Investigation On The Dielectric Properties Of CaCu₃Ti₄O₁₂ Ceramics

A high-dielectric-constant material, CaCu₃Ti₄O₁₂ ceramic, was fabricated by conventional ceramic solid-state-reaction processing techniques. We investigated the frequency dependences of the impedance, electric modulus, and dielectric constant of CaCu₃Ti₄O₁₂ ceramics sintered at 1100℃ for 24 h in air. We found that the outer surface of an unpolished sample was less conductive than the inner region, and the electrical properties of polished sample were independent on its thickness. The thickness independence indicates an internal barrier layer capacitor (IBLC) mechanism rather than Schottky barriers between sample and electrodes induce the giant dielectric constant.Microstructure investigation showed majority of grain boundaries in CaCu₃Ti₄O₁₂ ceramic were sharp and clean without a secondary phase. We did not find any twin boundary in CaCu₃Ti₄O₁₂ grains. So, we suggest that oxygen-deficiency causes the semiconductivity of grain; while for the grain boundary, it is insulating due to oxygen-stoichiometry and forms IBLC.Three conduction processes of parallel RC element in the impedance spectroscopy of polished CaCu₃Ti₄O₁₂ ceramic were detected. An equivalent circuit consisting of three parallel RC elements connected in series was proposed for explaining the impedance spectroscopy. We attributed the three RC elements to insulating grain boundaries, semiconducting grains, and partially oxygen-deficient transitional layers locate between grains and grain boundaries (or barrier layers within grains). The DC bias dependence of grain boundary resistance provides an indirect evidence for the IBLC model.In addition, the effects of sintering temperature, sintering period, and addition of CuO on grain sizes and dielectric properties of CaCu₃Ti₄O₁₂ ceramics were also investigated.