Shock wave synthesis and processing of high temperature superconductors with defect microstructures

The tetragonal (T _c=55 K) and orthorhombic (non-superconducting) Tl₂Ba₂CuO₆ (Tl 2201) phases have been synthesized by a ca 20 GPa planar shock wave in a microsecond time frame. Because of local rapid quench rates (up to 10⁶ K/s) defects related to metastable phases are frozen in the sample. In the 2201 phase the predominant defect identified by high resolution lattice imaging corresponds to a Tl-Ba-Cu-Tl-Cu (Tl 1212) phase which is metastable but can be synthesized via a low temperature reaction. Defects of this type may account for the enhanced flux trapping observed in the material using field modulated microwave absorption. Attempts at shock synthesizing the more complex Tl₂Ba₂CaCu₂O₈ and Tl₂Ba₂Ca₂Cu₃O₁₀ phases are also discussed. Shock processing pre-synthesized YBa₂Cu₃O₇ powder in the radial geometry followed by an O₂ anneal at 890 C produces near-theoretical density cylinders that sustain inter-grain critical currents at zero field of 1350 and 750 amps/cm² at 60 and 77 K respectively. The magnetic field dependence of the critical current and the magnitude of flux-pinning in this material relative to sintered pellets are enhanced by a factor of 2-3. Preliminary investigations of the defect microstructure of this material are discussed.