Abstract
The phases in Cu sheathed MgB
2 wires fabricated using very short annealing time are studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and critical current (
I
c) measurements. By comparison with the XRD pattern of our synthesized MgCu
2, the XRD line located at 2
θ
≈
36.1° for all Cu-sheathed MgB
2 wire samples is unambiguously identified to be due to the MgCu
2 phase. This line was previously unidentifiable due to its absence in the standard pattern of MgCu
2 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 CuMg
δ
(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 MgB
2 wires. For samples heat treated at 700
°C or below, the phases are basically the superconducting MgB
2 and impurity MgCu
2 phases. Quantitative analysis indicates that the molar percent of the MgB
2 phase in these samples is over 90%. For samples heat treated at 725
°C or above, two additional phases, CuMg
δ
and MgB
4 phases, are also present. The content of CuMg
δ
phase increases rapidly with the increase of the heat treatment temperature from 725
°C and 750
°C. This increase in CuMg
δ
content is one of the factors responsible for the dramatic decrease of
I
c. 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 MgB
2 fraction with the heat treatment temperature peaks at 700
°C, well correlated to the variation trend of
I
c with heat treatment temperature.