Phase Development in Ag-Clad Bscco Wires
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PHASE DEVELOPMENT IN Ag-CLAD BSCCO WIRES R.D. RAY II and E.E. HELLSTROM, Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, U.S.A. ABSTRACT We have studied phase evolution in Ag-clad BSCCO wires with nominal composition Bi 2 .48Sr 2 .05 Ca 0 .95 Cu 2 .27 Ov. The wires were initially processed at 920 C where the oxide was partially molten, then subsequently annealed at 840* C. The microstructure and composition of the phases present were studied at different points in the processing using optical and scanning electron microscopy, and electron probe microanalysis. The microstructure became simpler during the anneal resulting in two oxide phases and Ag. Ag from the cladding had dissolved into the melt at 920 C and subsequently precipitated in the core of the wire on cooling to the annealing temperature. INTRODUCTION Recent reports of Jc with weak field dependence in Ag-clad Bi-Sr-Ca-Cu-O4 (BSCCO) wires have created interest in these systems. Heine et al. [1] reported 1.5x10 A/cm 2 in 26T at 4.2K in BSCCO wires with nominal 2212 composition and Osamura et al. [2] reported values of 5x10 4 A/cm2 in 23T at 4.2K in Pb-doped samples of composition Bi0.8Pb0. 2 Sr0.8CaCul. 4 Oy, which show promise for these materials in high-field applications. Heine et al. [1] achieved their results by heating wire to 920* C, which is above the 2212 eutectic temperature, then annealing at a lower temperature. This wire contained multiple phases. We have studied [3] the composition of the phases formed in Ag-clad wires with a nominal 2212 composition after processing using the procedure of Heine et al. These wires contain a 2212-type phase of composition Bi2.48Sr2.05Cao.95Cu2 27 0y that is rich in Bi and Cu with respect to 2212, the solid solution (Sr 0 .6 9 Ca 0 .3 1)CuO 2 , and Ag particles. The work reported in the present study was done on Ag-clad wires made with BSCCO whose nominal composition was that found in the study of the 2212 wires [3]: namely Bi2.48Sr 2 o05Ca0o95Cu 2 .27 Oy. This composition was selected to eliminate (Sr 0 .6 9 Ca 0 .3 1 )Cu0 2 , which was not superconducting. These wires were processed in the same manner used by Heine et al. [1]. We studied the phase evolution in the wires at various points in the processing. EXPERIMENTAL PROCEDURES AND CHEMICAL ANALYSIS The BSCCO wire was prepared by mixing appropriate amounts of Bi 2 0 3 , SrCO 3 , CaCO 3 , and CuO to obtain a Bi:Sr:Ca:Cu molar ratio of 2.48:2.05:0.95:2.27. The powders were mixed then calcined successively at 800, 825, and 850 C in air for 24 hr at each temperature with intermediate grindings between each heating. An x-ray of this powder showed it to be predominantly the 2212-like phase observed in our previous study. This powder was ground and placed in a Ag tube, then sealed, swaged, and drawn into wire. The wire was reduced in diameter by successive drawings until the Ag broke. This wire was then rolled into tape 0.5mm thick by 3mm wide. The overall processing schedule was heating at 920 C for 0.5 hr, by placing the samp
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