Acoustic wave effects on catalysis: design of surfaces with artificially controllable functions for chemical reactions

The effects of surface acoustic wave (SAW) and resonance oscillation (RO) of bulk acoustic waves on the catalysis of metals were studied in an attempt to design a catalyst surface with artificially controllable functions for chemical reactions. In ethanol decomposition on a thin Cu film catalyst deposited on the propagation path of a shear horizontal leaky SAW, the SAW-on increased the activity for ethylene production remarkably but a little for acetaldehyde production. A poled ferroelectric z-cut LiNbO₃ with a thickness extensional mode RO (TERO) and a x-cut LiNbO₃ with a thickness shear mode RO (TSRO) were employed as a substrate, on which a thin Ag film catalyst was deposited. For ethanol decomposition, TERO increased ethylene production activity and the selectivity for ethylene production from 79 to 96%, whereas TSRO caused little activity enhancement for both ethylene and acetaldehyde production. The combination with the results of laser Doppler measurements showed that the activity enhancement and selectivity changes with SAW and RO of the acoustic waves are associated with dynamic large lattice displacement vertical to the surface.     

Analysis of spurious bulk waves in ball surface wave device

We analyzed the acoustic waves propagating in a sphere to establish a useful guideline for the design of NDE apparatus and ball surface acoustic wave (SAW) device exploiting the diffraction-free propagation of SAW on a sphere. First, we calculated the laser-generated acoustic displacements both under ablation condition and under thermoelastic condition and verified experimentally the validity of the calculation. Next, the acoustic waves excited by out-of-plane stress and those excited by in-plane stress were compared. The results showed that when the out-of-plane stress was applied, the relative amplitudes of the bulk waves to that of the SAW were larger and the number of bulk waves was larger than that when the in-plane stress was applied, while the SAW had similar waveforms in each case. The ratio of the relative amplitude of the bulk waves for the out-of-plane stress and the in-plane stress was 3.1:1 at phi(1)=90 degrees and 1.67:1 at phi(1)=0 degrees. The large amplitude for the out-of-plane stress can be explained by wide directivities of bulk waves. Consequently, we found that it is necessary for ball SAW device to select a piezoelectric material and form of interdigital transducer so that the in-plane stress becomes dominant.     

Interdigital transducer analysis using equivalent PSpice model

Interdigital transducers generating surface acoustic waves have found numerous applications. The present paper deals with an equivalent model of a single interdigital transducer as well as an SAW filter complying with the PSpice programme for the analysis of electronic systems. The suggested model makes it possible to take into account such effects as interelectrode reflections, losses along the acoustic path and the electromagnetic coupling between the transducers. The obtained results of the analysis of an interdigital transducer comply with the results obtained basing on an admittance model and delta-function model. The final aim is to model an SAW oscillator.     

Measurement of material nonlinearity using surface acoustic wave parametric interaction and laser ultrasonics

A dual frequency mixing technique has been developed for measuring velocity changes caused by material nonlinearity. The technique is based on the parametric interaction between two surface acoustic waves (SAWs): The low frequency pump SAW generated by a transducer and the high frequency probe SAW generated and detected using laser ultrasonics. The pump SAW stresses the material under the probe SAW. The stress (typically <5 MPa) is controlled by varying the timing between the pump and probe waves. The nonlinear interaction is measured as a phase modulation of the probe SAW and equated to a velocity change. The velocity-stress relationship is used as a measure of material nonlinearity. Experiments were conducted to observe the pump-probe interaction by changing the pump frequency and compare the nonlinear response of aluminum and fused silica. Experiments showed these two materials had opposite nonlinear responses, consistent with previously published data. The technique could be applied to life-time predictions of engineered components by measuring changes in nonlinear response caused by fatigue.     

Surface acoustic wave opto-mechanical oscillator and frequency comb generator

We report on realization of an efficient triply resonant coupling between two long lived optical modes and a high frequency surface acoustic wave (SAW) mode of the same monolithic crystalline whispering gallery mode resonator. The coupling results in an opto-mechanical oscillation and generation of a monochromatic SAW. A strong nonlinear interaction of this mechanical mode with other equidistant SAW modes leads to mechanical hyperparametric oscillation and generation of a SAW pulse train and associated frequency comb in the resonator. We visualized the comb by observing the modulation of the light escaping the resonator.     

Surface-acoustic-wave-induced space-charge waves in electron-hole systems

Space-charge waves in an electron-hole system are studied, which are excited by a moving grating provided by a surface acoustic wave (SAW). The SAW induces a constant current that may change its sign, when a constant electric field is applied opposite to the wave propagation direction. Current resonances are predicted to appear, when the SAW wavelength and frequency match the ones of the space-charge wave.     

The radiation efficiency of SAW periodic strips analysis at higher overtones based on the spectral theory

The harmonic response analysis of the surface acoustic waves interdigital transducer is presented in the paper. It is studied how the metallization ratio affects the element factor of periodic interdigital transducers. The spectral theory has been employed in the study. SAW wave-number for both the cases of isolated and short-circuited strips affects the element-factor characterizing generation and detection efficiency of periodic strips. The results for the radiation efficiency of strips for different metallization ratio is presented. This approach should be applicable to modeling the harmonic responses of the surface acoustic waves interdigital transducers. An example of the experimental measurement of the filter is presented in the paper.     

Comparative analysis of the search procedures for surface acoustic wave solutions in piezoelectric crystals

Various search procedures for finding the global extremum of a multivariate target function (TF) necessary for calculating the characteristics of surface acoustic waves (SAW) and leaky surface acoustic waves (LSAW) in crystals are analyzed. The search procedures aimed at determining the optimal orientations for SAW in crystals are considered. A comparative analysis of the promising methods for finding the global extremum of the TF is performed.     

Wideband high-frequency line-focus PVDF transducer for materials characterization

This paper presents the design, fabrication, operating characteristics and applications of a wideband, high-frequency, line-focus beam transducer we constructed using a 9 microm thick piezoelectric polyvinylidene fluoride (PVDF) film. This transducer possesses a focal length of 2.38 mm and an aperture angle of 84 degrees. The frequency spectrum of the signal measured at the focal point indicates that the transducer has a wide frequency response which extends from 10 MHz to over 100 MHz. When compensated for the frequency-dependent attenuation of the coupling medium, the operational frequency exceeds 150 MHz. The transducer can be operated in a time-resolved pulse mode or in a radio-frequency (rf) tone burst mode. An application of the transducer to determine the anisotropic elastic property of a silicon wafer is demonstrated. The phase velocities of surface acoustic waves (SAW) propagating along various directions on the (001) surface of cubic silicon are measured and compared to computed values.     

液体表面波振幅与激发深度的关系

研究了液体表面波振幅随激发深度的变化特性.采用探针式激发器激发,观察到液体表面波的衍射图样|改变激发深度,得到不同激发深度处的衍射图样|根据衍射图样分布与表面波振幅之间的关系并进行计算机编程,获得表面波振幅与激发深度之间的关系.用最小二乘法拟合,发现|液体表面波振幅随激发深度的增加而减小,并呈指数规律.     

Surface Acoustic Wave Induced Birefringence in In0.21Ga0.79As-GaAs Multiple Quantum Wells for Optical Modulation ≈1000 nm under Normal Incidence

This paper presents observation of quantum Confined Stark effect induced by the surface acoustic waves (SAW) propagating in multiple quantum well (MQW) layers. It also presents the evidence for the first time of SAW induced birefringence under surface normal mode operation in MQWs. MQW layers usually do not exhibit significant birefringence when the light is normally incident. The launching of surface acoustic waves in In_0.21Ga_0.79As-GaAs MQW layers produces significantly different strain along the direction of propagation (y) as compared to the lateral direction (x), and this in turn results in differential absorption coefficient (= _x - _y) and birefringence (n_bir). The enhanced birefringence due to excitonic effect depends on the magnitude of RF power exciting the SAW transducer. An interdigitated transducer operating at 119 MHz with an aperture (w) to wavelength () ratio of 25 was used to launch the surface acoustic waves. Typical values of n are in the range of 0.01-0.02 for polarized light at 1.01 m wavelength when the RF power is varied between 0.5-1.2 Watt. Computations of n are in agreement with experimental data. The SAW propagation also induces a perpendicular component E_z of electric field which results in conventional Stark effect producing absorption and index changes in MQWs that vary as a function of SAW power.     

研究激光激发的声表面波与材料近表面缺陷的振荡效应

根据激光激发声表面波的热弹运动方程及热传导方程, 采取有限元技术对方程进行求解, 得到声表面波传播波形图. 当声表面波经过近表面缺陷时, 声表面波与近表面缺陷之间产生一种振荡效应, 通过近表面缺陷的振荡波形幅值存在一个逐渐增加后又逐渐减小的过程. 当声表面波经过不同深度的近表面缺陷时, 振荡信号中心频率存在一定的变化规律. 数值仿真结果表明: 当近表面缺陷深度从0.1 mm到0.5 mm变化时, 振荡效应产生的振荡信号中心频率从0.4 MHz到0.76 MHz变化, 振荡信号中心频率与近表面缺陷深度呈近似线性关系, 这为近表面缺陷的定量检测提供了一种理论基础.     

Investigation of surface acoustic waves in laser shock peened metals

Laser shock peening is a well-known method for extending the fatigue life of metal components by introducing near-surface compressive residual stress. The surface acoustic waves (SAWs) are dispersive when the near-surface properties of materials are changed. So the near-surface properties (such as the thickness of hardened layers, elastic properties, residual stresses, etc.) can be analyzed by the phase velocity dispersion. To study the propagation of SAWs in metal samples after peening, a more reasonable experimental method of broadband excitation and reception is introduced. The ultrasonic signals are excited by laser and received by polyvinylindene fluoride (PVDF) transducer. The SAW signals in aluminum alloy materials with different impact times by laser shock peening are detected. Signal spectrum and phase velocity dispersion curves of SAWs are analyzed. Moreover, reasons for dispersion are discussed.     

Simulation and design on SAW reflective delay line for wireless sensor application using coupling of modes

This paper presents an optimal design on surface acoustic wave reflective delay line using coupling of modes（COM）model.The reflection coefficient S₁₁of the SAW device is deduced to evaluate the device performance.Typical SAW reflective delay line consists of a piezoelectric substrate,an interdigital transducer（IDT）and several reflectors positioned along the acoustic propagation direction.The influences of the design parameters of the device, such as structure of the IDT,reflector types,etc.on device performance are investigated,and the optimal design parameters are determined.Based on these parameters,a 434 MHz SAW reflective delay line with single phase unidirectional transducers and three shorted grating reflectors is fabricated on 41°YX LiNbO₃.The measured S₁₁agrees well with the simulated one.Sharp reflection peaks,high signal noise ratio（S/N）,and low spurious noise between the reflection peaks are observed.     

Characterization of lead zirconate titanate ceramics using surface acoustic wave

Lead zirconate titanate (PZT) is widely used as a piezoelectric transducer material for bulk and surface acoustic wave (SAW) devices. In this Paper a novel laser-ultrasound technique for measuring the SAW properties of PZT for quality control purposes is presented.     

谐振式检测器谐振腔对声表面波气相色谱仪灵敏度的影响

研究了谐振式检测器谐振腔对声表面波气相色谱仪灵敏度的影响。构建无液膜负载和有液膜负载下压电基片和金属指条结构的三维有限元模型,利用有限元方法提取不同周期结构的耦合模参量,结合P矩阵级联技术,推导出传感器灵敏度随检测器腔体结构改变的变化关系。实验制备了两种三换能器结构的声表面波谐振器,其输入换能器与输出换能器间距离分别为22.25倍波长和1.25倍波长,通过测试甲基膦酸二甲酯气体,验证理论计算结果。结果表明:缩短谐振器的谐振腔长度使器件中心处能量更为集中,有效地提高了声表面波气相色谱仪灵敏度。      

Visualization of surface acoustic wave scattering by dislocations

Stroboscopic X-ray topography at the synchrotron beam line was used to visualize the propagation of a 580 MHz surface acoustic waves (SAW) in LiNbO3 crystals. For this purpose, the X-ray bursts coming from the synchrotron storage ring with periodicity of 5.68 MHz were synchronized with the SAW frequency in a phase-locked mode. This method allowed us to "stop" the SAW in time and to observe the X-ray diffraction contrast caused by the dynamic deformation field of SAW. The X-ray topographic images showed well-resolved individual acoustic wave fronts of 6 microm SAW as well as their distortions due to SAW scattering by linear dislocations. Some of the images revealed an exceptional contrast of the concentric rings about the dislocation line, which is caused by coherent interaction of the secondary elastic waves. This contrast is similar to the Fresnel zones in optics, and this conclusion is confirmed by direct summation of secondary waves emitted by local elements of a vibrating dislocation string.     

Thermal stabilization of SAW devices by metal films

The characteristics of surface acoustic waves (SAW) propagating in a structure that consists of a metal layer overlying an YX-cut quartz crystal are numerically analyzed for a wide range of operating temperatures. It is shown that the presence of a metal film of a certain thickness on the surface of an YX-cut quartz crystal considerably improves the thermal stability of the characteristics of SAW propagating in such a structure.     

Measuring scanning acoustic microscope with a harmonic sounding signal

A concept of a scanning acoustic microscope with a harmonic sounding signal for measuring the parameters of local homogeneous regions of flat samples is proposed. The distinctive feature of the device is the utilization of the Doppler effect that occurs in the sounding wave reflected from the sample surface when the sample is uniformly moved relative to the focusing ultrasonic transducer of the microscope. It is theoretically demonstrated that the spectrum of the received signal is determined by the product of the reflection coefficient and the transfer function of the transducer. The errors of the measurement technique are considered, and the sources of signal distortions are analyzed. High sensitivity of the measurement results to the errors of the scanning system is demonstrated. The developed measuring microscope is described, in which an acoustic interferometer is used to provide the necessary precision of the scanning coordinate measurement. The microscope transfer function is measured for the frequency of the sounding signal 65 MHz, and the values of density and bulk wave velocities are determined for a homogeneous sample by the measured phase of the reflection coefficient using the technique of nonlinear estimation of parameters. With fused quartz used as an example, it is shown that the measurement error is 7.2% for density and 2.3 and 0.7% for the velocities of longitudinal and transverse waves, respectively. In addition, the velocity of a leaky surface wave (SAW) is determined by two methods. One method is based on measuring the position of the inflexion point for the experimental phase of the reflection coefficient, and the other is based on calculating the SAW velocity corresponding to the measured values of density and bulk wave velocities. The errors of these methods are found to be equal to 0.42 and 0.17%, respectively.     

由于表面粗糙引起的激光声表面波色散的实验和理论研究

表面粗糙是材料制造过程中必有的副产物, 粗糙表面会引起其中传播的声表面波的速度发生变化. 在利用激光声表面波对材料性质进行评估时, 常用宽带的激光声表面波速度频散特性对材料性质进行反演. 为了研究表面粗糙度是否能作为反演的特征参数之一, 本文建立了激光在表面粗糙样品中激发声表面波、聚偏氟乙烯换能器宽带接收声表面波的实验装置来研究不同粗糙度表面对声表面波速度的影响; 理论上建立了激光在粗糙表面中激发声表面波的计算模型, 利用有限元法得到声表面波的时域特征, 并进一步得到声表面波的速度色散曲线, 理论结果和实验结果能很好地拟合. 这为利用激光声表面波对表面粗糙的评估提供理论和实验依据. 关键词： 表面粗糙 激光声表面波 速度色散 聚偏氟乙烯传感器 有限元法