Abstract
The influence of bulk conductivity on the PTC effect of Nb-doped barium titanate is investigated. Both macroscopic and microscopic resistance measurements are performed as a function of temperature. The macroscopic measurement refers to the whole sample while the microscopic one, obtained by photolithography techniques, concerns only several grains. The results show that the increase of resistivity occurs in two steps: first, an abrupt variation at the Curie temperature T-c which is ascribed to the phase transition in the bulk; second, a continuous increase above T-c which is usually explained from grain boundary potential barriers only. However, our results suggest that a nonnegligible contribution of the bulk is also involved in this increase above T-c.
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