Thermodynamic approach for enhancing superconducting critical current performance

Abstract

The addition of artificial pinning centers has led to an impressive increase in the critical current density (Jc) of superconductors, enabling record-breaking all-superconducting magnets and other applications. The Jc of superconductors has reached ~0.2–0.3 Jd, where Jd is the depairing current density, and the numerical factor depends on the pinning optimization. By modifying λ and/or ξ, the penetration depth and coherence length, respectively, we can increase Jd. For (Y0.77Gd0.23)Ba2Cu3Oy ((Y,Gd)123), we can achieve this by controlling the carrier density, which is related to λ and ξ. We can also tune λ and ξ by controlling the chemical pressure in Fe-based superconductors, i.e., BaFe2(As1−xPx)2 films. The variation in λ and ξ leads to an intrinsic improvement in Jc via Jd, allowing extremely high values of Jc of 130 MA/cm2 and 8.0 MA/cm2 at 4.2 K, consistent with an enhancement in Jd of a factor of 2 for both incoherent nanoparticle-doped (Y,Gd)123 coated conductors (CCs) and BaFe2(As1−xPx)2 films, showing that this new material design is useful for achieving high critical current densities in a wide array of superconductors. The remarkably high vortex-pinning force in combination with this thermodynamic and pinning optimization route for the (Y,Gd)123 CCs reached ~3.17 TN/m3 at 4.2 K and 18 T (H||c), the highest values ever reported for any superconductor.

High-field critical current density enhancement in GdBCO coated conductors by cooperative defects

“…Above 20 K, S of the unirradiated sample deviates from its previous trend with further increasing T and has a minimum near 40 K, while the S values of the irradiated samples increase significantly with increasing dose (or total dpa). Introduction of non-superconducting nano defects is known to inhibit flux creep [34,35,37]. However, our data show a higher flux creep increases after the introduction of defects by irradiation.…”

Progress in superconducting REBa2Cu3O7 (RE = rare earth) coated conductors derived from fluorinated solutions

“…The main consequence of this reduced ratio is that the YBCO growth temperature is decreased and the superconducting properties improved due to the modified microstructure [31,53,57,[74][75][76][77]. Microstructural studies have shown that these non-stoichiometric compounds display a reduced porosity and a lower concentration of a/b nucleated crystals while the expected secondary phases (mainly Y 2 Cu 2 O 5 and CuO) do not perturb in a relevant way the epitaxial microstructure and so this modified metal stoichiometry has been widely used by many authors to grow CCs displaying very good superconducting performance [57,74,76,[78][79][80][81]. It is also worth to mention that the validity of the TFA approach has been widely proved for most of the Rare Earth compounds in REBCO films where high superconducting performances have been demonstrated [40,73,82].…”

“…It was shown that above a concentration of 10%-12% mol in the final YBCO layer there was a tendency towards NP aggregation and the size control of the NPs was compromised [90,91]. An alternative deposition approach was devised which hindered the NP diffusion and coarsening by taking advantage of the CuO interlayer formed at the intersurfaces of multideposited pyrolyzed films, called UTOC [40,[79][80][81]. This novel process selects ultrathin (∼30 nm) YBCO layers and a solution multideposition approach to reach competitive total film thickness and critical currents, for instance, 35 repetitions of the spin-coating/pyrolysis cycle were required to get a final film thickness of ∼0.6-0.7 µm [40,92].…”

“…The achieved particle size is very likely too large to have a direct effect on vortex pinning while the agglomeration effect decreases the specific interface with the YBCO matrix and so it decreases the positive influence on vortex pinning of the induced defects and it reduces the percolating critical current density J c . The NP sizes were later studied and the influence of modified thermal treatments was undertaken by several authors to decrease the particle size and improve the distribution homogeneity [40,80,81,91,155,194].…”

Recent progress on epitaxial growth of Fe-based superconducting thin films

“…Stangl et al concluded that the condensation energy was increased by over-doping. Furthermore, by combining a thermodynamic approach with microstructural modification by BaHfO 3 nano particles, Miura et al reported on a record pinning force density F p of 3.17 TNm −3 at 4.2 K and 18 T (H ∥ c) for (Y,Gd)Ba 2 Cu 3 O 7−δ grown on technical substrate [74]. As shown in figure 5, T c is almost constant in a wide range of doping for NdFeAs(O,H), which is the most suitable system for the improvement of J c by heavy carrier doping.…”