Influence of sintering atmosphere and thermobaric treatment (TBT) on dielectric behaviors of CaCu3Ti4O12 ceramics

Copper titanate (CaCu₃Ti₄O₁₂, or CCTO) ceramics sintered in oxygen and vacuum at 1100°C for 12 h have been treated by thermobaric treatment (TBT) at 9 GPa and 1000°C for 10 min and then quenched in liquid nitrogen (LN₂). Pure cubic body-centered perovskite-related structure was confirmed from XRD results. Besides, after TBT and quenching, additional minor peaks of TiO₂ and Cu were indexed, as well as a little cell volume expansion. A little reduction of the grain size as well as fuzzy grain boundaries can be observed in FE-SEM after TBT. The dielectric constant ε' of CCTO ceramics sintered in oxygen (～4600 @ 100 Hz) were a half of those treated by TBT and quenching (～9600 @ 100 Hz), whereas the value decreased almost by an order of magnitude (from 10⁵ to 10⁴) after TBT and quenching for the samples sintered in vacuum. The complex impedance spectra at high temperature showed three semicircles so a three RC model was used to explain the different relaxation regions consisting of grain, grain boundary and domain boundary. Besides, two relaxation peaks appeared in the frequency dependence of the imaginary part of the electric modulus formalism $M^{?}$ and the complex impedance $Z^{?}$. Moreover, the contributions of grain boundaries can be figured out by the relaxation activation energy $E_{re}$ (338–629 meV) and the conduction one $E_{dc}$ (396–823 meV) fitted by $M^{?}$ and $Z^{?}$ plots separately. As a result, the disorder and heterogeneity typical of grain boundaries should be responsible for the giant dielectric relaxation characteristics of CCTO ceramics, which can be affected by the sintering atmosphere as well as the extreme treating conditions.