Microstructural evolution of the BSCCO-2223 during powder-in-tube processing
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This paper reports a study of the microstructural changes that occur when BSCCO powder is processed by the powder-in-tube method. In this study the initial powder consisted of the 2212 phase plus second phases containing Ca, Cu, Sr, and Pb. When the material was drawn, there was no alignment of the 2212 phase and the second phase particles remained large and blocky. Rolling induced a small amount of alignment into the 2212 phase so that its c-axis was perpendicular to the rolling direction. Rolling also caused the second phase particles to become flatter. When these rolled samples were annealed at 828 °C, the core sintered into a platelet structure, and there was an increase in the amount of aligned material, particularly after annealing treatments of 16 and 32 h. After 8 h at 828 °C, the 2212 variant of the superconducting phase began to transform to the 2223 variant. Pressing this structure improved the alignment, and annealing after pressing allowed further conversion of the 2212 phase to the 2223 phase and apparently removed the strains produced by the pressing. Repeated pressing improved the alignment and repeated annealing allowed more conversion of 2212 to 2223. Both the improved alignment, produced by pressing, and the transformation of 2212 to 2223, produced by the anneals, caused the superconducting properties of the material to improve. I. INTRODUCTION
The discovery of high Tc superconducting materials has raised the possibility that superconductivity could be used in a broad range of applications. However, this wider usage requires that these materials be fabricated into a number of shapes and sizes. One of the most needed products is long wires or tapes. This product form is also one of the most difficult to make. One route to producing long tapes of these materials is the powder-in-tube (PIT) method.13 In this method precursor powders are placed in a tube that is usually made of silver or of a silver alloy; this packed tube is then deformed to the required shape. The material is then heat treated to produce the superconducting phase from the powder. This method has been successfully applied to the bismuth family of high Tc superconductors.16 These superconductors have the basic composition of (Bi-Pb)-Sr-Ca-Cu-O that is commonly abbreviated as BSCCO. If the cations have the ratio of 2212, with (Bi, Pb) taken as one cation type, the transition temperature is between 80 and 90 K.5 If the cation ratio is 2223, the transition temperature is about 110 K.5'7 Therefore, the 2223 phase is the more desirable. Like many of the other cuprate-based high Tc superconductors, the crystal structure, and hence the conductivity, in these BSCCO materials is highly anisotropic.57'8 It is consequently important to have
the polycrystalline material aligned so that the a-b directions of the tetragonal or orthorhombic structures are parallel to the current-carrying direction and the c directions of the grains are perpendicular to it. Wellaligned structures of the 2223 phase with Jc values above 104 A/cm2, measured at 77 K i
