Modélisation et caractérisation de nouveaux matériaux piézoélectriques (sans plomb et composites de connectivité 1-3) pour la transduction ultrasonore

Piezoelectric ceramics have been on the market for several decades and their use keepsgrowing. Nowadays, these materials are included in a wide range of devices, in particular forultrasonic applications. Since the discovery of lead zirconium titanate (PZT) sixty yearsago, a wide range of derived compositions coupled to efficient manufacturing processeshave been developed to enable their use in many devices. Thus, PZT ceramics are themost used piezoelectric materials, although the presence of lead in their composition isa growing health and environmental concern. Consequently, since 2003, E.U. has votedguidelines for the management of Waste Electrical and Electronic Equipment at the endof its lifecycle (WEEE) and Restrictions (Hazardous Substances, RoHS). In addition, inDecember 2012, the European CHemicals Agency (ECHA) added PZT in candidate listof the REACH directive. Similar steps are also followed by many countries in Asia andNorth America. In this international context, research on lead-free piezoelectric materialsand their applications becomes a major issue.This thesis focuses specifically on ultrasonic transducer applications (medical imagingand underwater acoustics) with, at first, functional characterization of lead-free piezoelectricmaterials. To this end, the KNbO3 lead-free single crystal is selected in its specificcutting (YXt)-45◦ which provides a coupling coefficient kt (in thickness mode) of 60%. Acharacterization of this material is carried out to provide a complete and consistent set ofelectromechanical properties, and thus avoid calculation errors during the simulation stageof the transducer. Its performance is demonstrated through its integration in a 30 MHzultrasound probe to obtain images of human skin in vivo.At the same time, a new algorithm for multimodal and multiphasic functional characterizationhas been developed thanks to the combination of a genetic algorithm and a finiteelement analysis. First, it is checked on a numerical reference : a 1-3 connectivity piezoelectriccomposite. Then, it is applied to real materials such as BaTiO3:Co rings that composea SONAR tonpilz transducer element. The good agreement between measurements andsimulations of electroacoustic responses of a 1-3 piezocomposite and displacements of apiston head mass from a tonpilz transducer reveals the advantage of this in situ characterization.Finally, an optimization is performed on 1-3 piezocomposite structure, which is generallya periodic one. Indeed, the periodicity leads a frequency limitation due to the presenceof spurious modes. The numerical study of a new pseudo-periodic structure shows that,by breaking the periodicity, the composite can be used in a wider frequency range. Thisobservation is confirmed by measurements on an optimized composite.Les céramiques piézoélectriques sont commercialisées depuis plusieurs décennies et leurs utilisations ne cessent de croître. Aujourd’hui, ces matériaux sont intégrés dans une large gamme de dispositifs et en particulier pour les applications ultrasonores. Depuis la découverte du zircono-titanate de plomb (PZT) il y a une soixantaine d’années, une large gamme de compositions dérivées couplée à des procédés de fabrication efficaces ont été développés permettant d’être utilisés dans de nombreux dispositifs. Les PZT sont ainsi les matériaux piézoélectriques les plus utilisés mais ils sont associés à des problèmes sanitaires et environnementaux en raison du plomb qu’ils contiennent. En conséquence, dès 2003, l’U.E. a voté des directives concernant la gestion des déchets d’équipements électriques et électroniques en fin de vie (WEEE) et leurs restrictions (substances dangereuses, RoHS). De plus, en décembre 2012, l’agence européenne des produits chimiques (ECHA) a placé les PZT dans laliste des candidats de la directive REACH. Des démarches similaires sont également suivies par de nombreux pays d’Asie et d’Amérique du Nord. Dans ce contexte international, la recherche sur les matériaux piézoélectriques sans plomb et leurs applications devient doncun enjeu majeur.Ce travail de thèse se concentre plus précisément sur les applications de transduction ultrasonore (imagerie médicale et acoustique sous-marine) avec au préalable la caractérisation fonctionnelle des matériaux piézoélectriques sans plomb utilisés. Pour ce faire, le monocristal sans plomb KNbO3 est choisi pour sa coupe spécifique (YXt)-45◦ qui délivre un coefficient de couplage kt (mode épaisseur) jusqu’à 60%. Afin deréaliser des simulations de transducteur ultrasonore intégrant ce matériau, il est caractérisé pour fournir une base de données complète et consistante de ses propriétés électromécaniques afin d’éviter les erreurs de calcul l’utilisant. Ses performances sont démontrées grâceà son intégration dans une sonde échographique de 30 MHz pour l’obtention d’images de la peau humaine in vivo. En parallèle, la prise en main de l’algorithme génétique, combinée avec le calcul par éléments finis, a permis d’élaborer un nouvel outil de caractérisation fonctionnelle multimodalet multiphasique. Il est d’abord validé sur une référence numérique de composite piézoélectrique de connectivité 1-3 (piézo-composite 1-3) avant de l’appliquer sur des matériaux réels comme des anneaux de BaTiO3:Co qui composeront un élément de SONAR (tonpilz). L’intérêt de cette caractérisation in situ est justifié par les diminutions d’écart entre des mesures et des simulations de réponse électroacoustique pour un piézo-composite 1-3 et de déplacement de pavillon pour un tonpilz.Enfin, un travail d’optimisation est réalisé sur la structure des piézo-composites 1-3 qui possède généralement une structure périodique. En effet, la périodicité crée une limite de fonctionnement en fréquence du composite (mode épaisseur) due à l’existence de certains modes parasites. L’étude numérique d’une nouvelle structure pseudo-périodique montre qu’en brisant la périodicité, le composite peut être utilisé dans une plus large gamme de fréquences en levant cette limite caractéristique des structures régulières. Cette observation est confirmée sur des mesures de cette nouvelle structure après sa fabrication

A combined genetic algorithm and finite element method for the determination of a practical elasto-electric set for 1-3 piezocomposite phases

International audience1-3 piezocomposites are widely used in ultrasonic transducers, particularly for imaging applications. The fabrication process is often based on the dice and fill method, leading to a periodic structure. This process can modify the initial properties of the two phases due to the machining of the piezoelectric bulk ceramic and setting of the polymer. A method is proposed to directly determine a practical set for 1-3 piezocom-posite properties and all the elastic, dielectric and piezoelectric parameters of the two piezoelectric (11 constants) and inert phases (3 constants). This method is based on a fitting process of the electrical impedance as a function of frequency (one thickness and two lateral modes). For this purpose, a genetic algorithm coupled with a finite element method (GA/FEM) was used in an iterative process to deduce all these parameters. This method was first performed on a numerical phantom (Pz21/epoxy resin). Comparisons showed that the GA/FEM obtained a good set of the 14 parameters, and the accuracy of several parameters was discussed. Finally, the GA/FEM algorithm was applied to a fabricated 1-3 piezocomposite (dice and fill method). The results showed that the fabrication process introduced several changes in the properties of the two phases (in particular, the dielectric constants of the ceramic and one elastic constant of the polymer) compared to the initial commercial data, while keeping the identical thickness coupling factor at 64%

Unified model for the electromechanical coupling factor of orthorhombic piezoelectric rectangular bar with arbitrary aspect ratio

Piezoelectric Single Crystals (PSC) are increasingly used in the manufacture of ultrasonic transducers and in particular for linear arrays or single element transducers. Among these PSCs, according to their microstructure and poled direction, some exhibit a mm2 symmetry. The analytical expression of the electromechanical coupling coefficient for a vibration mode along the poling direction for piezoelectric rectangular bar resonator is established. It is based on the mode coupling theory and fundamental energy ratio definition of electromechanical coupling coefficients. This unified formula for mm2 symmetry class material is obtained as a function of an aspect ratio (G) where the two extreme cases correspond to a thin plate (with a vibration mode characterized by the thickness coupling factor, kt) and a thin bar (characterized by k33′). To optimize the k33′ value related to the thin bar design, a rotation of the crystallogaphic axis in the plane orthogonal to the poling direction is done to choose the highest value for PIN-PMN-PT single crystal. Finally, finite element calculations are performed to deduce resonance frequencies and coupling coefficients in a large range of G value to confirm developed analytical relations

Optimized self-consistent full database of piezoelectric material: application to KNbO3 single crystal

International audienc

Optimized self-consistent full database of piezoelectric material: application to KNbO3 single crystal

International audienc

Electro-elastic moduli and frequency dependence of KN single crystal

International audience— Functional properties of KNbO 3 single crystal are characterized. Large polydomain plates cutted from an as-grown KNbO 3 crystal (oriented along [001] c (45°-cut) direction) are used. At the fundamental resonance (10 MHz), thickness coupling factor is 65% and frequency dependence is measured with overtone resonances. Its variation is relatively high and reduces to around 45% at 160 MHz. Electro-elastic moduli are also determined by mixing measured parameters (from thickness and lateral modes) and constants already published as initial data. Final constants are obtained by a minimization process to tend toward a self-consistent electro-elastic moduli which is an important condition for numerical modelling. Single crystal, lead-free, functional characterizatio

Complete electroelastic set for the (YXt)-45 cut of a KNbO3 single crystal

International audienceA complete and consistent set (elastic, dielectric, and piezoelectric tensors) of a commercial lead-free (YXt)-45 cut KNbO 3 single crystal is reported. These data were obtained using several samples and the resonance-antiresonance method. Particular attention was paid to the consistency of this delivered database. A genetic algorithm with an appropriate criterion was used. Electromechanical characterization revealed a high thickness coupling factor of approximately 60%. These properties make this single crystal a good candidate for several applications such as medical imaging. This complete set provides a basis for simulation designs of such devices integrating this piezoelectric lead-free material, especially for ultrasonic transducers. V C 2014 AIP Publishing LLC

Super-Cell Piezoelectric Composite With 1–3 Connectivity

International audience— The standard fabrication method for 1–3 piezo-composites for ultrasound transducers is the " dice and fill " method (DFM) in which lateral periodicity is introduced. This contributes to the appearance of spurious modes that can drastically affect the performance of the device if they appear near its thickness mode frequency, thus limiting the effective frequency range. A new 1–3 piezocomposite fabricated with a super-cell structure [1–3 super cell (13SC)] was designed in order to overcome these limitations. It consists of the merging of several periodic cells with 47% PZT volume fraction and epoxy resin as the matrix. Two lateral periodicities in one direction are defined as well as two different kerfs. The chosen cell shape is composed of five nonaligned square section rods (1 × 1 mm 2). For comparison of performance, two regular 1–3 piezocomposites (the same materials and equivalent periodicities) were fabricated by DFM. Electroacoustic responses in water were measured for the three composites being considered as transducers. Successive regular thinnings (from 2.8 to 1.1 mm) were carried out for each sample to increase the operating frequency (from around 0.4 to 1.3 MHz) and study the evolution of the characteristics (bandwidth and sensitivity). The experimental results confirmed the behavior of those obtained with numerical simulations, showing that the 13SC composite can be used in this entire frequency range, unlike regular composites

Lead-free high-frequency linear-array transducer (30 MHz) for in vivo skin imaging

International audience— This work presents the fabrication of a 30 MHz, linear-array transducer based on a KN, 1-3 piezocomposite. Performances of the transducer were characterized and compared to a PZT-based linear array with similar structure. The composites were designed to minimize lateral modes of vibration which can severely degrade imaging performances. Fabrication steps were optimized to achieve a 40 MHz resonant frequency in air with a composite thickness of 69 microns. The measured thickness coupling factor was around 50 %. A 128-element, linear array was then fabricated with 100 m pitch and 1,5 mm elevation aperture. The structure of the transducer (backing, matching layers, and electric components) was optimized to deliver good fractional bandwidth and sensitivity. The final probe was integrated in a prototype, real-time, 128-channel scanner to acquire high-resolution images of the human skin in vivo. Results showed that, compared to PZT ceramics, KN single crystals provide low density and high acoustic velocity, both highly desirable for the manufacturing of HF transducers. The central frequency of the linear-array transducer was 30 MHz despite the KN composite being 20% thicker than equivalent PZT-based composites and the relative bandwidth was about 50%. High-resolution images of the human skin were acquired. A large ultrasound penetration due to good signal sensitivity was obtained and detailed features could be visualized

High-Frequency Linear Array (20 MHz) based on Lead-Free BCTZ Crystal

International audienceCentimeter-sized BaTiO 3 -based crystals grown by top-seeded solution growth from the BaTiO 3 –CaTiO 3 –BaZrO 3 system were used to process a high-frequency (HF) lead-free linear array. Piezoelectric plates with (110)pc cut within 1° accuracy were used to manufacture two 1-3 piezo-composites with thicknesses of 270 and 78 μm for resonant frequencies in air of 10 and 30 MHz, respectively. The electromechanical characterization of the BCTZ crystal plates and the 10 MHz piezocomposite yielded thickness coupling factors of 40% and 50%, respectively. We quantified the electromechanical performance of the second piezocomposite (30 MHz) according to the reduction in the pillar sizes during the fabrication process. The dimensions of the piezocomposite at 30 MHz were sufficient for a 128-element array with a 70 μm element pitch and a 1.5 mm elevation aperture. The transducer stack (backing, matching layers, lens and electrical components) was tuned with the characteristics of the lead-free materials to deliver optimal bandwidth and sensitivity. The probe was connected to a real-time HF 128-channel echographic system for acoustic characterization (electroacoustic response, radiation pattern) and to acquire high-resolution in vivo images of human skin. The center frequency of the experimental probe was 20 MHz, and the fractional bandwidth at -6 dB was 41%. Skin images were compared against those obtained with a lead-based 20-MHz commercial imaging probe. Despite significant differences in sensitivity between elements, in vivo images obtained with a BCTZ-based probe convincingly demonstrated the potential of integrating this piezoelectric material in an imaging probe