Aging study of nickel‐copper‐manganite negative temperature coefficient thermistors by thermopower measurements

Ceramic nickel‐copper‐manganite based negative temperature coefficient thermistors are unstable under thermal constraint, which increases the resistivity of these devices. This unstability can be reduced by the addition of a small amount of barium. These ceramics have a spinel structure, and the conduction takes place by a hopping process involving [Mn3+]/[Mn4+] octahedral ions. This study by the use of thermopower measurements points out that the resistivity change under thermal constraint is principally due to the disproportionation of [Mn3+]/[Mn4+] ions in the octahedral sites. Furthermore, it seems that some nickel ions migrate from octahedral to tetrahedral sites.