ELSEVIER Journal of Electrostatics 39 (1997) 33-40 Journal of ELECTROSTATICS Double-layer electret transducer R. Kacprzyk a'*, A. Dobrucki b, J.B. Gajewski c Institute of Electrical Engineering and Technology, Technical University of Wroclaw, Wybrzeke S. Wyspiahskiego 27, 50-370 Wroclaw, Poland bInstitute of Telecommunications and Acoustics, Technical University of Wroclaw, Wybrze~ S. Wyspiahskiego 27, 50-370 Wroclaw, Poland cInstitute of Heat Engineering and Fluid Mechanics, Technical University of Wroclaw, Wybrze~e S. Wyspiahskiego 27, 50-370 Wroclaw, Poland Received 19 February 1996; accepted after revision 12 June 1996 Abstract A simplified analysis of the double-layer electret with a frozen space charge leads to the conclusion that such a structure can show a piezoelectric effect when the elasticity coefficients of the layers differ. Results of experiments carried out on the PTFE foil-PP non-woven fabric sandwich confirm the possibility of construction of a simple and relatively sensitive transducer using this effect. Keywords: Electret; Piezoelectric transducer; Electrical double-layer; Corona charging; PTFE foil 1. Introduction Monitoring the natural rhythms of the human body can sometimes require applica- tion of a transducer in the form of a thin, pliable sheet with a relatively large surface area. Piezosensitive foils [1] or double- and/or multi-layer electrets [2-4] can be utilized in the design of transducers. It has been shown [3-7] that a double-layer electret having layers with different mechanical and electrical properties should exhibit piezosensitivity when an electric charge is deposited in their interface. In the case when a perpendicular force is applied to the surface of an electret-sandwich and the resulting deformations are very small, one can write [3, 6] ~le2xlx2 El - - E 2 d3s = - qs (e2xl + elXz) 2 E1E-----~" (1) * Corresponding author. 0304-3886/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S0304-3 886 (96)00030-7