Phase formation in Cu-sheathed MgB2 wires

The phases in Cu sheathed MgB2 wires fabricated using very short annealing time are studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and critical current (Ic) measurements. By comparison with the XRD pattern of our synthesized MgCu2, the XRD line located at 2θ ≈ 36.1° for all Cu-sheathed MgB2 wire samples is unambiguously identified to be due to the MgCu2 phase. This line was previously unidentifiable due to its absence in the standard pattern of MgCu2 recorded in the current powder diffraction file (PDF) database. We found that the XRD lines previously attributed to Cu atoms by other groups, are actually due to the CuMgδ (with δ ≈ 6%) phase, indicating that copper does not exist in the form of un-reacted atoms in the core materials of these Cu-sheath MgB2 wires. For samples heat treated at 700 °C or below, the phases are basically the superconducting MgB2 and impurity MgCu2 phases. Quantitative analysis indicates that the molar percent of the MgB2 phase in these samples is over 90%. For samples heat treated at 725 °C or above, two additional phases, CuMgδ and MgB4 phases, are also present. The content of CuMgδ phase increases rapidly with the increase of the heat treatment temperature from 725 °C and 750 °C. This increase in CuMgδ content is one of the factors responsible for the dramatic decrease of Ic. These phase identification results are consistent with our SEM result and the published Cu–Mg phase diagram. It is also found that the variation of the MgB2 fraction with the heat treatment temperature peaks at 700 °C, well correlated to the variation trend of Ic with heat treatment temperature.