Silicone-based Dielectric Elastomer Research Articles

Largely Enhanced Energy Harvesting Performance of Silicone-Based Dielectric Elastomer Generators through Controlling the Uniaxial Device Structure

Largely Enhanced Energy Harvesting Performance of Silicone-Based Dielectric Elastomer Generators through Controlling the Uniaxial Device Structure

The Soft Ray-Inspired Robots Actuated by Solid-Liquid Interpenetrating Silicone-Based Dielectric Elastomer Actuator.

Dielectric elastomer actuators (DEAs) are widely used in robotics and artificial muscles because of their large energy densities and short response time. In this study, we developed two types of soft ray-inspired robots using solid-liquid interpenetrating silicone-based DEAs, named SIS DEAs. The optimized SIS DEA had an actuation strain of 79.8% at 20.43 kV/mm in a freestanding state, which was used as the muscle of the ray robot. To imitate the swimming behavior of the ray, the effect of the driving frequency on the velocity of the ray robot was explored. The ray robot achieved a maximum swimming rate of 5.7 mm/s when the driving frequency was ∼0.6 Hz. In addition, the steady-state and the transient simulation were carried out to reveal the mechanism of the ray robot's electro-swimming. The results revealed that the actuating deformation of the SIS DEAs caused the electro-deformation of the ray robot, and the periodical electro-deformation generated the high-speed vortex beneath the robot to push the ray robot forward. The high actuation strain in the freestanding state and the shape customizability of the SIS DEAs made it an ideal alternative to muscles for various soft robots.

Influence of electric field, temperature, humidity, elastomer material, and encapsulation on the lifetime of dielectric elastomer actuators (DEAs) under DC actuation

We present the effect of electric field, temperature, humidity, type of elastomer material, and encapsulation on the lifetime of silicone-based dielectric elastomer actuators (DEAs) under DC electric fields. DEAs are promising soft actuators, but little has been reported on their reliability under static electric fields. We report that humidity and electric field are the dominant accelerating factors for device failure, and that a thin encapsulation layer can increase lifetime by more than an order of magnitude with negligible reduction in actuation strain. Our samples are expanding circle, single layer, prestretched films with 5 mm diameter compliant electrodes, operated at electric fields from 80 V µm−1 to 110 V µm−1, with actuation strains from 2% to 6%. We compare four different silicone elastomers, finding highest lifetime with Momentive Electro 242-1, five times higher than Elastosil 2030. Typical mean time to failure (MTTF) for Elastosil 2030 based DEAs at 100 V μm−1, 85 °C and 85% RH are 1.6 h, but this value increases to over 200 h at 20% RH. At 85 °C and 85% RH, the MTTF decreases by a factor of 62 when increasing the electric field from 80 V μm−1 (2.1% actuation strain) to 100 V μm−1 (4.6% actuation strain). Adding a thin, soft silicone encapsulation layer is an effective yet simple strategy to increase DEA lifetime, increasing the MTTF by factors from 2.2 to 75 under humid conditions. Extrapolating from our data, we predict that DC lifetimes above 1000 h can be achieved at fields below 70 V μm−1 (i.e. ≈1.5% strain) for Elastosil 2030/20 DEAs, and below 85 V μm−1 (i.e. ≈2.5% strain) for encapsulated DEAs at 85 °C—85% RH.

Insights into the Complex Prebreakdown Actuation of Silicone Elastomers and its Influence on Breakdown Behavior.

Dielectric elastomer transducers can be applied in many different applications, but their current use is limited by either their electrical breakdown strength or by electromechanical instabilities in the case of soft elastomers. The breakdown process is never a single, simple process but rather—most likely—an ensemble of thermoelectric processes taking place in both elastomer and electrode materials, coupled with mechanical and potentially also chemical degradation. In this work, by using a high-speed camera, we follow silicone-based dielectric elastomers undergoing a ramp-up in voltage close to electrical breakdown strength, with differently constructed elastomers and electrodes. As such, we present experimental insights into the electromechanical processes immediately before the dielectric breakdown of elastomers and identify three different actuation mechanisms taking place prior to electrical breakdown, denoted prebreakdown actuation in the following. The prebreakdown actuation mechanisms observed herein include film thinning and stretching, as well as the formation of bubble- and ring-shaped structures from the elastomer surface, respectively. We furthermore present a theoretical explanation for the observed actuation mechanisms.

Silver thin films generated by Pulsed Laser Deposition on plasma-treated surface of silicones to get dielectric elastomer transducers

Due to their high stability over a wide range of temperature or moisture, silicone-based dielectric elastomer transducers (DETs) are of great scientific interest. In this regard, highly stretchable silicone elastomer films were prepared on the basis of a high molecular weight (Mn = 380,000 gmol−1) polydimethylsiloxane‑α,ω‑diol (PDMS) and stabilized by condensation crosslinking at room temperature, process which was monitored by IR spectroscopy. The obtained films were treated in oxygen radio-frequency (RF) plasma at 50 and 60 W for 3, 6 and 12 min. Depending on plasma treatment conditions, different surface morphologies were obtained, as evidenced by the atomic force microscopy (AFM), while chemical modifications were investigated by X-ray photoelectron spectroscopy (XPS). The elastomer films activated on both sides were further used as substrate for silver electrode deposition through Pulsed Laser Deposition (PLD) to obtain DETs. The deposited films were investigated from the point of view of mechanical and electrical behavior in normal state and after repeated mechanical stresses to assess the suitability of silver layer as stretchable electrode. Moreover, the achieved DETs were successfully tested in terms of actuation displacement.

Enhancing the electro-mechanical properties of polydimethylsiloxane elastomers through blending with poly(dimethylsiloxane-co-methylphenylsiloxane) copolymers.

In this work, improved electro-mechanical properties of silicone-based dielectric elastomers are achieved by means of adding so-called “voltage-stabilisers” prepared from phenyl-functional copolymers prepared using oxyanionic ring-opening polymerisation of octamethylcyclotetrasiloxane (D4) and either tetramethyltetraphenylcyclotetrasiloxane (T4) or octaphenylcyclotetrasiloxane (O4). The concentration of the voltage stabiliser was varied both by changing the molar ratio between methyl and phenyl groups in the copolymer and also by varying the amount of copolymer mixed into a PDMS-based elastomer. The phenyl-functional copolymers were generally found to disperse homogeneously in the PDMS matrix and this resulted in networks with improved mechanical and electrical properties. The developed elastomers were inherently extensible with enhanced tensile and tear strengths, due to phenyl-rich microphases acting as reinforcing domains. Furthermore, addition of phenyl-functional copolymers resulted in elastomers with increased relative permittivity and electrical breakdown strength compared to control elastomers while retaining a low dielectric loss. This demonstrates their efficiency as voltage stabilisers.

Degradation patterns of silicone-based dielectric elastomers in electrical fields

ABSTRACTSilicone elastomers have been heavily investigated as candidates for the flexible insulator material in dielectric elastomer transducers and are as such almost ideal candidates because of their inherent softness and compliance. However, silicone elastomers suffer from low dielectric permittivity. This shortcoming has been attempted optimized through different approaches during recent years. Material optimization with the sole purpose of increasing the dielectric permittivity may lead to the introduction of problematic phenomena such as premature electrical breakdown due to high leakage currents of the thin elastomer film. Within this work, electrical breakdown phenomena of various types of permittivity-enhanced silicone elastomers are investigated. Results showed that different types of polymer backbone chemistries lead to differences in electrical breakdown patterns, which were revealed through SEM imaging. This may pave the way towards a better understanding of electrical breakdown mechanisms of dielectric elastomers and potentially lead to materials with increased electrical breakdown strengths.

Synthesis and Characterization of Dielectric Elastomer Nanocomposites Filled With Multiwalled Carbon Nanotubes

Dielectric elastomers (DEs) have been attracting great attention in the field of electro-mechanical actuation and sensing. In this paper, we develop a new type of silicone-based DEs by incorporating multiwalled carbon nanotubes (MWNTs) to the DEs as fillers. The dispersion of MWNTs during the material processing plays a significant role in deciding the final properties of the nanocomposites. In this work, acetone and ultrasonication along with characterization tools such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are utilized to examine the MWNT dispersion quality within DE nanocomposites. Furthermore, microstructural MWNT dispersion and filler–matrix interfacial bonding as well as the overall dynamic mechanical responses are investigated to reveal the correlation between them. It is concluded that the processing of DE nanocomposites strongly affects the dynamic mechanical properties, which can inversely provide with microstructural information for the nanocomposites.

Voltage-stabilised elastomers with increased relative permittivity and high electrical breakdown strength by means of phase separating binary copolymer blends of silicone elastomers

Increased electro-mechanical properties of silicone-based dielectric elastomers are achieved by means of the addition of so-called voltage-stabilisers prepared from PDMS–PPMS copolymers.

Ageing of Silicone-Based Dielectric Elastomers Prepared with Varying Stoichiometric Imbalance: Changes in Network Structure, Mechanical, and Electrical Properties

Silicone‐based dielectric elastomers are promising dielectric electroactive polymers (DEAPs) applicable to various actuator applications. However, the lack of information concerning their long‐term performance still limits their industrial use. Here, the time‐dependent behavior of silicon‐based DEAPs under electromechanical cycling is investigated. A series of thin silicone films prepared with different stoichiometric imbalances are coated with compliant silver nanowire electrodes and then electromechanically cycled under alternating voltage (Vpp = 2 kV, VDC = 1 kV) over 105 cycles. Afterward, changes in cross‐linking density, elastic modulus, permittivity, and breakdown behavior are examined. With increasing load cycles, electrically induced cross‐linking of the hydrosilane groups occurs, which leads to increased cross‐linking density of the material. Concomitantly, increase in elastic modulus and decrease in permittivity are observed, resulting in a significant deterioration of actuation performance. The measured breakdown strength, which is related to an extrinsic breakdown strength based on electromechanical instability, increases too.image

Fabrication Process of Silicone-based Dielectric Elastomer Actuators

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an elastomeric dielectric membrane sandwiched between two compliant electrodes. The large actuation strains of these transducers when used as actuators (over 300% area strain) and their soft and compliant nature has been exploited for a wide range of applications, including electrically tunable optics, haptic feedback devices, wave-energy harvesting, deformable cell-culture devices, compliant grippers, and propulsion of a bio-inspired fish-like airship. In most cases, DETs are made with a commercial proprietary acrylic elastomer and with hand-applied electrodes of carbon powder or carbon grease. This combination leads to non-reproducible and slow actuators exhibiting viscoelastic creep and a short lifetime. We present here a complete process flow for the reproducible fabrication of DETs based on thin elastomeric silicone films, including casting of thin silicone membranes, membrane release and prestretching, patterning of robust compliant electrodes, assembly and testing. The membranes are cast on flexible polyethylene terephthalate (PET) substrates coated with a water-soluble sacrificial layer for ease of release. The electrodes consist of carbon black particles dispersed into a silicone matrix and patterned using a stamping technique, which leads to precisely-defined compliant electrodes that present a high adhesion to the dielectric membrane on which they are applied.

Fabrication Process of Silicone-based Dielectric Elastomer Actuators.

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an elastomeric dielectric membrane sandwiched between two compliant electrodes. The large actuation strains of these transducers when used as actuators (over 300% area strain) and their soft and compliant nature has been exploited for a wide range of applications, including electrically tunable optics, haptic feedback devices, wave-energy harvesting, deformable cell-culture devices, compliant grippers, and propulsion of a bio-inspired fish-like airship. In most cases, DETs are made with a commercial proprietary acrylic elastomer and with hand-applied electrodes of carbon powder or carbon grease. This combination leads to non-reproducible and slow actuators exhibiting viscoelastic creep and a short lifetime. We present here a complete process flow for the reproducible fabrication of DETs based on thin elastomeric silicone films, including casting of thin silicone membranes, membrane release and prestretching, patterning of robust compliant electrodes, assembly and testing. The membranes are cast on flexible polyethylene terephthalate (PET) substrates coated with a water-soluble sacrificial layer for ease of release. The electrodes consist of carbon black particles dispersed into a silicone matrix and patterned using a stamping technique, which leads to precisely-defined compliant electrodes that present a high adhesion to the dielectric membrane on which they are applied.

The Current State of Silicone-Based Dielectric Elastomer Transducers.

Silicone elastomers are promising materials for dielectric elastomer transducers (DETs) due to their superior properties such as high efficiency, reliability and fast response times. DETs consist of thin elastomer films sandwiched between compliant electrodes, and they constitute an interesting class of transducer due to their inherent lightweight and potentially large strains. For the field to progress towards industrial implementation, a leap in material development is required, specifically targeting longer lifetime and higher energy densities to provide more efficient transduction at lower driving voltages. In this review, the current state of silicone elastomers for DETs is summarised and critically discussed, including commercial elastomers, composites, polymer blends, grafted elastomers and complex network structures. For future developments in the field it is essential that all aspects of the elastomer are taken into account, namely dielectric losses, lifetime and the very often ignored polymer network integrity and stability.

Self-sensing Algorithms for Dielectric Elastomer Multilayer Stack-Transducers

Abstract: Due to their unique properties transducers based on dielectric elastomers (DE) can be used as actuators, generators, sensors or even in combined operation mode. In case of DE-based sensors especially the stretch-dependent DE capacitance is used for the identification of the mechanical state. Within this contribution, the authors present algorithms based on the recursive extended least squares method to estimate the electrical parameters of the DE transducer. The estimation results can be improved by using a variable forgetting factor that ensures on the one hand a fast adaption of the estimates during transient operation, while measurement noises is suppressed sufficiently, on the other hand. Finally, these algorithms are experimentally validated using a silicone-based DE multilayer stack-transducer.