Dielectric elastomers are widely investigated for use as actuators, stretch/force sensors and mechanical energy harvesters. As performance of such devices is limited by the elastomer’s dielectric strength, it is important to investigate the factors that mostly affect the electrical breakdown of those materials. In this paper, we present a preliminary study on the breakdown strength of a widely used poly-acrylic elastomer film, VHB 4905 by 3M with an equi-biaxial pre-strain of 300%. The breakdown was measured with two metal electrodes, one hemispherical and the other one planar, and was characterized under different conditions to investigate the effects of the hemispherical electrode’s curvature, the force applied by the two electrodes and the environmental humidity. With a given radius of curvature, the breakdown field increased by about 50% for a nearly ten-fold increase of the applied mechanical force, while, for a given mechanical force, the field decreased by about 20% for a two-fold increase of the radius of curvature. Furthermore, for a given radius of curvature, an increase of the environmental relative humidity from 0% to 80% caused a reduction of the breakdown field of about 20%. This study shows that the breakdown field of the studied dielectric elastomer is highly dependent on the boundary conditions of the breakdown test, as well as the environmental/storage conditions of the material. Therefore, such conditions must be reported carefully to allow for critical evaluations/comparisons of experimental results. As suggested by our data, variations of the compression, electrode’s curvature and environmental humidity are likely to cause a diversity of possible interplaying effects, some of which are preliminary proposed in this paper and are referred to as topics requiring deeper future investigations.
Damages in composite components of wind turbine blades and large-scale structures can lead to increase in maintenance and repair costs, inoperability, and structural failure. The vast majority of condition assessment of
composite structures is conducted by visual inspection and non-destructive evaluation (NDE) techniques. NDE
techniques are temporally limited, and may be further impeded by the anisotropy of the composite materials,
conductivity of the fibers, and the insulating properties of the matrix. In previous work, the authors have proposed a
novel soft elastomeric capacitor (SEC) sensor for monitoring of large surfaces, applicable to composite materials. This
soft capacitor is fabricated using a highly sensitive elastomer sandwiched between electrodes. It transduces strain into
changes in capacitance. Here, we present a fabrication method for fabricating the SEC. Different surface treatment
techniques for the nanoparticles are investigated and the effects on the mechanical and the electrical properties of the
produced film are studied. Results show that using melt mixing fabrication method was successful at dispersing the
nanoparticles without using any surface treatment, including coating the particles with PDMS oil or the use of Si-69
coupling agent. Yet, treating the surface would result in increasing the stiffness of the matrix as well as improving the
interaction between the filler particles and the matrix.
Health monitoring of civil structures is a process that aims at diagnosing and localizing structural damages. It
is typically conducted by visual inspections, therefore relying vastly on the monitoring frequency and individual
judgement of the inspectors. The automation of the monitoring process would be greatly beneficial by increasing
life expectancy of civil structures via timely maintenance, thus improving their sustainability. In this paper, we
present a sensing method for automatically localizing strain over large surfaces. The sensor consists of several
soft capacitors arranged in a matrix form, which can be applied over large areas. Local strains are converted
into changes in capacitance among a soft capacitors matrix, permitting damage localization. The proposed
sensing method has the fundamental advantage of being inexpensive to apply over large-scale surfaces. which
allows local monitoring over large regions, analogous to a biological skin. In addition, its installation is simple,
necessitating only limited surface preparation and deployable utilizing off-the-shelf epoxy. Here, we demonstrate
the performance of the sensor at measuring static and dynamic strain, and discuss preliminary results from
an application on a bridge located in Ames, IA. Results show that the proposed sensor is a promising health
monitoring method for diagnosing and localizing strain on a large-scale surface.
Since organic laser materials offer broad optical gain spectra they are predestined for the realization of widely
tunable laser sources. Here we report on a compact organic laser device that allows for voltage controlled
continuously wavelength tuning in the visible range of the spectrum by external deformation. The device
consists of an elastomeric distributed feedback (DFB) laser and an electro-active elastomer actuator also
known as artificial muscle. Second order DFB lasing is realized by a grating line structured elastomer
substrate covered with a thin layer of dye doped polymer. To enable wavelength tuning the elastomer laser is
placed at the center of the electro-active elastomer actuator. Chosen design of the actuator gives rise to
homogeneous compression at this position. The voltage induced deformation of the artificial muscle is
transferred to the elastomer laser and results in a decrease of grating period. This leads to an emission
wavelength shift of the elastomer laser. The increase of actuation voltage to 3.25 kV decreased the emission
wavelength from 604 nm to 557 nm, a change of 47 nm or 7.8%.
Dielectric elastomer actuators are soft electro-mechanical transducers with possible uses in robotic, orthopaedic and
automotive applications. The active material must be soft and have a high ability to store electrical energy. Hence, three
properties of the elastic medium in a dielectric elastomer actuator affect the actuation properties directly: dielectric
constant, electric breakdown strength, and mechanical stiffness. The dielectric constant of a given elastomer can be
improved by mixing it with other components with a higher dielectric constant, which can be classified as insulating or
conducting. In this paper, an overview of all approaches proposed so far for dielectric constant improvement in these soft
materials will be provided.
Insulating particles such as TiO2 nanoparticles can raise the dielectric constant, but may also lead to stiffening of the
composite, such that the overall actuation is lowered. It is shown here how a chemical coating of the TiO2 nanoparticles
leads to verifiable improvements. Conducting material can also lead to improvements, as has been shown in several
cases. Simple percolation, relying on the random distribution of conducting nanoparticles, commonly leads to drastic
lowering of the breakdown strength. On the other hand, conducting polymer can also be employed, as has been
demonstrated. We show here how an approach based on a specific chemical reaction between the conducting polymer
and the elastomer network molecules solves the problem of premature breakdown which is otherwise typically found.
Electrical breakdown due to electro-mechanical instability is the main intrinsic failure mechanism of dielectric
elastomer actuators (DEA). The same mechanism may also be responsible for failure in soft insulating materials
for other high voltage applications. We report on the validation of a model determining the electrical breakdown
in dependence of material properties. The model includes hyper-elastic material behavior and includes a proper
description of the experimental boundary condition.
An innovative approach for voltage-tunable optical gratings based on dielectric elastomer actuators (DEAs) using electro active polymers is presented. Sinusoidal surface gratings, holographically written into azobenzene containing films, are transferred via nanoimprinting to DEAs of different carrier materials. We demonstrate that the surface relief deformation depends on the mechanical and geometrical properties of the actuators. The tested DEAs were made using commercially available elastomers, including a tri-block copolymer poly-styrene-ethylene-butadiene-styrene (SEBS), a silicone polydimethylsiloxane rubber (PDMS) and commonly used polyacrylic glue. The polyacrylic glue is ready to use, whereas the SEBS and the PDMS precursors have to be processed into thin films via different casting methods. The DEA material was pre-stretched, fixed to a stiff frame and coated with stretchable electrodes in appropriate designs. Since the actuation strain of the DEA depends strongly upon the conditions such as material properties, pre-stretch and geometry, the desired voltage-controllable deformations can be optimized during manufacturing of the DEA and also in the choice of materials in the grating transfer process. A full characterization of the grating deformation includes measurements of the grating pitch and depth modulation, plus the change of the diffraction angle and efficiency. The structural surface distortion was characterized by measuring the shape of the transmitted and diffracted laser beam with a beam profiling system while applying an electro-mechanical stress to the grating. Such surface distortions may lead to decreasing diffraction efficiency and lower beam quality. With properly chosen manufacturing parameters, we found a period shift of up to 9 % in a grating with 1 μm pitch. To describe the optical behavior, a model based on independently measured material parameters is presented.
We present electro-mechanical characterizations of dielectric elastomer actuators (DEA) prepared from polystyrene-
ethylene-butadiene-styrene (SEBS) with comparison to the commonly used VHB 4905 tape. This study
discusses effects of boundary conditions, stiffness and voltage ramp rate on the actuation properties of both
materials. Measurements on samples in pure-shear configuration were made with variation in both load and
applied voltage, to achieve so-called '3D-plots'. A strong dependence of the actuation characteristics on the
voltage ramp rate was observed, leading to a large shift in the 'optimum load' for VHB, which was not found for
SEBS. This is due to the large difference in visco-elastic behavior between materials.
We discuss various approaches to increasing the dielectric constant of elastomer materials, for use in dielectric elastomer
actuators. High permittivity metal-oxide nano-particles can show elevated impact compared to larger size particles, but
suffer from water uptake. Composites with conducting particles lead to extremely high permittivity caused by
percolation, but they often suffer early breakdown. We present experiments on approaches combining metal-oxides and
metal particles, which compensate for the drawbacks, and may lead to useful DEA materials in which all relevant
properties are technologically useful. The key seems to be to avoid percolation and achieve a constant nearest-neighbor
separation.
Dielectric elastomer actuators deform due to voltage-induced Maxwell-stress, which interacts with the mechanical
properties of the material. Such actuators are considered for many potential applications where high actuation strain and
moderate energy density comparable to biological muscle are required. However, the high voltage commonly required to
drive them is a limitation, especially for biomedical applications. The high driving voltage can be lowered by developing
materials with increased permittivity, while leaving the mechanical properties unaffected. Here, an approach to lowering
the driving voltage is presented, which relies on a grafted nano-composite, in which conducting nanoparticles are
integrated directly into a flexible matrix by chemical grafting. The conducting particles are π-conjugated soft
macromolecules, which are grafted chemically to a polymer matrix flexible backbone. Dielectric spectroscopy, tensile
mechanical analysis, and electrical breakdown strength tests were performed to fully characterize the electro-mechanical
properties. Planar actuators were prepared from the resulting composites and actuation properties were tested in two
different modes: constant force and constant strain. With this approach, it was found that the mechanical properties of the
composites were mostly unaffected by the amount of nanoparticles, while the permittivity was seen to increase from 2.0
to 15, before percolation made further concentration increases impossible. Hence, it could be demonstrated that the socalled
"optimum load" was independent from the permittivity (as expected), while the operating voltage could be
lowered, or higher strains could be observed at the same voltage.
Dielectric elastomer actuators (DEA) of poly-styrene-ethylene-butadiene-styrene (SEBS) and commonly used VHB4910
tape were studied for voltage tunable optical transmission gratings. A new geometry is proposed, in which the grating is
placed in an area without electrodes, permitting for light transmission through the device. Experiments were performed
to implement surface relief gratings on DEA films from pattern masters made from holographic recorded gratings. Since
the actuation strain of the DEA depends strongly on the boundary conditions, the desired voltage-controllable
deformation of the grating can be achieved by choosing suitable manufacturing parameters. Conditions were found
permitting a shift of up to 9 % in a 1 μm grating. A model based on independently measured material parameters is
shown to describe the optical behavior.
Dielectric elastomer actuators (DEA) are a class of eletro-active polymers with promising
properties for a number of applications, however, such actuators are prone to failure. One
of the leading failure mechanisms is the electrical breakdown. It is already well-known that
the electro-mechanical actuation properties of DEA are strongly influenced by the mechanical
properties of the elastomer and compliant electrodes. It was recently suggested that also the
electrical breakdown in such soft materials is influenced by the mechanical properties of the
elastomer. Here, we present stress-strain measurements obtained on two tri-block thermoplastic
elastomers (SEBS 500040 and SEBS 500120, poly-styrene-ethylene-butadiene-styrene), with
resulting large differences in mechanical properties, and compare them to measurements on
the commonly used VHB 4910. Materials were prepared by either direct heat-pressing of the
raw material, or by dissolving in toluene, centrifuging and drop-casting. Experiments showed
that materials prepared with identical processing steps showed a difference in stiffness of about
20%, where centrifuged and drop-casted films were seen to be softer than heat-pressed films.
Electric breakdown measurements showed that for identically processed materials, the stiffness
seemed to be a strong indicator of the electrical breakdown strength. It was therefore found that
processing leads to differences in both stiffness and electrical breakdown strength. However,
unexpectedly, the softer drop-cast films had a much higher breakdown strength than the heatpressed
films. We attribute this effect to impurities still present in the heat-pressed films, since
these were not purified by centrifuging.
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