The effects of Mg doping on the microstructure and transport properties of internal tin-processed brass matrix Nb3Sn superconductors

Abstract

The addition of Zn into Cu matrix has been proved to enhance the Sn activity and accelerate the formation of a Nb3Sn layer in internal tin-processed Nb3Sn wires. However, in our samples, Ti is doped in Sn cores, and during the reaction, Ti was observed to accumulate between the inner and outer sub-elements of the 1980-Nb-core multi-filamentary brass matrix wires. With the addition of Mg into the brass matrix, Ti accumulation between the sub-elements was significantly suppressed, which promoted Ti incorporation into the Nb3Sn layers. Meanwhile, the formed Nb3Sn layers exhibited smaller grain sizes. Compared to the brass (Cu-12wt%Zn) matrix wires, an improvement of critical current density (Jc) was found in the Cu-12Zn-0.2Mg matrix wires due to the incorporation of Ti into the Nb3Sn layer and the refinement of the Nb3Sn grain size. The irreversibility field (Birr) and maximum bulk pinning forces (Fp,max) were calculated from the Kramer plot by analyzing the Jc(B) curves. A small amount of Mg (0.2 wt%) doping into the Cu-12Zn matrix wires can increase the Birr value by about 0.5 T and the Fp,max value by about 3 ∼ 4 GN m−3 after the wires were heat treated between 670 °C and 730 °C. The non-Cu Jc of the Cu-12Zn-0.2Mg matrix wires reaches the maximum value of 1533 A mm−2 @12 T after heat treatment at 685 °C and decreases at higher temperatures.