PTCR characteristics of single grain boundaries in barium titanate ceramics

Positive temperature coefficient of resistivity (PTCR) characteristics of strictly single grain boundaries in barium titanate ceramics have been investigated. The results indicate that in the ceramic samples there existed grain boundaries exhibiting typically three different PTCR characteristics, which can be described as follows: 1) PTCR characteristics similar to those observed for usual ceramic samples, 2) an abrupt increase in resistivity by more than three orders at the Curie point, then, immediately followed by a monotonous decrease of it, and 3) substantially little or no resistivity jump. The second type of the PTCR characteristic cannot be interpreted at all by the Heywang model, which is based on the temperature dependence of the grain boundary barrier height determined by the dielectric constant in barium titanate which steeply decreases above the Curie point according to the Curie-Weiss law. A new possible mechanism of the PTCR effect is discussed