The Potential of MgB2 Superconductors for Magnetic Levitation of Maglev Vehicles

Investigations carried out up to now on superconducting Maglev trains have always planned to use YBCO or REBCO oxides as superconducting materials^1,2. While MgB₂ enters more and more applications³, bulk MgB₂ has not yet been seriously considered for Maglev projects. Its main drawback is its T_c that is low as compared to that of HTS oxides. As a consequence, the cooling costs are assumed to be much higher, although cryo-coolers able to provide a high cooling power at 20K are now available⁴. Bulk MgB₂ presents, however, advantages that could offset this inconvenient. Magnesium and Boron are abundant and available worldwide and MgB₂ is a light, low cost and high hardness material. The fabrication process of MgB₂ bulks by Spark Plasma Sintering-SPS⁵ is simpler, takes much less time and is much less expensive than the techniques used for the growth of YBCO or REBCO bulks⁶. In addition, MgB₂ bulks can be made with<i> a priori</i> no limitation on their dimensions and shape.<br/>The non-conventional SPS process is based on the combination of a high current and a mechanical high pressure applied directly to the powder material. This technique presents the advantage to provide dense materials in a few minutes while mastering their microstructure. A lot of works have been reported since the introduction of SPS in research laboratories, and many groups have tried to understand the densification mechanisms involved. After a brief history of the SPS technique, the characteristics of this process will be detailed and compared to those of the other sintering techniques.<br/>The densification, and the functionnal properties of superconducting MgB₂cryo-magnets sintered by SPS will be discussed. Levitation forces up to 400 N at 25 K measured on a 70 mm diameter MgB₂ disc will be reported. The effect of the thickness and the diameter of cylindrical superconductors on the levitation force and the condition for stability in superconducting magnetic levitation will be also analysed.<br/><b>References</b><br/>¹G. G. Sotelo, D. H. N. Dias, E.S. Motta, F.Sass, et al., Physics Procedia <b>36</b> (2012) 943 – 947<br/>²F N Werfel, U Floegel-Delor, R Rothfeld, T Riedel, et al., Supercond. Sci. Technol. <b>25 </b>(2012) 014007<br/>³Yukikazu Iwasa, Physica C <b>445–448</b> (2006) 1088–1094<br/>⁴ https://www.cryomech.com/products/<br/>⁵J. G. Noudem, M. Aburras, P. Bernstein, X. Chaud, M. Muralidhar and M. Murakami, J. Appl. Phys. 116, 163916 (2014).<br/>⁶X. Chaud, S. Meslin, J-G. Noudem, C. Harnois, et al., Journal of Crystal Growth 275 (2005) 855-860.