A systematic experimental and theoretical investigation of the grain-boundary resistivities of n-doped BaTiO3 ceramics

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Abstract
The electrical resistivities, current‐voltage characteristics, and thermopowers of a series of n‐doped BaTiO3 ceramics having systematically varying resistivity anomalies (posistor or PTC effect) are analyzed in light of the theoretical description of the grain‐boundary resistivity by a Schottky‐type potential barrier model (Heywang’s barrier). The observed current‐voltage dependences agree well with this model over a wide voltage range. However, the agreement between the theoretical and experimental temperature curves of the low‐voltage resistivities is weak if the surface acceptor states, which result as recently suggested from an excess of barium vacancies in the immediate vicinity of the grain surface, are assumed to be at the same energy. Satisfactory agreement is obtained when the acceptor states are distributed over a certain energy interval. An estimate of the surface charge then reveals that the resistivity below the Curie point is governed by a residual potential barrier originating from an excess of surface charge which cannot be compensated even by an optimum alignment of the spontaneous polarization.