原子掺杂对单元材料结晶能力的影响

2009年我们建立了一个凝结势模型用以预测材料形成单晶体的能力,表明单元体材料(Ni,Al,Cu,Ar,Mg)的结晶能力随凝结势的增大而单调增强.本文将凝结势模型应用于二元材料体系,并结合分子动力学模拟研究了6% Al原子掺杂对于Ni单晶材料结晶能力的影响.模拟结果发现,Al元素的掺杂会大大减弱Ni单晶的结晶能力,在此基础上提出了二元材料体系凝结势的定义,表明凝结势模型可广泛应用于预测二元体材料的结晶能力.     

耐蚀合金Al3Ni和Ni3Al液态冷凝过程的比较研究

采用F S多体势对液态合金Al3Ni和Ni3Al在不同冷却速度下的微观结构及其转变机制进行了分子动力学模拟 ,得到了不同冷速下各温度的双体分布函数 ;采用HA键型指数法对其结构进行了分析 ,结果表明 :Al3Ni在两种冷速下均以非晶的形式出现 ,只是慢冷时体系的有序度略有升高 ;而Ni3Al的结构及能量转变受冷速影响较大 ,快冷时形成非晶 ,而慢冷时出现明显结晶 ;同样冷速下Al含量较少的Ni3Al体系的有序度高 ,更易形成晶体 ,晶体的形成过程中有能量突变 .     

利用超相对论量子分子动力学模型研究交变梯度同步加速器能区Au+Au碰撞中的核阻止效应

运用修正的超相对论量子分子动力学模型研究了交变梯度同步加速器(AGS)能区Au+Au碰撞中的核阻止效应. 该模型考虑了形成和"预形成"粒子的平均场势、核子-核子弹性散射反应截面的介质修正和碎块形成的判断条件. 研究发现: 在AGS能区, 核阻止效应受到形成和“预形成”粒子的平均场势和核子-核子弹性散射反应截面介质修正的影响; 在中心快度区自由质子的产额偏大, 考虑新的碎块形成判断条件后, 此模型的理论计算结果与自由质子的实验结果符合得很好. 关键词： 超相对论量子分子动力学模型 交变梯度同步加速器能区 Au+Au碰撞 核阻止效应     

压力对非晶铜形成影响的分子动力学模拟

用分子动力学模拟法研究了Cu熔体在冷却过程中形成非晶的能力. 结果表明，压力使Cu熔体在冷却过程中形成晶体的倾向增加. 高压下冷却形成的Cu非晶中含有较多的晶态原子团，有序度较高.     

金属Cu熔化结晶过程的分子动力学模拟

采用常温、常压分子动力学模拟技术,研究了在周期性边界条件下,由８６４个Ｃｕ原子构成的模型系统的熔化、结晶过程。原子间相互作用势采用ＥＡＭ势。模拟结果表明：在连续升温过程中,金属Ｃｕ在１５２０Ｋ熔化;以不同的冷速进行冷却,在较慢冷却条件下,液态Ｃｕ在１０１０Ｋ结晶;当冷速较快时,液态Ｃｕ形成非晶态。分析了升降温过程中熔体偶分布函数、原子体积、能量、ＭＳＤ随温度的变化特征。     

液态合金Ni3Al慢冷过程的计算机模拟

采用F-S多体势对液态合金Ni3Al在4×1011K·s-1的冷速下冷却时的微观结构及其转变机制进行了分子动力学模拟,得到了不同冷速下各温度的双体分布函数;采用HA键型指数法和键取向序对其结构进行了分析,结果表明Ni3Al出现明显结晶,晶体的形成过程中有能量突变.模拟得到的扩散系数值与实验值基本吻合.     

MEAM势与Tersoff势比较研究——碳化硅熔化与凝固行为

运用分子动力学方法对比模拟研究了碳化硅的体熔化、表面熔化和晶体生长过程.分别采用MEAM 势和Tersoff势两种势函数描述碳化硅.结果表明:体熔化时,两种势函数描述的SiC的原子平均能量、 Lindemann指数和结构有序参数与温度的变化关系相似,但MEAM势对应的体熔点(4250 K)比Tersoff势(4750 K) 的要高.表面熔化时,两种势函数描述的SiC在相同的过热度下熔化速度相近;而在相同的温度条件下,MEAM 作用的SiC表面熔化速度更快.这是由于MEAM势SiC的热力学熔点(3338 K)低于Tersoff势SiC的热力学熔点 (3430 K)的缘故.两种势函数作用的SiC在晶体生长方面差异很大.MEAM势SiC的晶体生长速度与过冷度有关, 过冷度约为400 K时晶体生长速度最快.但Tersoff势SiC晶体却在过冷度为0—1000 K的范围内均不能生长. 综合考虑,MEAM势比Tersoff势能更好地描述碳化硅的熔化和凝固行为.     

修正屏蔽库仑势下二维尘埃等离子体的动力学和结构特性

本文考虑等离子体密度分布变化, 得到了修正屏蔽库仑势的解析解. 数值分析以及分子动力学模拟表明, 在常见实验室参数情况下, 等离子体密度分布变化引起的屏蔽库仑势修正对二维尘埃等离 子体系统的动力学和结构特性影响很小. 在极限参数情况下, 本模型的计算结果表明二维尘埃等离子体系统的扩散能力明显降低, 并且系统组态呈圆形分布. 此外, 本文还研究了实验室常见大小磁场对二维尘埃等离子体系统的影响. 关键词： 修正屏蔽库仑势 二维尘埃等离子体 分子动力学模拟     

从昇华的NaCl蒸汽生长单晶体

半个世纪以来,从NaCl的蒸汽生长其单晶体的企图并没有得到正面的结果,解理面上的取向性结晶也尚未有超过数个分子层者。对于从溶液结晶出来时NaCl晶面发展的顺序,或者相对的重要性的研究已有肯定的结果。关于晶核或生长驼峰发端的位置从几率上看应在晶面何处的时尚理论未能从NaCl自溶液中结晶出来的实验加以证实或否定。为探索从蒸汽相生长NaCl单晶体的规律,本实验初步获得较大尺度的单晶体,并同时使取向性晶面上结晶达到1毫米以上的厚度。升华蒸汽来源于高纯度的从熔融体结晶出来的单晶体,并使母晶和结晶基面保持在近于NaC     

中能重离子碰撞中库仑相互作用对动量耗散的同位旋效应

利用同位旋相关的量子分子动力学模型，研究了中能重离子碰撞过程中的库仑相互作用对于 动量耗散的同位旋效应.计算结果表明: 由于库仑相互作用的存在，使得碰撞过程中的化学 不稳定区和力学不稳定区减小，明显地减弱了动量耗散；其减弱的程度与对称势的形式及强 度有关，对称势越强则动量耗散的减弱幅度越大.动量耗散灵敏地依赖于核子-核子碰撞截 面的同位旋相关性而较弱地依赖于对称势，这一特征不论有无库仑相互作用均存在. 关键词： 中能重离子碰撞 库仑相互作用 动量耗散 同位旋效应     

Effects of interface kinetics and anisotropy on the stability of the growing crystal face and dissolution face during crystallization from solution under microgravity

The stability of the shapes of the growing crystal face and dissolution face in a two-dimensional mathematical model of crystal growth from solution under microgravity is studied. Effects of the interface kinetics and anisotropy of crystallization and dissolution on the stability are also studied. It is proved that the stable shapes of crystal growth face and dissolution face do exist, which are of suitably shaped curves with their upper parts inclined backward properly no matter whether the interface kinetics and the anisotropy are taken into account or not. The stable shapes of the growing crystal faces and dissolution faces are calculated for various cases. The interface kinetics will make the inclination degree of stable crystal growth face reduce and that of stable dissolution face reduce slightly. The anisotropy of crystallization and dissolution may make the inclination degree of stable-growing crystal face smaller or larger, and that of the stable dissolution face varies very slightly.     

Dynamics of twist point defects with stretching in a polymer crystal

A molecular-dynamics simulation of the behavior of a twist point defect with stretching in a chain of an equilibrium polymer crystal (“united” atoms approximation for polyethylene) is performed for immobile and mobile neighboring chains. It is shown that such a defect in a cold polymer crystal possesses soliton-type mobility. The upper limit of the spectrum of soliton velocities is found, and it is the same for both cases. The maximum possible velocity of defects is three times lower than the theoretical limit of the spectrum (which is equal to the velocity of “ torsional” sound in an isolated chain). An explanation of the reason for this discrepancy is proposed: because of the interaction of two “degrees of freedom” of the defect (twisting and stretching) the energy of a nonlinear wave is dissipated in the linear modes of the system, which results in effective friction whose magnitude depends strongly on the velocity of the defect. The “boundary of the spectrum of soliton velocities” determines the transition between regimes of strong and weak braking of defects.     

Non-Isothermal Crystallization Behaviors of Ethylene Vinyl Acetate Copolymer and Ethylene Vinyl Acetate Copolymer-Graft-Maleic Anhydride

Maleic anhydride (MAH) was melt-grafted on ethylene-vinyl acetate copolymer (EVA) with a grafting degree of 6.3%. Through studying the crystal structure of the resulting EVA-g-MAH, the MAH groups had little influence on crystal structure. According to the Owaza model analysis of the non-isothermal crystallization kinetics of EVA-g-MAH and EVA, the MAH groups had a large blocking effect on crystallization. However, the Mo model showed that the crystallization rate of EVA-g-MAH was slightly faster than EVA. The crystallization activation energy was calculated by the Takhor and Kissinger models, and both results indicated that EVA had a larger effective energy barrier than EVA-g-MAH.     

Structural properties of complex (dusty) plasma upon crystallization and melting

A change in the local order of a bounded complex (dusty) plasma in the process of its crystallization and melting has been examined by molecular dynamics simulations. The dynamics of microparticles is considered in the framework of a Langevin thermostat, the pair interaction between charged particles is described by a screened Coulomb potential (Yukawa potential) with the hard wall potential as a confinement. It has been shown that the beginning of the crystallization of such a system is accompanied by the formation of clusters with the hexagonal close packed (hcp) structure; a noticeable number of these clusters are then transformed to the face centered cubic (fcc) phase. A plasma crystal formed after crystallization consists of the metastable hcp phase, fcc clusters, and a small number of clusters with a body centered cubic (bcc) crystal lattice. Beginning with a certain threshold value of the thermostat temperature, the number of fcc/bcc clusters decreases sharply with increasing temperature, which is an important signature of the beginning of the melting of the plasma crystal.     

Displacements and volume change in anisotropic elastic media due to a point defect

A simple perturbation method is used to calculate displacement vectors in an elastic medium of arbitrary crystal geometry due to a point defect. The resulting volume change is also calculated and a “selection rule” is seen to exist whereby only the external dipole singularity will contribute to the volume change. In addition the defect “strength matrix” is derived, for Bravais and for non-Bravais latices, by comparison of the continuum equations of equilibrium with the suitable particle model equations.     

Helium blistering of niobium: Large swelling measurements supporting the gas pressure model

Crystal potentials are usually obtained through a combination of theoretical calculation and the fitting of crystal properties. Because of thermal expansion of the crystal, the equilibrium condition is not satisfied at all temperatures unless the crystal potential is made temperature-dependent. A quasiharmonic potential that satisfies the equilibrium condition at all temperatures is derived from KC1 and used to calculate temperature-dependent defect energies (at constant volume). Other thermodynamic defect parameters are then derived by combining these results with the experimental Gibbs energies. The temperature-dependence of these parameters is displayed.     

Fullerene solar cells with cholesteric liquid crystal doping

This paper reports the doping effect of cholesteric liquid crystal 3β-Hydroxy-5-cholestene 3-oleate on polymer solar cells composed of the poly 3-hexyl thiophene and the fullerene derivative.With a doping ratio of 0.3 wt%,the device achieves an ideal improvement on the shunt resistor and the fill factor.Compared with the reference cell,the power conversion efficiency of the doped cell is improved 24%.The photoelectric measurement and the active layer characterization indicate that the self-assembly liquid crystal can improve the film crystallization and reduce the membrane defect.     

Phase transitions in a two-dimensional vortex system with defects: Monte Carlo simulation

AbstractThe phase states and phase transitions in a system consisting of a two-dimensional vortex lattice with defects are studied by the Monte Carlo method. It is shown that a “rotating lattice” phase, which is an intermediate phase between the vortex crystal and vortex liquid phases, is present. The dependence of the temperature of the transition from the rotating lattice phase into a vortex liquid on the strength of the defect potential is determined. The current-voltage characteristics of the system are calculated at various temperatures for point, square, and linear defects. It is shown that the phase state of the system strongly affects its transport properties.     

Magneto-optical activity of a one-dimensional photonic crystal with a magnetic defect

The influence of a magnetic defect on the field distribution and magneto-optical properties of a one-dimensional photonic crystal has been investigated. It has been shown that the maximum localization of the wave field in the defect layer is achieved in an asymmetric photonic crystal structure. A greater Faraday rotation, which significantly exceeds the angle of rotation of the polarization plane in an isolated magnetized layer, and a higher degree of localization of the wave field can be achieved when the magnetic layer is surrounded by layers of photonic crystal mirrors with a lower refractive index. An increase in the Faraday rotation angle is determined not only by an increase in the thickness of the magnetic defect but also by a symmetric increase in the number of periods in the photonic crystal mirrors.