COMPOSITION AND MICROSTRUCTURAL EVOLUTION OF NONSUPERCONDUCTING PHASES IN SILVER-CLAD (BI,PB)2SR2CA2CU3OX COMPOSITE CONDUCTORS

The composition and microstructural evolution of nonsuperconducting ph ases during the course of formation of (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) i n a silver sheath have been investigated by x-ray diffraction (XRD) an alysis, scanning electron microscopy (SEM), energy dispersive x-ray sp ectroscopy (EDX), and digital image analysis. Wire samples fabricated by the oxide-powder-in-tube technique were heat-treated under a variet y of conditions (time, temperature, and oxygen pressure). Backscattere d images taken on polished but unetched transverse cross sections were subjected to computerized image processing, which allowed determinati on of the stoichiometry and quantification of microstructural characte ristics (such as area fraction, size distribution, position, and orien tation) of each nonsuperconducting particle. The dominant nonsupercond ucting phases observed by SEM/EDX were CuO, (Ca, Sr)2CuO3 (2/1), and ( Ca, Sr)14Cu24O41 (14/24) in amounts that varied depending on the annea ling temperature, time, and oxygen partial pressure. Time evolution st udies performed at 825-degrees-C in 0.075 atm O2 showed that the area fraction of 2/1 decreased with reaction time, while that for 14/24 inc reased. In all cases, a substantial amount (> 10% area fraction) of no nsuperconducting phases was detected even after all the Bi2Sr2CaCu2Oy (Bi-2212) in the as-rolled composite conductor was fully converted to Bi-2223, as determined by XRD. High aspect ratio nonsuperconducting pa rticles were initially randomly oriented in the composite conductor co re but gradually aligned parallel to the silver/(Bi,Pb)-Sr-Ca-Cu-O int erface after extended annealing. They tended to segregate and exhibite d a much broader size distribution when processing was carried out at temperatures and oxygen partial pressures on the high end of the norma l processing range, most likely as a result of the occurrence of parti al melting in the system.