Co-addition of nano-carbon and nano-silica: An effective method for improving the in-field properties of magnesium diboride superconductor

• MgB2 pure and doped with n-C, n-SiC and n-C + n-SiO2 are prepared in bulk.
• Addition of n-SiO2 with n-C improves carbon substitution.
• Co-doped sample exhibits enhanced critical current density in entire range of field.
• Uniformly distributed nano sized secondary phases provide additional flux pinning.

MgB2 superconductor sample co-doped with nano-carbon (n-C) and nano-silica (n-SiO2) is prepared and its structural and superconducting properties are compared with pure and n-C and nano SiC (n-SiC) mono-doped samples. Shrinkage of a-lattice parameter is observed for all doped samples, among which the co-doped sample exhibits the maximum shrinkage, an evidence of effective carbon substitution at boron sites. All doped samples show significantly enhanced in-field critical current density JC(H), where the co-doped sample dominates others throughout the whole range of field studied. Lattice strains induced by carbon substitution and flux pinning caused by uniformly distributed nano sized Mg2Si particles formed by the reaction between Mg and n-SiO2, are the main reasons for the highly enhanced JC(H) behavior of the co-doped superconductor.