The high-Tccompound (Bi,Pb)2Sr2Ca2Cu3O10+ delta: features of the structure and microstructure relevant for devices in magnet and energy technology

The microstructure of Bi(Pb)-2223 tapes with j c ranging from 10-30 kA cm -2 has been studied by SEM and TEM methods. All different powder mixtures used for the preparation of the tapes had the same nominal chemical composition of Bi 1.8 Pb 0.4 Sr 2.0 Ca 2.1 Cu 3 O delta and the tapes were sintered in air for typically 300 h with controlled deformation processes in between. SEM analyses of cross sections were used to determine quantitatively the phase balance of second phases with a grain size larger than 1 mu m. This phase balance was found to vary significantly with the phase balance of the initial powder mixtures. This indicates that the established microstructure is strongly determined by the kinetics, and thermodynamic equilibria are not reached for the conditions applied. TEM analyses of Bi(Pb)-2223 crystallites revealed second-phase inclusions, intergrowth of the Bi(Pb)-2212 phase and the grain boundary and dislocation structure of the Bi(Pb)-2223 matrix. Small-angle grain boundaries formed by arrays of dislocations were observed in grains which showed significant bending of (001) lattice planes. The formation of these dislocations is attributed to plastic deformation of the material during the deformation processes. A systematic study of the sub- mu m second phases was carried out and the same compounds were observed in all investigated samples. The chemical composition of these second phases was determined by EDX in the TEM and a (Sr,Ca)CuO 2 compound, a Ca-rich Ca-Cu-O compound and Bi x Cu y O z compounds were identified. These sub- mu m second phases are considered to be either precursor phases involved in Bi(Pb)-2223 phase formation or recrystallized phases formed by partially melted material. The role of the microstructure with respect to the critical current densities is discussed. Tapes yielding critical current densities greater than 10 kA cm -2 contain (Sr 1-x Ca x )CuO 2 sub- mu m second phases and a planar Ag/Bi(Pb)-2223 interface with no intermediate layers present. The phase balance of mu m-sized second phases has a small effect on the critical current density but is very useful for monitoring the different stages of the tape preparation process.