THE HIGH-T-C COMPOUND (BI,PB)(2)SR2CA2CU3O10- FEATURES OF THE STRUCTURE AND MICROSTRUCTURE RELEVANT FOR DEVICES IN MAGNET AND ENERGY TECHNOLOGY(DELTA )

The microstructure of Bi(Pb)-2223 tapes with j(c) ranging from 10-30 k A 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 c hemical composition of Bi1.8Pb0.4ST2.0Ca2.1 Cu3Odelta and the tapes we re sintered in air for typically 300 h with controlled deformation pro cesses in between. SEM analyses of cross sections were used to determi ne quantitatively the phase balance of second phases with a grain size larger than 1 mu m. This phase balance was found to vary significantl y with the phase balance of the initial powder mixtures. This indicate s that the established microstructure is strongly determined by the ki netics, and thermodynamic equilibria are not reached for the condition s applied.TEM analyses of Bi(Pb)-2223 crystallites revealed second-pha se inclusions, intergrowth of the Bi(Pb)-2212 phase and the grain boun dary and dislocation structure of the Bi(Pb)-2223 matrix. Small-angle grain boundaries formed by arrays of dislocations were observed in gra ins which showed significant bending of (001) lattice planes. The form ation of these dislocations is attributed to plastic deformation of th e material during the deformation processes. A systematic study of the sub-mu m second phases was carried out and the same compounds were ob served in all investigated samples. The chemical composition of these second phases was determined by EDX in the TEM and a (Sr, Ca)CuO2 comp ound, a Ca-rich Ca-Cu-O compound and BixCuyOz compounds were identifie d. These sub-mu m second phases are considered to be either precursor phases involved in Bi(Pb)-2223 phase formation or recrystallized phase s formed by partially melted material. The role of the microstructure with respect to the critical current densities is discussed. Tapes yie lding critical current densities greater than 10 kA cm(-2) contain (Sr 1-xCax)CuO2 sub-mu m second phases and a planar Ag/Bi(Pb)-2223 interfa ce with no intermediate layers present. The phase balance of mu m-size d second phases has a small effect on the critical current density but is very useful for monitoring the different stages of the tape prepar ation process.