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

Abstract

Metal–organic chemical vapor deposition (MOCVD) on flexible, ion beam assisted deposition 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 of 0.7 µm 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 (Ics) of up 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. These high performance MOCVD films are characterized by closely spaced (Y,Sm)2O3 nanoparticle layers that may be tilted relative to the film normal and REBCO orientation. Small shifts in the angular dependence of Jc 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)2O3 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 Ic. 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.

Critical Current Densities of MOCVD Tapes for Different Current Directions

“…This behavior has again been reported previously [4], [5], [14] and was explained by an offset between the direction of the applied field and the magnetic flux density in the sample, given by the orientation of the vortices [15]. The maxima move from the -planes to the defect planes and then further towards the -axis, which corresponds to the "anisotropy-dominated" case described in [15].…”

“…The authors claimed that a complex interplay between these defects and the vortices was the reason for the occurrence of the peak and its shift with field. The samples investigated for the present study are known to contain -axis oriented threading dislocations [4], [5], which could well play a similar role as the columnar defects discussed in [13].…”

“…We have extended the angular measurements conducted by Holesinger et al [4] and Chen et al [5], in which the field was rotated in a plane perpendicular to the current direction, in three aspects: (i) in addition to an L track, a T track was measured; (ii) an angular range larger than 180 was investigated, covering the -peaks at and ; (iii) was also determined at negative applied fields. Fig.…”

“…The samples investigated consist of a 1 thick layer of (termed REBCO, where RE stands for rare earth [2]), which had been deposited on a -capped MgO buffer layer grown on a Hastelloy tape [3]. This process leads to a small vicinal tilt of the REBCO layer, with steps parallel to the tape direc- tion [4], [5]. In addition, various secondary phases are introduced, usually made up of rare earth oxides, which can act as flux pinning centers [6], [7].…”

“…In addition, various secondary phases are introduced, usually made up of rare earth oxides, which can act as flux pinning centers [6], [7]. In particular, layers of nanocrystalline precipitates are formed, tilted by with respect to the REBCO -planes [4], [5].…”