Defect chemistry of donor-doped BaTiO3 with BaO-excess for reduction resistant PTCR thermistor applications – redox-behaviour

The electric conductivity of donor-doped BaTiO3 thermistor ceramics with excessive BaO revealing a reduction-persistent PTCR effect has been carefully examined in dependence of materials composition and oxygen partial pressure at moderate temperatures between 973 and 1273 K. This thermal regime represents the range which is relevant for the realization of insulating grain boundaries in these electrically inhomogeneous ceramic materials through reoxidation. Based on the experimental results strong evidence for a general correlation between the PTCR characteristics, DC-conductivity and the herewith associated defect chemistry significant to thermistor applications is presented for the system (Ba, La)mTiO3, where m designates the BaO-excess (m ≧ 1). Nominal compositions with a relatively low (Ba+La)/Ti ratio m only show a rather poor PTCR effect and an overall donor-type response in conductivity can be observed at all levels of oxygen partial pressure considered in the present study. With increasing (Ba+La)/Ti ratio m the nonlinear resistivity jump with raising temperature strongly improves and acceptor-type behaviour seems to dominate the total conductivity at partial pressures of oxygen above approximately 10-6 MPa. This result for compositions with high BaO-excess can be understood by the local formation of point defect associates in the grain boundary regions that consist of both acceptor-type titanium vacancies and donor-type oxygen vacancies. Their origin is attributed to the preferential local incorporation of excessive BaO into the BaTiO3 lattice at the intergranular interfaces.