Jérôme Fortineau

Ultrasonic characterization of electrochemically etched porous silicon

This paper presents a non-destructive characterization of porous silicon wafer using ultrasound based on the strong relationship between porosity and acoustical parameters. Transmission coefficients are measured using broadband water immersion method and compared to theoretical ones using a genetic algorithm optimization. Retrieved parameters are thickness and acoustic impedance. Porosity is deduced from acoustic impedance. Two set of porous silicon wafers, with different porosities and thicknesses, have been etched using hydrofluoric acid based solution and galvanostatic etching method. Their thicknesses and porosities are measured using destructive methods after ultrasonic measurements. A good agreement is observed. Hypothesis to explain discrepancies are proposed.

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

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.

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.

Determination of microscopic parameters of piezoceramic materials under electrical loading using genetic algorithm

The purpose of this study is the characterization of microscopic parameters of piezoceramic materials, such as spontaneous polarization and spontaneous strain. In previous works, a model has been developed by bridging characteristics of microscopic domain distribution into the macroscopic behavior. It reproduces longitudinal strain and electrical displacement as a function of uniaxial electrical loading, according to material parameters and applied electrical field. Optimization is performed between theoretical and experimental hysteresis curves. This method is based on a genetic algorithm procedure in order to ensure robust convergence even if the system is multimodal. Materials used in this study are PLZT8/65/35 and PZT-5A. After optimization, experimental curves are well fitted to theoretical curves and a good agreement has been shown between retrieved parameters and values reported in literature. This validates domain wall modeling and genetic algorithm as an efficient way to characterize piezoceramic materials under harsh operating conditions.

Non linear parametric imaging using an ultrasonic focused transducer

In this work, a quantitative ultrasonic non linear parameter imaging technique in soft materials is developed. Using a self calibrated method in pulse echo mode, absolute local measurements of fundamental and harmonic pressures are determined and the non-linear parameter is locally measured with a focused transducer. The set up is coupled with an XY scanning system to produce images and the potential of the technique is demonstrated.

Monitoring of the nonlinear coefficient during silica gel formation using an ultrasonic self-calibrated method

The sol-gel process is currently used to develop new materials with a wide range of applications in chemical and food industries. An ultrasonic self-calibrated method is used to evaluate the behavior of the nonlinear coefficient during the sol to gel reaction. The method uses a single piezoelectric transducer in burst echo-mode. Assuming a classical non linearity, the propagation of ultrasonic waves in the sol-gel material can be modeled in a quasi-linear regime using the KZK equation. The temporal evolution of the nonlinear coefficient of silica gel is analyzed. A variation of around 6% of the nonlinear coefficient is observed during the formation of a sol-gel with an initial hydrolysis ratio of 4 and a surrounding temperature at 30°C. Nonlinear results are compared to linear measurements and to literature data, showing a good sensitivity of the nonlinear coefficient with the conventional linear approach of gelation process monitoring.

Characterisation and electroacoustic modelling of porous piezoceramics with graded porosity for ultrasonic transducer applications

Abstract Abstract In this study, a characterisation method for graded porosity piezoceramic discs is presented. A sample with graded porosity from about 5 to about 40% along the thickness has been produced. Two homogeneous samples with respectively low and high porosity are used as references. Density measurements have been performed in order to estimate the average porosity of samples. Using an equivalent circuit model including the mechanical and dielectric losses and a fitting technique, the electroacoustic properties of the constant porosity samples have been extracted. Then, based on a model of multilayer piezoceramics, the graded porosity sample properties are simulated assuming it is made of 10 layers stacked along the sample thickness.