Transmission Electron microscopy and atomic force microscopy observation of air-processed GdBa₂Cu₃O_7-δ superconductors doped with metal oxide nanoparticles (Metal = Zr, Zn, and Sn)

Single-domain, c-axis-oriented 30-rnrn-diameter GdBa₂Cu ₃O_7-δ (Gd123) melt-textured bulk superconductors have been successfully grown by the top-seeded melt growth method from precursors of Gd123, Gd₂BaCuO₅, and Ag₂O in air with doping of nanosized ZrO₂, ZnO, or SnO₂. Transmission electron microscopy (TEM) unveils a large amount of BaZrO₃ or BaSnO₃ particles with an average diameter of approximately 50 nm respectively embedded in ZrO₂ or SnO₂ doped samples, while no Zn-rich nanoparticles are observed in the ZnO-doped samples. The critical temperature T_c is almost unchanged up to a doping amount of 10 mol% tor ZrO₂- or SnO₂-doped Gd123 melt-textured bulks, while ZnO-doped Gd123 becomes non-superconductive at this doping level. By atomic force microscopy (AFM), nanostripes with a wavelength of 15nm are observed in the ZnO-doped Gd123 sample. Nanoscale particles in the grown Gd123 single domain, together with the micro-defects induced by nanoparticle doping, can account for the enhanced superconducting properties.