On almost-equidistant sets Polyanskii, A.
Linear algebra and its applications,
02/2019, Letnik:
563
Journal Article
Recenzirano
Odprti dostop
A finite set of points in Rd is called almost-equidistant if among any three distinct points in the set, some two are at unit distance. We prove that an almost-equidistant set in Rd has cardinality ...at most 5d13/9.
The K- and Co-doped BaFe(2)As(2) (Ba-122) superconducting compounds are potentially useful for applications because they have upper critical fields (H(c2)) of well over 50 T, H(c2) anisotropy γ < ...2and thin-film critical current densities J(c) exceeding 1 MA cm(-2) (refs 1-4) at 4.2 K. However, thin-film bicrystals of Co-doped Ba-122 clearly exhibit weak link behaviour for 001 tilt misorientations of more than about 5°, suggesting that textured substrates would be needed for applications, as in the cuprates. Here we present a contrary and very much more positive result in which untextured polycrystalline (Ba(0.6)K(0.4))Fe(2)As(2) bulks and round wires with high grain boundary density have transport critical current densities well over 0.1 MA cm(-2) (self-field, 4.2 K), more than 10 times higher than that of any other round untextured ferropnictide wire and 4-5 times higher than the best textured flat wire. The enhanced grain connectivity is ascribed to their much improved phase purity and to the enhanced vortex stiffness of this low-anisotropy compound (γ~1-2) when compared with YBa(2)Cu(3)O(7-x) (γ~5).
Methods of scanning electron microscopy, energy dispersive X-ray spectroscopy (EDS) analysis, and electron backscatter diffraction (EBSD) were used to study the microstructure and phase ...transformations in heatproof copper-nickel alloy
1
. C-shaped diagrams of kinetics of discharge of the metastable γ' phase and stable η phase were constructed, the temperatures of solvus (stability) phases, and the influence of phase composition on technological embrittlement of the alloy were determined. A regime of thermal treatment of the alloy making it possible to obtain semifinished products with a specified microstructure providing maximum strength and plasticity was proposed.
Using a statistical approach, we have estimated the size of water clusters and the number of free molecules participating in transport phenomena. The sizes of clusters and the number of molecules in ...them, as well as the number of free water molecules and their mean free paths, have been determined using experimental data on the temperature dependence of viscosity and density of water. It has been concluded that a temperature of 36.6°C is a special point in the range of 0–100°C. At temperatures higher than this value, the binding energy of a molecule with a cluster decreases abruptly, while the concentration of free molecules sharply increases. Quantitative data have been compared with the parameters of the superplastic state of titanium. As an example, we consider metabolism in functioning of erythrocytes of blood in an adult human.
Understanding new superconductors requires high-quality epitaxial thin films to explore intrinsic electromagnetic properties and evaluate device applications. So far, superconducting properties of ...ferropnictide thin films seem compromised by imperfect epitaxial growth and poor connectivity of the superconducting phase. Here we report new template engineering using single-crystal intermediate layers of (001) SrTiO(3) and BaTiO(3) grown on various perovskite substrates that enables genuine epitaxial films of Co-doped BaFe(2)As(2) with a high transition temperature (T(c,rho=0) of 21.5 K, where rho=resistivity), a small transition width (DeltaT(c)=1.3 K), a superior critical current density J(c) of 4.5 MA cm(-2) (4.2 K) and strong c-axis flux pinning. Implementing SrTiO(3) or BaTiO(3) templates to match the alkaline-earth layer in the Ba-122 with the alkaline-earth/oxygen layer in the templates opens new avenues for epitaxial growth of ferropnictides on multifunctional single-crystal substrates. Beyond superconductors, it provides a framework for growing heteroepitaxial intermetallic compounds on various substrates by matching interfacial layers between templates and thin-film overlayers.
The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a ...low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field-temperature (H-T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The mechanical behavior of a REBCO-coated conductor wire under uniaxial tension is largely determined by the two thickest component layers in the architecture, namely, the substrate and the ...stabilizer. A rather complicated stress-strain relationship is often observed when the composite conductor is under uniaxial tension, which is the result of the differences in elastic modulus and yield stress between the Hastelloy substrate and the electroplated Cu stabilizer. In this paper, the stress-strain relationships of a free standing Cu stabilizer and a bare REBCO wire, i.e., a tape without any stabilizer, were measured. The results are utilized for the calculation of the stress-strain relationships of electroplated Cu stabilized wires using a two-component composite model. Given the thicknesses of the substrate and stabilizer, the calculated results agree well with the measured stress-strain curves, which can be well fitted with the Ramberg-Osgood equation. The tensile strain (stress) dependence of the critical current (I c ) was measured at 77 K in liquid nitrogen. Depending on whether the critical current was measured with the wire under a stress or after unloading, a critical strain (stress) and an irreversible strain (stress) were determined, respectively.
Large-scale superconducting electric devices for power industry depend critically on wires with high critical current densities at temperatures where cryogenic losses are tolerable. This restricts ...choice to two high-temperature cuprate superconductors, (Bi,Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Ox, and possibly to MgB2, recently discovered to superconduct at 39 K. Crystal structure and material anisotropy place fundamental restrictions on their properties, especially in polycrystalline form. So far, power applications have followed a largely empirical, twin-track approach of conductor development and construction of prototype devices. The feasibility of superconducting power cables, magnetic energy-storage devices, transformers, fault current limiters and motors, largely using (Bi,Pb)2Sr2Ca2Cu3Ox conductor, is proven. Widespread applications now depend significantly on cost-effective resolution of fundamental materials and fabrication issues, which control the production of low-cost, high-performance conductors of these remarkable compounds.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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