Enhancing the performance of electromechanically active elastomers

Abstract

Dielectric elastomers are a promising material for solid-state actuators due to the high obtainable strain. Their action bases mainly on the Maxwell effect of electrostatics: The attraction between the electrodes in a condensor arrangement causes a compressing stress σ z in field direction as σ z = -εε 0 E 2 .( E q . 1 a ) Provided that the electrodes are compliant, a dielectric material deforms then under the influence of an electric field to an extent that the sum of deformation energy and electric field energy assumes a minimum, and the resulting deformation s x of the electrode area, assuming linear stress-strain relation, isHere, ε is the dielectric permittivity, ε 0 is the permittivity of the vacuum, and Y is Young's modulus. κ M is the corresponding electro-mechanical coupling coefficient. A quantitative evaluation of Eq. 1 proves that the Maxwell effect yields only in elastomers a verifiable or technically exploitable deformation. The effect has the same electric field dependence as electrostriction but is ruled by the ratio between dielectric permittivity and Young's modulus of the material. In elastomers, the electromechanical response is even ruled by the Maxwell effect, and true electrostriction plays almost no role. A route to increase the electro-mechanical performance of a dielectric elastomer is consequently to increase its dielectric constant by blending with a high dielectric constant filler while attempting not to increase its stiffness. In doing that, compounds of polydimethylsiloxane (PDMS) filled up to 10 wt-% with nano-scaled titanium dioxide (ε ≈ 80) were prepared. Pure PDMS yields a contraction s z of 10 % at 25 MV/m. The influence of the filler on the mechanical, dielectric, and electro-mechanical behaviour was investigated. The dielectric constant increased linearly with filler content (Fig. 1a). At the other hand, the elastic modulus of the compunds increased slightly in that composition range, too (Fig. 1b). The overall electromechanical response κ exp exhibited consequently a maximum for a filler level of 6 wt-% where it increased by 140 % with respect to that of the unfilled material (Fig. 1c). Considering the data of Figs. 1a,b and use of Eq. 1 allows moreover to split the experimental κ exp in the contributions of Maxwell effect (κ M ) and true electrostriction (κ E ). The approach enables to tune the mechanical and electromechanical properties of a dielectric elastomer to a desired ratio.