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 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. 2012 Elsevier B.V. All rights reserved.