Effect of Ta2O5 in (Ca, Si, Ta)-doped TiO2 ceramic varistors
Abstract TiO 2 varistors doped with 0.2 mol% Ca, 0.4 mol% Si and different concentrations of Ta were obtained by ceramic sintering processing at 1350 °C. The effect of Ta on the microstructures, nonlinear electrical behavior and dielectric properties of the (Ca, Si, Ta)-doped TiO 2 ceramics were investigated. The ceramics have nonlinear coefficients of α  = 3.0–5.0 and ultrahigh relative dielectric constants which is up to 10 4 . Experimental evidence shows that small quantities of Ta 2 O 5 improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 0.8 mol% Ta 2 O 5 leads to a low breakdown voltage of 14.7 V/mm, a high nonlinear constant of 4.8 and an ultrahigh electrical permittivity of 5.0 × 10 4 and tg δ  = 0.66 (measured at 1 kHz), which is consistent with the highest and narrowest grain boundary barriers of the ceramics. In view of these electrical characteristics, the TiO 2 –0.8 mol% Ta 2 O 5 ceramic 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 Ta 5+ for Ti 4+ .