The effect of Cu addition on the phase formation and critical current density in the sugar Doped MgB ₂ superconductor

With the aim of improving the critical current density (J _c) in theMgB ₂ superconductor, minor Cu (3 at%) was doped to the MgB ₂ 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 MgB ₂ superconductors were investigated. It is found that the minor Cu addition could obviously accelerated the MgB ₂ phase formation and improve the growth ofMgB ₂ grains during the sintering process of sugar-doped MgB ₂ 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 MgB ₂ crystal lattice. Hence, the connectivity between MgB ₂ grains was improved accompanying with the C substitution for B is decreased. At 20 K, the J _c of codoped samples at low fields was further increased whereas it is decreased at high fields, compared with the only sugardoped samples.