An approach for creating complex structures with embedded actuation in planar manufacturing steps is presented. Self-organization and energy minimization are central to this approach, illustrated with a model based on minimization of the hyperelastic free energy strain function of a stretched elastomer and the bending elastic energy of a plastic frame. A tulip-shaped gripper structure illustrates the technological potential of the approach. Advantages are simplicity of manufacture, complexity of final structures, and the ease with which any electroactive material can be exploited as means of actuation.
When exposed to sufficiently high electric fields, polymer-foam electret materials with closed cells exhibit ferroelectric-like behavior and may therefore be called ferroelectrets. In cellular ferroelectrets, the influence of the cell size and shape distributions on the application-relevant properties is not yet understood. Therefore, controlled inflation experiments were carried out on cellular polypropylene films, and the resulting elastical and electromechanical parameters were determined. The elastic modulus in the thickness direction shows a minimum with a corresponding maximum in the electromechanical transducer coefficient. The resonance frequency shifts as a function of the elastic modulus and the relative density of the inflated cellular films. Therefore, the transducer properties of cellular ferroelectrets can be optimized by means of controlled inflation.
The influence of the corona-charging process on the piezoelectric
transducer coefficient d33 of a cellular electret film has been investigated.
An increased corona voltage can be considered as a way to enhance the charge
density and thus also the resulting piezoelectric effect. Higher
corona-charging voltages are possible with increased ambient pressure or in
suitable dielectric gases. The effect of the gas inside the voids has also
been studied. Enhanced transducer coefficients were obtained by corona
charging in N2 or N2O gas atmospheres at 100-450 or 100-140 kPa
pressures, respectively. The highest transducer coefficients of about
790 pCN-1 were obtained when N2 gas was filled into the voids of a cellular
polymer film by means of consecutive vacuum and high-pressure treatments at
295 or 313 K.
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