隔声超构材料的研究进展

声学超构材料的概念起源于局域共振型声子晶体。作为一种新型复合人工结构材料,声学超构材料具备许多优异的特性,如相对于传统隔声材料可以灵活、精准地操控声波,采用小型化、轻质化的结构解决宽频带、低频隔声等问题。本文综述了声学超构材料在隔声方面的最新研究进展,从带隙理论和表征参数出发论述了声学超构材料隔声机理,重点介绍了相关代表性工作,包括Helmholtz式隔声超构材料、薄膜薄板式隔声超构材料、折叠卷曲空间式隔声超构材料以及组合式隔声超构材料的设计理念和方法,最后对这一新兴领域的应用进行了展望。     

单壁碳纳米管中稳态机械波的激发条件研究

通过分子动力学模拟,对单壁碳纳米管中稳态机械波的激发条件进行了详细的研究.利用所施加的周期性机械外力,使单壁碳纳米管产生局部径向形变,该形变将以单壁碳纳米管的管壁为弹性媒质传播开去,从而在单壁碳纳米管中形成机械波;机械波产生的条件依赖于单壁碳纳米管波动系统能量的维持和受迫振动区域形变方向的控制;稳定机械波形成的最佳条件是碳纳米管的形变度和受迫振动频率之间的最优匹配,以及由此形成的单壁碳纳米管在周期性外力作用下产生的共振.     

Soliton mechanism of the uranium nitride microdynamics and heat conductivity at high temperatures

The microdynamics of soliton waves and localized modes of nonlinear acoustic and optical oscillations in uranium nitride has been investigated. It is shown that, upon heating, the energies of solitons in the gap between the optical and acoustic phonon bands increase, while the energies of local modes decrease. The experimentally observed quasi-resonance features, which are shifted in the gap with a change in temperature, can be manifestations of the revealed soliton waves and local modes. The microdynamics of uranium nitride heat conductivity with the stochastic generation of the observed solitons and local modes at remote energy absorption have been investigated. The temperature dependence of the heat conductivity coefficient has been determined from the temperature gradient and energy flux within the standard approach (which is to be generalized).     

Coarsening Nature of Liquid-Ordered Domain in Model Membrane

We present the coarsening dynamics of a liquid-ordered (Lo) domain in a supported lipid membrane under an electric field. In a relatively small nanosmooth region surrounded by geometrical walls of nanocorrugated regions, full coarsening of the Lo domain was produced while in a large nanosmooth region, an intermediate, less-ordered Lo domain was developed so that the diffusion of charged phospholipids in the liquid-disordered (Ld) domain was allowed across the geometrical walls in the presence of an electric field. No appreciable diffusion of the charged lipids into a fully coarsened Lo domain by the electric field implies that the structural ordering of the membrane components plays a significant role on the formation of lipid rafts for biological processes.     

Possibilities of surface-sensitive X-ray methods for studying the molecular mechanisms of interaction of nanoparticles with model membranes

The processes of structural rearrangement in a model membrane, i.e., an arachic acid monolayer formed on a colloidal solution of cerium dioxide or magnetite, are studied in situ in real time by the methods of X-ray standing waves and 2D diffraction. It is shown that the character of the interaction of nanoparticles with the monolayer is determined by their nature and sizes and depends on the conditions of nanoparticle synthesis. In particular, the structure formation in the monolayer–particle system is greatly affected by the stabilizer (citric acid), which is introduced into the colloidal solution during synthesis.     

Nonlocal Continuum Theory of Liquid Crystals

A nonlocal continuum theory of liquid crystals is constructed to explain and predict the physical behavior of liquid crystals under long range intermolecular forces Balance laws consist of conservation of mass and mocroinertia, balance of momenta and energy. Constitutive equations are given for the equibilibirium and non-equilibirium parts of the stress, couple strees, free energy, entropy an nonlocal body force and couple. Thermodynamic restrinctions and material frame-indifference are studied. The theory is valid for liquid crystals having arbitrary shapes (inertia), Passage is made to the thread-like molecuels and to local theory. Applications are considered to two-dimensional problmes, steady, plane shear flows and disperison of twist waves.     

声学超表面的非对称声分束特性研究

非对称声分束超表面是由人工微单元结构按照特定序列构建的二维平面结构,可将垂直入射的声波分成两束传播方向和分束比自由调控的透射波,在声功能器件设计及声通信领域具有广泛的应用前景。本文系统研究了一种实现非对称声分束的设计理论和实现方法,基于局域声功率守恒条件研究了声分束器的设计理论、阻抗矩阵分布、法向声强分布、声压场分布等。利用遗传算法对四串联共振腔结构进行参数优化实现了声分束器所需的阻抗矩阵分布,声压场分布表明声波入射到声分束器后在入射侧激发出两列传播方向相反且幅值和衰减系数均相同的表面波,实现了入射侧与透射侧的局域声功率相互匹配。声波经过声分束器后被分为两束透射波,两束透射波的折射角和透射系数与理论值十分吻合,证明了设计理论及实现方法的正确性和可行性。本文的研究工作可以为新型非对称声分束结构设计提供理论参考、设计方法和技术支持,并促进其在工程领域的实际应用。     

Selective optical properties of a multilayered periodic gyrotropic structure at an arbitrary angle of incidence of waves

A multilayered periodic gyrotropic structure consisting of an arbitrary number of unit cells is considered. The gyrotropic properties of this multilayered structure are due to the external magnetic field. A matrix is calculated on the basis of the boundary conditions for waves in each layer, which relates a wave incident at an arbitrary angle with the transmitted and reflected waves. The complex amplitude transmittance and reflectance are found for the entire multilayered structure from the calculated matrix elements. The optimal parameters of a structure consisting of layers of potassium dihydrogen phosphate and europium-doped glass are calculated. The dependences of the intensity, ellipticity, and the angle of rotation of the major axis of the polarization ellipse of reflected and transmitted waves on the number of cells, light frequency, and magnetic field at an arbitrary angle of incidence of waves are obtained in graphical form. The structure considered is characterized by selective reflection of electromagnetic waves and can be used as a polarization converter controlled by a magnetic field.     

Resonance excitation of polaritons by bulk electromagnetic waves in the layered systems with anisotropic substrates

The possible resonance excitation of surface electromagnetic waves by the bulk waves at the interface between a positive transparent uniaxial crystal and an isotropic medium has been predicted. The existence of the surface waves is provided by anisotropy of one of the boundary media. The tensor relation between the vector amplitudes of an exciting bulk electromagnetic wave and the excited surface wave is established. The ratio of the moduli of the tangential components of these amplitudes is analyzed as a function of the angle of incidence and the polarization of the bulk wave. The numerical calculations are performed for the surface waves at the interface between paratellurite TeO₂ and a KRS-6 (TlBr 30%-TlCl 70%) crystal.     

X-ray diffraction in polydomain crystals modulated by transverse waves of atomic displacements. 2. Two-wave modulation of crystals

X-ray diffraction in an orthorhombic crystal modulated by two transverse-displacement waves propagating along one of the crystallographic directions has been numerically studied within the framework of the kinematic approximation. Crystal models are considered that allow the superposition of the displacement waves or the coexistence of domains modulated by the displacement waves with different parameters. It is shown that the formation of peaks additional to the satellite ones is possible not only due to the superposition of the displacement waves, but also due to a certain correlation of the displacement waves in each domain.     

Investigation into the seismoacoustic properties of specific polycrystalline materials used in nuclear reactors

The characteristics of the wave fields observed during the transmission of quasi-longitudinal ultrasonic waves through polycrystalline graphite samples have been studied. The specific features of propagation of elastic waves in a bilayer medium, where one of the layers (isotropic) is an acrylic glass hemisphere and the other (anisotropic) is a polycrystalline porous graphite hemisphere, are considered. The velocities and propagation times of quasi-longitudinal waves in polycrystalline graphite samples and a bilayer acrylic glass-graphite sample in different directions are experimentally measured by ultrasonic spatial sounding. The experimental results are compared with theoretical calculations using the data on graphite crystallographic texture obtained previously by neutron diffraction. The reasons for the discrepancy between the theoretical and experimental characteristics of elastic waves in the media under study are established and analyzed.     

田字形地震超材料甚低频宽带隙机理

在地球中传播的地震波主要有体波和表面波,而表面波中Rayleigh波对建筑物造成的破坏最为强烈。针对Rayleigh波的振动控制,提出一种田字形超材料结构。相比于传统的地震超材料,这种超材料屏障是由外部口字形框体内部嵌套十字形柱体组成,形成4个可填充区域,其外部框体采用部分埋入的方式,具有高强度、强稳定性、填充方式灵活的特点。应用有限元法计算了田字形超材料的能带结构和传输特性,并通过分析带隙边界处模态振型可知,带隙的打开是由于柱体的局域共振。结合带隙机理可知,柱体结构中土壤填充量不同可改变柱体的质量,形成不同的谐振频率,产生甚低频带隙。为进一步拓宽带隙,设计研究了正、负梯度的质量填充方式,均可得到3.3~13.1 Hz甚低频宽带隙,在谐振频率范围内两者的隔震方式分别为Rayleigh波彩虹捕获和Rayleigh波到体波的转化。最后,采用EI-Centro地震波对填充屏障进行了时程验证,加速度最大幅值衰减超过80%,为地震超材料在减震隔震方面应用提供了新的设计思路和方法。     

Temperature and potential-dependent structural changes in a Pt cathode electrocatalyst viewed by in situ XAFS

The paper presents an X-ray absorption fine structure (XAFS) investigation of Pt supported on Vulcan XC-72 operating as a cathode catalyst in a polymer electrolyte membrane fuel cell (PEM FC). XAFS spectroscopy was performed in situ using a cell developed by the authors and optimized for absorption measurements. Low-noise spectra were obtained using the transmission mode over the Pt L₃ XAFS range, enabling multiple-scattering XAFS data-analysis for the Pt nanocrystalline system under operating conditions. Accurate measurements of the local structure were performed at various potentials and working temperatures. Changes in the near-edge structures reflecting variations in the Pt electronic structure were observed for various potential values in the cathode activation region. The Pt average local geometric structure was found to be practically potential-independent. The increase in structural disorder caused by the higher working FC temperature was not found to affect the catalyst’s performance in the kinetically controlled region.     

On existence of one-partial bulk waves in semi-infinite piezoelectric media

The conditions for the existence of one-partial bulk waves satisfying the boundary conditions on a mechanically free surface of a semi-infinite piezoelectric medium have been analyzed. In purely elastic media, similar wave solutions are known to exist along the propagation directions m which form lines on the sphere m ? m = 1, passing through all the available degeneracy points (acoustic axes). It is shown that in a triclinic piezoelectric half-space with a metallized surface, one-partial bulk waves may exist solely along isolated propagation orientations, whereas for a nonmetallized surface, such waves can exist only if an additional condition for the material constants of the medium is fulfilled. It is also shown that the one-partial bulk solution may not exist along an arbitrary acoustic axis in a piezoelectric. These conclusions are valid in the general case of unrestricted anisotropy, i.e., they do not take into account the material symmetry. In addition, the orientations providing the propagation of one-partial bulk waves because of the existing symmetry are specified for piezoelectric media of various symmetry classes.     

Efficiency of excitation of dimensional resonances of magnetoelastic waves in layered structure

This paper reports the study of elastic and magnetoelastic waves in two-layered structure: magnetic film – non-magnetic substrate, restricted in one dimension. We have investigated behavior of elastic and magnetoelastic standing waves resonant frequencies dependent on the relative thicknesses of magnetic and non-magnetic layers. Magnetic susceptibility of magnetic film placed on non-magnetic substrate and character of its behavior in the vicinity of frequencies of dimensional resonances of magnetoelastic and elastic waves are studied. We have explored susceptibility dependence on the relation between magnetic and non-magnetic layers parameters. By change of the thickness of magnetic layer relative to overall thickness of a sample we can regulate magnetic susceptibility and therefore efficiency of excitation of definite harmonics of standing magnetoelastic and elastic waves. If even number of magnetoelastic half waves falls to magnetic layer corresponding harmonics of magnetoelastic and elastic wave disappears. In particular odd harmonics disappear when magnetic film occupies half space of a sample. Magnetic susceptibilities reveal peculiarities in their behavior dependent on material parameters change.     

Optical axes in various absorbing crystals

Analytical expressions have been derived for the components of the complex permittivity tensor as a function of the number of optical axes existing in an absorbing monoclinic crystal. Some specific features of the orientation of these axes are considered. The ellipticities of natural waves are calculated for these crystals, as well as the ellipticities of transmitted light for right- or left-hand circularly polarized waves normally incident on a crystal. A significant difference of particular cases of absorbing monoclinic crystals from the general case of crystal with four circular axes is demonstrated.     

Spectrum of natural electromagnetic waves in a periodic ferromagnet-dielectric structure

The specific features of propagation of natural TE waves in a periodic ferromagnet-dielectric structure are investigated in the following geometry: a bias magnetic field is parallel to the ferromagnet/dielectric interface and perpendicular to the wave propagation direction. The transformation matrix for the period of the structure, the dispersion relation, and the energy reflection coefficient are obtained for the case of normal incidence of a wave on a semi-infinite periodic medium by solving the boundary-value problem. It is shown that an external field can be used to control the spectrum and characteristics of reflected waves.     

Peculiarities of the structure of precrystallization state of amorphous and liquid metals

The phase and structural transformations in the molecular dynamics models of iron and Fe-B alloys are investigated using smooth and oscillating potentials for interaction of Fe-Fe pairs. To analyze the changes in the structure of materials during heat treatment, local time-averaged pair distribution functions and time-dependent van Hove functions are calculated, along with the standard functions. When studying the vibrational spectra, specific atoms are chosen from nanocrystals of different sizes or from the amorphous phase. The differences found in the periods of structural changes before the formation of bcc and fcc crystal phases, in the vibrational spectra of nanocrystals of different sizes, and in the distributions of waves over their wavelengths are described. A simple parameter is proposed to characterize the degree of structural quality of nanocrystals.     

Debye Temperature for Materials with Arbitrary Anisotropy As a Function of the Series of Elastic Moduli

Crystallography Reports - A compact approximation of an expression for an averaged sum of reciprocal cubed phase velocities of elastic waves, which determines the Debye temperature at 0 K in terms...     

Transformation of Light Wave in Absorbing Crystals with Low Symmetry and Determination of their Own Waves Parameters (I)

The decision of the direct and reverse problems in the optics of the low symmetry absorbing crystals are obtained and graphically represented by the specific drawing on the Poincare sphere. The direct problem is connected with the transmission and reflection of the normally incident light wave and reverse problem — with the determination of the own waves parameters. Instead of the usually used boundary conditions the equations of Fresnel and Airy are applied to each of the own waves separately, as the incident, reflected and transmitted waves are represented by the vector sum of the own waves. On the basis of the comparatively simple decision of the direct problem the simpliest method of the reverse problem decision, i.e. for the determination of the own waves parameters in the crystal plate is proposed. In the capacity of the experimental data for this purpose the measured polarization parameters, the reflection- and transmission coefficients of the reflected and transmitted waves must be used as well as the right and left circular polarized incident waves and also two different, but close thicknesses of the crystal plate.