A new focusing ultrasonic transducer and two foci acoustic lens for acoustic microscopy

The textured piezoelectric film of a new organic-based material produced by vapor deposition was used as an active element of a focusing ultrasonic transducer. The transducer exhibits near theoretical lateral and axial resolution, unipolar pulse response to a step voltage, and 30 dB insertion losses in an octave frequency band. The transducer is acoustically transparent over a wide frequency range and can be fabricated on a concave spherical surface of the standard acoustic lens. The resulting double transducer and two foci acoustic lens are capable of improving the quality of the surface structure imaging or of being used in continuous wave reflection acoustic microscopy.     

Finite element analysis of multilevel acoustic Fresnel lenses

Fresnel lenses have recently emerged as viable alternatives to conventional spherical lenses for focusing ultrasonic waves in acoustic microscopy systems. Although these lenses are relatively straightforward to manufacture, their bulk represents a major handicap. A remedy to this problem is to use multilevel acoustic Fresnel lenses. Multilevel lenses are surface relief structures that can be fabricated very precisely using existing VLSI semiconductor technology. However, accurate tools for designing lenses to achieve the desired efficiency and power specifications are not available. This paper presents a finite element study of multilevel acoustic Fresnel lenses. Results showing ultrasonic wave propagation through such surface relief structures together with the resulting diffraction profiles are presented. The high efficiency and focusing power of these lenses are also demonstrated. Simulation results together with a discussion on various multilevel lens design issues are presented. These results confirm the advantages of such lenses, and suggest that the finite element model can serve as a valuable tool for designing, simulating, and studying lens profiles prior to their fabrication     

线阵相控换能器阵列参数对聚焦性能的影响分析

针对超声相控阵检测中广泛使用的一维线阵换能器,通过建立其声场数学模型和引入指向性函数,仿真分析了阵列参数对声束聚焦性能的影响,提出了改善换能器声场性能的措施。最后对优化参数的阵列换能器声场进行实验测试,测试结果表明其具有良好的声束聚焦性能,为超声相控阵缺陷检测中换能器的选择提供了参考。     

The focusing of surface-acoustic-waves launched from a slanted chirped transducer. I. Isotropic substrate

The focusing behavior of surface acoustic waves launched from a slanted chirped transducer (SCT) is explored using both a continuum model and a discrete model for sources associated with a linear FM chirp transducer on a substrate with isotropic properties. The continuum model leads to the prediction of an understanding of effects that would arise in the radiation field of an SCT on an isotropic substrate. It is based on a representation of the transducer as a continuum of sources, with the radiation field determined by applying the stationary phase method, and is presented for infinitesimally short fingers. It is compared with a more direct and exact but less revealing method of determining the field based on a discrete array of sources. The effects of increasing finger lengths are considered. The results are related to the focusing of radiation by a lens.     

基于气体基压电复合材料的线聚焦空耦超声传感器研制与应用

空气耦合式超声检测技术因具有非接触、无损伤等特点,被广泛应用于材料的非接触检测。本文从晶硅太阳能电池的实际检测需求出发,设计并制作了一种气体基线聚焦空气耦合（空耦）式超声传感器,与传统的聚合物基空气耦合（空耦）式超声传感器相比,气体基线聚焦空耦传感器利用了3D打印技术将聚合物基框架改进为镂空结构,进一步降低了压电复合材料的声阻抗。所研制的传感器中心频率约为150 kHz,聚焦半径为20 mm,孔径为28 mm。对传感器进行了激励接收性能测试,并采用空耦超声Lamb波检测技术,对含有裂纹缺陷的单晶硅太阳能电池片进行非接触式检测,通过分析接收信号的幅值信息并利用相关系数法,完成了对裂纹缺陷的检出和定位,实现了气体基线聚焦空耦传感器在缺陷检测中的应用。     

A new ultrasonic composite transducer implemented with thick filmtechnology

A new ultrasonic transducer operating in air and fabricated by thick film technology is presented. It consists of a composite mechanical structure in which three active elements, made with planar alumina substrates supporting PZT-based piezoelectric films, behave as elastic guides. The active elements have a constant rectangular cross-section and are radially placed, 120° apart. They are mounted between a rigid base (operating as the acoustic backing) and an alumina disk. On both sides of each substrate a piezoelectric layer has interdigital thick-film electrodes embedded in layer itself, so that it can be polarized parallel to the long side of the active element; consequently, it allows the creation of periodic surface stresses responsible for symmetric extensional strains in the alumina plates. The elements, synchronously driven in phase, form, with the disk, a vibrating resonant structure, which radiates acoustic energy perpendicular to the disk surface. The assembly of active elements, acoustic backing and radiating disk forms a composite ultrasonic transducer operating in air without the need of any acoustic matching layer. Among the prominent features offered by the transducer design are: a transduction efficiency of 25% at an exiting voltage of 100 V_p and a high directivity of the emitted beam     

A performance study of a laser Doppler vibrometer for measuring waveforms from piezoelectric transducers

The stresses at transducer contacts were estimated from accurate particle velocity measurements by using a laser Doppler vibrometer (LDV). We then evaluated the performance of LDV for ultrasonic waveform measurements in physical model experiments that are employed for modeling seismic observations. For such experiments, the characteristics of the source and detector should be exactly known. Discshaped compression and shear-mode piezoelectric transducers were attached on a flat surface of a steel semicircular column, and ultrasonic waves were excited by single-shot sine waves with 0.25, 0.5, and 1 MHz frequencies. Radial and transverse components were measured by LDV at a distance of 150 mm from the source. The maximum amplitudes of waves with respect to radiation angle give a radiation pattern of a transducer. Each observed radiation pattern was fitted to the calculated radiation pattern by assuming a harmonically oscillating stress distributed uniformly on a flat circular area. The observed radiation patterns show fairly good agreement with the calculated radiation patterns for both radial and transverse components when the source frequencies are 0.25 and 0.5 MHz. Because the best-fit stress values were independently estimated from the radial and the transverse radiation patterns, the 2 stress values should be equal for each source and frequency. The discrepancy between the estimated radial and transverse stress values becomes larger as the source frequency increases. Provided that coincidence of the 2 stress values indicates the validity of waveform measurements, the results suggest that LDV is applicable for measuring the 3-D particle-velocity at frequencies up to 0.5 MHz.     

Analytical performance of a venturi-assisted array of micromachined ultrasonic electrosprays coupled to ion trap mass spectrometry for the analysis of peptides and proteins

The analytical characterization of a novel ion source for mass spectrometry named array of micromachined ultrasonic electrosprays (AMUSE) is presented here. This is a fundamentally different type of ion generation device, consisting of three major components: (1) a piezoelectric transducer that creates ultrasonic waves at one of the resonant frequencies of the sample-filled device, (2) an array of pyramidally shaped nozzles micromachined on a silicon wafer, and (3) a spacer which prevents contact between the array and transducer ensuring the transfer of acoustic energy to the sample. A high-pressure gradient generated at the apexes of the nozzle pyramids forces the periodic ejection of multiple droplet streams from the device. With this device, the processes of droplet formation and droplet charging are separated; hence, the limitations of conventional electrospray-type ion sources, including the need for high charging potentials and the addition of organic solvent to decrease surface tension, can be avoided. In this work, a Venturi device is coupled with AMUSE in order to increase desolvation, droplet focusing, and signal stability. Results show that ionization of model peptides and small tuning molecules is possible with dc charging potentials of 100 Vdc or less. Ionization in rf-only mode (without dc biasing) was also possible. It was observed that, when combined with AMUSE, the Venturi device provides a 10-fold gain in signal-to-noise ratio for 90% aqueous sample solutions. Further reduction in the diameter of the orifices of the micromachined arrays led to an additional signal gain of at least 3 orders of magnitude, a 2-10-fold gain in the signal-to-noise ratio and an improvement in signal stability from 47% to 8.5% RSD. The effectiveness of this device for the soft ionization of model proteins in aqueous media, such as cytochrome c, was also examined, yielding spectra with an average charge state of 8.8 when analyzed with a 100 Vdc charging potential. Ionization of model proteins was also possible in rf-only mode.     

Anomalies of the boundary reflection of ultrasound from a Maxwellian liquid

Reflection of continuous and pulsed longitudinal and transverse acoustic waves from a dissipative medium represented by a model of Maxwellian liquid in contact with a solid halfspace is considered theoretically. The substantial dependence of the modulus and phase of the reflection coefficient on both the viscosity and time of relaxation of stresses in the Maxwellian liquid is shown. Using computer programs, the acoustic pulsed signals reflected from the interface between the media and transmitted through it have been calculated. The calculations were performed for an asymmetric shape of the signal incident on the interface; this shape corresponds to that of a real signal emitted by an ultrasonic piezoceramic transducer.     

SAW focusing by circular-arc interdigital transducers on YZ-LiNbO(3)

Using the angular spectrum theory and experimental velocity data of surface acoustic waves (SAW) on YZ-LiNbO(3), the focusing characteristics of a circular-arc interdigital transducer have been demonstrated. The calculated results show that the depth of focus is long and the compressed acoustic beam width is very narrow. The concept of a caustic is shown to be an excellent way of characterizing SAW focusing by a circular-arc interdigital transducer on YZ-LiNbO(3). Comparison between theoretical and experimental results shows good agreement.     

c-Axis zig-zag ZnO film ultrasonic transducers for designing longitudinal and shear wave resonant frequencies and modes

A method for designing frequencies and modes in ultrasonic transducers above the very-high-frequency (VHF) range is required for ultrasonic non-destructive evaluation and acoustic mass sensors. To obtain the desired longitudinal and shear wave conversion loss characteristics in the transducer, we propose the use of a c-axis zig-zag structure consisting of multilayered c-axis 23° tilted ZnO piezoelectric films. In this structure, every layer has the same thickness, and the c-axis tilt directions in odd and even layers are symmetric with respect to the film surface normal. c-axis zig-zag crystal growth was achieved by using a SiO(2) low-temperature buffer layer. The frequency characteristics of the multilayered transducer were predicted using a transmission line model based on Mason's equivalent circuit. We experimentally demonstrated two types of transducers: those exciting longitudinal and shear waves simultaneously at the same frequency, and those exciting shear waves with suppressed longitudinal waves.     

Focused high frequency needle transducer for ultrasonic imaging and trapping

A miniature focused needle transducer (<1?mm) was fabricated using the press-focusing technique. The measured pulse-echo waveform showed the transducer had center frequency of 57.5 MHz with 54% bandwidth and 14?dB insertion loss. To evaluate the performance of this type of transducer, invitro ultrasonic biomicroscopy imaging on the rabbit eye was obtained. Moreover, a single beam acoustic trapping experiment was performed using this transducer. Trapping of targeted particle size smaller than the ultrasonic wavelength was observed. Potential applications of these devices include minimally invasive measurements of retinal blood flow and single beam acoustic trapping of microparticles.     

Modeling of ultrasonic wave propagation in teeth using PSpice: a comparison with finite element models

Ultrasound is used extensively in the medical field for the detection and characterization of a variety of features in the human body. Finite element models used to understand ultrasonic wave propagation in teeth have been developed so that ultrasound techniques could be realized in dentistry. This paper presents a hypothesis that underlies one possible design of an ultrasonic tool that can be used in a clinical environment, as well as several models that describe acoustic field simulation, propagation, and interaction with the layers of several tooth structures. A complete PSpice model of a single-element transducer based on Redwood's version of Mason's equivalent circuit, a focusing lens, and a multi-layer tooth structure is used to illustrate the validity of this hypothesis. Transmission line theory is employed as a basis for the models of the piezoceramic, the lens, and the different tooth layers. Results clearly depict the transmission and reflection of the ultrasonic waves as they travel through the layers within the tooth structure and point out the noticeable similarity to longitudinal L-wave signatures produced by axisymmetric finite element models presented in earlier studies.     

A parametric study of ultrasonic beam profiles for a linear phased array transducer

A numerical simulation model is presented to investigate the influences of design parameters of linear phased array transducers on beam focusing and steering performance. The characteristic of ultrasonic beam profiles has been simulated on the basis of the Huygen's superposition principle. For the simulation, a linear phased array is considered as the composition of finite number of elements separated by equidistance. Individual elements are considered as two-dimensional point sources. The waves generated from piezoelectric elements are considered as simplified transient ultrasonic waves that are constructed with the cosine function enveloped with a Hanning window. The characteristic of ultrasonic wave propagation into a medium from the phased array transducer is described. The effects of the number, the interelement spacing, steering angle, the focal length, and frequency bandwidth of the piezoelectric elements on beam directivity and ultrasonic pressure field in a linear phased array transducer are systematically discussed.     

Focusing with an optoacoustic transducer

A prototype of an ultrasonic transducer has been developed that uses an optical input to control the ultrasonic output. This transducer is called an optoacoustic transducer (OAT) and provides an ultrasonic pattern that is spatially similar to the optical pattern used to illuminate it. When a focus-inducing optical pattern, such as a zone plate, is used, an acoustic focus is achieved. The success of this procedure depends on the use of amplitude-modulated light at the input and on filtering the received signal to eliminate the primary frequency. This provides an increase, from 30 dB to 70 dB, in the ratio of acoustic pressure in the illuminated regions to that in the dark regions. The prototype operates at 2.8 MHz and has been used to provide a good acoustic focus in water. A 3-dB beamwidth of 3.5 mm was measured at a range of 92 mm. The construction techniques and materials used are discussed.     

Finite Element Evaluation of Acoustic Energy Penetration of Partially Submerged Tube Bundles

In this paper, the finite element software ANSYS is used to model ultrasonic waves propagating through a liquid volume containing partially submerged tubes. An immersible transducer is used to generate the waves. The goal of the investigation is to find an appropriate excitation frequency in order to perform ultrasonic cleaning of the tubes. Modal analysis of the coupled tubes–liquid system is conducted to evaluate the dynamic behavior of the tube structures under ultrasonic wave excitations. The frequency at which the acoustic waves efficiently penetrate the tube array with least energy loss and least deformation to tube structures is obtained, and will be used to probe the capability and the potential of utilizing ultrasonic energy as a non-destructive technique for cleaning tube bundles.     

开口凹球面聚焦声场分析

高强度聚焦超声（HIFU）治疗局部肿瘤存在聚焦换能器和定位探头同时旋转的问题，考虑到实际工程中必须考虑B超探头的放置对聚焦声场的影响。文章就此进行了研究。结果表明：开口的凹球面会降低焦区声压幅度。随着开孔半径的变大。焦区声压呈下降的趋势，焦区越长，越不利于聚焦；随着开孔位置距轴线的距离变大。焦区声压呈下降的趋势，而焦区的位置变化不大，文章同时对相应的温度场也进行了相关的分析。     

B超诊断仪凸阵阵元高度方向复合聚焦声场的研究

文章研究B超诊断仪凸阵阵元高度方向复合聚焦声场的计算推导了方便实用的计算公式。然后进行了实验和数值计算,实验结果与理论预测相当吻合,偏差不超过7％。最后在计算机上进行数值仿真,进一步研究了声透镜的几何参数影响声场参数的规律。     

Building three-dimensional images using a time-reversal chaotic cavity

The design of two-dimensional (2-D) arrays for three-dimensional (3-D) ultrasonic imaging is a major challenge in medical and nondestructive applications. Thousands of transducers are typically needed for focusing and steering in a 3-D volume. In this article, we propose a different concept allowing us to obtain electronic 3-D focusing with a small number of transducers. The basic idea is to couple a small number of transducers to a chaotic reverberating cavity with one face in contact with the body of the patient. The reverberations of the ultrasonic waves inside the cavity create at each reflection virtual transducers. The cavity acts as an ultrasonic kaleidoscope multiplying the small number of transducers and creating a much larger virtual transducer array. By exploiting time-reversal processing, it is possible to use collectively all the virtual transducers to focus a pulse everywhere in a 3-D volume. The reception process is based on a nonlinear pulse-inversion technique in order to ensure a good contrast. The feasibility of this concept for the building of 3-D images was demonstrated using a prototype relying only on 31 emission transducers and a single reception transducer.     

The scattering of surface acoustic waves by electrical effects in two-dimensional metal film structures

A theory is presented for the scattering of surface acoustic waves by electrical effects in thin metal films of arbitrary shape on the surface of a piezoelectric material. A Green's function approach is used, and an expression for the two-dimensional Green's function appropriate to the problem is given. General expressions are obtained for the far-field radiation pattern and for the response of an interdigital transducer to a single scatterer. Expressions for the reflection and velocity perturbation due to periodic arrays of scatterers are also presented. Good agreement is found when the theoretical predictions are compared with a wide range of experimental results on lithium niobate.