MgB2 multifilamentary tapes: microstructure, chemical composition and superconductingproperties

Abstract

Multifilamentary Ni-sheathed Cu-stabilizedMgB2 tapes with a criticalcurrent density of 2.0 × 105 A cm−2 (at 20 K and 1 T) were prepared by a powder in tube technique, using pre-reactedMgB2 powders. The microstructure and chemical composition of the superconducting core and theMgB2–Ni interface were studied using SEM, EPMA and TEM. A quick, reliable andstandard-less method of B quantification using SEM-EDX is established for the analysis ofMgB2 wires and tapes. Carbon-contamination-free sample preparation was crucial for the analysisof boron.The typical size of MgB2 colonies, i.e. the arrangement of several well connected grains, in theMgB2 filaments wasbetween 1 and 6 µm. The colonies are structurally well connected to each other, althoughsub-micrometre-sized voids are present. The B to Mg mole fraction ratio in theMgB2 colonies was found to be close to two and the O mole fraction is less than 1 at.%. The typical size of theMgB2 grains in thecolonies is about 0.5–1 µm; however, numerous grains of size 30–200 nm are also present.MgO precipitates of the size of 15–70 nm were found in theMgB2 grains. Long straight dislocations with a density of1 × 1010 cm−2 are observed. Non-superconducting layers which appear as oxidelayers in SEM and TEM samples were found on the surface of theMgB2 colonies and yield poor connectivity between the colonies. It is expectedthat these layers yield a significant reduction of the critical current densityJc.A 10 µm wide intermetallic reaction layer of B, Mg and Ni is formed at theMgB2–Ni interface. Reductionof the MgB2 grain size bymilling of starting MgB2 powder and elimination of non-superconducting layers aroundMgB2 colonies could further enhance the critical current density because of improved pinning andconnectivity between colonies.