣａｘＴｉＯ３（ＰＴＣ）纳米晶的拉曼光谱。同时在室温条件下测定了不同晶粒尺寸Ｐｂ０．８５Ｃａ０．１５ＴｉＯ３的拉曼光谱。讨论了Ｃａ掺入量、温度、晶粒尺寸等因素对声子频率的影响。声子频率的变化情况表明,对于ｘ≥０．２的ＰＴＣ纳米晶,与相同Ｃａ含量的陶瓷相比,存在着晶格膨胀的倾向。     

Elastic modulus of polypyrrole nanotubes

The first measurements of the tensile elastic modulus of polypyrrole nanotubes are presented. The nanotubes were mechanically tested in three points bending using atomic force microscopy. The elastic tensile modulus was deduced from force-curve measurements on different nanotubes with outer diameter ranging between 35 and 160 nm. It is shown that the elastic modulus strongly increases when the thickness or outer diameter of polypyrrole nanotubes decreases.     

Molecular dynamics analysis of the substrate temperature effect on thermomechanical characteristics of vapor deposited nanofilms

Modern engineering applications are in need for technologies of nanostructures and nanofilms with controllable properties. The detection of these structures requires methods of atomic research, among which are molecular dynamics techniques, Monte-Carlo simulation, and ab initio calculation. The most efficient method to deal with systems of about several thousands of atoms is molecular dynamics simulation. We used this method to analyze the formation of nanolayers on a Cu substrate in vapor deposition of Cu atoms. It is shown that the film deposited on the substrate surface replicates the crystalline structure of the substrate. It is found that at low deposition temperatures, the deposited layer reveals a great quantity of vacancies and vacancy clusters (nanopores). It is demonstrated that increasing the substrate temperature in metal vapor deposition ensures a more perfect lattice in the nanocoating, and the cohesive energy of atoms in the nanolayer thus approximates experimental values. It is also found that the increase in substrate temperature in the process causes Young’s modulus and elastic limit to tend to the values of a perfect crystal.     

Molecular dynamics simulation of the melting of a twist Σ=5 grain boundary

The existence of a possible grain boundary disordering transition of the melting type in a =5 (001) twist boundary of aluminium bicrystal below the melting temperature was investigated using a constant pressure molecular dynamics simulation. The calculated melting temperature T _cm of the bulk Al is about 960 K. The total internal energy, the structure factor, and the pair distribution function were calculated at different layers across the grain boundary. The mean atomic volume, the grain boundary energy, and the thermal expansion coefficients were also calculated using the same simulation method. This simulation also allows us to image the grain boundary structure at different temperatures. The equilibrium grain boundary structure at 300 K retains the periodicity of the coincident site lattice, so that the lowest energy structure corresponds to the coincident site arrangement of the two ideal crystals. With increasing temperature, the total internal energy of the atoms for both the perfect crystal and the grain boundary increases, as do the number of layers in the grain boundary. The grain boundary core exists and the perfect crystal structure still exists outside the grain boundary at 0.9375 T _cm. However, two atomic layers of the equilibrium grain boundary structure at 0.9375 T _cm lose the coincident site lattice periodicity and attain a structure with liquid-like disorder. Therefore, partial melting of the grain boundary has occurred at the temperature above 0.9375 T _cm which is in agreement with the experimental results.