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It is generally known that the piezoelectric effect in solids is tightly connected with the molecular structure of a solid dielectric. However, the short analysis of a simple model of this phenomenon provides a conclusion that it can appear in each material containing the frozen space charge and showing the Young's modulus space distribution. In such a situation changes of the space charge distribution, under the external stress, will not follow the changes in the molecular structure. Resulting changes in effective surface charge densities on both electrodes are the source of a piezosignal. As the simplest model supporting the above hypothesis, a thin double-layer electret was prepared. The layers were made of polymers with a significantly different mechanical properties (Young's moduli). The estimated piezo-coefficient for the double-layer corona-formed foil electret was of the order of that for the PVDF sensors. The most interesting results are: 1) opening of the new technological possibilities for the preparation of the highsensitive and low-cost piezoelements, and 2) spreading of the family of materials suitable for such sensors. I NTRODUCT I ONIt is generally known that the occurrence of the piezoelectric effect in solid dielectrics is closely related to their molecular structure. But the preliminary analysis of a simple dielectric with the frozen space charge [ l , 2 and 31 leads to a conclusion that this phenomenon can occur in each dielectric which is nonuniform from the point of view of the relative permittivity and Young's modulus. The wider analysis of the piezo-and piroelec-0-7803-1462-~/93$03.OO 0 1993IEEE tric coefficients for the generalized case of a dielectric with the space distribution of the above parameters [4, 51 shows fully that there is the possibility of occurrence of both effects in nonuniform dielectrics. Therefore, it seemed to be interesting to carry out tests of the model structure of a nonuniform dielectric (electret) in order to determine practical usefulness of both the model itself and the obtained structure.The simple (double-layer) model of a nonuniform dielectric, as well as the results of the effective piezoelectric coefficient measurements for the polypropylene-polyurethane (PP-PU) structures is presented below. While selecting materials for the tests, one was guided, amongst other things, by the mechanical properties of both layers. MODEL OF A DOUBLE-LAYER DIELECTRICLet us take a model of a planoparallel dielectric sample with the space charge in the form of the infinitely thin, and surface-uniform layer of an electric charge of the density QS into consideration. Let us also assume that the charge 9s divides a sample into two layers which thicknesses are xi and x2, as shown in Fig. 1. The layers 1 and 2 can have the different relative permittivities ~i and ~2 , respectively, and different mechanical properties, namely, different Young's moduli €1 and E2.The charge of the density QS induces the surface charge of the effective density qse in an electrode which is fastened ont...