Electrical properties of n-conducting barium titanate ceramics over a wide temperature range under voltage load
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Electrical properties of n-conducting barium titanate ceramics over a wide temperature range under voltage load Wolfgang Preis 1 Received: 2 September 2019 / Accepted: 9 March 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract 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). Keywords Barium titanate ceramics . PTC effect . Grain boundaries . Electrical properties . Double Schottky barriers
1 Introduction The technologically relevant field of functional electroceramic materials comprises PTCR (positive temperature coefficient of resistivity) thermistors based on n-type semiconducting BaTiO3 suitably doped (usually donor doped and acceptor codoped) and sintered under well-defined conditions in terms of atmosphere, dwell time, and cooling rate [1–6]. Basically, the electrical properties of PTCR ceramics are characterized by a steep increase of the resistivity by many orders of magnitude at temperatures above the ferroelectric (tetragonal) - paraelectric (cubic) phase transition (Curie-point Tc ≈ 120 °C depending on doping). At temperatures higher than approximately 200 °C the Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10832-020-00208-5) contains supplementary material, which is available to authorized users. * Wolfgang Preis [email protected] 1
Chair of Physical Chemistry, Montanuniversitaet Leoben, Franz-Josef-Strasse 18, A-8700 Leoben, Austria
NTC (negative temperature coefficient) regime is found, where the resistance is strongly diminish
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