Critical Current Anisotropy and Texture Gradients in ex situ ${\hbox {MgB}}_{2}/{\hbox {Fe}}$ Tapes

A correlation between the anisotropy of the critical current density J _c in ex situ monofilamentary MgB₂/Fe tapes and the degree of texturing of the MgB₂ filaments has been found. After removing the Fe sheath, X-ray diffraction measurements were performed at various depths of MgB₂ filaments. A texture gradient was detected, the degree of texturing being highest for the MgB₂ grains close to the Fe interface, the grains at the centre being almost untextured. The J _c anisotropy Gamma = J _c(par)/J _c(perp), was found to be highest for the larger initial particle grain size (average 4 mum), a strong decrease being observed for the smaller ones (1 mum). The texturing of MgB₂ grains (and thus the anisotropy) is caused by the contact with Fe sheath surface during the cold deformation of the tapes, while in hot rolled tapes, a random orientation of the MgB₂ grains was observed in the whole filament. From our data, it follows that almost no J _c anisotropy would be expected in ex situ Fe/MgB₂ tapes with submicron particle sizes and a sharp distribution. An anisotropic behavior of the irreversibility field B _irr was also found by inductive measurements, the highest value of B _irr being observed for the smallest particle diameters. The pinning force was found to scale only for measurements performed in perpendicular field. Using a simple model which takes into account the observed texture gradient, i.e. the filament volume consisting of a textured and an untextured part, scaling was also obtained in the case of fields parallel to the tape surface.