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

The thermodynamic stability of MgO-doped Nd1+xBa2−xCu3Oy (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, Tp, 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 Tp. 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 Tc 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.