Microstructural and superconducting properties of high current metal–organic chemical vapor deposition YBa₂Cu₃O_7−δcoated conductor wires

Metal-organic chemical vapor deposition (MOCVD) on flexible, ion beam assisted deposition MgO templates has been used to produce high critical current density (J(c)) (Y, Sm)(1)Ba2Cu3Oy (REBCO) films suitable for use in producing practical high temperature superconducting (HTS) coated conductor wires. Thick films on tape were produced with sequential additions of 0.7 mu m of REBCO via a reel-to-reel progression through a custom-designed MOCVD reactor. Multi-pass processing for thick film deposition is critically dependent upon minimizing surface secondary phase formation. Critical currents (I(c)s) of up to 600 A/cm width (t = 2.8 mu m, J(c) = 2.6 MA cm(-2), 77 K, self-field) were obtained in short lengths of HTS wires. These high performance MOCVD films are characterized by closely spaced (Y, Sm)(2)O-3 nanoparticle layers that may be tilted relative to the film normal and REBCO orientation. Small shifts in the angular dependence of J(c) in low and intermediate applied magnetic fields can be associated with the tilted nanoparticle layers. Also present in these films were YCuO2 nanoplates aligned with the YBCO matrix (short dimension perpendicular to the film normal), threading dislocations, and oriented composite defects (OCDs). The latter structures consist of single or multiple a-axis oriented grains coated on each side with insulating (Y, Sm)(2)O-3 or CuO. The OCDs formed a connected network of insulating phases by the end of the fourth pass. Subsequent attempts at adding additional layers did not increase I-c. There is an inconsistency between the measured Jc and the observed microstructural degradation that occurs with each additional layer, suggesting that previously deposited layers are improving with each repeated reactor pass. These dynamic changes suggest a role for post-processing to optimize superconducting properties of as-deposited films, addressing issues associated with reproducibility and manufacturing yield.