Top-seeded solution growth of Ca-doped YBCO single crystals

Abstract

The top-seeded solution growth (TSSG) method has been modified and developed for the growth of Y 1− x Ca x Ba 2Cu 3O 7− δ single crystals. The initial composition was investigated with a molar ratio of Y:Ba:Cu=1.0:7.2–24.0:11.8–54.0 plus additional Ca used in the melt to maintain the growth in a constant supersaturation. This allows crystals to grow at a constant rate of up to 2 mm/d for 100 h. Crystal sizes of up to 15×10×8 mm 3 were obtained. The growth was carried out in an oxygen environment and air, respectively. The melt temperature was found to increase significantly during the growth in oxygen atmosphere, suggesting that an increased crystallization temperature was required in comparison with that of the growth in air. Y-stabilized ZrO 2 crucibles were used. The concentration of Ca for the crystals was determined to be in the range of x=0.18–0.30. The solubility of Ca in YBa 2Cu 3O 7− δ is low which means Ca is hard to incorporate into the compound. The segregation coefficient of Ca for the compound was estimated to be ∼0.7 which is smaller than unity. The result of magnetic susceptibility measurement showed a broad superconducting transition due to the oxygen insufficiency in the as-grown crystals. By post-annealing, oxygen was taken over by the compounds, resulting in a sharp transition width of 4 K. The superconducting transition temperature T c reduces with increasing Ca content, 75 K at x=0.30, 78 K at x=0.20 and 79 K at x=0.18. The over-doped and high-quality crystals could be grown by the TSSG method.