Phase stability of MgO-doped Nd-Ba-Cu-O seed crystals for cold-seeded, OCMG processing

The thermodynamic stability of MgO-doped Nd_1+xBa_2-xCu₃O_y (x = 0-0.5) has been investigated as a function of solid solution level (x) under various oxygen partial pressures for use as a seed crystal composition in the melt growth of (LRE)-Ba-Cu-O (where LRE = light rare earth) bulk superconductors. The gradient of decomposition temperature, T_p, against solid solution level increases significantly with decreasing oxygen partial pressure, indicating that MgO-doped Nd-Ba-Cu-O (NdBCO) seed crystals should be grown under reduced pO₂ to obtain seeds with high T_p. Based on these results, the optimum cold-seeded processing conditions for the fabrication of single grain GdBCO bulk superconductors containing silver (GdBCO/Ag) under a controlled oxygen partial pressure of 0.01 atm have been determined. The onset T_c of GdBCO/Ag samples grown in air with the optimized seed crystal composition is improved significantly with increasing Ag content, indicating that silver addition to the precursor powder suppresses the formation of Gd-Ba solid solution.