Abstract
The effects of cooling rate and soaking time on the positive temperature coefficient of resistivity (PTCR) of donor-doped barium titanate ceramics have been well studied. Daniels and Wernicke presented a model which predicted that an increasing grain boundary thickness led to a continuous transition from a PTC thermistor to an intergranular capacitor. Common in grain boundary potential layer models of PTC behavior, is the assumption that the polarization vector is parallel to the domain boundary (180° domains). Haanstra and Ihrig observed only 90° domains (perpendicular polarization vectors in neighboring domains) in these materials. They did not observe any intergranular second phase in their microstructures, which had grain boundary thicknesses of 2 - 10 nm. They discarded the potential barrier model proposed by Hoffmann in which a 500 am thick second phase layer (possibly Ti-rich) was assumed to be present at the grain boundary. In the present investigation, the domain structures and grain boundary thicknesses in BaTiO3 PTC thermistor, and the intergranular capacitor specimens were studied.
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