Actuation strain

The actuation force or movement generated during redox cycling is directly related to the concomitant changes in mechanical properties. Using a simple linear elastic model of the small-strain mechanical properties of PPy, it has been shown that the actuation strain (eo) at a constant applied stress (a) is accurately predicted from Equation 3.3... 

Fig. 1.5 Actuation strain as a function of electric field for irradiated P(VDF-TrFE) copolymer (2), P(VDF-TrFE-CTFE) (4), P(VDF-CTFE) (5), and P(VDF-TrFE-CFE)...

Silicone elastomers have the advantage of lower viscoelasticity than acrylic films and can therefore be operated at higher frequencies with lower losses. Silicones show modest actuation strain when there is little to no prestrain and can be operated over a wide temperature range, making them more suitable to... 

Fig. 1.12 Characteristic stress of a dielectric elastomer film as a lunction of mechanical strain or electric field (constant voltage condition). The charts with origin at O are for a non-prestrained film and at O for the prestralned film. The cross (X) indicates dielectric breakdown and the bar (—) indicates stable actuation strain. Small o and large O represent the apparent breakdown field and actual breakdown strength, respectively...

Kofod used advanced materials models in an attempt to elucidate the effects that prestrain have on the actuation performance of a simple cuboid DE actuator [183]. The results are purely phenomenological however, they indicate that in the special case of a purely isotropic amorphous material, prestrain does not affect the electromechanical coupling directly. The enhancement in actuation strain due to prestrain occurs through the alteration of the geometrical dimensions of the acmator. Kofod also determined that the presence of an optimum load is related to the plateau region in the force-stretch curve and that prestrain is not able to affect the location of this region. 

Conventional acrylic films, such as the VHB 4910 series of elastomers from 3M, possess excellent actuation strain, energy density, and coupling efficiency. However, in order to achieve these high performance values, the film must be prestrained. The addition of bulky support frames required to maintain the prestrain on the film significantly increases the mass of VHB acryUc based devices, reducing their effective energy densities to more pedestrian values. VHB acrylic films also suffer from viscoelastic effects, which limit their maximum response frequency to the 10-100 Hz range. The viscoelastic nature of these films also limits their overall efficiency and results in time dependent strain that can make their performance somewhat erratic. 

In some generator applications it may not be possible or desirable to dry and seal the device. Nonetheless, data from dielectric elastomer actuator lifetime tests suggest that long lifetimes can still be achieved by a tradeoff in performance. For example. Fig. 3.14 shows operation of dielectric elastomer actuators submerged in salt water. In underwater operation, 6 out of 11 actuators survived for >10 million cycles with an electric field limited to 32 MV/m and approximately 2% strain (actuation strain). Operation while submerged in saline solution suggests the practicality of low-cost highly distributed ocean wave harvesters. 

The dielectric elastomer films presented here appear promising as actuator materials because their overall performance can be good. The available literature indicates that the actuated strains of silicone are greater than for any known highspeed electrically actuated material (that is, a bandwidth above 100 Hz). Silicone elastomers also have other desirable material properties such as good actuation pressures and high theoretical efficiencies (80-90%) because of the elastomers low viscoelastic losses and low electrical leakage [12]. 

The relationship between the driving voltage and actuation strain (s) could be expressed by Eq. (8.1) ... 

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