Direct measurement of electrical and compositional inhomogeneities in PTC thermistors

Abstract

Positive temperature coefficient (PTC) thermistors, based on donor doped barium titanate, show a large, reproducible increase in grain boundary resistance over a small temperature interval just above the Curie temperature ( T C). T C can be increased by additions of lead, and reduced by adding strontium. In recent times thermistors have been prepared containing additions of both lead and strontium along with other dopants to modify device performance. However, the distribution of these dopants is rarely homogeneous, giving rise to local differences in the functional properties of these devices, which affect the overall behaviour. Electron microprobe analysis of a commercial thermistor disc has revealed a depletion of lead, together with calcium and strontium enrichment in a zone ∼300 μm from the surface of the sintered pellet concurrent with a small decrease in grain size. Local resistance-temperature ( R– T) measurements have also demonstrated that T C is decreased and the pre-switching resistance increased in the near surface region.