Embedding nanostructure and colossal permittivity of TiO2-covered CCTO perovskite materials by a hydrothermal route

Abstract

In this article, hydrothermal methods were employed to synthesize CaCu3Ti4O12-based powders with a surface coverage of nano-sized TiO2 particles in molar ratio Ti4+/CCTO of 0, 0.1, 0.2, and 0.3. The phase structure, morphology, and compositional distribution of TiO2-covered CCTO powders were systematically characterized. Using the synthesized powders as starting materials, dense ceramics with very giant dielectric constants at room temperature (εr>7.0 ×105 at 20 Hz and εr>4.0 ×105 at 1 kHz) were successfully fabricated by sintering at 1050 °C for 8 h. Meanwhile, the embedding nanocrystalline structures were observed in grain interiors of TiO2-covered CCTO ceramics. The investigation of dielectric properties and impedance spectra confirm that, except the double-Schottky barriers on the grain boundaries, there is a low resistivity region in TiO2/CCTO composite ceramics, originated from the inhomogeneous nano-scale semiconducting regions, which formed by the dispersion of Ti4+ ions to replace Cu2+ positions in the grain interiors. According to the results, a model of nanocrystalline interface barrier layer capacitors is proposed to reasonably explain the colossal permittivity behaviors of nano-size TiO2-covered CCTO perovskite materials.