Electrode Impact on the Electrical Breakdown of Dielectric Elastomer Thin Films.

Dielectric Elastomer Actuators (DEAs) enable the realization of energy-efficient and compact actuator systems. DEAs operate at the kilovolt range with typically microampere-level currents and hence minimize thermal losses in comparison to low voltage/high current actuators such as shape memory alloys or solenoids. The main limiting factor for reaching high energy density in high voltage applications is dielectric breakdown. In previous investigations on silicone-based thin films, we reported that not only do environmental conditions and film parameters such as pre-stretch play an important role but that electrode composition also has a significant impact on the breakdown behavior. In this paper, we present a comprehensive study of electrical breakdown on thin silicone films coated with electrodes manufactured by five different methods: screen printing, inkjet printing, pad printing, gold sputtering, and nickel sputtering. For each method, breakdown was studied under environmental conditions ranging from 1 °C to 80 °C and 10% to 90% relative humidity. The effect of different manufacturing methods was analyzed as was the influence of parameters such as solvents, silicone content, and the particle processing method. The breakdown field increases with increasing temperature and decreases with increasing humidity for all electrode types. The stiffer metal electrodes have a higher breakdown field than the carbon-based electrodes, for which particle size also plays a large role.

Nanoscale Nickel-Based Thin Films as Highly Conductive Electrodes for Dielectric Elastomer Applications with Extremely High Stretchability up to 200.

This paper presents on electromechanical characterization of thin film nickel-based wrinkled electrodes for dielectric elastomer (DE) applications. The investigation of a sandwich composed of a very soft and flexible elastomer carrying an ultrathin metallic electrode, together with its prestretch-dependent wrinkled structure of the electrode, facilitates the understanding of some of its interesting properties. Compared to conventional screen-printed carbon black electrodes, nickel-based thin film electrodes offer an ohmic resistance that is about 2 orders of magnitude lower. This remarkable feature makes it an advantageous electrode material alternative for the development of energy-efficient and high-frequency DE applications. Ultrathin (10-20 nm) layers are sputter deposited as electrodes onto either biaxially or, under pure-shear conditions, uniaxially prestretched silicone membranes. After the sputtering process, the membranes are allowed to relax whereby wrinkled out-of-plane buckled surfaces are obtained. With an initial resistance smaller than 400 Ω/square and a strong adhesion to the silicone, some electrode configurations are able to withstand strains up to 200% while remaining electrically conductive. A linear dependence of the capacitance on strain is revealed, as well as a long-term stability over 10 million cycles of mechanical stretching. All investigated thin film configurations of nickel and nickel-carbon films are suitable as compliant electrodes for DE actuators, as demonstrated by measuring the force characteristics with and without a high voltage. An increased level of prestretch shifts the resistance threshold of the electrode layers to even higher strain levels. In general, the best performance is achieved with pure metallic electrodes deposited on biaxially prestretched silicone membranes.

Fully Polymeric Domes as High-Stroke Biasing System for Soft Dielectric Elastomer Actuators.

The availability of compliant actuators is essential for the development of soft robotic systems. Dielectric elastomers (DEs) represent a class of smart actuators which has gained a significant popularity in soft robotics, due to their unique mix of large deformation (>100%), lightweight, fast response, and low cost. A DE consists of a thin elastomer membrane coated with flexible electrodes on both sides. When a high voltage is applied to the electrodes, the membrane undergoes a controllable mechanical deformation. In order to produce a significant actuation stroke, a DE membrane must be coupled with a mechanical biasing system. Commonly used spring-like bias elements, however, are generally made of rigid materials such as steel, and thus they do not meet the compliance requirements of soft robotic applications. To overcome this issue, in this paper we propose a novel type of compliant mechanism as biasing elements for DE actuators, namely a three-dimensional polymeric dome. When properly designed, such types of mechanisms exhibit a region of negative stiffness in their force-displacement behavior. This feature, in combination with the intrinsic softness of the polymeric material, ensures large actuation strokes as well as compliance compatibility with soft robots. After presenting the novel biasing concept, the overall soft actuator design, manufacturing, and assembly are discussed. Finally, experimental characterization is conducted, and the suitability for soft robotic applications is assessed.

A Translucent Nanocomposite with Liquid Inclusions of a Responsive Nanoparticle Dispersion.

Active nanocomposites are created with liquid inclusions that contain plasmonic gold nanoparticles inside a polymeric matrix. The alkylthiol-coated gold particles are designed to reversible agglomerate at certain temperatures, which changes the plasmonic coupling and thus optical properties. It is found that particles confined to the liquid inclusions inside the active composite retain this capability and cause macroscopic, temperature-dependent color change of the solid. The transition is fully reversible for at least 100 times and tunable in temperature via particle size and ligand. This method is suitable to "package" responsive dispersion in solid composites to exploit their dynamic properties in materials.