POSITIVE TEMPERATURE-COEFFICIENT OF RESISTIVITY FOR BATIO3-BASED MATERIALS

In this paper electrical properties of BaTiO3 and its solid solutions are reported at room and moderate temperatures in the range 300-500 K. The electrical properties are mainly analyzed in terms of the positive temperature coefficient of resistivity (the PTC effect). Experimental data on the PTC effect have been reviewed and analyzed against several theoretical models. The Heywang-Jonker model and its further modifications are discussed in more detail. The models involve different explanations of the nature of the potential barrier which is formed across grain boundaries of polycrystalline BaTiO3. The grain boundary chemistry of BaTiO3 is considered, involving phenomena such as segregation, oxygen chemisorption, oxygen incorporation into the grain boundary layer and the formation of surface states. The effects of these processes on the temperature characteristics of resistivity are considered, with particular emphasis on the preparative procedure and post-preparative treatments such as annealing under gaseous atmospheres of controlled composition and cooling at different rates.