High temperature ac conductivity relaxations in dielectric ceramics: grain boundary/intergranular phase effects

The electric polarization and dc conductivity as two main factors cause electric relaxation in dielectric ceramic, which are difficult to be distinguished from each other at high temperatures. In this work, it is found that the two key factors can be separated via conjoint analysis of various complex planes such as complex dielectric permittivity, the impedance, the electric modulus, and the ac conductivity planes. Taking ZnO ceramics as a typical example, the ac conductivity relaxations caused by the long range and short-range migration of charge carriers are discussed as a function of frequency at high temperatures (433–473 K). Under the applied ac electric field, the migration of charge carriers within the ZnO ceramic can be restricted by two high-resistance barriers from grain boundaries and intergranular phases. These barriers result in two dispersion processes in conductivity response, which exhibit two relaxation peaks with activation energies of 0.75 eV and 0.89 eV. It was proposed that, in high temperature region, the ac conductivity relaxations of ZnO ceramic are the result of carrier migration localized between grain boundaries, and carrier migration localized between intergranular phases.