Temperature Dependence Of Critical Current Density Research Articles

Effect of K-doping on the superconducting properties of FeSe0.5Te0.5 single crystals

Fe1−xKxSe0.5Te0.5 ( x = 0, 0.025, 0.05, 0.075 and 0.1) single crystals were grown by self-flux method. The effects of K-doping on the length of c-axis, critical temperature, anisotropy and critical current density were investigated. With the increase of K-doping content x, the length of c-axis increases, the critical temperature rises at first and then falls, and the anisotropy decreases. The critical current density displays a significant increase at x = 0.075. Subsequently, the superconducting properties of Fe0.925K0.075Se0.5Te0.5 were thoroughly studied. The field-dependent pinning energy U(H) and the temperature-dependent upper critical field Hc2(T) were given. Additionally, the Dew-Hughes model was used to scale the flux pinning force, and the result suggests that the superconductor had a temperature-induced pinning mechanism from normal point pinning to normal surface pinning, in which twins play an important role. We also discussed the temperature-dependent critical current density Jc(T)=Jc(0)(1-T/T1)n and found that the n-value depends on different flux pinning regimes, including single flux pinning, strong collective pinning associated with twins and weak collective pinning associated with point defects.

Effect of temperature-dependent critical current density on the fracture behavior for a rectangular superconducting slab with a center crack

Superconducting cables made of high-temperature superconductors have great potential applications in space solar power station. Since the critical current density of high temperature superconductor is dependent of temperature and determines the distribution of the magnetic flux density, temperature change can affect the pinning force. In this paper, model-I crack problem of a long rectangular superconducting slab under thermal effect is investigated. The stress intensity factors of the slab during the field decent of pulsed field magnetization (PFM) and field cooling (FC) are calculated. Compared with that without considering thermal effect, the result shows that once the temperature of the slab rises, the variation of the stress intensity factor will have a peak during the applied magnetic field decent for the PFM. While for the FC, temperature rise causes the stress intensity factor to decrease.

Critical current and flux dynamics in Ag-doped FeSe superconductor

The measurements of DC magnetization as a function of the temperature M(T), magnetic field M(H), and time M(t) have been performed in order to compare the superconducting and pinning properties of an undoped FeSe0.94 sample and a silver doped FeSe0.94 + 6 wt% Ag sample. The M(T) curves indicate an improvement of the superconducting critical temperature and a reduction of the non-superconducting phase Fe7Se8 due to the silver doping. This is confirmed by the field and temperature dependent critical current density Jc(H,T) extracted from the superconducting hysteresis loops at different temperatures within the Bean critical state model. Moreover, the combined analysis of the Jc(T) and of the pinning force Fp(H/Hirr) indicate that the pinning mechanisms in both samples can be described in the framework of the collective pinning theory. The U*(T, J) curves show a pinning crossover from an elastic creep regime of intermediate size flux bundles, for low temperatures, to a plastic creep regime at higher temperatures for both the samples. Finally, the vortex hopping attempt time has been evaluated for both samples and the results are comparable with the values reported in the literature for high Tc materials.

Preparation and Properties of Si Doped $\hbox{Mg}{( \hbox{B}_{1 - x}\hbox{C}_{x})}_{2}$ Using Silica Sol-Gel and Carbon Chemical Vapor Coated Boron

Coating a precursor nano powder by depositing a thin layer of dopant can achieve a controllable homogeneous distribution of the dopant per nano grain. Previously, we have shown that a simple, scalable chemical vapor deposition method is capable of achieving a uniform deposition of carbon onto boron powder, since the dopant is evenly distributed on each precursor grain. In this paper, we implement a new technique using a sol-gel method (using tetraethyl orthosilicate as a precursor) to coat silica on nano-sized boron powder. The silica-coated boron is also subsequently coated with carbon using the technique detailed previously. Both silica-coated and silica+carbon coated powders are reacted with magnesium using the in situ process. Analysis using transmission electron microscopy, powder X-ray diffraction and the field and temperature-dependence of critical current density is used to evaluate the effectiveness and homogeneity of this method.

On a New Equation for Critical Current Density Directly in Terms of the BCS Interaction Parameter, Debye Temperature and the Fermi Energy of the Superconductor

Recasting the BCS theory in the larger framework of the Bethe-Salpeter equation, a new equation is derived for the temperature-dependent critical current density jc(T) of an elemental superconductor (SC) directly in terms of the basic parameters of the theory, namely the dimensionless coupling constant [N(0)V], the Debye temperature θD and, additionally, the Fermi energy EF—unlike earlier such equations based on diverse, indirect criteria. Our approach provides an ab initio theoretical justification for one of the latter, text book equations invoked at T = 0 which involves Fermi momentum; additionally, it relates jc with the relevant parameters of the problem at T ≠ 0. Noting that the numerical value of EF of a high-Tc SC is a necessary input for the construction of its Fermi surface—which sheds light on its gap-structure, we also briefly discuss extension of our approach for such SCs.

Critical current density and ac harmonic voltage generation in [formula omitted] thin films by the screening technique

The temperature and field dependence of harmonics in voltage Vn=Vn′-iVn″ using the screening technique have been measured for YBa2Cu3O7-δ superconducting thin films. Using the Sun model we obtained the curves for the temperature-dependent critical current density Jc(T). In addition, we applied the criterion proposed by Acosta et al. to compute Jc(T). Also, we made used of the empirical law Jc∝(1-T/Tc)n as an input in our calculations to reproduce experimental harmonic generation up to the fifth harmonic. We found that most models fit well the fundamental voltage but higher harmonics are poorly reproduced. Such behavior suggests the idea that higher harmonics contain information concerning complex processes like flux creep or thermally assisted flux flow.

Determination of the relative influences of carbon doping and disorderon field and temperature dependent critical current density ofMgB2

SiC was mixed with Mg and B and reacted by either a one-step in situ or two-step method at 650 or850 °C. By doing so, it was possible to determine the extent to which scattering viaC doping influences the magnitude of field dependent critical current density,Jc(H),compared to pinning via generation of microstructural disorder. The one-step reaction method leadsto Mg2Si formation and at the same time to more C doping ofMgB2 than the two-step method. Carbon increases both the irreversibility field,Hirr, and uppercritical field, Hc2 (T<28 K). However, for the temperatures (6 and 20 K) and fields (up to 7 T) studied, pinningrather than scattering overwhelmingly dominates the magnitude of the field dependentcritical current density.

Superconducting trapped-field magnets: Temperature and field distributions during pulsed-field activation

We calculate the temperature and magnetic field distributions in a bulk superconductor during the process of pulsed-field magnetic activation. The calculations are based on the heat diffusion equation with account of the heat produced by flux motion, and the critical state model with temperature dependent critical current density. For a given activation time, the total amount of trapped flux Φ is maximum for an optimal value Bopt of the maximal applied field. We analyze how Bopt and Φ depend on the material parameters and the field ramp rate.

Study of Nd substitution in Bi–(Pb)–Sr–Ca–Cu–O high-Tc superconductors

Measurements of complex AC susceptibility χ=χ′+χ″ as a function of temperature and AC field amplitude have been carried out on rectangular bar-shaped high-temperature superconductors with nominal composition of Bi1.6Pb0.4Sr2(Ca1−xNdx)2Cu3Oδ superconducting samples prepared by the solid-state reaction method. The effect of Nd-substitution on the Bi–(Pb)–Sr–Ca–Cu–O system has been investigated in terms of an AC susceptibility study. It was found that as the amount of Nd content x increases, the transition temperature decreases. Temperature-dependent critical current density was deduced from AC susceptibility data. In spite of a slight decrease of Tc, the highest pinning strength parameters (Jcm (T=0) and temperature exponent n) were obtained for the sample with x=0.02.

Critical current density in the Tl2212 film

A method of numerical simulation combined with experimental curves of AC susceptibility (ACS) is proposed to determine the temperature dependent critical current density J c0(T) without flux creep. Based on this method and ACS curves, we have determined the J c0(T) of the Tl2212 thin film.

Magnetic Relaxation Studies on Grain-Aligned HgBa2Ca2Cu3Oy Superconductor

Temperature dependent critical current density and flux-creep measurement were carried out in magnetically aligned HgBa2Ca2Cu3Oy . For 20 K<T<60 K, we observed quasi-exponential behavior of the critical current density J c. The normalized relaxation rate S vs T curve has an N-like shape with the minimum around T=15 K and the peak at 50 K. The theoretical U(J) given by the collective flux creep model describes the experimental result well.

Effect of flux creep on the temperature dependence of the current density in Y-Ba-Cu-O crystals.

Temperature-dependent critical current density and normalized flux-creep rate, together with the current dependence of the vortex pinning energy, were studied in as-grown and proton-irradiated Y-Ba-Cu-O crystals. A consistent interpretation is found within the collective-pinning or vortex-glass models.

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>

Temperature-dependent critical current density of Bi(Pb)-Sr-Ca-Cu-O tapes in fields up to 20 tesla

The transport critical current density (Jc) of silver-sheathed polycrystalline Bi(Pb)-Sr-Ca-Cu-O (BPSCCO) tapes has been measured at approximately 10 K intervals from 4.2 K up to 77 K in fields of 1-20 T. The tapes were produced by swaging, drawing and multiple final pressing and sintering steps, which developed a preferred orientation of the basal planes of the predominantly 2223 BPSCCO phase within the tape Jc values greater than 104 A cm-2 at 4.2 K were obtained, and these decreased smoothly as the field and temperature were increased. No evidence of any catastrophic change in Jc(H) was seen at any temperature, thus suggesting that flux line lattice melting is not controlling the Jc of these tapes.

Scaling law for critical currents and magnetization in high temperature superconductors

The field and temperature dependent critical current density, j c( B, T), and magnetization, M( B, T), of melt processed Bi 2Sr 2CaCu 2O 8 samples have been examined in fields up to 8 T. In addition, published data on an epitaxially grown YBa 2Cu 3O 7 film have been analysed. Both sources of information show universal field dependence of j c and M proportional to (1 − T/ T c) m exp [− B/ B o( T)] above 0.5 T. The scaling field, B o( T), contains the essential part of the temperature dependence. It is shown that neither weak links nor thermal activation of single pinning sites alone can explain this behaviour. Although it is inferred from the data at 4.2 K that a large number of pinning centres must be present in the sample, it is assumed that their efficiency is drastically reduced with rising field at elevated temperatures. It is concluded that the role of statistical summation of pinning forces to give an effective volume force should be reconsidered, and it is proposed that thermal activation reduces the number of active pinning interactions with rising temperature and field, in addition to the expected effects of flux creep.