The trifluoroacetate metal-organic deposition (TFA-MOD) method has been commonly recognized as a low-cost technique for production of REBCO coated conductors (CCs), and also considered as having a slightly lower superconducting property comparing with the CCs by the vapour method. In our previous study, we have developed a Ba-deficient starting solution for improvement of superconducting property of CCs by TFA-MOD method [1]. One of the effects of Ba-deficient starting solution is that coarsening of intermediate products of Ba-compounds is suppressed during a crystallization step. This leads to restrain the residual of Ba-compounds in the grown film, which inhibits the formation of large pores and accompanying degradation of superconducting property of CCs. This approach is effective to the previous process which is difficult to completely prevent the entrapped intermediate phases. However, application of the Ba-deficient starting solution with excess Y and Cu elements in TFA-MOD method has created a new problem of deposition of Y and Cu oxides (Y₂O₃, CuO, and Y₂Cu₂O₅) on the top surface of the REBCO layer in the UTOC-MOD process [2] that we recently developed for the significant improvement of the in-field superconducting property. The reaction to form REBCO phase can be almost completed in the UTOC-MOD. Therefore, excess Y and Cu in the solution is pushed up and segregated on the surface. This oxides layer functions as a resistance layer when a high current flows through the CCs, and may cause a burn-out. In order to solve this issue, the several combinations of different compositional layers such as gradation, rather than the simple stoichiometric composition. In this study, we have investigated the optimization of precursor film composition to reduce the surface resistance with maintaining high performance.

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) and SWCC Showa Cable System Co., Ltd.

[1] K. Nakaoka et al., Physica C 463–465 (2007) 519–522
[2] K. Nakaoka et al., Supercond. Sci. Technol. 30 (2017) 055008

Keywords: TFA-MOD process, Intermediate phases, REBCO, Precursor