Porous polytetrafluoroethylene (PTFE) films were positively or negatively corona charged at room or elevated temperatures. Their charge storage behavior was investigated by means of isothermal surface potential measurements in direct comparison to nominally nonporous samples of the same polymer. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities. Direct piezoelectricity was studied on quadruple, double, and single layer samples by means of quasi-static measurements. For the determination of indirect piezoelectricity, frequency-dependent acoustical-transducer experiments were carried out. Both applications-relevant measurements yielded piezoelectric d33 coefficients of up to approximately 600 pClN or 600 pmlV. These values are more than one order of magnitude higher than in conventional piezoelectric polymers such as polyvinylidenefluoride (PVDF) and almost comparable to the highest known values of inorganic piezoelectrics. Consequently, the novel piezoelectric porous-fluoropolymer spacecharge electrets exhibit an outstanding potential for various device applications that are very briefly discussed.

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Piezoelectric cellular polypropylene films, socalled ferroelectrets, are assembled in a stack with two active transducer layers. The stack is characterized with respect to its linear and quadratic response in a frequency range from 1 kHz to 80 kHz. A relatively smooth frequency response in the sound-pressure level is found for the individual layers as well as for both layers driven in phase. The piezoelectric response of the two-layer stack is twice the response of an individual layer over a rather broad frequency range. Furthermore, the influence of the preparation conditions on the resonance frequency and the effect of the quadratic distortion on the radiated sound are investigated both for the individual transducer films in the stack and for the stack system as a whole.

For the first time, site-selective distortion has been investigated for two different structural units in the ternary compound alpha-GaPO(4) under the influence of a permanent external electric field. Based on 54 measured reflection intensities, the electric-field-induced distortion of PO(4) and GaO(4) tetrahedra in alpha-GaPO(4) crystals is evaluated using a model of pseudoatomic displacements introduced recently [Gorfman, Tsirelson & Pietsch (2005). Acta Cryst. A61, 387-396]. A stronger variation of the P-O bond lengths in the PO(4) tetrahedron was found compared to the bonds in the GaO(4) tetrahedron. The different distortions of the tetrahedra owing to the electric field were analysed in terms of the valence charge density of alpha-GaPO(4) and its topological characteristics. The larger charge of the P pseudoatom compared to the Ga atom was recognized as the main reason for the higher sensitivity of the PO(4) tetrahedron to a permanent external electric field.

In order to understand the atomistic origin of the inverse piezoelectric effect, the changes of integrated intensities of selected Bragg reflection of α-SiO2 and α-GaPO4 were studied, which were induced by an external high electric field of up to E = ±8 kV/mm. Because the model of the field-induced displacement of ionic sublattices against each other fails for the interpretation of experimental data, we propose a model of the inverse piezoelectric effect, which considers the strong covalent bond between Si and O atoms in α-SiO2. Here the main effect of screening the external electric field is a change in the Si-O-Si bonding angles, i.e. the rotations of rigid SiO4 tetrahedra. The same model holds for α-GaPO4, which is an isostructural compound to α-SiO2. For the first time a similar experiment was performed at low temperatures. Between 50 K ⩽ T⩽300 K the piezoelectric coefficient d111 of both substances behaves nearly temperature independent. On the other hand, the field-induced change of the intensities increases for decreasing temperature. This can be interpreted by the rotation of tetrahedra, which is partially originated by the temperature decrease and by the external electric field, respectively, accompanied by a field-induced deformation of tetrahedra.