Enhanced higher temperature irreversibility field and critical current density in MgB₂ wires with Dy₂O₃ additions

Bulk samples of magnesium diboride (MgB₂) doped with 0.5 wt% of the rare earth oxides (REOs) Nd₂O₃ and Dy₂O₃ (named B-ND and B-DY) prepared by standard powder processing, and wires of MgB₂ doped with 0.5 wt% Dy₂O₃ (named W-DY) prepared by a commercial powder-in-tube processing were studied. Investigations included x-ray diffractometry, scanning- and transmission electron microscopy, magnetic measurement of superconducting transition temperature (T _c), magnetic and resistive measurements of upper critical field (B _c2) and irreversibility field (B _irr), as well as magnetic and transport measurements of critical current densities versus applied field (J _cm(B) and J _c(B), respectively). It was found that although the products of REO doping did not substitute into the MgB₂ lattice, REO-based inclusions resided within grains and at grain boundaries. Curves of bulk pinning force density (F _p) versus reduced field (b = B/B _irr) showed that flux pinning was by predominantly by grain boundaries, not point defects. At all temperatures the F _p(b) of W-DY experienced enhancement by inclusion-induced grain boundary refinement but at higher temperatures F _p(b) was still further increased by a Dy₂O₃ additive-induced increase in B _irr of about 1 T at all temperatures up to 20 K (and beyond). It is noted that Dy₂O₃ increases B _irr and that it does so, not just at 4 K, but in the higher temperature regime. This important property, shared by a number of REOs and other oxides promises to extend the applications range of MgB₂ conductors.