压强对液态InGaAs快速凝固过程中微观结构变化的影响

采用分子动力学方法模拟不同压强下液态InGaAs的快速凝固过程,并采用双体分布函数、键角分布函数、配位数统计以及可视化等方法,从微观结构的不同层面分析了压强对凝固过程微观结构的影响机制。结果表明：对于InGaAs体系,压强对最近邻和次近邻的原子排布都有影响,但对次近邻原子排列的影响更为明显,通过次近邻原子键角的调整,使得原子排列更加紧密,体系的短程有序度增强。在原子的配位数结构上,随着压强的增加,部分三配位向四配位发生转变,从而使整个体系达到致密的结构。     

不同冷速下SiGe纳米液滴快速凝固过程中的微观结构演变

采用分子动力学方法模拟了7种不同冷速下Si50Ge50纳米液滴快速凝固过程,利用双体分布函数、原子平均能量、最大标准团簇分析法、键角分布函数、二面角分布及可视化方法等对体系微观结构的演变进行了分析。研究结果表明: 当冷速R≥1×1010 K/s时,系统形成非晶态结构; R≤5×109 K/s时,系统形成以闪锌矿和纤锌矿结构相互嵌套的近似球形的纳米晶体,且纳米液滴起始结晶温度随冷速降低而降低。     

用FS多体势模型模拟金属铜的冷却过程

采用常温、常压分子动力学模拟方法模拟了在周期性边界条件下由500个原子构成的液态Cu模型系统的凝固过程,考察了不同降温速率下液态Cu的凝固行为.模拟结果很好地重现了实验值.模拟中原子间作用势采用FS势,结构分析采用双体分布函数、对分析技术、内能、均方位移等方法,得到了原子体系微观结构组态变化的重要信息,并利用能量分析的方法对体系微观结构的变化进行了说明.     

热镀铜层冷凝过程的分子动力学模拟

采用常温、常压分子动力学模拟方法和FS(Finnis Sinclair)势 ,研究了在周期性边界条件下由 5 0 0个原子构成的液态Cu模型系统的凝固过程 ,考察了不同降温速率下Cu的凝固行为 ,得到了不同温度、不同冷却速率下Cu的双体分布函数 ;采用HA键型指数法统计了各种小原子团在不同温度下所占比例 ,采用键取向序分析了体系降温全过程的局域取向对称性 ,得到原子体系微观结构组态变化的重要信息 ;最后 ,利用能量分析的方法对体系微观结构的变化进行了说明 ,给出了液态Cu冷凝过程中微观结构转变的重要信息 .     

液态金属凝固过程原子团簇结构表征的新方法（英文）

采用分子动力学方法对液态金属钠的凝固过程进行了模拟计算,运用团簇结构表征新方法――团簇类型指数法(CTIM)对凝固过程中的团簇结构进行了识别.为了阐明CTIM在识别团簇结构上的准确性和效率,将其与广为采用的Voronoi多面体方法 (VPM)进行比较.结果表明:当采用CTIM和VPM分别对液态金属钠凝固结构中的原子团簇结构进行表征时,它们所得到的微观结构特征是一致的.非晶态结构中,原子团簇类型的分布呈现明显的区段特征,每一区段都存在一种主要团簇类型,它们分别是二十面体或其缺陷结构.晶体结构中,体系形成以BCC团簇为主体的晶态结构.同时发现,VPM不易区分不同团簇构型之间的细微差别,不同构型的原子团簇可能被归为同种结构类型;而CTIM根据近邻原子之间相对位置关系,直接准确描述原子团簇的构型.不但由CTIM分析获得的凝固体系结构特征与VPM的分析结果一致;而且CTIM抓住了体系微观结构特征的主要方面,简化了团簇结构的表征形式,这在大尺度模拟体系的结构分析中将具有较高效率.     

物理截断与电子局域函数结合法研究非晶态结构中的原子成键

本文将常用的键长物理截断方法与电子局域函数方法相结合,用于分析共价系统的非晶态结构,得到了更合理的原子成键信息,提高了非晶态结构模型分析的可靠性.以相变存储材料GeTe合金为例,通过采用上述方法详细研究了GeTe合金非晶态中原子间成键及结构信息,确定了其合理的成键物理截断长度为3.05?,电子局域函数阈值为0.63.研究结果显示,当电子局域函数的数值由0.58逐渐增大至0.63时,结构中所截断的化学键大部分是强度较弱的非同质键(即Ge—Te键),而强度较强的Ge—Ge键的数量几乎不变.对Ge原子配位数和其键角分布等结构信息的分析表明,Ge原子以3配位和4配位为主,其中3配位的Ge原子主要是以缺陷八面体形式存在,而4配位的Ge原子则主要以四面体的形式存在.此外,在本研究工作中所建立的确定成键物理截断长度及电子局域函数阈值的方法也可以应用于其他具有共价键性质的非晶态材料研究.     

CaO-Al2O3-SiO2熔渣结构和热导率的分子动力学模拟

利用分子动力学模拟研究CaO-Al₂O₃-SiO₂三元熔渣的微观结构和热导率。针对六组不同Al₂O₃含量的熔渣,基于BMH势函数分析了径向分布函数、键长、键角分布函数、氧类型和微结构单元Qⁿ等微观结构信息。结果表明硅和铝在熔渣中以四面体网络结构存在,硅氧四面体较铝氧四面体更为稳定。随着铝含量增加,熔体中的键长和四面体中心角无明显变化,Al-O-Al和Si-O-Al键角逐渐减小;网络中的桥氧(BO)和三配位氧(TO)含量增加,Q³和Q⁴含量增多,在高铝区出现较多的五配位铝(Al^V)。采用Muller-Plathe方法计算了熔渣的热导率,随着铝替代硅原子,网络的聚合程度增加,强化了熔体的热输运能力,热导率呈现增大趋势。     

Au20Si80 合金类Si结构的模拟研究

采用分子动力学模拟方法研究液态富硅Au20Si80 合金的快速凝固过程.在此基础上,使用双体分布函数、配位数、最大标准团簇、能量和三维可视化等方法研究Au-Si合金在凝固过程中的微观结构规律.结果指出,体系中形成种类繁多的非常规非晶结构,以类Si结构为主.类Si结构具有如下特点:中心原子几乎都为Si;以Si原子为中心的类Si-A、类Si-B、类Si-C、类Si-D配位数为5,其外围原子元素组成类型主要为4个Si和1个Au;以Si原子为中心的类Si-E配位数为4,其外围原子元素组成类型主要为4个Si;五种类Si结构的稳定性程度由高到低顺序依次为类Si-E、类Si-B、类Si-C、类Si-A、类Si-D.这为研究过渡金属与半导体组合的非晶结构提供新的分析思路和方法     

冷速对液态合金Ca50Zn50快速凝固过程中微观结构演变的影响

采用分子动力学方法对六种不同冷速对原子尺寸相差较大的液态合金Ca50Znso凝固过程中微观结构演变的影响进行了模拟研究,并采用双体分布函数、Honeycutt—Andersen（HA）键型指数法、原子团类型指数法（CTIM一2）、可视化等方法进行了深入分析,结果表明：系统存在一个临界冷速,介于和5×10^11K／s与1×10^11K／s之间,在临界冷速以上（如1×10^11K／s,1×10^11K／s,1×10^11K／s和5×10^11K／s）时,系统形成以1551,1541,1431键型或二十面体基本原子团（12012000）等为主体的非晶态结构;在临界冷速以下时,系统形成以1441和1661键型或bcc基本原子团（1460800）为主体（含有少量的hcp（1200066）和fcc（12000120）基本原子团）的部分晶态结构．在非晶形成的冷速范围内,其总双体分布函数的第一峰明显分裂成与近邻分别为Zn—Zn,Ca—Zn,Ca—Ca相对应的三个次峰;且随着冷速的下降,同类原子近邻的次峰峰值升高、异类原子近邻的次峰峰值下降;Zn原子容易偏聚,随着冷速降低,二十面体的数量增多,非晶态结构也越稳定．在晶态形成的冷速范围内,Zn原子己大量偏聚形成大块bcc晶态结构,Ca原子也部分形成hcp和fcc晶态结构．     

熔体初始温度对液态金属Ni凝固过程中微观结构演变影响的模拟研究

采用量子 Sutton-Chen多体势, 对熔体初始温度热历史条件对液态金属Ni快速凝固过程中微观结构演变的影响进行了分子动力学模拟研究. 采用双体分布函数g(r)曲线、键型指数法、原子团类型指数法和三维可视化等分析方法对凝固过程中微观结构的演变进行了分析. 结果表明: 熔体初始温度对凝固微结构有显著影响, 但在液态和过冷态时的影响并不明显, 只有在结晶转变温度T_c附近才开始充分显现出来. 体系在1×10¹² K/s的冷速下, 最终均形成以1421和1422键型或面心立方(12 0 0 0 12 0)与六角密集(12 0 0 0 6 6) 基本原子团为主的晶态结构. 末态时, 不同初始温度体系中的主要键型和团簇的数目有很大的变化范围, 且与熔体初始温度的高低呈非线性变化关系. 然而, 体系能量随初始温度呈线性变化关系, 初始温度越高, 末态能量越低, 其晶化程度越高. 通过三维可视化分析进一步发现, 在初始温度较高的体系中, 同类团簇结构的原子出现明显的分层聚集现象, 随着初始温度的下降, 这种分层现象将被弥散开去. 可视化分析将更有助于对凝固过程中微观结构演变进行更为深入的研究. 关键词： 液态金属Ni 熔体初始温度 微观结构 分子动力学模拟     

Correlation between liquid structure and glass forming ability in glassy Ag-based binary alloys

The atomic structures of liquid Ag-based binary alloys have been investigated in the solidification process by means of X-ray diffraction. The results of liquid structure show that there is a break point in the mean nearest neighbor distance r1 and the coordination number Nmin for glass-forming liquid, while the correlation radius rc and the coordination number Nmin display a monotone variational trend above the break point. It means glass-forming liquids have a steady changing in structure above liquidus a...     

Structure of Y and Zr Segregated Grain Boundaries in Alumina

Grain boundary segregation of Y and Zr in -Al₂O₃ and the atomic structural environment around the Y and Zr atoms have been investigated using high resolution STEM and EXAFS. At dilute concentrations, the Y ions in -Al₂O₃ grain boundaries, on average, are coordinated by 4 oxygens, at a distance of 2.30 Å, which corresponds nearly to the Y-O bond length in cubic Y₂O₃, and Zr ions are coordinated by 5 oxygens at a distance of 2.14 Å, which is approximately the same as the average Zr-O bond length in monoclinic ZrO₂. However, in the EXAFS radial distribution function, the Y-cation and Zr-cation next nearest neighbor shell cannot be clearly identified. These results suggest that Y and Zr at dilute concentrations in -Al₂O₃ occupy grain boundary sites with well defined nearest neighbor cation-oxygen bond lengths similar to those in their parent oxides, but with the next nearest neighbor cation-cation distances varying considerably from site to site. Grain growth can cause grain boundaries to become supersaturated by Y. In this case, both the Y-O nearest neighbor coordination number and the ordering of Y with respect to Al ions beyond nearest neighbor O are increased. This Y-Al distance is the same as that expected for the Y-Al distance when Y substitutes for Al while relaxing the Y-O distance to that in Y₂O₃. This may indicate that as the Y concentration increases, Y begins to occupy near-boundary sites in planes on each side of the geometrical boundary. In these near-boundary planes, the nearest neighbor ordering extends at least to nearest neighbor cations. Nucleation of the YAG phase leads to the depletion of these partially ordered layers.     

"Devil's staircase"-type phase transition in NaV2O5 under high pressure

The "devil's staircase"-type phase transition in the quarter-filled spin-ladder compound NaV2O5 has been discovered at low temperature and high pressure by synchrotron radiation x-ray diffraction. A large number of transitions are found to successively take place among higher-order commensurate phases with 2a x 2b x zc type superstructures. The observed temperature and pressure dependence of modulation wave number q(c), defined by 1/z, is well reproduced by the axial next nearest neighbor Ising model. The q(c) is suggested to reflect atomic displacements presumably coupled with charge ordering in this system.     

Local structure of supercooled liquid Ga90In10 alloy

The local structure of liquid and supercooled liquid Ga₉₀In₁₀ alloy was studied by the X-ray absorption fine-structure (XAFS), X-ray diffractometer (XRD) and the ab initio molecular dynamics (AIMD) simulation. The ab initio FEFF8 code was applied to simulate the In k-edge X-ray absorption near edge structure (XANES) spectrum of the melt. The local atomic arrangement models have been obtained. The results indicate that the structural evolution from the liquid to supercooled liquid Ga₉₀In₁₀ alloy is mainly caused by the change of the In atom local structure. As the temperature decreases, the nearest neighbor coordination number decreases and the nearest neighbor interatomic distance increases. The nearest neighbor coordination number around In atom decreases from 13 to 12, and there is obvious Ga aggregation under the undercooled condition.     

EXAFS study of the atomic structure of amorphous Tb20Fe80

The atomic structure of amorphous Tb₂₀Fe₈₀ thin films has been studied by Extended X-ray Absorption Fine Structure (EXAFS) of both FeK and TbL _III absorption edges. The local site geometry around Fe atoms shows predominantly Fe nearest neighbors with an Fe-Fe distance distribution centered on 2.50±0.02 Å and a coordination number of 9.1±1. In contrast, the radial structure function (RSF) obtained at the Tb edge is broad and asymmetric. The peak in the RSF corresponds to a Tb-Fe near neighbor distance of 2.94±0.1 Å with no evidence for Tb-Tb nearest neighbor coordination. The width and the shape of the RSF suggest that the Tb-Fe atomic environment is anisotropic and strained probably as a consequence of the growth process. This distorted atomic environment is suggested to be responsible for the magnetic anisotropy in these alloys. Thermal annealing at 200 °C leads to reduction inK _u. We propose that this results from reordering of the Tb local environment such that the average structural anisotropy in the distribution is reduced. EXAFS data show that annealing at 400°C causes precipitation of bcc polycrystalline Fe. The addition of 7 at.% Au to the alloy prevents this recrystallization and preserves the amorphous state but does not prevent the structural relaxation which reducesK _u at lower temperatures.     

Entanglement in the three-qubit Heisenberg model with next nearest neighbor interaction and a nonuniform magnetic field

Pairwise thermal entanglement in the three-qubit XXX Heisenberg model with next nearest neighbor interaction and a nonuniform magnetic field has been studied. It's found that the next nearest neighbor interaction has a great effect on the entanglement between the next nearest neighbor sites, but has slight effect on the nearest neighbor entanglement (NNE). Applying a magnetic field at the middle site enhances the next nearest neighbor entanglement (NNNE) sharply when there is a small field at the side sites and the next nearest neighbor coupling constant is positive. A staggered magnetic field helps to maintain nearest neighbor entanglement obviously.     

Magnetic structure factors Heisenberg model for magnetic ordered graphene like structure

We have theoretically studied the magnetic structure factors of Heisenberg model on honeycomb lattice in the presence of anisotropic Dzyaloshinskii–Moriya interaction and next nearest neighbor coupling exchange constant. A sublattice antiferromagnetic long range ordering has been considered for localized electrons on honeycomb lattice structure. In particular, the frequency dependence of both longitudinal and transverse dynamical spin susceptibilities has been investigated for various physical parameters in the model Hamiltonian. Using Holstein–Primakoff bosonic transformations, the behavior of magnetic susceptibilities properties has been studied by means of excitation spectrum of mapped bosonic gas. Furthermore we have studied the dependence of static spin susceptibilities on Dzyaloshinskii–Moriya interaction strength for various next nearest neighbor interaction strengths. We have found the dependence of static longitudinal spin structure factor on Dzyaloshinskii–Moriya interaction strength shows a divergence behavior at phase transition point for various next nearest neighbor exchange constants. Also our results show the position of peak in the dynamical transverse spin structure factor at fixed value for Dzyaloshinskii Moriya interaction moves to lower frequency with next nearest neighbor coupling constant.     

Thermodynamic properties of Heisenberg model for magnetic ordered graphene sheet

We study the thermodynamic properties of two dimensional Heisenberg antiferromagnet on the honeycomb lattice in the presence of anisotropic Dzyaloshinskii-Moriya interaction and next nearest neighbor coupling exchange constant. A sublattice antiferromagnetic long range ordering has been considered for localized electrons on honeycomb lattice structure. In particular, the temperature dependence of specific heat has been investigated for various physical parameters in the model Hamiltonian. Using Holstein-Primakoff bosonic transformations, the behavior of thermodynamic properties has been studied by means of excitation spectrum of mapped bosonic gas. Furthermore we have studied the dependence of specific heat and magnetization on Dzyaloshinskii-Moriya interaction strength for various next nearest neighbor interaction strengths. At low temperatures, the specific heat is found to be monotonically increasing with temperature. We have found the dependence of specific heat on Dzyaloshinskii-Moriya interaction strength shows a monotonic increasing behavior for various next nearest neighbor exchange constants. Also we have studied the temperature dependence of staggered magnetization for different next nearest neighbor coupling constants. Our results show the critical temperature moves to higher amounts with reduction of Dzyaloshinskii-Moriya interaction strength.     

DETERMINATION OF THE Mn2+-F- BOND LENGTH USING A COMPLETE SPECTRUM CRITERION AND A NEXT NEAREST NEIGHBOR MODEL

A complete spectrum criterion and a next nearest neighbor model are proposed. Research indicates: 1) Using this criterion can remove the long existing puzzle that different parameters produce different bond lengths. The criterion greatly improve the reliability of optical method for determining the Mn²⁺-F^- distance R. 2) The exponent n and coefficient K in the inverse power law 10Dq=KR^-n are obviously affected by next nearest neighbor. 3) Considering the effect can lead to a better result, the next nearest neighbor model is thus more strict and reliable.