Local property measurement in PTC thermistors

The local electrical properties of a positive temperature coefficient (PTC) thermistor based on donor doped BaTiO3 with additions of Sr, Pb and Ca, have been characterised and correlated with the results of compositional, microstructural and crystallographic analyses. Wavelength dispersive spectroscopy showed a significant depletion in lead near the pellet surface, caused by volatilisation during sintering. Compensation for the lead loss was principally by Ba substitution, with only a small increase in the levels of Ca and Sr. Local resistance-temperature (R-T) measurements showed a slight reduction in switching temperature, together with an increase in the pre-switching resistance, within the lead depleted zone. Electron backscatter pattern (EBSP) analysis revealed a significant proportion of Σ=3 and 9 grain boundaries, which are reported to be PTC inactive, in the microstructure. The proportion of Σ=3 grain boundaries was significantly higher in the lead-depleted zone than in the bulk of the pellet