Improved critical current densities in ex situ processed MgB 2 tapes fabricated with chemically treated powder
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K. Ozawa Photocatalytic Materials Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan (Received 13 November 2006; accepted 16 January 2007)
Fe-sheathed MgB2 tapes were fabricated using powder chemically treated through an ex situ process employing a powder-in-tube (PIT) technique. The treatment of MgB2 powder in a benzene solution of benzoic acid caused the pulverization and disappearance of large grains. Compared with the pristine powder without treatment, the tape made from this powder showed a fivefold increase in critical current density (Jc) in 10 T. This was due to the pulverization of grains by the chemical treatment and the removal of surface MgO layers from MgB2 grains by dissolution in the acid solution. The removal of MgO layers was effective in promoting substitution of carbon from the solvent for boron.
I. INTRODUCTION 1
Since the discovery of the MgB2 superconductor, the powder-in-tube (PIT) technique has been widely used for the fabrication of wires and tapes. Two processes are used in the PIT technique. One is an ex situ process, and the other is an in situ process. In the former process, MgB2 powder is used for packing into metallic tubes. Although the as-fabricated ex situ processed wires and tapes carry supercurrent, heat treatment of these wires and tapes drastically increases critical current density (Jc) values. In the latter process, Mg and B sources, such as Mg and B powders, are used for packing, and subsequent heat treatment after deformation to wires and tapes is required to form MgB2 inside the tubes. Currently ex situ and in situ processed wires and tapes show comparable Jc values, but there is much room for improvement. To increase the Jc values, several issues must be resolved. One of the most critical issues for Jc improvement for the in situ process is how to increase the density of MgB2 core layers. The density of the core layer of wires and tapes fabricated through the in situ process is much lower than that of the wires and tapes fabricated through the ex situ process. This is because volume contraction occurs by formation of MgB2 by heat treatment in the in situ process. The low density causes a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0159 J. Mater. Res., Vol. 22, No. 5, May 2007
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the low Jc. It is difficult to avoid this volume contraction in the in situ process. On the other hand, for the ex situ process, one of the most critical issues is how to improve grain coupling. In the in situ process, relatively strong coupling occurs during the formation of MgB2, even with heat-treatment temperatures as low as 600 °C, when MgH2 is used as the Mg source instead of metallic Mg.2,3 In contrast, the coupling is much weaker in the ex situ process, even with heat-treatment at temperatures above 900 °C.4–6 Some grains in commercially available MgB2 powder are completely surrounded by MgO layers, or MgO layers attach partially to other grains.7
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