Design of a hexagonal air-coupled capacitive micromachined ultrasonic transducer for air parametric array

An air parametric array can generate a highly directional beam of audible sound in air,which has a wide range of applications in targeted audio delivery.Capacitive micromachined ultrasonic transducer(CMUTs)have great potential for air-coupled applications,mainly because of their low acoustic impedance.In this study,an air-coupled CMUT array is designed as an air parametric array.A hexagonal array is proposed to improve the directivity of the sound generated.A finite element model of the CMUT is established in COMSOL software to facilitate the choice of appropriate structural parameters of the CMUT cell.The CMUT array is then fabricated by a wafer bonding process with high consistency.The performances of the CMUT are tested to verify the accuracy of the finite element analysis.By optimizing the component parameters of the bias-T circuit used for driving the CMUT,DC and AC voltages can be effectively applied to the top and bottom electrodes of the CMUT to provide efficient ultrasound transmission.Finally,the prepared hexagonal array is successfully used to conduct preliminary experiments on its application as an air parametric array.

ACOUSTICAL TRANSMISSION LINE MODEL FOR ULTRASONIC TRANSDUCERS FOR WIDE-BANDWIDTH APPLICATION

An improved model for ultrasonic transducers is proposed. By considering only the first symmetric mode, each layer is represented as an acoustical transmission line in modeling of bulk wave transducers. In imaging applications, wide bandwidth and short time duration are required. The approach we have used consists of impedance matching the front face of the piezoelectric transducer to the propagating medium with a quarter wavelength impedance matching layer and inserting an nnmatching quarter wavelength acoustical layer between the rear face and backing material. A heavy backing would degrade the wide-band phenomena, but show a time duration shorter than 0.5 μs for imaging applications. PSPICE code of the controlled source model is implemented to precisely predict the performance of the matched transducers such as impedance, insertion loss, bandwidth and duration of the impulse response. Good agreement between the simulation and experimental results has been achieved.

Structure of Pb(Zr,Ti)O3(PZT)for Power Ultrasonic Transducer

To improve the performance of ultrasonic transducer, the samples of PZT were improved by doping. The doped PZT was observed and analyzed from the following aspects： the crystal phase structures, the surface morphologies and the dielectric constant. According to the transducer parameter requirements for ultrasonic machining, there are also requirements for the parameters of piezoelectric ceramics. The high performance PZT was prepared by doping the elements of Ga, Ba, Nb, Sn, and Sr in PZT. The doped PZT is suitable for power ultrasonic machining at 20 kHz through analysis using X-ray diffraction（XPD）, a scanning electron microscope（SEM） and LCR meter. Therefore, the excellent performance of transducer for power ultrasonic machining is guaranteed.

Concurrent photoacoustic and ultrasound microscopy with a coaxial dual-element ultrasonic transducer

Simultaneous photoacoustic and ultrasound(PAUS)imaging has attracted increasing attention in biomedical research to probe the optical and mechanical properties of tissue.However,the resolution for majority of the existing PAUS systems is on the order of 1 mm as the majority are designed for clinical use with low-frequency US detection.Here we developed a concurrent PAUS microscopy that consists of optical-resolution photoacoustic microscopy(OR-PAM)and high-frequency US pulse-echo imaging.This dual-modality system utilizes a novel coaxial dual-element ultrasonic transducer(DE-UST)and provides anatomical and functional information with complementary contrast mechanisms,achieving a spatial resolution of 7μm for PA imaging and 106μm for US imaging.We performed phantom studies to validate the system’s performance.The vasculature of a mouse’s hind paw was imaged to demonstrate the potential of this hybrid system for biomedical applications.

Modification of Pb(Zr,Ti)O3 by Doping Mo based on High Power Ultrasonic Transducer

In view of the special requirements of transducers for power ultrasonic processing,the piezoelectric ceramic material Pb(Zr,Ti)O3 was modified by adding other effective elements.The piezoelectric ceramic piece has a good crystal phase structure,a stable piezoelectric constant,a higher dielectric constant,and a lower dielectric loss by adding a small amount of Sr^2+,Ba^2+,Ga^2+,Sn^4+,Nb^5+,etc.These properties are well suited to requirements of power ultrasonic transducer more than 1 000 W.The crystal phase structure and surface morphology of the modified piezoelectric ceramic chips were analyzed by X-ray and scanning electron microscopy (SEM) to prove that the piezoelectric ceramic chips have good crystal structure and density.The dielectric constant of the polarized piezoelectric ceramic chips was measured using an LCR meter.Finally,the piezoelectric ceramic chips were used to design the ultrasonic transducer,and the transducer was analyzed and measured by the impedance analyzer.The measurement results show that the performance of the piezoelectric ceramic chips is reasonable and feasible.

Study on aerosol agglomeration using the airborne ultrasonic transducer

Acoustic agglomeration technology use high-intensity acoustic field to make aerosol particles collide and condense rapidly. Existing studies have shown that 70%–90% of fine particles can be eliminated within minutes using compression drives and air-jet generators. Currently, there are limitations to the sound sources used. In this paper, an airborne ultrasonic transducer with a resonant frequency of 15 kHz is designed, followed by the corresponding numerical simulation and experiments for the evaluation of the vibration modal and sound pressure field. The sound pressure levels (SPL) of the open space and the agglomeration chamber can reach 150 dB and 156 dB, respectively. The agglomeration effect of water droplets, liquid phase smoke, solid phase smoke and mixed smoke is experimentally investigated, and the light transmittance rapidly increases from 8% to 60% within 4 s, 8 s, 5 s and 6 s, respectively. Agglomeration is also effective in the high-frequency range, and we infer that the acoustic wake effect is the predominant mechanism. The elimination effect is promoted with the increasing of SPL until the corresponding secondary acoustic effect is enhanced. Moreover, the agglomeration rate of higher concentration aerosol is significantly better than that of diluted aerosols in ultrasonic agglomeration process.

High piezoelectricity performance in PSBZT ceramic for ultrasonic transducer

Lead zirconate titanate(PZT)ceramics possess great potential for practical applications and thus improving their piezoelectric properties is crucial.Pb0.99−xSm0.01BaxZr0.53Ti0.47O3(PSBZT)ceramics with high Curie temperature and excellent piezoelectric properties were fabricated via a conventional solid-state method,and the effect of Ba2+doping on the structural,dielectric,piezoelectric and ferroelectric properties was studied in detail.It is shown that doping of Ba2+significantly enhanced the piezoelectric properties of PSZT,the maximum d33~533 pC/N and Tc~361°C at x=0.02 were acquired.Furthermore,PSZT and PSBZT ceramics were used to prepare single element ultrasonic transducers,and their performance were compared and evaluated.The results demonstrate that the PSBZT ceramic-based transducer possesses better sensitivity and bandwidth than the PSZT ceramic-based transducer.

Calculation Model for the Steady-State Vibration Amplitude of a New Type of Cascaded Composite Structure-Based Ultrasonic Transducer

The steady-state vibration amplitude is an important performance indicator of high-frequency ultrasonic transducers for ultrasonically assisted manipulating,machining,and manufacturing.This work aimed to develop a calculation model for the steady-state vibration amplitude of a new type of dual-branch cascaded composite structure-based ultrasonic transducer that can be used in the packaging of microelectronic chips.First,the steady-state vibration amplitude of the piezoelectric vibrator of the transducer was derived from the piezoelectric equation.Second,the vibration transfer matrices of the tapered ultrasonic horns were obtained by combining the vibration equation,the continuous condition of the displacement,and the equilibrium condition of the force.Calculation models for the steady-state vibration amplitude of the two working ends of the transducer were then developed.A series of exciting trials were carried out to test the performance of the models.Comparison between the calculated and measured results for steady-state vibration amplitude showed that the maximum deviation was 0.0221μm,the minimum deviation was 0.0013μm,the average deviation was 0.0097μm,and the standard deviation was 0.0046μm.These values indicated good calculation accuracy,laying a good foundation for the practical application of the proposed transducer.

The application of laser reflective tomography in near-field measurements of ultrasonic transducers

With Laser Reflective Tomography（LRT）,the near fields of ultrasonic transducers were measured and analyzed.The principle of LRT measurement of ultrasonic field distribution was introduced and an experimental system was set up.Acoustic pressure of a multiple element piston transducer was measured by using of a laser vibrometer.Its distribution in amplitude and phase was obtained.The acoustic pressure in the same region was measured with a needle hydrophone to validate the LRT method.Furthermore,through reconstruction of acoustic fields,it indicated that LRT method is suitable for predicting the distribution on transducers＇surface and conditions of active elements.

Performance enhancement of ultrasonic transducer made of textured PNN-PZT ceramic

In this paper,0.36Pb(Ni_(1/3)Nb_(2/3))O_(3)-0.24PbZrO3-0.40PbTiO_(3)(PNN-PZT)ceramic was prepared,and texture engineering was performed on this PNN-PZT ceramic to improve its electromechanical properties and temperature stability.Single element ultra-sonic transducers were prepared using PNN-PZT,PNN-PZT textured ceramics,and their performance were evaluated and com-pared using a PZT-5H ceramic based transducer as the benchmark.It is shown that the sensitivity and bandwidth of the PNN-PZT textured ceramic-based transducer are much superior to regular PNN-PZT ceramic and PZT-5H ceramic based transducers.

Study on the sandwiched longitudinal-torsional ultrasonic transducer

The longitudinal-torsional composite mode ultrasonic transducer is studied. The transducer consists of the front and back exponential metal horns, and the axially and tangen-tially polarized piezoelectric ceramic piles. The resonance frequellcy equations fo f the longitu-dinal and torsional vibrations in the transducer are derived. By changing the cross sectional radius decny coefficient of the exponeotial metal horns, the longitudinal and torsional vibrations in the transducer could resonate at the same resonance frequency Experimental results show that the measured resonance frequency is in good agreement with that of theoretical results, and the resonance frequency for the longitudinal vibration is basically the same as that for the torsional vibration in the transducer. This kind of transducer could be used in ultrasonic processing and other high power ultrasonic technologies.

Measurement of acoustic field radiated by low frequency power ultrasonic transducer with laser-interferometer

Based on the piezo-optic effect of medium, the refractive index of medium is the function of its density, and so it's also the function of acoustic pressure. Therefore, acoustic pressure in the optical path everywhere can be determined absolutely by laser-interferometric technique and relative distribution of pressure in the middle and far acoustic field, which can be obtained from theory or experiment respectively. Theory and experiment of measurement of pressure in acoustic field with laser-interferometer are introduced. Distribution of pressure radiated by a power ultrasonic transducer is determined by laser interferometric technique. The theoretical and experimental results are in good agreement. The receiving sensitivity of a PVDF (Polyvinylidene fluoride) transducer in free field is also calibrated absolutely due to above results and its sensitivity is -118.5 dB.

Lead-free piezoelectric materials and ultrasonic transducers for medical imaging

Piezoelectric materials have been vastly used in ultrasonic transducers for medical imaging.In this paper,firstly,the most promising lead-free compositions with perovskite structure for medical imaging applications have been reviewed.The electromechanical properties of various lead-free ceramics,composites,and single crystals based on barium titanate,bismuth sodium titanate,potassium sodium niobate,and lithium niobate are presented.Then,fundamental principles and design considerations of ultrasonic transducers are briefly described.Finally,recent developments in lead-free ultrasonic probes are discussed and their acoustic performance is compared to lead-based transducers.Focused transducers with different beam focusing methods such as lens focusing and mechanical shaping are explained.Additionally,acoustic characteristics of lead-free probes including the pulse-echo results as well as their imaging capabilities for various applications such as phantom imaging,in vitro intravascular ultrasound imaging of swine aorta,and in vivo or ex vivo imaging of human eyes and skin are reviewed.

A fast implementation for computing spatial impulse response of ultrasonic transducers

The spatial impulse response（SIR） method is often used as the ＇gold standard5 in simulation of transient acoustic wave fields due to its high accuracy in the linear domain.However, a high sampling frequency is often required in order to achieve the high accuracy. As a result, a large amount of data has to be processed. In this paper a fast approach for computing spatial impulse response is proposed to reduce the computation burden. The proposed approach is developed by employing the relationship of SIRs at observed points and SIRs of the projection points on the transducer surface. Two critical parameters used in the proposed approach, the calculation sampling frequency and the interpolation sampling frequency, are then analyzed.Results show that for a 2.25 MHz rectangular transducer with the size of 5 mm×10 mm,a calculation sampling frequency of 1000 MHz and an interpolation sampling frequency of500 MHz can achieve superior performance while improving the computation efficiency 18 times than the direct solving.

Temperature and pressure characteristics of ultrasonic transducer and the amplitude spectra correction of echoes

In this paper,the transfer functions of ultrasonic transducers under different temperatures are imitated according to Mason equivalent circuit. The relevant experiments are carried out. The results show that the transfer characteristic of ultrasonic transducer varies with temperature and pressure. Therefore, we present an approach to correct the amplitude spectra of ultrasonic echoes got in different temperature and pressure environmeots. The theoretical simulation and experimental results prove that the approach is simple, effective and practical.

Operating characteristics analysis of a dual-excited full-wavelength piezoceramic ultrasonic transducer

A dual-excited full-wavelength piezoceramic ultrasonic transducer as a cascade of two half-wavelength sandwich piezoceramic transducers is studied.The relevant parameters＇ expressions of the figure of merit N for the transducer are derived,and the effects of the structure and material parameters of the transducer on its characteristics are further analyzed by numerical calculation.The results show that when the two piezoceramic stacks are respectively located at the displacement nodes of their own half-wavelength transducers,or the two piezoceramic stacks have the same number of pieces in the case of a certain number of piezoceramic pieces,the figure of merit of the transducer can reach a maximum.With increasing of the number of piezoceramic pieces in a fairly large range,the figure of merit of the transducer slightly decreases,but the force factor of the transducer increases rapidly.The metal materials of the transducer have little effect on its figure of merit.Thus it can be seen that the dual-excited full-wavelength transducer can effectively increase the volumes of the piezoceramic stacks in the case of that the transducer＇s comprehensive performance has only a little bit of degradation,so it＇s power capacity and load capability can be dramatically improved,which means the transducer is more suitable for high power and heavy load applications.

Circular PVDF Airborne Transducer

With the required increased audio pressure of the parametric ultrasonic transducer array and the difficulty to theoretically analyse the complex ultrasonic structure in audio beam application, an computationally efficient model is desired to describe the characteristic of the parametric ultrasonic transducer array for the system design and optimization. By applying the symmetry boundary conditions at the mid-plane in the thickness direction, a finite element model based on the haft thickness simplification is presented to analyze the parametric circular transducer which is designed by gluing the poly Vinylidene fluoride film （PVDF）. The validity of the proposed model is confn＇med by a comparison of finite element analysis results with the theoretical value and experimental data, which show that they are making a good agreement with each other.

Simulation Study of CMUT for Pressure Sensing Applications

Capacitive micromechanical ultrasonic transducers(CMUTs)have been widely studied because they can be used as substitutes for piezoelectric ultrasonic transducers in imaging applications.However,it is unclear whether and how CMUTs can be developed for sensors incorporating other functions.For instance,researchers have proposed the use of CMUTs for pressure sensing,but fundamental and practical application issues remain unsolved.This study explored ways in which a pressure sensor can be properly developed based on a CMUT prototype using a simulation approach.A three-dimensional finite element model of CMUTs was designed using the COMSOL Multiphysics software by combining the working principle of CMUTs with pressure sensing characteristics in which the resonance frequency of the CMUT cell shifts accordingly when it is subjected to an external pressure.Simultaneously,when subjected to pressure,the CMUT membrane deforms,thus the pressure can be reflected by the change in the capacitance.

Properties study of LiNbO_3 lateral field excited device working on thickness extension mode

This paper investigates the properties of thickness extension mode excited by lateral electric field on LiNbO3 by using the extended Christoffel-Bechmann method. It finds that the lateral field excitation coupling factor for amode （quasi-extensional mode） reaches its maximum value of 28% on Xmcut LiNbO3. The characteristics of a lateral field excitation device made of X-cut LiNbO3 have been investigated and the lateral field excitation device is used for the design of a high frequency ultrasonic transducer. The time and frequency domain pulse/echo response of the LiNbO3 lateral field excitation ultrasonic transducer is analysed with the modified Krimholtz-Leedom-Matthae model and tested using traditional pulse/echo method. A LiNbO3 lateral field excitation ultrasonic transducer with the centre frequency of 33.44 MHz and the -6 dB bandwidth of 33.8% is acquired, which is in good agreement with the results of the Krimholtz-Leedom-Matthae model. Further analysis suggests that the LiNbO3 lateral field excitation device has great potential in the design of broadband high frequency ultrasonic transducers.

Application of Micro Electro Mechanical System(MEMS)Technology in Photoacoustic Imaging

Photoacoustic imaging(PAI)is a new biomedical imaging technology that provides a mixed contrast mechanism and excellent spatial resolution in biological tissues.It is a non-invasive technology that can provide in vivo anatomical and functional information.This technology has great application potential in microscopic imaging and endoscope system.In recent years,the devel-opment of micro electro mechanical system(MEMS)technology has promoted the improvement and miniaturization of the photoacoustic imaging system,as well as its preclinical and clinical appli-cations.This paper introduces the research progress of MEMS technology in photoacoustic micro-scope systems and the miniaturization of photoacoustic endoscope ultrasonic transducers,and points out the shortcomings of existing technology and the direction of future development.