Microstructures of BaTiO3 based PTC thermistors with Ca, Sr and Pb additions

BaTiO3 based PTC thermistors undergo a large and rapid increase in resistance just above the Curie temperature, TC. The resistance increase is predominantly associated with an increase in grain boundary barrier height, therefore it is important to gain information about the grain boundary character in these devices. In this study the microstructures of three different BaTiO3 PTC thermistor formulations were compared by transmission electron microscopy. The samples differed in the A-site substitutions, giving the formulae: (1) BaTiO3, (2) (Ba,Ca)TiO3, (3) (Ba,Ca,Sr,Pb)TiO3. No grain boundary films were observed, though rounding of the grains at the grain junctions was evidence of a liquid second phase being present during sintering. EDX measurements were made across grain boundaries but no significant segregation of elements to the boundaries was observed.