ANALYSIS OF THE AC ELECTRICAL RESPONSE FOR (BA,PB)TIO3 POSITIVE TEMPERATURE-COEFFICIENT CERAMICS

Abstract

AC electrical properties of positive temperature coefficient of resistance (PTCR) ceramics of composition Ba0.347Pb0.65La0.003Ca0.005Ti0.995O3 with Curie point Tc almost-equal-to 380-degrees-C were studied. A combined imaginary impedance/modulus plot was adopted to identify the appropriate equivalent circuit. Analysis of such circuit modified with a frequency dependent capacitor C(n) gives satisfactory interpretation of the commonly observed inclined semicircle in the complex impedance plane. The exponent n, which is extracted from the depression angle and suggested to be associated with the ''loss'' degree of the material, shows a temperature dependence similar to that of a useful parameter extracted from the ac conductivity-frequency measurements. Both are ascribed to he conduction characteristics of the grain boundary, whose resistivity anomaly is the main contribution to PTCR jump. The validity of the present methodology is assessed not only by fitting well the observed non-ideality of real ac response, but also by confirming the Curie-Weiss behavior of the calculated grain boundary capacitance, which is the essence of well-accepted barrier theory (Heywang-Jonker model) proposed to explain the PTCR effect.