Achraf Kachroudi

Piezoelectric cellular micro-structured PDMS material for micro-sensors and energy harvesting

This paper reports a novel low-cost fabrication process of a charged cellular microstructured polydimethylsiloxane (PDMS) material referred as piezo-electret or ferro-electret for micro-sensors applications. The dielectric spectra reached on these structures exhibit a high piezoelectric longitudinal coefficient d33 of 350pC/N. A mechanical characterization method proves the reliability of this material for low-frequencies applications around 100Hz.

Micro-structured PDMS piezoelectric enhancement through charging conditions

Micro-structured cellular polydimethylsiloxane (PDMS) materials were prepared by a low-cost molding process allowing us to control geometry and sample size. Cellular structures are charged with a triangular quasi-static voltage with amplitudes between 1 kV and 4 kV and a frequency of 0.5 Hz fixed after having evaluated the conditions enhancing the piezoelectric response of the cellular PDMS. The piezo-electret PDMS material charged at room temperature has a piezoelectric coefficient d 33 of 350 pC/N, which is ten times larger than that of polyvinylidene fluoride. The high piezoelectric coefficient with a very low elastic modulus of 300 kPa makes these materials very useful for wearable device applications. The piezoelectric coefficient d 33 of the samples poled at high temperatures improves thermal stability but reduces PDMS piezo-electret piezoelectricity, which is explained by the structures stiffness. These results are useful and allow us to set the conditions for the preparation of the piezo-electret materials according to desired applications.

Dielectric properties modelling of cellular structures with PDMS for micro-sensor applications

Electro-active polymers are emerging in the fields of actuators and micro-sensors because their good dielectric and mechanical properties makes them suitable for such applications. In this work, we focus on micro-structured (cellular) polymer materials (referred as piezoelectrets or ferroelectrets) that need prior charging to attain piezoelectric behaviour. The development of such applications requires an in-depth knowledge of the intrinsic dielectric properties of such structures and models to enable the accurate prediction of a given micro-structured material’s dielectric properties. Various polymers including polypropylene, polytetrafluoroethylene, fluoroethylenepropylene, cyclo-olefines and poly(ethylene terephthalate) in a cellular form have been studied by researchers over the last fifteen years. However, there is still a lack of information on the intrinsic dielectric properties of the most recently used dielectric polymer (polydimethylsiloxane, PDMS) over wide frequency and temperature ranges. In this work, we shall propose an exhaustive equivalent electrical circuit model and explain how it can be used to predict the micro-structured PDMS complex permittivity versus frequency and temperature. The results obtained from the model were found to be in good agreement with experimental data for various micro-structured PDMS materials. Typically, for micro-sensor applications, the dielectric constant and dielectric losses are key factors which need to be minimized. We have developed a configuration which enables both to be strongly reduced with a reduction of 16% in the dielectric constant of a micro-structured PDMS compared with the bulk material. In addition, the phenomena responsible for dielectric losses variations with frequency and temperature are discussed and correlated with the theoretical model. Our model is thus proved to be a powerful tool for the control of the dielectric properties of micro-structured PDMS material for micro-sensor applications.

Autonomous electrostatic generator for energy harvesting applications under inertial load

Vibration-based energy harvesters have been extensively studied and investigated to harvest the energy produced by environmental mechanical vibration sources as mean to produce low electrical energy, thereby supplying low-power sensors and actuators. Different devices have been proposed as energy harvesters, cantilevers-based geometries have been pursued frequently in the literature. Here, we propose the geometry of an elastomeric circular membrane coupled with an electret (soft electrostatic generator) with a central proof mass. By soliciting the designed device around its resonance frequency of 14Hz with an acceleration of 0.4g for a mass of 9.5g, the system produced an average electric power of 24μW for an optimal resistance of 150MΩ. An analytical study developed closely with a finite element simulation with Comsol® allowed to validate the obtained experimental results, suggesting that this approach can be used as a tuning method to develop other geometrical shapes and conceive large-scale devices for vibration energy harvesting applications.

Air-spaced PDMS piezo-electret cantilevers for vibration energy harvesting

This paper reports a design of a new prototype of air-spaced cantilevers made from a micro-structured PDMS piezo-electret material for accelerometer and energy harvesting applications. The test performed on these cantilevers in a sensor mode exhibits a stable sensitivity of 385 mV/g for a frequency ranging from 5 Hz to 200 Hz that encompass most macro-scale vibrations. In the energy harvesting mode, the cantilever generates a power of 103 nW with a load resistance of 217 MΩ.