Evolution of low sigma grain boundaries in PTC thermistors during sintering

Abstract

BaTiO 3-based positive temperature coefficient (PTC) thermistors undergo a large and rapid increase in grain boundary resistivity at temperatures just above the Curie temperature, T c. 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 BaTiO 3, 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. In the current study, the formation and retention of low Σ grain boundaries in a PTC thermistor based on doped BaTiO 3 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. A significant 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. An increase 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.