Mechanism Investigation of Grain Boundary Reoxidation of Barium-lanthanum Titanate Ceramics

BaTiO3 ceramics doped with different La concentrations were sintered under a reducing atmosphere of H2/Ar (1:99), subsequently, reoxidized in oxygen partial pressure Po2=260Pa (mixture with Ar and O2 gas).The behaviour of oxygen in the reoxidation process was investigated by an oxygen coulometry. The microstructure of the raw sintered and reoxidized samples were analysed via TEM. Temperature dependence of resistivity and complex impedance spectroscopy of reoxidized samples at different maximum temperature were measured. Results showed that three oxygen uptake peaks with different onset temperature could be observed during the heating circle. Peak I (onset about 250℃) was attributed to the filling up of oxygen vacancy. Peak II and III were related to the oxidizing of reduced phase. Respectively, Peak II (onset about 800℃) was the formation of oxidized phase at the vicinity of the grain boundaries, which was controlled by grain boundary diffusion. Peak III (onset about 1250℃) was the oxidized phase expanding towards the grain centre and controlled by oxygen bulk diffusion, meanwhile, the rich-Ti phase Ba6Ti17O40 was precipitated. During the transformation from reduced phase to oxidized phase the development of two potential barriers at grain boundaries resulted in the strong PTCR effect.