Characterisation of the R–T response of BaTiO3 thermistors on three different length scales

Scatter in the resistance–temperature (R–T) characteristic of BaTiO3-based PTC thermistors was investigated on three scales of measurement, corresponding to bulk pellet, a region approximately six grains wide, and single grain boundaries. On the bulk pellet scale the R–T responses were highly consistent with only small variations in peak resistance. Comparison of R–T measurements across pairs of electrodes separated by approximately six grain diameters revealed systematic changes with position across the pellet, as well as local scatter, corresponding to local resistivity differences of up to a factor of five on the 50 μm scale in the PTC region. These were attributed to differences both in local composition and the local level of porosity, which affects oxidation during cooling. The R–T responses of individual high angle grain boundaries showed significant variations in PTC curve shape and degree of field dependence between boundaries.