Gd-Ba-Cu-O bulk superconductors fabricated by a seeded infiltration growth technique under reduced oxygen partial pressure

Single-grain Gd-Ba-Cu-O (GdBCO) bulk superconductors have been grown by a seeded infiltration and growth (SIG) technique under a 1% O₂+N ₂ atmosphere using a generic MgO-doped Nd-Ba-Cu-O (MgO-NdBCO) seed placed on the sample surface at room temperature (the so-called the cold-seeding method). Partial melting of the MgO-NdBCO seeds fabricated in air under notionally identical thermal processing conditions, however, limited the reliability of this bulk GdBCO single-grain process. The observed seed decomposition is attributed to the dependence of the peritectic temperature T_p of MgO-doped Nd_1+xBa_2-xCu₃O _y solid solution (MgO-doped Nd-123ss, where ss indicates solid solution) compounds on both oxygen partial pressure during the melt process and the level of solid solution (x). The peritectic decomposition temperature of MgO-doped Nd-123ss, with x ranging from 0 to 0.5 under p(O₂) ≤ 1.00 atm, was observed to remain constant at 1120 °C. T_p was observed to decrease linearly as a function of solid solution level, on the other hand, under oxygen partial pressures of both p(O₂) ≤ 0.21 and 0.01 atm. Based on these results, MgO-doped NdBCO seed crystals should be grown under reduced oxygen partial pressure in order to obtain a stable MgO-doped NdBCO seed crystal suitable for cold-seeding processes of large-grain (RE)BCO bulk superconductors (where RE is a rare earth element).