The dimensional effect of dielectric performance in CaCu₃Ti₄O₁₂ ceramics: Role of grain boundary

The relationship between dielectric performance and sample thickness in CaCu₃Ti₄O₁₂ ceramics was investigated. The geometric dimensional effect, observed in gradually grinded CaCu₃Ti₄O₁₂ samples, is referred to as the responsible parameter for the change in breakdown field and nonlinear coefficient with the variation in the thickness. An inflection point behavior of the breakdown electric field, analogous to ZnO based varistor, can be observed with reduced thickness. When the thickness was reduced from 2.2 mm to 0.45 mm, the low-frequency (<100 Hz) dielectric permittivity and dielectric loss was increased while high frequency dielectric relaxation can be barely affected. Impedance spectroscopy results show that the grain resistance remained as ∼40 Ω cm, while grain boundary resistance decrease sharply from 79 MΩ cm to 14 MΩ cm. Scanning electron micrographs illustrate that there exists great difference between microstructure of surface area and interior area along thickness direction, which can be possibly caused by heterogeneous diffusion of oxygen and liquid phase sintering of ceramics. It was proposed that grain boundary play a key role in the dimensional effect, which can also be verified by the results of as-sintered CaCu₃Ti₄O₁₂ samples with varied thickness.