Nanostructural Superconducting Materials for Fault Current Limiters and Cryogenic Electrical Machines

Abstract

Materials of the Y-Ba-Cu-O (melt-textured YBa2Cu3O 7−δ -based materials or MT-YBCO) and Mg-B-O (MgB2-based materials) systems with high superconducting performance, which can be attained due to the formation of regularly distributed nanostructural defects and inhomogeneities in their structure can be effectively used in cryogenic technique, in particular in fault current limiters and electrical machines (electromotors, generators, pumps for liquid gases, etc.). The developed processes of high-temperature (900-800• C) oxygenation under elevated pressure (16 MPa) of MT-YBCO and high-pressure (2 GPa) synthesis of MgB2-based materials allowed us to attain high superconductive (critical current densities, upper critical fields, fields of irreversibility, trapped magnetic fields) and mechanical (hardness, fracture toughness, Young modulus) characteristics. It has been shown that the effect of materials properties improvement in the case of MT-YBCO was attained due to the formation of high twin density (20-22 µm −1 ), prevention of macrocracking and reduction (by a factor of 4.5) of microcrack density, and in the case of MgB2-based materials due to the formation of oxygen-enriched as compared to the matrix phase fine-dispersed Mg-B-O inhomogeneities as well as inclusions of higher borides with near-MgB12 stoichiometry in the Mg-B-O matrix (with 15-37 nm average grain sizes). The possibility is shown to obtain the rather high Tc (37 K) and critical current densities in materials with MgB12 matrix (with 95% of shielding fraction as calculated from the resistant curve).

AC losses in high pressure synthesized MgB2bulk rings measured by a transformer method

“…It is expected that bulk MgB 2 -based materials will be able to successfully compete with high-temperature bulk superconductors in fault current limiters, electrical motors, generators, superconducting magnets, passive levitated bearings and shields for AC and DC magnetic fields [1][2][3][4][5][6][7][8]. Recently developed manufacturing technologies use high pressure and various doping additions to prepare bulk MgB 2 materials with a high critical current density J c .…”

Effect of Nanostructural Inhomogeneities on the Superconducting Characteristics of <inline-formula> <tex-math notation="TeX">$\hbox{MgB}_{2}$</tex-math></inline-formula> With Enhanced Grain Connectivity

“…The method of preparation (pressure-temperature-time conditions) can affect the stoichiometry of MgB x inclusions, their amount and sizes [6]. The MgB x inclusions in the materials prepared at 2 GPa have near MgB 12 stoichiometry, are well crystallized and their nanohardness and Young modulus are higher than that of sapphire [3], [8]. The presence of higher borides in MgB 2 cannot be revealed by traditional X-ray diffraction analysis due to their fine dispersion in the matrix and the large number of atoms in their unit cells of low symmetry resulting in many "reflecting planes", which essentially reduce the intensities of the X-ray reflections as compared to those of MgB 2 having a simple hexagonal unit cell.…”

MgB2-based superconductors for fault current limiters

“…A low ratio of the primary coil inductances in the current limitation regime and nominal one, 0.09/0.28≈3.2, is explained by peculiarities of the design of the magnetic system for the big ring: ratio of the height of the magnetic core to its diameter was about only 0.39. For more realistic SFLC model with other rings the inductance ration increases up to [5][6][7]. In liquid helium all tested models returned into an initial state for 1-2 s. A quenching current in rings, I q , changes in the range of 4,500 ÷ 24,000 A, corresponding to the quenching current density J q = 19,000÷63,200 A/cm 2 .…”

“…Devices based on the superconductor magnesium diboride MgB 2 with critical temperature T c = 39 K can operate at temperatures of liquid helium, hydrogen or even neon. This compound can thus be envisaged for many applications, as fault current limiters, MRI magnets, cables, electromotors, magnetic bearings, etc [2,[5][6][7]. Note that hydrogen is already used for cooling conventional generators in electrical power stations.…”

“…Magnesium diboride is cheaper than the high temperature Y-and Bi-based superconductors (the technological process is cheaper); superconductivity can be realized within a wide range of Mg-B stoichiometries and some impurities do not impede to achieve high critical currents; the compounds do not contain toxic elements (such as As and F in iron-based superconductors). A high critical current densities j c and good mechanical properties can be achieved in bulk MgB 2 samples synthesized under high quasihydrostatic pressure (at 2 GPa), spark plasma sintering (at 50 MPa) or by hot pressing (at 30 MPa) [6,8].…”