2694 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011 High-Pressure Synthesized Nanostructural MgB Materials With High Performance of Superconductivity, Suitable for Fault Current Limitation and Other Applications Tatiana A. Prikhna, Wolfgang Gawalek, Wilfried Goldacker, Yaroslav M. Savchuk, Jacques Noudem, Alexander Soldatov, Mikhael Eisterer, Harald W. Weber, Vladimir Sokolovsky, Maxim Serga, SergeyN. Dub, Michael Wendt, Shujie You, Nina V. Sergienko, Viktor E. Moshchil, Vasiliy N. Tkach, Jan Dellith, Friedrich Karau, Mikhael Tomsic, Christa Shmidt, Igor P. Fesenko, Tobias Habisreuther, Doris Litzkendorf, Viktor Meerovich, and Vladimir B. Sverdun Abstract—A variety of samples made via different routes were investigated. Samples are nanostructured (average grain sizes are about 20 nm). The advantage of high-pressure (HP)-manu- factured (2 GPa, 800–1050 C, 1 h) MgB bulk is the possibility to get almost theoretically dense (1–2% porosity) material with very high critical current densities reaching at 20 K, in 0–1 T A/cm (with 10% SiC doping) and A/cm (without doping). Mechan- ical properties are also very high: fracture toughness up to MPa m and MPa m at 148.8 N load for MgB undoped and doped with 10% Ta, respectively. The HP-synthesized material at moderate temperature (2 GPa, 600 C, 1 h) from B with high amount of impurity C (3.15%) and H (0.87%) has A/cm in 8 T field at 20 K, highest irreversibility fields (at 18.4 K T) and upper critical fields (at 22 K T) but 17% porosity. HP materials with stoichiometry near MgB can have K and A/cm at 0 T and T at 20 K. The spark plasma synthesized (SPS) material (50 MPa, 600–1050 C 1.3 h, without additions), Manuscript received August 06, 2010; accepted November 22, 2010. Date of publication January 06, 2011; date of current version May 27, 2011. This work was supported in part by the Projects STCU 3665, of Ukrainian-German Cooperation IB/BMBF UKR 06/004, Ukrainian-Austrian Cooperation OEAD UA 13/2009 (M12-2009), Ukrainian-French Cooperation “Dnipro” No1969856 XF (M15-2009), H.C. Starck GmbH, Germany. T. A. Prikhna, Ya. M. Savchuk, S. N. Dub, M. Serga, N. V. Sergienko, V. E. Moshchil, V. N. Tkach, I. P. Fesenko, and V.B. Sverdun, are with the Institute for Superhard Materials of the National Academy of Sciences of Ukraine, Kiev 07074, Ukraine (e-mail: prikhna@mail.ru; prikhna@iptelecom.net.ua). W. Gawalek, M. Wendt, J. Dellith, Ch. Shmidt, T. Habisreuther, and D. Litzk- endorf are with Institut für Photonische Technologien, Jena, D-07745, Germany (e-mail: gawalek@ipht-jena.de). W Goldacker is with Karlsruhe Institute of Technology (KIT), 76344 Eggen- stein, Germany (e-mail: wilfried.goldacker@kit.edu). J. Noudem is with CNRS/CRISMAT/ISMRA, CNRS UMR 6508, 14050 Caen, France (e-mail: jacques.noudem@ensicaen.fr). A. Soldatov and Sh. You are with Luleå University of Technology, Depart- ment of Applied Physics & Mechanical Engineering, SE-971 87 Luleå, Sweden (e-mail: Alexander.Soldatov@ltu.se). H. W. Weber and M. Eisterer are with TU Wien-Atominstitut, Vienna Uni- versity of Technology Institute of Atomic and Subatomic Physics, 1020 Wien, Austria (e-mail: weber@ati.ac.at). V. Sokolovsky and V. Meerovich are with Ben-Gurion University of the Negev, Beer-Sheva 8410,5 Israel (e-mail: sokolovv@bgu.ac.il). F. Karau is with H.C. Starck GmbH, Goslar 38642, Germany (e-mail: Friedrich.Karau@hcstarck.com). M. Tomsic is with Hyper Tech Research, Inc., Columbus, OH 43212 USA (e-mail: tomsic@voyager.net). Digital Object Identifier 10.1109/TASC.2010.2096494 demonstrated at 20 K, in 0–1 T A/cm . Dispersed inclusions of higher magnesium borides, which are usually present in MgB structure and obviously create new pinning centers can be revealed by Raman spectroscopy (for the first time a spectrum of MgB was obtained). Tests of quench behavior, losses on MgB rings and material thermal conductivity show promising properties for fault current limiters. Due to high critical fields, the material can be used for magnets. Index Terms—Boron compounds, FCL behavior, magnetic vari- able measurement, pressure effects, Raman spectroscopy, super- conducting material growth. I. INTRODUCTION T HE coherence length of MgB is in the range of 6–12 nm. Therefore, the clean grain boundaries are not the obsta- cles for superconductive (SC) current flow. It is considered that smaller grains (or larger intergrain surface area) are promoting flux pinning. Obviously the denser material with clean grain boundaries should exhibited a higher critical current density and should be more stable against degradation. Besides, pinning can be improved by different additions (SiC, C, etc.). In our previous studies it was shown that an increase of can be realized by the presence of fine dispersed higher magnesium borides and Mg-B-O inclusions (oxygen enriched compared to the surrounding MgB or Mg-B-O matrix) [1]–[3] and that addi- tion of Ti or Ta together with a change of synthesis temperature can promote their formation. In addition, it has been observed that the quality of initial boron or magnesium diboride plays a crucial role in achieving improved superconducting properties in MgB and oxygen distribution in the material structure plays a more important role than the total oxygen content in the ma- terial. In this paper the MgB microstructure, pressure effect during synthesis, remaining porosity, and role of C, H, and N impu- rities in the initial boron powders is discussed and correlated with the variation of the sample properties, microstructure and superconductivity, due to different pressures and manufacturing methods. As regards applications, it is known and proved that MgB wires can be used for resistive type fault current limiters (FCL). Here we present some results of quench behavior and AC losses 1051-8223/$26.00 © 2011 IEEE