50 research outputs found

    Superconducting joining of melt-textured Y-Ba-Cu-O bulk material

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    The Tm-Ba-Cu-O solder can be successfully used to produce a superconductive joint between MT-YBCO parts. The peculiarities of solidification, phase formation, structure transformations and electromagnetic properties of MT-YBCO soldered with TmBa2Cu3O7-d are discussed.Comment: PS of 6 pages text and 5 figures, presented at ICMC'2000, Brasi

    MgBâ‚‚-based superconductors for fault current limiters

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    A promising solution of the fault current problem in power systems is the application of fast-operating nonlinear superconducting fault current limiters (SFCLs) with the capability of rapidly increasing their impedance, and thus limiting high fault currents. We report the results of experiments with models of inductive (transformer type) SFCLs based on the ring-shaped bulk MgB2 prepared under high quasihydrostatic pressure (2 GPa) and by hot pressing technique (30 MPa). It was shown that the SFCLs meet the main requirements to fault current limiters: they possess low impedance in the nominal regime of the protected circuit and can fast increase their impedance limiting both the transient and the steady-state fault currents. The study of quenching currents of MgB2 rings (SFCL activation current) and AC losses in the rings shows that the quenching current density and critical current density determined from AC losses can be 10-20 times less than the critical current determined from the magnetization experiments

    Thermobaric Effect on Melt-Textured MBa2_2Cu3_3O7−δ_{7-\delta} (M = Y, Nd)

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    The effect of a short (10-30 min) thermobaric action (in the 1-5 GPa pressure and 700-1300∘^\circC temperature range) on the structure, superconductive and mechanical properties of melt-textured-MBa2_2Cu3_3O7−δ_{7-\delta} (M=Y, Nd) or MT-MBCO have been studied. The existence has been established of pressure-temperature--time conditions (2 GPa, 800∘^\circC for 30 min and 900-950∘^\circC for 15 min for MT-YBCO; 5 GPa, 850-900∘^\circC for 15 min for MT-NdBCO) the treatment under which allows superconductive properties of the materials (because of the contact with zirconia and high pressure) to be preserved or improved, mechanical characteristics increased and the materials condensed.Comment: 2 pages (tex), 2 figures (eps), to be presented at LT2

    Structure and properties of MgB2bulks: Ab-initio simulations compared to experiment

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    Analysis of XRD patterns by Rietveld refinement has been shown that the main phase of superconducting MgB2_{2}-based bulk materials (with high level of superconducting characteristics) has AlB2_{2} type structure and near MgB1.8−1.68_{1.8-1.68}O0.2−0.32_{0.2-0.32} stoichiometry. The materials demonstrated the critical current densities up to 0.9 – 0.4 MA/cm2^{2} jc (at 0 - 1 T, 20 K); up to 15 T Bc2_{c2} (at 22.5 K) and Birr_{irr} (at 18 K). The ab-initio simulation confirmed (1) benefits in binding energy and enthalpy of formation if stoichiometry of the solid solution is near MgB1.75_{1.75}O0.25_{0.25}; (2) energetic advantage in case if impurity oxygen present only in each second boron plane of MgB2 cell while the first boron plane of the same cell stays pristine and location of substituted oxygen atoms in the nearby positions. Besides, the results of ab-initio modeling allow explanation of the tendency towards segregation of O-impurity in MgB2_{2} structure during synthesis or sintering, and formation of Mg-B-O inclusions or nanolayers (with MgO type of structure) which effect pinning. Calculated transition temperatures, Tc_{c}, for MgB1.75_{1.75}O0.25_{0.25} occurred to be 23.3 K, while for MgB2_{2} it was 21.13 K only. Experimental Tc_{c} of the bulk materials was 35.7-38.2 K

    Pinning and trapped field in MgB2- and MT-YBaCuO bulk superconductors manufactured under pressure

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    The relevant pinning centers of Abrikosov vortices in MgB2_{2}–based materials are oxygen-enriched Mg-B-O inclusions or nanolayers and inclusions of MgBx_{x} (x>4) phases. The high critical current densities, jc_{c}, of 106^{6} and 103^{3}A/cm2^{2} at 1 and 8.5 T, respectively, at 20 K can be achieved in polycrystalline materials (prepared at 2 GPa) containing a large amount of admixed oxygen. Besides, oxygen can be incorporated into the MgB2_{2} structure in small amounts (MgB1.5_{1.5}O0.5_{0.5}), which is supported by Auger studies and calculations of the DOS and the binding energy. The jc_{c} of melt textured YBa2_{2}Cu3_{3}O7−δ_{7-δ } (or Y123)-based superconductors (MT-YBaCuO) depends not only on the perfectness of texture and the amount of oxygen in the Y123 structure, but also on the density of twins and micro-cracks formed during the oxygenation (due to shrinking of the c-lattice parameter). The density of twins and microcracks increases with the reduction of the distance between Y2_{2}BaCuO5_{5} (Y211) inclusions in Y123. At 77 K jc_{c}=8⋅104^{4} A/cm2^{2} in self-field and jc_{c}=103^{3} A/cm2^{2} at 10 Т were found in materials oxygenated at 16 MPa for 3 days with a density of twins of 22–35 per μm (thickness of the lamellae: 45-30 nm) and a density of micro-cracks of 200–280 per mm. Pinning can occur at the points of intersection between the Y123 twin planes and the Y211 inclusions. MTYBaCuO at 77 K can trap 1.4 T (38×38×17 mm, oxygenated at 0.1 MPa for 20 days) and 0.8 T (16 mm in diameter and 10 mm thick with 0.45 mm holes oxygenated at 10 MPa for 53 h). The sensitivity of MgB2_{2} to magnetic field variations (flux jumps) complicates estimates of the trapped field. At 20 K 1.8 T was found for a block of 30 mm in diameter and a thickness of 7.5 mm and 1.5 T (if the magnetic field was increased at a rate of 0.1 T) for a ring with dimensions 24×18 mm and a thickness of 8 mm
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