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. [1], [2]. 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 [3]. 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 [4]. 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 [5]. 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.
[1] Y. Ishiwata et al., IEEE Trans. Appl. Supercond. 23 (2013) 7500804.
[2] L. Soler et al., NATURE COMMUNICATIONS, 11 (2020) 334.
[3] M Miura et al., Supercond. Sci. Technol. 23 (2010) 014013.
[4] N. Chamorro et al., RSC Adv.,10 (2020) 28872.
[5] K. Ohki and T. Nagaishi, Ceramics 47 (2012) No.4 280.
Keywords: coated conductor, REBCO, FF-MOD, APC