Piezoelectric micromachined ultrasonic transducers based on PZT thin films

This paper describes fabrication and characterization results of piezoelectric micromachined ultrasonic transducers (pMUTs) based on 2-microm-thick Pb(Zr0.53Ti0.47O3) (PZT) thin films. The applied structures are circular plates held at four bridges, thus partially unclamped. A simple analytical model for the fully clamped structure is used as a reference to optimize design parameters such as thickness relations and electrodes, and to provide approximate predictions for coupling coefficients related to previously determined thin film properties. The best coupling coefficient was achieved with a 270-microm plate and amounted to kappa2 = 5.3%. This value compares well with the calculated value based on measured small signal dielectric (epsilon = 1050) and piezoelectric (e3l,f = 15 Cm(-2)) properties of the PZT thin film at 100 kV/cm dc bias. The resonances show relatively large Q-factors, which can be partially explained by the small diameters as compared to the sound wavelength in air and in the test liquid (Fluorinert 77). A transmit-receive experiment with two quasi-identical pMUTs was performed showing significant signal transmission up to a distance of 20 cm in air and 2 cm in the test liquid.     

Piezoelectric ceramics with high dielectric constants for ultrasonic medical transducers

Complex system ceramics Pb(Sc(1/2)Nb(1/2))O3-Pb(Mg(1/3)Nb(2/3))O3-Pb(Ni(1/2)Nb(1/2))O3-(Pb0.965,Sr0.035) (Zr,Ti)O3 (PSN-PMN-PNN-PSZT abbreviated PSMNZT) have been synthesized by the conventional technique, and dielectric and piezoelectric properties of the ceramics have been investigated for ultrasonic medical transducers. High capacitances of the transducers are desired in order to match the electrical impedance between the transducers and the coaxial cable in array probes. Although piezoelectric ceramics that have high dielectric constants (epsilon33t/epsilon0 > 5000, k'33 < 70%) are produced in many foundries, the dielectric constants are insufficient. However, we have reported that low molecular mass B-site ions in the lead-perovskite structures are important in realizing better dielectric and piezoelectric properties. We focused on the complex system ceramics PSMNZT that consists of light B-site elements. The maximum dielectric constant, epsilon33T/epsilon0 = 7, 200, was confirmed in the ceramics, where k'33 = 69%, d33 = 940 pC/N, and T(c) = 135 degrees C were obtained. Moreover, pulse-echo characteristics were simulated using the Mason model. The PSMNZT ceramic probe showed echo amplitude about 5.5 dB higher than that of the conventional PZT ceramic probe (PZT-5H type). In this paper, the electrical properties of the PSMNZT ceramics and the simulation results for pulse-echo characteristics of the phased-array probes are introduced.     

Broadband EMFi-based transducers for ultrasonic air applications

In this work, we explore the possibilities of electromechanical film (EMFi) as a new material for developing broadband transducers for ultrasonic air applications. The advantages of the EMFi film are its wide usable frequency range and easiness to use, making it highly suitable for self made, customizable ultrasonic sensors. This paper presents theoretical and experimental information focused on the needs of the sensor's end user, namely, frequency response, actual dynamic mass and Young's modulus, bandwidth, sensitivity, electromechanical dynamical model, acoustic response, and directivity. It is found empirically that the behavior of the film as an almost ideal piston-like acoustic source permits accurate prediction of the characteristics of transducers built on a developable surface. The results obtained represent the first step to more complex geometries, and, ultimately, to completely customizable field ultrasonic transducers.     

Micromachined ultrasonic capacitance transducers for immersion applications

Investigations into the characteristics of water-coupled ultrasonic capacitance transducers have been undertaken for a range of transducer configurations. The radiated fields have been scanned in water using a miniature hydrophone detector, and the results compared to theory based on a plane piston approach. Micromachined backplates in conjunction with thin Mylar and mica membranes have been investigated, together with aperture modifications such as an annulus and Fresnel zone plate. The measured results agree well with theory, thus demonstrating that wideband predictable performance (>8 MHz) is obtainable with such transducers. Additionally, pulse-echo C-scans of a Plexiglas plate containing an artificial defect have been undertaken with the capacitance transducer, in order to demonstrate that the capacitance transducer has sufficient sensitivity to allow routine nondestructive testing within immersion applications.     

High frequency properties of passive materials for ultrasonic transducers

The acoustic properties of passive materials for ultrasonic transducers have been measured at room temperature in the frequency range from 25 to 65 MHz using ultrasonic spectroscopy. These materials include alumina/EPO-TEK 301 composites and tungsten/EPO-TEK 301 composites. Experimental results showed that the acoustic impedance of the composites monotonically increased with the volume fraction of the particle filler, which is in agreement with the Denavey model. The attenuation, however, peaked between 7 and 9% volume fraction of particle filler. For comparison, several other passive materials were also fabricated and measured. The results suggest that materials that possess a higher attenuation also appear to have a larger velocity dispersion     

Interdigital pair bonding for high frequency (20-50 MHz) ultrasonic composite transducers

Interdigital pair bonding is a novel methodology that enables the fabrication of high frequency piezoelectric composites with high volume fractions of the ceramic phase. This enhancement in ceramic volume fraction significantly reduces the dimensional scale of the epoxy phase and increases the related effective physical parameters of the composite, such as dielectric constant and the longitudinal sound velocity, which are major concerns in the development of high frequency piezoelectric composites. In this paper, a method called interdigital pair bonding (IPB) is used to prepare 1-3 piezoelectric composite with a pitch of 40 microns, a kerf of 4 microns, and a ceramic volume fraction of 81%. The composites prepared in this fashion exhibited a very pure thickness-mode resonance up to a frequency of 50 MHz. Unlike the 2-2 piezoelectric composites with the same ceramic and epoxy scales developed earlier, the anticipated lateral modes between 50 to 100 MHz were not observed in the current 1-3 composites. The mechanisms for the elimination of the lateral modes at high frequency are discussed. The effective electromechanical coupling coefficient of the composite was 0.72 at a frequency of 50 MHz. The composites showed a high longitudinal sound velocity of 4300 m/s and a high clamped dielectric constant of 1111 epsilon 0, which will benefit the development of high frequency ultrasonic transducers and especially high frequency transducer arrays for medical imaging.     

Bio-characterisation of tosylate-doped polypyrrole films for biomedical applications

Electroactive polymers such as polypyrrole (PPy) are receiving particular interest because they can combine inherent biological properties that can specifically trigger desired cellular responses for many biological applications. The aim of the present work was to evaluate more efficient solutions, achieved by different oxidisations of tosylate-doped PPy films in different chemico-physical conditions, in view of their long-term biomedical applications. The conductivity and the release of the tosylate ion were assessed. Morpho-functional studies were also performed. A gradual increase in impedance of the oxidised tosylate-doped PPy films soaked in MEM and a reduced conductivity after over-oxidation was found. Scanning electron microscopy (SEM) analysis showed the presence of a hierarchical organisation of the PPy films and a partially different morphology between oxidised and over-oxidised films. Cell behaviour was also randomly affected in relation to the oxidative state of the studied Ppy films. On the basis of the results, growth of these tosylate-doped Ppy films directly in the sol–gel matrix or on the surface of metal alloys could be hypothesised in order to improve bioactive materials of potential use in modern regenerative medicine. However, in view of their in vivo application, over-oxidation must be considered as it may represent the degraded material that is likely to form under a biological oxidising environment.     

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)     

Spinel ferrite films by a novel solution process for high frequency applications

Highly crystalline spinel MnZn-ferrite films applicable in high frequency devices were prepared by a simple and novel wet-chemical process. A stable precursor solution was prepared using dextran as complexing agent. Thus prepared solution was sprayed onto substrates fixed on a rotating table. The reaction temperature of 90 °C and slightly alkaline pH of the precursor solution were suitable for the formation of MnZn-ferrite films. The as-prepared films were highly crystalline and hence post-annealing was not required to obtain spontaneous magnetization. The real and imaginary permeability (μ′ and μ″), and the resonance frequency (f_r) could be tuned by varying the Zn concentration in the film. Since the film exhibits large magnetic loss, they can be used as electromagnetic compatible (EMC) materials, by which conducted noise in mobile computer and cellular phone can be suppressed effectively.     

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。     

Piezoelectric generators for biomedical and dental applications: Effects of cyclic loading

This paper presents the results of a study of the effect of cyclic loading parameters on the performance of piezo crystals. The output power of the crystals was observed to increase with parameters such as the cyclic frequency and the dynamic load range. However, the output power also decreased with increasing mean load. The efficiency of the crystal was calculated based on the mechanical energy applied to the piezo crystal. The ratio of the electrical output to mechanical energy input was taken as the efficiency of the crystal. This ratio was seen to increase with the cycling frequency, and also with the dynamic load range. However, increasing mean load caused the efficiency to drop significantly. The implications of the results are discussed for possible applications implanted bioMEMS and microelectronics systems.     

A dual frequency ultrasonic probe for medical applications

A dual frequency probe using a multilayer ceramic is proposed for simultaneously obtaining a high resolution B mode and a high sensitivity Doppler mode image. This ceramic consists of two layers in which the poling directions are opposite and the individual thicknesses are different. It is possible to control the values of relative electromechanical coupling factors in the fundamental and the second harmonic by changing the thickness ratio. A thickness ratio of 1:0.7 was decided from computer simulation based on the Mason's model. A sufficient resolution has been shown from the fact that the intima of the carotid artery could be distinguished by an actually fabricated probe with dual frequencies of 3.75 and 7.5 MHz. Also, the sensitivity of this probe in the Doppler mode at 5 cm depth from the surface has been improved as much as 5 dB over that of a conventional one     

Narrowband impedance matching layer for high efficiency thickness mode ultrasonic transducers

A new matching layer design concept has been proposed for narrowband continuous wave (CW) devices. Analysis has shown that the mechanical impedance of a resonant-type transducer in thickness mode CW operation does not equal its acoustic impedance ρVs but roughly equals ρVs/Q, where ρ is density, Vs is acoustic velocity, and Q is the mechanical quality factor. The value of ρVs/Q is much lower than the acoustic impedance of water for any transducer material, including lead zirconium titanate (PZT), single crystals, or polyvinylidene fluoride (PVDF). With this new approach, the impedance of the matching layer must also be between water and ρVs/Q, but there are few such practical low impedance materials. To realize equivalent low impedance structure, a novel double layer design is presented: a relatively low impedance material (such as polyethylene or polyurethane) on the inside and a relatively high impedance material (such as polyester or metal) on the outside. A high power CW transducer structure was designed and fabricated with PVDF-TrFE (polyvinylidene fluoride trifluoroethylene) to operate at 1.4 MHz     

Ballistic nano-devices for high frequency applications

In this paper, we present a study on three-terminal ballistic junction and their applications to rectifiers and MUX/DEMUX. Rectifying effect is observed up to 94 GHz at room temperature. Although THz frequency performance has been demonstrated by Monte Carlo simulation, the high impedance of the nano-device combined with the parasitic capacitances is a limiting factor.     

功率超声换能器电声效率及辐射声功率的测量

在高频电功率计方法的基础上,本文提出了一种能够直接测量大功率压电超声换能器在实用状态下的辐射超声功率以及电声效率的新方法。与传统的高频电功率计法相比,本方法避免了介电以及机械损耗功率的测试,简化了测试步骤。