Multi-step process to prepare bulk BSCCO (2223) superconductor with improved transport properties

BSCCO (2223) superconducting powders were prepared by three different techniques (solid state, pyrolysis and sol-gel) and with three different stoichiometries, especially based on variations of Ca/Sr ratio and Cu concentration. Pressureless sintering was performed on cold uniaxially pressed (1 GPa) samples, avoiding the drawbacks connected to hot-pressing; different hot-forging cycles were then applied to attain higher density and orientation factor. It was found that the process phenomenology is strictly linked to the powder starting stoichiometry: when composition is very near to the theoretical (2223), effects of secondary phase extrusion are observed during hot-forging, yielding a purification and inhibition of (2212) formation accompanied by an appreciable increase of J_c respect to hot-pressed samples. When a Ca/Sr ratio is considerably > 1, recrystallisation of (2223) from the liquid takes place, with a remarkable improvement of critical current density (J_c > 10⁴ A/cm²). In this case the non-superconducting secondary phases act as intrinsic oxygen reservoir. Finally, when composition diverges even more from (2223) (Ca/Sr ≫ 1 and Cu excess), the formation of too many precipitates of non-superconducting phases hinders the texturing process and more in general deteriorates the intergranular properties.