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| {{Use dmy dates|date=July 2013}}
| | I would like to introduce myself to you, I am Andrew and my spouse doesn't like it at all. She works as a journey agent but soon she'll be on her personal. Mississippi is exactly where his home is. The favorite pastime for him and his kids is to play lacross and he would never give it up.<br><br>Feel free to visit my web page live psychic reading ([http://www.taehyuna.net/xe/?document_srl=78721 Full Piece of writing]) |
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| [[Image:dielectric elastomers.gif|thumb|right|300px|Working principle of dielectric elastomer actuators. An elastomeric film is coated on both sides with electrodes. The electrodes are connected to a circuit. By applying a voltage <math>U</math> the electrostatic pressure <math>p_{el}</math> acts. Due to the mechanical compression the elastomer film contracts in the thickness direction and expands in the film plane directions. The elastomer film moves back to its original position when it is short-circuited.]]
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| '''Dielectric elastomers''' (DEs) are [[smart material]] systems that produce large [[Strain (materials science)|strains]]. They belong to the group of [[electroactive polymers]] (EAP). DE actuators (DEA) transform electric energy into mechanical work. They are lightweight and have a high elastic energy density. They have been investigated since the late 1990’s. Many prototype applications exist. Every year, conferences are held in the US<ref>{{cite web|url=http://spie.org/app/program/index.cfm?fuseaction=conferencedetail&export_id=x12536&ID=x12233&redir=x12233.xml&conference_id=1040757&event_id=997497 |title=Conference Detail for Electroactive Polymer Actuators and Devices (EAPAD) XV |publisher=Spie.org |date=2013-03-14 |accessdate=2013-12-01}}</ref>{{Registration required|date=December 2013}} and Europe.<ref>[http://www.euroeap.eu/conference European conference]</ref>
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| == Working principles ==
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| A DEA is a compliant [[capacitor]] (see image), where a passive [[elastomer]] film is sandwiched between two compliant [[electrode]]s. When a [[voltage]] <math>U</math> is applied, the [[electrostatic]] pressure <math>p_{el}</math> arising from the Coulomb forces acts between the electrodes. The electrodes squeeze the elastomer film. The equivalent electromechanical pressure <math>p_{eq}</math> is twice the electrostatic pressure <math>p_{el}</math> and is given by:
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| <center><math>p_{eq}=\varepsilon_0\varepsilon_r\frac{U^2}{z^2}</math></center>
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| where <math>\varepsilon_0</math> is the [[vacuum permittivity]], <math>\varepsilon_r</math> is the [[dielectric constant]] of the [[polymer]] and <math>z</math> is the thickness of the elastomer film. Usually, strains of DEA are in the order of 10–35%, maximum values reach 300% (the acrylic elastomer VHB 4910, commercially available from [[3M]], which also sports a high elastic energy density and a high [[electrical breakdown]] strength.)
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| === Ionic ===
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| Replacing the electrodes with soft [[hyrdogels]] allows ionic transport to replace electron transport. Aqueous ionic hydrogels can deliver potentials of multiple kilovolts, despite the onset of electrolysis at below 1.5 V.<ref name=sci1308>{{cite doi|10.1126/science.1243314}}</ref>
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| The difference between the capacitance of the double layer and the dielectric leads to a potential across the dielectric that can be millions of times greater than that across the double layer. Potentials in the kilovolt range can be realized without electrochemically degrading the hydrogel.<ref name=sci1308/>
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| Deformations are well controlled, reversible, and capable of high-frequency operation. The resulting devices can be perfectly transparent. High-frequency actuation is possible. Switching speeds are limited only by mechanical inertia. The hydrogel's stiffness can be thousands of times smaller than the dielectric's, allowing actuation without mechanical constraint across a range of nearly 100% at millisecond speeds. They can be biocompatible.<ref name=sci1308/>
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| Remaining issues include drying of the hydrogels, ionic build-up, hysteresis, and electrical shorting.<ref name=sci1308/>
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| Early experiments in semiconductor device research relied on ionic conductors to investigate field modulation of contact potentials in silicon and to enable the first solid-state amplifiers. Work since 2000 has established the utility of electrolyte gate electrodes. Ionic gels can also serve as elements of high-performance, stretchable graphene transistors.<ref name=sci1308/>
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| == Materials ==
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| Films of carbon powder or grease loaded with [[carbon black]] were early choices as electrodes for the DEAs. Such materials have poor reliability and are not available with established manufacturing techniques. Improved characteristics can be achieved with sheets of [[graphene]], coatings of carbon nanotubes, surface-implanted layers of metallic nanoclusters and corrugated or patterned metal films.<ref name=sci1308/>
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| These options offer limited mechanical properties, sheet resistances, switching times and easy integration. [[Silicone]]s and [[acryl group|acrylic]] elastomers are other alternatives.
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| The requirements for an elastomer material are:
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| * The material should have low [[stiffness]] (especially when large strains are required);
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| * The [[dielectric constant]] should be high;
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| * The [[electrical breakdown]] strength should be high.
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| Mechanically prestretching the elastomer film offers the possibility of enhancing the electrical breakdown strength. Further reasons for prestretching include:
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| * Film thickness decreases, requiring a lower voltage to obtain the same electrostatic pressure;
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| * Avoiding compressive stresses in the film plane directions.
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| The elastomers show a visco-hyperelastic behavior. Models that describe large strains and [[viscoelasticity]] are required for the calculation of such actuators.
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| Materials used in research include graphite powder, silicone oil / graphite mixtures, gold electrodes. The electrode should be conductive and compliant. Compliance is important so that the elastomer is not constrained mechanically when elongated.<ref name=sci1308/>
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| Films of polyacrylamide hydrogels formed with salt water can be laminated onto the dielectric surfaces, replacing electrodes.<ref name=sci1308/>
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| == Configurations ==
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| Configurations include:
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| * 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 is mounted around the film. Examples are expanding circlesp and lanars (single and multiple phase.)
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| * Cylindrical/Roll actuators: Coated elastomer films are rolled around an axis. By activation, a force and an elongation appear in the axial direction. The actuators can be rolled around a compression spring or without a core. Applications include artificial muscles ([[prosthetic]]s), mini- and [[microrobot]]s, and valves.
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| * 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.
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| * Shell-like actuators: Planar elastomer films are coated at specific locations in the form of electrode segments. With a well-directed activation, the foils assume complex three-dimensional shapes. Examples may be utilized for propelling vehicles through air or water, e.g. for blimps.
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| * Stack actuators: Stacking planar actuators can increase deformation. Actuators that shorten under activation are good candidates.
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| * Thickness Mode Actuators: The force and stroke moves in the z-direction (out of plane). Thickness mode actuators are a typically a flat film that may stack layers to increase displacement.
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| == Applications ==
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| Dielectric elastomers offer multiple potential applications with the potential to replace many electromagnetic actuators, pneumatics and piezo actuators. A list of potential applications include:
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| {{columns-list|3|
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| * Haptic Feedback
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| * Pumps
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| * Valves
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| * Robotics
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| * Prosthetics
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| * Power Generation
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| * Active Vibration Control of Structures
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| * Optical Positioners such for auto-focus, zoom, image stabilization
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| * Sensing of force and pressure
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| * Active Braille Displays
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| * Speakers
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| * Deformable surfaces for optics and aerospace
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| * Energy Harvesting
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| * Noise-canceling windows<ref name=sci1308/>
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| * Display-mounted tactile interfaces<ref name=sci1308/>
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| * Adaptive optics<ref name=sci1308/>
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| }}
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| == References ==
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| {{Reflist}}
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| == Further reading ==
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| * {{ cite doi|10.1126/science.287.5454.836}}
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| * {{ cite journal | author1 = Carpi | author2 = De Rossi | author3 = Kornbluh | author4 = Pelrine | author5 = Sommer-Larsen | title = Dielectric elastomers as electromechanical transducers: Fundamentals, materials, devices, models & applications of an emerging electroactive polymer technology | publisher = [[Elsevier]] | year = 2008 | url = http://www.sciencedirect.com/science/book/9780080474885}}
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| == External links ==
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| * [http://empa.ch/plugin/template/empa/*/39286/---/l=2 Smart Materials & Structures (EAP/AFC)] program at [[Empa]]
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| * [http://esnam.eu European Scientific Network for Artificial Muscles]
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| * [http://www.euroeap.eu/conference EuroEAP - International conference on Electromechanically Active Polymer (EAP) transducers & artificial muscles]
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| * [http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-web.htm WorldWide Electroactive Polymer Actuators * Webhub]: [[Yoseph Bar-Cohen]]'s link compendium at [[JPL]]
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| * {{ cite doi|10.1039/b605525g }}
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| * [http://www.PolyPower.com Danfoss PolyPower]
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| * [http://www.abi.auckland.ac.nz/en/about/our-research/projects/biomimetics.html The Biomimetics Laboratory] at The [[University of Auckland]]
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| * [http://www.emk.tu-darmstadt.de/en/mems/research/electroactive-polymers Dielectric Elastomer Stack Actuators (DESA)] at [[Technische Universität Darmstadt]]
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| * [http://www.polywec.org PolyWEC EU Project: New mechanisms and concepts for exploiting electroactive Polymers for Wave Energy Conversion]
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| {{DEFAULTSORT:Dielectric Elastomers}}
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| [[Category:Smart materials|Smart materials]]
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| [[Category:Conductive polymers]]
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| [[nl:Smart material]]
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