Disposable PVDF ultrasonic transducers for nondestructive testingapplications

Disposable ultrasonic contact transducers have been constructed with inexpensive PVDF films for nondestructive testing (NDT) applications. This paper reports the temperature-dependent ultrasonic performance of commercial polyvinylidene fluoride (PVDF) films and PVDF sensors. PVDF film was evaluated for its material properties of interest for ultrasonic transducer performance including the relative dielectric constant ε_r, dielectric loss tangent tan δ_e , electromechanical coupling constant k_t, mechanical quality factor Q_m, and acoustic impedance Z. Disposable PVDF transducers were then coupled to both low and high acoustic impedance test panels and their ultrasonic insertion loss/sensitivity and frequency bandwidth measured as a function of temperature in both the pulse-echo and pitch-catch modes. The results yielded center frequencies in the 3.5-16 MHz range, with -6-dB fractional bandwidths for some of the transducers exceeding 100%. The temperature dependence of the transducers' performance is reported for the -40 to 80°C range and shows an approximate linear decrease in center frequency and increase in fractional bandwidth with increasing temperature. These measured results are compared with predictions from computer simulations based on Mason's model. These tests showed that the same disposable transducers can be used for both ultrasonic and acoustic emission sensing NDT applications. This paper summarizes the quantitative ultrasonic and vibration sensing performance for all of the disposable PVDF film transducers which were tested     

铁电驻极体空气耦合声换能器的研制

基于多孔聚丙烯铁电驻极体薄膜系统研制了平面型和球型聚焦空气耦合超声波换能器。平面型换能器孔径为20mm,两个球型聚焦换能器的孔径和焦距分别为20mm和35mm、30mm和40mm。使用激光干涉仪测得了三个换能器作为发射器工作时的带宽和谐振频率,并且将在脉冲回波模式下测得的换能器作为接收机工作时的响应与激光干涉仪测试结果进行比较。最后选择孔径为20mm的球型聚焦换能器,在脉冲回波模式下对不同直径孔的聚乙烯阶梯楔进行扫描成像。     

High-frequency transducers based on integrated piezoelectric thick films for medical imaging

A screen-printed PZT thick film with a final thickness of about 40 microm was deposited on a porous PZT substrate to obtain an integrated structure for ultrasonic transducer applications. This process makes it possible to decrease the number of steps in the fabrication of high-frequency, single-element transducers. The porous PZT substrates allow high acoustic impedance and attenuation to be obtained, satisfying transducer backing requirements for medical imaging. The piezoelectric thick films deliver high electromechanical performance, comparable to that of standard bulk ceramics (thickness coupling factor over 45%). Based on these structures, high-frequency transducers with a center frequency of about 25 MHz were produced and characterized. As a result, good sensitivity and axial resolution were obtained in comparison with similar transducers integrating a lead titanate (PT) disk as active material. The two transducers were integrated into a high-frequency imaging system, and comparative skin images are shown.     

Cylindrical PVDF film transmitters and receivers for air ultrasound

Cylindrical polyvinylidene fluoride (PVDF) film transducers for transmission and reception of 40-kHz ultrasonic waves in air have been investigated. A key feature of such transducers is their omni-directional polar response. An optimized structure comprises a cylindrical PVDF film element resting on a spool without a mechanical bond to it. Various key design equations to obtain the required ultrasonic performance both as transmitter and receiver are shown, which include resonance frequency, acoustic pressure, angle performance, back air cavity effect, and receiver sensitivity. Measurements of actual frequency response of transmitter output and receiver sensitivity, angular performance, back air space effect, and temperature effect are presented. The results agree well with the theoretical predictions. It has been shown that this device is well-suited for practical application as an ultrasonic ranging device     

Surface acoustic wave characteristics and electromechanical coupling coefficient of lead zirconate titanate thin films

Lead zirconate titanate (PZT) thin films were created on ST-X quartz using radio frequency magnetron sputtering deposition. PZT films deposited on quartz are used as a new piezoelectric substrate for surface acoustic wave (SAW) devices. Microelectromechanical technique was used to fabricate interdigital transducers on the surface of the substrate to be used as a SAW delay line device. The results show that the PZT film was successfully deposited on ST-X quartz, and that the PZT film on ST-X quartz can enhance the electromechanical coupling coefficients of SAW.     

Ferroelectric nylon materials and their feasibility for ultrasoundtransducers

Measurements of the high frequency electrical and acoustic properties of oriented ferroelectric nylon-7 and nylon-11 are reported for the frequency range of 10 to 50 MHz and temperature range of 20 to 160°C. The dielectric properties of nylon films were measured over a broadband frequency range including the fundamental halfwave resonance frequency for each sample, and a curve-fitting resonance technique was used to determine the properties of interest for ultrasound transducer design. Despite the high remanent polarization measured in the nylon films (1.25 mC/cm²), the measured electromechanical coupling constant (k_t=0.11), and mechanical quality factor (Q_m =8) were lower than typically seen for PVDF and P(VDF-TrFE) copolymer. Poor thickness uniformity of the nylon film samples is suspected of dampening the features of the dielectric spectra near resonance, resulting in lower measured values for k_t and Q _m. Comparisons of both simulated and actual measured pulse-echo performance showed the nylon materials to be competitive with PVDF for ultrasonic transducer performance. Actual pulse-echo tests with nylon-7 yielded a -6 dB fractional bandwidth of 144%. Since the ferroelectric nylons are known to be hydrophilic, tests were conducted to quantitatively determine the effects of water absorption on the ultrasonic performance of nylon-7 transducers. The results showed the water exposure slightly reduces the pulse-echo amplitude response and resonant frequency of a nylon-7 transducer. The water absorption effects were shown to be fully reversible upon drying the nylon film     

Acoustic properties of alumina colloidal/polymer nano-composite film on silicon

Alumina colloidal/polymer composite films on silicon substrates have been successfully fabricated using the sol-gel method, in which the crystallite sizes of alumina are between 20 and 50 nm. The density and ultrasonic phase velocities in these films with different alumina ratios from 14% to 32% were measured at the desired operating frequency. We have proved that the density, acoustic phase velocities, and hence the acoustic impedance of the nano-composite films increase with the alumina content, which gives us another option of tailoring the acoustic impedance of the nano-composite film for making the matching layer of high-frequency medical ultrasonic transducers.     

Modeling 1-3 composite piezoelectrics: thickness-mode oscillations

A simple physical model of 1-3 composite piezoelectrics is advanced for the material properties that are relevant to thickness-mode oscillations. This model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer are derived. A number of examples illustrate the implications of using piezocomposites in medical ultrasonic imaging transducers. While most material properties of the composite roughly interpolate between their values for pure polymer and pure ceramic, the composite's thickness-mode electromechanical coupling can exceed that of the component ceramic. This enhanced electromechanical coupling stems from partially freeing the lateral clamping of the ceramic in the composite structure. Their higher coupling and lower acoustic impedance recommend composites for medical ultrasonic imaging transducers. The model also reveals that the composite's material properties cannot be optimized simultaneously; tradeoffs must be made. Of most significance is the tradeoff between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced.     

Simple model for piezoelectric ceramic/polymer 1-3 composites used in ultrasonic transducer applications

A theoretical model is presented for combining parameters of 1-3 ultrasonic composite materials in order to predict ultrasonic characteristics such as velocity, acoustic impedance, electromechanical coupling factor, and piezoelectric coefficients. Hence, the model allows the estimation of resonance frequencies of 1-3 composite transducers. This model has been extended to cover more material parameters, and they are compared to experimental results up to PZT volume fraction ν of 0.8. The model covers calculation of piezoelectric charge constants d₃₃ and d₃₁. Values are found to be in good agreement with experimental results obtained for PZT 7A/Araldite D 1-3 composites. The acoustic velocity, acoustic impedance, and electromechanical coupling factor are predicted and found to be close to the values determined experimentally     

Piezoelectric films for high frequency ultrasonic transducers in biomedical applications

Piezoelectric films have recently attracted considerable attention in the development of various sensor and actuator devices such as nonvolatile memories, tunable microwave circuits and ultrasound transducers. In this paper, an overview of the state of art in piezoelectric films for high frequency transducer applications is presented. Firstly, the basic principles of piezoelectric materials and design considerations for ultrasound transducers will be introduced. Following the review, the current status of the piezoelectric films and recent progress in the development of high frequency ultrasonic transducers will be discussed. Then details for preparation and structure of the materials derived from piezoelectric thick film technologies will be described. Both chemical and physical methods are included in the discussion, namely, the sol-gel approach, aerosol technology and hydrothermal method. The electric and piezoelectric properties of the piezoelectric films, which are very important for transducer applications, such as permittivity and electromechanical coupling factor, are also addressed. Finally, the recent developments in the high frequency transducers and arrays with piezoelectric ZnO and PZT thick film using MEMS technology are presented. In addition, current problems and further direction of the piezoelectric films for very high frequency ultrasound application (up to GHz) are also discussed.     

圆管超声换能器响应的激光超声检测

文章着重叙述一种用激光超声检测圆管超声换能器响应的方法。为激励和接收轴对称声场，采用压电圆管作为压电换能元件，并分别用环氧、环氧加钨粉为背衬，制作了两种压电圆管换能器。由激光超声方法和自发自收脉冲反射法实验测定了所制作的换能器的频率响应，并比较了频谱分析结果。实验结果不仅在一定程度上说明了两种背衬的效果，同时也表明激光超声检测换能器响应的可行性。     

Fabrication of high sensitivity ZnO thin film ultrasonic devices byelectrochemical etch techniques

In this experiment, high quality thin film acoustic devices were constructed utilizing zinc oxide (ZnO) thin film deposition, integrated circuit (IC), and electrochemical etch techniques. The bridge type ZnO thin film ultrasonic devices produced showed acute high frequency response sensitivity, with the high frequency response at 600 KHz attaining -124 dBV μBar. The highest response was near 8 MHz. Sensitivity increased in direct proportion to frequency, indicating that the bridge-type ZnO thin film acoustic devices were suitable for application as ultrasonic wave sensors at high operating frequencies. Further experimentation verified that the bridge-type ZnO thin film acoustic devices fabricated also had ultrasonic signal transmitting capability and signal bidirectional devices having both transmission and reception functions were successfully constructed. Therefore, bridge-type ZnO thin film acoustic devices will continue to be developed in the future due to the reliable control possible over the structural transformations involved; and, since IC fabrication techniques can be readily adopted in the process, the manufacturing of such devices will be greatly assisted     

Radial vibration of the composite ultrasonic transducer of piezoelectric and metal rings

The radial composite ultrasonic transducer of a piezoelectric ceramic ring and a metal ring is studied. The radial vibrations of a piezoelectric ceramic ring polarized in the thickness direction and a metal ring are analyzed and their electromechanical equivalent circuits are obtained. On the basis of the electromechanical equivalent circuits of the piezoelectric and the metal ring and the radial boundary conditions, the total electromechanical equivalent circuit for the composite ultrasonic transducer is also obtained. The resonance frequency and anti-resonance frequency equations of the composite ultrasonic transducer are given. Some radial composite ultrasonic transducers are designed and manufactured and their resonance and anti-resonance frequencies are measured. Experiments show that the measured resonance frequencies are in a good agreement with the theoretical results.     

L型匹配网络对超声换能器振动性能的影响

探讨了L型匹配网络对超声换能器振动频率的影响,从理论和实验两个方面给出了L型匹配网络与机电偶合系数的关系。通过对串联、并联的比较,并对几种方式下的匹配电路参数对系统有效机电耦合系数的影响进行分析,结果表明：通过合理选择匹配电路参数,L型匹配网络能提高压电换能器的有效机电耦合系数。     

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.     

80-MHz intravascular ultrasound transducer using PMN-PT free-standing film

[Pb(Mg(1/3)Nb(2/3))O(3)](0.63)[PbTiO(3)](0.37) (PMN-PT) free-standing film of comparable piezoelectric properties to bulk material with thickness of 30 μm has been fabricated using a modified precursor coating approach. At 1 kHz, the dielectric permittivity and loss were 4364 and 0.033, respectively. The remnant polarization and coercive field were 28 μC/cm(2) and 18.43 kV/cm. The electromechanical coupling coefficient k(t) was measured to be 0.55, which was close to that of bulk PMN-PT single-crystal material. Based on this film, high-frequency (82 MHz) miniature ultrasonic transducers were fabricated with 65% bandwidth and 23 dB insertion loss. Axial and lateral resolutions were determined to be as high as 35 and 176 μm. In vitro intravascular imaging on healthy rabbit aorta was performed using the thin film transducers. In comparison with a 35-MHz IVUS transducer, the 80-MHz transducer showed superior resolution and contrast with satisfactory penetration depth. The imaging results suggest that PMN-PT free-standing thin film technology is a feasible and efficient way to fabricate very-high-frequency ultrasonic transducers.     

流体动力式次声换能器机电类比分析与试验研究

流体动力式换能器相较于其他形式的换能器在产生低频声音方面具有更大的优势,为了预测流体动力式换能器的声学性能,提出一种基于机电类比的流体动力式换能器等效电路模型。设计并搭建流体动力式换能器试验台,将机电类比模型模拟结果与空气中进行的试验结果进行了比较。结果显示,该机电类比模型在空气中1~4 Hz频率范围内的声源级预报值与实测值的误差低于10%,证明了机电类比分析模型的有效性和可靠性。     

Properties of transverse ultrasonic transducers of ferroelectricpolymers working in thickness shear modes

Shear piezoelectric properties of poly(vinylidene fluoride) (PVDF) and copolymer of vinylidene fluoride and trifluoroethylene [P(VDF/TrFE)] have been studied precisely in a wide temperature range from 10 to 400 K. It was found that these polymers have the shear electromechanical coupling factors k₁₅ and k₂₄ large enough to be utilized for transverse ultrasonic transducers operating in a wide frequency range and in a wide temperature range below 400 K. Shear mode ultrasonic transducers of P(VDF/TrFE) were fabricated and their performances were studied both experimentally and theoretically. The shear polymer transducers are useful for generation and detection of transverse ultrasonic waves     

Micromachined high frequency ferroelectric sonar transducers

Millimeter-sized ferroelectric monomorph sonar transducers have been built using sol-gel PZT on micromachined silicon wafers. First generation transducer arrays with diaphragms varying in size from 0.2 to 2 mm were tested. Second generation 8×8 arrays have also been built and tested in water in the frequency range of 0.3 to 2 MHz. Improvements to the sol-gel process have yielded high-quality, crack-free PZT films up to 12 μm in thickness, which leads directly to higher sensitivity and figure of merit for acoustic transducers. The longitudinal piezoelectric coefficient d₃₃ is 140 to 240 pC/N, measured through a double beam laser interferometer. Remanent polarization of 28 μC/cm², a coercive field of 30 kV/cm, and dielectric constant of 1400 were measured on 4-μm thick films. Test results are presented, including frequency response, beam patterns, and sensitivity. High-resolution acoustic images have been generated using these transducers and a four-element underwater acoustic lens. Potential applications for these transducers include high-frequency imaging sonars, medical ultrasound, ultrasonic communication links, and flaw detection (NDT)     

VHF/UHF range bioultrasonic spectroscopy system and method

A new system and method for characterizing biological tissues in vitro and liquids in the VHF and UHF ranges is described. Bulk acoustic properties such as the sound velocity, attenuation, acoustic impedance, and density are determined in reflection and transmission modes, with the biological tissue/liquid specimen sandwiched between the parallel surfaces of synthetic silica glass buffer rods having ZnO piezoelectric film transducers on their opposite ends. The method is an ultrasonic transmission line comparison method wherein the reference medium is distilled water, for which all acoustic properties are known. Measurement errors due to diffraction losses in the acoustic media and to mode conversion at the buffer/sample interfaces are corrected. Special techniques for achieving precise parallelism between the two rod surfaces, for movement to adjust the gap distance, and for signal processing are employed in order to obtain high measurement accuracy. Attenuation and reflection coefficients are determined using the gated pulse echo method. The sound velocity is determined with the gated pulse interference method by sweeping the ultrasonic frequency, or by changing the gap distance. Results of measurements on castor oil, cottonseed oil, silicone oil, and bovine liver, in the frequency range from 10 to 500 MHz, are presented and compared with results of earlier reports