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

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The influence of bulk conductivity on the PTC effect of calcinated at 800ЊC. The sol-gel method is based on controlled hydrolysis of a titanium alkoxide under basic conditions proNb-doped barium titanate is investigated. Both macroscopic and microscopic resistance measurements are performed as duced by hydrated barium hydroxide in the solid state. 8,9 Niobium oxide, citrate, or alkoxide is added in adequate propora function of temperature. The macroscopic measurement refers to the whole sample while the microscopic one, tions (0.1 to 0.4 at.%) as donor dopant and manganese oxide (less than 0.1 at.%) as acceptor dopant for the solid-state reacobtained by photolithography techniques, concerns only several grains. The results show that the increase of resistion method only. The powders prepared by the solid-state and citrate methods were pressed at 400 MPa, sintered at 1400ЊC tivity occurs in two steps: first, an abrupt variation at the for 30 min, then annealed at 1300Њ and at 1220ЊC, both for 30 Curie temperature T c which is ascribed to the phase transimin. The powders prepared by the sol-gel method were sintered tion in the bulk; second, a continuous increase above T c at 1200ЊC for 2 h followed by furnace cooling. which is usually explained from grain boundary potentialMacroscopic electrical measurement is performed as a funcbarriers only. However, our results suggest that a nonneglition of temperature on the ceramic samples coated with a gible contribution of the bulk is also involved in this increase "French alloy" (42% Bi-18.1% In-10.6% Sn-8.3% Cd-21% above T c .Pb) which is well known 10 to ensure ohmic contact. On the other hand, using photolithographic techniques, microscopic