Dielectric elastomers

Dielectric elastomers (DE) are adaptive material systems that can generate high elongations (up to 300%). They belong to the group of electroactive polymers (EAP). Based on their simple functional principle, dielectric elastomer actuators (DEA) convert electrical energy directly into mechanical work. DE are very light, have a high elastic energy density and have been researched since the late 1990s. Many potential applications are currently in the prototype stage. Every spring, a SPIE conference takes place in San Diego at which the latest research results in the field of DEA are exchanged.

Working principle

Functional principle of a dielectric elastomer actuator. An elastomer film is coated with electrodes on both sides. The electrodes are applied to an electrical circuit. The electrostatic pressure is set by applying an electrical voltage . The elastomer film is compressed in the thickness direction and expands laterally. When the circuit is short-circuited, the elastomer film returns to its original state. ${\displaystyle U}$ ${\displaystyle p_{\mathrm {el} }}$

A DEA is basically a compliant electrostatic capacitor (see picture). A passive elastomer film is sandwiched between two flexible electrodes . When an electrical voltage is applied, the opposing electrodes attract each other due to the electrostatic pressure (Coulomb forces). The incompressible elastomer film is compressed in the thickness direction and expands in the lateral direction. ${\displaystyle U}$ ${\displaystyle p_{\mathrm {el} }}$

The electrostatic pressure force on the spaced plane-parallel plates of a capacitor when the voltage is ${\displaystyle p_{\mathrm {el} }}$ ${\displaystyle z}$ ${\displaystyle U}$

${\displaystyle p_{\mathrm {el} }={\frac {1}{2}}\varepsilon _{0}\varepsilon _{r}{U^{2} \over z^{2}}}$

with - permittivity ${\displaystyle \varepsilon _{0}\varepsilon _{r}}$

The pressure is therefore dependent on the square of the field strength and can be greatly increased by using puncture-proof materials . In addition, there is the electrostatic repulsion of charges of the same name within the electrodes. The equivalent electromechanical pressure is therefore twice as large as the electrostatic pressure and is ${\displaystyle \varepsilon _{r}}$ ${\displaystyle p_{eq}}$

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

Usual unidirectional elongations of DEA are 10–35%, maximum values ​​go up to 300%.

materials

Silicones or acrylics are often used as elastomer materials . The acrylic elastomer VHB 4910 ( 3M ) showed elongations of up to 300%. However, acrylics have the disadvantage. that they are visco-elastic and have a shorter shelf life.

Basically, there are the following requirements for an elastomer which is used as a DEA:

• The material should have a low modulus of elasticity have
• The dielectric constant should be high
• The dielectric strength should be high.

Layer thicknesses in the one to two-digit micrometer range and voltages up to about 1 kV are used.

Many of the elastomers used show a visco-hyperelastic behavior. For the calculation of such actuators, models are required which describe both rubber elasticity and viscoelasticity .

For example, graphite powder, silicone oil-graphite mixtures or gold electrodes are used for the electrode. Ionic gels are also in use. The electrode must be sufficiently electrically conductive and flexible. The flexibility of the electrode is important so that the elastomer is not hindered by the electrode during expansion or is not destroyed.

Configurations and Applications

There are different configurations for dielectric elastomers:

• Planar actuators: A planar actuator is a film that is coated with two electrodes.
  • Stack actuators: By stacking several planar actuators, a higher displacement can be generated with high force. When a voltage is applied, tensile stress is created in the actuator and the stack is shortened.
    • Several of these stacks are bundled. Analog of the muscle
• Rolled actuators: Coated elastomer foils are rolled up around an axis. When activated, there is a stretch in the axial direction.
• Shell-shaped actuators: Elastomer foils are selectively coated so that several electrode cells are created. By individually controlling these cells with electrical voltages, the films can take on three-dimensional shapes.

Individual evidence

1. ^ Arnold Führer, Klaus Heidemann, Wolfgang Nerreter: Time-dependent processes , basic areas of electrical engineering , Volume 2, ISBN 978-3-446-43038-9 , page 62
2. ↑ Tristan Schlögl: Modeling, simulation and optimal control of systems with dielectric elastomer actuators , dissertation 2018 at the University of Erlangen , page 106 (PDF)
3. ↑ Empa - Materials for dielectric elastomer actuators. (No longer available online.) In: www.empa.ch. Archived from the original on February 18, 2016 ; Retrieved December 25, 2009 . 
4. ↑ ^{a b} Sebastian Reitelshöfer, Maximilian Landgraf, Jörg Franke, Sigrid Leyendecker: Qualification of dielectric elastomer actuators for use as artificial muscles in highly dynamic N-DOF robot kinematics , page 4 (PDF)
5. ^ JA Rogers: A Clear Advance in Soft Actuators . In: Science . tape 341 , no. 6149 , 2013, p. 968-969 , doi : 10.1126 / science.1243314 , PMID 23990550 .

literature

• Federico Carpi, Elisabeth Smela: Biomedical Applications of Electroactive Polymer Actuators . John Wiley and Sons, 2009, ISBN 978-0-470-77305-5 , p. 389 ( digitized version )
• Joseph Ayers, Joel L. Davis, Alan Rudolph: Neurotechnology for biomimetic robots . MIT Press, 2002, ISBN 0-262-01193-X , p. 152 ( digitized version )
• Dielectric elastomer stack actuators at the Institute for Electromechanical Constructions (EMK) Technical University Darmstadt

• Dielectric elastomer stack converters at the Fraunhofer Institute LBF in Darmstadt