Improved Critical Current Densities in Ex-Situ Processed ${\rm MgB}_{2-x}{\rm C}_{x}$ Tapes Using Powders Treated in Chemical Solutions

We have studied grain coupling and critical current density (J _c) in ex-situ processed Fe-sheathed MgB_{2 - x}C_x (x = 0, 0.05, and 0.10) tapes fabricated by a powder-in-tube (PIT) technique. Compared with commercially available MgB₂ powder, homemade MgB_2-xC_x powders contained much larger amounts of MgO. Therefore, tapes using these homemade powders showed much lower J _c values. Chemical treatment of the homemade powders in acid solutions brought about a large enhancement in J _c of tapes, mainly due to the dissolution of MgO. Although tapes using powders with x ges 0.05 showed small field dependence in J _c due to carbon substitution, the transport J _c values of tapes at 4.2 K and 10 T were almost the same as those of tapes with x = 0 , when using chemically treated powders. This is due to weak grain coupling in tapes with x ges 0.05. Microstructural analyses revealed amorphous fine particles causing the weak coupling present in these tapes.