Papers by Author: Vladimir Sokolovsky

Abstract: Our investigations show that the addition of Ti, polyvalent titanium oxides or TiC powders (0.1-40 mm, amount: 10 wt%) can affect the formation of MgB_x (x³4) inclusions and a redistribution of admixed oxygen resulting in the appearance of dispersed inclusions with near MgBO stoichiometry in the MgB₂ matrix and thus influencing the critical current density and the critical magnetic fields of the materials. The highest B_c2 and B_irr were obtained when powdered polyvalent titanium oxides (synthesized by electroerosion dispersion) or powdered titanium were added, but the critical magnetic fields were somewhat lower in the case of Ti additions. We show that Mg diffuses during the synthesis inside the grains of polyvalent titanium oxide, titanium carbide or titanium, which can affect the redistribution of boron and oxygen in the superconducting matrices and thus influence pinning and the superconducting properties.

Abstract: Fault current limiters (FCL) require superconducting (SC) materials which can provide a definite rate of response to a fault event resulting in the SC – normal state reversible transition. The main characteristics determined the material suitability are the critical current density, j_c, thermal conductivity and capacity which are strongly determined by manufacturing technology, in particular, of MgB₂. In the paper we estimate the j_c of bulk MgB₂ samples by the vibrating magnetometer and inductive, contactless transformer, method using ring samples. The bulk MgB₂ samples were produced under 30 MPa (hot pressing) and 2 GPa (quasihydrostatic pressing) at 800-1050 ^оС from different initial ingredients (Mg and B or MgB₂ with and without additions). It is shown that the technology process and initial ingredients strongly influence the distribution of boron-and oxygen-enriched nanosized inhomogenities in MgB₂ matrix, connectivity between SC grains, material porosity and, as result, the SC properties. The transformer method gives the j_c in the range from 1.6·10⁴ up to 6.3·10⁴ A/cm² at about 4 K while using magnetometer measurements the j_c is estimated from 2.24·10⁵ up to 5.1·10⁵ A/cm² at 10 K in self-magnetic fields. The contradictions in the j_c estimated by different methods can be explained by instability of the SC state of MgB₂, caused by variation of the applied magnetic field. Using the transformer method AC losses per a cycle before quenching for the best materials were estimated around 0.75-1 J/cm³, while the power of losses was about 200 W. The FCL model with rings cut out from SC MgB₂ materials prepared using various technologies demonstrated that MgB₂ is a promising material for application in inductive FCLs.

Abstract: Auger and SEM studies show that with increasing of MgB₂ manufacturing temperature from 600÷800 °C to 1050÷1100 °C the Mg-B-O nanolayers which are present in the MgB₂ matrix transform into distinct dispersed Mg-B-O inclusions. On the other hand the sizes of inclusions of higher magnesium borides (MgB_x, x=7 ÷ 25) which are also present in the MgB₂ matrix. The tendency is observed in a wide range of synthesis pressures (0.1 MPa-2 GPa). The described structural transformations are accompanied by an increase in critical current density, j_c, in low and medium magnetic fields and by transition from the grainboundary to the point pinning. The Ti addition results in a further increase in j_c due to: Ti promotes the formation of higher magnesium boride inclusions and localization (or segregation) of oxygen in MgB₂ matrix, and, hence, facilitates the formation of a homogeneous MgB₂ matrix with lower oxygen content, but with an increased number of Mg-B-O and MgB_x pinning centers. At low synthesis temperature Ti absorbs hydrogen forming titanium hydrides, thus preventing the formation of MgH₂ and provides the material densification. The positive effect of Ti addition is connected with the high ability of Ti to absorb hydrogen, oxygen, and magnesium. The results of the critical current and AC loss study by transformer method using rings from MgB₂ are discussed.

Abstract: The high pressure (50 MPa - 2 GPa) – high temperature synthesized MgB2 bulk materials are characterized by nearly theoretical density (1-2% porosity), 80-98% connectivity, extremely high critical current densities (e.g. at 20 K, in 0–1 T jc=1.3–1.0 MA·cm-2 (with 10% SiC) and jc= 0.92 – 0.73 MA·cm-2 (without doping)), large irreversibility fields (Birr(18.4 K) =15 T and Birr (0 K) = 32.5 T) and high upper critical fields (Bc2 (22 K) =15 T and Bc2(0 K) ~ 42.1 T). The transformation of grain boundary pinning to point pinning in MgB2-based materials with increasing manufacturing temperature (from 800 to 1050 oC) under pressures from 0.1 to 2 GPa correlates well with an increase in critical current density in low external magnetic fields caused by the redistribution of boron and the oxygen impurities in the material structure. As the manufacturing temperature increases, the discontinuous oxygen enriched layers transform into distinct Mg-B-O inclusions and the size and amount of inclusions of higher magnesium borides MgBX (X>4) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC additions.

Abstract: It was experimentally shown that joint application of a solar station and a superconducting flywheel (a kinetic energy storage device) can solve the main problem of renewable energy sources: to provide uninterrupted and controlled power supplying. The storage device changes its operation mode (stand-by, charging, discharging) practically without any delay and its use leads to an increase of the transient stability of a power system.

Abstract: The results of experimental study of the critical current and AC losses in high pressure-synthesized MgB₂ bulk superconductors are presented. The method is based on the use of the transformer configuration and suitable for samples in the form of hollow superconducting cylinders and rings. It was shown that the critical current measured by the transformer method is an order of magnitude lower than the value given by the magnetization technique. The obtained dependencies of the losses on current and frequency reveal the deviations from the critical state model behavior and reflect structural peculiarities of the investigated MgB₂ samples.

Abstract: Superconducting (SC) and mechanical properties of spark plasma (or SPS) produced MgB₂ –based materials allow their efficient applications in fault current limiters, superconducting electromotors, pumps, generators, magnetic bearings, etc. The synthesized from Mg and B at 50 MPa, 1050 °C for 30 min material has a density of 2.52 g/cm³, critical current density, j_c = 7.1•10⁵ A/cm² at 10 K , 5.4 •10⁵ A/cm² at 20 K, and 9•10⁴ A/cm² at 35 K in zero magnetic field; at 20 K its field of irreversibility B_irr(20)=7 T and upper critical field B_c2(20)=11 T; microhardness H_V=10.5 GPa and fracture toughness K_1C =1.7 MPa•m^1/2 at 4.9 N-load. SPS-manufactured in- situ MgB₂-based materials usually have somewhat higher jc than sintered ex-situ. The pressure variations from 16 to 96 MPa during the SPS-process did not affect material SC characteristics significantly; the j_c at 10-20 K was slightly higher and the material density was higher by 11%, when pressures of 50-96 MPa were used. The structure of SPS-produced MgB₂ material contains Mg-B-O inclusions and inclusions of higher borides (of compositions near MgB₄, MgB₇, MgB₁₂, MgB₁₇, MgB₂₀), which can be pinning centers. The presence of higher borides in the MgB₂ structure can be revealed by the SEM and Raman spectroscopy.

Abstract: MgB2-based nanostructural materials with rather high oxygen concentration (5-14 wt.%) and dispersed grains of higher borides (MgB12, MgB7) high-pressure (2 GPa or 30 MPa) synthesized (in-situ) or sintered (ex-situ) demonstrated high superconducting characteristics (critical current density, jc, up to 1.8-1.0106 A/cm2 in the self magnetic field and 103 in 8 T field at 20 K, 3-1.5105 A/cm2 in the self field at 35 K, upper critical field up to HC2 = 15 T at 22 K, field of irreversibility Hirr =13 T at 20 K). The additives (Ti, SiC) and synthesis or sintering temperature can affect the segregation of oxygen and formation of oxygen-enriched Mg-B-O inclusions in the material structure, thus reducing the amount of oxygen in the material matrix as well as the formation of higher borides grains, which affects an increase of the critical current density. The record high HC2 and Hirr have been registered for the material high-pressure (2 GPa) synthesized from Mg and B at 600 oC having 17% porosity and more than 7 wt.% of oxygen. The attained values of the critical current, AC losses and thermal conductivity make the materials promising for application for fault current limiters and electromotors. The structural and superconducting (SC) characteristics of the material with matrix close to MgB12 in stoichiometry has been studied and the SC transition Tc=37 K as well as jc= 5×104 A/cm2 at 20 K in the self field were registered, its Raman spectrum demonstrated metal-like behavior.