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

The effect of the addition of niobium oxide (NbO
₂
) 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 NbO
₂
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 10
³
A/cm
²
for the NbO
₂
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.