71 research outputs found
Superconducting joining of melt-textured Y-Ba-Cu-O bulk material
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
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 MBaCuO (M = Y, Nd)
The effect of a short (10-30 min) thermobaric action (in the 1-5 GPa pressure
and 700-1300C temperature range) on the structure, superconductive and
mechanical properties of melt-textured-MBaCuO (M=Y, Nd) or
MT-MBCO have been studied. The existence has been established of
pressure-temperature--time conditions (2 GPa, 800C for 30 min and
900-950C for 15 min for MT-YBCO; 5 GPa, 850-900C 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
Analysis of XRD patterns by Rietveld refinement has been shown that the main phase of superconducting MgB-based bulk materials (with high level of superconducting characteristics) has AlB type structure and near MgBO stoichiometry. The materials demonstrated the critical current densities up to 0.9 – 0.4 MA/cm jc (at 0 - 1 T, 20 K); up to 15 T B (at 22.5 K) and B (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 MgBO; (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 MgB 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, T, for MgBO occurred to be 23.3 K, while for MgB it was 21.13 K only. Experimental T of the bulk materials was 35.7-38.2 K
Pinning and trapped field in MgB2- and MT-YBaCuO bulk superconductors manufactured under pressure
The relevant pinning centers of Abrikosov vortices in MgB–based materials are oxygen-enriched Mg-B-O inclusions or nanolayers and inclusions of MgB (x>4) phases. The high critical current densities, j, of 10 and 10A/cm 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 MgB structure in small amounts (MgBO), which is supported by Auger studies and calculations of the DOS and the binding energy. The j of melt textured YBaCuO (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 YBaCuO (Y211) inclusions in Y123. At 77 K j=8⋅10 A/cm in self-field and j=10 A/cm 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 MgB 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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