Alain Sylvestre

Optimization of an electret-based energy harvester

Thanks to miniaturization, it is now possible to imagine self-powered systems that use vibrations to produce their own electrical energy. Many energy harvesting systems already exist. Some of them are based on the use of electrets: electrically charged dielectrics that can retain charge for years. This paper presents an optimization of an existing system and proves that electret-based electrostatic energy scavengers can be excellent solutions to power microsystems even with low-level ambient vibrations. Thereby, it is possible to harvest up to 200??W with vibrations of acceleration lower than 1g (typically 50? ?mpp at 50?Hz) using thin SiO2 electrets with an active surface of 1?cm2 and a mobile mass of 1?g. This paper optimizes such a system (geometric, electrostatic and mechanical parameters), using FEM (finite element method) software (Comsol Multiphysics) and Matlab to compute the parameters, and proves the importance of such an optimization to build efficient systems. Finally, it is shown that the use of electrets with high surface potential is not always the best way to maximize output power.

Dielectric properties of polyacrylate thick films used in sensors and actuators

Dielectric polymers are emerging electro-active materials used in high performance applications such as micropumps, robots and artificial muscles. The development of such applications requires the use of models taking into account the electrical parameters of the material. However, there is still some controversy over the dielectric constant of the most widely used dielectric polymer (VHB 4910, 3M, USA). In this paper, we present an exhaustive study relating to changes in the dielectric constant of VHB 4910 over wide frequency and temperature ranges. We found that the permittivity was a function of: frequency, temperature, the nature of the electrodes and the pre-stress applied to material. Mechanisms of dielectric polarization (β-relaxation) explain the behaviour in temperature and frequency of this parameter. The use of silver grease-compliant electrodes induces an increase in the dielectric constant which moves to a value of 5.4 (against 4.7 with gold electrodes). A pre-strain applied to the material shows a reduction up to 15% in the value of the dielectric constant. Short-range dipolar relaxation, local mechanical constraints in the material and a possible crystallization of material induced by the stretching are suggested to explain these behaviours. Analytic equations of the dielectric constant according to the temperature and pre-strain are then proposed and used to validate the behaviour of these materials for actuator and scavenger devices.

Space charge distribution measurement methods and particle loaded insulating materials

In this paper the authors discuss the effects of particles (fillers) mixed in a composite polymer on the space charge measurement techniques. The origin of particle-induced spurious signals is determined and silica filled epoxy resin is analysed by using the laser-induced-pressure-pulse (LIPP), the pulsed-electroacoustic (PEA) and the laser-induced-thermal-pulse (LITP) methods. A spurious signal identified as the consequence of a piezoelectric effect of some silica particles is visible for all the methods. Moreover, space charges are clearly detected at the epoxy/silica interface after a 10-kV/mm poling at room temperature for 2 hours.

The influence of filler particles on space charge measurements

The effects of filler particles on the signal measured using the pressure-wave-propagation method to determine the space charge distribution are analysed. A special test sample geometry is used to classify the influence of particles, and in particular, the role of piezoelectricity is discussed. Applications with Epoxy resin samples loaded either by silica or alumina particles are presented. Silica particles are shown to exhibit piezoelectricity which strongly modifies the measured signals.

Field and temperature dependence of space charge injection in a silicone rubber

The generation of space charges in a silicone elastomer has been characterized under dc voltage as a function of the temperature using the laser induced pressure pulse (LIPP) method. The analysis of the LIPP spectra has been detailed thanks to acoustical considerations, taking into account the reflection, attenuation and dispersion of the pressure wave. Both heterocharges and homocharges have been observed depending on the field magnitude (5 to 20 × 106 V m−1), polarization duration and temperature (20, 40 or 60 °C). With temperature increasing, charges are observed at lower fields. Heterocharges appear in less stressed conditions. Then, homocharges take the place of these heterocharges. Positive homocharges are observed in lower stressed conditions than negative homocharges. Mobility of these charges has not been observed below 60 °C.

Silicone rubber subjected to combined temperature and humidity effects

Room-temperature-vulcanization silicone rubbers have been subjected to short duration, (1sec) negative corona discharges in different temperature and relative humidity environments. We have investigated the time dependence of the surface potential decay, in situ, in a climatic chamber. This study has revealed a strong influence of the initial deposited surface charge density, temperature and relative humidity. Bulk conduction mechanisms appear to be preponderant. A non homogeneous evolution of surface potential decay is observed at increased relative humidity.

Dielectric relaxation study in Tantalum Pentoxide capacitors

In this paper MIM (Metal-Insulator-Metal) Ta2O5 capacitor has been studied in term of dielectric relaxation over several thicknesses with a low frequency dielectric spectroscopy and current versus time measurement. A Maxwell-Wagner mechanism and resistance degradation has been identified. The resistance degradation follows the Space-Charge-Limited (SCL) theory. These two behaviours have the same activation energy and have been attributed to electrode polarisation mechanism created by mobile charges in the dielectric. Finally according to physical characterization the origin of these defects has been attributed to oxygen vacancies.

Space charge measurements in epoxy resin under DC voltage

Various regimes of homocharge and heterocharge generation have been observed in the range 5-40 kV/mm dc. Three temperature domains have been studied depending on the glass transition temperature of the epoxy resin (Tg/spl sim/65/spl deg/C): 20-40/spl deg/C for glassy state properties, 55/spl deg/C (just below T/sub g/), and 80/spl deg/C (above T/sub g/).

Temperature dependence of space charge behavior in silicone

The generation of space charges in a silicone elastomer has been characterized under DC voltage as a function of the temperature using the LIPP method. Both heterocharges and homocharges have been observed depending on the field magnitude, polarization duration and temperature. Heterocharges appear in less stressed conditions. Then, homocharges take place of these heterocharges. The phenomena depend on the electrode polarity.

Estimation of surface degradation under immersion plasma by surface potential decay method [polymer outdoor insulation applications]

The surface potential decay (SPD) method is frequently used to investigate the properties of conduction and the electrostatic phenomena on polymer surfaces. Generally, the influence of the hydrophobicity change on the SPD is not considered. However, this parameter must be taken into account when using this method to study the aging of materials, in particular for insulating polymers used for outdoor insulation. To this end, we have analyzed by using the SPD, the hydrophobicity recovery of silicone rubber subjected previously to a plasma treatment which changes its wettability. The conclusions of this work clearly show the influence of hydrophobicity recovery on the SPD behavior.