Superconducting fibers from organometallic precursors: Part III. Pyrolytic processing of precursor fibers
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Superconducting fibers from organometallic precursors: Part III. Pyrolytic processing of precursor fibers Zhi-Fan Zhanga) and Richard A. Kennish Polymeric Materials Laboratory of the Washington Technology Center and the Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195
Kay A. Youngdahl Blohowiak Polymeric Materials Laboratory of the Washington Technology Center and the Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, and Boeing Corporation, Seattle, Washington
Martin L. Hoppe and Richard M. Lainea)- b) Polymeric Materials Laboratory of the Washington Technology Center and the Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195 (Received 14 August 1992; accepted 16 April 1993)
Sixty-70 yam diameter preceramic fibers, extruded from THF solutions containing 1 : 2 : 3 stoichiometric mixtures of yttrium, barium, and copper carboxylates, were pyrolytically transformed into ceramic fibers using controlled heating schedules and reactive atmospheres. The objectives of the work reported here were to identify appropriate processing conditions such that during pyrolysis the preceramic fibers would (1) eliminate the organic ligands without pore or void formation, (2) reach full density with a controlled grain size, and (3) form orthorhombic phase, 123 fibers with reasonable mechanical properties. The mechanisms of organic ligand decomposition and loss were examined using mass spectral fragmentation and TGA. Microstructural and phase evolution were correlated with heating schedules and atmospheres, using XRD, DTA, SEM, and limited flux exclusion studies. The mechanisms of decomposition of the spinnable mixtures suggest intermolecular rather than intramolecular decomposition pathways. Different pyrolysis atmospheres were also examined to explore methods of controlling the degradation process. Slow pyrolysis in air followed by oxygen anneals appears to give superior fibers in terms of controlled grain size and phase. The pyrolyzed fibers exhibit the appropriate orthorhombic phase according to x-ray powder diffractometry. Preliminary flux exclusion measurements demonstrate that the fibers are superconducting although the measured ATC is not exceptional.
I. INTRODUCTION The optimal ceramic superconducting fiber must be dense, thin, flexible, and single phase. Commercial ceramic fibers that meet the "flexible" criterion are typically less than 20 ^im in diameter.3 The current methods of producing superconducting "wires", with few exceptions, provide filaments (50-100 fim) rather than flexible fiber. These methods include powder-intube extrusion,4 melt processing,5 laser-heated pedestal growth6 techniques, and suspension spinning.7a The processes presently used to fabricate thin ceramic fibers (600 °C, the ceramic yield is 29.5%, which corresponds to the formation of Cu 2 O. However, this is unlikely given that the DTA shows two exotherms (at =290 and 300 °C) of equivalent size, w
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