Novel Polymer Electrets
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G. M. SESSLER and J. HILLENBRAND Institute for Communications Technology, Darmstadt University of Technology, Merckstrasse 25, D-64283 Darmstadt (Germany)
ABSTRACT Permanently charged films with a cellular or porous structure represent a new family of polymer electrets. These materials show piezoelectric properties with high transducer constants. The electromechanical response equations of such films are derived for their operation as sensors and as actuators. Experimental results are also presented for cellular polypropylene. In particular, measurements of the direct and inverse transducer constants, the thermal stability of the charge, and Young's modulus are discussed. Assuming reasonable charge distributions and charge densities, the calculated transducer constants are in good agreement with the measured values. Both the theoretical model and the measurements show the reciprocity of the transducer constants.
1. INTRODUCTION There has been considerable recent interest in the permanent charging of films with a cellular or porous structure [1-151. It was shown that such films of polypropylene, polytetrafluoroethylene, and silicon dioxide, when charged with corona or other methods, show relatively good electret behavior. These films can be used, either as single layers or together with another, less compliant layer, as reversible electromechanical or electroacoustic transducers. They exhibit piezoelectric properties and possess transducer constants or sensitivities comparable to those of piezoelectric ceramics, but are mechanically better matched to air or water. This makes such materials attractive for a variety of applications [1]. In the present paper, the electromechanical response equations of such films are derived for
their operation as sensors and as actuators and the reciprocity of the transduction is shown. Following this, various experimental results are reported on cellular films of polypropylene. These results demonstrate the operation of such films in both directions and allow one to determine the transducer constants. Finally, a comparison of the theoretical and experimental results yields information on the amount of charge and on its distribution in the cellular structure of the films. 2.
RESPONSE EQUATIONS
2.1 Model of Cellular Film Typical cross sections of a cellular film are shown in Fig. 1. To allow for a ready analysis of the electromechanical operation of such films, a simplified structure, as shown in Fig. 2 is considered. The charged material, electroded on top and bottom, comprises plane parallel solid layers and air layers of thicknesses din and d 2m, respectively, with n = 1, 2, .. N and m = 1, 2, .. N-1, where N is the total number of solid layers. It is further assumed that the two solid surfaces confining the m-th air layer carry a total planar charge density of arn and -am, respectively, and that no volume charges exist. The quantity arm includes all permanent charges (surface charges and the ends of polarization chains, see [16], p. 13 if). The permanent charges on the two sid
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