Improved Critical Current Density in Ex Situ Processed Carbon-Substituted MgB2 Tapes by Mg-Addition

Grain coupling and critical current density (J(c)) property were improved in ex situ processed MgB2 tapes by using a mixture of MgB2 and Mg powders with a planetary ball-milling treatment with mineral and silicon oils. The milling process with the oils chemically and mechanically probably peels off the damaged surface layers of the filling powder. The mineral oil is much more effective for the carbon substitution than the silicon oil. Nearly all of the residual mineral oil after drying the filling powder is used for a source for a carbon substitution for B in MgB2, whereas the residual silicon oil hardly reacts with MgB2. Although the B released from MgB2 on the substitution reaction works as obstacles to the supercurrent path, the carbon substitution raises the upper critical field (B-c2), bringing about J(c) enhancement. Mg-addition to the filling powder brings about the grain coupling and further J(c) enhancement, probably because of the consumption of the released B. The optimized amount of Mg added to the filling powder is MgB2 : Mg = 10 : 1 in molar ratio.