Phase formation and superconductive properties of Cu doped MgB2 sintered with milled B powder

Based on the thermal analysis, microstructure observation and superconductivity measurement, the phase formation and superconductive properties of Cu doped MgB2 samples sintered with milled B powder were investigated. The minor Cu addition could prompt the solid-solid reaction between Mg and milled B and the formation of MgB2 phase is almost completed before Mg melting. The MgB2 grains near the local Mg-Cu liquid could grow faster than others during the phase formation of MgB2 before Mg melting. Furthermore, abnormal growth of these preferentially formed MgB2 grains could occur after Mg melting. Hence, the MgB2 grains in the Cu-doped samples are larger and less homogeneous than that in the undoped sample sintered with milled B powder. As a result, the pinning of grain boundaries is reduced obviously and the Jc becomes more sensitive to the applied magnetic fields in the Cu-doped sample sintered with milled B powder. On the other hand, the accelerated solid-solid reaction between Mg and B would accompany with less residual Mg and thus the volatility of Mg is weaker after melting, which result in a higher density and fewer voids in the Cu-doped samples and thus enhance the grain connectivity between MgB2, finally lead to the improvement of Jc at low fields.