Enhanced pinning effect in air-processed Gd-123 bulk superconductors with BaTiO3 addition

Introducing additives into the superconducting matrix is an effective way to enhance the flux pinning performance of superconductors. In this article, we introduce a typical ferroelectric material, namely BaTiO3, into the GdBa2Cu3O7−δ (Gd123)/Ag matrix to increase the inhomogeneous state with stronger pinning effect via polarities or ion substitutions. The top-seeded melt growth processed Gd123/Ag bulk superconductors with different doping amount of BaTiO3 ranging from 0.0 mol.% to 0.4 mol.% were fabricated with a cold seeding method. The effect of BaTiO3 addition on microstructure and superconducting properties was investigated systematically. Critical current density, Jc, shows that there are two optimized doping levels. One is 0.1 mol.% with BaTiO3, in the bulk, Jc was enhanced both around zero field and middle field of 1.5 T. The enhanced peak effect (δTc pinning) is rising from the Gd3+/Ba2+ solid solutions, which was further strengthened by a little amount of entrant of Ba2+ and caused a reduction of Tc. While the enhanced Jc around zero field (δl pinning) is coming from the atomic scale pinning centers formed in the CuO plane due to part of Cu2+/3+ ions were substituted by Ti4+. The other optimized doping level is 0.3 mol.%, in this doping amount, Jc value is close to that in 0.1 mol.% doped bulk at self field, but has a lower peak. Two reasons are involved here. The first is that the Gd3+/Ba2+ solid solutions were suppressed by the increased amount of Ba2+ and lead to a suppression of peak effect. The second is the non-superconducting phases (including the survived BaTiO3) formed in the melting process, which act as core pinning centers and contribute to the enhanced δl pinning. The novel doping effect of BaTiO3 is that it may enter as an interstitial among the crystal lattice of Gd-123. The properties of the final products depend on the doping amount. Microstructure analysis shows that Gd-211 particles are fined by proper doping amounts ranging from 0.1 mol.% to 0.3 mol.%.

► BaTiO3 doping improves flux pinning in bulk Gd-123 superconductors. ► The enhanced pinning originated from different two-step pinning mechanisms. ► BTO may act as interstitial between Ba–O layers in Gd123 tri-provskite structure. ► The Jc–B properties of the products depend on the doped amounts of BTO particles.