Advances in artificial flux pinning of MOD-REBCO superconducting coated conductors
Abstract
<p indent="0mm">In the past decades, REBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−</sub><sub><italic>δ</italic></sub> (REBCO; RE=Y, rare earth element) second-generation high-temperature superconducting coated conductors have been considered one of the most promising superconducting materials. Although much progress and breakthroughs have been made, there remains a need to artificially introduce effective flux pinning centers in REBCO thin films for higher field applications, such as maglev trains, fusion reactors, and generators. Elemental doping is a common method for introducing artificial flux pinning centers. In REBCO films prepared by metal-organic deposition (MOD), the growth, size, and distribution of the doped phases formed by elemental doping are generally more difficult to control at high temperatures. Various approaches, such as medium-temperature treatment, ultrathin coating, and multi-doping strategy, can be adopted in MOD. However, the ability to optimize the doped phases is limited. For satisfying the high requirements of size control and distribution of the doped phases, the pre-prepared nanocrystal addition technique can solve the problems encountered in conventional elemental doping, yielding REBCO nanocomposite films containing randomly distributed nanocrystals of controllable sizes. Ba<italic>M</italic>O<sub>3</sub> (<italic>M</italic>=Zr or Hf) nanocrystals with sizes in the range of <sc>3–15 nm</sc> can be obtained by adjusting prefabrication conditions and then monodispersed in the REBCO precursor solution and crystallization film after modification such that each nanocrystal can act as an effective flux pinning center. The nanocrystal addition technique can also be applied to REBCO films of different thicknesses. At <sc>30 K,</sc> the in-field<italic> I</italic><sub>c</sub> of a YBCO nanocomposite tape with a superconducting layer thickness of 3.2 μm is improved by a factor of ~5 compared with that of the conventional YBCO tape with a superconducting layer thickness of <sc>1.5 μm.</sc> A 4-mm-wide YBCO tape can carry a current of <sc>290 A</sc> at <sc>4.2 K</sc> in a magnetic field of <sc>20 T.</sc> Furthermore, the anisotropy of the in-field <italic>I</italic><sub>c</sub> at <sc>77 K</sc> of YBCO tapes in different magnetic field directions is significantly suppressed and tends to become isotropic because of randomly distributed and size-controlled nanocrystals acting as pinning centers. The nanocrystal addition technique can be easily introduced into the industrial production process. Moreover, it enables the stable manufacturing of long REBCO tapes up to lengths of <sc>300–500 m,</sc> making them highly suitable for commercial and scientific applications.</p>
