Nieliniowe właściwości dielektryczne wybranych kryształów ferroicznych

Abstract

The development of modern electronics bases on the usage of the materials of refined properties. One of the group of materials displaying such properties are smart materials. In contrast to traditional materials, the smart ones change their properties under the influence of external forces. Owing to that, they simultaneously perform the function of a sensor and actuator. The sine qua non of having such properties is the appearance of nonlinear effects. The work involves the results of measurements and analyses of nonlinear dielectric properties of a wide range of ferroic crystals. The nonlinear properties of ferroelectrics with continuous or discontinuous ferroelectric phase transitions and ferroelectric relaxors were presented and compared. Among relaxors both canonic relaxors, i.e. the ones in which a spontaneous macroscopic structural phase transition does not take place, and those in which spontaneous phase transitions do occur were considered. The results obtained for ferroelectric crystals confirmed the assumptions of the mean field theory of ferroelectric phase transitions. What was observed was the change of the sign of a third order nonlinear dielectric susceptibility during the continuous ferroelectric phase transition. Such a change was not detected in the case of the discontinuous phase transition. Also, the signs of a third order nonlinear electric susceptibility in respective phases complied with theoretical assumptions. However, in the case of a canonic PMN relaxor the existence of a discrepancy of an a3 nonlinear coefficient in the freezing temperature was denied. Such a discrepancy resulted from the expectations of the SRBRF model. The work also showed how the analysis of nonlinear dielectric properties makes it possible to determine the nature of ferroelectric phase transitions. Besides, an original dynamic method and measurement system allowing for measuring complex nonlinear dielectric susceptibilities is presented.