Impedance spectroscopy study of niobium-doped strontium titanate ceramics

Impedance spectroscopy was used to study the temperature dependence of the resistance and capacitance of niobium-doped strontium titanate ceramics and also to identify the main contributions determining potential applications. Nyquist plots (Z" versus Z') show a main contribution in the intermediate-frequency range, with a peak at similar to1 kHz at 500degreesC. The activation energy of this resistance contribution is close to 1.5 eV. Modulus representations (log M" versus logf) show a second contribution in the high-frequency range, with a peak at similar to100 kHz at 500degreesC. This resistance contribution is much smaller, and its activation energy is also lower (close to 0.8 eV). The capacitance of the intermediate-frequency term increases from similar to0.1 muF/m at 700degreesC to similar to1 muF/m at 425degreesC. The capacitance values of the high-frequency contribution are also > 0.1 muF/m, much higher than expected for the bulk. These contributions might be related to differences between the intergrain interfaces and outer grain shells, as suggested by SEM microstructures with core-shell formations. Representations of 1/C against temperature suggest a Curie-Weiss law for the main contribution in the intermediate-frequency range, with a T-C of similar to400degreesC. However, the temperature dependence of the overall dielectric constant measurements (at constant frequency) fail to show a clear maximum, probably due to an additional low-frequency contribution ascribed to the Pt/SrTi1-xNbxO3-delta interface.