• R Sharma

      Articles written in Bulletin of Materials Science

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

      Z Iqbal N N Thadhani K V Rao R Puzniak R Sharma B L Ramakrishna H Eckhardt F J Owens

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      The tetragonal (Tc=55 K) and orthorhombic (non-superconducting) Tl2Ba2CuO6 (Tl 2201) phases have been synthesized by aca 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.

    • Effect of heat treatment procedures on site preferences of Fe in YBa2Cu3O7−D

      S C Bhargava R Sharma J S Chakrabarty C V Tomy S K Malik

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      Mössbauer spectra of YBa2Cu2·85Fe0:15O7−d prepared using different heat treatment procedures have been obtained. The transition temperature varies from 23 to 53 K, depending on the procedure adopted for preparation. Associated with this are changes in the lattice structure. Mössbauer spectra reveal population of the four components and their relation to the decrease in the transition temperatures.

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