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

Characterisation of P(VDF-TrFE) material taking into account dielectric relaxation: application to modelling of high frequency transducers

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Matériaux piézoélectriques sans plomb pour transducteurs acoustiques

International audienceLes céramiques PZT sont utilisées pratiquement dans tous les dispositifs du fait de leur faible cout de production et de leur performance. Les excellentes propriétés piézo-électriques de ces oxydes perovskite complexes (PbZr x Ti y 0 3 ou PZT, PbMg 1/3 Nb 2/3 O 3 -PbTiO 3 ou PMN-PT,…) sont dues à la présence du plomb qui est un élément principal. Du fait de la politique sanitaire et environnementale relative à l'élimination du plomb dans les matériaux, l'alternative des céramiques sans plomb est une question à résoudre pour les industriels. L'inscription de PZT dans l'annexe 15 de REACH en décembre 2012 conduit à l'impératif projet de leur substitution. Cet exposé présentera l'état de l'art sur les matériaux piézoélectriques sans plomb et les pistes de travail des céramiques aux monocristaux et céramiques texturées. Les propriétés des matériaux à base de BaTiO 3 et K/NaNbO 3 issus des projets français seront comparées pour éclairer le potentiel de substitution. Quelques exemples des applications potentiels, LF Langevin transducteur et HF médical transducteur, seront présentés

Texturation of lead-free BaTiO3-based piezoelectric ceramics

International audienceNowadays, piezoelectric ceramics are integrated in a wide range of devices, in particular in ultrasonic applications (underwater sonar systems, medical imaging, non-destructive testing…). Most of them use Pb(Zr,Ti)O3 (PZT). However, due to health care and environmental problems, lead content must be reduced in such applications [1]. Recent reviews demonstrated that few lead-free materials families can be considered: the alkaline-niobates (K0.5Na0.5NbO3), the alkaline-bismuth-titanates (Na0.5Bi0.5TiO3) and barium titanate based materials (BaTiO3) [2, 3]. One of the limitations of ceramic materials is their isotropic nature. This is the reason why texturation process has been developed in order to improve their properties in particular electromechanical parameters. The aim of the present study is thus to obtain textured BaTiO3 based materials by using the templated grain growth process (TGG) and to measure their piezoelectric properties. Nanosized BaTiO3 powders were prepared by classical solid state route at relatively low temperature. BaTiO3 templates of different morphologies were elaborated by a molten salts process. Dispersing agent, binder and plasticizer, the mixture of the templates and matrix particles was then dispersed in the appropriated solvent using non-aqueous formulation. The slurry was then tape-casted on plastic film. After drying, the green sheet was cut, stacked, thermo compressed and then sintered at the appropriated temperature. This process allowed obtaining highly-oriented materials with a texturation degree around 95% (fig. 1), presenting Kt values around 35-40%, higher than for non-textured BaTiO3 ceramics. Fig. 1: X-ray diffraction diagram of (tetragonal) textured sample BaTiO3 and (tetragonal) non-textured BaTiO3 ceramic as reference

Theoretical electroelastic modili of porous textured piezoceramics

International audienceNowadays, piezoceramics are commonly used in ultrasonic transducers (underwater applications, medical imaging, NDT...). Most of them are PZT-based materials (Pb(Zr,Ti)O3), which causes environmental issues due to the presence of lead. To comply with international regulations on the use of lead, numerous studies are conducted on lead-free piezoceramics (K0.5Na0.5NbO3, BaTiO3) which can offer similar electromechanical performance to lead-based materials. Fabrication of lead-free textured ceramics enables the production of large area of piezoelectric materials with good electromechanical capabilities. Textured ceramics can be simply modeled as a 3-phases composite: one for the single-crystal that increases with the texturation degree, another one for the ceramic matrix and a last one for porosity. The amount of porosity depends on the sintering conditions. In the long wavelenght approximation, this material can be considered as homogeneous and characterized by effective properties. In this context, few studies were carried out for this homogeneization step and, in this paper, a homogeneization scheme is developped, based on the generalization of series and parallel connections. For the modeling, a unit cell is defined and used to determine the effective electroelastic moduli. Then, a validation step for this model is performed using FEM calculation. Results show that a high texturation is necessary and that the presence of porosity is not always damaging to optimize the targeted performance. Finally, theoretical properties of two single-element transducers based on textured piezoceramics are determined and compared to those based on PZT