Superconducting Phases and Modulated Structures in (Bi, Pb)-Sr-Ca-Cu-O Wires
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SUPERCONDUCTING PHASES AND MODULATED STRUCTURES IN (Bi, Pb)-Sr-Ca-Cu-O WIRES Y. FENG*, R. E. SMALLMAN*, I. P. JONES*, F. WELLHOFER*, N. McN. A1FORD** AND T. W. BUTTON** *Superconductivity Research Group, and School of Metallurgy and Materials, University of Birmingham, P. O. Box 363, Birmingham B15 27T, UNITED KINGDOM **ICI Advanced Materials, Runcom, Cheshire WA7 4QE, UNITED KINGDOM. ABSTRACT (Bi, Pb)-Sr-Ca-Cu-O '2223' wires with varying Bi/Pb ratios and sinter times have been investigated by parallel physical measurements and analytical electron microscopy. The numbers of phases present, their relative amounts, compositions and structures are reported and correlated with the superconducting properties. In particular, the recently reported long period (010) modulation (2b*/q*=7.7) is found to be connected with the "2223" high Tc phase. INTRODUCTION Evidence in diffraction patterns of modulations along the b* axis has been shown by many authors [1-4]. Matsui et al[1] reported the existence of two types of modulation in both low Tc(75K) and high Tc (105K) phases. Kang et al [2,3] reviewed the modulated structures published by previous authors and described 3 types of modulated structure. They found 2 of these modulations in their Pb-free "2212" samples. Ikeda et al[4] reported that substituting Pb for Bi encouraged the formation of a variant of the high Tc phase where the lattice parameter of the supercell was increased from 2.7 nm to about 5 nm along the b axis. However, there is no very direct evidence connecting either the low and high Tc phase with a specific modulation in the Pbdoped "2223" composition. In this paper we report on a series of lead stabilized BSCCO wires, with nominal 2223 composition, where the proportion of Pb and the time of sintering have been varied systematically. We attempt to relate the modulated structures to varying Pb content and sintering time. EXPERIMENTAL Two series of specimens in the (Bi2-xPbx)Sr2Ca2Cu3010 system have been investigated: series P contained varying amounts of Pb (x = 0, 0.4, 0.8, and 1.2) and series S consisted of samples with a fixed Pb content (x=0.8) but with varying sintering times. The mixed powders were initially calcined for 5h at 845°C in air. The calcined powders were processed into rods approximately 1 mm in diameter as reported elsewhere for YBa2Cu3Oy(5]. The rods were then sintered in air at 850°C for various times between 2 and 116h and furnace cooled. The field for the A. C. susceptibility measurements was 5mOe at a frequency of 1kHz and the sample dimensions were 6mm (length) and 1mm (diameter). The powder X-ray diffraction data were collected on a Philips diffractometer using CuKox radiation. Specimens for electron microscopy were prepared by grinding and mounting either across a copper oval with silver paste, on a copper folding grid, or (for chemical analysis) on a holey carbon film supported by an aluminium grid. The microscope was a JEOL 4000 FX 400 kV AEM with a LINK AN10000 EDX system. For quantitative results, the hole spectrum was first strip
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