Transformation of acoustic waves in periodic metal grating sandwiched between piezoelectric and dielectric

The mechanism of SAW transformation with variation of film thickness is investigated in a piezoelectric substrate with a metal grating overlaid by a dielectric film, via simulation and visualization of the acoustic fields. By way of example, two orientations of lithium niobate substrates are analyzed, YX-LN and 128°YX-LN, with a Cu grating and an isotropic silica glass overlay. The motions, which follow the wave propagation in the sagittal plane, are visualized within two periods of the grating, with added contour plots showing the shear horizontal displacements. The continuous transformation of the wave's nature is investigated for each wave propagating in the analyzed material structures when the film thickness is increased from zero to a few wavelengths. The examples of the SAW transformation into boundary waves and into plate modes of different polarization have been found and investigated. The behavior of the SAW characteristics in the grating is correlated with transformation of the wave structure with increasing overlay thickness.     

Characteristic parameters of surface acoustic waves in a periodic metal grating on a piezoelectric substrate

The coupling problem between electrodes due to the propagation of acoustic waves in a periodic metal grating on a piezoelectric substrate is theoretically studied. A method for the determination of the mutual admittance between the grating electrodes is presented, and an analytical expression for the contribution of surface acoustic waves (SAWs) is proposed. The SAW characteristic parameters are determined with a numerical technique that is able to deal with a leaky SAW as well as a true SAW (conventional Rayleigh type or Bleustein-Gulyaev type). Using this technique, the SAWs' contribution to the mutual admittance can be removed, and the analysis of other contributions becomes possible. In particular, the amplitude decay rate of the residual mutual admittance with electrode separation gives information about the propagation of the surface skimming bulk waves (SSBWs). The method presented is applied to several currently used material-cut configurations.     

The peculiarities of energy characteristics of acoustic waves in piezoelectric materials and structures

This paper is devoted to detailed theoretical investigation of energy density and power flow of homogeneous (bulk) and inhomogeneous (surface and plate) plane acoustic waves in piezoelectric materials and structures. The analysis of these waves in different materials of various crystallographic orientations allowed us to establish some energy regularities. These regularities are the same for instantaneous energy characteristics of homogeneous waves and for time-average energy characteristics on unit of aperture of inhomogeneous waves if the electrical energy and power flow in vacuum are taken into account. It has been shown that, for strong piezoactive waves, the electric energy density may exceed the mechanical energy density more than three times.     

Electromechanical coupling constant extraction of thin-film piezoelectric materials using a bulk acoustic wave resonator

Thin-film piezoelectric materials such as ZnO and AlN have great potential for on-chip devices such as filters, actuators and sensors. The electromechanical coupling constant is an important material parameter which determines the piezoelectric response of these films. This paper presents a technique based on the Butterworth Van-Dyke (BVD) model which, together with a simple one-mask over-moded resonator, can be used to extract the bulk, one-dimensional electromechanical coupling constant K(2) of any piezoelectrically active thin-film. The BVD model is used to explicitly define the series resonance, parallel resonance, and quality factor Q of any given resonating mode. Common methods of defining the series resonance, parallel resonance, and Q are shown to be inaccurate for low coupling, lossy resonators such as the over-moded resonator. Specifically, an electromechanical coupling constant K(2) of (2.6+/-0.1)% was measured for an (002) c-axis textured AlN film with an X-ray diffraction rocking curve of 7.5 degrees using the BVD based extraction technique.     

Transversely isotropic piezoelectric materials with an arbitrarily shaped boundary

Summary In this paper, a two-dimensional electro-elastic analysis is performed on transversely isotropic piezoelectric materials containing an arbitrarily shaped boundary under out-of-plane mechanical and in-plane electrical loads at infinity. General closed-formed solutions are provided in terms of complex functions by using the conformal mapping technique. Using such solutions, several numerical examples are shown by graphical representation for various arbitrarily shaped boundaries. The effects of stress concentration on arbitrarily shaped boundaries are discussed. The stress intensity factors of mode III at the cusp are computed.     

Analysis of piezoelectric boundary acoustic wave in Cu electrode/Y-cut X-propagating LiNbO3 substrate structure partially covered with SiO2

This paper describes the existence of piezoelectric boundary acoustic wave (PBAW) propagating in a Cu electrode/Y-cut X-propagating (YX) LiNbO(3) substrate structure partially covered with a SiO2 layer. In the analysis, two types of structures are taken into consideration: one is the so-called slotted structure with SiO2 pillars placed in the grating slots; the other is the so-called topped structure with SiO(2) pillars placed on the top of grating electrodes. The top surface could be fully covered with an additional layer (like epoxy) to bridge the grating slots for encapsulation. Results show that SH-type PBAW begins to propagate in the slotted structure when the SiO(2) thickness exceeds 0.3 wavelength. Strong electromechanical coupling factor K(2) of 21%, and temperature coefficient of velocity (TCV) of -33 ppm/°C are obtained. In the topped structure, on the other hand, the boundary acoustic wave mode is not supported. Instead, the thickness resonance modes in the SiO2 pillar do exist. Comparison of the obtained results with those in the structure fully covered with the SiO(2) layer indicates that, as for the PBAW mode, the slotted structure offers improved K(2) but with worse TCV compared with the fully covered SiO(2) structure.     

二维复合压电型表面波声子晶体带隙特性

设计了在镍圆柱体底部添加薄环氧树脂并沉积在铌酸锂基底的二维复合压电型声子晶体.通过有限元法计算了该结构的能带结构、分析了其带隙特性.计算结果显示:通过引入环氧树脂来减小散射体的有效弹性模量,从而降低了声表面波(SAW)完全带隙,同时打开了多条SAW方向带隙.计算了传输损失来验证带隙的存在性且讨论了其传输特性.利用布里渊...     

Integrated high-temperature piezoelectric plate acoustic wave transducers using mode conversion

Piezoelectric thick (>66 mum) films have been directly coated onto aluminum (Al) substrates using a sol-gel spray technique. With top electrode, these films serve as integrated ultrasonic transducers (IUT), which normally operate as thickness longitudinal wave transducers. When such IUT are located at the edges of the metallic plates, they can excite and detect symmetrical, antisymmetric and shear horizontal types of plate acoustic waves (PAW) using mode conversion methods. In 2 mm thick Al plates, 2 line defects of 1 mm width and 1 mm depth were clearly detected at temperatures up to 150degC in pulse-echo mode. Results indicated that, for 2 mm thick aluminum plates, shear horizontal PAW were the best for the line defect detection. Also, the experimental results agree well with those obtained by a finite-difference-based method.     

The energy density and power flow of acoustic waves propagating in piezoelectric materials

It is shown that, for a plane bulk acoustic wave propagating in arbitrary piezoelectric media, the densities of mechanoelectrical and electromechanical energies are always equal in absolute value and have opposite signs. However, in general, the mechanoelectrical and electromechanical power flows of such a wave calculated by the traditional expression for the Poynting vector do not compensate each other, although the total density of these energies is always equal to zero. A discovered discrepancy based on the dissymmetry of piezoelectric constants with respect to the electrical and mechanical indexes may cause difficulties for calculation of important parameters for practical applications such as energy transport velocity of acoustic waves in piezoelectric materials.     

多孔金属材料高温吸声性能测试及研究

以多孔金属材料在高温环境下的吸声特性研究为背景,以不锈钢纤维烧结型多孔金属材料为主要研究对象,首先在理论上研究了计算多孔材料吸声特性随温度变化的特性,然后设计并研制了一种温度可控的高温阻抗管测量装置。通过实验测试,获得了具有不同声学参数的金属纤维板在不同环境温度下的吸声系数,研究了温度对其吸声性能的影响规律。理论结果与实验结果基本吻合,证明了理论分析以及实验装置设计的合理性。     

带声学放大器的行波热声发动机声阻抗特性

通过分析带有声学放大器的行波热声发电系统中直线发电机的电-力-声类比图,发现直线发电机的最佳工作状态与行波热声发动机的输出声阻抗特性相关。采用DeltaEC软件计算带有声学放大器的行波热声发动机（以下简称系统）的输出声阻抗特性。计算结果发现,输出声阻抗虚部X_a为-1×10⁷ Pa·s·m^-3时,系统的最大输出声功率545.47 W,最大热声转换效率为7.2%;当输出声阻抗虚部X_a在-3.9×10⁶~-1×10⁷ Pa·s·m^-3之间变化,实部R_a在1.37×10⁶~2.31×10⁷ Pa·s·m^-3之间时,等效位移在1.89~6 mm之间变化,符合直线发电机的位移要求;结合输出声阻抗对压力与体积流率的相位差及系统工作频率的影响,发现声阻抗实部R_a应在1.37×10⁶~2.31×10⁷ Pa·s·m^-3之间,声阻抗虚部X_a在-7.5×10⁶~-1.0×107 Pa·s·m^-3之间时,系统具有较好的工作状态。     

Comparison of nondestructive microfailure evaluation of fiber-optic Bragg grating and acoustic emission piezoelectric sensors using fragmentation test

Nondestructive evaluation of microfailure mechanisms in two-diameter SiC fibers/epoxy composites is investigated using a directly embedded fiber-optic sensor attached with an acoustic emission piezoelectric (AE-PZT) sensor. Interfacial shear strength by fragmentation test, and optical failure observation inside microcomposite can contribute to analyze two sensors quantitatively. Although fiber Bragg grating (FBG) sensor exhibits sudden wavelength shift due to plastic deformation by larger diameter SiC fiber breakage, AE-PZT monitors much more precise microfailure process, such as the fiber break or matrix cracking. Since the FBG sensor can measure the strain at only a single point, whether it can detect a fiber break in single-fiber composite specimen depends on its proximity to the failure location. In addition, micro-strain measurement at one single point may not provide enough information on the whole microfailure process including multiple fiber breakage and matrix crack. It can be considered that FBG sensor can be somewhat effective in measuring the continuous micro-strain change due to the internal disturbance such as resin curing, whereas AE-PZT sensor can be effective in detecting the microfailure by elastic wave propagation through the composite materials.     

Full-wave analysis of piezoelectric boundary waves propagating along metallic grating sandwiched between two semi-infinite layers

This paper describes full-wave analysis of piezoelectric boundary acoustic waves (PBAWs) propagating along a metallic grating sandwiched between 2 semi-infinite layers. In the analysis, the finite element method (FEM) is used for the grating region while the spectral domain analysis (SDA) is applied for an isotropic overlay region as well as a piezoelectric substrate region. The combination of the FEM and SDA makes the numerical analysis very fast and precise. As an example, the analysis was made on the PBAWs propagating in an SiO₂ overlay/ Cu grating/rotated Y-cut LiNbO₃ structure. It is shown that both the shear-horizontal (SH) type and Rayleigh-type PBAWs are supported in the structure, and that their velocities are very close to each other. Thus spurious responses due to the Rayleigh-type PBAW should completely be suppressed for device implementation. Discussions are made in detail on the influence of Cu grating thickness, substrate rotation angle, and metallization ratio on excitation and propagation characteristics of the SH- and Rayleigh-type PBAWs.     

Characterization of superhydrophobic materials using multiresonance acoustic shear wave sensors

Various superhydrophobic (SH) surfaces, with enhanced superhydrophobicity achieved by the use of nanoparticles, were characterized by a new acoustic sensing technique using multiresonance thickness-shear mode (MTSM) sensors. The MTSM sensors were capable of differentiating SH properties created by nano-scale surface features for film, exhibiting similar macroscopic contact angles.     

Functionally graded piezoelectric materials for modal transducers for exciting bulk and surface acoustic waves

We show that functionally graded piezoelectric materials can be used to make modal actuators through theoretical analyses of the excitation of extensional motion in an elastic rod and Rayleigh surface waves over an elastic half-plane. The results suggest alternatives with certain advantages for the excitation of bulk and surface acoustic waves.     

A boundary element analysis of metal matrix composite materials

The numerical modelling of metal matrix composites is an important part of the research now being conducted on these materials. Due to the numerical complexity of a fully three-dimensional analysis, two-dimensional approximations are normally used with finite element methods. While these analyses are informative, they cannot treat complex particle shapes or examine three-dimensional effects in the composite. The use of boundary element methods in place of the more widely used finite element methods significantly reduces the computing power necessary to obtain a solution to a given problem, making it possible to simulate fully three-dimensional geometries. In the present paper a two-dimensional form of the BEM is applied to the study of metal matrix composite materials, and its performance compared with that of similar FEM stadies. We also compare the predicted composite properties with existing and new experimental results. We conclude that the BEM is an effective tool for the analysis of this class of problems.     

Dispersion and mirror transmission characteristics of bulk acoustic wave resonators

A heterodyne laser interferometer is used for a detailed study of the acoustic wave fields excited in a 932-MHz solidly mounted ZnO thin-film BAW resonator. The sample is manufactured on a glass substrate, which also allows direct measurement of the vibration fields from the bottom of the acoustic mirror. Vibration fields are measured both on top of the resonator and at the mirror-substrate interface in a frequency range of 350 to 1200 MHz. Plate wave dispersion diagrams are calculated from the experimental data in both cases and the transmission characteristics of the acoustic mirror are determined as a function of both frequency and lateral wave number. The experimental data are compared with 1-D and 2-D simulations to evaluate the validity of the modeling tools commonly used in mirror design. All the major features observed in the 1-D model are identified in the measured dispersion diagrams, and the mirror transmission characteristics predicted for the longitudinal waves, by both the 1-D and the 2-D models, match the measured values well.     

Transmitting electric energy through a metal wall by acoustic waves using piezoelectric transducers

The feasibility of transmitting electric energy through a metal wall by propagating acoustic waves using piezoelectric transducers is examined by studying the efficiency of power transmission and its dependence upon the relevant system parameters for a simplified system consisting of an elastic plate sandwiched by two piezoelectric layers. One of these layers models the driving transducer for generating acoustic wave, and the other layer models the receiving transducer for converting the acoustic energy into electric energy to power a load circuit. The output voltage, the output power, and the efficiency of this system are expressed as explicit functions of the system parameters. A numerical example is included to illustrate the dependence of the system performance upon the physical and geometrical parameters.