Microstructural and superconducting properties of high current metal–organic chemical vapor depositionYBa2Cu3O7−δ coated conductor wires

Abstract

Metal–organic chemical vapor deposition (MOCVD) on flexible, ion beam assisteddeposition MgO templates has been used to produce high critical current density(Jc) (Y,Sm)1Ba2Cu3Oy (REBCO) films suitable for use in producing practical high temperature superconducting(HTS) coated conductor wires. Thick films on tape were produced with sequential additions of0.7 µm of REBCO via a reel-to-reel progression through a custom-designed MOCVDreactor. Multi-pass processing for thick film deposition is critically dependentupon minimizing surface secondary phase formation. Critical currents(Ics) ofup to 600 A/cm width (t = 2.8 µm, Jc = 2.6 MA cm−2, 77 K, self-field) were obtained in short lengths of HTS wires. Thesehigh performance MOCVD films are characterized by closely spaced(Y,Sm)2O3 nanoparticle layers that may be tilted relative to the film normaland REBCO orientation. Small shifts in the angular dependence ofJc in low and intermediate applied magnetic fields can be associatedwith the tilted nanoparticle layers. Also present in these films wereYCuO2 nanoplates aligned with the YBCO matrix (short dimension perpendicularto the film normal), threading dislocations, and oriented compositedefects (OCDs). The latter structures consist of single or multiplea-axis oriented grains coated on each side with insulating(Y,Sm)2O3 or CuO. The OCDs formed a connected network of insulating phases by the end of thefourth pass. Subsequent attempts at adding additional layers did not increaseIc. There is an inconsistency between the measuredJc 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 superconductingproperties of as-deposited films, addressing issues associated with reproducibility andmanufacturing yield.