Improved REBa2Cu3O7−x (RE = Y, Gd) structure and superconducting properties by addition of acetylacetone in TFA-MOD precursor solutions

Abstract

For developing commercially utilized high-performance high-temperature superconductors, the fabrication of biaxially textured (RE)Ba₂Cu₃O_7−x (REBCO, RE = Y, Gd) coated conductors via metal–organic decomposition of trifluoroacetate precursors (TFA-MOD) has become an interesting strategy for industrial scale-up due to low costs and simple operation. However, the hygroscopic nature of commonly used precursor solutions makes them very sensitive to water pollution through air humidity. This can lead to a degradation of the final microstructure, which in return deteriorates critical current densities. Here, we present a new method to overcome that problem by using a moderator of 2,4-pentanedione (acac) in a pre-existing REBCO precursor solution. Our results show that even initially low-performance solutions can be enhanced to such an extent that they finally outperform standard high-performance solutions and the temperature window for their optimal growth widens significantly. Scanning electron microscopy gives evidence of considerable microstructural improvements, e.g. avoidance of pore formation and grooves, reduction of buckling and surface granularity. X-ray investigations indicate texture improvements, and electrical measurements reveal that transport critical current densities (J_c) increase in self-field and applied magnetic fields. For YBCO, a molar ratio of acac/RE = 0.64 is most effective and leads to an increase of the maximum pinning force density F^max_p from 1.0 to 2.4 GN m⁻³ at 77 K. For GdBCO, a broad window of annealing temperatures (790–840 °C) is possible for films with J_c values above 2.9 MA cm⁻² and F^max_p above 3 GN m⁻³ at 77 K.