Comparative analysis of particle irradiation and second phases additions on the critical current densities of YBa₂Cu₃O₇ single crystals, thin films, and coated conductors: implications for fusion reactors magnets

Abstract The study of irradiation effects in cuprate high temperature superconductors (HTS) has been a topic of interest since their discovery. Enormous progress in the understanding of vortex physics and pinning mechanisms was made in the early 1990s through the irradiation of HTS single crystals with a variety of particles over broad ranges of energies. For YBa 2 Cu 3 O 7 (YBCO), the overall conclusion was that irradiation could increase the critical current density ( J c ) by orders of magnitude. The interpretation of the results was simplified by the fact that the pristine crystals were very clean, with few pinning centers and quite low J c , thus essentially all pinning in the irradiated crystals could be attributed to the controllably added disorder. The case of the Re Ba 2 Cu 3 O 7 ( Re BCO, where Re = Y, a Rare Earth, or combinations of them) epitaxial thin films and coated conductors (CC) is more complex, because the pre-irradiation samples already have high J c due to the presence of large densities of strong pinning centers, which are fabrication-method and processing dependent. The most popular and efficient method to further increase J c in CC has been the incorporation of artificial pinning centers (APC) by chemical incorporation of second phases. Efforts by many groups worldwide demonstrated that a diversity of APC can be effective, and it is now clear that mixed pinning landscapes, nanoengineered by the combination of defects of various shapes and sizes, produce the best results. In some cases, particle irradiation is still effective at enhancing J c in CC, by more modest factors than in the single crystals. Interaction with pre-existing defects cannot be ignored, resulting in both cooperating and competing effects. In this work, I review the vortex pinning generated in YBCO by defects of various geometries (point defects, randomly distributed nanoparticles, aligned or splayed columnar) created either by particle irradiation, incorporation of second phases, or combinations of both routes, and discuss some implications of those results for the design of fusion reactors CC magnets.