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.
Harvesting human kinetic energy to produce electricity is an attractive alternative to batteries for applications in wearable electronic devices and smart textile. Dielectric elastomers generators (DEGs) represent one of the most promising technologies for these applications. Nevertheless, one of the main disadvantages of these structures is the need of an external polarization source to perform the energetic cycle. In the present work, a hybrid electret-dielectric elastomer generator in DEG mode is presented. In this configuration, the electret material is used as polarization source of a classical DEG, i.e. an electrostatic generator based on electrical capacitance variation. The electrical energy output in this mode (1.06mJ.g−1) could be higher than the one obtained using a classical electret mode (0.55mJ.g−1), i.e. charges recombination. In this paper, the operation principle of the hybrid generator will be fully described and the design rules for the realization of the prototype will be presented. The experimental data obtained from the prototype will be compared to the results of FEM simulations.
Dielectric elastomers generators (DEGs) constitute promising systems due to their high energy density. This latter is influenced by viscoelasticity and the leakage current. An understanding of this leakage current and how it can be influenced by the stretch state of material is required to predict or optimize DEGs. In this context, our work consisted in studying the evolution of the leakage current in commercial electroactive polymer (3 M VHB4910) using silver grease as electrodes. This analysis has been performed in order to evaluate the influence of three different factors: the biaxial prestretch (λ2 = 4, 9 and 16), the temperature (from 20°C to 80°C) and the high electric field (from 1MVm-1 to 20MVm-1). Main results are (i) the increase in the leakage current at higher pre-stretch due to the increase of the electric field, (ii) a predominant Schottky conduction mechanism (iii) a lower current at room temperature for asymmetric pre-stretch compared to an equivalent area surface ratio with symmetric pre-stretch, (iv) the point iii fails when the material works at temperatures higher than room temperature. Probable changes in the molecular chains with strain explain these results.
Dielectric elastomers such as 3M VHB4910 acrylate film have been widely used for electromechanical energy conversion such as actuators, sensors and generators, due to their lightweight, high efficiency, low cost and high energy density. Mechanical and electric properties of such materials have been deeply investigated according to various parameters (temperature, frequency, pre-stress, nature of the compliant electrodes…). Models integrating analytic laws deduced from experiments increase their accuracy. Nevertheless, leakage current and electrical breakdown reduce the efficiency and the lifetime of devices made with these polymers. These two major phenomena are not deeply investigated in the literature. Thus, this paper describes the current-voltage characteristics of acrylate 3M VHB4910 and investigates the stability of the current under high electric field (kV) for various temperatures (from 20°C to 80°C) and over short (300 s) and long (12h) periods. Experimental results show that, with gold electrodes at ambient temperature, the current decreases with time to a stable value corresponding to the conduction current. This decrease occurs during 6 hours, whereas in the literature values of current at short time (less than 1 hour) are generally reported. This decrease can be explained by relaxations mechanisms in the polymer. Schottky emission and Poole-Frenkel emission are both evaluated to explain the leakage current. It emerges from this study that the Schottky effect constitutes the main mechanism of electric current in the 3M VHB4910. For high temperatures, the steady state is reached quickly. To end, first results on the leakage current changes for pre-stretch VHB4910 complete this study.
Subject to a voltage, dielectric elastomers deform by the effect of Maxwell stress which is depended directly
on the dielectric constant of the material. The combination of large strain, soft elastic response and good
dielectric properties has established VHB 4910 elastomer as the most used material for dielectric elastomer
actuators. However, the effect of stretch on the dielectric constant for this elastomer is much debated topic while
controversy results are demonstrated in the literature. The dielectric constant of this material is studied and
demonstrated that it decreases slightly or hugely among the stretch but any pertinent response and any physic
explications are validated by the scientific community. In this paper, we presented a detail study about dielectric
behavior of VHB 4910 elastomer versus a broadband of stretch and temperature. We found that the dielectric
constant of this material depends strongly on the stretch following a polynomial law. Among all the explanations
of stretch dependence of the dielectric constant of VHB 4910 in the literature: the crystallization, the change
of glass transition temperature, the decrease of dipole orientation, the electrostriction effect under stress; and
based on our experimental result, we conclude that the decrease of dipole orientation seems the main reason to
the drop of dielectric constant of VHB 4910 elastomer versus the stretch. We proposed also an accurate model
describing the dielectric constant of this material for a large range of stretch and temperature.
Dielectric elastomer generators (DEGs) are light, compliant, silent energy scavengers. They can easily be incorporated
into clothing where they could scavenge energy from the human kinetic movements for biomedical applications.
Nevertheless, scavengers based on dielectric elastomers are soft electrostatic generators requiring a high voltage source
to polarize them and high external strain, which constitutes the two major disadvantages of these transducers. We
propose here a complete structure made up of a strain absorber, a DEG and a simple electronic power circuit. This new
structure looks like a patch, can be attached on human’s wear and located on the chest, knee, elbow… Our original strain
absorber, inspired from a sailing boat winch, is able to heighten the external available strain with a minimal factor of 2.
The DEG is made of silicone Danfoss Polypower and it has a total area of 6cm per 2.5cm sustaining a maximal strain of
50% at 1Hz. A complete electromechanical analytical model was developed for the DEG associated to this strain
absorber. With a poling voltage of 800V, a scavenged energy of 0.57mJ per cycle is achieved with our complete
structure. The performance of the DEG can further be improved by enhancing the imposed strain, by designing a stack
structure, by using a dielectric elastomer with high dielectric permittivity.
Dielectric elastomers can work as a variable capacitor to convert mechanical energy such as human motion into electrical energy. Nevertheless, scavengers based on dielectric elastomers require a high voltage source to polarize them, which constitutes the major disadvantage of these transducers. We propose here to combine dielectric elastomer with an electret, providing a quasi-permanent potential, thus replacing the high voltage supply. Our new scavenger is fully autonomous, soft, lightweight and low cost. Our structure is made of a dielectric elastomer (Polypower from Danfoss) and an electret developing a potential of -1000V (Teflon from Dupont). The transducer is designed specifically to scavenge energy from human motion. Thus, it works on pure-shear mode with maximum strain of about 50% and it is textured in 3D form because electret is not deformable. The shape of the hybrid structure is critical to insure huge capacitance variation and thus higher scavenged energy. We present in this paper our process for the optimization of the 3D shape that leads us to the developpment and characterization of our first prototype. From an appropriate electromechanical analytical model, an energy density of about 1.48mJ.g-1 is expected on an optimal electrical load. Our new autonomous dielectric generator can produce about 0.55mJ.g-1 on a resistive load, and can further be improved by enhancing the performance of dielectric elastomer such as dielectric permittivity or by increasing the electret potential.
Dielectric elastomer generators are a promising solution to scavenge energy from human motion, due to their lightweight, high efficiency low cost and high energy density. Performances of a dielectric elastomer used in a generator application are generally evaluated by the maximum energy which can be converted. This energy is defined by an area of allowable states and delimited by different failure modes such as: electrical breakdown, loss of tension, mechanical rupture and electromechanical instability, which depend deeply on dielectric behaviors of the material. However, there is controversy on the dielectric constant (permittivity) of usual elastomers used for these applications. This paper aims to investigate the dielectric behaviors of two popular dielectric elastomers: VHB 4910 (3M) and Polypower (Danfoss). This study is undertaken on a broad range of temperature. We focus on the influence of pre-stretch in the change of the dielectric constant. An originality of this study is related to the significant influence of the nature of compliant electrodes deposited on these elastomers. Additionally, the electrical breakdown field of these two elastomers has been studied as a function of pre-stretch and temperature. Lastly, thanks to these experiments, analytic equations have been proposed to take into account the influence of the temperature, the pre-stretch and the nature of the compliant electrodes on the permittivity. These analytic equations and the electrical breakdown field were embedded in a thermodynamic model making it possible to define new limits of operation closer to the real use of these elastomers for energy harvesting applications.
Thanks to their high energy density and their flexibility, scavenging energy with dielectric polymer is a promising
alternative to ensure the autonomy of various sensors such as in e-textiles or biomedical applications. Nevertheless, they
are passive materials requiring a high bias voltage source to polarize them. Thus, we present here a new design of
scavenger using polymer electrets for poling the dielectric polymer. Our scavenger is composed of commercial dielectric
polymer (3M VHB 4910) with Teflon electrets developing a potential of -300V, and patterned grease electrodes. The
transducer works in a pure shear mode with a maximal strain of 50% at 1Hz. The typical "3D-textured" structure of the
scavenger allows the electrets to follow the movement of the dielectric. A complete electromechanical analytical model
has been developed thank to the combination of electrets theory and dielectric modelling. Our new autonomous structure,
on an optimal resistance, can produce about 0.637mJ.g-1.