Influence of the upper critical-field anisotropy on the transport properties of polycrystalline MgB2

Abstract

The intrinsic properties of MgB2 form the basis for all applications of this superconductor. We wish to emphasize that the application range of polycrystalline MgB2 is limited by the upper critical field Hc2 and its anisotropy. In wires or tapes, the MgB2 grains are randomly oriented or only slightly textured and the anisotropy of the upper critical field leads to different transport properties in different grains, if a magnetic field is applied and the current transport becomes percolative. The irreversibility line is caused by the disappearance of a continuous superconducting current path and not by depinning as in high temperature superconductors. Based on a percolation model, we demonstrate how changes of the upper critical field and its anisotropy and how changes of flux pinning will influence the critical currents of a wire or a tape. These predictions are compared to results of neutron irradiation experiments, where these parameters were changed systematically.

Ti and Zr doped MgB2bulk superconductors

“…We find very significant J c enhancements at 5 K at all fields above ~1 T. At 20 K, these effects are less pronounced and occur at higher fields (2 -3 T). The results are in general agreement with the percolation model for current transport in polycrystalline MgB 2 [7] and provide further evidence for the interplay between anisotropy, mean free path effects and flux pinning, as outlined recently [8]. …”

Neutron Irradiation of SiC Doped and Magnesium Rich MgB$_{2}$ Wires

“…The iron sheath of a small piece of the SiC-doped wire was removed mechanically, in order to assess . Otherwise the low resistivity of the metallic sheath masks the onset of the transition [5] and the upper critical field cannot be defined properly. In our case the resistivity of the filament was found to be about 90 , while the whole wire only had .…”

Superconducting transition width under magnetic field in MgB2 polycrystalline samples

“…The linear behavior of ⌬T C can be well understood based on a percolation model, [10][11][12] which predicates the superconducting transition width as:…”