Microstructure and dielectric properties of (Nb + In) co-doped rutile TiO₂ ceramics

The (Nb + In) co-doped TiO₂ ceramics recently attracted considerable attention due to their colossal dielectric permittivity (CP) (∼100,000) and low dielectric loss (∼0.05). In this research, the 0.5 mol. % In-only, 0.5 mol. % Nb-only, and 0.5–7 mol. % (Nb + In) co-doped TiO₂ ceramics were synthesized by standard conventional solid-state reaction method. Microstructure studies showed that all samples were in pure rutile phase. The Nb and In ions were homogeneously distributed in the grain and grain boundary. Impedance spectroscopy and I-V behavior analysis demonstrated that the ceramics may compose of semiconducting grains and insulating grain boundaries. The high conductivity of grain was associated with the reduction of Ti⁴⁺ ions to Ti³⁺ ions, while the migration of oxygen vacancy may account for the conductivity of grain boundary. The effects of annealing treatment and bias filed on electrical properties were investigated for co-doped TiO₂ ceramics, where the electric behaviors of samples were found to be susceptible to the annealing treatment and bias field. The internal-barrier-layer-capacitance mechanism was used to explain the CP phenomenon, the effect of annealing treatment and nonlinear I-V behavior for co-doped rutile TiO₂ ceramics. Compared with CaCu₃Ti₄O₁₂ ceramics, the high activation energy of co-doped rutile TiO₂ (3.05 eV for grain boundary) was thought to be responsible for the low dielectric loss.