Evolution of low sigma grain boundaries in PTC thermistors during sintering

BaTiO3-based positive temperature coefficient (PTC) thermistors undergo a large and rapid increase in grain boundary resistivity at temperatures just above the Curie temperature, Tc. Ample evidence exists for variability in the magnitude and form of the resistivity increase between different grain boundaries, and it has been noted that many of the grain boundaries showing a weak PTC effect have low Σ values when indexed using coincidence site lattice notation. It has also been reported in the literature that, in undoped BaTiO3, there is a strong preference for the formation of Σ = 3 grain boundaries in the microstructure, with many more present than would be expected by chance. current study, the formation and retention of low Σ grain boundaries in a PTC thermistor based on doped BaTiO3 have been characterised through interrupted sintering experiments. Electron backscatter pattern (EBSP) analysis was used to establish grain boundary misorientation distributions in a series of samples prepared during an interrupted sintering study. ificant proportion of Σ = 3, 5 and 9 boundaries was observed, with Σ = 3 boundaries being systematically preferred over other low Σ boundaries. Since Σ = 3, 5 and 9 grain boundaries are believed to be PTC inactive, their presence in significant numbers in the microstructure is likely to be deleterious to the overall performance of a thermistor, particularly during transient loading. rease in the proportion of Σ = 3 twin boundaries was noted with sintering time, however, the proportion of Σ = 3 grain boundaries remained fairly constant, although occurring with a higher frequency than would be expected in a random population. The proportion of Σ = 5 and 9 boundaries also remained approximately constant during the sintering process, indicating that the density of low Σ boundaries in the microstructure is fixed at an early stage of sintering and is not affected significantly by grain growth.