François Vander Meulen

Preparation and electromechanical properties of PZT/PGO thick films on alumina substrate

PZT thick films with lead germanate as low temperature sintering aid were prepared on alumina substrates. The thickness of films after sintering procedures reached 50 μm. Chemical compatibility and microstructure of layers was studied by EDS/SEM analysis. A fit between theoretical and experimental electrical impedance of several samples as a function of frequency is used to determine the elastic, dielectric and piezoelectric properties in thickness mode of the ceramic layers. Results for different poling fields (3 and 12 kV/mm) and sintering temperatures are obtained. Finally, the KLM equivalent circuit is used to obtain simulations of transducers integrating these thick films and their performance for medical imaging applications is evaluated.

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.

Nonlinear constant evaluation in a piezoelectric rod from analysis of second harmonic generation

The design of transducers requires a clear understanding of their electromechanical behavior. This involves precise linear modeling as well as characterization. With the development of novel techniques such as harmonic imaging as well as high-power applications, nonlinear aspects must also be taken into account. In this study, harmonic generation in the mechanical displacement of a piezoceramic rod under high sinusoidal electric fields was measured. Theoretically, the nonlinearity can come from various sources: dielectric, mechanical, and electromechanical. The nonlinearity coming from external sources being eliminated or taken into account, it is shown here that the analysis, over a wide frequency range, of 2 parameters related to the harmonic distortion enables the respective identification of these sources and, at the same time, the evaluation of third-order constants of the material.

Celerity and thickness measurements by ultrasound in protons exchange membranes

Acoustic measurements of protonic membranes thicknesses are performed by using Insertion/Substitution technique. Ultrasound celerity in this material is firstly determined from time domain analysis of the received signal of a thick membrane. This celerity is then used to measure the different thickness of the samples from frequency analysis. Indeed, the ratio between the reference and transmitted spectra shows resonances due to the multiple transit echoes in the layer. The measurements on the dry membranes give a celerity of 1240 m/s. The measurements on fully hydrated membranes show an increased thickness of 10 - 15 %. This latter has to be compared to the typical swelling of 10 % reported in literature.

P2I-4 Experimental Study of the Non Linearity from Ultrasonic Transducers

The quantitative evaluation of the non linearity from a material is a characterization method currently used in various applications such as NDE or medicine. Such a measurement must be performed using a set of devices exempted of non linearity themselves. The purpose of the present work is to investigate the non linear behavior of transducers. A measurement method is proposed to evaluate the second harmonic generation in an ultrasonic transducer submitted to a high electrical excitation. Displacement at the surface of the transducer is measured by a laser probe, while the real electrical excitation is acquired. The method is based on measurement of fundamental and harmonic quantities on the acoustical axis of the transducer for aluminum samples of various thicknesses. The experimental set up includes a high power filter to avoid second harmonic electrical excitation. However, the possibility to take into account the presence of component at twice the fundamental frequency is considered in the proposed method. Then the second harmonic generation due to the loaded transducer itself is compared to the generation in a solid sample

Efficient algorithm for discrimination of overlapping ultrasonic echoes

HIGHLIGHTSOverlapped ultrasonic echoes are identified using an iterative algorithm.One echo is identified at each iteration.The effectiveness and the robustness of the method are studied.Measurement can be achieved in a wide range of situations.Ultrasonic velocities and thicknesses are simultaneously determined. ABSTRACT We propose a method to identify the different echoes of an overlapped ultrasonic signal. This method is based on an iterative algorithm that compares the experimental signal to a realistic dictionary of trial functions and allows identification of one overlapped echo at each iteration. Adding physical parameters to the dictionary such as sample attenuation and ultrasound beam diffraction allows the method to be applied to various materials and sample geometries. Measurements at 500 kHz and 5 MHz on a ABS material and a copper plate are reported. The effectiveness and the robustness of the method are studied as a function of time delay between the different echoes. We show that taking into account the experimental set‐up and material properties in the development of the dictionary are critical to identifying a round‐trip signal when overlapping occurs.

Ultrasonic self-calibrated method applied to monitoring of sol–gel transition

In many industrial processes where online control is necessary such as in the food industry, the real time monitoring of visco-elastic properties is essential to ensure the quantity of production. Acoustic methods have shown that reliable properties could be obtained from measurements of velocity and attenuation. This paper proposes a simple, real time ultrasound method for monitoring linear medium properties (phase velocity and attenuation) that vary in time. The method is based on a pulse echo measurement and is self-calibrated. Results on a silica gel are reported and the importance of taking into account the changes of the mechanical loading on the front face of the transducer will be shown. This is done through a modification of the emission and reception transfer parameters. The simultaneous measurement of the input and output currents and voltages enables these parameters to be calculated during the reaction. The variations of the transfer parameters are in the order of 6% and predominate other effects. The evolution of the ultrasonic longitudinal wave phase velocity and attenuation as a function of time allows the characteristic times of the chemical reaction to be determined. The results are well correlated with the gelation time measured by rheological method at low frequency.

Linear and nonlinear acoustic parameters measurements in plates with various moisture contents

Variation of the moisture content is known to modify many of the mechanical properties of solids. For this reason, the estimation of the absorbed quantity of water is a subject of growing interest in characterization of materials. In this work, the water content in phenolic resin plates, placed under variable relative moisture conditions, is investigated through linear and nonlinear acoustic parameter measurements. Linear measurements are performed using an insertion/substitution spectroscopy technique. Nonlinear measurements consist of an evaluation of the phase modulation obtained by a collinear interaction between a high frequency tone burst and a low frequency wave. Results show that many of the studied parameters have a good sensitivity to the absorbed water content.

Contribution of reflector interfaces to the measurement sensitivity improvement of nonlinear B/A parameter for echo mode

The B/A parameter is a second order distortion ratio defined in nonlinear acoustics. Pulse echo method has been shown to be a suitable way for the parameter measurement using a simple experimental set-up. We show how to improve the sensitivity of the method without any change in the apparatus and modus operandi.

Ultrasonic material characterization using Matching Pursuit algorithm with experimental dictionary

Adaptation of the Matching Pursuit algorithm is presented to simulate the successive overlapped echoes of an ultrasonic signal. Crucial step of sparse representation algorithm such as MP algorithm is the choice of the dictionary. Here, the dictionary is built from experimental measurements in a transmission technique. Acoustic measurements on copper and ABS sample at 500 kHz, 10 MHz, 15 MHz and 20 MHz are performed to test the efficiency of the method. Simultaneously measurements of thickness and velocity of the copper sample are reported at different frequency. These results show that using a dictionary construct from the reference signal is enough to obtain sample characteristics with low attenuating material. The robustness of the method is studied as a function of time delay between the different echoes. In the case of the ABS sample, taking into account the attenuation of this material in the development of the dictionary is critical to identifying a round-trip signal when overlapping occurs.