Critical Current Density Of Superconductor Research Articles

Nonlinear motion characteristics of the high-temperature superconducting levitation system with boundary crack

High-temperature superconducting levitation systems have nonlinear behaviors, such as the period-doubling bifurcation and chaotic vibration under external excitation, in connection with the nonlinear hysteresis interaction between the high-temperature superconductors (HTSC) and permanent magnet. The HTSC is a non-ideal type II superconductor in which the fracture is generated internally during manufacturing, and the material properties are brittle. The HTSC tends to crack under a strong magnetic field and electromagnetic force, and its internal defects are likely to cause structural damage with the variable Lorentz force. The fracture performance of the superconducting magnetic levitation system will affect the temperature variation of HTSC, in which the temperature is coupled with the internal electromagnetic force. In this paper, we analyze the fracture characteristics of the HTSC in nonlinear vibration with thermal effect. A superconducting magnetic levitation system model with boundary cracks is applied to study the coupling of multiple physical fields in dynamic processes. The Maxwell equation and superconducting electromagnetic constitutive equation are used to obtain the superconductor’s current density, and the temperature is calculated through the heat conduction equation. The superconducting magnetic flux flow and creep model is applied to analyze the magnetic flux motion inside the superconductors. We compare the superconducting levitation system’s electromagnetic force, temperature, and current density with four critical current densities. The numerical results show that the critical current density of superconductors significantly impacts bifurcation motion, and the temperature of the crack tip of the superconductors varies greatly during vibration, causing the superconductor to lose its superconductivity.

Large-scale vibrating coil magnetometer for the magnetic characterization of bulk superconductors

This work presents the design and validation of a vibrating coil magnetometer for the characterization of the field dependence of the critical current density of centimeter-sized bulk superconductors as an alternative to the destructive methods typically used. The magnetometer is also shown to be capable of measuring the magnetic moment in an applied field of up to 5 T for diverse magnetic materials, such as soft and hard ferromagnets and high-temperature superconducting pellets. The vibrating coil magnetometer was first optimized using finite element simulations and calibrated using a commercial vibrating sample magnetometer. The vibrating coil magnetometer was benchmarked with hysteresis measurements of a Nd2Fe14B disk made with a commercial hysteresisgraph, showing good agreement between the different setups. The magnetic hysteresis of a YBa2Cu3O7−x superconducting pellet was measured at 77 K, showing a penetration field of 1 T and an irreversibility field of 4 T. The field dependent critical current density of the superconductor was then inferred from the magnetic hysteresis measurements and extrapolated at low fields. Finally, the resulting critical current density was used to successfully reproduce the measured magnetization curve of the pellet at 2 T with finite element simulations.

Thermally activated flux flow and inter-granular coupling properties in superconducting (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1−x (ZnO NPs)x composites

In high-temperature ceramic superconductors, vortices motion is induced by the strong thermal fluctuations because of the thermally activated flux flow (TAFF). The TAFF impedes the transport properties and critical current density of superconductors. It has been reported that adding nano-scale impurities can induce artificial pining centers that may improve inter-granular connections and flux pinning strength in ceramic superconductors. Here, the effects of different amounts (0.0–1.0 wt%) of ZnO nanoparticles on the TAFF behavior and zero temperature activation energy of the Bi1.6Pb0.4Sr2Ca2Cu3O10+δ superconducting phase have been studied using the modified TAFF model. Moreover, the impacts of the additive on the inter-granular traits and the Josephson coupling energy of the superconducting phase have been investigated using AC susceptibility measurements. Vortex phases analysis indicates that all composites show a vortex glass to vortex liquid phase transition at Tg. The vortex liquid phase is divided into the critical region existing in a finite temperature region just above Tg and the TAFF region present in the finite temperature region above it. It was found that the TAFF region is shifted to the higher temperatures and gets narrower, as the ZnO nanoparticles concentration enhances from 0.0 to 0.2 wt%. The vortex glass to vortex liquid transition temperature, Tg, increases from 93.8 K for the sample without additive to 101.0 K for the composite with 0.2 wt% ZnO nanoparticles. In addition, the zero-temperature activation energy (U0/KB) increases from ~0.4 × 105 K for the sample without additive to ~1.4 × 105 K for the composite with 0.2 wt% ZnO nanoparticles and then decreases for more ZnO concentrations. Moreover, it was found that the Josephson coupling energy Ej increases from ~0.039 eV for the sample without additive to ~0.136 eV for the composite with 0.2 wt% ZnO nanoparticles. These results point out a significant enhancement of the activation energy, flux pinning capability, and inter-granular coupling of the Bi1.6Pb0.4Sr2Ca2Cu3O10+δ superconducting phase with the addition of the 0.2 wt% ZnO nanoparticles.

Features of the thermal stabilization theory of composite superconductors: The nonlinear description of the dissipative states

The features of the Joule's energy dissipation in a composite high-temperature superconductor with a power equation of the voltage-current characteristic cooled by liquid helium or hydrogen are discussed. It is shown that the non-standard thermal stabilization conditions of composite superconductors can exist if the nonlinear temperature dependences of the electrical resistivity of the stabilizing matrix and heat flux to liquid refrigerant are considered. First, there may be no minimum currents of the existence and propagation of a normal zone. Second, the intensive cooling of a composite superconductor by liquid refrigerant significantly increases the range of the stable currents, which leads to the existence of the supercritical applied currents. Third, an increase in the temperature of a composite superconductor may occur under the conditions close to adiabatic during irreversible propagation of the thermal instability despite its intensive cooling by liquid refrigerant. This effect must be considered when determining the burnout conditions of a composite superconductor. The performed numerical experiments are compared with the results obtained from the existing thermal stabilization theory of the composite superconductors, which is based on the linear temperature dependence of the critical current density of superconductor and jump-like transition from the superconducting state to the normal.

Numerical Analysis of the Influence of Superconductor and Magnet Material Parameters on the Dynamic Stability of Maglev Systems

In previous research, a two-dimensional numerical model based on Newton’s second law and Maxwell’s equations was developed to analyze the dynamic behavior of superconducting maglev systems. Studies showed that vibration and drift phenomena will occur in the vertical and lateral directions triggered by external disturbances, resulting in a change in the levitation point and even instability. In this paper, a numerical analysis is further carried out to alleviate these situations. The dependence of the drifts of vertical and lateral vibration centers of the levitated body upon system material parameters, such as the magnetization of permanent magnets and the critical current density of superconductor, is studied in terms of a power law model. In addition, the influence of the configuration of permanent magnets on the dynamic stability is analyzed.

Investigation of the levitation force of field-cooled YBCO and MgB2 disks as functions of temperature

We report levitation force cycles resulting from measurements carried out on a YBCO and a MgB2 disk cooled down in the field of a permanent magnet. In both cases the amplitude of the levitation force tends toward maximum values as the temperature decreases. Otherwise, the cycles are almost closed at low temperature and strongly hysteretic in the high temperature range. The hysteresis of the force cycles is attributed to the distribution of the currents induced in the sample by the field of the magnet. The saturation of the levitation forces at low temperature is related to that of the magnetic moment of the disks. We show that this type of measurement allows for the determination of the critical current density of superconductors in a restricted domain of temperatures.

Vibration effect on magnetization and critical current density of superconductors

In this work the effect of vibrations on critical current density (Jc) of superconductors has been studied. The vibrations are shown to affect Jc of all types of superconductors during their measurements, employing a vibrating sample magnetometer (VSM). Increasing vibration frequency (f) and/or amplitude (A) leads to progressive reduction of Jc as a function of magnetic field (Ba). The effect of vibrations is substantially stronger in thin films. It leads to development of unexpected kinks on curves. Analysis of magnetization loops and relaxation of magnetization in YBCO films revealed that the vibration effect can be treated as the effective reduction of pinning potential. The asymmetry of the vibration effect in ascending and descending Ba is observed, indicating differences in free energy of the corresponding vortex structures. Thermal effects induced by vibrations with large f and A are shown to have rather insignificant influence, while the vibrational vortex dynamics exhibit a strong impact. The irreversibility field () is shown to be instrumentally defined, and its value depends on VSM settings. In addition, the practical importance of for Jc modeling is demonstrated.

Thermal Mechanisms of Stable Macroscopic Penetration of Applied Currents in High Temperature Superconductors and their Instability Conditions

Macroscopic mechanisms of applied current penetration in high-T c superconductors are discussed to understand the basic physical trends, which are characteristic for the stable and unstable formation of the thermo-electrodynamics states of high-T c superconductors placed in DC external magnetic field. The performed analysis shows that the definition of the current stability conditions of high-T c superconductors must take into consideration the development of the interconnected thermal and electrodynamics states when the temperature of superconductor may stably rise before instability. As a result, the thermal degradation effect of the current-carrying capacity of superconductor exists. The boundary values of the electric field and the current above which the applied current is unstable are defined taking into account the size effect, cooling conditions, non-linear temperature dependences of the critical current density of superconductor. It is proved that the allowable stable values of electric field and current can be both below and above those determined by a priori chosen critical values of the electric field and current of the superconductor. The violation features of the stable current distribution in high-T c superconductors cooled by liquid coolant are studied. The necessary criteria allowing one to determine the influence of the properties of superconductor and coolant on the current instability onset are written.

Point Contact Based Method to Measure Critical Current Density of Superconductor Bulk

Abstract In this paper a novel method to measure the critical current density of superconductor bulks through a small point contact to the surface of the samples has been presented. Based on the proposed method a localized current is applied to the superconductor at the center of its surface in the absence of magnetic field where there is a second ring shape electrode in the outer area of the surface of the superconductor bulk. By increasing the pulse current through the central electrode beyond the critical current density of the material, normal zones develop in the superconductor. Theoretical results anticipate semi oblate spheroid normal zones as also confirmed by the obtained experimental data. Based on the used theoretical model, the form of the voltage-current relation is obtained as also verified by the measurements. Considering the area of the central electrode and the voltage current relation, a model is proposed to obtain critical current density of the bulk material, while the used electrical approach and technique avoid heat generation in the sample.

Magnetic Flux Penetration and Transport AC Loss in Superconductor Coated Conductor on Ferromagnetic Substrate

The current distribution and the AC loss in a composite superconducting tape containing a layer from magnetic material is calculated and compared with experiments, showing a very good agreement. The situations of an alternating uniform applied field or a transport current are studied. The newly developed numerical model is an approximation to the critical state model, adapted for the applicability to commercial finite elements codes that solve the vector potential. Substantial feature of this procedure is that it can be carried out in the case when the critical current density in superconductor depends on the magnetic field and the magnetic layer material is non- linear. Additionally, the hysteresis loss in the magnetic material is estimated, based on its measured magnetization loops. Measurements on Bi-2223 multifilamentary tapes covered on edges by nickel confirmed our predictions, showing a substantial ac loss reduction in both the investigated regimes.

Preparation of Yba2Cu3OxSuperconductor Prepared with Additives of PbO and Ag2O

The improvement of preparation process of <TEX>${YBa_2}{Cu_3}{O_x}$</TEX> superconductor and its conducting properties is important for practical applications. In this study, the additives such as <TEX>$Ag_{2}O$</TEX> and PbO were used to improve the preparation conditions of <TEX>${YBa_2}{Cu_3}{O_x}$</TEX> superconducting bulk samples and the properties of <TEX>${YBa_2}{Cu_3}{O_x}$</TEX> superconductors prepared with powders using sol-gel method and solid state reaction method were studied. The effects of the different powders and the additives to the density, grain alignment, and porosity of samples, that affect the critical current density of superconductor, also have been investigated. It is found that the properties of <TEX>${YBa_2}{Cu_3}{O_x}$</TEX> prepared with sol-gel synthesized powder and the additives showed better superconductivities than those of conventionally prepared superconductors.

Drift of Levitated/Suspended Body in High-$T_{c}$ Superconducting Levitation Systems Under Vibration—Part II: Drift Velocity for Gap Varying With Time

To study levitation drift further, i.e., the gap between a superconductor and a permanent magnet varying with time in high-Tc superconducting levitation systems, drift velocity is introduced. Based on the numerical simulations of the dynamic response of a levitated body, and according to the essential reasons for drift, the drift velocity is first divided into two regimes: Vff [related to flux flow (FF)] and Vtc [related to flux creep (FC)]. The drift velocity is shown to be mainly dependent upon properties of superconductivity (such as the critical current density of superconductors), initial disturbances, and applied excitations (such as the amplitude and the frequency of external excitations). Furthermore, the corresponding influences of the drift velocities Vff and Vfc have been investigated quantitatively in this paper.

Influence of Flux Creep on Dynamic Behavior of Magnetic Levitation Systems With a High-&lt;tex&gt;$T_c$&lt;/tex&gt;Superconductor

Significant flux creep may be generated in some high-T/sub c/ superconductors with weak pinning, which could yield an influence on the dynamic behavior of a high-T/sub c/ superconductor-magnet levitation system. To investigate this influence, this article presents a numerical analysis of dynamic features of the levitation generated by an interaction between a high-T/sub c/ superconductor (HTSC) and a permanent magnet (PM) after the flux creep in the superconductor is taken into account in a macro-model of superconductivity. The influence is comprehensively displayed by comparing the predictions of dynamic responses of such systems in which the flux creep in the superconductor is and is not taken into account. The obtained results show that whether or not the flux creep results in a noticeable influence to the levitation of superconductor-magnet systems is mainly dependent upon properties of superconductivity and applied excitation, e.g., critical current density of superconductors, and amplitude and frequency of external excitations. When the critical current density is less than 4.5/spl times/10/sup 8/ A/m/sup 2/, and the system is subjected to a periodic excitation, the influence of flux creep should be taken into account in the theoretical analysis.

Interaction region magnets for VLHC

The interaction region (IR) magnets for the proposed Very Large Hadron Collider (VLHC) require high gradient quadrupoles and high field dipoles for high luminosity performance. Moreover, the IR magnets for high energy colliders and storage rings must operate in an environment where the amount of energy deposited on superconducting coils is rather large. In the case of doublet IR optics with flat beams, the design of the first 2-in-1 quadrupole defines the geometry and pole tip field in this and other IR magnets. This paper will present a novel design of this magnet that allows a very small separation between the two apertures. A brief discussion of the conceptual magnetic design of this and other magnets for interaction regions is given. The influence of critical current density in superconductor (a higher value of which is most beneficial to high performance IR magnet design) is also discussed. Since high temperature superconductors (HTS) retain most of their critical current density at high fields and at elevated temperatures, they offer an attractive possibility for the IR magnet designs of future colliders or upgrades of present colliders.

Critical current density of superconductors as a function of strain in high magnetic fields

A probe has been designed and built to measure the critical current density ( J C) of short superconducting wires and tapes as a function of strain ( ϵ) in small bore high field magnets. The probe can measure currents of up to 100 Amps and strains of ±1.0% on both low and high temperature superconductors. Measurements can be made in our 40 mm bore 17 T high magnetic field system in pumped cryogens. Strain is measured using commercial strain gauges pasted directly onto the samples. A Ag-sheathed Bi 2Sr 2Ca 2Cu 3O X tape has been measured at 4.2 K in magnetic fields up to 12 T. J C is hysteretic in high magnetic fields and is weakly dependent on magnetic field and tensile strain up to ∼0.29%. Beyond ∼0.29% strain, the filaments break and J C drops precipitously. Results are also presented for the effect of tensile and compressive strain for a 0.37 mm diameter Nb 3Sn wire at 4.2 K and 2.9 K from 10 T up to 14 T. The data confirm that a simple scaling law that includes the upper critical field ( B C2( T, ϵ)) and the reduced field ( B/ B C2( T, ϵ)) alone cannot accurately describe the field, temperature and strain dependence of J C.

A probe for the measurement of the transport critical current density of superconductors in high magnetic fields and at temperatures between 2 and 150 K

A new probe has been designed for the measurement of the critical current density (Jc(B,T)) of both high- and low-temperature superconductors as a function of field and temperature. With this probe, voltage-current characteristics can be measured in small bore high-field magnets in the temperature range from 2-150 K with noise levels of typically 350 nV RMS. Excellent temperature accuracy and control in high magnetic fields is achieved such that at 4.2 K, for currents up to 300 A, the critical currents measured are accurate to the equivalent of +or-50 mK; at 20 K, up to 30 A, the accuracy is +or-70 mK and at 80 K up to at least 50 A the accuracy is +or-100 mK. The material under study can be short sections of superconducting cables, wires, tapes or thin films. The important design features of the probe are described. The probe's capabilities are demonstrated by presenting Jc(B,T) data obtained from a commercial multi-filamentary Nb46.5% wtTi wire and a high-temperature Bi2Sr2Ca2Cu3Ox silver-dad tape in magnetic fields up to 15 T.

Novel simple method to determine critical current density of superconductor in the critical state

The simple and exact expression which connects J c with the differential magnetic susceptibility ( ϰ d = dM/ dH) of supercondactive slab has been derived for a virgin part of magnetization. The magnetic experimental results for Nb 3 Zr and Bi 2 Sr 2 CaCu 2 O 8 were processed according to the proposed method. The received J c ( H ) curves at field H < H p (full penetration field) are sewed together exactly with the J c ( H) curves obtained by Bean method at field H < H p [1-2].

Concerning factors which determine whether flux-lattice shear or pin breaking limits the critical current density of superconductors

An elementary model that illustrates the conditions under which flux-lattice shear, rather than pin breaking, limits the critical current density is presented. An expression for the shear strength of the flux-lattice, based on the plasticity of metals and alloys, is used to derive the critical current density, including the effect of thermal activation in the flux creep regime. Expressions are also derived for the flow stress and dislocation density of the flux-line lattice (FLL) and are combined to yield the magnetic-field- and temperature-dependent critical current density in the absence of thermal activated flux creep. The rate theory of plastic deformation is used to derive the electric-field vs. current-density relation and J/sub c/ to be expected when thermally activated processes make the dominant contribution to FLL shear.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electron diffraction contrast of fluxons

Partial penetration of an applied external magnetic field occurs in type-II superconductors. The properties of magnetic fluxons are important in determining the critical current density of type-II superconductors as it is the mobility of the fluxon lattice that limits the high value of critical current density of superconductors. There have been various experimental techniques in use to study the fluxons, e.g. the decoration technique, neutron diffraction, electron holography and scanning tunneling microscopy.Noting that in the thin crystal case the magnetic fluxes have a tangential component which deflects incident electrons, we explore the possibility of using conventional electron diffraction contrast technique to observe the fluxons. This is accomplished by using the London model for the vortex in thin crystals and the classical electromagnetic theory based on Maxwell equations to construct the magnetic field of a fluxon.

Factors affecting critical currents determined by the transformer method for the YBa2Cu3Oy high-Tc superconductors

Abstract Critical current measurements give quantitative information regarding the dissipationless electrical current carrying capability of bulk high- T c materials. We report an improvement of the contactless transformer technique for measuring directly the bulk transport critical currents of superconductors, and report critical currents of bulk high- T c YBa 2 Cu 3 O y -based materials as a function of sample geometry, composition, and preparation conditions. We found that the critical current density of superconductors is not independent of sample geometry, and we propose a supercurrent/magnetic flux pinning field model of the critical current state of superconductors to explain our results.