The electrical properties of the bulk as well as the grain boundaries of n-conducting barium titanate, BaTiO_3, containing fairly high additions of strontium and lead have been determined as a function of temperature ranging from 25 to 400°C by impedance spectroscopy. Donor doped and acceptor co-doped samples of almost equal dimensions and grain sizes but different compositions have been investigated. Whereas the bulk resistivity is almost independent of temperature with typical values around 10 Ω cm, the grain boundary resistivity increases by several orders of magnitude when the temperature is raised through the ferroelectric-paraelectric phase transition temperature (PTC effect). The specific heat of the PTC resistors has been measured over the temperature range from 50 to 300°C by means of DSC. The observed shift of the onset temperatures can be interpreted in terms of different mole fraction ratios x_(Sr)/x_(Pb). The PTC effect is described theoretically by application of a double-Schottky-barrier model.
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