Electrical properties of n-conducting barium titanate ceramics over a wide temperature range under voltage load
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Bulk and grain boundary resistivities as well as grain boundary capacitances of PTCR (positive temperature coefficient of resistivity) thermistors have been investigated as a function of voltage load and temperature ranging from 30 to 820 °C by application of impedance spectroscopy. In addition, current – voltage curves have been measured and the resistivities extracted from these dc measurements are in close agreement with those obtained from impedance spectroscopy. The resistance – temperature characteristics are typical for n-type barium titanate – based PTCR ceramics, viz. a steep increase of the grain boundary resistance above the Curie – temperature (PTCR effect) and decreasing resistance with increasing temperature in the NTC (negative temperature coefficient) regime above approximately 200 °C. The grain boundary capacitance shows a sharp peak at the Curie – temperature (around 120 °C) and obeys the Curie – Weiss law in the paraelectric state. Basically, the grain boundary resistivities decrease significantly under voltage load. However, at elevated temperatures (above 600–700 °C) this non-linear effect vanishes and linear ohmic (or even sub-ohmic) behavior can be observed. The electrical properties can be interpreted in terms of a modified double Schottky barrier model. Reasonable coincidence between simulated and measured current - voltage curves as well as grain boundary conductivities has been found in a wide temperature range (up to 800 °C) under high field conditions (up to an external field strength of 1000 V cm −1).