Fluorine-free MOD (FF-MOD) is a promising technique for low cost REBCO fabrication because inexpensive 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. Moreover, the FF-MOD method can obtain highly textured film at a high growth rate of ~ 100 nm/sec. , . However, it has been hard to introduce artificial pinning centers (APCs) needed for magnetic field applications because the addition of dilute metals (M = Hf, Zr, etc.) to the starting solution often disturbed orientation of the REBCO matrix.
During the REBCO growth process, BaMO3 (BMO) formation occurs . The size of the BMO can change depending on the conditions during heating. To avoid the misorientation of REBCO films and a variation in BMO size, we used BMO nanoparticles made using a solvothermal method . By adding BMO nanoparticles to the starting solution, the size of BMO particles can be kept in the nanometer order. Furthermore, we introduced a microcrystallization sintering process for decomposed BaCO3 before making oriented REBCO films, because inhomogeneous decomposition of BaCO3 leads to a disturbance in the homogeneous crystallization of REBCO films.
By utilizing the above techniques, we succeeded in fabricating epitaxial REBCO films which include more than 7 mol% BMO on a clad type tape with buffer layers . The clad type tape consists of textured electro-plated Ni film and rolled Cu foil on a 100 μm thick stainless base tape. 120-m-long APC-added REBCO tapes with IC exceeding 200 A/4 mm-width at 77 K can be achieved by the FF-MOD method. The highest IC at 77 K, s.f. is 337 A/4 mm and IC at 20 K, 20 T (B//c) is 194 A/4 mm with 3.4 mol% BaZrO3. Further improvement of critical current properties at low temperature and with high magnetic fields is expected by optimizing the concentration and size of nanoparticles.
Acknowledgements: A part of this work was performed at High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University (Project No 202112-HMKPC-0006). We appreciate Dr. Obradors and Prof. Puig of the ICMAB for technical discussions.
 Y. Ishiwata et al., IEEE Trans. Appl. Supercond. 23 (2013) 7500804.
 L. Soler et al., NATURE COMMUNICATIONS, 11 (2020) 334.
 M Miura et al., Supercond. Sci. Technol. 23 (2010) 014013.
 N. Chamorro et al., RSC Adv.,10 (2020) 28872.
 K. Ohki and T. Nagaishi, Ceramics 47 (2012) No.4 280.
Keywords: coated conductor, REBCO, FF-MOD, APC