The positive temperature coefficient resistivity of BaTiO3 ceramics as a function of the amount of titanium-rich second phase

The positive temperature coefficient resistivities of barium titanate ceramics are investigated as a function of the titanium–barium ratio. In the concentration range between 0 and 50 mol% titanium excess the specific electrical low temperature resistivities of the ceramics are roughly constant and the PTC effect is present. The intensity of the PTC effect, but not the functional slope of the current–voltage characteristics, depends on the amount of second phase. The second phase has a titanium–barium ratio of 3:1 and is insulating. The latter causes longer current paths and smaller effective current areas because the nonconducting second phase encloses more and more the barium titanate grains with increasing titanium excess. A discussion of the resistivity date in the framework of Heywang's model reveals that the grain surface acceptor state density decreases at higher titanium excesses. This causes a decrease of the grain boundary resistivities which roughly cancels the expected increase of the sample resistivity due to the geometrical reasons.