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 wa s investigated for the BaTiO3-Nb2O5-Co3O4 ternary system with two diff erent Nb/Co ratios, in terms of sintering characteristics. The tempera ture dependence of the dielectric constant, the densification behavior and the grain growth rate were considerably influenced by the Nb/Co r atio, but the core-shell structure was observed in both compositions. Densification occurred after the additives reacted with BaTiO3, as con firmed by X-ray diffraction measurement. The densification behavior wa s normal with Nb-rich composition but showed two steps in Cu-rich comp osition due to the change of the diffusion mechanism. The liquid phase formed by the secondary-phase Ba6Ti17O40 did not promote densificatio n with these compositions because of its poor wettability with the mat rix. The diffusion rate of Nb into the center of BT was smaller than t hat of Co, and Co was trapped with Nb in the simulation experiment. A model of the formation mechanism of the ''core-shell'' structure has b een developed.