MgB2 multifilamentary tapes: microstructure, chemical composition and superconducting properties

Multifilamentary Ni-sheathed Cu-stabilized MgB2 tapes with a critical current density of 2.0 x 105 A cm(-2) ( at 20 K and 1 T) were prepared by a powder in tube technique, using pre-reacted MgB2 powders. The microstructure and chemical composition of the superconducting core and the MgB2-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 MgB2 wires and tapes. Carbon-contamination-free sample preparation was crucial for the analysis of boron. The typical size of MgB2 colonies, i.e. the arrangement of several well connected grains, in the MgB2 filaments was between 1 and 6 mu m. 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 MgB2 colonies was found to be close to two and the O mole fraction is less than 1 at.%. The typical size of the MgB2 grains in the colonies is about 0.5 - 1 mu m; however, numerous grains of size 30 - 200 nm are also present. MgO precipitates of the size of 15 - 70 nm were found in the MgB2 grains. Long straight dislocations with a density of 1 x 1010 cm-2 are observed. Non-superconducting layers which appear as oxide layers in SEM and TEM samples were found on the surface of the MgB2 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 mu m wide intermetallic reaction layer of B, Mg and Ni is formed at the MgB2 - Ni interface. Reduction of the MgB2 grain size by milling of starting MgB2 powder and elimination of non-superconducting layers around MgB2 colonies could further enhance the critical current density because of improved pinning and connectivity between colonies.