Fabrication of high T.sub.C superconducting helical resonator coils

A method for producing a superconducting rare earth barium copper oxide based helical resonator coil comprising the steps of:

• mixing a rare earth barium copper oxide superconductor as a powder and a binder as a melt at a solids loading of about 45.degree.-65% by volume to form a homogeneous extrudable powder/binder mixture, wherein said binder consists essentially of an ester wax with no more than about 0.25 weight percent extrusion-aiding additives, and said ester was has the general formula RCOOR' with said R and said R' each being independently selected from the group consisting of long chain hydrocarbon groups of at least 6 carbons, and said ester wax has a melting point of about 40.degree.-100.degree. C. and a viscosity of about 94-2000 centipoise at its melting point;
• extruding said binder/power mixture to form an as-extruded wire;
• simultaneously with said extruding step, wrapping said as-extruded wire around a cylindrical mandrel to form an extruded helical coil;
• removing said extruded coil from said mandrel;
• heating said extruded coil, while said extruded coil is embedded in a setter powder, in an oxidizing atmosphere at a heating rate of up to about 2.degree. C./min to a binder removal temperature of about 450.degree.-650.degree. C., and holding said extruded coil in said oxidizing atmosphere at said binder removal temperature for a time sufficient to remove said binder from said extruded coil and to form a green coil having a density of at least 50% of theoretical density, wherein said setter powder is selected to avoid contamination of said green coil;
• heating said green coil in said oxidizing atmosphere at a heating rate of up to about 3.degree. C./min to about the incongruent melting temperature of said superconductor, and holding said green coil in said oxidizing atmosphere at about said incongruent melting temperature for a time just sufficient to begin partial melting of at least one component in said superconductor;
• and
• cooling said partially melted coil in said oxidizing atmosphere at a rate below about 4.degree. C./min to a consolidation temperature of about 940.degree.-969.degree. C., and holding said coil in said oxidizing atmosphere at said consolidation temperature for a time sufficient to consolidate said coil to a density of at least 90% of theoretical density and sufficient to achieve an average grain length of at least about 0.3 mm and grain alignment along the length of said wire;
• cooling said densified coil in said oxidizing atmosphere at a rate below about 2.degree. C./min to a cooled temperature at or below about 550.degree. C.;
• and
• holding said cooled coil at an annealing temperature of about 450.degree.-550.degree. C. in an atmosphere of flowing oxygen for a time sufficient to convert the crystal structure of said cooled coil to at least 50 v/o high T.sub.c phase superconducting perovskite crystal;
• wherein said extruded helical coil produced by said extruding and wrapping steps is of a geometry selected to produce, after subsequent densification, a helical resonator coil about 3-17 mm in inner diameter, and of a total length n .lambda./2, wherein n is an integer and .lambda. is the operating wavelength of said helical resonator coil, said length being suitable for a resonant frequency of about 100 MHz-1.5 GHz.