Dielectric elastomers

Summary

Dielectric elastomers (DEs) are smart material systems that produce large strains. They belong to the group of electroactive polymers (EAP). DE actuators (DEA) transform electric energy into mechanical work. They are lightweight and have a high elastic energy density. They have been investigated since the late 1990s. Many prototype applications exist. Every year, conferences are held in the US and Europe. Working principles A DEA is a compliant capacitor (see image), where a passive elastomer film is sandwiched between two compliant electrodes. When a voltage U is applied, the electrostatic pressure p_{el} arising from the Coulomb forces acts between the electrodes. The electrodes squeeze the elastomer film. The equivalent electromechanical pressure p_{eq} is twice the electrostatic pressure p_{el} and is given by: where \varepsilon_0 is the vacuum permittivity, \varepsilon_r is the dielectr

Official source

https://en.wikipedia.org/wiki/Dielectric_elastomers

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Ultra-high voltage, low power and energy recovering electronics for dielectric elastomer actuators

Raphaël Johannes Charles Mottet

Heart failure is a cardiovascular disease affecting between 1 and 2% of the population of developed countries alone - more than 10 million people - with this number bound to increase due to the ageing of the population. This condition starts to develop itself when the heart is not capable to properly pump the blood in the circulatory system anymore, and it can be due to other cardiovascular diseases, hypertension, heart attacks, etc. When cases become very severe, surgical procedures are then necessary to install cardiac assist devices to help reduce the stress experienced by the heart and allow it to potentially heal itself. However, the devices currently used are bulky and very invasive with risks of infections and rejections.As such, the Center for Artificial Muscles (CAM) was tasked to investigate and develop a new kind of cardiac assist devices which would be much less invasive to install. To do so, the idea envisioned consisted of working with Dielectric Elastomer Actuators (DEAs) which are often nicknamed artificial muscles due to their close resemblance with the natural ones as they, amongst other things, light, thin and can deform themselves significantly. One of the main challenges of working with this technology comes from the fact that DEAs have very uncommon needs concerning the driving electronics. Indeed, to operate to the maximum of their abilities, DEAs require voltages of several thousands of volts - between 5 and 20 kV depending on the actuator - to be applied to them. Currently, these actuators are powered using extremely large and impractical laboratory power supplies which is unacceptable for portable and medical applications. It was therefore critical to investigate and uncover a solution for the electronics so that the DEAs' needs could be met with a system as compact and efficient as possible.Thus, this thesis presents the investigating process followed to obtain in the end an electronics system capable of supplying at least 8 kV to a DEA and recover unused electrical energy stored in the actuator. Process which consisted at first of an investigation of power electronics topologies capable of amplifying a given input voltage to a higher level in order to determine the most suitable one. It was concluded that the DC/DC flyback converter was the best candidate for the job.This was followed by an in depth analysis of the working principle of this converter topology to determine the limiting factors regarding the supply of high voltages. This analysis was confirmed through the development of an analytical model depicting the behavior of the converter when it charges the actuator to the required voltage. Thanks to this a list of solutions to overcome these factors could be given.Then, due to the voltage levels worked with and the necessity to recover the unused electrical energy stored in the actuator, it was revealed that commercially available solutions were scarce to nonexistent to properly control the electronics. As such, control strategies and custom devices were ultimately conceived to resolve this problem.In the end, the final ultra-high voltage bidirectional flyback converter prototype was used to operate a DEA pump placed in a test bench recreating the pressure conditions and blood flow which are expected to be found in a living body. These tests showed that the electronics performed very well and managed to operate without major issues the new cardiac assist device created by the CAM.

EPFL2022

High Permittivity Silicones for Dielectric Elastomer Actuators

Simon Johannes Dünki

Many technical advances of modern society sprout from the creation of specially customized polymer materials exhibiting unique combinations of properties uncharted so far. Amid this plurality of highly versatile materials, the class of electroactive polymers (EAPs) bears the special ability to undergo large shape changes when exposed to electrical stimulation. Such materials offer new perspectives in automation, robotics and medicine. Especially the dielectric elastomer actuators (DEA) technology is considered as highly promising among EAPs due to the simple working principle and actuation characteristics similar to mammalian muscles. Despite great research efforts and remarkable progress in regard to materials and devices, the commercial breakthrough of the DEA technology is still outstanding. A major obstacle for their application is the high driving voltage needed to obtain a reasonable actuation. The development of tailored elastomers that overcome this deficiency is therefore a main research focus in academia and industry. The exceptional elastic and dielectric characteristics of silicones nominate them as an ideal vantage point to start this endeavor. Their low permittivity inherently decreases the actuation performance and should therefore be optimized with all other properties remaining unaffected. In the present work, dipoles were covalently attached to a siloxane backbone in order to increase the permittivity thereof and achieve a material with an enhanced actuation performance at low driving voltages for DEAs. The introduction of the polar groups to the polymer side chains was carried out via thiol-ene chemistry, which enabled a quantitative functionalization of the polymer backbone. Three different polar groups, nitrile, thioether and sulfones, were investigated and the corresponding functionalized polysiloxanes were prepared and characterized. Thorough dielectric, mechanical and electromechanical investigations were conducted on polysiloxane based elastomers with different content of nitrile groups. In addition, first results were obtained with thioethers and sulfones which are also promising candidates to enhance the permittivity of polysiloxanes. Most of the synthesized polysiloxanes show glass transition temperatures well below room temperature, an important prerequisite to ensure elastic properties. Polysiloxanes that carry a butylthioether moiety at every repeat unit doubled the permittivity of the elastomer. With the same amount of the nitrile or sulfone dipoles, a six respectively eight times higher permittivity as compared to regular siloxane is achieved. However, with enhanced dipole content also an increase of the electrical conductivity of the polymer was observed. A simple one-step process to prepare cross-linked polar silicone films was developed using UV light triggered thiol-ene addition. A strong correlation between the dipole content in the material and the resulting actuation strain was found. Finally, a polar silicone elastomer was prepared that showed up to 20.5% lateral strain under a field as low as 10.8 V/um. Compared to commercial silicone this corresponds to an effective voltage reduction by about 70%. Processing such materials in films the thickness of which is below 10 um would result in an actuator that can be driven below 100 V.

EPFL2017

Dielectric elastomer transducers consist of a dielectric elastomer between two compliant electrodes. When the elastic capacitor is electrically charged, the dielectric elastomer elongates until a balance of elastic and electrostatic force is reached. Electrical energy is converted into mechanical work. Contrary, in the generator mode, the capacitor is stretched by an outside mechanical force. The mechanical work is converted into electrical energy due to the change of capacitance. The need of a high voltage power supply is the main obstacle for the elastomer transducer technology in various applications. Siloxane elastomers are the most frequently tested elastomers with excellent mechanical stability, processability and electrically insulating properties. It is well-known that an increase in permittivity of the silicone elastomer would allow a reduction of the driving voltage of the elastic transducer. Hence, manifold research activities have been initiated to enhance the permittivity of siloxane elastomers. In the present work, siloxane elastomers were developed that combined increased permittivity with high mechanical stability and cost-efficient processability of the siloxane materials. The synthesis of polar thiols as the first step of siloxane functionalization was studied. Serious challenges in the synthesis of polar groups that can be integrated by thiol-ene addition as side group of polysiloxanes were found. Therefore, commercially available alkylthiols were selected for the functionalization of siloxane due to their chemical stability. The alkylthiols were grafted onto the polysiloxane by thiol-ene addition and subsequently cross-linked in the well-established organotin-catalyzed condensation reaction. The impact of different types of cross-linker on the electro-mechanical properties of the polar siloxane elastomers was analyzed. A significant improvement in the performance of actuator test devices was demonstrated. In addition, the electromechanical stability of the polar siloxane elastomer was illustrated by 50.000 actuation cycles. However, solvent had to be used in the production process of the siloxane elastomers and the cross-linking reaction time was about two days. Apart from their application in thin film actuators, these polar siloxane elastomers were studied with respect to their performance in dielectric elastomer generators together with siloxane composites. Nanospring carbon nanotubes were blended as high permittivity fillers in siloxanes subsequent to cross-linking. Indeed, improved performances of the high permittivity silicone elastomers were found and a deeper understanding for the design of dielectric elastomers for generator applications was gained. The most significant result of this work was the development of polar siloxane elastomers that could be processed into thin film elastomers without any solvent within a few minutes. Two different synthetic approaches were developed based on low viscosity mixtures of cross-linkable oligo(ethylthioethyl)(methyl)siloxanes and multifunctional thiol cross-linkers. The cross-linking reaction was selectively initiated by UV-induced radical formation. These siloxane elastomers possessed an increased permittivity and showed a high electromechanical stability.

EPFL2019