SEM observation on the nucleation and grain growth of Bi-2223 phase in Ag-sheathed BSCCO tapes

The SEM observation on the surface morphologies of quenched Bi-2223/Ag tapes after various sintering time indicates that Bi-2223 grains nucleated on the partially melted Bi-2212 grains at the initial stage of reaction and some of them were in the form of twin grains. Then they grew into plate-like grains with the growth of the twin grains and the consumption of the mother Bi-2212 grains. Remarkably, at the growth fronts of Bi-2223 grains the growth steps show the existence of preferential growing sites, which played an important role during the whole growing process of Bi-2223 grains. The appearance of preferential growing sites is thought to be related to the inhomogeneously distributed chemical composition and liquid phase in the reaction system. Based on the microstructure observation and kinetic analysis, it is suggested that both the intercalation process and the two dimensional diffusion controlled nucleation and growth process might be responsible for the nucleation of Bi-2223 phase, but with only the later accounting for the 2223 grain growth.