Preparation of MgB 2 powder for ex situ tape with high J c - B performance
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The relationship between the preparation conditions of MgB2 powder and the performance of the critical current density (Jc) of an applied magnetic field (B) of the final ex situ tape produced from it was investigated. The pelletizing pressure of the precursor is crucially important for improving the Jc-B performance of ex situ tape. The higher the pressure, the greater the resulting Jc of the ex situ MgB2 tape. We have shown that the pelletizing pressure used for the precursor mixture during the powder preparation process affects the phases formed in the MgB2. On the other hand, the heating time used is effective in changing the slope of the Jc-B curve of the final tape. This is thought to be due to a change in the crystallinity of the prepared MgB2 powder. The experimental data gathered here will become the basis for investigating the establishment of a guideline for preparing the starting powder used for the manufacture of ex situ MgB2 tape.
I. INTRODUCTION
Magnesium diboride (MgB2) tape is one of the most promising materials for the next generation of superconductor applications. Thus, numerous efforts have been undertaken to improve the critical current density, Jc, of these tapes under an applied magnetic field, B.1–11 The MgB2 tape is commonly fabricated by a powder-in-tube (PIT) technique,12,13 which can be classified into two approaches, depending upon the identity of the starting material. The first approach is the so-called in situ route, which uses unreacted starting powders12; the second approach is the ex situ route, which employs MgB2 powder as the starting material.13 The ex situ route has several advantages in industrial applications with respect to, for example, tape homogeneity, the variety of sheath materials available, and the determination of the heattreatment conditions. However, this route has been deemed unsuitable for obtaining MgB2 tape with a high Jc in a high magnetic field. Recently, we succeeded in significantly improving the Jc-B performance of ex situ MgB2 tape: at 4.2 K and in high magnetic fields over 9 T, the Jc levels of ex situ tapes exhibited no major differences from those of in situ tapes.14–17 In these studies, we prepared MgB2 powder from the superconductor core of an in situ tape by re-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0052 486
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 2, Feb 2008 Downloaded: 07 Apr 2015
moving the sheath material and crushing it. This result demonstrated the possibility of fabricating a high-Jc MgB2 tape via the ex situ route, and it revealed the importance of upgrading the starting MgB2 powder for the ex situ tape. However, this preparation method for the starting powder is not applicable to the fabrication of long-length MgB2 conductors. Therefore, to produce a MgB2 tape with a high Jc on a large scale, researchers must focus on further development of the technique. Fabrication of these MgB2 powders has been quite difficult, because clear guidelines for the pow
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