We have been developing high critical current REBCO tapes and high engineering current density (Je) REBCO wires for high magnetic field applications. REBCO tapes made by our Advanced Metal Organic Chemical Vapor Deposition (MOCVD) method have yielded critical current over 1830 A/4-mm-width at 4.2 K, 20 T in short samples and over 1,270 A/4-mm-width in 50-m-long tapes. A-MOCVD method has also enabled fabrication of double-sided REBCO tapes with critical current over 1,000 A/4-mm-width at 20 K, 20 T. Double-sided tapes also exhibit excellent current sharing because a face-to-face contact between tapes is always present. Consequently, a superior quench stability has been achieved in these tapes. A slot-and-fill technique has also been demonstrated to improve current sharing and quench stability. A fast quench detection method has been developed using the REBCO tape itself as a microwave transmission line. Since the mechanical strength is a limiting factor for use of high-current REBCO conductors in high magnetic field applications, we have developing methods to significantly improve the yield strength of our tapes. Highlights of recent progress in our projects on Advanced REBCO conductors will be presented.

This work was supported by multiple awards from the U.S. Department of Energy and the U.S. Navy.

Coated conductors (CC) of REBa₂Cu₃O₇ (REBCO, RE= Rare Earth) are an exceptional achievement in materials science which encompassed many scientific and engineering challenges. These superconducting materials have emerged as the most attractive opportunity to reach unique performances at high and low temperatures, particularly at high magnetic fields, while reducing the cost/performance ratio continues to be a key objective for a large scale marketability.

To address the challenge of reducing the cost/performance ratio it is unavoidable to develop ultrafast growth rate processes which will lead to high throughput manufacturing of CCs with high performance. Liquid assisted growth of epitaxial REBCO films appears as a very promising approach to reach growth rates beyond 100 nm/s. We have recently created a novel concept, the Transient Liquid Assisted Growth (TLAG) [1-3], which differs from previous growth paths because it is a non-equilibrium process, i.e. the Ba-Cu-O transient liquid with different stoichiometries leading to the formation of REBCO is not an equilibrium one and its properties can be manipulated through kinetic parameters [4]. We will show that different REBCO (RE= Y, Gd, Yb) films can be grown through TLAG using either the temperature or the PO₂ routes and also with different liquid compositions. The TLAG process is fully compatible with the use of preformed BaMO₃ (M=Zr, Hf) nanoparticles to prepare nanocomposite CCs when propionate metalorganic solutions are used in a Chemical Solution Deposition (CSD) route. Finally, we show that the TLAG process can also be extended to other precursors such as amorphous phases deposited by Pulsed Laser Deposition (PLD) at low temperatures [5]. The growth process has been analyzed by in-situ synchrotron X-ray diffraction analysis which have confirmed that ultrafast growth rates (> 1.000 nm/s) can be achieved. We will show that high critical current densities have been achieved up of 3-5 MA/cm² at 77K in thin films and CCs and the process has been transferred to thicker films and metallic substrates. An overall overview of the features of the TLAG process, as compared to other growth approaches, will be presented [6], together with an outlook of the future potential and the pending challenges of this novel technique.

[1] L. Soler et al, Nature Communications 11, 344 (2020)
[2] A. Queraltó et al, ACS Applied Materials and Interfaces 13, 9101 (2021)
[3] L. Saltarelli et al, ACS Applied Materials and Interfaces 14, 48582 (2022)
[4] S. Rasi et al, Advance Science, 9, 2203834 (2022)
[5] A. Queraltó et al, Superconductor Science and Technology 36, 025003 (2023)
[6] T. Puig et al, Nature Review Physics 6, 132 (2024)

We acknowledge the European Research Council for the ULTRASUPERTAPE project (ERC-2014-ADG-669504), IMPACT project (ERC-2019-PoC-8749) and SMS-INKS (ERC-2022- PoC-101081998). We also acknowledge the financial support from the Spanish Ministry of Science and Innovation and the European Regional Development Fund, MCIU/AEI/FEDER for SUPERENERTECH (PID2021–127297OB-C21), “Severo Ochoa” Program for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S and Matrans42 CEX2023-001263-S) and HTS-JOINTS (PDC2022–133208-I00) and PTI+TransEner CSIC programme for Spanish NGEU.

Keywords: coated conductor, nanocomposite, synchrotron radiation, critical currents, TLAG, chemical solution deposition

The fluorine-free metal organic decomposition (FF-MOD) method is considered to be most suitable for mass production of REBCO thin film layer with low cost among various methods such as Pulsed-Laser-Deposition (PLD), Metal-Organic-Chemical-Vapor-Deposition (MOCVD), trifluoroacetate (TFA)-MOD and Reactive Co-Evaporation by Deposition & Reaction (RCE-DR), because of fast crystal growth, flat and clean surface, relatively low cost production system and less material loss. However, development of FF-MOD processed REBCO tapes was not very active due to low Jc under magnetic field originated in very high crystallinity of REBCO layer with less defects acting as pinning centers. Recent successful studies on the introduction of fine nonsuperconducting precipitates in GdBCO layer, increase in the thickness of the layer by multiple sintering process and coating of raw solutions on long metallic tapes [1] suggested that high future potential of FF-MOD method. A problem remains in starting raw materials used to prepare solutions of FF-MOD. Acetates or acetylacetonates (acac) of constituent metals are used in most of studies, however, they are not inexpensive and some of them are hydrates, resulting in difficulty in precise control of metal composition. In the present study, we have attempted to develop a new method to prepare FF-MOD solutions with low cost, high productivity, high homogeneity, high cation concentration and well-controlled cation compositions.

The discovery of easy dissolution of YBCO fine powder in propionic acid at room temperature was a breakthrough of this study. In addition, we found the various REBCO powder and sintered bulks can be dissolved in propionic acid with high concentration by increasing temperatures up to the boiling point of the acid 414 K. After evaporating excess solution, propionate crystals containing RE, Ba and Cu were obtained. By using new FF-MOD solutions, which were prepared by dissolving these propionate crystals in a mixed solvent of methanol, butanol and water, high J_cREBCO films on SrTiO₃ single crystals and IBAD substrates were successfully synthesized with high reproducibility under wider sintering conditions expanding lower temperature down to 993 K. Details of the superconducting properties of REBCO films and preparation process of the new FF-MOD solutions will be shown and future prospects of the FF-MOD processed REBCO films will be discussed.

[1] T. Yoshihara, G. Honda, T. Nagaishi, S. Kobayashi, K. Kanie, T. Okada, and S. Awaji, IEEE Trans. Appl. Supercond., 33, 6600205 (2023)

Recently high field magnet coil especially for MRI, NRM and especially fusion strongly demand low-cost REBCO tape. Sumitomo Electric Industries, Ltd. (SEI) has been developing REBCO tape fabrication by Fluorine-free MOD (FF-MOD) method which is a promising technique for low cost REBCO fabrication. This is because inexpensive and easily prepared solutions can be used, there is a nearly 100% material yield, and no special energy source is required except for a conventional tube furnace. It used to be hard to include artificial pinning centers (APCs) in the process needed for high magnetic field applications. However, we recently reported success in making FF-MOD films with BaMO3 (BMO, M = Hf, Zr, etc.) nanoparticles as APCs by applying a poly-crystallization sintering process along with other processes [1]. We developed BMO nanoparticles with a minimum diameter of 2 nm by utilizing the solvothermal method [2]. So far, we have succeeded in fabricating epitaxial REBCO films with BMO on a clad type tape with buffer layers and 120-m-long REBCO tapes with Ic exceeding 200 A/4 mm-width at 77 K have been achieved. The highest Ic at 20 K, 20 T (B//c) is 230 A/4 mm with 2.7 mol% BaHfO3. In addition, we have demonstrated high Ic potential of FF-MOD films on IBAD substrates [3]. The Ic of over 500 A/4-mm-width at 77 K has been achieved, which is expected to be over 180 A at (20 K, 20 T), meeting a fusion requirement. However, the texture requirement of IBAD-MgO template for FF-MOD has not been clarified. In this study, the REBCO films were fabricated various texture template of IBAD-MgO and the effect of its texture on critical current were investigated.

[1] T. Yoshihara et al., IEEE Trans. Appl. Supercond., 33 (2023) 6600205.
[2] N. Chamorro et al., RSC Adv., 10(2020) 28872.
[3] M. Inagaki et al., WB-6-5-INV, ISS2023, Nov. 29, 2023.