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 Co-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 Nb and Co macroscopically distributed homogeneously. X-ray diffraction analysis showed that the shell formation preceded the densification and completed at about 1280°C for both Comp.N and Comp.C as determined from differential scanning calorimetry. A diffusion couple experiment disclosed that Co had a larger diffusivity than Nb and that the diffusion of Co was suppressed when the sample was codoped with a sufficient amount of Nb. On the basis of these experimental results, new mechanisms of the formation and collapse of core–shell structure in the BT–Nb₂O₅–Co₃O₄ system were proposed.