Sintering Characteristics and Formation Mechanisms of “Core-Shell” Structure in BaTiO3-Nb2O5-Co3O5 Ternary System

Abstract

The formation mechanism of the “grain core-grain shell” structure was investigated for the BaTiO₃-Nb₂O₅-Co₃O₄ ternary system with two different Nb/Co ratios, in terms of sintering characteristics. The temperature dependence of the dielectric constant, the densification behavior and the grain growth rate were considerably influenced by the Nb/Co ratio, but the core-shell structure was observed in both compositions. Densification occurred after the additives reacted with BaTiO₃, as confirmed by X-ray diffraction measurment. The densification behavior was normal with Nb-rich composition but showed two steps in Co-rich composition due to the change of the diffusion mechanism. The liquid phase formed by the secondary-phase Ba₆Ti₁₇O₄₀ did not promote densification with these compositions because of its poor wettability with the matrix. The diffusion rate of Nb into the center of BT was smaller than that of Co, and Co was trapped with Nb in the simulation experiment. A model of the formation mechanism of the “core-shell” structure has been developed.