Improved Flux Pinning in Y–Ba–Cu–O Superconductors Containing Niobium Oxide

The effect of the addition of niobium oxide (NbO₂) on the critical current density of large, single grain bulk YBCO superconductors fabricated by top seeded melt growth (TSMG) has been investigated. The high field critical current density, J_c, of NbO₂ doped YBCO is improved significantly even though J_c in the low field region is generally lower than in undoped samples. Under these conditions, when field is applied parallel to the c-axis (H//c) , J_c is enhanced by more than an order of magnitude at 4 T at 77 K to 2 times 10³A/cm² for the NbO₂ containing sample. This enhancement in J_c is accompanied by a large increase in irreversibility field, H_irr, from 4 T for the undoped sample to over 6 T. Enhanced flux pinning at high field is due to the formation of a nano-scale Nb-rich phase during high temperature sintering, melting and solidification of the bulk superconductor.