Superconducting fibers from organometallic precursors: Part III. Pyrolytic processing of precursor fibers

Abstract

Sixty−70 μm 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 δTc is not exceptional.