Phase formation and superconductive properties of Cu doped MgB ₂ sintered with milled B powder

Based on the thermal analysis, microstructure observation and superconductivity measurement, the phase formation and superconductive properties of Cu doped MgB ₂ 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 MgB ₂ phase is almost completed before Mg melting. The MgB ₂ grains near the local Mg-Cu liquid could grow faster than others during the phase formation of MgB ₂ before Mg melting. Furthermore, abnormal growth of these preferentially formed MgB ₂ grains could occur after Mg melting. Hence, the MgB ₂ 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 J _c 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 MgB ₂, finally lead to the improvement of J _c at low fields.