Melt processing of bulk high Tc superconductors and their application

The authors report a melt-powder-melt-growth (MPMG) process which results in high J_c for bulk Y-Ba-Cu-O superconductors. The Y-Ba-Cu-O pellets or powders are melt quenched. The quenched plates are crushed into powder and mixed well. The powder is then compacted into desired shapes, remelted, and slowly cooled in a thermal gradient. When the starting composition is changed from the 1:2:3 stoichiometry toward the Y₂BaCuO₅ (211) rich region, the 211 inclusions can be dispersed in the YBa₂Cu₃O_x matrix, which contributes to increases in both flux pinning force and fracture toughness. A J_c value exceeding 3×10⁸ A/m² has been achieved at 77 K and 1 T. Another attractive feature of the MPMG process is that other components such as fine Ag powders can be added during solid-state mixing. Fine dispersion of Ag particles can effectively reduce the amount of cracking. MPMG-processed Y-Ba-Cu-O with Ag doping can levitate a mass of 3-kg at 1-mm height using a repulsive force against a 0.4-T magnet. A noncontacting rotation device such as a magnetic bearing can be made utilizing bulk high-J_c materials. A superconducting permanent magnet is also a promising candidate for future application. MPMG-processed Y-Ba-Cu-O can generate 0.25 T at 77 K