Phase formation in Cu-sheathed MgB 2 wires G. Liang a, * , H. Fang b , D. Katz a , Z. Tang c , K. Salama b a Department of Physics, Sam Houston State University, Huntsville, TX 77341, USA b Department of Mechanical Engineering and Texas Center for Superconductivity, University of Houston, Houston, TX 77204-4006, USA c Department of Chemistry, University of Houston, Houston, TX77204-5003, USA Received 17 October 2005 Available online 19 June 2006 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 2h  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 d (with d  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. Quanti- tative 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 d and MgB 4 phases, are also present. The content of CuMg d phase increases rapidly with the increase of the heat treatment temperature from 725 °C and 750 °C. This increase in CuMg d content is one of the factors responsible for the dra- matic 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. Ó 2006 Elsevier B.V. All rights reserved. PACS: 74.70.Ad; 61.10.Nz; 74.62.Bf; 74.70.b; 74.25.Sv Keywords: MgB 2 superconductor; X-ray diffraction; SEM graph; Crystallographic database; Critical current 1. Introduction Ever since the discovery of superconductivity at 39 K in magnesium diboride [1], MgB 2 , much effort has been made in the fabrication and characterization of metal-sheathed MgB 2 wires/tapes [2–10] for large-scale high current appli- cations [11]. Among all of the sheath metals such as iron (Fe), nickel (Ni), silver (Ag), copper (Cu) and their alloys [3–9,11–21], iron seems to be the best metal sheath material in achieving high critical current density (J c ) due to its best chemical compatibility (almost no reaction) with MgB 2 or Mg at high temperatures. For Fe-sheathed tapes, J c with value above 10 5 A/cm 2 has been achieved by several groups [3,7–9,12,14]. Recently, however, much attention has been drawn to the study of the Cu-sheathed MgB 2 wires/tapes [13,14,16–20] due to the fact that Cu is superior than Fe in terms of the following four advantages: (1) high thermal and electrical conductivity, (2) high ductility, (3) non mag- netic nature, and (4) cost effectiveness. These four factors are critical to the cryogenic stability, wire drawing work- ability, AC loss, and large scale production of practical superconductors, respectively. Compared with Fe, the only drawback of Cu as a sheath material for MgB 2 is that Cu can react with MgB 2 or Mg more actively than Fe, result- ing in a substantial degradation of J c . While Fe does not 0921-4534/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.05.023 * Corresponding author. Tel.: +1 936 294 1608; fax: +1 936 294 1585. E-mail address: phy_gnl@unx1.shsu.edu (G. Liang). www.elsevier.com/locate/physc Physica C 442 (2006) 113–123