Dielectric properties of colossal permittivity materials: An update

During the last ten years, a lot of interest has been devoted to the so-called colossal dielectric constant (CDC) material. The first materials exhibiting this behavior were the perovskyte-based ceramics based on the CaCu₃Ti₄O₁₂ composition (CCTO). Relative dielectric permittivity can attain values up to (or even larger than) 10⁵. Nevertheless, their losses are still high, the lower values ranging 10%, in a narrow frequency range, thus limiting their applications. Most work on this type of materials aims to reduce these losses and widen their useful frequency range. On the other hand, the underlying physical mechanisms responsible of the CDC are still under study. While the analysis of broadband impedance spectroscopy measurements leads most of the authors to propose an interfacial polarization mechanisms (at the electrodes or at internal barriers), there is a limited number of complementary electrical characterization techniques, for the moment, they comfort the proposed interfacial polarization mechanisms. In the present work, several characterization techniques like I-V, Sawyer-Tower (ST) measurements, and time-domain polarization are used to characterize these materials. One of our main results is the observation of a non-symmetrical response of these materials related to the direction of the polarization. These results are observed for both macroscopic level on bulk polycrystalline material and within individual grains of the same samples. These results do not fit current accepted models for polarization for CDC materials.