Grain coupling and critical current density in Fe-sheathed carbon-substitutedMgB2 tapes fabricated through an ex situ process using chemically treated powder

Abstract

We have studied grain coupling and critical current density(Jc) in ex situ processedFe-sheathed MgB2−xCx (x = 0, 0.05and 0.10) tapes fabricated by a powder-in-tube (PIT) technique. Compared with commercially availableMgB2 powder, homemadepowders of MgB2−xCx contained larger amounts of MgO. Therefore, tapes using these homemade powders showed much lowerJc values.Chemical treatment of the homemade powders in acid solutions brought about a large enhancementin Jc of tapes, mainly due to the dissolution of MgO. The transportJc valuesof tapes were almost the same at 4.2 K and 10 T for both powders with chemical treatment, independent ofx, although tapesusing powders with x≥0.05 showed a large enhancement in normalized volume pinning force(Fp) in the high-field region and small field dependence inJc dueto carbon substitution. This can be attributed to high temperature heat treatment for ex situ processedMgB2 tapes. At sintering,MgB2−xCx probably partiallydecomposes into MgB2, MgB4 and C, producing precipitates at grain boundaries. This brings about weakcoupling of grains and hence negligible enhancement in the low-field region inJc of tapeswith x≥0.05.