Formation and retention of low (sigma) interfaces in PTC thermistors

The distribution of grain boundary types in a barium titanate based positive temperature coefficient of resistance (PTC) thermistor was established using electron backscatter pattern analysis in a scanning electron microscope. A higher proportion of (sigma) = 3 interfaces than would be expected from a random grain distribution was observed, together with numerous (sigma) = 5 and (sigma) = 9 grain boundaries. All of these grain boundary types are reported in the literature to be PTC inactive, and hence should not contribute to the overall PTC response of the device. The (sigma) = 3 interfaces were further characterised on the basis of their microstructure as grain boundaries or annealing twins, from which it was established that the proportion of (sigma) = 3 grain boundaries in the thermistor increased only slightly during the sintering process, whereas the proportion of annealing twins increased significantly during the first 30 min of soak at the peak sintering temperature. (sigma) = 3 grain boundaries were found to be immobile during grain growth, in contrast with high angle grain boundaries, which showed considerable migration.