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[en] Iron-based superconductors (IBSs) with ultrahigh upper critical fields and low anisotropies have attracted significant attention in terms of the novel mechanism of superconductivity and high-field applications. A major concern for practical research is the fabrication of long wires with enhanced critical current density and low cost. In this paper, Cu/Ag composited sheathed Ba0.6K0.4Fe2As2 (Ba122) tapes were fabricated through a hot isostatic pressing method, which is feasible for long-wire manufacturing. The Cu/Ag composite sheath can lower the tape cost by reducing the use of expensive Ag. A high-transport critical current density (J c) up to 5.8 × 104 A cm−2 under 10 T at 4.2 K was achieved in our tapes. Evidence has shown that these tapes have pure Ba122 phase, homogeneous element distribution, orientated grains and good grain connectivity. Our work shows that low-cost Cu/Ag-sheathed IBS tapes have great promise for practical applications in the future. (paper)
[en] Magnetic epitaxial (CoFe2O4)0.1(CeO2)0.9 nanocomposite layers were incorporated into superconducting FeSe0.1Te0.9 thin films as either a cap layer or a buffer layer. Both capped and buffered samples show an enhancement of the superconducting property compared to the reference sample without the incorporated layer, while the capped one shows the best pinning properties of all the samples. Specifically for the capped sample, the critical temperature is ∼12.5 K, while the self-field critical current density increases to as high as 1.20 MA cm−2 at 4 K. Its value shows a slower decrease with increasing applied magnetic field, with the lowest power-law exponent α values (derived following by the plot) of 0.20, 0.23 and 0.33 at 2 K, 4 K and 8 K, respectively. This nanocomposite capped sample also exhibits a high upper critical field of 166 T, which indicates its potential in high field applications. This pinning method provides an effective way of enhancing the superconducting property of iron chalcogenide thin film. (paper)
[en] This article reports the influence of isostatic pressure (from 0.1 MPa to 1.1 GPa), low annealing temperature of 570 ∘C, and annealing time for the formation of high-field pinning centers in 2% C-doped MgB2 wires. Measurements indicate that 1.1 GPa pressure significantly increases the density of high-field pinning centers below 20 K. However, lower pressure (0.6 GPa) slightly increases the density of high-field pinning centers. Increasing the annealing time from 120 to 210 min leads to a reduction of critical temperature (Tc), irreversibility field (Birr), critical current density (Jc), and upper critical field (Bc2), suggesting that a long annealing time leads to a reduction of high-field pinning center density and the number of connections between superconducting grains. The high pressures and low annealing temperature lead to a high critical current density of 1000 A/mm2 in 7.2 T and 100 A/mm2 in 12.5 T at 4.2 K in MgB2 wires.
[en] The transport ac loss Q of an elliptical thin strip of critical current I c with a power-law relation is accurately computed as a function of current amplitude I m and frequency f. The resulting is normalized to following the Norris critical-state formula, and converted to at a critical frequency f c based on a transport scaling law. Having a set of at several values of n as a base, a general expression of is obtained, which can be used to easily calculate for any practical purposes. (paper)
[en] Critical Heat Flux (CHF) data collected at pressures near the critical point of water (i.e., 22.1 MPa) have been compiled from available literature. Variations of these data with experimental conditions (i.e., thermodynamic quality, mass flux, and pressure) were examined. Overall, data obtained recently have exhibited some scatter compared to those obtained before the 90s.The compiled database was applied in assessing the prediction accuracy of a CHF look-up table.Within the applicable range of the CHF look-up table, the data were predicted with an average and a root mean square (rms) errors of 1.7% and 26%, respectively. However, the prediction accuracy deteriorates further when extrapolating beyond the applicability limits of the look-up table. Outside the applicable range, the CHF look-up table over predicted the data with an average and an rms errors of 63% and 66%, respectively. (author)
[en] Highlights: • The effects of small-scale of the both nanoflow and nanostructure on the vibrational response of fluid flowing single-walled carbon nanotubes are investigated. • Critical flow velocity decreases as the wave number increases, employed. • Kn effect has considerable impact on the reduction of critical velocities especially for the air-flow flowing through the CNT. In this study, the effects of small-scale of the both nanoflow and nanostructure on the vibrational response of fluid flowing single-walled carbon nanotubes are investigated. To this purpose, two various flowing fluids, the air-nano-flow and the water nano-flow using Knudsen number, and two different continuum theories, the nonlocal theory and the strain-inertia gradient theory are studied. Nano-rod model is used to model the fluid-structure interaction, and Galerkin method of weighted residual is utilizing to solve and discretize the governing obtained equations. It is found that the critical flow velocity decreases as the wave number increases, excluding the first mode divergence that it has the least value among of the other instabilities if the strain-inertia gradient theory is employed. Moreover, it is observed that Kn effect has considerable impact on the reduction of critical velocities especially for the air-flow flowing through the CNT. In addition, by increasing a nonlocal parameter and Knudsen number the critical flow velocity decreases but it increases as the characteristic length related to the strain-inertia gradient theory increases.
[en] The effect of α-particle irradiation on a NdFeAs(O,F) thin film has been investigated to determine how the introduction of defects affects basic superconducting properties, including the critical temperature T c and the upper critical field H c2, and properties more of interest for applications, like the critical current density J c and the related pinning landscape. The irradiation-induced suppression of the film T c is significantly smaller than on a similarly damaged single crystal. Moreover H c2 behaves differently, depending on the field orientation: for H//c the H c2 slope monotonically increases with increasing disorder, whereas for H//ab it remains constant at low dose and it increases only when the sample is highly disordered. This suggests that a much higher damage level is necessary to drive the NdFeAs(O,F) thin film into the dirty limit. Despite the increase in the low temperature H c2, the effects on the J c(H//c) performances are moderate in the measured temperature and field ranges, with a shifting of the pinning force maximum from 4.5 to 6 T after an irradiation of 2 × 1015 cm−2. On the contrary, J c(H//ab) is always suppressed. The analysis demonstrates that irradiation does introduce point defects (PD) acting as pinning centres proportionally to the irradiation fluence but also suppresses the effectiveness of c-axis correlated pinning present in the pristine sample. We estimate that significant performance improvements may be possible at high field or at temperatures below 10 K. The suppression of the J c(H//ab) performance is not related to a decrease of the J c anisotropy as found in other superconductors. Instead it is due to the presence of PD that decrease the efficiency of the ab-plane intrinsic pinning typical of materials with a layered structure. (paper)
[en] We calculate the energy of threshold fluctuation which triggers the transition of a superconducting current-carrying bridge to the resistive state. We show that the dependence , found by Langer and Ambegaokar for a long bridge with length , holds far below the critical temperature in both dirty and clean limits (here I dep is the depairing current of the bridge and ξ is a coherence length). We also find that even a ‘weak’ local defect (leading to a small suppression of the critical current of the bridge ) provides , typical for a short bridge with or a Josephson junction. (paper)
[en] Nowadays, in situ- and ex situ-processed MgB2 superconducting wires are commercially available. A premix process is a hybrid of the in situ and ex situ processes, in which a mixture of magnesium, boron, and MgB2 powders is used as a precursor. Hence, the premix process should cause less trouble in the manufacturing of practical wires. In this study, to clarify the potential regarding the critical current density of the premix process, the mono-filamentary MgB2 wires are prepared at premix ratios, x p, of 0.25, 0.50, and 0.75 by drawings with a large total area reduction ratio of 99.92%, and their electromagnetic properties and microstructures are investigated. Here, x p is defined as the mass ratio of MgB2 to the total mixture; that is, the composition of the mixture is Mg: B: MgB2 = 1 − x p: 2(1 − x p): x p on a molar basis. Although a high MgB2 packing factor is obtained by the premix process, it does not lead to an increase in electrical connectivity of MgB2 filaments because of the poor sinterability of the premixed MgB2 powder. On the other hand, strong electron scattering, which is introduced to the premixed MgB2 particles in the pulverisation process, leads to the enhancement of the upper critical field in the low temperature region. As a result, the critical current density in a low temperature and high magnetic field region is improved in wires prepared at x p = 0.50. (paper)
[en] In this paper we explore the effects of 3.5 MeV proton irradiation on Fe(Se,Te) thin films grown on CaF2. In particular, we carry out an experimental investigation with different irradiation fluences up to 7.30 · 1016 cm−2 and different proton implantation depths, in order to clarify whether and to what extent the critical current is enhanced or suppressed, what are the effects of irradiation on the critical temperature, resistivity, and critical magnetic fields, and finally what is the role played by the substrate in this context. We find that the effect of irradiation on superconducting properties is generally small compared to the case of other iron-based superconductors. The irradiation effect is more evident on the critical current density Jc, while it is minor on the transition temperature Tc, normal state resistivity ρ, and on the upper critical field Hc2 up to the highest fluences explored in this work. In more detail, our analysis shows that when protons implant in the substrate far from the superconducting film, the critical current can be enhanced up to 50% of the pristine value at 7 T and 12 K; meanwhile, there is no appreciable effect on critical temperature and critical fields together with a slight decrease in resistivity. On the contrary, when the implantation layer is closer to the film–substrate interface, both critical current and temperature show a decrease accompanied by an enhancement of the resistivity and lattice strain. This result evidences that possible modifications induced by irradiation in the substrate may affect the superconducting properties of the film via lattice strain. The robustness of the Fe(Se,Te) system to irradiation-induced damage makes it a promising compound for the fabrication of magnets in high-energy accelerators. (paper)