Dynamic Ferroelectric–Like Softening Due to the Conduction in Disordered and Inhomogeneous Systems: Giant Permittivity Phenomena

Abstract

After briefly summarising the temperature dependent dynamics in the dielectric spectra connected with proper ferroelectric phase transitions, relaxor ferroelectrics and dipolar glasses in macroscopically homogeneous dielectric materials, the situation is discussed for the case of macroscopically homogeneous materials with non-negligible conduction (particularly disordered (semi)conductors). Special attention is paid to the cases of increasing low-frequency permittivity (Maxwell-Wagner and giant-permittivity phenomena). Connection to Jonscher's universal conductivity behavior is mentioned. The main part is devoted to macroscopically inhomogeneous conducting systems, particularly to conductor—dielectric composites and nanocomposites. On the example of two-component composites with a dielectric loss-less component of frequency-independent real permittivity and conducting component of frequency-independent AC conductivity, various composites are discussed using the modeling based on effective medium approach. Particular attention is paid to the electrical percolation threshold, the corresponding divergence of static permittivity and the related dynamic behavior in the dielectric spectra. The critical dynamics is of relaxation type (Debye relaxation in case of the coated-spheres model and Cole-Davidson relaxation in case of the Bruggeman model) and on approaching the threshold, the relaxation frequency passes rapidly from the IR range down to zero. Similarities and differences compared with the dielectric softening near ferroelectric phase transitions are pointed out. From the application point of view, the most perspective systems (high permittivity and low loss) appear to be the core-shell composites consisting of well-conducting cores and thin as possible high-permittivity dielectric shells.