Downloaded from orbit.dtu.dk on: Oct 07, 2021 The Current State of Silicone-Based Dielectric Elastomer Transducers Madsen, Frederikke Bahrt; Daugaard, Anders Egede; Hvilsted, Søren; Skov, Anne Ladegaard Published in: Macromolecular Rapid Communications Link to article, DOI: 10.1002/marc.201500576 Publication date: 2016 Document Version Peer reviewed version Link back to DTU Orbit Citation (APA): Madsen, F. B., Daugaard, A. E., Hvilsted, S., & Skov, A. L. (2016). The Current State of Silicone-Based Dielectric Elastomer Transducers. Macromolecular Rapid Communications, 37(5), 378-413. https://doi.org/10.1002/marc.201500576 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Complete Manuscript DOI: 10.1002/marc.((insert number)) ((or ppap., mabi., macp., mame., mren., mats.)) 1 2 Review 3 4 5 The current state of silicone-based dielectric elastomer transducers 6 7 Frederikke B. Madsen, Anders E. Daugaard, Søren Hvilsted and Anne L. Skov* 8 9 10 11 ––––––––– 12 13 Dr. F. B. Madsen, Prof. A. E. Daugaard, Prof. S. Hvilsted, Prof. A. L. Skov 14 15 Technical University of Denmark, DTU, Department of Chemical and Biochemical 16 Engineering, Søltofts Plads, building 227, 2800 Kgs. Lyngby, Denmark 17 E-mail: [email protected] 18 19 ––––––––– 20 21 22 23 Silicone elastomers are promising materials for dielectric elastomer transducers (DETs) due to 24 25 26 superior properties such as high efficiency, reliability and fast response times. DETs consist of 27 28 thin elastomer films sandwiched between compliant electrodes, and they constitute an 29 30 interesting class of transducer due to their inherent lightweight and potentially large strains. For 31 32 33 the field to progress towards industrial implementation, a leap in material development is 34 35 required, specifically targeting longer lifetime and higher energy densities to provide more 36 37 38 efficient transduction at lower driving voltages. In this review the current state of silicone 39 40 elastomers for DETs is summarised and critically discussed, including commercial elastomers, 41 42 43 composites, polymer blends, grafted elastomers and complex network structures. For future 44 45 developments in the field it is essential that all aspects of the elastomer are taken into account, 46 47 48 namely dielectric losses, lifetime and the very often ignored polymer network integrity and 49 50 stability. 51 52 53 54 55 56 57 58 59 60 61 62 63 - 1 - 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 - 2 - 64 65 1. Introduction 1 2 3 The search for materials that exhibit properties similar – or even superior – to the human muscle 4 5 6 has been the focus of engineers and researchers for several decades. The challenge lies in the 7 8 many excellent properties of human muscles, such as high energy densities and large, 9 10 obtainable strains. Furthermore, human muscles generate linear motion, whereas currently used 11 12 13 electric motors generate rotational motion.[1–3] Another feature of human muscles is that they 14 15 have self-repairing properties, which is also highly desired for soft transducers. 16 17 18 Polymers offer interesting prospects as smart materials for artificial muscles, as they are 19 20 lightweight, noiseless and low in cost. Electroactive polymers (EAPs) consist of soft polymeric 21 22 23 materials with relatively large energy densities, relatively fast response times and the ability to 24 25 undergo large deformations when stimulated by electrical fields.[4,5] EAPs can be divided into 26 27 28 two main groups: ionic and electronic. Ionic EAPs respond to an electrical field through the 29 30 diffusion of ions in a material. In addition, they generally require low driving voltages (as low 31 32 as 1-2 V) but require an electrolyte, which means that encapsulating the aqueous environment 33 34 35 is necessary. Furthermore, there is a need to maintain wetness, and it is difficult to maintain 36 37 DC-induced displacements.[6–9] However, recent developments[10] have proven it possible to 38 39 40 create semi-interpenetrating networks from polyionic liquids, which eliminates the need for an 41 42 aqueous environment and thereby opens up the possibility of a wide range of applications where 43 44 45 large forces are not needed. The advantages of electronic EAPs over their ionic counterparts 46 47 include higher energy density, that they can be operated in air without major constraints, large 48 49 actuation forces, rapid response times and long lifetimes. Their major disadvantage is that they 50 51 [4,11] 52 require high driving voltages to actuate (typically 500 V to 10 kV), and so a great deal of 53 54 research focuses on reducing this issue. Electronic EAPs can be divided further into subclasses, 55 56 [12–14] [15] 57 such as polymer electrets, electrostrictive graft elastomers and dielectric elastomers. In 58 59 the field of electronic EAPs, dielectric elastomers are a very interesting class of material, due 60 61 62 63 - 3 - 64 65 to high obtainable actuation speeds, large strains and high work densities, a high degree of 1 2 electromechanical coupling and good overall performance.[16,17] As a result of this unique 3 4 5 combination of properties, dielectric elastomers occupy a niche in transducer technologies, 6 7 mainly due to their low weight coupled with large strains. The focus of this review is therefore 8 9 10 exclusively on dielectric elastomers. 11 12 Dielectric elastomers are promising materials for advanced electromechanical applications such 13 14 as actuators, generators and sensors, due to their simple and flexible working principle. They 15 16 17 consist of an elastomer film sandwiched between two thin and compliant electrodes, thereby 18 19 forming a capacitor capable of energy transduction. The electrodes can be made from a variety 20 21 [18,19] [17] 22 of conducting materials such as silver and gold and carbon grease. For more details on 23 24 electrode systems an excellent recent review on electrode materials by Rosset and Shea[20] 25 26 27 exists. In this review we refer to the system of elastomer and electrode as a ‘dielectric elastomer 28 29 transducer’ (DET). When an external voltage is applied to the electrodes, electrostatic forces 30 31 are created. Electrostatic pressure acting on the film squeezes the elastomer in thickness, and 32 33 34 as the elastomer is incompressible it consequently expands in planar directions. Electrical 35 36 energy, in this case, is thus converted into mechanical energy. The driving force for the 37 38 39 actuation process is the resulting reduction in charge density when the electrode area is 40 41 enlarged.[21] When the external voltage is switched off, the elastomer film returns to its original 42 43 44 shape. The actuation working principle can be seen in Figure 1. 45 46 Dielectric elastomers can also be used for generators to create electrical energy, if the DET is 47 48 [22,23] 49 stretched by an external force and electrical charges are placed on the film. When the DET 50 51 is allowed to relax, elastic stresses work against electrical field pressure, thereby increasing 52 53 electrical energy. In this process opposite charges on the two electrodes are pushed further apart 54 55 56 as the film contracts and increases in thickness, while similar charges are brought closer 57 58 together as a result of the decreasing area of the elastomer, thus increasing energy density. 59 60 [24] 61 These changes increase the voltage of the charge, and the created energy can be harvested. 62 63 - 4 - 64 65 DETs have the potential to be used in novel applications which have not been possible 1 2 previously or feasible through the use of other technologies. Especially within the actuator 3 4 [25] 5 application area, DETs could provide novel solutions such as flat screen loud speakers, 6 7 lightweight morphing structures and prosthetic-like actuation, especially due to the flexibility 8 9 10 of the material, which offers the ability to be formed into complex shapes and still provide 11 12 actuation.[26–29] Other DET applications range from valves and pumps[30] to (bio-mimicking) 13 14 robotics,[31–33] Braille displays,[34–36] adaptable lenses and gratings,[37] motion sensors,[38] wave 15 16 [24] [39] 17 energy harvesting and active vibration control. The main advantage of DETs in these 18 19 applications over other transducers, however, is their ability for large displacements. In Figure 20 21 22 2 two examples are presented showing the extraordinary behaviour of dielectric elastomers, 23 24 namely a flying fish by the Swiss Federal Laboratories for Materials Science and Technology, 25 26 [40] 27 EMPA, illustrating the lightweight nature, and a wave-harvesting unit capable of large 28 29 strains.[41] 30 31 One of the most important components of a DET is the elastomer, as it governs dielectric 32 33 34 permittivity, dielectric breakdown strength and (in combination with the electrodes) obtainable 35 36 strain.