Anisotropic connectivity and its influence on critical currentdensities, irreversibility fields, and flux creep in in situ processedMgB2 strands

Abstract

The anisotropy of the critical current density(Jc) and its influence on the measurement of the irreversibility field(Birr) has been investigatedfor high quality, in situ MgB2 strands. Comparison of transport and magnetization measurements hasrevealed the onset of a regime where large differences exist between transportand magnetically measured values of the critical current density andBirr. These effects, initially unexpected due to the lack of crystallinetexture in these in situ processed strands, appear to be due to a fibrousmicrostructure, connected with the details of the wire fabrication andMgB2 formation reactions.Scanning electron micrographs of in situ processedMgB2 monocore strands have revealed a fibrous microstructure. Grains(∼100 nm) are randomly oriented, and there is no apparent local texture of the grains. However, this randomlyoriented polycrystalline material has a fibrous texture at a larger length scale, with stringers ofMgB2 (∼60 µm longand ∼ 5 µm in diameter) partially separated by elongated pores—the spaces previously occupiedby stringers of elemental Mg. This leads to an interpretation of the differencesobserved between transport and magnetically determined critical currents, inparticular a large deviation between the two at higher fields, in terms of differenttransverse and longitudinal connectivities within the strand. The different valuesof the connectivity also lead to different resistive transition widths, and thusirreversibility field values, as measured by transport and magnetic techniques. Finally,these considerations are seen to influence estimated pinning potentials for thestrands.