Preparation, structure and properties of the high T_c Bi-based and Y-based cuprates

The structure and some properties of the new bismuth-based high T_c superconductor are discussed and compared to those of the 40K and 90K materials. While the superconducting behavior of the 90K is very sensitive to the oxygen composition and therefore to the annealing procedure, the Bi-based phase is dramatically less affected. But it is more sensitive to the solid state reaction temperature and starting composition. Thus a better control of the chemistry using a solution technique should enhance the performances of this material. The Y-based 90K superconducting oxide has been synthesized using a novel solution technique (Sol-Gel process). Physical properties, including resistivity and critical current, are compared to those obtained by solid state reaction in which the starting materials include carbonates, acetates and nitrates. The solution technique produces YBa₂Cu₃O_7-y powders at lower reaction temperatures and shorter times. The smaller particle size obtained by this method (1μm) yields denser compacts upon sintering. The solution derived materials are made from inexpensive reagent-grade (98% pure) starting materials and they superconduct at 91K with narrow transition widths (0.6K). Although the material is homogeneous, the critical currents do not significantly exceed those reported for solid-state reacted materials and we attribute this to the presence of carbon at the grain boundaries. Finally, the ability to produce stable viscous gels allows us to prepare large-area thick films on various substrates. Best results are obtained on SrTiO₃ or MgO substrates, and problems encountered in making thick films on substrates of technological interest are discussed.