Study on the formation of MgB2 and other phases in Cu-sheathed /MgB2 wires.

We report the results from a systematic x-ray diffraction, SEM, and critical current density measurements for Cu-sheathed MgB$_{2 }$wires fabricated using the powder-in-tube method and ultra-fine Mg and B precursor prepared by high-energy ball milling. The samples were sintered at temperatures ranging from 450 \r{ }C to 900 \r{ }C for 5 minutes. It is found that MgB$_{2}$ phase can be formed in this whole temperature range. Below 550 \r{ }C, the weight of the Mg$_{2}$Cu phase increases with sintering temperature while the Mg-phase decreases. Between 550 \r{ }C and 725 \r{ }C, the Mg$_{2}$Cu phase disappears, only MgB$_{2}$ and MgCu$_{2}$ phase co-exist. At or above 725 C, MgB$_{2}$, MgB$_{4}$, and Mg$_{1+y}$Cu$_{32+\delta }$ phase coexist, and the fraction of the Mg$_{1+y}$Cu$_{32+\delta }$ phase increases with sintering temperature while the other two phases decrease. These results are supported by our SEM and $J_{c}$ measurement results.