Influence of Reoxidation on PTC Effect of Porous BaTiO3

Relation between PTC (Positive Temperature Coefficient) effect and oxygen adsorption in porous BaTiO3 ceramics was investigated. The semiconducting samples were prepared by reduction of sintered body in 50% H2-Ar atmospheres followed by gradual reoxidization in air. The magnitude of the PTC effect increased with increasing reoxidizing temperature. Barrier height, which was estimated by taking Arrhenius plots of resistivity above Curie temperature, showed a similar tendency to PTC effect with the reoxidizing temperature. This barrier height is thought to be generated by chemisorbed oxygen on BaTiO3 grain surfaces and to determine the PTC effect. The behavior of the adsorbed oxygen was confirmed by means of ESR at room temperature. ESR spectra indicate that the chemisorbed oxygen exist as O2-. ESR intensity assigned to O2- became stronger with increasing reoxidizing temperature. It seems that low-temperature type of the chemisorbed oxygen O2- contribute to formation of energy barrier.