Self-Healing, High-Permittivity Silicone Dielectric Elastomer

Dielectric elastomers (Des) are a type of EAPs with unique electrical properties and mechanical properties: high actuation strains and stresses, fast response times, high efficiency, stability, reliability and durability. The excellent figures of merit possessed by dielectric elastomers make them the most performing materials which can be applied in many domains: biomimetics, aerospace, mechanics, medicals, etc. In this paper, we present a kind of electroactive polymer composites based on silicone Dielectric elastomers with a high dielectric constant. Novel high DEs could be realized by means of a composite approach. By filling an ordinary elastomer (e.g. silicone) with a component of functional ceramic filler having a greater dielectric permittivity, it is possible to obtain a resulting composite showing the fruitful combination of the matrix's advantageous elasticity and the filler's high permittivity. Here we add the ferroelectric relaxor ceramics (mainly BaTiO3) which has high dielectric constant (>3000) to the conventional silicone Dielectric elastomers, to get the dielectric elastomer which can exhibit high elastic energy densities induced by an electric field of about 15 MV/m. Tests of the physical and chemical properties of the dielectric elastomers are conducted, which verify our supposes and offer the experimental data supporting further researches.

Dielectric elastomer actuators (DEAs) are usually operated at high voltage to induce sufficient electric pressure between two compliant electrodes sandwiching the dielectric elastomer. However, a harsh environment (e.g. humid environment combined with high voltage) often induces electrical breakdown (EB) of the DEAs, which results in pinhole formation or even tearing of the device, followed by macroscopic failure. Therefore, it is ideal for DEAs to be self-healing to extend robustness and lifetime, such as observed for biological muscles, which can be healed from injuries by inherent biological processes. Herein, we prepared a soft (Young’s modulus: 187 kPa) polydimethylsiloxane (PDMS) thermoplastic elastomer (TPE) to demonstrate an autonomous self-healing ability. The system exploits hydrogen bonding (H-bonding) in two types of transient cross-linkers: urea group serves as a sacrificial bond under loading and ureidopyrimidone (UPy) serves as a strong, load-carrying crosslinker. The PDMS TPE shows a crossover of the elastic moduli at 119 °C and highly frequency dependent elastic modulus. The DEA is prepared with compliant, corrugated silver electrodes on both sides of a corrugated PDMS TPE film. A maximum actuation strain (6.8 % in longitudinal direction) is achieved at 18.8 V μm-1. A further increase in potential does not increase the actuation strain, but EBs are observed on the electrodes and the actuation is maintained without any detrimental effects to the film despite the previous breakdown. The EBs do not form permanent pinholes, nor do they cause tearing of films. Instead, only the removal of the silver electrode is observed, which is the so-called self-clearing effect commonly observed for metallic electrodes. In combination with the self-clearing effect, the heat generated by the EBs allows the PDMS TPE to soften. The polymer molecules are then capable of flowing into the voids that were created at the initiation of the EBs. As a result, further propagation of the EB is hindered, and instead, an instantaneous and autonomous self-healing of the DEA is observed.

Dielectric elastomer is a kind of typical electro-active polymer material. Under external electric field it can produce large electrostriction deformation and possesses the advantages of high elastic energy density, super short response time, high efficiency, and so on. It is widely used in the artificial muscles, facial expressions, actuators, energy harvesters, sensors, robots and Braille display devices, and also shows huge application potential in the aerospace and intelligent bionic areas. We built the free energy of the dielectric elastomer electrical-mechanical coupling system and investigated its constitutive relation and stability behavior. Then we calculated the elastomer’s critical deformation suffering from the voltage. If electrical breakdown, electromechanical instability and snap-through instability can be avoided, the large electrostriction deformation can induce adiabatic temperature change and isothermal entropy change of the dielectric elastomer. We used the entropy-temperature or electric displacement-electric field plane to describe the temperature change and entropy change of dielectric elastomer undergoing large electrostriction deformation. With the influence of temperature, we developed a temperature and deformation coupling thermodynamical free energy model to calculate the electric field induced variation of temperature and entropy in dielectric elastomers. The results should offer great help in guiding the design and fabrication of excellent actuators featuring soft dielectric elastomers.

Dielectric elastomers are novel soft smart material that could deform sustainably when subjected to external electric field. That makes dielectric elastomers promising materials for actuators. In this paper, a spring-roll actuator that would bend when a high voltage is applied was fabricated based on dielectric elastomer. Using such actuators as active parts, the flexible grippers and inchworm-inspired crawling robots were manufactured, which demonstrated some examples of applications in soft robotics. To guide the parameters design of dielectric elastomer based spring-roll bending actuators, the theoretical model of such actuators was established based on thermodynamic theories. The initial deformation and electrical induced bending angle of actuators were formulated. The failure of actuators was also analyzed considering some typical failure modes like electromechanical instability, electrical breakdown, loss of tension and maximum tolerant stretch. Thus the allowable region of actuators was determined. Then the bending angle-voltage relations and failure voltages of actuators with different parameters, including stretches of the dielectric elastomer film, number of active layers, and dimensions of spring, were investigated. The influences of each parameter on the actuator performances were discussed, providing meaningful guidance to the optical design of the spring-roll bending actuators.

Mechanical energy and electrical energy can be converted to each other by using a dielectric elastomer transducer. Large voltage-induced deformation has been a major challenge in the practical applications. The voltage-induced deformation of dielectric elastomer is restricted by electromechanical instability (EMI) and electric breakdown. We study the loading path effect of dielectric elastomer and introduce various methods to achieve giant deformation in dielectric elastomer and demonstrate the principles of operation in experiments. We use a computational model to analyze the operation of DE generators and actuators to guide the experiment. In actuator mode, we get three designing parameters to vary the actuation response of the device, and realize giant deformation with appropriate parameter group. In the generator mode, energy flows in a device with inhomogeneous deformation is demonstrated.

As a widely used configuration for dielectric elastomer (DE) actuators, DE tube actuators (or cylindrical actuators) are also found to be susceptible to electromechanical instability (EMI), which may lead to a premature electrical breakdown (EB), and inhibit the potential actuation of DE actuators. This work investigates the electromechanical response of a DE tube actuator with and without boundary constraints to demonstrate an alternative to avoid EMI while achieving large actuation. Our simulation results based on the Gent strain energy model show that the EMI of a DE tube actuator can be eliminated, and larger actuation deformation can be achieved by applying boundary constraints. As a result of these constraints, consideration is also given to the possible mechanical buckling failure that may occur. Mechanisms of possible failure modes of constrained and unconstrained DE tube actuators, such as electromechanical instability, electrical breakdown and mechanical buckling, are elucidated. This paper should provide better theoretical guidance on how to improve the actuation performance of DE actuators, thus leading to the optimal design of DE-based devices.

Dielectric elastomers can exhibit load deformation of more than 1692% percent. The deformation is made possible when an external electric field interacts with the dielectric elastomer material, this leads to electrical polarization within the bulk media of the material and charge accumulation on a compliant electrode. This action causes a compressive force that squeezes the film thickness and expands the surface area leading to actuation of the material. The geometric configuration of the complaint electrode on the surface area of the dielectric elastomer can directly influence the electromechanical response of the film. The surface of the dielectric elastomer can be divided into two regions: the active region, which is the surface area of the film that is covered by the compliant electrode, and the passive region, which is the area of the film that remains uncovered. The purpose of this research is to determine the optimal ratio of active vs passive regions and its impact on the electromechanical performance (actuation, dielectric and electromechanical breakdown).

Silicone dielectric elastomer with improved actuated strain at low electric field and high self-healing efficiency by constructing supramolecular network

Giant voltage-induced deformation in dielectric elastomers near the verge of snap-through instability

ABSTRACTOperation of a dielectric elastomer (DE) transducer is limited by the way it fails, and any other limits imposed by the user. For example, material rupture and dielectric breakdown are two examples of transducer failure, while tension loss is a user-imposed limit. Electromechanical instability may or may not result in transducer failure, but is often considered as a boundary to denote the onset of state transition. For a DE undergoing homogeneous deformation, constructing operational limits on work-conjugate state diagrams are rather straight-forward. However, for DE subjected to non-homogeneous deformation, the process is more complex. We present a method to plot the operational boundaries of a DE subject to non-homogeneous deformation, known commonly as the Universal Muscle Actuator or the loudspeaker configuration. Our analysis show that tension loss need not be imposed as a hard operational limit, and that there is further operational space for the transducer, post tension loss. By plotting the operational limits on work-conjugate planes, we determine an optimal inner-to-outer ring ratio and the required pre-stretches that maximize energy conversion.

Development of a predictor for fatigue crack nucleation of dielectric viscoelastomers under electromechanical loads

This paper introduces a fabrication method and the experimental characterization of a soft polymeric energy converter manufactured using a combination of dielectric and conductive polydimethylsiloxane elastomers. The presented system is an inflated circular diaphragm dielectric elastomer generator; i.e., a deformable electrostatic transducer that converts the mechanical work done by a time-varying pressure into electricity. A prototype of the system is realized on the basis of a simple fabrication procedure that makes use of commercially available silicone dielectric elastomer films and custom-prepared deformable conductive electrodes. A test-bench is developed and employed to estimate the energy conversion performance. Remarkable results are obtained, such as an amount of energy converted per cycle of up to 0.3 J, converted power of up to 0.15 W, energy per unit of employed elastomer mass of up to 173 J/kg, and fraction of the input mechanical work converted into electricity of 30%.

Optimization of Load-driven Soft Dielectric Elastomer Generators

Modeling of static reliability assessment in dielectric elastomer transducers subject to electric loads

One prominent method of modifying the properties of dielectric elastomers (DEs) is by adding suitable metal oxide fillers. However, almost all commercially available silicone elastomers are already heavily filled with silica to reinforce the otherwise rather weak silicone network and the resulting metal oxide filled elastomer may contain too much filler. We therefore explore the replacement of silica with titanium dioxide to ensure a relatively low concentration of filler. Liquid silicone rubber (LSR) has relatively low viscosity, which is favorable for loading inorganic fillers. In the present study, four commercial LSRs with varying loadings of silica and one benchmark room-temperature vulcanizable rubber (RTV) were investigated. The resulting elastomers were evaluated with respect to their dielectric permittivity, tear and tensile strengths, electrical breakdown, thermal stability and dynamic viscosity. Filled silicone elastomers with high loadings of nano-sized titanium dioxide (TiO2) particles were also studied. The best overall performing formulation had 35 wt.% TiO2 nanoparticles in the POWERSIL® XLR LSR, where the excellent ensemble of relative dielectric permittivity of 4.9 at 0.1 Hz, breakdown strength of 160 V µm−1, tear strength of 5.3 MPa, elongation at break of 190%, a Young’s modulus of 0.85 MPa and a 10% strain response (simple tension) in a 50 V μm−1 electric field was obtained.