The angular dependence of critical current densities in bulk magnetically melt textured YBaCuO in magnetic fields up to 20 teslas

Bulk magnetically melt textured YBaCuO at 77 K shows critical current densities of up to 19000 A/cm² in 20 teslas when the field is parallel to the(a, b) planes. The angular dependence ofJc is shown to support a recent interpretation of the high superconducting transport currents measured above the irreversibility line in terms of intrinsic surface superconductivity. The behavior when the field is applied very close to the(a, b) planes is shown to follow a two dimensional model meaning that the maximum current density is determined by thec-axis component of the applied field. The size and density of 211 inclusions are probably the main factors which determines the maximum critical current density.     

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.     

On the orientation of the moving flux line lattice in a superconducting film

The experimental counter-evidence is presented for Schmid and Hauger's theory which predicts the orientation of the moving flux line lattice relative to the direction of its flow. The pinning current and the current-versus-voltage characteristics of the flux flow state, together with the interference effects of the moving flux line lattice of indium films, are measured under two different experimental conditions, where either externally regulated dc voltage or current is supplied to the specimen. No difference is found between the two conditions. The theoretical arguments which predict a different orientation according to the conditions are no longer valid.     

Optimization of critical current density of bulk YBCO superconductor prepared by coprecipitation in oxalic acid

YBa₂Cu₃O_7–x(YBCO) superconductor powder was prepared from mixtures of solutions containing yttrium, barium, and copper nitrates by coprecipitation in oxalic acid. Single-phase YBCO was obtained from a solution mixture of 14 vol% excess of barium nitrate and 12 vol% excess of copper nitrate relative to the stoichiometry of YBCO. The optimal pH in the coprecipitation process was determined to be 6.6–6.7 which was obtained by using 12 vol % excess oxalic acid anhydrous solution of the required oxalic acid to convert all metal cations to oxalates and adding a dilute ammonium hydroxide solution. The measurement of critical current density, J _c, of bulk YBCO prepared by the coprecipitation, showed a trend that the J _c increased together with the degree of orthorhombic distortion of YBCO phase which depended on the sample density and the content of impurity phases.     

Superfluid density and critical current of3He in confined geometries

Superfluid³He in severely confined geometries is studied in the Ginzburg-Landau limit. Both A and B phases are analyzed in narrow slabs and in small cylinders. Physically measurable quantities include the superfluid density and the critical current, which are significantly suppressed by the boundaries. Related alterations in the phase diagram are also considered. For small lateral dimensions, the A phase can be favored even well below the polycritical pressure.     

Properties of the flux line lattice in hysteretic type II superconductors. I. Neutron diffraction experiments

A detailed study of neutron rocking curves on flux line lattices was made in single-crystalline hysteretic type II superconductors with Ginzburg-Landau parameters between 1.0 and 2.5. Information was obtained about the symmetry and the quality of the flux line lattice as a function of crystal orientation, sample purity, and magnetic field. The flux line lattice is always observed to form a single crystal. The width of the rocking curves, which is a measure of the misorientation of flux line crystallites in the plane perpendicular to the external field, is found to decrease strongly with increasing field along the initial magnetization curve, and to be much smaller upon decreasing the field from aboveH _c2 . This width varied between 8 and 1.5°. The minimum size of a coherently scattering flux line crystallite was estimated to be about 2–3 µm. Along the initial magnetization nearH _c1 the flux is found to form flux line bundles of constant lattice spacing. Although large flux density gradients have been observed earlier with field probes nearH _c1 ^1,2 , the lattice parameter measured with neutrons remained constant in this field range.     

Properties of the flux line lattice in hysteretic type II superconductors. II. Neutron depolarization experiments

Neutron depolarization experiments made on the same samples as investigated previously by neutron diffraction experiments (part I of this report) are presented. The correlations between crystal lattice and flux line lattice are shown to lead to a different field dependence of the average misalignment of the flux lines compared with polycrystalline materials. The average tilting angles are found to depend strongly on the magnetic field and to differ for increasing and decreasing fields in the same way as the mutual misorientations of singlecrystalline flux line blocks in the plane perpendicular to the field. For a known defect structure the tilting angles may be used to calculate the average displacement of the flux lines and the basic pinning forces.     

Longitudinal critical current in type II superconductors. I. Helical vortex instability in the bulk

The vortex lattice in type II superconductors is unstable against the growth of helical perturbations if the current along the vortices exceeds a critical value. The longitudinal critical current, the pitch, and the spatially varying amplitude of the elliptically polarized helices are calculated from the London theory at the onset of instability in planar current distributions far from the surface. For weak pinning (_L² c ₆₆) the wavelength and width of the mode extend over the entire specimen, and the critical current is 2H(c ₆₆/c ₁₁)^1/4. For moderate pinning (c _{66 L}² c ₁₁) the wavelength and width are close to Campbell's pinning length (c ₁₁/_L)^1/2, and the critical current times its mean density is 2H ²(_L/c ₁₁)^1/2. For strong pinning (_L² c ₁₁) helical instability occurs at pin-free vortex sections, the helix wavelength is 2.2d, and the critical current density is 0.47Hd/² (H, d, c ₁₁ and c ₆₆), and _L are the magnetic field, spacing, elastic moduli, and pinning parameter of the vortex lattice, and is the magnetic penetration depth).     

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.     

Effect of a cerium/tin doping mixture on the texturing process and on the superconducting properties of top seeded melt textured YBaCuO

The influence of a cerium/tin doping mixture on top seeded melt textured pellets of YBa₂Cu₃O_7–/Y₂BaCuO₅ (Y123/Y211) was investigated and compared with cerium doping. The thermal cycle of the texturing process has been adapted to this new composition, considering the variations of the decomposition temperature and of the melt viscosity. Growth rates of YBCO crystals appear to be quadrupled by the presence of tin. We manage to grow domains of diameter 30 mm from a SmBa₂Cu₃O₆₊ seed. SEM observations have shown no modification of the Y₂BaCuO₅ inclusions either in size or in morphology compared to Y211 particules in cerium doped YBCO. However, EDS analyses have revealed the existence of submicron particles containing Y, Ba, Cu, Sn, Ce and O distributed in the matrix. Superconducting properties are greatly improved by the addition of a moderate amount of tin to the cerium doped YBCO. The critical temperature is shifted toward 92 K with a transition width of 0.5 K. Critical current densities at 77 K reach values as high as 9 × 10⁴ A/cm² in self field, 5 × 10⁴ A/cm² under 1 Tesla and still 2.5 × 10⁴ A/cm² under 2 T, which are among the best results ever obtained for top seeded doped YBCO.     

Hydrodynamic aspect of the effect of the thermophysical properties of the heater on the critical heat flux in bulk pool boiling of helium

The article examines the hydrodynamic aspect of the effect of the thermophysical properties of the heater on the critical heat flux in pool boiling of helium.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 45, No. 2, pp. 263–267, August, 1983.The authors are obliged to S. S. Kutateladze and Yu. A. Kirichenko for fruitful discussions.     

Microstructural factors important for the development of high critical current density Nb3Sn strand

Nb₃Sn is the primary candidate for the next generation of accelerator magnets as well as for NMR and other applications that require magnetic fields between 11 and 20 T. Since 1999 the layer critical current density available in long length accelerator quality strand has almost doubled. The microstructural and microchemical factors that are important for high critical current density Nb₃Sn are reviewed. The highest critical current density strands have a Nb₃Sn layer that minimizes chemical and microstructural inhomogeneities and has a high fraction of the layer close to stoichiometric Sn content. Only the internal Sn process has yielded critical current densities beyond 3000 A/mm² at 12 T (4.2 K) and only with interfilamentary Cu thicknesses that are too low to separate the filaments after the final reaction heat treatment. The result of the reaction heat treatment is to produce a continuous ring of Nb₃Sn from hundreds of Nb or Nb-alloy filaments and thus a major ongoing challenge of Nb₃Sn conductor design is to reduce the effective filament diameter to acceptable levels for intended applications. Recent successful attempts to reduce the cost of alloying the Nb₃Sn for high field application are also examined and the potential for future improvements discussed.     

Dielastic interaction of the flux line lattice with internal stresses of crystal imperfections. II. Second-order interaction force between flux lines and crystal dislocations

The elementary dielastic interaction force (E effect) between crystal dislocations (screw and edge dislocations) and one flux line as well as the flux line lattice is investigated by means of the phenomenological Ginzburg—Landau theory. The dependence of the maximum pinning force on the reduced induction b ; B/B _c2 is calculated. It turns out that the dielastic pinning force increases with decreasing coherence length , p ^E 1/ and therefore becomes especially large for the technologically important high-field superconductors. Numerical values for the maximum pinning force are presented for Nb and Nb₃Sn.     

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.     

Superfluid3He-B in a realistic thin film. Gap and critical current

We examine³He-B confined to a thin film on a rough substrate with a freestanding upper surface by using quasiclassical techniques. First, the dependence of the gap on surface roughness, film width, and boundary condition formulation is examined in detail for the current-free state. The evaluation is conducted using both the Randomly Rippled Wall and the Thin Dirty Layer surface models in order to compare them. Second, the current bearing state for a submicron film resting on a rough substrate is calculated using the Thin Dirty Layer surface model. Several unusual and previously unreported features appear in both cases. Numerical estimates of the critical current densities are in favorable agreement with corresponding experiments.     

Effect of platinum on the critical current density of fluorine-doped YBa2Cu3Ox superconductors

The critical current density, Jc, in fluorine-doped YBa2Cu3Ox bulk superconductors has been measured at several temperatures. The measured Jc value is 108 A m-2 at 77 K and 0.5 T. A sample with a larger critical current density can be produced by the addition of a small amount of platinum during the sintering process. The added platinum has the effect of evenly distributing Y2BaCuO5 particles and voids in the matrix in comparison with the undoped sample. These distributions of Y2BaCuO5 particles and voids are reflected in an increase in the critical current density. However, these distributions are not directly related to the observed peak effect in the critical current density. If the fluorine-doped YBa2Cu3Ox superconductor consists of the matrix and a phase with a Ginzburg–Landau parameter, that is slightly different from that of the matrix, then the occurrence of the peak effect can be explained.