About the Interpretation of the Ptc Effect in Nb-doped Batio3 Ceramics

In order to check the consistency of the well known Heywang model for explaining the PTC effect, DC resistivity, AC impedance and thermoelectric power measurements have been performed on Nb-doped BaTiO3 ceramics. It is found that the Heywang model can fit the shape of the experimental DC resistivity data, but it appears that the donor density in the grain is much lower than the nominal one, while impedance measurements emphasize the role of the grain boundary resistance. The thermoelectric power exhibits an anomalous behavior in contradiction with the usual semiconductivity mechanism of electrical transport.