Sintering characteristics in the BaTiO3-Nb2O5-Co3O4 ternary system: II, stability of so-called "core-shell" structure

The sintering characteristics and the reaction of additives with BaTiO3 (BT ) were examined for two materials having Nb-rich composition (Comp.N) and G o-rich composition (Comp.C) to elucidate the relation between the stability of the core-shell microstructure and the Nb/Co ratio in the BT-Nb2O5-Co3O4 system. TEM observation revealed that the concentration gradient of Nb and Co existed in the shell region although Nh and Co macroscopically distribu ted homogeneously. X-ray diffraction analysis showed that the shell formati on preceded the densification and completed at about 1280 degrees C for bot h Comp.N and Comp.C as determined from differential scanning calorimetry. A diffusion couple experiment disclosed that Co had a larger diffusivity tha n Nh and that the diffusion of Co was suppressed when the sample was codope d with a sufficient amount of Nb. On the basis of these experimental result s, new mechanisms of the formation and collapse of core-shell structure in the BT-Nb2O5-Co3O4 system were proposed.