Dielectric Elastomers for Actuation and Energy Harvesting
Dielectric elastomers are an emerging transducer technology that fit an interesting niche. They are soft and compliant by nature, and can be stretched, either via mechanical or electrically-induced means, to very large strains. They are capable of large strains, high energy densities, and low costs, while possessing the additional advantages of being light weight and conformable. Dielectric elastomers are well suited for both actuator and generator applications, with the actuators being heralded as artificial muscles. There remains, however, several issues that prevent the widespread adoption of dielectric elastomer technology. Each applications has its set of technical issues; for actuator purposes, the primary issues are that the driving voltage required for high performance is in excess of 2000 V for most dielectric elastomer actuators, and that the lifetime when operated at high performance levels is rather short. The requirement to prestrain the elastomers to achieve high performance actuation in most cases can also add significant bulk and mass to the actuator as well as reduce its shelf-life. Thus, materials capable of high-strain performance without the requirement for prestrain are sought after. The ability to operate over a wide range of temperatures and environmental conditions, as well as operate over a wide range of frequencies, is also very important for most actuator applications.
The first part of this dissertation looks to tackle the aforementioned issues. Silicone materials are chosen as the base dielectric elastomer material as they are generally capable of operating over wide ranges of temperature and frequency and are generally unaffected by moisture and have good environmental stability. The work focuses on free-standing linear soft silicone actuators as this configuration is the most relevant for real applications. A particular soft silicone has been isolated a good candidate and was extensively tested in a free-standing linear actuator configuration to determine the effects of pre-stretch and the application of mechanical loads on its actuation performance. It is shown that when the mechanical loads are properly applied, large linear actuation strains of 120% and work density of 0.5 J/cm3 can be obtained. Furthermore, we demonstrate that when coupled with single wall carbon nanotube (SWNT) compliant electrodes, fault-tolerance is introduced via self-clearing leading to significantly improved operational reliability. Driven at moderate electric fields, the actuators display relatively high linear actuation strain (25%) without degradation of the electromechanical performance even after 85,000 cycles.
An issue related to the use of soft silicone actuators is that the work density is not very high when compared with many of the high-performance dielectric elastomer materials available. It is shown that the force output and work density of soft silicone actuators can be significantly enhanced by the addition of high permittivity titanium oxide nanoparticles. The nanocomposites are capable of maintaining the actuation strain performance of the pure silicone at relatively low electric fields while increasing the force output and work density due to the additive effects of mechanical reinforcement and permittivity enhancement.
The high performance of the aforementioned soft silicone actuators requires the application of rather large levels of prestrain. In order to eliminate this requirement a novel all-silicone prestrain-locked interpenetrating polymer network (S-IPN) elastomer was developed. The elastomer is fabricated using a combination of two silicones: a soft room temperature vulcanizing silicone that serves as the host elastomer matrix, and a more rigid high temperature vulcanizing silicone that acts to preserve the prestrain in the host network. The free-standing prestrain-locked silicones show a more than twofold performance improvement over standard free-standing silicone films, with a linear strain of 25% and an area strain of 45% when tested in a diaphragm configuration.
The S-IPN procedure was leveraged to improve electrode adhesion and stability as well as improve the interlayer adhesion in multilayer actuators. It is demonstrated that strongly bonded SWNT electrodes are capable of fault tolerance through self-clearing, even in multilayer actuators. The fault-tolerance and improved interlayer adhesion was used to fabricate prestrained free-standing silicone actuators capable of stable long life actuation (>30,000 cycles at >20% strain and >500 cycles at ~40% strain) while driving a load. Issues related to gradual electrode degradation are also addressed through the use of quasi-buckled electrodes.
For generator purposes, the primary concerns are ensuring environmental stability, increasing energy density, lowering losses, and determining effective methods to couple the dielectric elastomer to natural energy sources. The factors that affect the energy density and efficiency are explored and it is found that energy density can be increased by applying larger mechanical strains and using stiffer materials. Increasing the permittivity of the material and bias voltage can also improve the energy density but reach a peak and decrease the energy density thereafter. The efficiency is shown to depend primarily on the shape of the stress-strain curve of the material, the applied strain, and a lumped parameter containing the stiffness, permittivity and nominal bias electric field. For a particular applied strain, the efficiency shows a peak at a particular value of the lumped parameter. Increasing stiffness shifts the location of the peak to higher values, and thus higher required electric fields, while increasing permittivity shifts it to lower values, and thus to lower electric fields.
Using the results of this analysis, two material systems are explored: VHB acrylic elastomers at various prestrains and with various amounts of a stiffening additive, and a high energy density silicone-TiO2 nanocomposite elastomer with various amounts of additive. It is shown that increasing prestrain in the VHB acrylic system increases the energy density, while the stiffening additive has the effect of making the acrylic stiffer but results in increased losses, result in poorer performance. The silicone TiO2 composite demonstrates an increase in permittivity and stiffness with increasing additive while maintaining very high dielectric breakdown strength values. These increases are partially offset by small increases in mechanical and electrical losses. Calculations based on a simple model show that the generator energy density can be improved by a factor of 3 for a 20wt.% TiO2 loading at a strain of 50% in area. The calculated generator energy density values exceed the maximum values measured experimentally for highly prestrained VHB4910 acrylic elastomers.
The focus on high energy density materials ignores the fact that not all applications require such a material, and that some applications may, in fact, benefit from the use of a softer material that is less intrusive. However, for lower energy density materials, parasitic losses due to electrode resistance and viscoelasticity play a larger role as relative energy gains are lower. With this is mind, a soft silicone single walled carbon nanotube composite is developed. The composite is very soft, has low viscoelastic losses, and is capable of being stretched up to >150% strain with only a marginal increase in resistance. The composite is also capable of being repeatedly stretched at up to 120% strain for over 1000 cycles without any appreciable change in resistance and is capable of being stretched at strain rate up to 100%/s or higher without affecting the performance of the electrode.
Two applications for dielectric elastomer actuators are also explored. A pressure-based ocean wave energy harvesting concept was introduced that does not use any moving parts. A footfall energy harvester concept is also developed capable of being deployed inside the sole of a shoe or as a flexible floor tile.