SINTERING CHARACTERISTICS AND FORMATION MECHANISMS OF CORE-SHELL STRUCTURE IN BATIO3-NB2O5-CO3O5 TERNARY-SYSTEM

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 measurement. The densification behavior was normal with Nb-rich composition but showed two steps in Cu-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.