Mechanistic investigations on the densification behaviour of barium titanate ceramics

The processes occurring during the densification of La0.002Ba0.998TiO3 powd ers with different SiO2-containing additives (TiO2+SiO2), (CaO+TiO2+SiO2), (2BaO+TiO2+2SiO(2)) have been investigated using a step-like annealing of t he compacts. The reactions taking place during isothermal dwelling and thei r effects on the shrinkage rates in the following heating period have been characterized. The samples dwelt for 30 minutes exhibited a higher shrinkag e rate than the compacts dwelt for 120 minutes. X-ray diffraction measureme nts revealed that the differences between both dwelling times are caused by the differences in the structural activity of the powders, i.e. by the rea ction-mediated formation and degradation of defects in the surface regions of the BaTiO3 grains. The process of formation of the Ba2TiSi2O8 phase whic h 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 intermedi ates are created. These structures favour the sliding of whole grains and c onsequently result in high shrinkage rates. The gradual degradation of thes e thermodynamically unstable structures by the formation of the Ba4Ti13O30 phase during prolonged dwelling times deteriorates the sliding properties o f the grains and reduces the shrinkage rate. (C) 2001 Editions scientifique s et medicales Elsevier SAS. All rights reserved.