Crystallization of hard spheres near wall

The classical molecular dynamics method has been used to study a thermodynamic system of hard spheres occurring near an ideal wall. It is established that, at an initial mean number density of spheres above a certain value, the system exhibits relaxation to an equilibrium state that consists of two phases. The closer the wall, hard spheres form a primitive pyramidal crystal belonging to a trigonal system; farther from the wall, hard spheres occur in an anisotropic liquid phase that is separated from the crystal by a transition layer. As the mean density increases, the process of crystallization in the configuration space takes place and the system passes to a new equilibrium state with increased thickness of the near-wall crystalline layer, at unchanged densities of both the crystal and isotropic liquid phase.     

On the accuracy of the Hertz model to describe the normal contact of soft elastic spheres

The finite element method is used to investigate the validity of the Hertz model for normal contact under conditions that violate some of the assumptions on which the Hertz model was derived. The conditions investigated are: frictional contact, non-flat contact surface and large strains. The investigations were performed for the case of two elastic spheres in contact as well as that of a sphere in contact with a flat plate. The existence of friction for normal contact does not appear to lead to any erroneous predictions of the force-deflection relationship by the Hertz model. The Hertz model also does not appear to predict significant errors when the contact surface is not flat. However, large strains were found to cause important prediction errors by the Hertz model. Calibration curves for the corrections on the Hertz model are proposed.     

交换耦合软/硬磁双层膜体系磁动力学性质的研究

基于Laudau-Lifshitzt方程,在一维原子链模型的框架内研究了交换耦合软/硬磁双层膜体系的磁动力学性质,得到了软磁层在磁反转前处于一致磁结构和交换弹性磁反转过程中处于螺旋磁结构时的本征自旋波模式的频率及其空间分布特点,以及自旋波色散关系曲线。研究表明,软磁层的反磁化机制与自旋波的软模现象有密切关系,一阶自旋波模式的软化诱导了软磁层的磁反转,且磁反转的形式与一阶自旋波模式的空间分布状态相关。由磁动力学方法得到的磁反转临界场与近似的解析解得到的结果一致。     

The transmission properties of elastic waves through multilayers of spheres with planar defects

The work presented in this paper discusses the transmission properties of elastic waves through multilayers of spheres which are periodically arranged in a homogeneous host material. The multilayers of spheres can have different kinds of planar defects. These defects are formed by removing one layer of spheres or by changing the radiuses or the material of scatterers in some layers. First, the reflection and transmission matrices of a single layer of spheres are obtained by the multiple scattering approaches and then the reflection and transmission matrices of multilayers of spheres are derived based on the polymerization method. Numerical examples are presented for 15 layers of spheres with one central planar defect layer or two symmetrically arranged planar defect layers. The influences of these planar defects on the frequency-dependent transmission curves are discussed. It is observed that the band gap can be widened evidently by introducing the planar defect, and the defect states appear in the band gap consequently. So the elaborately arranged planar defects are important in designing specific acoustic filters.     

Stress distribution in orthotropic plates with coupled elastic properties

Summary If the roots of the characteristic equation of the governing differential equation for the stress function of an orthotropic plate under generalized plane stress conditions are equal classical solutions of anisotropic elasticity theory do not hold anymore. The general form of the stress function for such orthotropic materials is discussed and the exact solution is given for the plate with an elliptic opening loaded in tension.Comparison of the stress distribution for the material with distinct characteristic roots indicated that in produces much higher stress concentrations than the model with equal roots. For the latter case solution of any boundary value problem may be obtained very easily through an existing solution for the isotropic material.Given also that there are several problems involving stress concentrations in orthotropic plates as yet unsolved, the analysis presented here might be of considerable help in the, designing of the structure of composite laminates.With 8 Figures     

Conservation laws and the material momentum tensor for the elastic dielectric

It is shown that a Langrangian formulation of continuum mechanics can provide not only the equations of motion, but the conservation laws related to the material symmetries in a perfect continuum interacting with an external electric field. These conservation laws in the presence of defects lead to the path-independent integrals widely used in fracture mechanics. They are basically related to the “material force” on a defect in a continuum. The quantity playing the role of the physical stress tensor in this formulation is the material momentum tensor. A material force in the form of a path-independent integral for the elastic dielectric is derived employing Toupin's [1] formulations.     

Tensile fracture of soft and hard PZT

Power applications generate high stresses which can damage piezoceramic components. In this study tensile fracture of several types of PZT (hard/soft) is investigated. After validation of the specimen geometry by means of numerical simulation, samples are led to failure using a specific device. Weibull law parameters enable the characterisation of the tensile strength distribution and highlight clear differences between soft and hard ceramics. A fractographic approach emphasises the specificities of the fracture mode and the fracture origin for each type of samples.     

Computer simulation of amorphous thin films of hard spheres

The simulation of Henderson et al. of the structural anisotropy of amorphous thin films is modified. As a result, the density is increased while preserving the anisotropic columnar growth in the simulated film.     

A linear theory for soft ferromagnetic elastic solids

A general theory of magnetoelasticity is developed for soft ferromagnetic materials of multidomain structure, for which the hysteretic loss and exchange effect may be neglected. The general equations are linearized by assuming infinitesimal strains, linear constitutive equations, and that all magnetic variables (magnetic intensity, induction and magnetization) in the deformed body may be divided into two parts: a rigid body state and a perturbation state. The former are the same as those in the magnetostatics for a rigid body and the latter which are the added corrections due to deformations, are coupled with strains and stresses in a set of linear differential equations and boundary conditions. Two versions of the linear theory are given for materials with isotropic, cubic or uniaxial symmetry. One of them is applied to investigate the buckling of an elastic, isotropic plate in a transversally applied uniform magnetic field. The calculated results agree with previous observations.     

Integration of hydrogels with hard and soft microstructures

Hydrogels, i.e., water-swollen polymer networks, have been studied and utilized for decades. These materials can either passively support mass transport, or can actively respond in their swelling properties, enabling modulation of mass and fluid transport, and chemomechanical actuation. Response rates increase with decreasing hydrogel dimension. In this paper, we present three examples where incorporation of hydrogels into solid microstructures permits acceleration of their response, and also provides novel functional capabilities. In the first example, a hydrogel is immobilized inside microfabricated pores within a thin silicon membrane. This hydrogel does not have a swelling response under the conditions investigated, but under proper conditions it can be utilized as a part of an electrolytic diode. In the second example, hydrogels are polymerized under microcantilever beams, and their swelling response to pH or glucose concentration causes variable deflection of the beam, observable under a microscope. In the third example, swelling and shrinking of a hydrogel embedded in a microfabricated valve structure leads to chemical gating of fluid motion through that valve. In all cases, the small size of the system enhances its response rate.     

硬弹性聚丙烯中空纤维的形成

通过应力场下聚丙烯熔体结晶制备出硬弹性聚丙烯中空纤维 .利用对样品弹性回复率及力学性能测试等手段 ,研究了硬弹性聚丙烯中空纤维的形成 .结果表明 :纺丝温度下降、熔体拉伸比增加及热处理等均有利于硬弹性聚丙烯中空纤维的形成 ,聚丙烯原料的分子量、纺丝冷却条件等对硬弹性的形成也有影响 .所制备的硬弹性中空纤维具有典型的应力 -应变行为 .     

Sediments of soft spheres arranged by effective density

Colloidal sedimentation has been studied for decades from both thermodynamic and dynamic perspectives. In the present work, binary mixtures of colloidal spheres are observed to separate spontaneously into two distinct layers on sedimentation. Both layers have a high volume fraction and contain distinct compositions of particles. Although predicting these compositions using settling dynamics is challenging, here we show that the compositions are readily predicted thermodynamically by minimizing the gravitational energy of the system. As the random packing fraction of a mixture of spheres exceeds that of monodisperse spheres of either type, the mixture produces a denser suspension that forms the bottom phase. Experimentally, the use of charged particles and low-ionic-strength solutions provides interparticle repulsions that keep the packed particles mobile, avoiding a glassy state that would prevent particles from reaching their equilibrium configuration. We extend this work beyond binary systems, showing similar separated layers for a five-component mixture of particles.     

Bulk and interior packing densities of random close packing of hard spheres

The packing densities of random close packing of equal hard spheres (RCPHS) are studied. The RCPHS is generated by a rearrangement algorithm with an optimization subroutine. Traditionally defined packing density, bulk density, is found to be 0.635 ± 0.002 by extrapolation to infinite number of spheres. We propose that there exist a characteristic packing density without boundary effects. This interior packing density is calculated by two methods, resulting in values without statically significant difference. Interior packing density deduced from Voronoi diagram is 0.6690 ± 0.0006. Local packing density for each sphere is defined as ratio of its volume to volume of its corresponding Voronoi cell and is sensitive to sphere's local configuration and overlapping.     

Electrochemical properties of solid polymer electrolytes prepared by the hard/soft acid base principle

In order to improve the electrochemical properties including ionic conductivity of SPEs (solid polymer electrolytes), understanding of the interaction between the polymer and salt in the SPE is important. In this study, four types of polymer matrices and four types of salts were used according to the hard/soft acid base (HSAB) principle. The results of impedance measurement reveal that the ionic conductivities are affected by the HSAB principle at low salt concentration. With increasing salt content, however, the SPEs are influenced by the ion hopping property of salt rather than by the solubility of the polymer with salt. In contrast, the PPS-based SPE shows different characteristics because it is prepared as a slurry phase at high salt content.     

Laser-trapping properties of dual-component spheres

The optical trapping properties of dual-component spheres consisting of a cocentered outer transparent dielectric spherical shell and internal solid sphere are examined on the basis of the enhanced ray optics model. It is shown that stable trapping can occur on axis, off axis, or at multiple axial positions and depends on the dual-sphere and laser beam parameters. Computation results are also presented for an internal reflecting sphere surrounded by an outer dielectric spherical shell.     

新型炭微球的制备及性能研究

采用悬浮缩聚法制得烯丙基COPNA-BMI树脂微球,然后经过炭化处理后得到新型炭微球.采用SEM观察这种树脂微球及炭微球的表面形貌;采用FT-IR分析该树脂微球炭化过程官能团的变化;采用元素分析仪对不同温度下该树脂微球C、H、O、N元素的含量进行测定;采用XRD对炭微球的微晶结构进行分析.研究表明,炭微球到具有很好的球形和表面光洁度,其结构为无定形炭;随着炭化温度的升高,炭微球的C含量逐渐增加,H、O含量明显降低.     

Torsional buckling of elastic cylinders with hard coatings

The torsional buckling characteristic of an elastic cylinder with a hard surface coating layer is addressed in this paper. Deformations of the core and surface layer are obtained analytically through the Navier??s equation and thin shell model, respectively. Both infinitely and finitely long cylinders are studied and the effects of the surface layer??s stiffness, thickness, residual stresses, as well as the cylinder lengths on the critical torsional angle and buckling morphologies, are discussed. It is found that either the surface rippling or global buckling mode may occur when there exist residual stresses within the surface layer. The critical torsional angle increases when the surface layer becomes stiffer and thinner. In addition, higher-order rippling modes frequently occur for a finite-length cylinder with stiffer and thinner surface layer.     

Effect of electrical and mechanical poling history on domain orientation and piezoelectric properties of soft and hard PZT ceramics

Abstract

The superior piezoelectric properties of all polycrystalline ferroelectrics are based on the extent of non-180° domain wall motion under electrical and mechanical poling loads. To distinguish between 180° and non-180° domain wall motion in a soft-doped and a hard-doped lead zirconate titanate (PZT) ceramic, domain texture measurements were performed using x-ray and neutron diffraction after different loading procedures. Comparing the results to measurements of the remanent strain and piezoelectric coefficient allowed the differentiation between different microstructural contributions to the macroscopic parameters. Both types of ceramic showed similar behavior under electric field, but the hard-doped material was more susceptible to mechanical load. A considerable fraction of the piezoelectric coefficient originated from poling by the preferred orientation of 180° domains.