A shell model for the filament structure of Bi-2223 conductors

Phase development during the first few minutes of heat treatment of first generation (Bi,Pb)₂Sr₂Ca₂Cu₃O_y (Bi-2223) multifilamentary wires determines to a large extent the overall microstructure, the efficiency of primary phase development, competing secondary phase development, texture evolution, and grain-to-grain connectivity. It is shown that the filament structure of these wires can be described in terms of a shell model. The shell next to the silver sheath is comprised of well-formed Bi-2223 colonies grapho-epitaxially aligned to the silver sheath and only a few grains (average one colony) thick. Inside this shell is a less well-aligned, less phase pure, more porous Bi-2223 structure through which current is carried in a nonuniform manner as shown by MOI.