Dynamics of bi-periodic structures

Many structures considered for space applications are bi-periodic in their construction. Bi-periodicity means that two types of structural subassemblies, alternating in one or more directions, make up the structure. To gain insight into the dynamics of bi-periodic space structures a variety of one and two dimensional bi-periodic structures are considered. Results indicate that bands in which natural frequencies lie for periodic structures are further subdivided as a consequence of the bi-periodicity. Analytical solutions for the modes and frequencies of finite length one dimensional bi-periodic structures are obtained for general boundary conditions. A transmission method is developed to simplify the application of boundary conditions. It is found that some modes occur at frequencies which are outside the frequency bands predicted for bi-periodic structures. Two dimensional bi-periodic crossed beam grillage and truss structures are considered in this study. For cases where a separation of variables solution is possible the two dimensional structures exhibit similar properties to the one dimensional bi-periodic structures. Analytical solutions for the one and two dimensional bi-periodic structures considered above lead to a compact solution form similar to that of periodic structures analysis.     

Statistical properties of thermodynamically predicted RNA secondary structures in viral genomes

By performing a comprehensive study on 1832 segments of 1212 complete genomes of viruses, we show that in viral genomes the hairpin structures of thermodynamically predicted RNA secondary structures are more abundant than expected under a simple random null hypothesis. The detected hairpin structures of RNA secondary structures are present both in coding and in noncoding regions for the four groups of viruses categorized as dsDNA, dsRNA, ssDNA and ssRNA. For all groups, hairpin structures of RNA secondary structures are detected more frequently than expected for a random null hypothesis in noncoding rather than in coding regions. However, potential RNA secondary structures are also present in coding regions of dsDNA group. In fact, we detect evolutionary conserved RNA secondary structures in conserved coding and noncoding regions of a large set of complete genomes of dsDNA herpesviruses.     

Visualization of sinusoidal and varicose instabilities of streaks in a boundary layer

Nonlinear instabilities of boundary layer streaks are investigated experimentally. Extensive measurements visualizing the sinusoidal and varicose instabilities of streaky structures at nonlinear stage of the breakdown process in boundary layer are presented. The flow behaviour in the course of spatial evolution of the streaky structures with a secondary high-frequency disturbance generated on them is discussed. Various scenarios of origination and development of coherent vortex structures examined in physical experiments are considered. Specific features of the development of sinusoidal and varicose cases of destruction of the steady streamwise streaks are demonstrated, such as transverse and streamwise modulations of the streak by the secondary-disturbance frequency, appearance of new streaky structures in the downstream direction, and emergence and evolution of unsteady Λ-shaped structures localized in space in both cases.     

Electron microscopy study of incommensurate modulated structures in misfit ternary chalcogenides

The study of misfit structures by means of transmission electron microscopy and associated techniques is reviewed. It is complementary to X-ray crystallography and provides a local and direct view of the beautiful complexity of these systems. Three different types are presented in this work. Misfit columnar structures can be contemplated as a case of one-dimensional misfit structures while misfit layer structures are considered as two-dimensional misfit structures. Some extra dimensionality is added when these misfit layer structures wrap to give rise to tubular crystals. Electron microscopy/diffraction shows clearly in many examples the presence of structural modulations as weak satellite reflections that are very difficult to detect by X-ray diffraction methods. Besides, high-resolution images show the presence of stacking defects in some of these misfit layer structures.     

激光全息光刻技术在微纳光子结构制备中的应用进展

微纳光子结构研究随着光子学、半导体物理学及微加工技术的发展而逐渐蓬勃开展,并在其结构、理论、制备技术等方面取得了系列进展。受限于目前的微加工技术水平,要成功制备大尺度、高质量的光子材料仍然存在着一定挑战。激光全息光刻技术作为一种简便快捷的微结构制作技术已经发展成为一种经济快速制作大面积微纳超材料及光子晶体模板的重要手段。介绍了激光全息光刻技术的原理,详细阐述了该技术在制作三维面心立方、木堆积结构、金刚石结构光子晶体以及光学周期类准晶、手性超材料、周期性缺陷结构等微纳光子结构中的应用研究进展。激光全息光刻技术成功制作微纳光子结构为光子材料在更多领域的广泛应用提供了基础和方法。     

Optical scanning of dusty 3D-structures formed in a glow discharge

3D-quasi-crystals formed in strata of a glow discharge are scanned in the optical range with the help of a moving laser knife and high-speed videorecording. The spatial positions of dusty grains are determined. The ordering of structures and the type of arrangement of particles are determined from a comparison of pair correlation functions constructed for the structures under study with correlation functions corresponding to ideal crystalline structures. Several types of unit cells are found through the visual collation of separate parts of structures. As compared to data from the literature on experiments in a high-frequency discharge, the structures under study have a clearly pronounced anisotropy.     

微穿孔板吸声结构水下应用研究

马大猷教授提出的微穿孔板吸声结构在空气噪声降低和隔离方面得到了广泛的应用,但未见水下应用的相关研究和报道。本文将空气中微穿孔板理论应用到水中,得到了水下微穿孔板吸声结构的吸声公式。通过理论分析,得出了微穿孔板结构直接应用于水中无法获得宽频吸收的结论。提出了通过匹配液将微穿孔板间接应用到水下的设想。设计了单层板和双层板吸声结构,并对它们的吸声特性进行了理论分析与仿真。结果表明,本文设计的微穿孔板吸声结构在水中能够获得优于空气中的宽频带吸声效果。实验测量了自制的微穿孔板吸声结构,吸声系数的测量值与理论曲线基本吻合,从而验证了理论分析的正确性。     

Performances and design of ironless loudspeaker motor structures

This paper presents several kinds of structures of ironless loudspeaker motors. The proposed devices go from simple structures, in terms of manufacturing process, to structures in which the magnet rings can be stacked, with different magnetization directions or values. The structures are studied and compared according to their created induction level and uniformity across the coil displacement range, which are directly linked to the loudspeaker performances (force factor). The model used is the coulombian model of magnets to calculate the magnetic field created, therefore all the calculations are analytical. The paper shows that no structure is universal when it comes to the loudspeaker design: some are well adapted to micro displacements of the coil while other structures are adapted to large displacements.     

Analysis on band gap properties of periodic structures of bar system using the spectral element method

In this paper, the band gap properties of the periodic structures of bar system, which include the rod-joint, truss, and frame structures, are studied using spectral element method (SEM). The spectral equations of the rod, beam, and joint elements are established and the spectral equations of the whole structures are further assembled. The frequency responses of the whole structures are calculated and the results are compared with those calculated by the finite element method (FEM). It can be observed that the SEM is more accurate in high frequency ranges. The band gap properties of the three types of periodic structures are studied, respectively. Furthermore, the effects of structural length, unit cell number, structural configurations, load conditions, and structural damping on the band gap properties are investigated.     

中等尺寸硫化锌团簇结构和稳定性

通过第一性原理方法研究中等尺寸硫化锌团簇(ZnS)_n(n=24,28,36,和48)的结构和稳定性.团簇初始结构主要通过手工搭建和从体材料中切割得来.研究表明:对于中等尺寸的团簇,空心的笼状和管状结构能量最低;随着团簇尺寸的增大,洋葱结构能量越来越低.此外,对中等尺寸硫化锌团簇,纤锌矿结构比闪锌矿结构更稳定.说明在小尺寸下,纤锌矿硫化锌结构更稳定.     

Nd:YAG纳秒激光诱导硅表面微结构的演化

利用Nd:YAG纳秒激光(波长为532和355 nm)对单晶硅在真空中进行了累积脉冲辐照,研究了表面微结构的演化情况.在激光辐照的初始阶段,532和355 nm激光脉冲均在硅表面诱导出了波纹结构,后者辐照硅表面后形成了近似同心但稍显混乱的环形波纹结构.随着脉冲数的增加,波纹结构逐渐演化为一种类似珠形的凹凸结构,最后形成准规则排列的微米量级锥形结构,该微结构的生长依赖于表面张力波和结构自组织.分析发现,形成的交叉环形结构主要是在355 nm激光辐照硅的过程中,表面张力波导致波纹结构部分叠加的结果.     

Novel structures and properties of gold nanowires

The structures of free-standing gold nanowires are studied by using molecular-dynamics-based genetic algorithm simulations. Helical and multiwalled cylindrical structures are found for the thinner nanowires, while bulk-like fcc structures eventually form in the thicker nanowires up to 3 nm in diameter. This noncrystalline-crystalline transition starts from the core region of nanowires. The vibrational, electronic, and transport properties of nanowires are investigated based on the optimal structures. Bulklike behaviors are found for the vibrational and electronic properties of the nanowires with fcc crystalline structure. The conductance of nanowires generally increases with wire diameter and depends on the wire structure.     

可见光波段超材料的平面聚焦效应

基于超材料双鱼网结构物理模型,设计了树枝状结构+树枝状结构、树枝状结构+银膜和树枝状结构+氧化铟锡玻璃三种可见光波段超材料复合结构.采用双模板辅助化学电沉积方法,制备出金属银树枝状结构阵列和银膜,分别实现了这三种复合结构.透射和反射实验表明,三种复合结构具有类似的行为.比较发现,树枝状结构+氧化铟锡玻璃复合结构具有损耗低和平面聚焦效应明显的优点.通过调节实验条件,分别制备出能够实现平面聚焦的红绿蓝三种可见光波段超材料. 关键词： 超材料 银树枝状结构 可见光 平面聚焦     

Experimental,semi-experimental and ab initio equilibrium structures

The determination of equilibrium structures of molecules by spectroscopic methods or by quantum mechanical calculations is reviewed. The following structures are described in detail: experimental equilibrium structures, empirical structures, semi-experimental structures and ab initio structures. The approximations made by the different methods are discussed and their accuracies are compared.     

Light propagation characteristics in quantum well structures of photonic crystals

Two-dimensional （2D） closed-cavity single quantum well （SQW） and multiple quantum well （MQW） structures are proposed based on the traditional 2D open-cavity SQW structures of photonic crystals. The numerical calculation results show that the proposed structures can greatly improve the optical characteristics compared with the traditional structures. It is found that the barrier thickness has a great impact on the optical characteristics of the closed-cavity MQW structures： when the barrier thickness is narrower, each resonant peak which appears in the SQW would split, the number of split times is just equal to the number of wells, and each well in the MQW structures is a travelling-wave-well, similar to the well in the open-cavity SQW structures; when the barrier thickness is wider, there is no effect of spectral splitting, and each well in the MQW structures is a standing-wave-well, just like the well in the closed-cavity SQW. The physical origin of different field distributions and the effect of the spectral splitting are provided.     

Hf-C体系的高压结构预测及电子性质第一性原理模拟

本论文中, 采用晶体结构预测软件USPEX结合第一性原理方法全面地搜索了Hf-C体系在高压下的晶体结构, 预测得到了两种新的化合物及HfC在高压下的相变路径. 压力低于100 GPa 时, 除了常压下的结构HfC, Hf₃C₂, Hf₆C₅, 并没有得到新的热力学稳定结构. 在200 GPa时, 预测得到了一种新化合物——Hf₂C, 空间群为I4/m; 且HfC的结构发生了相变, 空间群由Fm3m变为C2/m. 在300 GPa时, 预测得到了另一种新化合物——HfC₂, 空间群为Immm. 而在400 GPa时, HfC的结构再次发生相变, 空间群为Pnma. 通过能量计算, 得到了Hf-C体系的组分-压力相图: 在压力分别低于15.5 GPa和37.7 GPa时, Hf₃C₂和Hf₆C₅是稳定的; 压力分别大于102.5 GPa和215.5 GPa时, Hf₂C和HfC₂变成稳定化合物; HfC的相变路径为Fm3m→C2/m→Pnma, 相变压力分别为185.5 GPa 和322 GPa. 经结构优化后, 得到了这四种高压新结构的晶体学数据, 如晶格常数、原子位置等, 并分析了其结构特点. 对于Hf-C 体系中的高压热力学稳定结构, 分别计算了其弹性性质和声子谱曲线, 证明是力学稳定和晶格动力学稳定的. 采用第一性原理软件VASP模拟高压结构的能带结构、态密度、电子局域函数和Bader 电荷分析, 发现HfC(C2/m, Pnma结构), Hf₂C 和HfC₂ 中Hf-C 键具有强共价性、弱金属性和离子性, 且C-C 间存在共价作用.     

水下均匀材料复合层吸声特性研究

本文对水下阻尼板和其它介质层复合的多层吸声和结构吸声，反声性能进行了研究，利用各层界面压力，振速的四端网络矩阵及层间边界条件导出吸声结构表面阻抗和反射系统数随频率的变化关系，数值计算分析了不同边界条件，多种层以及改变各层参数情况，反声，吸声系数的变化，发现多层结构中厚的水层引起强烈的共振反射峰，并提出了抑制共射的方法。     

Electron emission from crystal surfaces of condensed aromatics under the impact of metastable rare gas atoms

Energy distributions of electrons ejected from polycrystalline surfaces of naphthacene, perylene and coronene by the impact of metastable He ^*, Ne ^* and Ar ^* atoms have been measured. Two types of peaks, which are similar to the “non-moving” and “moving” structures in photoelectron spectra, are observed in each spectrum. The non-moving structures (ca. 1 eV) for perylene and coronene are similar to those in the photoelectron spectra, whereas the relative intensities of the two non-moving structures for naphthacene (0.6 and 1.7 eV) are remarkably different from the corresponding structures in the photoelectron spectrum. The peak positions (but not necessarily their intensities and widths) for the moving structures for all the samples (> 2 eV) agree with those of the corresponding photoelectron spectra. The origin of these moving structures is ascribed to Penning ionization on the solid surface.     

结构声强技术测量墙体结构中弯曲波耦合功率流

虽然结构声强技术已成功地应用于匀质薄壁结构，但在建筑结构的应用研究只是一些定性的探讨，本文应用结构声强技术研究建筑结构中弯曲波声强的测量，以传统方法获得的结构总损耗能量测量值为参考，基于９５％合成置信区间研究弯曲波声强的测量精度，初步探讨了结构声强技术在建筑中应用的可行性。     

Effects of thickness and delamination on the damping in honeycomb-foam sandwich beams

In engineering applications where the use of lightweight structures is important, the introduction of a viscoelastic core layer, which has high inherent damping, between two face sheets, can produce a sandwich structure with high damping. Sandwich structures have the additional advantage that their strength to weight ratios are generally superior to those of solid metals. So, sandwich structures are being used increasingly in transportation vehicles. Knowledge of the passive damping of sandwich structures and attempts to improve their damping at the design stage thus are important. Some theoretical models for passive damping in composite sandwich structures are reviewed in this paper. The effects of the thickness of the core and face sheets, and delamination on damping are analyzed. Measurements on honeycomb-foam sandwich beams with different configurations and thicknesses have been performed and the results compared with the theoretical predictions.