Two new compounds, GdBa4Cu3O8.5+δ (Gd143) and DyBa4Cu3O8.5+δ (Dy143), were synthesized from precursors Gd2O3, Dy2O3, BaO2, and CuO at 1000 °C in an oxygen atmosphere. The oxygen stoichiometric value δ was found to be 0.68 for Gd143 and 0.6 for Dy143 by iodometric titration. Rietveld refinement of x-ray powder diffraction data showed that Gd143 belongs to the space group Pm3 while Dy143 belongs to the space group P23. The two space groups, Pm3 and P23, are very similar. Their main difference is that P23 does not have the inversion symmetry of Pm3. Both compounds have a cubic unit cell with a lattice parameter of 8.16528 ± 0.00006 Å for Gd143 and 8.10807 ± 0.00010 Å for Dy143. Superconducting quantum interference device (SQUID) measurement indicated that neither compound was superconductive down to 5 K.
The conventional processing of oxide powder-in-tube Bi2223 superconductor tapes involves iterative cycles of mechanical deformation to develop texture and reduce porosity, followed by thermal annealing to react components, relieve stress and heal cracks. However, texture and deformation hardening and cracking may limit the ability to obtain the desired grain texture through deformation alone. In addition, desintering of the oxide core occurs during annealing. We have developed a compressive anneal processing (CAP) technique in which a continuous uniaxial pressure is applied during the annealing. The new process is found to promote texture formation, enhance reaction kinetics and prevent desintering and bloating. Our initial results show that the CAP technique significantly improves the critical current density.
Isothermal melt processing (IMP) has been used to produce phase-pure, high critical current density (J,) Bi-2212 tapes at temperatures as low as 780°C. Bi-2212 tapes processed by IMP have yielded J, values up to 250 kA/cm2 (I, = 345 A) at liquid helium temperatures. Small test coils have been produced with 9, values up to 150 kA/cm2. The effects of the oxidation rate, processing temperature, and time spent in the partial melt on the superconducting properties and microstructure were investigated. Optimal superconducting properties result from the interplay of these parameters such that the grain size of the Bi-2212 phase in the polycrytalline core is allowed to coarsen and align itself with the silver sheath to form a well-connected superconductor. Deviations from optimal conditions result in either too little coarsening to form a well-aligned structure or instabilities in the coarsening process that lead to the formation of large secondary phases which disrupt connectivity..
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