Evidence for the control of the critical current density of Nb-25% Zr superconducting wires by the dislocation substructure

Transmission electron microscopy of longitudinal sections of 250μm diameter Nb-25% Zr wire has revealed that annealing treatments which maximiseJ _c can cause polygonisation of the as-drawn dislocation substructure, precipitation ofα _Zr, precipitation ofβ _Zr, or recrystallisation ofβ _Nb. It is argued that the polygonised structure is the one which is most effective in controllingJ _c. T _c data has provided some very useful information for assessing the constitutional state of the material.     

Surface effect on the critical current density of superconducting niobium

Experiments are described which measure the critical current density of single and polycrystal niobium samples. The density measured is related to the ac losses of the sample caused by an applied ac field in the presence of a dc magnetic field. It was found that the critical current density depends upon the surface of the sample increasing with the surface smoothness.     

Anisotropy of the critical current density in superconducting niobium-titanium wires

The critical current density in wires of an Nb-64 at % Ti alloy was determined in axial and circular directions by various methods. Owing to the cell structure due to cold deformation an anisotropy in the critical current density (j_{c, axially} <j_c, circularly) is present in the wires There is no major difference between magnetization currents and transport currents. Therefore it should be possible in principle to obtain the high critical current densities determined from circular magnetization currents by suitable defect structure with axial transport currents too.     

The effect of heat-treatment and nitrogen addition on the critical current density of a worked niobium 44 wt % titanium superconducting alloy

Mechanical working of a single phase superconducting alloy leads to an increase in the critical current density,J _c, After cold-rollingJ _c exhibits anisotropy as the angle of the magnetic field to the rolled surface is changed. This paper studies the effects of interstitial addition and heat-treatment onJ _c and its anisotropy in both cold-rolled and wire-drawn alloy. The variations inJ _c are correlated with the observed changes in the microstructure.     

Temperature dependence and microwave enhancement of the critical current of superconducting microbridges

The temperature dependence of superconducting tin, thin-film, narrow bridges has been investigated. NearT _c the critical current varies with (1 – T/T_c ), with a distribution of values between 1 and 3/2. It is suggested that the values can be used to characterize the bridges. The microwave enhancement ofI _c andT _c was measured and these data were compared with a recent theory by Christiansenet al. on the microbridges. We found that their theory gives a correct prediction for the Dayem effect in the bulklike bridges (with =3/2) while their theoretical values for the junction-like bridges (with =1) are not in agreement with our data.Work supported in part by the National Science Foundation.     

The optimization of superconducting Helmholtz coils taking into account the dependence of critical current on magnetic induction

A means of optimizing the volume of the windings of superconducting Helmholtz coils is described which takes into account the dependence of the critical current on the maximum induction in the windings. The calculation is carried out for a superconducting Nb + Ti alloy. The method proposed enables the amount of superconductor used to be reduced by up to ～ 6%, or the maximum induction in the centre to be increased by 1 to 2% compared with the way suggested earlier.¹     

On the study of phase formation and critical current density in superconducting MgB2

Superconducting bulk MgB₂ samples have been synthesized by employing sintering technique without using any additional process steps, generally undertaken in view of the substantial loss of magnesium, during heat treatment. Starting with Mg rich powders having different atomic ratios of Mg : B, as against the nominally required Mg : B = 1:2 ratio, we have obtained superconducting MgB₂ samples of different characteristics. The effect of excess Mg in the starting mixture and processing temperature on the phase-formation, transition temperature (T _C) and critical current density (J _C) have been investigated by electrical transport and a.c. susceptibility measurements. The X-ray diffraction and X-ray photoelectron spectroscopic analyses of MgB₂ bulk samples have been carried out to understand the role of excess Mg and the effect of processing temperature. It is established that MgB₂ samples with high critical current density can be synthesized from a Mg rich powder having Mg : B in 2:2 ratio, at temperatures around 790°C. Critical current density has been found to vary systematically with processing temperature.     

Microstructure,composition and critical current density of superconducting Nb3Sn wires

The production of superconducting Nb₃Sn multifilamentary wires with optimized critical currents requires a detailed knowledge of various processes, involving both the mechanical deformation and the reaction kinetics. The physical properties of the Nb₃Sn phase are briefly reviewed, the precise knowledge of their variation as a function of the Sn content being essential for the optimization of the critical current density, J_c. The variation of the transition temperature T_c, the upper critical field, B_c2(0), and the normal state electric resistivity ρ_o, as a function of the Sn content, β, in the binary system Nb_1?βSn_β is carefully analyzed. The effect of Ta, Ti and Ta + Ti additives to Nb₃Sn on the physical properties is discussed in detail. Low temperature specific heat measurements are introduced for determining the T_c distribution inside Nb₃Sn filaments while avoiding shielding effects.The microstructure of the superconducting phase in Bronze Route and Internal Sn Diffusion processed wires is studied, taking into account the unique microstructure of Bronze Route filaments, comprising an equiaxed and a columnar grain region, their areas being comparable. The Sn content increases gradually, from 18 to 22 at.% in columnar and from 22 to 25 at.% in equiaxed grains. Taking into account the equiaxed grains only in Bronze Route wires, it is found that the pinning force density F_GB is essentially the same as in the superconducting part of Internal Sn and PIT wires. The lower values of the overall F_GB in Bronze Route wires is due to the presence of columnar grains, with lower T_c and B_c2. The presence of columnar grains also explains the deviation from linearity of the Kramer rule in Bronze Route wires.The mechanism leading to the variation of J_c vs. ε, where ε is the uniaxial applied strain, is correlated to the elastic tetragonal distortion of the A15 phase in the filaments, caused by the matrix precompression or by Lorentz forces. The behavior of J_c (ε) is found to show systematic differences between Bronze Route and Internal Sn processed wires. Possible reasons for the stronger variation of J_c (ε) up to 21 T in Internal Sn wires are discussed.     

On the limiting factors of the critical current density in high-T c superconducting ceramics

The temperature dependence of the critical current density at high temperatures and in weak applied magnetic field for YBa₂Cu₃O_7?y ceramic samples with a pronounced granular character is analyzed. The experimental results can be explained in terms of thermally activated motion of the intergrain Josephson vortices at grain boundaries, which may be an indication that the actual limiting factor for the critical current density in ceramic samples results from a weak pinning force density for the intergrain vortices rather than from the weak-link quenching.     

The structure and properties of a cold-rolled and annealed Al-0.8 wt % Zr alloy

The effect of annealing at 400? C on the microstructure of a cold-worked Al-0.8 wt % Zr alloy is reported. It is shown that the initially high dislocation density in the cold-rolled material is progressively reduced, although the grains and subgrains were exceptionally resistant to coarsening. Precipitation of the metastable cubic Al₃Zr phase occurred, both discontinuously in the form of fan shaped precipitates and also on the grain boundaries and within the grains as small, nearly spherical particles. The mechanical properties of the alloy at 400? C are consistent with a major dislocation contribution to the overall deformation process, in contrast with most other fine grained materials which are superplastic.     

The hysteresis of the critical current in superconducting tin whiskers

Voltage-current characteristics are measured for fixed temperatures down to 0.5 K below the critical temperature. From a plot of the hysteretic behavior of the critical current as a function of the bath temperature, the temperature hysteresis for fixed current is evaluated and compared with theoretical predictions.     

Magnetic field dependence of the critical current density for the bismuth-based bulk high-Tc superconductors

Three types of bismuth-based bulk samples were prepared through uniaxial pressing at room temperature, hot isostatic pressing (HIP) and drawing and rolling. Transport current properties were characterized in a steady field up to 1.12 T at 77 K (T/T _c=0.75). The Josephson weak-link decoupling fields have been found to be 5 mT for the cold-pressed pellet and 30 mT for the HIPed pellet and the rolled tape. At the decoupling field the transport critical current density,J _c, drops 80% from 124 (OT) to 29 A cm^–2 (5 mT) for the cold-pressed pellet, 80% from 582 (OT) to 126 A cm^–2 (30 mT) for the HIPed sample and 50% from6500 (0 T) to 2850 A cm^–2 (30 mT) for the rolled tape. In the flux flow regime, whereB is perpendicular to thec-axis a modified Kim's modelJ _c=(/B ₀)/[(1+B/B ₀)] ⁿ can be used to describe the field dependence of the critical current density, J_c, in the field range 0.2–1.12 T. The effective upper critical fields were estimated to be 0.98, 1.54 and 1.94 T for the three types of samples, respectively. An adjustable range ofB _c2 for bismuth-based bulk highT _c superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. WhenB is parallel to thec-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample.     

The time-decaying critical current density of a type-II superconductor and its measuring method

The paper will show that a critical current which has been thought to be defined only in the critical state of a type-II superconductor is able to measure by making use of its time-decaying behavior below onset currents of the resistive state due to flux creep. This is a kind of adiabatic process, in which the measurement of critical currents is done by waiting for a quench which takes place when the persistent current induced in a superconducting sample goes cross the time-decaying critical current and also is a method measuring life of the persistent current in the sample. According to this method, potential barrier models for flux pinning and characteristic parameters about superconductors will be determined.     

The propagation of the resistive region in high current density superconducting coils

A method is given describing the propagation phenomenon of the resistive region within the winding of superconducting coils wound from semi-stabilized filamentary conductors. The method permits the approximate calculation of the velocities of propagation of the normal conducting front in superconducting coils of high current density. The calculation is carried out assuming that the parameters of the conductor materials, the short sample data, and the heat transfer coefficient from the conductor into the ambient medium are given. The propagation velocities of the normal front have been measured in insulated wires freely immersed in liquid helium, in test samples using conventional interleaving material, and in a compact multilayer coil wound from the same wire. The calculated results are compared with experimental data.     

The influence of the first heat-treatment on the critical current density of (Bi,Pb)-2223/Ag superconducting tapes

Optimization heat-treatments have been performed on multi-filament Bi-2223/Ag superconducting tapes under 1 bar total gas pressure, the oxygen partial pressure being 8.5%. In a first heat-treatment (HT1), the tapes were sintered within 822-838 °C for 1-50 h. After intermediate deformation, all the samples underwent the second heat-treatment (HT2) at 825 °C and 830 °C for 20 h followed by a thermal sliding procedure. The relative content of the phases present in HT1 samples was measured by XRD. It was found that the Bi-2212 phase content after HT1 strongly influences the values of J_c after HT2. There is a correlation between the amount of Bi-2212 phase after HT1 and the final J_c values after HT2. A maximum of J_c was found for a ratio of 0.15 between Bi-2212 and Bi-2223.     

The temperature and magnetic field dependence of superconducting critical current densities of multifilamentary Nb3Sn and NbTi composite wires

Commercial NbTi and Nb3Sn multifilamentary superconducting wire is becoming increasing important for use in research and commercial magnet systems. In both materials the temperature dependence of Jcplays a major role in the determination of magnet system operating parameters and design stability margins. We report here critical current density measurements as a function of temperature from 4.2 to 19 K and of applied magnetic field upto 8 T for multifilamentary Nb3Sn wire and for 2 alloys of NbTi superconducting wire. From this data [partial J_{c}(H_{a})/partialT] and[partialH_{c2}/partialT]T=T_{c}can be obtained and stability criteria and other superconducting parameters of the wires may be extracted.     

Further investigations of the upper critical field and the high field critical current density in Nb-Ti and its alloys

The work described in this report was undertaken, within the context of the 12 Tesla program, to investigate the possibility of increasing the upper critical field, Hc2, of Nb-Ti through alloying additions. A preliminary report has previously been given by us. In the first part of this paper we report further measurements of Hc2in the Nb-Ti-Ta and Nb-Ti-Hf systems. Whilst we find only small enhacements of ∼0.3 Tesla in μoHc2(4.2K) compared to binary Nb-Ti, at 2K there is a wide composition range in the Nb-Ti-Ta system where μoHc2(2K) exceeds 15 Tesla, reaching a maximum of 15.5 Tesla. This represents an enhancements of 1.3 Tesla over unalloyed Nb-Ti. By comparison alloys in the Nb-Ti-Hf system show a maximum enhancement in μoHc2(2K) of only 0.3 Tesla. The reasons both for the enhancements in Hc2and for the differences in behavior systems shown by alloys containing Ta and Hf are briefly discussed. In part II we discuss common features in the behavior of the high field critical current density, Jc, of four commercial Nb-Ti composites and upon the basis of this behavior predict the enhancements in the high field Jcto be expected from using Nb-Ti-Ta and its alloys.     

On the limiting factors of the critical current density in high-Tc superconducting ceramics

The temperature dependence of the critical current density at high temperatures and in weak applied magnetic field for YBa₂Cu₃O_7–y ceramic samples with a pronounced granular character is analyzed. The experimental results can be explained in terms of thermally activated motion of the intergrain Josephson vortices at grain boundaries, which may be an indication that the actual limiting factor for the critical current density in ceramic samples results from a weak pinning force density for the intergrain vortices rather than from the weak-link quenching.