Non-Planar Pad-Printed Think-Film Focused High-Frequency Ultrasonic Transducers for Imaging and Therapeutic Applications

Pad-printed thick-film transducers have been shown to be an interesting alternative to lapped bulk piezoceramics, because the film is deposited with the required thickness, size, and geometry, thus avoiding any subsequent machining to achieve geometrical focusing. Their electromechanical properties are close to those of bulk ceramics with similar composition despite having a higher porosity. In this paper, padprinted high-frequency transducers based on a low-loss piezoceramic composition are designed and fabricated. High-porosity ceramic cylinders with a spherical top surface are used as the backing substrate. The transducers are characterized in view of imaging applications and their imaging capabilities are evaluated with phantoms containing spherical inclusions and in different biological tissues. In addition, the transducers are evaluated for their capability to produce high-acoustic intensities at frequencies around 20 MHz. High-intensity measurements, obtained with a calibrated hydrophone, show that transducer performance is promising for applications that would require the same device to be used for imaging and for therapy. Nevertheless, the transducer design can be improved, and simulation studies are performed to find a better compromise between low-power and high-power performance. The size, geometry, and constitutive materials of optimized configurations are proposed and their feasibility is discussed

Influence des propriétés de céramiques piezoélectriques sur des mesures ultrasonores non-linéaires

Lors de la détection de phénomènes non-linéaires faibles dans les solides, dans le cadre de l évaluation non-destructive par ultrasons, les non-linéarités des céramiques piézoélectriques, éléments actifs des transducteurs ultrasonores, peuvent introduire des effets parasites. L objectif de la thèse est de caractériser les propriétés non-linéaires de ces céramiques et de prédire l influence de celles-ci sur la propagation d ondes ultrasonores. Une méthode de caractérisation non-linaire basée sur l analyse de la génération de second harmonique sur un résonateur piézoélectrique sous excitation électrique est ici développée. On montre que l analyse des taux de distorsions permet de distinguer différentes sources de non-linéarité, mécanique ou électromécanique, impliquées dans la génération harmonique. L influence de la charge acoustique sur le comportement vibratoire non-linéaire de la céramique est ensuite étudiée. L ensemble des résultats obtenus est utilisé pour prédire l influence de la non-linéarité de la céramique piézoélectrique sur la propagation d ondes acoustiques dans un milieu extérieur.During the detection of nonlinear phenomena in solids, under the non-destructive evaluation by ultrasound, nonlinearities of piezoelectric ceramics might introduce parasitic effects that could disrupt the measurements. The objective here is to characterize the nonlinear properties of these ceramics and predict their influence on the propagation of ultrasonic waves. A method to characterize nonlinear properties based on the analysis of the generation of second harmonic on a piezoelectric resonator is developed here. It is shown that the analysis of the distortions for a wide range of excitation frequencies, allow to distinguish different sources of nonlinearity. Thereafter, the influence of an acoustic load on the nonlinear behaviour of vibration is studied. Finally, the previous results are used to predict the influence of non-linearity of piezoelectric ceramic on the propagation of acoustic waves in an environment outside.TOURS-Bibl.électronique (372610011) / SudocSudocFranceF

Electrical control of the elastic wave propagation in a piezoelectric plate: reduction of the crosstalk in a multi- element ultrasonic transducer

International audienceThe cross couplings in a multi-element ultrasonic transducer create perturbations in its radiation, affecting image quality. The crosstalk phenomenon increases the number of vibrating elements by comparison to the number of electrically excited elements and produces spurious guided modes. Solutions have been proposed to reduce the crosstalk, for example, the passive mechanical decoupling between the elements, i.e. varying the depth of the periodic cut of the piezoelectric ceramics and also an active electrical decoupling [1], in applying to the other elements, except to the active one, a suitable electric voltage. This work proposes another possibility by using a passive electrical decoupling. A homogeneous piezoelectric plate, covered on one side by a 1D periodic arrangement of thin metallic electrodes and on the other side by a full metallic electrode, is considered. Through electrical boundary conditions applied to the electrodes, band gap location and width of guided elastic waves can be tuned to a range including the resonance frequency of the first piezoelectric thickness mode. Finite element analysis and experimental measurements show the appearance of band gaps in the dispersion curves, which limits the inter-element couplings and thus the parasitic signals in the radiation. Bibliography: [1] BYBI, A., et al. Electrical method for crosstalk cancellation in transducer arrays. NDT & E International, 2014, vol. 62, p. 115-121

Exact second order formalism for the study of electro-acoustic properties in piezoelectric structures under an initial mechanical stress

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Stress cycle on a ZnO nanowire-based nanogenerator: a phenomenological study

International audienceThe aim of this work is to search influent parameters on the energy conversion in a ZnO nanowire-based nanogenerator and transfer to the external load during a quasi-static stress cycle. The studied parameters are: mechanical excitation characteristic times, ZnO doping level, polymer dielectric permittivity and external load resistance

Piezo‐Semiconductor Coupled Model for the Simulation of Zinc Oxide 1–3 Piezo‐Composite

International audienceAbstract Semiconducting effects in zinc oxide nanowires‐based nanogenerators greatly impact their performances. A coupled model integrating piezoelectric and semiconducting properties is developed with a finite element method to better understand the phenomena involved in this kind of device made with a process including chemical bath deposition of the nanostructures and their polymer encapsulation. Free carriers and surface traps are taken into account to give some keys to explain differences between previous theoretical and experimental works. This model is first validated on a single nanowire with a 10 16 cm −3 n‐doping level by comparison with analytical calculations. For this comparison, surface traps are not taken into account as no analytical solution is available considering them. Second, n‐doping levels (between 10 12 and 10 18 cm −3 ) and surface traps densities (between 10 8 and 10 12 cm −2 ) lead to variations of the generated piezopotential of two orders of magnitude. The control of these two semiconducting properties is a real challenge for this kind of application. Furthermore, simulation results show that the benefic effect of 1–3 piezo‐composite is still present even for a high n‐doping level of 10 18 cm −3

Improved Properties of Doped BaTiO 3 Piezoelectric Ceramics

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Modelling of hysteretic behavior of piezoceramic materials under electrical loading

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Electromechanical Behavior of a Doped Barium Titanate Piezoceramic Under Mechanical Stress: Modeling and Comparison With Experimental Measurements

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Calculation of dissipation resistances in a single-element transducer

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