Anisotropy of the critical current in MgB2 tapes made of high energy milled precursor powder

For applications of MgB2 wires or tapes, high critical currents in high magnetic fields are essential. By using tapes in superconducting coils the anisotropic behaviour of the critical current, i.e. the dependence on the direction of the external field in relation to the tape surface, has to be taken into account. The anisotropy of MgB2 tapes with mechanically alloyed (MA) precursor powder and different sheath materials which can be much higher than the intrinsic anisotropy is discussed. Furthermore tapes with pure and C doped MA-MgB2 precursor are compared. Tapes with a hard Fe-sheath and undoped precursor show a high extrinsic anisotropy of the critical current density which can be reduced considerably by carbon doping. A texture of the MgB2 phase in the tape filaments introduced by flat rolling was observed by synchrotron x-ray diffraction. Using this texture information the observed macroscopic Ic-anisotropy of the tapes can be explained by calculations based on the percolation model. (Some figures in this article are in colour only in the electronic version)