Texture gradients in Fe-sheathed ex situ produced MgB₂ tapes

Superconducting Fe-sheathed MgB₂ monofilamentary tapes have been fabricated by the powder-in-tube technique, varying the particle size of the starting MgB₂ powder and applying either cold or hot rolling during the last deformation process. Measurements of the critical current density J_c with the magnetic field applied parallel or perpendicular to the tape surface revealed a pronounced anisotropy for the cold rolled tapes, which was found to increase with increasing particle size and magnetic field strength. The microstructural origin of the J_c anisotropy was confirmed by means of x-ray diffraction performed on the filaments after mechanical removal of the sheath. The local texture was studied in a series of diffraction patterns collected at different distances from the filament centre, removing each time some 25 µm of the filament thickness. In the cold rolled tapes, the average orientation of the MgB₂ grains was found to approach a preferred orientation with the crystallographic c-axis perpendicular to the tape surface, near the interface with the sheath; however, the misalignment angle increased towards the centre of the tape. In the hot rolled tapes, for which no J_c anisotropy was observed, the grains were found to be approximately randomly oriented. Roughness measurements performed on the side of the Fe sheath in contact with the MgB₂ filament are in agreement with the difference in texture observed for cold and hot rolled tapes.