Influence of Y_2O_3 and CeO_2 additions on growth of YBCO bulk superconductors

摘要

The Top-Seeded Melt-Growth process for preparation of bulk YBCO single-grain samples from the nominal composition 1 mol YBa_2Cu_3O_x, 0.25 mol Y_2O_3 and 1 wt% CeO_2 was analysed by optical and electron microscopy, EDAX microanalyses and thermal analyses. It is shown that 4.2 wt% of CuO is formed after reaction of constituents. During an isothermal dwell at the optimised temperature, the growth of Y123 single-crystal stops due to decreasing peritectic temperature with increasing CuO concentration in the melt. The growth at furnace cooling leads to trapping of droplets of melt into the growing crystal, which finally epitaxially crystallises on Y123 as BaCu_2O_2 phase. The last part of the sample solidifies as Y123 elongated spheroids with Y211, BaCu_2O_2 and CuO phases. Additional slow cooling 1 ℃/h from the temperature of isothermal growth leads to epitaxial growth of Y123 crystal over the entire sample. Only very narrow part at the rim of the sample contains CuO phase and blocky Y123 crystals are formed there due to self-nucleation.