Nonlinear electrical properties of Ta-doped titania capacitor–varistor ceramics

The electrical properties of TiO2-based ceramics with various Ta2O5 contents have been investigated. It was found that an optimal doping composition of 0.25 mol% Ta2O5 leads to a low breakdown voltage of 6 V/mm, a high nonlinear constant of 8.8 and an ultrahigh electrical permittivity of 6.2×104 (measured at 1 kHz), which is consistent with the highest and narrowest grain boundary barriers of the ceramics. The frequency dependence of the capacitance and the resistivity also shows that ceramics doped with 0.25 mol% Ta2O5 exhibit the highest capacitance and resistivity at low frequencies and comparatively low ones in comparison with the ceramics doped by other Ta2O5 concentration at high frequencies. In view of these electrical characteristics, the TiO2–0.25 mol% Ta2O5 ceramics is a viable candidate for capacitor–varistor functional devices. The characteristics of the ceramics can be explained by the effect and the maximum of the substitution of Ta5+ for Ti4+. In order to illustrate the grain boundary barrier formation in TiO2–Ta2O5 varistors, an interface defect model was introduced.