A simple analytical relationship for the efficiency of a constant charge dielectric elastomer energy
harvesting cycle is derived for the case of pure shear. The relationship takes into account the nonlinear
nature of elastomer materials and the effects of electrically induced strains during relaxation. It
is explicitly shown that efficiency is dependent on the applied strain, the shape of the stress-strain
curve, and on a lumped parameter (Z') containing the applied electric field, stiffness and permittivity.
We show that any term in the lumped parameter can be offset by the other terms; thus a stiff material
may require a higher permittivity or electric field to attain the same efficiency as a similar soft
material.
Poly(t-butyl acrylate) is a bistable electroactive polymer (BSEP) capable of rigid-to-rigid actuation. The BSEP combines
the large-strain actuation of dielectric elastomers with shape memory property. We have introduced a material approach
to overcome pull-in instability in poly(t-butyl acrylate) that significantly improves the actuation lifetime at strains greater
than 100%. Refreshable Braille display devices with size of a smartphone screen have been fabricated to manifest a
potential application of the BSEP. We will report the testing results of the devices by a Braille user.
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.
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.
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.
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) based on Maxwell-stress induced deformation are considered for many potential
applications where high actuation strain and energy are required. However, the high electric field and voltage required
to drive them limits some of the applications. The high driving field could be lowered by developing composite
materials with high-electromechanical response. In this study, a sub-percolative approach for increasing the electromechanical
response has been investigated. Composites with conductive carbon black (CB) particles introduced into a
soft rubber matrix poly-(styrene-co-ethylene-co-butylene-co-styrene) (SEBS) were prepared by a drop-casting method.
The resulting composites were characterized by dielectric spectroscopy, tensile tests, and for electric breakdown
strength. The results showed a substantial increase of the relative permittivity at low volume percentages, thereby
preserving the mechanical properties of the base soft polymer material. Young's modulus was found to increase with
content of CB, however, due to the low volume percentages used, the composites still retain relatively low stiffness, as
it is required to achieve high actuation strain. A serious drawback of the approach is the large decrease of the composite
electric breakdown strength, due to the local enhancement in the electric field, such that breakdown events will occur at
a lower macroscopic electric field.
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