小尺寸金属团簇熔化过程的分子动力学模拟

运用分子动力学方法模拟了小尺寸金属团簇的熔化过程, 原子之间的作用采用嵌入原子法(EAM)模型, 计算了均方根键长涨落δ随温度的变化, 以及升温过程中团簇热容的变化. 包含55、56个原子的面心立方(FCC)结构Au团簇的熔化过程是基本相同的. 而同样结构和数目Cu团簇的熔化过程却呈现出不同的趋势. Cu55、Cu56在模拟过程中都出现了FCC结构到二十面体结构的转变. 但由于表面多出了一个原子, Cu56的热容曲线比Cu55多了一个峰, 体系出现了预熔化现象. 这表明小尺寸团簇的固液转变的过程与团簇的原子类型、几何结构和原子数目密切相关.     

(AgI)n团簇熔化行为的分子动力学模拟

用遗传算法结合经验势搜索了(AgI)n(n=3-15)团簇的可能稳定结构, 并用微正则分子动力学方法研究了它们的熔化行为. (AgI)n团簇的稳定结构主要以四元环和六元环相接的笼状结构为主. 大多数(AgI)n会在一个较大的温度范围内随温度升高结构不断扭曲, 原子间距涨落及动能涨落不断增大, 直到在某个温度下熔化, 结构变得完全无序. (AgI)6的结构具有很高的对称性, 熔化发生在一个较窄的温度范围. 对于(AgI)5, 会在熔化前较大的温度范围内发生最稳定结构与能量较高的环状异构体之间的转化, 并可能出现负热容现象.     

Molecular Dynamics Investigation of Differences in Melting Behaviors of Cu57 and Cu58 Clusters

Within the framework of the embedded-atom method, we performed molecular-dynamics calculations to investigate the structural transformation during melting of two copper clus- ters containing 57 and 58 atoms. The simulation results reveal how their different structural changes can strongly influence internal energy and radial distribution functions. The local structural patterns of different regions during the temperature increase, determined by atom density profiles, are identified for the melting of each cluster. The simulations show sensi- tivities of the structural changes for these two small size clusters with different structures.     

Infuence of New Interaction Potential on MD Simulation of MgSiO3 Perovskite Thermodynamic Properties

The interaction potential plays an important role in molecular dynamics (MD) simulations. Pair potentialhas been used to simulate the melting temperature of MgSiO3 perovskite in previous studies, but considerablediscrepancy of melting temperature exists between these simulations. Comparisons of potential energy curvesare performed to explain the discrepancy. To further investigate the infuence of the interaction potentialparameters on the MD simulation result, a new set of potential parameters is developed based on two fitting potential parameters of previous studies, and is applied in the present study. The simulated molar volume MgSiO3 perovskite agrees well with the study by Belonoshko and Dubrovinsky at ambient condition. The equations of state, constant-pressure heating capacity and the constant-pressure thermal expansivity of MgSiO3 perovskite are close to the experimental data. Calculated melting temperatures are also comparable with those derived from previous studies.     

高压下MgO熔化特性的分子动力学模拟

Molecular dynamics simulation was used to study the melting of MgO at high pressures. The melting temperature of MgO was accurately obtained at elevated temperature and high pressure after corrections based on the modern theory of melting. The calculated melting curve was compared with the available experimental data and other theoretical results at the pressure range of 0-135 GPa. The corrected melting temperature of MgO is in good agreement with the results from Lindemann melting equation and the twophase simulated results below 15 GPa.     

金属Ni熔化前后结构变化的分子动力学模拟

使用Tight-binding势函数, 对FCC-Ni升温熔化过程的结构变化进行了分子动力学模拟. 在定压条件下模拟得到的Ni的熔点在1850 K与1900 K之间. 计算得到了体系在各温度下的径向分布函数和配位数分布等静态结构信息以及动力学性质. 计算得出的液体Ni的扩散系数在1900 K时约为5.02×10−9 m2•s−1, 与实验数据相符. 对液态体系中FCC短程有序结构可能发生的畸变以及由此导致的H-A键型变化进行了分析, 结合配位体构型搜索和键对分析方法计算了各温度下不同短程有序结构的分布. 计算表明, Ni在熔化之后仍保留有部分晶态短程结构, 但发生了较大的畸变, 同时液态中有少量的缺陷二十面体结构存在. 而液体Ni中大多数的配位体的几何构型介于FCC与缺陷二十面体之间.     

Molecular van der Waals symmetry affecting bulk properties of condensed phases: melting and boiling points

Qualitative analysis of the trends in melting temperatures T _m and boiling temperatures T _b of organic compounds and related substances supports a concept of van der Waals symmetry suggested earlier by the authors. Data for substituted methanes, ethanes, ethylenes, benzenes and cyclohexanes, reveal linear T _b versus M ^α patterns but usually non-uniform T _m versus M ^α patterns (where M is the molecular mass, α varies from 0.5 to 1). In the same set of isomers, T _m and T _b often change in different succession. To explain the observed trends, a local van der Waals field U that acts on a molecule in a solid or liquid phase, was represented by a sum of an averaged long-range field U ₀ from all molecules, and a short-range “contact” term W determined by a molecular shape. Variations of T _m and T _b among closely related substances correspond to different strength of perturbation of U ₀ ^(C) (in a crystal) and U ₀ ^(L) (in a liquid) by W. Higher T _b of isomers with medium-symmetry molecules (like 1,2-disubstituted benzenes and cyclohexanes) reflect the better fitting of their molecules to a local molecular environment in a liquid (that contains vacancies). Highly symmetrical quasispherical molecules (e.g., C₂Cl₆ or C₆F₁₂) produce a stabilised solid and destabilised liquid state, hence their molecular crystals easily sublime rather than melt at ambient pressure. Dedicated in memory of Professor Petr M. Zorky     

中介尺度Au纳米团簇熔化的分子动力学模拟

采用分子动力学模拟技术，研究了原子个数为16～8628的 Au纳米团簇的熔化过程.采用 Johnson的EAM (embedded atom method) 模型,模拟结果表明，金属纳米团簇存在一中介尺度区域.对Au纳米团簇而言，当原子个数N >456时，团簇的热力学性质与团簇尺寸呈线性关系，熔化首先从表面开始，逐步向中心区域推进，且满足Tmb－Tmc(N)＝aN(－1/3)的关系.另外，计算了中介区域的团簇的尺寸、熔化温度、表面能、熵、焓等热力学量以及均方根位移（RMSD）等动力学量，为研究纳米团簇提供定量数据.     

离子液体[emim]Br熔点的分子动力学模拟

运用分子动力学模拟, 采用直接加热法和微正则(NVE)系综法计算离子液体[emim]Br的熔点, 以期获得较好的熔点预测方法. 直接加热法通过分析体系的非键合能、密度、径向分布函数、扩散系数和平动序参数随温度的变化关系判断熔点; NVE系综法则通过获得固液共存体系判断熔点. 直接加热法中, 体系易出现过热问题; NVE系综法则能有效克服过热问题, 是在模拟研究中应优先选择的离子液体熔点预测方法.     

Room-temperature spin-transition iron compounds

Abstract  Since the discovery of the spin transition phenomenon in tris(N,N-dialkyldithiocarbamato) iron(III) complexes [1], numerous investigations have been devoted to this field of molecular magnetism. The spin transition phenomenon is probably the most spectacular example of bistability in molecular chemistry. However, it is a challenge to obtain spin transition materials when working under ambient conditions (e.g. room temperature and pressure), which would be highly advantageous for potential applications. So far, only some Fe(II) and Fe(III) molecular systems have shown temperature-induced spin transitions around and even above room temperature. Within this review we discuss the characteristics of this class of bistable compounds in detail and we try to draw more general conclusions regarding the integration, implementation and application of spin transition compounds as switching elements in hybrid molecular devices. Graphical abstract        

金属银在高升温速率下的熔化和过热行为

采用分子动力学方法和QSC(quantum Sutton-Chen)力场研究了升温速率对金属银的熔化和过热行为的影响.模拟中考虑了缺陷和表面对熔化和过热行为的影响.研究结果表明,升温速率对金属银的熔化和过热行为有很大影响,随着升温速率的升高,金属银的熔点有所升高.高的升温速率会导致金属银体系内部无序化程度增加,降低了熔化相变的能垒.升温速率导致的银完美晶体的过热极限大约为1450 K.     

升温速率对金属铅的熔化和过热行为的影响

采用分子动力学方法和QSC(quantum Sutton-Chen)力场研究了升温速率对金属铅的熔化和过热行为的影响.模拟中考虑了缺陷和表面对熔化和过热行为的作用.结果表明，升温速率对金属铅的熔化和过热行为影响很大，随着升温速率的升高，金属铅的熔点有所升高.快的升温速率会导致金属铅体系内部无序化程度增加，进而使体系能量增加，降低了熔化相变的能垒.升温速率导致的金属铅的过热极限大约为780 K.     

Size dependent structural and electronic properties of MgO nanotube clusters

MgO nanotube clusters which cross sections are composed of two‐, three‐, four‐, and five‐membered rings are constructed and studied by the density functional theory at B3LYP/6‐31G(d) level. The variations of bond length present anisotropic effect. Three‐membered ring nanotube cluster is the most stable tube among these MgO isomers. Mixed covalent and ionic bonding always exists in MgO nanotube clusters. With increasing length of MgO nanotube clusters, the averaged atomic charge increases, and converge to 1.227; the s‐p separation of O bands decreases; whereas energy gap nearby frontier orbitals present dramatic difference corresponding to various structure family. It is possible that MgO nanotube clusters show electronic properties of semiconductor. An interpretation for MgO nanotube clusters fabricated by simply thermal methods is proposed. The structural and electronic properties of MgO nanotube clusters are discussed systematically in details. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

SrF2和BaF2熔化温度的分子动力学模拟

利用壳层模型分子动力学方法,考虑萤石结构分子中的预熔化现象,对SrF2和BaF2的分子动力学模拟熔化温度进行修正,获得了高压下SrF2和BaF2的熔化温度．同时给出了300K、0．1MPa-7GPa和10．1MPa-3GPa时SrF2和BaF2的状态方程,与已有研究结果的最大误差分别为0．3％和2．2％．计算所得SrF2和BaF2常压下的熔点与已有的实验结果符合较好．对于SrF2和BaF2分子体积变化和已有的熔化模拟的差别也做了比较和讨论．     

Quasi-isothermal measurement of frequency dependent heat capacity of semicrystalline polyethylene at the melting temperature using light heating modulated temperature DSC

Frequency dependence of the heat capacity of semicrystalline polyethylene was measured using light heating modulated temperature DSC (LMDSC). The quasi-isothermal measurement was carried out in the melting temperature range of the crystal. Two decades of the frequency, from 0.01 to 1 Hz, was covered by the LMDSC instrument constructed in the author's laboratory. It was found that in the melting temperature range polyethylene exhibited Debye relaxation with the relaxation time of 14 s and had excess heat capacity independent of the kinetics. The excess heat capacity and the relaxation strength could be attributed to the disordered crystals generated during rapid cooling from the molten state and/or its surrounding amorphous region instead of the stable crystals reorganized during the quasi-isothermal measurement.     

Nucleation Mechanism of Iron in an External Magnetic Field

In this work, the solidification of liquid iron with or without external magnetic field was investigated by using two molecular dynamics methods, namely direct cooling and two-phase simulation. The influence of external magnetic field on the solidification is characterized by the critical temperature and radial distribution functions. Our computational results show that under external magnetic field, the solidification point tends to decrease significantly. By further analyzing the diffusion coefficients and viscosity, we attribute the effect to the stronger fluctuation of liquid iron atoms driven by the external magnetic field.     

The mechanism of reduction of iron oxide by hydrogen

Precipitated iron oxide samples were characterized using temperature-programmed reduction. H₂ was used as the reduction agents. The two-stage reduction was observed: Fe₂O₃ was reduced to Fe₃O₄ and then reduced to metallic Fe. The activation energy for the two reduction steps of iron oxide are 89.13 and 70.412 (kJ mol⁻¹), respectively. The simulation by reduction models of the TPR patterns presents well fitting of unimolecular model for Fe₂O₃→Fe₃O₄ reduction and two-dimensional nucleation according to Avarmi–Erofeev model for Fe₃O₄→Fe.     

Crystallite orientation during melting of oriented ultra-high-molecular-weight polyethylene

The changes in crystallite orientation during melting of oriented ultra-high-molecular-weight polyethylene (UHMW PE) were investigated by means of wide-angle X-ray scattering. The orientation distribution of crystallites in drawn UHMW PE is composed of two components differing in width. The narrow and broad components revealed in this study indicate the existence of two classes of crystallites with different orientability. Some of the crystallites are oriented almost perfectly even at low-draw ratios, while the others do not orient so effectively. The analysis of melting behaviour of such a texture composed of orthorhombic crystals indicates that highly oriented crystallites are formed by taut molecules and transform first to the hexagonal phase, while the molecules constituting low-oriented crystallites melt directly to the typical amorphous phase. The increase in orientation of highly oriented crystallites during their partial melting, observed in the samples kept at constant length and even those allowed to shrink under constant load, can be explained by the kinetic factor proposed by Ziabicki. Received: 11 September 1998 Accepted in revised form: 18 February 1999     

Synthesis and properties of oligomers of iron-manganese carbonyl complexes with bridging disulfido ligands

The reactions of activated CpFeMn(CO)₇1a and Cp*FeMn(CO)₇1b, Cp=C₅Me₅ with thiirane yielded the new dimeric mixed metal disulfido complexes: [CpFeMn(CO)₅(μ₃-S₂)]₂ (2) and [Cp*FeMn(CO)₅(μ₃-S₂)]₂ (3). Compounds 2 and 3 both contain two triply bridging disulfido ligands. When heated at 40 °C, compound 2 was transformed into a trimeric compound Cp₃Fe₃Mn₃(CO)₁₅(μ₃-S₂)(μ₄-S₂)₂, 4. Compound 4 contains three disulfido ligands, each of which has a different bridging coordination mode in the six atom metal cluster. There are three inequivalent CpFe(CO)₂ groupings linked to a central Mn₃(S₂)₃ core by the disulfido ligands. In solution, compound 4 exhibits a dynamical intramolecular exchange process that interconverts two of the three CpFe(CO)₂ groups on the NMR timescale.