Electrical conduction in dielectric elastomer transducers
Résumé
Dielectric elastomers are considered to be promising materials for applications in energy harvesting, haptic interface or biomedical actuation, thanks to their high energy density and attractive efficiency. Nevertheless, the performance of dielectric elastomer transducers (DETs) are affected by losses, especially losses induced by leakage current. This work reports an extensive investigation into the stability of the current-time characteristics of VHB4910 dielectric elastomers, currently in use for such applications. It should be noted that conductive pastes that act as compliant electrodes play an important role in the behavior of the current. The Schottky current is thus disturbed at the electrode - elastomer interface and an increase in current is observed over time when the device is subjected to electrical voltages over long periods of time. Pre-stress, commonly applied for application using VHB4910, influences also significantly the electrical response. Thus, for a same stretching of the material, asymmetric stretching is preferred over symmetric stretching in order to obtain a lower leakage current. This difference in behavior is explained by the fact that the polymer chains are stretched in preferential directions which will then lead to specific responses of both the dipoles and other conduction mechanisms such as hopping. Finally, a simple analytical model is proposed to report the current behavior in these materials according to the nature of the electrodes and the pre-stretching applied.