Guy Feuillard

Lateral resonances in 1–3 piezoelectric periodic composite: Modeling and experimental results

The objective of this work is to provide an accurate model of the lateral resonance modes in 1–3 piezoelectric composite materials. These materials are widely used in ultrasonic transducers and the lowest lateral mode frequency gives the upper limit for the usable transducer bandwidth. Considering the propagation of purely transverse waves in a 2-D periodic medium of infinite thickness, two different approaches for obtaining the solutions are presented and compared. The first approach is based on the use of the Bloch waves theory. The second is a straightforward method (a so-called membrane method) which consists in numerically solving the propagation equation in the two-phase medium while taking into account the periodic boundary conditions. Methods based on both models are described that allow the calculation of the dispersion curves and the stop band limits, as well as the frequencies and the displacement fields of the lateral modes. A test case is used to compare and discuss the theoretical prediction...

Mass sensitivity of acoustic plate mode in liquids

In this paper, the concept of electrical effective permittivity function is used to calculate the eigen-frequencies and the particle displacements of piezoelectric acoustic plate modes (APM). These results allowed us to determine the mass sensitivities of the first order vibration modes using a first order perturbation theory. Theoretical results are discussed and compared to those of a variational method and isotropic two-layer composite analysis in the case of a shear horizontal APM sensor on a singly rotated cut quartz substrate. Experimental measurements by a copper electrodeposition are carried out and show that the perturbation method leads to a better understanding of the APM behavior.

Theoretical and experimental study of the influence of the particle size distribution on acoustic wave properties of strongly inhomogeneous media

The ultrasonic method is particularly suitable to characterize diffusive media, as acoustic properties (velocity and attenuation) are related to the properties and concentrations of the homogeneous phase and scatterers. Thus, ultrasonic characterization can be useful in the study of sedimentation or flocculation processes, in concentration measurements, and granulometry evaluation. Many models have been developed for media where particles are very small compared to the incident wavelength. When the diameter of the particles is close to the wavelength, multiple-scattering theories have to be used to describe the propagation of waves. In this paper, the case where the ratio of wavelength to scatterer size is around unity is studied. First, the particle size distribution is taken into account in two types of multiple-scattering theories based on the effective field approximation or on the quasicrystalline approximation and theoretical results are produced. The T-matrix formalism has been used to calculate the amplitude of the wave scattered by a single sphere. The calculation of the complex wave number in the effective medium has been implemented, using in particular the Percus-Yevick equation as a spatial pair-correlation function between scatterers, and a normal particle-size distribution. The influence of these parameters is discussed. Finally, attenuation and phase velocity measurements are performed in moving suspensions of acrylic spheres in ethylene glycol, at various concentrations and for different particle-size distributions. A good agreement between the theoretical results and the measurements is found for both velocity and attenuation. These results show that the size distribution is a critical parameter to understand velocity and attenuation behavior as function of frequency and volume fraction.

Comparative performance of piezoceramic and crystal SAW filters

Bulk elastic, piezoelectric, and dielectric constants of four lead zirconate titanate piezoceramics, Pz24, Pz26, Pz27, Pz28, and a modified lead titanate, PTS, are measured and used to theoretically compute the effective permittivity curve of each material from which the surface acoustic wave (SAW) properties are deduced. In parallel, experimental measurements of the SAW properties are carried out by using a curve fitting algorithm on the real and imaginary parts of the electrical input impedance of an unapodised single electrode SAW transducer. The SAW propagation losses are also measured using a SAW delay line. For these ceramics, the effects of a hot isostatic pressing (HIP) post sintering process on the performances of the device are also studied. All these results are discussed and show that ceramic materials, particularly PTS, have potential for SAW applications.

Modeling of the influence of a prestress gradient on guided wave propagation in piezoelectric structures

The objective of this study is to model the propagation of guided waves in piezoelectric structures subjected to a prestress gradient. The constitutive equations for a piezoelectric bulk material are first modified to take into account a uniform prestress on a given cross section. Then, these modified constitutive equations are used to derive a formalism for the propagation of guided waves in piezoelectric structures under a prestress gradient. In particular, we modify the recursive stiffness matrix method to introduce a gradient of stress in a piezoelectric structure. Numerical studies are then led for a bending and for an exponential stress profile. For a piezoelectric plate, the Lamb and shear horizontal modes are found to be sensitive to the prestress gradient. In particular, some key features of dispersion curves appearing in the presence of a gradient of properties are highlighted. In the last part, these results are extended to a piezoelectric film laid down on a substrate in order to model the imp...

Acoustic wave transmission through piezoelectric structured materials

This paper deals with the transmission of acoustic waves through multilayered piezoelectric materials. It is modeled in an octet formalism via the hybrid matrix of the structure. The theoretical evolution with the angle and frequency of the transmission coefficients of ultrasonic plane waves propagating through a partially depoled PZT plate is compared to finite element calculations showing that both methods are in very good agreement. The model is then used to study a periodic stack of 0.65 PMN-0.35 PT/0.90 PMN-0.10 PT layers. The transmission spectra are interpreted in terms of a dispersive behavior of the critical angles of longitudinal and transverse waves, and band gap structures are analysed. Transmission measurements confirm the theoretical calculations and deliver an experimental validation of the model.

Electromechanical Properties of Piezoelectric Integrated Structures on Porous Substrates

The fabrication of piezoelectric transducers for high resolution medical imaging applications requires a backing material to damp the piezoelectric resonance, resulting in a shorter time response, i.e. improved resolution, but lower sensitivity. Thus, the choice of such a substrate must be made according to its acoustical properties, namely the ratio of acoustic impedance of the backing material and the piezoelectric layer. Moreover, this backing material must have a relatively high attenuation, and provide good surface properties such as a low roughness and diffusion potential. Finally, the substrate material must have a good mechanical behavior at high temperature (around 900°C for the sintering of the piezoelectric thick film). In this context a review of available materials is first given, and the corresponding list is found to be quite limited. Several PZT/PGO piezoelectric thick films deposited by screen-printing have been fabricated on dense alumina [1] and have delivered good electromechanical performance, but the substrate attenuation was too low to be used as a backing for high frequency transducers. In this paper, new porous Al 2 O 3 substrates have been fabricated and used for the fabrication of a batch of piezoelectric films. The effects of a barrier layer and bottom electrode (conductive material type and thickness) are studied and related to the electromechanical performance of the piezoelectric thick film and then to the electro-acoustic properties of the ultrasonic transducer integrating these structures. Finally, these results are compared with previous studies [2] using porous PZT substrate. These comparisons are performed in terms of fabrication facility, electromechanical constants of the thick films (in thickness mode) and electro-acoustic responses of ultrasonic transducers with center frequencies over 20 MHz.

Propagation of Lamb waves in 1–3 piezocomposites and their application to liquid sensors

Abstract This paper deals with the Lamb modes in 1–3 piezocomposites. The dispersion curves in the cases where the plate is free and loaded by water, as well as the electromechanical coupling factor for a piezocomposite with 36% volume fraction of ceramic have been calculated. It is found that the first antisymmetric mode has a very low coupling factor, whereas that of first symmetric mode has a high value. Experimental measurements of the phase velocity of the four lowest order modes of Lamb wave devices based on piezocomposites are found to be in fair agreement with theory. Furthermore, as predicted by theory, no significant attenuation of the first symmetric mode is observed when the plate is loaded by water.

Electromagnetic Surface Wave Attenuation Caused by Acoustic Wave Radiation

Abstract In theoretically studying electromagnetic surface wave propagation in crystals possessing piezoelectric properties, we found that, if the frequency is smaller than typical phonon frequencies (not exceeding 1012–1013 Hz), then electric fields are able to generate elastic vibrations because of the piezoelectric effect. In this case, an electromagnetic surface wave is coupled with bulk acoustic modes propagating from the interface to the interior of the structure. As a result, the surface wave attenuates because of the radiation of acoustic waves. The attenuation factor is proportional to the parameter τ2 va/vel, where τ is the electromechanical coupling coefficient, and va and vel are typical speeds of acoustic and electromagnetic waves in the crystal, respectively.

Principles and Applications of High-Frequency Medical Imaging

In echographic systems, the axial resolution (i.e., along the propagation axis) is limited by the bandwidth of the received signals, and the lateral resolution (i.e., perpendicular to the propagation axis) is governed by the diffraction of the tranducer aperture. The resolutions will be on the order of a few wavelengths. The speed of sound in most tissues of the human body is quite constant and close to that in water (1540 m/s). One can try to improve image resolution by increasing the center frequency of the ultrasonic devices, but there is a limitation due to attenuation in the tissues, which is roughly proportional to the frequency (0.5 to 1 dB/cm per MHz depending on the tissue). Other limitations linked to the emitted ultrasonic power and the thermal and electronic noises of the system lead to a typical dynamic range on the order of 100 dB. This allows an exploration of a maximum depth of a few hundreds of wavelengths. Typically the abdomen of an adult is examined using a frequency of 3.5 MHz and superficial blood vessels such as the carotids are observed at 7.5 or 10 MHz. The development of ultrasonic techniques in medicine has been essentially linked to relatively low-frequency imaging, first at 2.5 and 3.5 MHz, and later at 5 MHz, for abdominal, obstetrical, and cardiological applications.