Improved critical current densities in MgB 2 tapes fabricated through an ex situ process using powders treated in variou
- PDF / 770,268 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 47 Downloads / 213 Views
K. Ozawa Photocatalytic Materials Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan (Received 14 August 2007; accepted 6 November 2007)
We have studied grain coupling and critical current density (Jc) in ex situ processed Fe-sheathed MgB2 tapes fabricated by a powder-in-tube (PIT) technique, using MgB2 powder soaked in various chemical solutions. The grain coupling and the Jc are strongly influenced by the chemical solutions. Compared with the chemical treatment in a benzene solution of benzoic acid, the use of a cyclohexane solution of benzoic acid doubles the Jc value. Cyclohexane is less stable and hence effectively removed from the surface of MgB2 grains, bringing about the improved coupling of grains and the Jc enhancement.
I. INTRODUCTION 1
Since the discovery of MgB 2 superconductor, powder-in-tube (PIT) technique has been widely adopted 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 packed into metallic tubes. Although the as-fabricated ex situ processed wires and tapes carry supercurrent, heat treatment of these wires and tapes increases critical current density (Jc) values drastically. 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. The ex situ process has fewer fabrication parameters and has been less studied than the in situ process. The Jc values of the ex situ processed tapes used to be lower than those of the in situ processed tapes, although the ex situ processed tapes have higher density in the core layers. This is due to the weak coupling of grains, even heat treated at a temperature as high as above 900 °C.2–4 This is in contrast to the optimized heat treatment temperature of 600 °C for the in situ process.5–8 For both processes, the selections of the powders for packing are important to obtain high Jc.
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0059 J. Mater. Res., Vol. 23, No. 2, Feb 2008
http://journals.cambridge.org
Downloaded: 18 Mar 2015
Some grains in commercially available MgB2 powder are completely surrounded by MgO layers, or MgO layers attached partially to other grains.9 Moisture and oxygen in the air gradually oxidize the surface of the grains. These MgO layers prevent the coupling of MgB2 grains during the sintering process and hence act as obstacles for supercurrent path. The use of freshly prepared powder and the ball-milling process is effective to improve reactivity or coupling of the grains.10–12 Thus, the Jc values of the ex situ processed tapes have reached nearly the same level as those of the in situ processed tapes. We previously reported that chemical treatment of MgB2 powder in an acid solution is effective in improving the Jc property of tapes fabricated through an ex situ process.13 The
Data Loading...
