Dielectric elastomers

Introduction

Dielectric elastomers (DEs) are smart material systems which produce large strains (up to 300%) and belong to the group of electroactive polymers (EAP). Based on their simple working principle dielectric elastomer actuators (DEA) transform electric energy directly into mechanical work. DE are lightweight, have a high elastic energy density and are investigated since the late 90’s. Many potential applications exist as prototypes. Every year in spring a SPIE conference takes place in San Diego where the newest research results concerning DEA are exchanged.

Working Principle

A DEA is basically a compliant capacitor (see image), where a passive elastomer film is sandwiched between two compliant electrodes. When a voltage $U$ is applied, the electrostatic pressure $p_{el}$ arising from the Coulomb forces acting between the electrodes. Therefore the electrodes squeeze the elastomer film. The equivalent electromechanical pressure $p_{eq}$ is twice the electrostatic pressure $p_{el}$ and is given by the following equation:

p_{eq}=\varepsilon _{0}\varepsilon _{r}{\frac {U^{2}}{z^{2}}}

where $\varepsilon _{0}$ is the vacuum permittivity, $\varepsilon _{r}$ is the dielectric constant of the polymer and $z$ is the thickness of the elastomer film. Usually, strains of DEA are in the order of 10-35 %, maximum values are up to 300%.

Materials

For the elastomer often silicones and acrylic elastomers are used. In particular, the acrylic elastomer VHB 4910, commercially available from the company 3M has shown the largest activation strain (300%), a high elastic energy density and a high electrical breakdown strength. Basically, the requirements for an elastomer material for DEA are:

The material should have a low stiffness (especially when large strains are required)
The dielectric constant should be high
The electrical breakdown strength should be high

A possibility to enhance the electrical breakdown strength is to prestretch the elastomer film mechanically. Further reasons for prestretching the elastomer are the following:

The thickness of the film decreases. A lower voltage has to be applied to obtain the same electrostatic pressure
The prestrain avoids compressive stresses in the film plane directions which might be responsible for failure

The elastomers show a visco-hyperelastic behavior. Models which describe large strains and viscoelasticity are required for the calculation of such actuators.

Several different types of electrodes are used in the research (e.g. graphite powder, silicone oil / graphite mixtures, gold electrodes, etc.). The electrode should be conductive and compliant. The compliance of the electrode is important in order that the elastomer is not constrained mechanically in its elongation by the electrode.

Configurations and Applications

Several configurations exist for dielectric elastomer actuators:

Planar actuators: A planar actuator is a foil coated with two electrodes. With planar actuators the working principle of DEA is obvious
Cylindrical actuators: Coated elastomer films are rolled around an axis. By activation a force and an elongation appear in axial direction. The application of such cylindrical actuators are artificial muscles (prosthetics) and mini- and microrobots
Shell-like actuators: Planar elastomer films are coated at specific locations in form of different electrode segments. With a well directed activation of the cells with the voltage, the foils assume complex three-dimensional shapes. Such shell-like actuators may be utilized for the propulsion of vehicles through air or water, e.g. for blimps.
Stack actuators: By pilling-up several planar actuators the force and the deformation can be enlarged. Especially an actuator which shortens by activation can be realized.

Current applications of this technology include Braille pads and carbon-elastomer active valves. Researchers have recently proposed the use of these elastomers as actuating devices for MEMS devices because at microscale thicknesses, the rise in energy density allows the electrostatic energy input to actuate the compression of the material to occur at far lower voltages.

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