Papers by Author: Nina Sergienko

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: 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: The critical current density, jc, of high-pressure synthesized MgB2-based balk materials correlates with the amount and distribution of higher borides (MgB12) and Mg-B-O inclusions, which in tern correlates with the synthesis temperature and presence of additions (Ti, Ta, SiC). High-pressure-synthesized materials with near MgB12 composition of matrix exhibited superconducting transition temperature, Tc, of about 37 K, rather high jc (5∙105 and 103 A/cm2 in 0 T and 3.5 T, respectively, at 20 K) and doubled matrix microhardness: 25±1.1 GPa at 4.9 N –load as compared to materials with MgB2).

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.

Abstract: Addition of Ti and Zr to high-pressure (HP) synthesized MgB2 results in an increase of critical current density of the material due to the absorption of impurity hydrogen coming most likely from the materials of a high-pressure cell. The results of the studies of structure, critical current density, trapped field and mechanical characteristics are discussed. High-pressure synthesized MgB2 (with Ti additions) blocks were for the first time used in a SC electromotor at 20 K and demonstrated the efficiency similar to that of MT-YBCO bulk (at the same working temperature).