Impedance Spectroscopy of Undoped BaTiO3Ceramics

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Abstract
The electrical properties of ceramic BaTiO3were investigated by ac impedance spectroscopy over the ranges 25°‐330°C and 0.03 Hz‐1 MHz. Results are compared with those obtained from fixed‐frequency measurements, at 1 kHz and 100 kHz. Fixed‐frequency Curie‐Weiss plots show deviations from linearity at temperatures well abovetc. The ac measurements show that grain boundary impedances influence Curie‐Weiss plots in two ways: at high temperatures, they increasingly dominate the fixed‐frequency permittivities; at lower temperatures, closer toTc, the high‐frequency permittivity contains a contribution from grain boundary effects. Methods for extraction of bulk and grain boundary capacitances from permittivity and electric modulus complex plane plots are discussed. The importance of selecting the appropriate equivalent circuit to model the impedance response is stressed. A constriction impedance model for the grain boundary in BaTiO3ceramics is proposed: the grain boundary capacitance is neither temperature‐independent, nor shows Curie‐Weiss behavior. The grain boundary is ferroelectric, similar to the grains, but its impedance is modified by either air gaps or high‐impedance electrical inhomogeneity in the region of the necks between grains; the activation energy of theconstriction grain boundaryimpedance differs from that of the bulk, suggesting differences in defect states or impurity levels.