硅功能化石墨烯负极材料的粗粒模型

硅功能化石墨烯(硅化烯)作为锂离子电池的负极材料, 一旦发生分层或粉化等损伤现象, 会严重地降低材料的电子输运能力和储锂容量, 减少电池的使用寿命, 因此要求负极材料具有较强的力学可靠性. 考虑到传统分子动力学方法的模拟尺度很难达到硅化烯负极材料的真实尺度, 首先采用Tersoff 势函数和Lennard-Jones 势函数建立了多种硅化烯的全原子数值模型, 计算材料的各种弹性模量和吸附能; 然后采用珠子-弹簧结构, 根据力学平衡条件和能量守恒定律, 结合全原子模型的计算结果, 建立了硅化烯粗粒模型及其系统的能量方程; 最后, 通过对比石墨烯粗粒模型与其全原子模型的拉伸性能, 验证了硅化烯粗粒模型的有效性.     

原子力显微镜在轻敲模式下的动力学模型

基于Hamaker假设、Lennard-Jones势能定律及经典弹性理论建立了一种新型的球体与平面黏着接触的弹性模型，该模型显示黏着力在原子力显微镜(AFM)针尖趋近和撤离样品表面，即加载和卸载的两个过程中存在黏着滞后现象，表明了AFM在轻敲工作模式中存在能量耗散.同时，根据所建的黏着接触弹性模型，建立了AFM在轻敲工作模式下的动力学模型，研究了AFM在轻敲工作模式下的振动幅度、相位差及耗散功率随针尖与样品表面间距的变化规律，仿真结果与现有的实验结果相一致.     

A nonlinear magneto-thermo-elastic coupled hysteretic constitutive model for magnetostrictive alloys

This paper presents a general hysteretic constitutive law of nonlinear magneto-thermo-elastic coupling for magnetostrictive alloys. The model considered here is thermodynamically motivated and based on the Gibbs free energy function. A nonlinear part of the elastic strain arising from magnetic domain rotation induced by the pre-stress is taken into account. Furthermore, the movement of the domain walls is incorporated to describe hysteresis based on Jiles–Atherton's model. Then a set of closed and analytical expressions of the constitutive law for the magnetostrictive rods and films are obtained, and the parameters appearing in the model can be determined by those measurable experiments in mechanics and physics. Comparing this model with other existing models in this field, the quantitative results show that the relationships obtained here are more effective to describe the effects of the pre-stress or in-plane residual stress and ambient temperature on the magnetization or the magnetostriction hysteresis loops.     

Comparative assessment of the ELBA coarse-grained model for water

The ELBA force field for water consists of a single spherical site embedded with a point dipole. This coarse-grained model is assessed here through the calculation of fundamental properties of bulk liquid water and the water–vapour interface. Accuracy and efficiency are evaluated and compared against simulations of standard three- and four-site atomistic models. For bulk liquid systems, ELBA reproduces accurately most of the investigated properties. However, the radial distribution function deviates from atomistic and experimental data, indicating a loss of local structure. The water–vapour interface, simulated over a range of temperatures from 300 to 600 K, is captured realistically in terms of its density distribution, and the accuracy in reproducing the experimental surface tension is as high as that of the best atomistic model. The critical temperature of ELBA is also found to be in excellent agreement with experiment. However, the interfacial electric field and surface potential are missing. The computational speed-up of ELBA compared to traditional atomistic models is estimated to be between one and two orders of magnitude.     

One-dimensional model of the quasicrystalline alloy

A one-dimensional chain of atoms of two types is investigated. It is proven exactly for the model of attracting hard spheres that if the ratio of the numbers of atoms of the two types is irrational, then the state of absolutely minimal energy is quasicrystalline. The quasicrystalline state is also investigated in the case of the Lennard-Jones interatomic potential. All the microscopic values (interatomic spacing, electronic density, etc.) are shown to be quasiperiodic functions varying on Cantor sets. Diffraction patterns, electronic properties, and vibrational spectra are also discussed.     

Sublattice identification in scanning force microscopy on alkali halide surfaces

We propose and apply to the KBr(001) surface a new procedure for species recognition in scanning force microscopy (SFM) of ionic crystal surfaces which show a high symmetry of the charge arrangement. The method is based on a comparison between atomistic simulations and site-specific frequency versus distance measurements. First, by taking the difference of force-distance curves extracted at a few judiciously chosen surface sites we eliminate site-independent long-range forces. The obtained short-range force differences are then compared with calculated ones assuming plausible tip apex models. This procedure allows for the first time identification of the tip apex polarity and of the positive and negative sublattices in SFM images of the (001) cleavage surface of an ionic crystal with the rock salt structure.     

微机械纳米接触连续方法的建模和计算

为解决微机械中"微碰撞"、"纳米接触"问题,建立了纳米接触的刚性球-面模型,根据Hamaker3个假设和Lennard Jones势,利用连续方法推导出球同平面第1和第N层原子之间的作用力表达式,仿真出粘着力同原子层之间的关系,得出影响纳米接触的主要原子为接触区域少数几层原子的结论,从而为纳米接触、纳米摩擦的进-步研究提供理论基础.     

Identification of gradient elasticity parameters based on interatomic interaction potentials accounting for modified Lorentz-Berthelot rules

The identification algorithm developed here for scale parameters of gradient elasticity combines solutions for a deformed heterogeneous composite fragment in a continuous one-dimensional model and for a diatomic chain in a discrete atomistic model. For the identification, the models are taken equivalent and the effective stiffnesses of equivalent composite fragments are compared. In the discrete model, only the nearest neighbor atoms interact and the interaction between dissimilar atoms are determined by a modified Lorentz-Berthelot rule. As a result, the effective stiffness of the discrete composite represented as a nonuniform atomic chain is found. The continuous model is a gradient one and takes into account nonlocal effects in the volume and adhesive properties of phase boundaries. The problem of determining the effective stiffness of a composite fragment is solved analytically in the one-dimensional approximation. The study is aimed to develop a procedure of identifying the scale parameters of gradient theories such that the parameters would be independent of the choice of potentials used in discrete modeling. On the example of modeling using the Morse and Lennard-Jones potentials, we propose an identification methodology invariant with respect to the choice of potentials. It is shown that the invariance is provided if the potentials in discrete modeling are coincided in the vicinity of equilibrium points. It is demonstrated that for unambiguous determination of the scale parameters of gradient elasticity, it suffices to use the simplest two-parameter potentials approximating any other potentials subject to equal equilibrium bond distances and equal second derivatives at the equilibrium point (i.e., force constants). An example of identifying the gradient elasticity parameters is presented for a two-phase W-Si composite.     

Atomistic simulation of aging and rejuvenation in glasses

Slow structural relaxation ("aging") observed in many atomic, molecular, and polymeric glasses substantially alters their stress-strain relations and can produce a distinctive yield point. Using Monte Carlo simulation for a binary Lennard-Jones mixture, we have observed these phenomena and their cooling-rate dependences for the first time in an atomistic model system. We also observe that aging effects can be reversed by plastic deformation ("rejuvenation"), whereby the system is expelled from the vicinity of deep minima in its potential energy surface.     

Bi-resonance functions in the theory of collisional broadening of the spectral lines of molecules

In the model of parabolic trajectories, expressions are derived for the bi-resonance functions that appear in the theory of collisional broadening of molecular spectral lines in the case of their interference. The functions required for calculating the cross-relaxation parameters of molecular spectral lines in the case of their broadening by rare gas atoms are obtained numerically in the model of exact trajectories for the isotropic Lennard-Jones 6–12 potential. Analytic models are proposed for these functions, and the corresponding parameters are obtained.     

Wigner-Seitz模型微观连续性分析

针对Wigner-Seitz单元模型同微观连续介质理论体积无限小的连续性矛盾,通过比较面心立方体和体心立方体双Wigner-Seitz单元和经典Lennard-Jones双原子势函数之间引力（长程力）和斥力（短程力）的相对误差,发现当双Wigner-Seitz模型相互重合一半后,相对误差仍满足工程要求;将Wigner-Seitz模型同离散模拟法计算得到的铝单原子球-面作用力结果相比较,两者结果符合;根据Wigner-Seitz模型,建立包含黏附的数字微镜（DMD）动力方程,仿真并同试验结果比较;从而得到Wigner-Seitz模型满足微、纳技术工程分析中微观连续性要求的结论,为微、纳技术的研究提供理论基础. 关键词： Wigner-Seitz模型 微观连续介质 Lennard-Jones势     

Interfaces and ripple states in ferroelastic crystals—A simple model

A simple atomistic model with two harmonic potentials between layers of atoms describes surface relaxations equivalent to those found as kink solitary waves in ferroelastic crystals. The parameters of the model are expressed in terms of the entropy term in a Landau potential and the Ginzburg gradient energy. We discuss the possibility that metastable ripple states exist in which a modulation is superimposed to an underlying uniform order parameter. Such ripple states can occur at temperatures close to the transition point between a ferroelastic phase and an incommensurate phase. In the ripple state, domain walls consist of kinks with modulations on either side of the kink.     

Simple pair potential model for real fluids. II. Transport properties of dilute gases

The collision integrals required for evaluating the dilute gas transport properties have been computed for the hybrid pair potential of Nezbeda. Calculations of viscosity and self-diffusion coefficients of Ar, Kr, Xe, and CH₄ have been made and the results compared with those based on the Lennard-Jones potential. It is found that (i) the potential parameters determined from the viscosity give the attractive force constant,C ₆, in good agreement with experimental data, (ii) the accuracy of calculated viscosity is reasonable even at low temperatures, and (iii) the hybrid model gives in all cases much better results than the Lennard-Jones potential.     

Crack formation in composites through a spring model

When two or more different materials are combined to form composites, the property of the composite may be quite different. This depends on relative interaction strengths of cohesion between similar and adhesion between dissimilar particles. We study crack formation in composites through a very general spring network model on a square lattice. The spring model shows the distortion of the lattice, prior to fracture and this affects the pattern of subsequent cracks, unlike the prevalent random fuse models, where no distortion is allowed. Two different types of sites (A and B) are distributed randomly on the lattice, representing two different constituents. There are springs of three types connecting them (A-A, B-B and A-B). We assign two spring parameters for each type of spring. These are a spring constant and a breaking threshold. We show that intermediate compositions may require higher energy to induce the first sample spanning break than either pure A or pure B. So the breaking energy goes through a maximum as the concentration of one component varies from 0% to 100%. The position and height of the peak depend on the spring parameters.     

利用电子结构数据拟合H2分子的电子壳模型势函数参数

电子壳模型势函数在离子晶体的原子级计算机模拟中有广泛应用,其势参数主要通过拟合晶体的实验数据或电子结构数据得到.提出了通过拟合双原子分子的量子化学从头计算电子结构数据来获得该势函数的方法,并由H2分子的电子结构数据建立了H原子间的电子壳模型势函数.此外,还应用该势函数对H+2分子离子进行了计算.该势函数拟合方案更适合于共价键型的分子.     

Transport Coefficients in Some Stochastic Models of the Revised Enskog Equation

A stochastic model of the revised Enskog equation is considered. A choice of the smearing function suggested by the work of Leegwater is used to apply the model to the repulsive part of the Lennard-Jones potential and the inverse-power soft-sphere potential. The virial coefficients obtained from the equilibrium properties of the models are in excellent agreement with the known exact coefficients for these models. The transport coefficients for the repulsive Lennard-Jones (RLP) model are also computed and appear to be of comparable accuracy to the Enskog-theory coefficients applied directly to a hard-sphere system, although exact results for the RLP with which to make an extensive comparison are not yet available. The pressure and the transport coefficients obtained from the model (shear viscosity, thermal conductivity, and self-diffusion) are compared with the pressure and the corresponding transport coefficients predicted by the Enskog and square-well kinetic theories.     

Hamaker微观连续介质理论的数字密度和Hamaker常数分析

针对Hamaker微观连续介质理论在微观接触力计算中存在的问题,根据Hamaker假设,用连续介质法计算2个原子之间的相互作用力,发现作用力同经典的Lennard-Jones势所反映的作用力不一致.通过分析数字密度,发现数字密度并非是如Hamaker所认为的常数,而是随间距变化的;并得到Hamaker微观连续介质理论仅在间距大于7倍的原子半径时才成立的结论.通过分析Hamaker常数,发现Hamaker常数也随间距变化.     

Calculated line broadening coefficients in the ν2 band of CH3D perturbed by helium

Line broadening coefficients have been calculated, at room temperature, for lines in the P and R branches of the ν₂ band of monodeuterated methane. A properly symmetrized semiclassical model with parabolic relative trajectories has been used. Two interaction potential models have been considered. The first is a Lennard-Jones type atom-atom potential, while the second one was derived from ab initio calculations. The calculated line widths were compared to the available experimental data and a satisfactory agreement was found, although the model contains no other adjustable parameters than the four atomic Lennard-Jones ones. Nonetheless, failures of calculations have also been evidenced for the highest rotational quantum numbers.     

利用电子结构数据拟合H2分子的电子壳模型势函数参数

电子壳模型势函数在离子晶体的原子级计算机模拟中有广泛应用,其势参数主要通过拟合晶体的实验数据或电子结构数据得到.提出了通过拟合双原子分子的量子化学从头计算电子结构数据来获得该势函数的方法,并由H₂分子的电子结构数据建立了H原子间的电子壳模型势函数.此外,还应用该势函数对H⁺₂分子离子进行了计算.该势函数拟合方案更适合于共价键型的分子. 关键词： 电子壳模型势 参数拟合 共价键 2分子')" href="#">H₂分子     

Data-driven techniques to estimate parameters in a rate-dependent ferromagnetic hysteresis model

The quantification of rate-dependent ferromagnetic hysteresis is important in a range of applications including high speed milling using Terfenol-D actuators. There exist a variety of frameworks for characterizing rate-dependent hysteresis including the magnetic model in Ref. [2], the homogenized energy framework, Preisach formulations that accommodate after-effects, and Prandtl–Ishlinskii models. A critical issue when using any of these models to characterize physical devices concerns the efficient estimation of model parameters through least squares data fits. A crux of this issue is the determination of initial parameter estimates based on easily measured attributes of the data. In this paper, we present data-driven techniques to efficiently and robustly estimate parameters in the homogenized energy model. This framework was chosen due to its physical basis and its applicability to ferroelectric, ferromagnetic and ferroelastic materials.