次世代超電導線材の開発と特性評価 改良型ＴＦＡ‐ＭＯＤ法におけるＹ１２３膜組織形成機構と水蒸気分圧の影響

Abstract

YBa2Cu3O7-δ (Y123) films were deposited under various water vapor partial pressures (P(H2O)) in the crystallization process by an advanced metalorganic deposition (MOD) method using trifluoroacetates. Microstructures of the films were evaluated by means of transmission electron microscopy. As a result, in the Y123 film crystallized under lower P(H2O) such as 2.1%, large pores were remarkably observed. Furthermore, there were many amorphous layers of non-superconducting phases in the film. In contrast, small pores were distributed in the films deposited under higher P(H2O) of 4.2, 6.3 and 13.5%. Considering that the critical current (Ic) values become higher as P(H2O) increases, the existence of large pores and non-superconducting phase compounds causes a reduction in Ic. By investigating the microstructures of the films quenched during the crystallization process, it was found that the pores in the films are generated by the reaction of the non-superconducting phases trapped in the growing Y123 film, which then transform to Y123 afterwards accompanied by volume reduction of about 30%. The sizes of the entrapped phase crystals in the Y123 film depend on the P(H2O), and this may determine the pore sizes in the final film.