Dielectric elastomers
Encyclopedia
Dielectric elastomers 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 have been 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.
(see image), where a passive elastomer
film is sandwiched between two compliant electrode
s. When a voltage
is applied, the electrostatic pressure 
arising from the Coulomb forces acting between the electrodes. Therefore the electrodes squeeze the elastomer film. The equivalent electromechanical pressure 
is twice the electrostatic pressure 
and is given by the following equation:
where
is the vacuum permittivity, 
is the dielectric constant
of the polymer and
is the thickness of the elastomer film. Usually, strains of DEA are in the order of 10–35%, maximum values are up to 300%.
s 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
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 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.
Smart material
Smart materials or designed materials are materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields....
systems which produce large strains
Strain (materials science)
In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
(up to 300%) and belong to the group of electroactive polymers
Electroactive polymers
Electroactive Polymers, or EAPs, are polymers that exhibit a change in size or shape when stimulated by an electric field. The most common applications of this type of material are in actuators and sensors. A typical characteristic property of an EAP is that they will undergo a large amount of...
(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 have been 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 a compliant capacitorCapacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
(see image), where a passive elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...
film is sandwiched between two compliant electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s. When a voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
is applied, the electrostatic pressure arising from the Coulomb forces acting between the electrodes. Therefore the electrodes squeeze the elastomer film. The equivalent electromechanical pressure is twice the electrostatic pressure and is given by the following equation:where
is the vacuum permittivity, is the dielectric constantDielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
of the polymer and
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 siliconeSilicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
s and acrylic
Acryl group
In organic chemistry, the acryloyl group is the functional group with structure H2C=CH–C–; it is the acyl group derived from acrylic acid. The preferred IUPAC name for the group is prop-2-enoyl, and it is also known as acrylyl or simply acryl...
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
Electrical breakdown
The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....
strength. Basically, the requirements for an elastomer material for DEA are
- The material should have a low stiffnessStiffnessStiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom when a set of loading points and boundary conditions are prescribed on the elastic body.-Calculations:...
(especially when large strains are required); - The dielectric constantDielectric constantThe relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
should be high; - The electrical breakdownElectrical breakdownThe term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....
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
Viscoelasticity
Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just...
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
Several configurations exist for dielectric elastomer actuators:- Framed/In-Plane actuators: A framed or in-plane actuator is an elastomeric film coated/printed with two electrodes. Typically a frame or support structure will be mounted around the film. Examples are expanding circles, planars (single and multiple phase)
- Cylindrical/Roll actuators: Coated elastomer films are rolled around an axis. By activation, a force and an elongation appear in axial direction. The roll actuators can be rolled around a compression spring or rolled without a core. The application of such cylindrical actuators are artificial muscles (prosthetics), mini- and microrobots, and valves.
- Diaphragm actuators: A diaphragm actuator is made as a planar construction which is then biased in the z-axis to produce out of plane motion. Artificial Muscle, Inc. offers a Universal Muscle Actuator (UMA) that is a double diaphragm. http://www.artificialmuscle.com/products/reflex-products.php
- 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 stacking up several planar actuators the force and the deformation can be enlarged. Especially an actuator which shortens by activation can be realized.
- Thickness Mode Actuators: The force and stroke of DEA's can be taken in the z-direction. Thickness mode are a typically a flat film that may have a stack of multiple layers to increase displacement.
Applications
Dielectric elastomers offer a wide variety of potential applications as a novel actuator technology that can replace many electromagnetic actuators, pneumatics, and piezo actuators. And dielectric elastomers can enable actuators to be integrated into applications that were previously infeasible. A list of some applications for dielectric elastomers are:- Haptic Feedback
- Pumps
- Valves
- Robotics
- Prosthetics
- Power Generation
- Optical Positioners such for auto-focus, zoom, image stabilization
- Sensing of force and pressure
- Active Braille Displays
- Speakers
- Deformable surfaces for optics and aerospace
External links
- "Artificial Muscle, Inc. - High-Volume EPAM Actuator Developer and Manufacturer
- http://empa.ch/plugin/template/empa/*/39286/---/l=2
- http://esnam.eu
- http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-web.htm
- Loverich, Kanno, and Kotera, "Concepts for a new class of all-polymer micropumps". Lab Chip, 2006, 6, 1147–1154, DOI: 10.1039/b605525g
- Danfoss PolyPower
- Biomimetics Laboratory