Polycrystalline High-temperature Superconductors Research Articles

Equilibrium magnetic properties of the Fe-doped HTSC YBaCuO

For the polycrystalline high-temperature superconductor Y1 –xFexBa2Cu3Oy (0 ≤ x ≤ 0.05), where a significant increase in the intragrain critical current density has recently been found, the effect of iron doping on the equilibrium magnetic characteristics is studied. The field dependences of the magnetization are measured in the temperature range of 40–85 K and in the field range up to 60 kOe with increasing and decreasing fields. The temperature dependences of the London penetration depth λab, the superconducting carrier density ns, and the thermodynamic critical field Hc are determined from an analysis of the middle lines of the magnetic hysteresis loop. It is shown that the thermodynamic properties of Fe substituted samples do not change at x ≤ 0.03. At a higher Fe content, a slight decrease in Hc and the superconducting transition temperature is observed. However, changes in the condensation energy turn out to be much smaller than values expected due to an increase in the density of effective pinning centres created by substitutional atoms in both the yttrium and copper sublattices. It is concluded that the pinning regions in chain CuO planes near substituted Cu sites are two-dimensional in nature and do not affect the superconducting pairing in CuO2 planes. Another possible pinning mechanism, which does not affect Hc, is the interaction of vortices with the magnetic moments of Fe3+ ions.

Critical Current Density Through Grain Boundaries in High-Temperature Superconductors

In this paper, a theoretical study is proposed based on the assumption that the vortices on low-angle grain boundaries (GBs) in high-temperature superconductor (HTS) are mixed Abrikosov-Josephson (AJ) vortices. The critical current density through GB is obtained on the basis of the Bean critical model and the assumption that the periods of AJ vortices coincide with the ones of Abrikosov (A) vortices. The model also enables us to calculate J c of HTS with an inclined GB. In addition, the effect of strain on critical current density is also taken into account in this model by considering the strain dependence of deparing current density within GB. There is a good agreement of our results with the classical power-law expression. The model proposed in this work can be used for simultaneous studies of the effects of misorientation angles, GB-inclined angles, and applied fields on the critical current density of polycrystalline HTS.

Magnetoresistance of porous polycrystalline HTSC: Effect of the transport current on magnetic flux compression in intergranular medium

The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi1.8Pb0.3Sr1.9Ca2Cu3Ox have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current.

Physical processes in high-temperature superconductors at the interface between the vortex-filled and meissner regions

A technique has been proposed for investigating the magnetic microstate of high-temperature superconductors with a simultaneous analysis of the crystalline microstate of the sample with the aim of elucidating the specific features of the interaction between the crystalline and magnetic microstructures of polycrystalline high-temperature superconductors. Qualitatively new results have been obtained for samples with different microstructures. In particular, it has been found that the magnetic field dependences of the trapped magnetic flux density Btr(H0) of polycrystalline and epitaxial films of high-temperature superconductors exhibit regular steps for both increasing and decreasing magnetic fields. The obtained results have demonstrated that, in strong magnetic fields, the studied epitaxial films, as well as bulk and thin-film polycrystalline high-temperature superconductors, “break down” into single domains, crystallites, and subcrystallites with different demagnetization factors. It has been revealed that the dependences Btr(H0) also exhibit steps due to the simultaneous penetration of vortices into crystallites of approximately the same sizes and into more regularly arranged subcrystallites. As the quality of the samples increases, these steps become more pronounced because of the increase in the short-range order. The absence of steps in the dependence Btr(H0) of the polycrystalline bulk samples clearly demonstrates the absence of long-range order in these samples. It is the vitreousness of the crystallographic microstructure of high-temperature superconductors which is responsible for the observed transformations in the vortex system. The similarity of the results obtained for samples with different microstructures indicates that the penetration (escape), distribution, and trapping of the magnetic flux in these samples occur through a universal mechanism. It has been found that the polycrystalline high-temperature superconductors are actually multi-step rather than two-step systems. It has been shown that the vitreousness of the microstructure of high-temperature superconductors and the presence of close-packed twin boundaries in samples lead to the penetration of a magnetic flux in the form of hypervortices into the sample and cause the formation of a superconducting glass state on a different physical basis as compared to the Ebner-Stroud model of a granulated glass.

The critical current density of grain boundary channels in polycrystalline HTS and LTS superconductors in magnetic fields

We provide evidence that a single mechanism—flux flow along channels—can explain the functional form of the critical current density (Jc) in the low-temperature superconductor Nb3Sn and in the high-temperature superconductors (HTS) YBa2Cu3O7−δ (YBCO) and (Bi,Pb)2Sr2Can−1CunOx (BiSCCO) in low and high magnetic fields. In this paper, we show that standard flux pinning theories, used for the past four decades to describe Jc in low-temperature superconductors (LTS), cannot explain the strain dependence of Jc in YBCO because Jc is a function of strain but the average superconducting properties are not. We conclude that in the polycrystalline samples presented here, the channels are grain boundaries that are narrow and metallic in Nb3Sn and YBCO but wide and semiconducting in BiSCCO. In Nb3Sn, strain alters Jc by changing the superconducting properties of the grains, whereas in YBCO, strain alters Jc by changing the properties of the grain boundaries.

Phonon echo probing of high-temperature superconductors containing magnetic ions

Polycrystalline high-temperature superconductors (HTSCs) containing iron and gadolinium ions have been studied using the phonon echo technique, whereby a powdered sample is probed by a special sequence of RF pulses and the response having an acoustic nature is measured. It is shown that the behavior of this signal in the iron-containing HTSC materials exhibits some distinctive features in comparison to the response of a conventional yttrium-based ceramic. The superconducting gap estimated for the samples with magnetic ions does not differ significantly from the value for a usual HTSC. This fact is related to a spatial separation of the magnetic and superconducting phases in the materials studied.

Phenomenological description of flux pinning in non-uniform high-temperaturesuperconductors in magnetic fields lower than the self-field

Critical currents measured on a commercial Bi-2223/Ag tape in applied magnetic fieldsranging from zero to 100 mT are analysed in order to demonstrate the validity of thephenomenological description proposed by the authors for flux pinning in polycrystallinehigh-temperature superconductors in low magnetic fields. The four-parameter model is ageneralization of the Kim formula describing the magnetic field dependence of the criticalcurrent density. When combined with the finite-element calculation, it is able to predict thecritical currents measured in different field orientations with an average accuracy betterthan 0.2%. The volume density of the flux pinning force is easily evaluated from theparameters of the model. Particular attention is paid to the problem of a possible samplenon-uniformity. In fact, the experimental data can be better explained assuming thehypothesis of better filament texturing in outer filaments. This also means that theaccurate determination of critical currents in low magnetic fields can be used toreveal a non-uniformity in the current-carrying capability of the superconductingmaterial.

Andreev reflection and experimental temperature dependences of the critical current in heterogeneous high-temperature superconductors (polycrystals and related composites)

The temperature dependences of the critical current of Y3/4Lu1/4Ba2Cu3O7 polycrystalline high-temperature superconductors subjected to annealing for different times and those of Y3/4Lu1/4Ba2Cu3O7 + BaPbO3 composites were measured. The results obtained were analyzed in terms of the Gunsenheimer-Schussler-Kummler theory considering the Andreev reflection of carriers in a “superconductor-normal metal-superconductor” (S-N-S) junction. Good agreement between the experimental and theoretical temperature dependences of the critical current over a wide temperature range for polycrystals with both natural and artificially created boundaries (composites) made it possible to estimate the effective length of grain boundaries in the high-temperature superconductors. It was revealed that the critical current density measured at 4.2 K is an exponential function of the length of grain boundaries in samples of both types.

High-field Josephson magnetic resonance in high-temperature superconductors

The possibility of high-field Josephson magnetic resonance (HFJMR) in high-temperature super-conductors (HTSCs) is theoretically demonstrated. The HFJMR mechanism is related to the Josephson oscillations in intergranular junctions of a polycrystalline HTSC, which are caused by the penetration of Abrikosov vortices into the grains. In fields within H c1 ≤ H < H x (where H c1 is the first critical field and H x is the field corresponding to vortex separation from pinning centers in the grains), measurements of the microwave absorption spectra of a sample can be used for investigations of the HTSC macrostructure and the properties of vortex matter. In the latter aspect, the HFJMR method is analogous to the SQUID technique.

Study of current–voltage characteristics of Bi-based high-temperature superconductors with fractal cluster structure

We study the effect of the structure on critical currents and current–voltage characteristics (CVC’s) of foamed bismuth-based polycrystalline high-temperature superconductors (HTSC). The fractal cluster structure of superconducting foams has been observed and the fractal dimension of boundaries between superconducting and normal clusters has been determined. Based on the magnetic and transport properties of the foamed polycrystalline superconductors, we have shown that the initial parts of CVC’s of the superconducting foams are described well by the model that accounts the magnetic flux trapping in fractal clusters of a normal phase.

Magnetization and Critical Current of High-Temperature Superconductors with Artificial Pinning Centers

We present new data on magnetization at T = 4.2 and 77 K for polycrystalline high-temperature superconductors of the Bi2Sr2Ca2Cu3O10+δ system containing various amounts of nanodimensional inclusions of tantalum carbide, niobium carbide, or niobium nitride. The introduction of these nanoparticles (≤30 nm in size) leads to an increase in the magnetization and the critical current density. It is established for the first time that the dependence of the normalized critical current on the bulk concentration of indicated dopants is described by the same universal curve. The interval of the optimum dopant concentrations is found, in which the additives lead to the maximum increase in the critical current density.

Temperature evolution of the hysteresis in the current-voltage characteristic of a polycrystalline high-temperature superconductor with 1-2-3 structure

The temperature evolution of the current-voltage (I-U) characteristic of a contact of the break-junction type with direct conduction is investigated on a polycrystalline HTSC of the Y-Ba-Cu-O system. The experimental I-U characteristics possessing a hysteresis are correctly described in the framework of the Kummel-Nicolsky theory for an S-N-S contact (S stands for a superconductor; N, for a normal metal) in which the Andreev reflection of quasiparticles from the N-S interface is considered. It is shown that the shape of the I-U curve, as well as the existence of a hysteresis, is determined by the ratio of the number of “long” and “short” intergranular boundaries in the polycrystal under investigation. The coincidence of the calculated and experimental I-U curves made it possible to estimate the effective length of “natural” intergranular boundaries in polycrystalline HTSC materials. The estimate is obtained from the experimental temperature dependence of the critical current in the sample under investigation.

Computer simulation of current percolation in polycrystallinehigh-temperature superconductors

YBCO-coated conductors were modelled in a computer simulation using aresistor network concept, with the resistors representing the grainboundaries. Dissipation above the critical current, accompanied by fluxpenetration into the grain boundaries, was described by a linear (flux-flow)resistivity. The model allowed calculation of the combined percolation ofcurrent and magnetic flux. Current-voltage data showed scaling in agreementwith percolation theory for two-dimensional systems. The influence of grainalignment and electromagnetic parameters on conductor performance wasinvestigated.

Theory of percolative conduction in polycrystalline high-temperature superconductors

Conduction in bulk polycrystalline high-${T}_{c}$ superconductors with relatively high critical currents has been shown to be percolative. This phenomenon is due to weak links at grain boundaries. These weak links are the major limiting factor for technological applications that require high-current densities. We formulate a model of these materials that can be reduced to a nonlinear resistor network. The model is solved by analytical approximations and a new numerical technique. The numerical technique is variational, which makes it capable of solving a wide variety of nonlinear problems. The results show that the presence of a distribution of critical currents in the sample does not erase all information about the dissipative electrical properties of individual boundaries. This means that an unambiguous connection can be made between the $I\ensuremath{-}V$ characteristics at the microscopic level and the macroscopic electrical properties.

Thermomechanical response of polycrystalline BaZrO 3

Thermomechanical properties of fine-grained, 96% dense, polycrystalline BaZrO 3 specimens were studied. The average thermal expansion coefficient of BaZrO 3 at 25–1000°C was 7.72×10 −6°C −1. At 25°C, Young's modulus was 240.5 GPa and the shear modulus was 97.2 GPa. These values decreased by ≈10% with heating to 1015°C. Poisson's ratio was 0.237 and was independent of temperature. The BaZrO 3 specimens exhibited a thermal expansion coefficient of about one half, and elastic moduli more than twice, those of polycrystalline high-temperature superconductors. Diffusional creep was observed when BaZrO 3 was compressed at 1300–1400°C; the activation energy for steady-state deformation was 460±30 kJ/mole. The mechanism was probably grain-boundary sliding, controlled by diffusion of one of the cations along grain boundaries. Deformation rates below 1000°C were too low to relax thermally generated stresses.

Role of c-axis field components in making the transport E(J) characteristics of (Bi,Pb)2Sr2Ca2Cu3Ox tapes insensitive to the direction of field within the tape plane

We report extended (over four decades) electric field versus current density characteristics of (Bi,Pb)2Sr2Ca2Cu3O10 tapes in magnetic fields applied along each of the three principal tape axes at 77 K, and along the two principal axes in the tape plane at 4.2 K. The characteristics are almost independent of field direction in the tape plane at both temperatures. Similar insensitivity of the critical current to magnetic field direction observed in other polycrystalline high temperature superconductors has been ascribed to the percolative nature of current flow, while the insensitivity in (Bi,Pb)2Sr2Ca2Cu3O10(2223) films and Bi2Sr2CaCu2O8 single crystals has been attributed to c-axis conduction across Josephson junctions. We show that the insensitivity of the critical current density to the field direction of these tapes follows naturally from the fact that the dissipation depends only on the field component lying along the c axis of individual 2223 grains. Lorenz-force driven motion of the pancake vortices that form within each 2223 grain provides a dissipation mechanism that involves only the c-axis components of the field. The angular dispersion of the grains in the tape ensures a c-axis field component in most grains, regardless of the applied field direction. This produces the remarkable insensitivity of E(J) over some four orders of magnitude of the electric field.

Improvement of the cation-distribution in the polycrystalline high-temperature superconductor LaSrCuO

Results obtained by high resolution EDX measurements which demonstrate that polycrystalline samples of the LaSrCuO-system prepared by means of a refined wet-chemical preparation method exhibit a much better homogeneity of the cation distribution than samples prepared by standard ceramic routes. Other characteristic properties like FC flux expulsion, phase purity and behaviour of resistivity are at least preserved. The proposed wet-chemical method is similar to several spray-drying techniques described in the literature but easier to handle and optimized for LSCO.

Isolation techniques and electrical characterization of single grain boundaries of Bi 2Sr 2CaCu 2O 2 high-temperature superconductor

We have isolated single grain boundaries of a bulk material of polycrystalline high-temperature superconductor Bi 2Sr 2CaCu 2O 2 (Bi-2212) tapes. The tapes were grown on a single crystal of MgO or Ag-foil by partial melting of the superconductor. The isolation techniques consisted of fabricating metal micro-contact pads, wire bonding, and laser patterning of the Bi-2212 superconductor. Results of electrical and magnetic measurements of single grain boundaries suggested a possible existence of Josephson-like junctions coexisting with flux flow junctions. In contrast to bulk samples, T c of a single grain boundary did not change by applying magnetic field. J c and T c were found to be greatly dependent on heat treatment and growth conditions. The current–voltage characteristics of a single patterned bridge followed a relation of the form V∼( I− I c) n for I> I c with n∼1.2.

Current-influenced voltage noise in bulk polycrystalline high- Tc superconductors

The 1/ f voltage noise in bulk polycrystalline high-temperature superconductors (HTSC) under bias current and magnetic field has its origin in the noise current-dependence of the grain boundary junctions (GBJs), due in turn to the correlated effects of junction critical current and normal resistance fluctuations. The analogy between the results obtained by varying the bias current through the specimen and those performed with temperature as variable is evidenced. The noise maxima obtained in both sets of measurements turn out to be caused by the junction critical current fluctuations, which dominate when the currents flowing through the GBJs are close to the Josephson critical current. The anti-phase correlation between the normal resistance and the critical current fluctuations is responsible for the monotonical decrease of the noise at constant bias current, with the temperature exceeding the value corresponding to the noise maximum. In contrast, varying the bias current at fixed temperature, the voltage noise exhibits a local minimum followed by an increasing tendency after passing through the maximum.

Correlation of grain size and ac susceptibility in the Bi 2Sr 2CaCu 2O 8 system

Here we report the influence of small grain size (from 9–125 nm) of the Bi 2Sr 2CaCu 2O 8 system on the ac susceptibility. The mean grain size of the samples was determined by powder x-ray diffraction (XRD). The results showed that unlike the usual polycrystalline high-temperature superconductors, our present samples with small grain sizes displayed paramagnetism, and the paramagnetism became weaker and weaker by increasing the grain size. The peculiar paramagnetic properties of our samples was thought to be caused by the small grain size effect like other nanocrystalline materials.