Mesure de l'atténuation et de la vitesse de propagation ultrasonore par une technique spectroscopique

A technique of F.F.T. ultrasonic spectroscopy is described which allows determination of the absorption and velocity of sound in the condensed phase over a wide frequency range. The experiment was checked using a comparison of these data and those obtained using classical ultrasonic methods on 1.1.2 trichloroethane and certain aqueous mixtures of alcohol. The use of this technique to study the change in relaxation spectrum for systems changing with time and in particular those undergoing gelation are described.Une spectroscopie ultrasonore par F.F.T. est présentée qui permet de mesurer la vitesse et l'absorption des ondes ultrasonores dans les milieux condensés sur une large gamme de fréquences. La validation expérimentale de cette technique est testée sur des liquides connus par comparaison avec les grandeurs obtenues par les méthodes ultrasonores classiques, comme le 1.2.2 trichloroéthane ou certains mélanges aqueux d'alcools. Cette technique est utilisée également pour la détermination de l'évolution dans le temps des spectres de relaxation des systèmes qui évoluent, en particulier pour ceux dont la croissance est gouvernée par des processus d'agrégation

Study of the electrical conductivity in fiber composites

Three-dimensional simulations have been conducted to predict the percolation threshold in fiber composite materials. It has been shown that the expansion of the sample's size increases the sharpness of distributions curves of the percolation threshold. Decreasing the percolation threshold with longer fiber is also verified. A method is proposed to evaluate the electrical resistance of fibrous composites. Assuming meandering paths, the calculation is based on detecting conductive pathways through the insulating matrix. The percolation is detected by the height of the conducting cluster instead of its number at the two electrodes. The electrical resistivities and the conduction thresholds of the carbon fiber reinforced polycarbonate composites have been characterized. The simulation results are in good agreement with an experimental study result found in the literature

CARACTÉRISATION ULTRA-ACOUSTIQUE DE RÉSEAUX POLYURÉTHANE

La formation et les propriétés finales de réseaux macromoléculaires covalents sont étudiés au moyen de techniques de propagation ultrasonore. Dans le cas de réseaux polyuréthanne, l'évolution de la vitesse de propagation et de l'absorption des ondes longitudinales montrent l'intérêt de la spectroscopie ultrasonore utilisée comme méthode d'investigation de la croissance du réseau ainsi que des propriétés du matériau à l'état achevé

The inserted piezoelectric sensor method for monitoring thermosets cure

An original method enabling the in-situ and real time monitoring of a polymer structure evolution using an inserted piezoelectric sensor is presented. The validation of this technique is done at constant temperature by comparison to the results obtained on several polymers by classical ultrasonic methods in practically the same conditions. The complete validation is achieved by monitoring the complex cure of a polymer matrix fibre reinforced composite PMR-15 under sever conditions of temperature

Flexoelectric response in soft polyurethane films and their use for large curvature sensing

International audienceThe flexoelectric effect is simply defined as the coupling between the strain gradient and polarization in solid dielectrics. It may be seen as an alternative transduction mechanism to the piezoelectric effect to directly sense the curvature of bent flexible thin structures. In the case of large curvatures, flexible and compliant sensors are required and soft polar elastomers may be suitable for curvature sensing. In this study, we report the flexoelectric characterization of soft semi-crystalline polyurethane (PU) films with thicknesses ranging from 1.7 μm to 350 μm. Dynamic bending experiments have been performed on PU films deposited onto rigid steel substrates in the vicinity of the mechanical resonance frequency of the cantilever beams. Quasi-static flexoelectric coefficients of PU films could be obtained by using a classical oscillating model. A global large increase of μ′12 with the decreasing film thickness was found, especially for thicknesses lower than 25 μm. The variation of μ′12 is explained by the presence of a Young's Modulus gradient through the thickness of PU films. Besides, a concomitant uncommon dramatic decrease in the dielectric constant is observed. The combination of these two effects contributes to enhancing the flexocoupling “F” constant with the decreasing thickness. At last, the potential use of a 6.6 μm-thick soft PU film as a large curvature sensor has been experimentally demonstrated by subjecting a flexible Aluminum foil/Polyethylene terephthalate bilayered cantilever to large deflections. A curvature of about 80 m−1 (radius of curvature of ∼1.2 cm) could be sensed under low frequency (3 Hz) bending motion. These results may pave the way for the development of low cost and easy to implement soft flexoelectric elastomer-based large curvature sensors on highly flexible metallic structures

Enhanced Piezoelectric Response in Nanostructured Ni/PVDF Films

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Enhanced Piezoelectric Response in Nanostructured Ni/PVDF Films

International audiencePoly(vinylidene fluoride) (PVDF) composites have recently emerged as excellent candidates to fabricate flexible and small piezoelectric generators for portable devices. Among various techniques used to nanostructure polarized PVDF, the track-etching represents a new route for manufacturing nanostructured composite thin films. The moderate influence of irradiation on the piezoelectric response of polarized PVDF makes possible the use of this technique. In this way, a nanostructured composite based on polarized thin PVDF films comprising embedded nickel nanowires (Ni NWs) was fabricated. The nanostructured PVDF/Ni NWs composites were tested under bending conditions using a homemade pressure cell. Due to the presence of NWs, an increase of five-fold the initial dielectric permittivity, in the low-frequency range, was observed. It suggested the presence of an interfacial polarization at the PVDF/Ni interface. With respect to the etched PVDF, the nanostructured PVDF/Ni NWs composites exhibited a non-negligible enhancement by 2.5 times the piezoelectric efficiency. This result was attributed to the increased Au/Ni NWs electrode surface