Actuators high actuation strain

Recently single crystal relaxors have become available offering very high electrostrictive strains (see Table 6.3). There are many forms of actuator exploiting the different types of piezoelectric/electrostrictive response and the following choice is made to illustrate principles. 

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]. 

Figure 15.9 Illustration of the high cathodic strains seen with larger cations in a PPy actuator formed in the presence of an emulsion TMA —tetramethylammonium, TEA —tetraethylammonium, TBA —tetrabutylammonium. (Reprinted with permission from Chemistry Letters, Improved cathodic expansions of electrochemomechanical behavior in polypyrrole films electrodeposited from Aerosol OT emulsion by W. Takashima, S. S. Pandey and K. Kaneto, Chem. Lett., 33, 8, 996-997. Copyright (2004) Chemical Society of Japan)...

The simplest actoation mechanism is the result of an electrostatic effect on a dielectric elastomer. Dielectric elastomers are a subclass of EAPs and are able to be manufactured as deformable actuators with high active strains (up to 380%), high active stresses (up to 1 MPa) and low response times. Usually, an applied field of above 100 M V/m is required. When a voltage is applied across the material, the attraction and repulsion between charges generates stress in the dielectric, which causes compression and elongation of the material. Often, the employed elastomers are... 

It is expected that the high prevalence of large pores in the polymer grown on platinum or gold enables easier passage of electrolyte into the bulk of the polymer, thus improving the rate of diffusion of ions into the sub-micron sized free volume of the polymer and improving the actuation strain rate. An earlier study by Pandey et al. [53] showed that the polymerization electrode also influenced the balance of anion/cation movement in the polymer. In this case PPy doped with naphthalene sulfonic acid (NSA) was prepared on three different electrodes. Anion movement was favoured in those films that had a more open, porous structure. The actuation was performed in aqueous NaCl electrolyte, so that the mobile cation (Na ) was smaller than the mobile anion (NSA). 

While these improvements are impressive, the maximum performances in each area have not been achieved simultaneously. The highest stress actuators, for example, produce an actuation strain of only 2 % [56]. It is particularly useful to have actuators that give simultaneously high stroke, fast response and can operate against high stresses. A web based resource for tracking the published actuator performances of ICPs (and other actuator materials) has been developed by the Molecular Mechatronics Group at the University of British Columbia [74]. 

---