Effects of Secondary Phases on the High-Performance Colossal Permittivity in Titanium Dioxide Ceramics

The intensive demands of microelectronics and energy-storage applications are driving the increasing investigations on the colossal permittivity (CP) materials. In this study, we designed a new system of Dy and Nb co-doped TiO₂ ceramics [(Dy_0.5Nb_0.5)_xTi_1-xO₂] with the formation of secondary phases, and then the enhancement of overall dielectric properties (ε_r ∼ 5.0-6.5 × 10⁴ and tan δ < 8%) was realized in the broad composition range of 0.5 ≤ x ≤ 5%. More importantly, effects of secondary phases on microstructure, dielectric properties, and stability were explored from the views of defect-dipoles and internal barrier layer capacitance (IBLC) effect. According to the defect-dipoles theory, the CP should mainly originate from Nb⁵⁺, and the Dy³⁺ largely contributes to the decreased dielectric loss. Both CP and low dielectric loss were obtained for co-doping with Dy³⁺ and Nb⁵⁺. Besides, the Dy enrichment induced the formation of secondary phases, which were regarded as the low loss unit dispersed into the ceramic matrix, and largely facilitate the decreased dielectric loss. In particular, the analysis of temperature-dependent complex impedance spectra indicated that a stronger IBLC effect caused by the increased grain boundary resistance can also contribute to the optimized CP and low dielectric loss under appropriate contents of secondary phases.