Grain boundary reoxidation of donor-doped barium titanate ceramics

The reoxidation process of donor-doped BaTiO3 ceramics which is crucial for the development of the potential barrier at the grain boundary was investigated in situ by oxygen coulometry. Samples with nominal composition of 0.98Ba1−xLaxTiO3 + 0.02TiO2 (0.0025 ≤ x ≤ 0.05) were prepared by sintering under highly-reducing conditions and subsequently reoxidized by heating up to maximum 1380 °C at an oxygen partial pressure of 260 Pa. During the heating cycle three oxygen uptake peaks were observed. These effects could be attributed (1) to the filling up of oxygen vacancies which were formed during sintering, (2) to the transformation from the reduced to the oxidized phase of BaTiO3 in the direct vicinity of the grain boundaries, and (3) to the formation of oxidized phase inside the grains which is controlled by oxygen bulk diffusion. Process (3) is accompanied with Ti-rich precipitations which were detected by TEM methods. The microscopic model of the development of the reoxidation process was confirmed by temperature-dependent dc and ac electrical measurements.