Spatial variations in composition in high-critical-current-density Bi-2223 tapes
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ARTICLES Spatial variations in composition in high-critical-current-density Bi-2223 tapes T.G. Holesinger and J.F. Bingert MST-6, Metallurgy Group, Materials Science Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
M. Teplitsky, Q. Li, R. Parrella, M.P. Rupich, and G.N. Riley, Jr. American Superconductor Corporation, Westborough, Massachusetts 01581 (Received 25 November 1998; accepted 26 November 1999)
A detailed compositional analysis of high-critical-current-density (Jc) (55 and 65 kA/cm2 at 77 K) (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223) tapes was undertaken by energy dispersive spectroscopy in the transmission electron microscope. Structural features were coupled with characteristic compositions of the Bi-2223 phase. The average of all compositional measurements of the Bi-2223 phase was determined to be Bi1.88Pb0.23Sr1.96Ca1.95Cu2.98Oy. However, spatial variations in the Bi-2223 composition and differing phase equilibria were found throughout the filament structure. In particular, a considerable range of Bi-2223 compositions can be found within a single tape, and the lead content of the Bi-2223 phase is significantly depressed in the vicinity of lead-rich phases. The depletion of lead in the Bi-2223 phase around the 3221 phases may be a current-limiting microstructure in these tapes. I. INTRODUCTION
Rolled multifilamentary (Bi,Pb)2Sr2Ca2Cu3Oy (Bi2223) tapes manufactured by the oxide powder-in-tube (OPIT) process are of technological importance for the development of superconducting motors, transmission lines, magnets, and transformers. Bi-2223 is one of several bismuth-based superconducting (BSCCO) phases based on the general formula Bi2Sr2Ca1−nCunOy (n ⳱ 1, 2, or 3; short notation, Bi-2201, Bi-2212, or Bi-2223). Of all the high-temperature superconducting phases discovered so far, only Bi-2212 and Bi-2223 are currently produced in lengths suitable for application development. Regarding these two superconductors, most of the research has centered on the Bi-2223 phase because of its higher transition temperature (Tc ⳱110 K), higher critical current densities (Jc), and better magnetic field performance at liquid-nitrogen temperatures compared to those of the Bi-2212 phase (Tc ⳱ 70–94 K). Currently, Jc values up to the level of 70 kA/cm2 can be obtained in rolled, multifilamentary Bi-2223 tapes.1,2 However, these values only represent approximately 2% of the Jc values reported for Bi-2223 single-crystal thin films,3,4 suggesting that substantial increases in the Jc performance of Bi-2223 tapes are still possible. The Jc level of a Bi-2223 wire or tape is determined by current flow across grain boundaries in the polycrystalline conductor. The short coherence length and anisotropy of the superconducting properties dictate that the grain boundaries must be clean and of low misorientation to ensure J. Mater. Res., Vol. 15, No. 2, Feb 2000
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good coupling or connectivity. Hence, controlling the grain bounda
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