Fabrication and characteristics of long MgB₂ wire with high superconducting properties

Appling the in situ powder-in-tube method, 11 meter long Fe-sheath MgB₂ superconducting wires with outer diameter 1.75 mm were fabricated, using 99.5% Mg powder and 99.9% amorphous boron powder doped with 30 nm SiC powder as starting materials. This procedure begins with ball-milling of the mixture powder immersed in acetone solution in a jar and subsequently drying in vacuum. Then the iron tube loaded with the powder mixture was grooved, swaged and drawn to the final wire size. The transition temperature, microstructure, lattice constant and chemical element distribution were evaluated by SQUID, XRD, SEM and EDS, respectively. It was found that T_c(onset) and ΔT_c are 35.1 K and 5.3 K, respectively. All samples containing a small amount of MgO, exhibit rather uniform microstructure. Besides, by nano-SiC doping, the MgB₂ lattice distortion due to substitution of boron by carbon, results in forming effective flux pining centers, which significantly improves the critical current, and the C element was uniformly distributed in the MgB₂ matrix. The standard four-probe measurement shows that all the 10 sample points, which were evenly cut at 1 meter intervals of the 11 meter length wire, have good superconducting homogeneity with critical current values fluctuating within 10% under the applied fields varying from 10 to 18 Tesla, and all J_c values are above 10⁴ A/cm², with the highest value reaching 1.2 × 10⁴ A/cm² at 4.2 K and 10T.