PTC EFFECT IN DOPED BATIO3 CERAMICS - EVIDENCE FOR A BULK CONTRIBUTION

Using photolithographic techniques, four-probe electrical resistivity measurements of Nb-doped BaTiO3 ceramics, at the microscopic scale, ar e presented from room temperature up to 180 degrees C. The PTC effect displayed by several grains exhibits an abrupt jump at the Curie tempe rature, followed by a monotonic quasi linear increase of the resistanc e. This behaviour looks quite different from the usual PTC effect obse rved on macroscopic ceramics. The jump is clearly due to a change of r esistance within the bulk. It appears, from the microscopical results, that the bulk itself must be partially involved in the increase of re sistance above T-c contrary to the generally accepted theory.