Viewpoint on fast track communication by V Selvamanickam et al: Critical current density above 15 MA cm⁻² at 30 K, 3 T in 2.2 μm thick heavily-doped (Gd,Y)Ba₂Cu₃O_x superconductor tapes

This is a viewpoint on the fast track communication by Selvamanickam et al [5].

The addition of nanosize flux pinning defects to YBa₂Cu₃O_7−x (YBCO) thin films to dramatically improve the critical current density (J_c) properties in high magnetic fields began in 2003 and 2004, with the addition of Y₂BaCuO₅ [1, 2] and BaZrO₃ [3] phases. The field of YBCO flux pinning has expanded greatly since then, with research papers published on many types of single-phase additions, mixed double phases additions, point-site substitutions, modeling, mixed (Y,RE) where RE = rare-earth, and others. Over 6200 citations and patents have been listed from 2003 to 2015 for the subfield 'flux pinning YBCO', and the field of 'flux pinning' has over 60 000 citations from 2003 to 2015 (source, Google Scholar [4]) Despite the many excellent papers published, there are still many interesting types of additions, flux pinning mechanisms, and processing methodologies to explore. The Fast Track Communication by Selvamanickam et al [5] presents a new advance in this field, by studying an off-stoichiometric Y–Gd–Ba–Zr–Cu–O composition and achieving a new world record for engineering current density of (Y,Gd)BCO coated conductors, and processed with an established manufacturing technology.

The University of Houston research team continues to systematically study and optimize the addition of Zr to (Gd,Y)BCO films, and the Fast track communication is a follow-on of a paper publishing a detailed composition map varying the Ba, Zr and Cu contents [6]. In the Fast track communication [5], the composition was optimized for 20 mol% addition of Zr, and an experimentally measured composition for an optimal film was (Y_0.586Gd_0.561)Ba_2.057Zr_0.166Cu₃O_z, where the composition is normalized for Cu = 3. The paper didn't quantify the precise volume % of different phases formed, since it is difficult to image and accurately analyze <3 nm size particles. However it's of interest to estimate the volume % additions that might form in equilibrium conditions, or might be occurring. Assuming all the Zr reacted with Ba to form BaZrO₃, and GdBCO and YBCO were formed equally, the composition would be (Y_0.5Gd_0.5)Ba₂Cu₃O_z + (Y₂O₃)_0.120 + (Gd₂O₃)_0.093 + (BaZrO₃)_0.165 + (CuO)_0.172. In terms of Volume %, the composition would be (Y_0.5Gd_0.5)BCO = 83.6 Vol%, Y₂O₃ = 4.3 Vol%, Gd₂O₃ = 3.8 Vol%, BaZrO₃ = 6.6 Vol%, and CuO = 1.7 Vol%. From the phase equilibrium diagrams of YBCO thin films processed at 800–850 °C and reduced O₂ atmospheres, Y₂O₃, Y₂Cu₂O₅, and CuO are stable second phase defects observed to be in equilibrium (tied to) the YBCO phase, and the Y₂BaCuO₅ 'green' phase which is normally tied to YBCO was not observed [7]. The previous paper by the U. of Houston group observed a high density of Cu and Zr rich nanoparticle phases [6], which is consistent in part with the phase equilibria study [7]. It is interesting that the film composition is significantly off-stoichiometry from (Y,Gd)Ba₂Cu₃O_z + (BaZrO₃)_M and it is possible the secondary phases are balancing the lattice mismatch strains of each other, which can be positive or negative; e.g. BaZrO₃ has a positive lattice mismatch of +8.3% and can be balanced to some degree by Y₂O₃ or Gd₂O₃ with negative lattice mismatches of −2.5% and −1.7%, respectively. The complicated composition would suggest that more studies and analysis are needed, and additional studies for other mixed (Y,RE)(Ba,Ta,Nb) family of additions and compositions could be of interest.

In addition to exploring a new composition space, another aspect of the Fast track communication is the strong positive impact it can have on technology development and applications. The increase of current density in applied fields is required to raise the device operation temperature for a given current density (typical of magnets), and the increase of current density (itself) can dramatically improve the device performance and lower costs. The Fast track paper is reporting an almost 6-fold increase of current density at 30 K 3 T which is required for 10 MW windmill generators, which would reduce the wire length (and mass and wire cost) required about 4.5× [6]. Very similarly, for superconducting magnetic energy storage (SMES) devices, the mass-specific energy density is proportional to $\sim {{J}_{c}}^{\mathrm{2/3}}$ [8]. So a 6-fold increase of J_c reduces the SMES mass ∼4-fold, and enables SMES to be much more competitive to compete for energy storage applications requiring light weight.

The dramatic increase of current density achieved can also be realized by plotting in figure 1 the J_e(H) properties for the Zr = 20% film compared to present commercially available YBCO coated conductors. For H_appl < 7 T similar or higher J_es can be achieved, amazingly at 50 K instead of 4.2 K. This is very significant since cryocooler size and power requirements are dramatically lower at 50 K compared to 4.2 K; the Carnot efficiency reduces from 69 to 4.9, and the refrigerator efficiency increases from <1% of Carnot to ∼30% of Carnot; and therefore the cryocooler power requirement reduces by over 400×. These increases of performance and benefits provide a critical difference that could enable new applications and capabilities to be practically realized.

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