Enhanced breakdown field in CaCu₃Ti₄O₁₂ ceramics: Effect of in-situ secondary phase

CaCu₃Ti₄O₁₂ (CCTO) ceramics shows both varistor performance and giant dielectric constant, which makes it possible candidate for varistor and capacitor application. However, the electric breakdown field of CCTO ceramics is too low and the dielectric loss tarô is too high to meet present requirements. In this paper, the electric breakdown field of CCTO ceramics was increased from conventional 1.0-2.0 kV-cm^-1 to 21 kV-cm^-1. The extreme increase of breakdown field was accompanied with low dielectric loss at low frequency. This result was caused by a secondary phase of CuAhO4, which was introduced by dispersion-precipitation of Al₂O₃ around CCTO powders and optimized sintering process. As comparisons, CuAl₂O₄ powders was also directly added to prepare CCTO-CuAl₂O₄ composite ceramics. For both in-situ and direct methods, the highest breakdown fields can be found in the samples which were sintered at 1100 °C with 50 mol% CuAl₂O₄ addition, while the direct sample exhibited lower breakdown field than in-situ sample. It is indicated that this enchantment is due to both the secondary phase and consumption of Cu-rich phase of in-situ reaction to avoid abnormal growth during sintering. In addition, it was found that the activation energy of conduction increased from 0.60 eV to 0.81 eV, which is caused by secondary phase and consequently lead to much higher breakdown field.