Effect of Mutual Positions of Individual Contacts on the Overall Resistance and Elastic stiffness of a Cluster of Contacts

The paper provides statistical analysis on the effect of mutual positions of individual contacts on the overall resistance and incremental elastic stiffness of a cluster of contacts. We consider an example of a cluster with 76 contact spots of the same radii. The regular lattice of the contacts is compared with the perturbed ones (random clusters). The constriction resistance is determined as a sum of self resistance of the individual contacts and interactions between them. It is shown that the effect of perturbations is very small and, therefore, the mean distance between the centers of individual contacts can be used to estimate the overall cluster resistance. Using elasticity-conductivity cross-property connections, this result is transferred to the incremental elastic stiffness of the cluster. Thus, we established a correspondence between cluster of regular structure and cluster of randomly located contacts. The geometrical parameters governing elastic compliance and spreading resistance of the clusters are number of contact spots and the average distance between individual contacts.     

基于社会网络理论的集群创新网络中的创新扩散行为

利用社会网络理论构建了集群网络中的创新扩散模型,运用仿真技术分析了集群创新网络结构对创新扩散行为的影响,研究表明集群网络结构使得创新信息的扩散效率更高、创新扩散过程的速度更快。集群网络的社会影响对集群创新扩散速度产生重要影响,当集群主体对社会影响的敏感程度较低时,社会影响有助于创新信息在集群网络中的扩散,而当集群主体对社会影响的敏感程度较大时,社会影响将会阻碍创新信息在集群网络中的扩散。     

Combining Maxwell’s methodology with the BEM for evaluating the two-dimensional effective properties of composite and micro-cracked materials

Maxwell’s methodology is combined with the boundary element method (BEM) for evaluating the two-dimensional effective elastic properties of composite, porous, and microcracked isotropic materials with periodic or random structure. The approach is based on the idea that the effective properties of the material can be deduced from the effects that a cluster of fibers, pores, or cracks embedded in an infinite matrix has on the far-fields. The fibers, pores, or cracks can have arbitrary shapes, sizes, and elastic properties, provided that the overall behavior is isotropic, and their effects are assumed to be the same as those of an equivalent circular inhomogeneity. The key aspect of the approach is to precisely account for the interactions between all the constituents in the cluster that represent the material in question. This is done by using the complex-variables version of the BEM to solve the problem of a finite cluster of fibers, pores or cracks embedded an infinite isotropic, linearly elastic matrix. The effective properties of the material are evaluated by comparing the far-field solutions for the cluster with that of the equivalent inhomogeneity. It is shown that the model adequately captures the influence of the micro-structure of the material on its overall properties.     

On the Mechanism of the BCC-HCP Transformations in Small Lennard-Jones Crystals

The structure relaxation of the initially body-centered cubic clusters to the most favorable configurations has shown that the transition to the hexagonal close-packed structure starts at the cluster surface and propagates to the cluster center. The intermediate orthorhombic structure was found and forms in application to helium the regions with delocalized atoms, which are presumably important in connection with the phenomenon of ‘supersolidity’. The time to complete the transition depends on the cluster size and shape.      

Molecular dynamics investigations of structural changes accompanying with freezing a molten Cu135 cluster on cooling

We use molecular dynamics (MD) simulations within the framework of the embedded-atom method (EAM) to investigate structural changes during freezing a molten Cu cluster containing 135 atoms. The simulations show how the structural changes can strongly cause internal energy to change accordingly, and reveal that continuous interchange positions of atoms are the key in the formation of the icosahedral (Ih)-like Cu₁₃₅ cluster. By using visual inspection on atomic packing according to atomic density profiles, we analyze crystallization processes during freezing the molten cluster. At the initial stage of the freezing, one atom moves into the center of this cluster. Then the packing in interior atoms is changed into an ordered Ih structure, while outer atoms are becoming locally ordered with a fivefold Ih symmetry. Subsequently, nanocrystallization at lower temperatures propagates outward from the interior Ih structure, leading to the Ih-like cluster.     

Role of vapor flow in the mechanism of levitation of a droplet-cluster dissipative structure

The rate of water evaporation has been experimentally determined under conditions that ensure the formation of a dissipative structure of the droplet cluster type [1, 2]. It is shown that, in the region of localization of a droplet cluster, the velocity of the vapor-air flow is sufficient to maintain the levitation of droplets over the liquid surface according to the Stokes mechanism.     

晶体形态对金属涂层阻尼特性的影响

研究了金属涂层阻尼性能的微观机理。采用磁控溅射技术在不锈钢表面制备了Al、Ti、TiAl合金涂层,利用X射线衍射仪、扫描电子显微镜和能谱分析仪研究了金属涂层的物相、微观结构和元素组成;利用动态热机械分析仪(DMA)分析了涂层的阻尼性能。结果表明:单靶磁控溅射制备Al涂层为典型的片状晶体结构,而Ti涂层为柱状微晶结构;双靶共溅射制备的TiAl合金涂层为Al晶体为核心包裹微晶Ti的合金微团结构。在金属涂层和基体结构阻尼性能测试实验中,频率为31~35 Hz区间Al、Ti、TiAl合金和不锈钢基体四种试样均出现阻尼品质因子倒数(Q^-1)峰;然而在频率为57 Hz时, Al、Ti及TiAl合金涂层的试件出现明显Q^-1峰,其峰值约为0.016,未镀膜的不锈钢基体试样未出现峰值;在57 Hz频率下的应变幅与Q^-1关系曲线中,微团状晶态TiAl合金涂层阻尼性能最强,柱状晶态Ti涂层阻尼性次之、相对而言片状晶态Al涂层最弱。由此可见,晶体结构对金属涂层的阻尼性能的影响存在直接相关性,乱序晶体结构弛豫和晶界纳米缝隙的内摩擦是金属涂层产生阻尼的主要原因。     

Modulations in the abundance of salt clusters in electrosprays

The abundance of tetraheptylammonium bromide clusters produced from formamide solutions by electrospray ionization (ESI) is investigated. It exhibits a wavy pattern when plotted as a function of cluster size, successive waves being ordered according to the initial cluster charge z. This structure confirms the ion evaporation mechanism of Iribarne and Thomson and simplifies drastically the determination of the distribution of cluster charge z and radius R, from which the kinetics of ion evaporation from charged drops can be inferred.     

An experiment investigating the application of clustering procedures and similarity coefficients to the GT machine cell formation problem

This paper reports the development of a number of similarity-based coefficients designed for applying hierarchical cluster analysis to the group technology machine cell formation problem. The paper also discusses an experimental investigation applying these and other well-known similarity coefficients in conjunction with some well-known clustering algorithms. The mixture model experimental approach is used for the investigation. A number of problems were generated via simulation, randomly ‘mixed’ to hide the original cellular structure, and the clustering techniques applied. Extensions of prior research include the development of new similarity coefficients, their comparative evaluation, and the incorporation of the concept of part ‘weighting’ into the cluster analysis, and hence, cell formation     

The role of the structure and surface properties of Sn–Ge melts in the germanium crystallization

By analyzing the local atomic structure using models reconstructed from the experimental structure factor curves by the reverse Monte-Carlo method, we have established that microsegregation in the liquid state takes place in the Sn–Ge system and that tin and germanium atoms form chains with atomic spacings close to the covalent bond in solid state. The increase in the number of germanium atoms participating in cluster formation in diluted melts of germanium noticeably surpasses the increase in the germanium content of the melt, which is the reason for the appearance of extremums in the isotherms of density and surface tension of the Sn–Ge melt that is characterized by small positive deviations of activity from the ideal solution. In parallel with the interfacial energy and supersaturation, the cluster formation of the crystallizing substance with spacings close to the bond of this substance in the solid state contributes to an increase of the crystal growth rate.     

Coupling or lock-in? Co-evolution of cultural embeddness and cluster innovation-exploratory case study of Shaoxing textile cluster

Based on an exploratory case study of the development of Shaoxing textile cluster for more than 30 years, through the cross-level and multi-dimensional deconstruction of cultural embeddednesss(CE), this article dynamically analyzes the co-evolution relationship and results of its effect on the innovation capabilities of the cluster. The case study results show that CE affects the network structure and network behavior of cluster firms, which leads to the differences of innovation capabilities between cluster firms and the overall clusters. There exists a co-evolution relationship between CE and cluster innovation capabilities, and the result will lead to co-evolution coupling or lock-in. Co-evolution coupling appears in the early stage of cluster development; while co-evolution lock-in appears in the later stage of cluster development. So the embeddedness of same regional cultures may have completely different roles at different stages of cluster development. The root cause is that the network structure and behavior influenced by CE cannot dynamically match the demands for innovation in different stages of cluster's development, and the path dependence of CE evolution leads to change difficultly.     

Effects of relaxation and phase transition on the resistivity of chemically deposited Ni66B34 layers

The effects of thermal treatment on the conductivity and the temperature coefficient of resistivity of thin amorphous layers of Ni-B metallic glasses (66% Ni, 34% B) are analysed in terms of the dependence on thickness and ageing temperature. The layer exhibits a macroscopical structure consisting of clusters in an amorphous medium; it is assumed that the cluster resistivity is temperature independent up to 350° C and that the amorphous medium is altered markedly by the ageing processes. These consequences are deduced which are in agreement with a previously proposed model for a layer structure.     

Effect of electron overheating on the tunneling current of a molecular transistor

The effect of overheating of the electron subsystem on the Coulomb blockade in a structure (molecular transistor) based on a metal cluster containing a finite number of atoms has been theoretically studied. The electron energy spectrum in such quantum grains of cylindrical and spherical shape has been calculated. An increase in the electron subsystem temperature in the cluster leads to vanishing of the current gap and pronounced smoothening of the quantum and Coulomb steps on the current-voltage characteristic of the structure, in agreement with experimental observations.     

Ion beam nanopatterning in graphite: characterization of single extended defects

The morphology and the electronic structure of a single focused ion-beam-induced artificial extended defect is probed by several methods including micro-Raman spectroscopy, atomic force and scanning tunneling microscopies and Monte Carlo and/or semi-analytical simulation within standard codes. The efficiency of the artificial defect for deposited metallic cluster pinning is also investigated. We show a correlation between the ion dose, morphology, electronic structure and cluster trapping efficiency. At room temperature, cluster pinning is efficient when the displacement per atom is one or more. Well-ordered patterned cluster networks are considered for potential applications.     

Thermophysical and Acoustic Liquid Properties in the Framework of the Cluster Theory

A relation for the distribution function of clusters in a liquid, earlier proposed by the authors, allows one to find the number of particles in a cluster and its dependence on state parameters and to find the frequencies of IR-spectra of organic liquids. The authors have improved the method of finding the number of particles contained in a cluster. The proposed modification of the cluster theory allows one to estimate the energy of clusters, to reveal features of their structure, and to calculate the basic thermophysical properties of liquids, such as the difference of isobaric and isochoric heat capacities, the dynamic viscosity, and other properties. The authors have determined the cluster component of full absorption of ultrasonic waves, the cluster relaxation time, and the dimer formation energy for liquid noble gases and organic liquids. The comparison of the obtained results with experimental data demonstrates the reasonableness of the proposed model for prediction of thermophysical and acoustic properties of liquids.     

Molecular dynamics simulations of impact of energetic silicon clusters onto crystalline silicon

Cluster deposition is a promising technique for the growth of nanometric structures and is therefore the objective of many theoretical and experimental investigations. In this work molecular dynamics calculations have been performed to study the deposition of silicon clusters onto a crystalline silicon substrate. The evolution of the structure of the deposited cluster has been studied as a function dependence of the initial direction of incidence of the cluster, of the orientation of the exposed surface, of the cluster shape and of the interatomic forces. The comparison of deposition onto (100) and (110) shows the presence of channelling effects which lead to the implant of the cluster atoms and limit their spreading on the substrate surface. Compact clusters, obtained from the minimization of the quantum mechanical energy, show an enhanced resilience against fragmentation in comparison with clusters with a crystalline lattice. A similar resistance is obtained when using an interatomic potential accounting for a coordination larger than the one of crystalline silicon. At variance with the known behavior of metallic clusters, the effects of the incidence direction are hardly perceptible.     

Dynamics of the Cluster of Vortex Points in Two-Dimensional Superfluid 4He

We have simulated the dynamics of cluster of quantized vortex points with the same circulation in two-dimensional superfluid ⁴He. In two-dimensional quantum turbulence, vortices are always distributed uniformly, but often form some clusters. We study the dynamics of a cluster at zero temperature and finite temperatures. At zero temperature the cluster rotates. The radius and the density of the cluster hardly change because of the conservation of the energy and the angular momentum. On the other hand, at finite temperatures, the cluster rotates with forming a lattice structure and expanding because of the mutual friction.     

Hardness and the nature of microplasticity of hydroxyapatite

The hardness of thin (1.0–4.0 μm) hydroxyapatite (HA) coatings with different structures (nanocrystalline, amorphous-crystalline, and amorphous) grown on Ti and Si by rf magnetron sputtering has been studied using nanoindentation. In all of the coatings, deformation was observed to have an elastoplastic nature. The hardness of the nanocrystalline coatings corresponds to medium hardness values of HA microcrystals. The structure of the nanocrystalline coatings has been studied by high-resolution transmission electron microscopy in the indent zone and away from it. Comparison of the hardness values of coatings with different structures and analysis of the intragranular structure leads us to assume a nondislocation mechanism of plastic deformation. Its nature is interpreted in terms of a cluster representation of the structure of HA and amorphous calcium phosphates and cluster-boundary sliding in the course of deformation.     

Novel approach for measuring the effective shear modulus of porous materials

A new approach is proposed for the experimental study of the effective shear modulus of porous elastic materials using the uniaxial tension test. The idea is to measure strains at a few points surrounding a cluster of holes that represents the structure of the material. The representative cluster is placed in the material with the same elastic properties as those of the matrix. The measured strains lead to the properties of the equivalent circular inhomogeneity, which would produce the same elastic fields as from the cluster. An aluminum plate containing a cluster of seven circular or hexagonal holes was used. The effective shear modulus obtained from the strain data was compared with theoretical predictions and various bounds, and it was shown that the laboratory estimated values are quite accurate. The experimental technique can be used for the determination of the effective Poisson’s ratio of porous media and/or cellular solids if more detailed strain data are obtained.     

Computer simulation studies of Ar clusters

Results of the solid-liquid transition of Ar₁₃ cluster in a spherically symmetric external potential have been presented. The transition temperature is observed to show an elevation with pressure. The broadening of the heat capacity peaks indicate the transition becoming more diffused with pressure. The icosahedral structure of the cluster remains unaltered under pressure. Ar₅₅ cluster has also been studied by similar approach. A possible connection between glass transition phenomenon and melting of clusters under pressure has been examined. Communication no. 1413 from Solid State and Structural Chemistry Unit.