Dielectric elastomers

Dielectric elastomers ( DE) are adaptive material systems, which can generate high strains (up to 300 %). They belong to the group of electroactive polymers (EAP). Based on its simple operating principle convert dielectric elastomer actuators (DEA ) electrical energy directly into mechanical work. DE are very lightweight, have a high elastic energy density and are being researched since the late 1990s. Many potential applications are currently in the prototype stage. In the spring a SPIE conference held every year in San Diego, where the latest research results to be replaced in the field DEA.

Principle of operation

A DEA is basically a compliant electrostatic capacitor ( see picture). A passive elastomer film is sandwiched between two compliant electrodes. When an electrical voltage is applied to the opposite electrodes ( Coulomb forces) pull due to the electrostatic pressure. The incompressible elastomer film is compressed in the thickness direction and expands in the lateral direction. The equivalent electromechanical pressure is twice as large as the electrostatic pressure and can be calculated as

Wherein the permittivity of the vacuum, the dielectric constant and z is the layer thickness of the elastomeric film. Usual unidirectional elongations of DEA are at 10-35 %, maximum values ​​go up to 300%.

Materials

As the elastomer materials or silicones acrylics are commonly used. Particularly the 4910 VHB acrylic elastomer available from 3M is commonly used in research, because it has been shown up to now the most strains ( up to 300 %). Basically, there are the following requirements for an elastomer which is used as DEA:

• The material should have a low modulus of elasticity (especially if high strains are desired )
• The dielectric constant should be high
• The dielectric strength should be high.

One way to increase the dielectric strength is the mechanical stretching of the film. Other reasons for the pre-strain of the elastomer are as follows:

• The layer thickness decreases. Thus, a lower voltage is required to generate the same electrostatic printing
• Compressive stresses in the lateral direction, which can lead to failure of the actuator, are thus avoided.

The elastomers exhibit a visco- hyperelastic behavior. Models are required for the calculation of such actuators which describe both rubber elasticity and viscoelasticity. For the electrode, various materials (for example, graphite powder, silicone oil, graphite mixtures, gold electrodes, etc.) used in research. The electrode should be good electrically conductive and flexible. The compliance of the electrode is important for the elastomer is not hindered in the expansion of the electrode.

Configurations and applications

There are various configurations and applications for dielectric elastomers:

• Planar actuators: a planar actuator is a film which is coated with two electrodes. On the basis of planar actuators, the principle of DEA can be seen.
• Rolled actuators: coated elastomer sheets are rolled up around an axis. When activated, sets a strength and an elongation in the axial direction. The application of such rolled actuators are artificial joints for people ( prosthetics) and robot joints.
• Cup-shaped actuators: planar elastomeric films are coated at certain locations, so that different electrode cells are present. Targeted activation of these cells to the electric power, the slides can assume complex three-dimensional shapes. Cupped actuators can be used for the propulsion of vehicles in the water or air, such as a blimp.
• Stack actuators: by lamination of a plurality of planar actuators increased force and displacement can be generated. In particular, can be generated by the activation of a tensile stress in the actuator, and thus a shortening of the actuator.