Structure and dielectric relaxations of CaCu3Ti4O12 ceramics by heat treatments in different atmospheres

Abstract

CaCu3Ti4O12 (CCTO) ceramics have drawn much attention for their potential application in energy storage components and overvoltage protection devices. In order to improve the electrical properties and understand the colossal dielectric behavior, several CCTO ceramic samples were prepared and heat treated in different atmospheres. The results showed that the breakdown field and nonlinear coefficient were effectively intensified, and the dielectric loss was reduced after heat treatments in the oxidizing atmosphere including air and oxygen. However, CCTO phases will decompose into TiO2, CuO and CaTiO3 phases when heat treated in the neutral atmosphere such as nitrogen, losing its colossal dielectric behaviors and excellent electrical properties. It was found that the oxygen atmosphere played the most beneficial effect in improving the characteristics of CCTO ceramics in this research. The maximum stored energy density of CCTO can be promoted from 7.7 to 26.5 kJ/m3, and the barrier height between grain boundaries were increased from 0.46 to 0.62 eV which may be closely related with its high breakdown filed. Three dielectric relaxations were observed in CCTO ceramics. After heat treatment in oxygen atmosphere, the activation energies of the two relaxation peaks (peak 1 and peak 2) in high frequencies kept nearly constant, at 0.11 and 0.51 eV respectively, while the activation energy of relaxation peak 3 at low frequency were enhanced from 0.75 to 0.85 eV, which may be related to the oxygen vacancy defects. Moreover, the contribution of dc conductivity to dielectric loss can be effectively restrained. An equivalent circuit model was proposed according to impedance analysis to expound the correlation between electrical properties and dielectric relaxations.