Flux Flow Models Research Articles

Thermal effects associated with the crack of high-temperature superconducting magnetic levitation system

Based on the magnetic flux pinning characteristics of the non-ideal type II superconductor YBa2Cu3O7−x, the high-temperature superconducting magnetic levitation system has the advantages of self-stability in levitation and low energy consumption. Thermal stress, electromagnetic force, and other mechanical stress may cause the micro-cracks to expand and eventually lead to fractures in the application of superconducting materials, significantly affecting the superconductor’s ability to transmit current. The superconducting magnetic levitation system with low damping is prone to nonlinear vibration of large amplitude under external interference, which affects the system’s regular operation. Due to the limitations of experimental conditions, it is difficult to analyze complex physical phenomena with cracks and obtain the distribution characteristics of electromagnetic, heat, and force inside the superconductor in the nonlinear vibration process, as theoretical modeling can compensate for this deficiency. In this paper, we study the fracture behavior of the YBa2Cu3O7−x bulk superconductor under nonlinear vibration based on the flux creep and flow models. The temperature of the superconductor in the nonlinear vibration of the superconducting levitation system is calculated with center cracks. The flux flow phenomenon in the bulk superconductor for various cracks under the bifurcation vibration is presented. The results show that the temperature of the superconductor will dramatically rise in nonlinear vibration under thermal insulation conditions, and the distribution position is affected by the cracks. For the 15 and 12 mm center cracks, a large amount of heat is generated around the crack and causes the temperature to rise above the critical temperature.

Effect of thickness on the superconducting properties of the thin films of FeTe0.55Se0.45 deposited on YSZ substrates using Pulsed laser deposition (PLD) technique

We report the successful fabrication of thin films of thickness ∼26 nm, 59 nm, 76 nm, and 158 nm of the composition FeTe0.55Se0.45 on the (100)-oriented Yttria-stabilized zirconia (YSZ) substrate using the pulsed laser deposition (PLD) technique. The effect of thickness on the structure and superconducting properties of the thin films has been investigated. X-ray diffraction study reveals the growth of thin films along the c-axis. The temperature-dependent resistivity measurements confirm superconductivity in all the fabricated thin films, and the highest ∼ 16.2 K has been observed for the thin film of thickness ∼59 nm, which is higher than the corresponding value of the polycrystalline target sample. The superconducting parameters such as upper critical magnetic field coherence length thermally activated energy (TAE) U0, and vortex phase state have been measured by magnetotransport measurements near in the different applied magnetic fields up to 12 T. The TAE values determined from the conventional and modified thermally activated flux flow (TAFF) models are discussed and compared. A crossover from 2-dimensional (2D) vortex to 3-dimensional (3D) vortex behaviour has been observed. The vortex phase diagrams are used to observe the transition from the vortex liquid to the vortex glass state in the grown thin films. Moreover, magnetoresistance (MR) measurements show non-saturating linear magnetoresistance (LMR) in the high field region, indicating the presence of the possible topological state in the grown thin films. The values of magnetic field dependent transport critical current density, have been evaluated from the current-voltage (I-V) curves. Furthermore, the normalised pinning force density has been used to characterize the pinning processes using the Dew Hughes’ scaling rule which reveals that the grown thin films contain δl-surface pinning centers.

Thermally activated flux mechanism in Mg-doped InN epitaxial film

The superconducting behaviour of InN has been observed in many experiments where the origin of superconductivity is addressed to presence of (i) In–In chains in ab-plane, (ii) specific carrier density range limited Mott transition critical carrier density and (iii) presence of In2O3 impurities. Although the superconductivity can be observed when the above conditions are enough for epitaxial grown InN films, the superconductivity properties of InN, so far, have not worked comprehensively. Here, we report the magneto-resistance, upper critical field and thermally activated flux mechanism of superconductor Mg doped InN epitaxial film grown by Molecular Beam Epitaxy. The superconducting phase transition temperature was observed at ~3.9 K at zero magnetic field. The carrier density of the film is found in the range of Mott transition and superconductivity to metal transition. The effect of magnetic field on the superconductivity of Mg-doped InN film is studied by employing the magnetoresistance and Hall resistance measurement with a typical Hall-bar shape device. The magnetoresistance analysis has been carried out by flux-flow and flux-creep models. The activation energy is found as highly sensitive with field in a range of 0.0 to 1.0 T. The upper critical field at zero temperature and coherence length estimated by Ginzburg–Landau relation were found as around 0.8 T and 216.9 Å, respectively. The superconducting properties of the epitaxial growth Mg-doped InN film are discussed through the manuscript.

Magnetic shielding analysis of high-T/sub c/ superconducting plates by power law, flux-flow, and flux-creep models

The magnetic shielding performance of a high-T/sub c/ superconducting (HTS) plate is investigated numerically. By taking account of the crystallographic anisotropy in the shielding current density of the melt-powder-melt-growth YBa/sub 2/Cu/sub 3/O/sub x/ plate, the multiple-layer structure is introduced to modelize the HTS plate. In this case, the shielding current density is governed by the integral-differential equation of the scalar potential. In addition, the power law is used as the J-E constitutive equation for description of the characteristics of the Type-II superconductor. When the equation is descretized by using the finite element method and the weighted average method, the resulting algebraic equation is solved by using the decelerated Newton method. A numerical code for analyzing the time evolution of the shielding current density has been developed and, by use of the code, the shielding performance of HTS plates is investigated. In addition, the results obtained by using the power law, the flux-flow and the flux-creep models are compared.

Distributed steady and dynamic modelling of dry-expansion evaporators: Modèlisation du régime stable et du régime transitoire des évaporateurs à détente sèche

A general distributed parameter model is presented to describe both steady and dynamic behaviors of dry-expansion evaporators. The homogeneous and three different non-homogeneous two-phase flow models are used to evaluate the impact of different flow models on the accuracy of the simulation. The experimental work was carried out on a full-scale refrigeration system with R-134a as the working fluid and without frost formation at the evaporator. Comparison between the modelling and experimental measurements shows that the drift flux flow models give satisfactory predictions. The simulation results indicate that an even air temperature distribution off the evaporator may be obtained by controlling liquid dry-out point at the two ends of the coil. The study also indicates that the counterflow configuration provides a higher heat exchange efficiency with a slower transient response compared with the cocurrent-flow configuration. A general distributed parameter model is presented to describe both steady and dynamic behaviors of dry-expansion evaporators. The homogeneous and three different non-homogeneous two-phase flow models are used to evaluate the impact of different flow models on the accuracy of the simulation. The experimental work was carried out on a full-scale refrigeration system with R-134a as the working fluid and without frost formation at the evaporator. Comparison between the modelling and experimental measurements shows that the drift flux flow models give satisfactory predictions. The simulation results indicate that an even air temperature distribution off the evaporator may be obtained by controlling liquid dry-out point at the two ends of the coil. The study also indicates that the counterflow configuration provides a higher heat exchange efficiency with a slower transient response compared with the cocurrent-flow configuration.

Application of infrared diode laser absorption spectroscopy to the F4 high enthalpy wind tunnel

A tunable diode laser spectrometer has been used to probe the ONERA-F4 arc-driven high-enthalpy wind tunnel. By fast wave number tuning of the laser beam passing through the flow, free-stream measurements from NO or H 2O absorption lines can be made at high repetition rates (1 kHz) in the infrared domain near 5 μm. Temperature and density measurements are obtained from the absorption line shapes, and the flow velocity is deduced from the Doppler shift at the line centre position. This line-of-sight technique gives averaged values over the absorption path through the flow. Herein we report results concerning medium and high enthalpy runs, where velocities inferred from the Doppler shifts range between 2500 and 4500 m/s. The results are also compared to calculations from heat flux probes measurements and flow models.

Mechanical stability of a high-T/sub c/ superconducting levitation system

This research deals with dynamics of a permanent magnet freely levitated above an excited high-T/sub c/ superconductor. Vertical restoring forces acting on the magnet were evaluated by analysis based on the critical state and flux flow models. The results show nonlinearity and frequency-dependence of the forces, which were observed in experiments. Vertical vibration properties of the system were also evaluated analytically by considering the above properties of the restoring forces. The results show good agreement with experimental ones, which include effect of the frequency-dependent stiffness on the resonant frequency, the primary resonance characteristics, stability of the steady-state motions and so on.

Effect of the Lorentz force on dissipation in YBa 2Cu 3O x thin films

The current-voltage characteristics of YBa 2Cu 3O x , (YBCO) thin films have been measured, with both current and magnetic field applied parallel to the ab-plane, to study the effect of the Lorentz force on dissipation. The angle (θ) between the field and the current is varied to obtain the angular dependence of the I–V curve. A power law behavior for the I–V curves is observed for different fields and θ values studied. The results cannot be explained on the basis of the existing flux flow and flux creep models. A model for the vortex motion has been developed to explain the results.

AC magnetic susceptibility measurements near the superconducting transition temperature of YBCO sintered samples

The temperature dependence of the ac susceptibility of sintered YBCO pellets is experimentally analysed in detail few Kelvin below the transition temperature. The quite complex dependence of χ ′ and χ ″ on the frequency and the amplitude of the ac magnetic field cannot be explained by an ohmic Flux Flow or Critical State models, so that intermediate regimes or different non linear phenomena are to be considered.

Flux-motion related AC losses in high T/sub c/ superconductors

The AC losses of high-temperature superconductors in the flux-depinned mixed-state have been treated using the classical magnetic diffusion equation in conjunction with various models of flux-motion. With the imaginary part representing the AC losses, the field- and frequency-dependent AC susceptibilities were investigated. The imaginary component was found to obey a scaling rule with a characteristic frequency, estimated to be approximately 10/sup 5/-10/sup 9/ Hz, that depended on the sample size and normal state resistivity. This frequency range agrees with earlier experimental results, which could not be accounted for previously based upon the notion of thermally activated hopping of vortices. The frequency scaling behaviors using flux-creep and flux-flow models are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Log to non-log variation of magnetic relaxation: studies in steady and varying fields

Magnetic relaxation studies have been performed in low fields for small times (t<or=300 s). Two separate cases are reported. In the first the relaxation is observed in constant applied fields, while in the second, the 'relaxation' due to a steady but small field decrease has been studied. For the case of steady fields, a logarithmic time variation is observed, while in decreasing fields the variation is of a stretched exponential type. Effects of H and H on the relaxation are also discussed. The data are analysed in the spirit of the critical state and thermally activated flux flow models.

Superconducting fluctuation effects on the magnetoconductivity in single-crystalline YBa 2Cu 3O 7−δ} and Bi 2Sr 2CaCu 2O 8+δ}

Precise resistivity measurements in magnetic fields up to 6 T have been performed on single crystalline YBa 2 Cu 3 O 7−δ and Bi 2 Sr 2 CaCu 2 O 8+δ samples. The results show that the resistivity change becomes anomalously broad over a wide temperature region around T c in both cases in a similar fashion; the broadening is most significant when the magnetic field is applied along the crystallographic c -axis, irrespective of the relative orientation between the current and the magnetic field. This behavior is discussed in the superconducting fluctuation model and associated with the extremely short coherence length in highly two dimensional nature of the electronic structure. The broadening cannot be ascribed to the traditional Lorentz force driven mechanisms such as flux flow and flux creep models.

Anomalous Hall effect in superconductors near their critical temperatures.

Measurements on epitaxial ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ thin films show that the Hall voltage just below the superconducting transition temperature has sign opposite to the Hall voltage in the normal state. This can result either from quasiparticle effects, or from unusual vortex motion. Because we observe a similar effect in Nb thin films, we argue that vortex motion is responsible. A simple interpretation of the Hall effect then requires that the vortex velocity have a component antiparallel to the transport current. This motion, while contrary to existing flux flow models, could result from vortex motion damping similar to that expected in superfluid $^{4}\mathrm{He}$.

Transport-critical-current behaviour and grain boundary microstructure in YBa2Cu3Ox

The microstructures of polycrystalline YBa2Cu3Ox samples were varied through different sintering mechanisms to study transport-current characteristics in magnetic fields. TEM experimental results indicate that the transport critical current in polycrystalline YBa2Cu3Ox is closely correlated with grain boundary microstructures. Specifically, the Jc(H) is severely suppressed as the grain boundaries are contaminated with the second phases. The field required for the Jc(H) values to decrease to zero is also strongly affected by the grain boundary microstructure. The critical-current behaviour and closely related microstructural changes are explained with Josephson weak-link and flux-flow models.

Empirical formula for the measured hall angles of dirty type II superconductors

Abstract Measurements of the magnetic field and current dependence of the Hall angle in the mixed state of Pb-Bi alloys are presented. These and other measurements published in literature are analyzed in terms of the flux flow models where the mixed state Hall angle equals the “normal” state Hall angle in the vortex core. It is found that the values of the electron relaxation time in the core, needed to fit the measured data, are in excellent agreement with the superconducting order parameter relaxation time τ g = h /πΔ (which in dirty type II superconductors is orders of magnitude larger than the bulk normal relaxation time). The modification of the electron density, now needed to fit the resistivity of the quasinormal core, is in agreement with the requirements for a local description and with the Ginzburg-Landau equations.