Mechanism of enhanced critical fields and critical current densities of MgB 2 wires with C/Dy 2 O 3 co-additions.

A series of monofilamentary powder-in-tube MgB 2 wires were fabricated with 2 mol. % C doping and co-additions of 0-3 wt. % Dy 2 O 3 . Irreversibility fields ( μ 0 H irr ), upper critical fields ( μ 0 H c 2 ), and transport critical currents were measured, and from these quantities, anisotropies ( γ ) and electronic diffusivities ( D π , σ ) were estimated. The addition of 1 wt. % Dy 2 O 3 to already optimally C-doped MgB 2 wires produced higher H c 2 //ab , H c 2 //c , and H irr values at 4.2 K. In addition, the critical current density, J c , increased with Dy 2 O 3 concentration up to 1 wt. % where non-barrier J c reached 4.35 × 10 4  A/cm 2 at 4.2 K, 10 T. At higher temperatures, for example, 20 K and 5 T, co-additions of 2 mol. % C and 2 wt. % Dy 2 O 3 improved non-barrier J c by 40% and 93% compared to 2 and 3 mol. % C doping, respectively. On the other hand, measurements of T c showed that C/Dy 2 O 3 co-additions increase interband scattering rates at a lower rate than C doping does (assuming C doping levels giving similar levels of low-T μ 0 H c 2 increase as co-addition). Comparisons to a two-band model for μ 0 H c 2 in MgB 2 allowed us to conclude that the increases in H c 2 //ab , H c 2 //c , and H irr (as well as concomitant increases in high-field J c ) with Dy 2 O 3 addition are consistent with increases primarily in intraband scattering. This suggests C/Dy 2 O 3 co-addition to be a more promising candidate for improving non-barrier J c of MgB 2 at temperatures above 20 K.