Mechanistic investigations on the densification behaviour of barium titanate ceramics

The processes occurring during the densification of La_0.002Ba_0.998TiO₃ powders with different SiO₂-containing additives (TiO₂+SiO₂), (CaO+TiO₂+SiO₂), (2BaO+TiO₂+2SiO₂) have been investigated using a step-like annealing of the compacts. The reactions taking place during isothermal dwelling and their effects on the shrinkage rates in the following heating period have been characterized. The samples dwelt for 30 minutes exhibited a higher shrinkage rate than the compacts dwelt for 120 minutes. X-ray diffraction measurements revealed that the differences between both dwelling times are caused by the differences in the structural activity of the powders, i.e. by the reaction-mediated formation and degradation of defects in the surface regions of the BaTiO₃ grains. The process of formation of the Ba₂TiSi₂O₈ phase which acts as a defect source is kinetically preferred to the formation of the Ba₄Ti₁₃O₃₀ phase which acts as a defect drain. Thus, during short dwelling times defect-rich surfaces of the BaTiO₃ grains and/or glass-like intermediates are created. These structures favour the sliding of whole grains and consequently result in high shrinkage rates. The gradual degradation of these thermodynamically unstable structures by the formation of the Ba₄Ti₁₃O₃₀ phase during prolonged dwelling times deteriorates the sliding properties of the grains and reduces the shrinkage rate.