NANOPHASE PRECURSORS FOR HIGH-J(C) AG-CLAD SUPERCONDUCTING BIPBSRCACUO TAPES

A controlled coprecipitation technique has been used to synthesize precursors of superconducting BiPbSrCaCuO ceramics. The oxalate precursors were nano-sized powders with high homogeneity, low crystallinity, and well-defined stoichiometry. These precursors have been used to fabricate superconducting tapes by the powder-in-tube technique, which involves the use of Ag sheaths. The nano-sized homogeneous powders require considerably reduced tape-processing (sintering) times for Bi-2223 formation, and provide 'clean' intergrain boundaries within the sintered tapes. As a result, high critical transport current density, J(c), can be obtained even in multiphase composites that do not necessarily exhibit a high degree of texture. To gain further insight into the role of microstructural details at each step of the tape fabrication, extensive x-ray diffraction analyses and scanning electron microscopy measurements have been made. The influence of homogeneity and grain size distribution on the superconducting properties of the Ag-clad tapes is discussed. We have reproducibly achieved, within 60 h of the thermal processing, J(c) values as high as 4.4 x 10(4) A cm(-2) at 77 K in zero applied magnetic field.