Ca-containing additives in PTC-BaTiO3 ceramics: effects on the microstructural evolution

Two series of Ca2+-modified BaTiO3 ceramics have been prepared of the gross composition La0.002Ba0.998–xCaxTi1.01O3.02 (0<x<0.24). In the first series, CaCO3, BaCO3, TiO2 and La2(C2O4)3·9H2O were used as starting materials. The calcination of mixtures with x⩽0.08 resulted in the formation of the corresponding titanate solid solution (Ba1–xCax)TiO3. With values of x higher than 0.08, CaTiO3 was observed as an additional phase. In the second series, a La0.002Ba0.998TiO3 starting powder was hydrothermally recrystallized in Ca(NO3)2 solution. High resolution imaging and analytical methods revealed that the BaTiO3 grains are surrounded by small CaTiO3 crystallites, which influence the evolution of the microstructure of the ceramic in the sintering process strongly by acting as seeds during the recrystallization of the matrix material. Thus, it was possible to optimize the microstructural and electrical characteristics of a ceramic of the second series by adding only 4 mol% Ca, while in case of the first series 16 mol% Ca are necessary.