Composition and microstructural evolution of nonsuperconducting phases in silver-clad (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x compo
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W. L. Carter and G. N. Riley, Jr. American Superconductor Corporation, Westborough, Massachusetts 01581 (Received 4 May 1994; accepted 30 August 1994)
The composition and microstructural evolution of nonsuperconducting phases during the course of formation of (Bi,Pb)2Sr2Ca2Gi3Ox (Bi-2223) in a silver sheath have been investigated by x-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), and digital image analysis. Wire samples fabricated by the oxide-powder-in-tube technique were heat-treated under a variety of conditions (time, temperature, and oxygen pressure). Backscattered images taken on polished but unetched transverse cross sections were subjected to computerized image processing, which allowed determination of the stoichiometry and quantification of microstructural characteristics (such as area fraction, size distribution, position, and orientation) of each nonsuperconducting particle. The dominant nonsuperconducting phases observed by SEM/EDX were CuO, (Ca,Sr)2Cu03 (2/1), and (Ca, Sr)14Cu24O4i (14/24) in amounts that varied depending on the annealing temperature, time, and oxygen partial pressure. Time evolution studies performed at 825 °C in 0.075 atm O2 showed that the area fraction of 2/1 decreased with reaction time, while that for 14/24 increased. In all cases, a substantial amount (>10% area fraction) of nonsuperconducting 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 particles were initially randomly oriented in the composite conductor core but gradually aligned parallel to the silver/(Bi,Pb)-Sr-Ca-Cu-0 interface after extended annealing. They tended to segregate and exhibited a much broader size distribution when processing was carried out at temperatures and oxygen partial pressures on the high end of the normal processing range, most likely as a result of the occurrence of partial melting in the system.
I. INTRODUCTION
Superconducting (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) composite conductors fabricated by the oxide-powderin-tube (OPIT) technique have demonstrated the capacity to support large critical currents at liquid nitrogen temperature.1^ Fabricating such composite conductors is a materials science challenge that requires the achievement of a precisely engineered microstructure with correct chemical, mechanical, and electromagnetic properties. One of the keys to preparing OPIT Bi-2223 composite conductors that exhibit high critical current density (Jc) is to control the size and distribution of the nonsuperconducting phases remaining in the superconducting core matrix after thermomechanical processing. The usual processing sequence is as follows5: The silver sheath in tubular form is loaded with J. Mater. Res., Vol. 9, No. 12, Dec 1994 http://journals.cambridge.org
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an oxide precursor powder (overall stoichiometry «* Bi-2223) that has been sintered to produce a mixture c
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