Mechanistic investigations on the densification behaviour of barium titanate ceramics

The processes occurring during the densification of La0.002Ba0.998TiO3 powders with different SiO2-containing additives (TiO2+SiO2), (CaO+TiO2+SiO2), (2BaO+TiO2+2SiO(2)) 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. (C) 2001 Editions scientifiques et medicales Elsevier SAS. All rights reserved.