Effect of the carbon addition to filling powder ball-milled with oils on the critical current density in ex situ processed MgB2 tapes

We have studied the effect of planetary ball-milling of filling powder with mineral and silicon oils on the carbon substitution in MgB2 and the critical current density (J(c)) of Fe-sheathed MgB2 tapes fabricated through an ex situ process in a powder-in-tube (PIT) technique. The mineral oil is much more effective for the carbon substitution and hence the J(c) enhancement of tapes than the silicon oil. The silicon oil hardly refines MgB2 and causes a slight carbon substitution, compared with the mineral oil. Low-temperature drying of the as-milled powder to remove the mineral oil is more effective for the J(c) enhancement due to an enhanced carbon substitution. This is probably because the low-temperature drying leaves more organic material from decomposed oil which acts as a carbon source. However, the carbon substitution amount is just x = 0.04-0.05 in MgB2-xCx. Although the carbon addition to filling powder is effective in increasing the carbon substitution, the J(c) properties of tapes degrade due to induced weak coupling.