The Effect of Cu Addition on the Phase Formation and Critical Current Density in the Sugar Doped MgB2 Superconductor

With the aim of improving the critical current density (J(c)) in the MgB2 superconductor, minor Cu (3 at%) was doped to the MgB2 samples in-situ sintered with Mg powder and sugar-coated amorphous B powder. Combined with thermal analysis, phase identification, microstructure observation and J(c) measurement, the effect of minor Cu addition on the sintering mechanism, microstructure and critical current density of sugar-doped MgB2 superconductors were investigated. It is found that the minor Cu addition could obviously accelerated the MgB2 phase formation and improve the growth of MgB2 grains during the sintering process of sugar-doped MgB2 due to the appearance of Mg-Cu liquid at low sintering temperature. On the other hand, the Mg-Cu liquid hindered the reactive C released from sugar entering in the MgB2 crystal lattice. Hence, the connectivity between MgB2 grains was improved accompanying with the C substitution for B is decreased. At 20 K, the J(c) of co-doped samples at low fields was further increased whereas it is decreased at high fields, compared with the only sugar-doped samples.