Bi-based high T c superconducting fibers by melt extraction
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J. Cave Institut de Recherche d'Hydro Quebec (IREQ), 1800 Montee Ste-Julie, Varennes, Quebec, Canada, JOL 2P0 (Received 19 November 1991; accepted 3 June 1992)
Bismuth-based high Tc superconductors have been prepared as fibers by a technique of melt extraction. As-made, the fibers are amorphous with diameters ranging from 0.7 /xm to 100 /Ltm and lengths of up to 5 cm. The fibers were subsequently transformed into high Tc superconductors by heat treatment in air. Superconducting transitions at 105 K and 82 K were measured in annealed fibers of initial composition Bi! 8Pbo.2Sr2Ca3Cu40x by SQUID magnetometry. The volume fractions of superconducting phases were estimated to have lower bounds of 30% for 2212 and 5% for 2223. The crystallization process has been studied by differential scanning calorimetry, electron microscopy, and x-ray diffraction. Crystallization involves first the formation of the Bi-2201 phase and a bcc phase with lattice parameter a = 0.425 nm before finally significant fractions of both the Bi-2212 and Bi-2223 phases are formed.
I. INTRODUCTION The demand for robust high Tc superconducting wires has led to much work in the development of fibers and tapes from the B i - S r - C a - C u - 0 compound. The glass-forming properties of the bismuth-based material make feasible the casting of amorphous precursors into shapes desirable for the production of multifilamentary conductors.1"3 Originally devised by Maringer and Mobley,4 the melt-extraction technique has been developed by Rudkowski, Rudkowska, and Strom-Olsen to allow continuous casting of fine metallic fibers down to less than 5 /xm in diameter.5 This method is not limited by composition or viscosity as are other fiberization techniques (e.g., gasjet 9 ). In this work we adapted this procedure to produce amorphous fiber precursors for subsequent crystallization of the B i - S r - C a - C u - 0 high TC superconductors.6 Characterization studies were performed on as-made and heat-treated fibers. The amorphous-to-crystalline transition in Bi-based materials was investigated, as well as the growth of 2212 and 2223 superconducting phases. Through this research, we hope that a process for the production of multifilamentary conductors using amorphous preform fibers can be developed. II. EXPERIMENTAL The solid state method was used to prepare precursor pellets for use in the melt-extraction procedure. Constituent oxide (and/or carbonate) powders were mixed by ball milling for 10 to 15 min (isopropyl alcohol was used as a wetting agent). The mixed powder was dried J. Mater. Res., Vol. 7, No. 9, Sep 1992
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and then cold pressed at —20000 psi to form pellets (1 cm diameter, —0.5 cm thick). Pellets were prereacted by heating at 1020 K for 12 to 14 h in air, ball milled, and pressed again to ensure maximum homogeneity, and finally sintered at 1125 K for 12 to 14 h in air. The resulting precursor pellets were 2212 majority phase with the remainder made up of 2201, 2223, and various impurity phases. Pre
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