Carbon substitution and superconducting properties in ex-situ processed tapes using MgB2 powder treated in organic acid solutions

Fe-sheathed MgB2 superconducting tapes were prepared through an ex-situ process and powder-in-tube (PIT) technique using powders treated in organic acid solutions. The treatment realizes the substitution of carbon, derived from the remnants of the organic solvents being adsorbed onto the surface of MgB2 grains, into boron in MgB2 and pulverization of MgB2 grains. The amount of carbon substitution is x = 0.02-0.03 in MgB2-xCx. The carbon substitution is effective for increasing Bc2, and grain boundaries act as pinning centers. Therefore, this improves the Jc property of tapes in the high-magnetic-field region. Depending on the solution, the Jc values at 4.2 K and 10 T increase up to ten times compared to tapes using powders without treatment. These values are comparable to those obtained for the ex-situ processed tapes with the best-ever reported performance. Tapes using MgB2-xCx (x ≥ 0.05) powders clearly show weak grain coupling, and hence the enhancement of transport Jc is limited. (author)