Processing–microstructure–transport correlations in Y-358 ceramics: Effects of low-level Co substitution under optimized thermal conditions

This study examines the processing–microstructure–transport relationships in Y 3 Ba 5 Cu 8-x Co x O 18-δ (Y-358) bulk ceramics synthesized via the solid-state reaction method, with emphasis on the influence of low-level cobalt substitution (x = 0.00–0.05). X-ray diffraction confirmed the orthorhombic Y-358 phase for all compositions. Microstructural analysis showed that light Co substitution (x = 0.01–0.03) improved grain alignment and densification, promoting stronger intergranular contact, whereas excessive substitution (x = 0.05) introduced porosity, microcracks, and disrupted grain connectivity. Electrical resistivity measurements revealed superconducting transitions across all samples, with the undoped composition exhibiting T c-onset ∼93 K and T c-zero ∼86 K. The optimally substituted sample (x = 0.03) demonstrated enhanced grain connectivity and a sharp single-step transition, indicating well-established intergranular coupling. Magnetic hysteresis measurements at 20 K further verified enhanced flux pinning for this composition, yielding a critical current density of 28.25 kA/cm 2 compared with 21.20 kA/cm 2 for the undoped sample (∼33% improvement). These findings demonstrate that controlled low-level Co substitution, combined with optimized thermal processing, provides an effective route to improve phase stability, microstructural refinement, flux pinning, and current-carrying capability in Y-358 ceramics, offering a practical pathway for advancing cuprate superconductor performance.