MgB2multifilamentary tapes: microstructure, chemical composition and superconducting properties

Multifilamentary Ni-sheathed Cu-stabilized MgB 2 tapes with a critical current density of 2.0 × 10 5 A cm −2 (at 20 K and 1 T) were prepared by a powder in tube technique, using pre-reacted MgB 2 powders. The microstructure and chemical composition of the superconducting core and the MgB 2 –Ni interface were studied using SEM, EPMA and TEM. A quick, reliable and standard-less method of B quantification using SEM-EDX is established for the analysis of MgB 2 wires and tapes. Carbon-contamination-free sample preparation was crucial for the analysis of boron. The typical size of MgB 2 colonies, i.e. the arrangement of several well connected grains, in the MgB 2 filaments was between 1 and 6 µm. The colonies are structurally well connected to each other, although sub-micrometre-sized voids are present. The B to Mg mole fraction ratio in the MgB 2 colonies was found to be close to two and the O mole fraction is less than 1 at.%. The typical size of the MgB 2 grains in the colonies 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 the MgB 2 grains. Long straight dislocations with a density of 1 × 10 10 cm −2 are observed. Non-superconducting layers which appear as oxide layers in SEM and TEM samples were found on the surface of the MgB 2 colonies and yield poor connectivity between the colonies. It is expected that these layers yield a significant reduction of the critical current density J c . A 10 µm wide intermetallic reaction layer of B, Mg and Ni is formed at the MgB 2 –Ni interface. Reduction of the MgB 2 grain size by milling of starting MgB 2 powder and elimination of non-superconducting layers around MgB 2 colonies could further enhance the critical current density because of improved pinning and connectivity between colonies.