Claire Jean-Mistral

Standards for dielectric elastomer transducers

Dielectric elastomer transducers consist of thin electrically insulating elastomeric membranes coated on both sides with compliant electrodes. They are a promising electromechanically active polymer technology that may be used for actuators, strain sensors, and electrical generators that harvest mechanical energy. The rapid development of this field calls for the first standards, collecting guidelines on how to assess and compare the performance of materials and devices. This paper addresses this need, presenting standardized methods for material characterisation, device testing and performance measurement. These proposed standards are intended to have a general scope and a broad applicability to different material types and device configurations. Nevertheless, they also intentionally exclude some aspects where knowledge and/or consensus in the literature were deemed to be insufficient. This is a sign of a young and vital field, whose research development is expected to benefit from this effort towards standardisation.

Dielectric polymer: scavenging energy from human motion

More and more sensors are embedded in human body for medical applications, for sport. The short lifetime of the batteries, available on the market, reveals a real problem of autonomy of these systems. A promising alternative is to scavenge the ambient energy such as the mechanical one. Up to now, few scavenging structures have operating frequencies compatible with ambient one. And, most of the developed structures are rigid and use vibration as mechanical source. For these reasons, we developed a scavenger that operates in a large frequency spectrum from quasi-static to dynamic range. This generator is fully flexible, light and does not hamper the human motion. Thus, we report in this paper an analytical model for dielectric generator with news electrical and mechanical characterization, and the development of an innovating application: scavenging energy from human motion. The generator is located on the knee and design to scavenge 0.1mJ per scavenging cycle at a frequency of 1Hz, enough to supply a low consumption system and with a poling voltage as low as possible to facilitate the power management. Our first prototype is a membrane with an area of 5*3cm and 31µm in thickness which scavenge 0.1mJ under 170V at constant charge Q.

Impact of the nature of the compliant electrodes on the dielectric constant of acrylic and silicone electroactive polymers

Dielectric elastomers are emerging electroactive materials used in high performance applications such as robots, artificial muscles and energy harvesting. The development of such applications requires the use of accurate, predictive, reliable models which take into account the dielectric constant (permittivity) of these materials. This dielectric constant is not clearly defined for such applications and depends on many parameters. This leads to values dispersed in the literature for the same electroactive polymer. This paper shows that the nature of the compliant electrodes can influence this dielectric constant significantly. However, the reduction generally observed in this permittivity according to the stretching of elastomer cannot be imputed to the nature of these electrodes, and rather confirms an effect of the volume of the elastomer. This tends to prove that the influence of the compliant electrode is located at the electrode–elastomer interfaces. In addition, the nature of the metallic particles embedded in the electrode grease seems not to influence the value of the dielectric constant. Lastly, we propose analytic laws to describe changes of the dielectric constant as a function of the temperature and the deformation of the material. This makes it possible to define new limits of operation for these polymers for actuators and energy harvesting applications.

Electrets substituting external bias voltage in dielectric elastomer generators: application to human motion

Dielectric elastomer generators offer great potential for soft applications involving fluid or human interactions. These scavengers are light, compliant, have a wide range of functions and develop an important energy density. Nevertheless, these systems are passive and require an external bias source, namely a high voltage source and complex power circuits. This cumbersome polarization complexes the system in a drastic way and slows down the development of dielectric generators. In order to remove these problems, we propose here new transducers based on the use of an electret coupled with dielectric elastomer, thus avoiding the use of a high external voltage source, and leading to the design of a oft autonomous dielectric generator. By combining a dielectric model and the electret theory, an electromechanical model was developed to evaluate the capabilities of such a generator. This generator was then produced starting from Teflon TM as electret and silicone PolyPower TM as electroactive polymer. A good agreement between the model and the experiment were obtained. An experimental energy density of 0:55 mJ g-1 was reached for 50% strain (electret potential of -1000 V). Once optimized in its design, such a soft generator could produce energy density up to 1:42 mJ g-1. An energy density of 4:16 mJ g-1is expected with an electret potential of -2000 V.

New operating limits for applications with electroactive elastomer: effect of the drift of the dielectric permittivity and the electrical breakdown

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.

Experimental identification of smart material coupling effects in composite structures

Smart composite structures have an enormous potential for industrial applications, in terms of mass reduction, high material resistance and flexibility. The correct characterization of these complex structures is essential for active vibration control or structural health monitoring applications. The identification process generally calls for the determination of a generalized electromechanical coupling coefficient. As this process can in practice be difficult to implement, an original approach, presented in this paper, has been developed for the identification of the coupling effects of a smart material used in a composite curved beam. The accuracy of the proposed identification technique is tested by applying active modal control to the beam, using a reduced model based on this identification. The studied structure was as close to reality as possible, and made use of integrated transducers, low-cost sensors, clamped boundary conditions and substantial, complex excitation sources. PVDF (polyvinylidene fluoride) and MFC (macrofiber composite) transducers were integrated into the composite structure, to ensure their protection from environmental damage. The experimental identification described here was based on a curve fitting approach combined with the reduced model. It allowed a reliable, powerful modal control system to be built, controlling two modes of the structure. A linear quadratic Gaussian algorithm was used to determine the modal controller‐observer gains. The selected modes were found to have an attenuation as strong as 13 dB in experiments, revealing the effectiveness of this method. In this study a generalized approach is proposed, which can be extended to most complex or composite industrial structures when they are subjected to vibration. (Some figures may appear in colour only in the online journal)

Scavenging energy from human motion with tubular dielectric polymer

Scavenging energy from human motion is a challenge to supply low consumption systems for sport or medical applications. A promising solution is to use electroactive polymers and especially dielectric polymers to scavenge mechanical energy during walk. In this paper, we present a tubular dielectric generator which is the first step toward an integration of these structures into textiles. For a 10cm length and under a strain of 100%, the structure is able to scavenge 1.5μJ for a poling voltage of 200V and up to 40μJ for a poling voltage of 1000V. A 30cm length structure is finally compared to our previous planar structure, and the power management module for those structures is discussed.

On the power management and electret hybridization of dielectric elastomer generators

Dielectric elastomer generators offer great potential for applications involving human or fluid interaction. Nevertheless, they are passive materials requiring to be poled by a high external bias voltage or by electronic power circuits. These power circuits complicate the system drastically and have slowed down the development of dielectric elastomer generators. Replacing the high-voltage source by an electret is a promising alternative, leading to simple flexible generators. In this paper, after a brief review of the classic electronic circuits used to supply dielectric elastomer generators, we present a complete theoretical study on a new hybridization: named the ‘edge mode’. In this mode, we prove the feasibility of flexible 2D autonomous dielectric generators using the edge effects of the electric field induced by electrets (Teflon) to polarize a dielectric elastomer (Danfoss Polypower). To this end, different 2D structures were modelled as a function of various geometric parameters of the structure. After these structures were subjected to a strain of 50%, a scavenged energy of a few J g 1 can be obtained. (Some figures may appear in colour only in the online journal)

Modelling of soft generator combining electret and dielectric elastomer

Dielectric elastomer generators (DEGs) are a promising technology for soft applications involving fluid or human interactions thanks to their high energy density and ability to couple directly on mechanical ambient sources. Electret can be used to replace the high external voltage source necessary to this electrostatic generator. These soft hybrid generators require thus a reliable non-linear multi-physic models to estimate their performances and to design new innovative structure. We present here an analytic and numeric model for structures combining dielectric elastomer and electrets.

How does static stretching decrease the dielectric constant of VHB 4910 elastomer

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.