Grain coupling and critical current density in Fe-sheathed carbon-substituted MgB2 tapes fabricated through an ex situ process using chemically treated powder

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