Design, fabrication, and evaluation of high frequency, single-element transducers incorporating different materials

The performance of high frequency, single-element transducers depends greatly on the mechanical and electrical properties of the piezoelectric materials used. This study compares the design and performance of transducers incorporating different materials. The materials investigated include 1-3 lead zirconate titanate (PZT) fiber composite, lead titanate (PbTiO₃) ceramic, poly(vinylidene fluoride) (PVDF) film, and lithium niobate (LiNbO₃) single crystal. All transducers were constructed with a 3-mm aperture size and an f-number between 2 and 3. Backing and matching materials were selected based on design goals and fabrication limitations. A simplified coaxial cable tuning method was employed to match the transducer impedance to 50 Ω for the PZT fiber composite and PbTiO₃ ceramic transducers. Transducers were tested for two-way loss and -6 dB bandwidth using the pulse/echo response from a flat quartz target. Two-way loss varied from 21 to 46 dB, and bandwidths measured were in the range from 47 to 118%. In vitro ultrasonic backscatter microscope (UBM) images of an excised human eye were obtained for each device and used to compare imaging performance. Both press-focusing and application of a lens proved to be useful beam focusing methods for high frequency. Under equal gain schemes, the LiNbO ₃ and PbTiO₃ transducers provided better image contrast than the other materials     

匹配层厚度振动换能器研究

随着水声信号处理技术的发展,对宽带信号的需求日益增长。换能器的带宽是宽带信号处理的基础,而匹配层技术是拓宽厚度振动换能器频带最有效的方法。研究了多匹配层厚度振动换能器的设计方法,建立了复数形式的多匹配层厚度振动换能器等效网络模型,仿真研究了匹配层参数对换能器响应带宽的影响,指出了单、双匹配层在拓宽换能器带宽方面的限制,还指出匹配层的最佳厚度并非一定就是四分之一波长。在此基础上,制作了单、双匹配层换能器样品,对仿真结果进行了试验验证。测试结果和仿真结果符合得很好。     

Acoustic impedance matching of piezoelectric transducers to the air

The purpose of this work is threefold: to investigate material requirements to produce impedance matching layers for air-coupled piezoelectric transducers, to identify materials that meet these requirements, and to propose the best solution to produce air-coupled piezoelectric transducers for the low megahertz frequency range. Toward this end, design criteria for the matching layers and possible configurations are reviewed. Among the several factors that affect the efficiency of the matching layer, the importance of attenuation is pointed out. A standard characterization procedure is applied to a wide collection of candidate materials to produce matching layers. In particular, some types of filtration membranes are studied. From these results, the best materials are identified, and the better matching configuration is proposed. Four pairs of air-coupled piezoelectric transducers also are produced to illustrate the performance of the proposed solution. The lowest two-way insertion loss figure is -24 dB obtained at 0.45 MHz. This increases for higher frequency transducers up to -42 dB at 1.8 MHz and -50 at 2.25 MHz. Typical bandwidth is about 15-20%.     

高频水声换能器匹配层技术研究

在复杂的海洋环境中,为提高对目标的探测效果以获得更多的信息,高频声呐常需要更宽的工作频带.首先,通过声透射原理对不同匹配层材料进行选取与设计;其次,通过有限元仿真对不同匹配层换能器的透射系数、电导谱和发送电压响应展开了分析计算;最后,在理论分析、解析计算和有限元仿真计算的基础上成功研制了单层、双层和三层匹配层高频宽带水...     

Characterizing ultra-thin matching layers of high-frequency ultrasonic transducer based on impedance matching principle

The quarter-wavelength (lambda/4) acoustic matching layer is a vital component in medical ultrasonic transducers, which can compensate for the large acoustic impedance mismatch between the piezoelectric material and the human body. At high frequencies (approximately 100 MHz), the lambda/4 matching layers become extremely thin, and the characterization of their properties becomes very challenging. We report a method to measure the phase velocity and attenuation of ultra-thin layers using the lambda/4 matching principle, in which the acoustic impedance of the thin layer is between the substrate and water. The method has been successfully used to characterize epoxy films on glass substrate. The experimental results show good agreement in the phase-velocity measurement between our proposed method and the conventional ultrasonic spectroscopy method, but the attenuation measurement is sensitive to the properties of the substrate and water medium as well as the alignment of the sample.     

Characterization of air-coupled transducers

This paper describes a theoretical and experimental study for determination of the through-air system impulse response and insertion loss with different air-coupled ultrasonic transducers. Wide-band piezopolymer transducers (PVDF) are employed in both transmission and reception modes and their behavior assessed by means of mathematical modeling and experiment. Specifically, a linear systems approach, modified to include the influence of attenuation in the propagation medium, was used to design suitable PVDF transducers for wide-band operation in air. Suitable devices were then manufactured for determination of the transmission and reception response characteristics of piezocomposite and electrostatic transducers when operating in the air environment. A range of transducers was evaluated, including 1-3 connectivity composites of different ceramic volume fraction and mechanical matching conditions, in addition to electrostatic devices of varying design. To complement the investigation, relative performances for narrow-band operation are also presented under transmission and transmit-receive conditions. Despite the obvious measurement difficulties, good agreement between theory and experiment was observed and the methodology is shown to provide a convenient and robust procedure for comparison of through-air transducers operating in the frequency range 50 KHz to 2 MHz. Although highly resonant, the most effective composite transducers under consideration demonstrate an improvement in two-way insertion loss of 22.4 dB and 11.5 dB over a corresponding electrostatic pair, under narrow-band and wide-band operation, respectively     

25 MHz ultrasonic transducers with lead- free piezoceramic, 1-3 PZT fiber-epoxy composite, and PVDF polymer active elements

This paper presents the fabrication and characterization of single-element ultrasonic transducers whose active elements are made of lead-free piezoceramic, 1-3 PZT/polymer composite and PVDF film. The lead free piezoelectric KNNLT- LS(K_0.44Na_0.52Li_0.04)(Nb_0.84Ta_0.10S_0.06b)O₃ powders and ceramics were prepared under controlled humidity and oxygen flow rate during sintering. Due to its moderate longitudinal piezoelectric charge coefficient (175 pC/N) and k_t of 0.50, the KNN-LT-LS composition may be a good candidate for highfrequency transducer applications. PZT fibers with 25 μm diameter formed by the viscose suspension spinning process were incorporated into epoxy to fabricate 1-3 composites with the averaged k_t = 0.64 and d₃₃ = 400 pC/N. Using KNN-LS-LT ceramic, 1-3 PZT fiber composite, and PVDF film, 3 different unfocused single element transducers with center frequencies of 25 MHz were fabricated. The acoustic characterization of the transducers demonstrated that wideband and low insertion loss could be obtained employing KNN-LS-LT ceramic. The ?6 dB bandwidth and insertion loss were 70% and ?21 dB, respectively. In comparison, the insertion loss of the ceramic transducer was much smaller than those made with 1-3 composite and PVDF film. This was attributed to closer electrical impedance match to 50 Ω and higher thickness coupling coefficient of the ceramic transducer.     

Self-focused ZnO transducers for ultrasonic biomicroscopy

A simple fabrication technique was developed to produce high frequency (100 MHz) self-focused single element transducers with sputtered zinc oxide (ZnO) crystal films. This technique requires the sputtering of a ZnO film directly onto a curved backing substrate. Transducers were fabricated by sputtering an 18 μm thick ZnO layer on 2 mm diameter aluminum rods with ends shaped and polished to produce a 2 mm focus or f-number equal to one. The aluminum rod served a dual purpose as the backing layer and positive electrode for the resultant transducers. A 4 μm Parylene matching layer was deposited on the transducers after housing and interconnect. This matching layer was used to protect the substrate and condition the transfer of acoustic energy between the ZnO film and the load medium. The pulse-echo response for a representative transducer was centered at 101 MHz with a -6 dB bandwidth of 49%. The measured two way insertion loss was 44 dB. A tungsten wire phantom and an adult zebrafish eye were imaged to show the capability of these transducers.     

Computer optimization of transducer transfer functions usingconstraints on bandwidth, ripple, and loss

Transducers, having one piezoelectric layer near its half-wave resonance and N quarter-wave layers, are designed using computer optimization to adjust the thicknesses and impedances of the various layers so as to fit the resulting transfer function to a target function. An augmented Mason model is used to evaluate the transducer. Optimization of fit is by a steepest descent algorithm. Essentially error-free fits are achieved for target functions that match the underlying dynamics. By applying classical filter theory to a lumped-element transducer model, the transducers dynamics are identified as all-pole filters, which are characterized by polynomials of order N to N+1. The design methodology is tested by designing a series of low-loss transducers that explore fractional bandwidths from 45 to 116%. From these studies there appears to be constraints on the minimum Q of the poles, and other properties. Typical power transfer efficiencies of -1 dB are achieved by impedance scale matching. Using a second-order Fano bound, it is shown that the matching layers function as an optimal compensation network for low-loss flat bandpass transducers. Finally, by the inclusion of loss, lower Q poles are demonstrated with a Bessel transducer     

Multilayer piezoelectric ceramics for two-dimensional array transducers

In medical ultrasound imaging, 2-D array transducers have become essential to implement dynamic focusing and phase-correction in the elevation dimension as well as real-time volumetric scanning. Unfortunately, the small size of a 2-D array element results in a small clamped capacitance and a large electrical impedance near resonance. These elements have poor sensitivity because their impedance is much higher than the electrical impedance of the transmit and receive circuitry. Sensitivity can be improved by using an N layer structure of PZT ceramic with the layers connected acoustically in series and electrically in parallel. For the multilayer ceramic (MLC), the damped capacitance is multiplied by a factor of N(2) and the electrical impedance by 1/N(2) compared to a single layer element of the same dimensions. A 3x43 phased-array transducer has been fabricated using 3 layer PZT-5H material. Each element had a thickness of 0.66 mm and an area of 0.37x3.5 mm. The MLC was manufactured using thick film technology with plated-through vias to electrically interconnect the electrode layers. The completed transducer was compared to a single layer control array of similar dimensions. With a light epoxy backing and a lambda/4 matching layer, the MLC array elements had an impedance of 100 Omega at series resonance of 2.25 MHz, compared to 800 Omega for the control elements. The lower impedance of the MLC elements resulted in a minimum round-trip insertion loss of 24.0 dB, compared to an 34.1 dB for the control array elements. These results were consistent with KLM modeling. B-scan images were made of cysts in a tissue-mimicking phantom and of the left kidney in vivo. The images clearly showed a higher signal-to-noise ratio for the MLC array compared to the control. As a result, 2-D arrays made of multilayer ceramics can be used to form images at a higher frequency and greater range than single layer arrays.     

Design considerations for piezoelectric polymer ultrasound transducers

Much work has been published on the design of ultrasound transducers using piezoelectric ceramics, but a great deal of this work does not apply when using the piezoelectric polymers because of their unique electrical and mechanical properties. The purpose of this paper is to review and present new insight into seven important considerations for the design of active piezoelectric polymer ultrasound transducers: piezoelectric polymer materials selection, transducer construction and packaging requirements, materials characterization and modeling, film thickness and active area design, electroding selection, backing material design, and front protection/matching layer design. Besides reviewing these design considerations, this paper also presents new insight into the design of active piezoelectric polymer ultrasonic transducers. The design and fabrication of an immersible ultrasonic transducer, which has no adhesive layer between the active element and backing layer, is included. The transducer features direct deposition of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer onto an insulated aluminum backing substrate. Pulse-echo tests indicated a minimum insertion loss of 37 dB and -6 dB bandwidth of 9.8 to 22 MHz (71%). The use of polymer wear-protection/quarter-wave matching layers is also discussed. Test results on a P(VDF-TrFE) transducer showed that a Mylar/sup TM/ front layer provided a slight increase in pulse-echo amplitude of 15% (or 1.2 dB) and an increase in -6 dB pulse-echo fractional bandwidth from 86 to 95%. Theoretical derivations are reported for optimizing the active area of the piezoelectric polymer element for maximum power transfer at resonance. These derivations are extended to the special case for a low profile (i.e., thin) shielded transducer. A method for modeling the non-linear loading effects of a commercial pulser-receiver is also included.     

高频宽带换能器多匹配层研究

在水声应用中,高频换能器往往需要较宽的工作带宽,以获得更多的目标信息。文章首先建立了等效电路模型,利用粒子群算法对匹配层材料和厚度进行初步选定,使得换能器具有最宽的工作频带;其次,通过有限元方法对匹配层换能器的导纳和发射电压响应进行分析计算;最后,在理论分析的基础上成功制得三匹配层高频宽带换能器,其工作频段约为150～430 kHz,相对带宽为93%,带内发送电压响应起伏为-6 dB。实验结果表明,三匹配层设计方案可以有效拓宽高频换能器的工作频段。     

高频宽带换能器研究

主要研究高频换能器的匹配层技术,通过匹配层技术拓宽换能器的频带,首先利用等效电路法分析高频匹配层换能器,其次通过Matlab仿真分析匹配层材料的密度、声速、厚度变化对换能器电声参数性能的影响,进而对其电声性能进行优化设计,最终制作出一高频宽带换能器。通过实验测得结果与仿真结果基本一致,实验测得换能器的最大发送电压响应为178dB,工作频带为260～370kHz,带内发送电压响应起伏为-3dB,300kHz时换能器指向性-3dB开角为6．5°。     

Characterization and assessment of an integrated matching layer for air-coupled ultrasonic applications

A novel ultrasonic matching layer for improving coupling between piezoelectric transducers and an air load is presented and the results of a theoretical and experimental program of work are provided. A combination of a porous material that has very low acoustic impedance with a low-density rubber material forms the basis of the approach. These matching layers were first analyzed experimentally using scanning electron and optical microscopy to determine the microscopic structure. Air-coupled resonance measurements were then performed to reveal the acoustic parameters of the individual layers that were identified within this multilayered structure. These data were then incorporated into a conventional linear model, and this has been verified and used to study performance and produce designs. Close correlation between experiment and theory is demonstrated. The most efficient designs have been implemented in a pitch/catch air-coupled system, and an improvement in received signal amplitude of 30 dB was achieved when compared with the unmatched case.     

玻璃基掩埋式光波导堆栈的制作与表征

在硅酸盐光学玻璃基片上制作了光波导堆栈, 这种光波导堆栈通过Ag⁺/Na⁺熔盐离子交换和电场辅助离子扩散技术顺次制作了两层掩埋式光波导. 对光波导堆栈的横截面显微结构进行了观察, 并对堆栈中两层波导的损耗特性进行了测试. 所获得的光波导堆栈中的上、下两层波导芯部分别位于玻璃表面以下14和35 μm处; 上层光波导芯部尺寸约为12 μm×7 μm; 下层光波导芯部尺寸约为9 μm×8 μm. 通光测试显示两层波导在1.55 μm工作波长下均为单模光波导, 且两者之间没有相互耦合. 损耗测试分析结果显示: 堆栈中两层光波导的传输损耗均约为0.12 dB/cm,与单模光纤之间的耦合损耗分别为0.78和0.73 dB. 分析表明, 这种光波导堆栈在玻璃基集成光芯片的高密度集成方面具有很好的应用前景.     

废玻璃再生利用制备长余辉蓄光釉面砖及其性能的研究

利用传统陶瓷制备方法合成了SrAl2O4:Eu,Dy长余辉发光粉体,该磷光体主发射波长位于520nm,余辉时间长达20h以上,将发光粉掺入适量的低熔点玻璃料,经780℃烧成30min合成了性能较好的低温发光釉料,以废玻璃,粘土为主要原料添加其它少量助剂,经过成型,预烧,将低发光釉料涂覆在其上,在一定温度下烧成,制得长余辉蓄光釉面砖。     

低损耗离子交换玻璃基光波导制备与分析

考虑到离子交换和离子扩散工艺的特殊要求, 设计并熔制了适合于离子交换工艺的硅酸盐玻璃材料SiO₂-B₂O₃-Al₂O₃-R’O-R₂O（R’=Ca, Mg; R=Na, K）. 采用Ag⁺/Na⁺熔盐离子交换和电场辅助离子扩散工艺在这种玻璃材料基片上获得了掩埋式条形光波导. 光学显微镜和电子探针分析表明高折射率的Ag⁺扩散区位于玻璃基片表面以下约10μm处, 形成光波导的芯部. 光波导芯部尺寸约为8μm×8μm, 与单模光纤芯径尺寸相当, 保证了较低的光纤耦合损耗. 对光波导的测量结果得出：在波长为1.5μm处条形光波导的传输损耗约为0.1dB/cm, 与单模光纤的耦合损耗约为0.2~0.3dB. 条形光波导的传输损耗与材料本身的损耗接近, 表现出掩埋式光波导的低损耗特征. 分析表明, 经过进一步优化, 这种光波导制备技术可用于低损耗光波导器件的制作.     

A high-frequency annular-array transducer using an interdigital bonded 1-3 composite

This paper reports the design, fabrication, and characterization of a 1-3 composite annular-array transducer. An interdigital bonded (IB) 1-3 composite was prepared using two IB operations on a fine-grain piezoelectric ceramic. The final composite had 19-μm-wide posts separated by 6-μm-wide polymer kerfs. A novel method to remove metal electrodes from polymer portions of the 1-3 composite was established to eliminate the need for patterning and aligning the electrode on the composite to the electrodes on a flexible circuit. Unloaded epoxy was used for both the matching and backing layers and a flexible circuit was used for interconnect. A prototype array was successfully fabricated and tested. The results were in reasonable agreement with those predicted by a circuit-analogous model. The average center frequency estimated from the measured pulse-echo responses of array elements was 33.5 MHz and the -6-dB fractional bandwidth was 57%. The average insertion loss recorded was 14.3 dB, and the maximum crosstalk between the nearest-neighbor elements was less than -37 dB. Images of a wire phantom and excised porcine eye were obtained to show the capabilities of the array for high-frequency ultrasound imaging.     

A miniaturized catheter 2-D array for real-time, 3-D intracardiac echocardiography

The design, fabrication, and characterization of a 112 channel, 5 MHz, two-dimensional (2-D) array transducer constructed on a six layer flexible polyimide interconnect circuit is described. The transducer was mounted in a 7 Fr (2.33 mm outside diameter) catheter for use in real-time intracardiac volumetric imaging. Two transducers were constructed: one with a single silver epoxy matching layer and the other without a matching layer. The center frequency and -6 dB fractional bandwidth of the transducer with a matching layer were 4.9 MHz and 31%, respectively. The 50 omega pitch-catch insertion loss was 80 dB, and the typical interelement crosstalk was -30 dB. The final element yield was greater than 97% for both transducers. The transducers were used to acquire real-time, 3-D images in an in vivo sheep model. We present in vivo images of cardiac anatomy obtained from within the coronary sinus, including the left and right atria, aorta, coronary arteries, and pulmonary veins. We also present images showing the manipulation of a separate electrophysiological catheter into the coronary sinus.     

A surface micromachined electrostatic ultrasonic air transducer

Airborne ultrasound has many applications such as, ranging, nondestructive evaluation, gas flow measurement, and acoustic microscopy. This paper investigates the generation and detection of ultrasound in air at a few MHz. Conventional plane piston lead zirconium titanate (PZT) based transducers perform poorly for this application due to the lack of proper matching layer materials. Electrostatic, or capacitive, transducers promise higher efficiency and broader bandwidth performance. The device structure in this work consists of a capacitor where one plate is a circular silicon nitride membrane coated with gold and the other is a rigid silicon substrate. By applying a voltage between the membrane and the silicon substrate, an electrostatic force is exerted on the membrane which sets it in motion, thus generating a sound wave in air. Presented here is an electrical equivalent circuit model for electrostatic transducers which is based on the early work of Mason (1942). The electrostatic transducers were designed and constructed for operation at 1.8 and 4.6 MHz. The transducers were fabricated using standard micromachining techniques. An optical interferometer was used to measure the peak displacement of the 1.8 MHz electrostatic transducer at 230 Å/V. A transmit-receive system was built using two electrostatic transducers. The system had a signal to noise ratio of 34 dB at a transducer separation of 1 cm. Each transducer had a 3-dB bandwidth of 20%, and a one-way insertion loss of 26 dB. There is excellent agreement between the measured device performance and theoretical predictions