Intergranular Critical Currents Research Articles

Structural, thermal and superconducting properties of Ag2O-doped YBa2Cu3O7-x composite materials

A composite material with superconducting properties, consisting of YBa2Cu3O7-x doped with Ag2O, was obtained after 233 h of sintering at 960 °C and followed by a special cooling program in oxygen atmosphere. Preferential orientation of YBa2Cu3O7-x (YBCO-123) crystallites was evidenced through X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Small increase of c parameter for orthorhombic YBCO-123 crystalline unit cell together with a slight decrease of oxygen content (7-x) versus Ag content, were encountered. The dimensions of YBCO-123 crystallites are dependent on Ag content. Thermogravimetric analysis (TGA) during heating in flowing air atmosphere from room temperature (RT) to 1100 °C, showed that total mass loss is depending on Ag content. The cooling (from 1100 °C to RT, in air atmosphere) endows optimal oxygen accumulation only for 0 and 5 wt% Ag, while final masses for 10 and 20 wt% of Ag content, are more than 10% less. Differential scanning calorimetry (DSC) showed peritectic melting points for YBa2Cu3O7-x dependent on Ag percentage. DSC graphics also highlighted the endothermic effects: silver melting; melting and decomposition of Ag2O (in 225.81–345.12 °C temperature range) only for 10 and 20 wt%, but completely missing in the case of 5 wt% Ag content. We concluded that some of Cu sites from YBCO-123 superconductor were substituted by Ag ions. Quasi de Almeida-Thouless lines in the range of magnetic field amplitudes from 0.4 to 800 A/m, in the temperature range from 77 K to RT, were also investigated. Two cases are found concerning intergranular critical currents (Jci): depinning mechanism available for 5 wt% Ag content; the decoupling mechanism for 10, and 20 wt% Ag content.

The superconducting properties of YBa2Cu3Oy ceramics fabricated using ultrahigh dilution technology

YBa2Cu3Oy (Y-123) is one of the most intensively studied high-temperature superconductors (HTS) and has great potential for applications in electronics. Its superconducting properties significantly depend on the oxygen index (y), the peculiarities of the atomic crystal and microgranular structure, the type and concentration of contaminating atoms. Therefore, of particular interest are the techniques of HTS synthesis that significantly affect the superconducting properties, as well as modifications of these techniques. The aim of this study was to investigate the effect of ultrahigh dilution (UHD) technology on the Y-123 HTS properties.We applied UHD technology to treat Y2O3·4BaO2·6CuO composition (before and/or after calcination) and used solid-phase reaction to obtain batches of ceramic samples of the Y-123 phase. Main characteristics of the synthesized HTS specimens (the orthorhombic unit cell parameters, temperature dependence of electrical resistance, Meissner effect, and critical current density) were studied.X-ray phase analysis demonstrated that the samples were practically single-phase and consisted of the Y-123 phase. Depending on the treatment method, the modifying effect varied. Positive and most significant results were demonstrated for specimens obtained from precursor powder mixture treated with UHD. For some of such specimens an increase in the temperature of the beginning of the transition to the superconducting state by 3.8 K, the Meissner effect by 9%, and the density of the intergranular critical current by 35% were observed. We conclude that UHD technology can be used to improve properties of HTS. However, methodology for implementation of UHD technology should depend on which specific superconductor property needs to be changed.

Investigation of the effect of Ag addition on the critical current density of the high-temperature superconductor Nd1.85Ce0.15CuO4

We have synthesized a Nd1.85Ce0.15CuO4 + Ag composite system with the aim of studying the effect of Ag addition in the electron-doped system Nd1.85Ce0.15CuO4 on its superconducting and magnetic properties. Measurements of magnetization using a vibration sample magnetometer indicate a systematic increase in diamagnetic shielding upon Ag addition. A subsequent analysis of the critical current density using the Bean model indicates a small but significant increase in intra-grain critical current density upon Ag addition. From the study of the dependence of electrical resistivity on the temperature and electrical current, an increase in inter-granular critical current has also been evidenced upon Ag addition. These results correlate well with the earlier reports of a similar improvement in the properties on the hole-doped ceramic superconductor/Ag composites. Possible reasons for the enhancement of JC have been described and discussed. Magnetic characterization by AC susceptibility using a SQUID magnetometer has been presented for a representative composition, which enabled delineation of the inter- and intra-granular transitions.

AC Losses in Sn-Doped Bi1.6Pb0.4Sr2(Ca1-xSnx)2Cu3Oδ Superconductors

Measurements of complex AC susceptibilityχ = χ + χas a function of temperature have been carried out on Sn-doped Bi1.6Pb0.4Sr2(Ca1-xSnx)2Cu3Oδsuperconductor samples prepared via the conventional solid state reaction method. All the samples exhibit perfect diamagnetism below 109 K. Theχ (T)curves display two-step features, indicating the presence of mixed phases and therefore weakening of the grains' coupling. The amount of shielded volume in Sn-free samples is greater than that in Sn-doped samples. The intrinsic peak due to the small AC losses within the grain was not observed in theχ (T)curves for all samples. However, the coupling peak,TP, for Sn-free samples at an applied field of 1.0 Oe was observed at 89 K and shifted to a lower temperature ranging from 59 K to 64.2 K in Sn-doped samples. The amounts of hysteresis losses above theTPin all doped samples were smaller than that of the Sn-free sample. Therefore, the effect of Sn doping suppressed the inter-granular critical current,Jcm, and the presence of weak links that coupled the superconducting grains.Keywords: BSCCO, AC Losses, Sn-doped, Superconductor

The effect of the return fields of magnetized grains on flux trapping in type IIsuperconductors

Several workers have reported on the observation of a peak, in the magnetic flux trappedby polycrystalline type II superconductors, when plotted versus the cooling fields fromTC, and versus the magnitude of the half-cycle fields applied after zero-field cooling. Wereport on our observations of this behavior in a niobium sample and present a detailedmodel which accounts for these observations. The crucial feature of the model is that ittakes into account the effect of the return magnetic field of the magnetized grains on themacroscopic continuous intergranular persistent critical currents induced to circulate in thespecimen by the externally applied magnetic field. Our analysis indicates thatattempts to trap magnetic fields, greater than the calculated peaks, inside hollowand solid cylinders can lead to significantly diminished results. Our analysis mayalso apply in calculations of hysteresis losses in type II superconducting cables.

Effective demagnetization factor of quasi-single-crystal and granular YBa2Cu3O7−x thin disks

The effective demagnetization factor neff of quasi-single-crystal and granular YBa2Cu3O7−x thin disks with different values of the edge barrier height, bulk pinning, and demagnetization is measured by a Hall probe with the use of a trapped magnetic flux as a test field. As the trapped magnetic flux increases, neff reaches a maximum as a function of the external field; the qualitative behavior of this maximum is independent of the morphology of a sample. It is shown that, when the densities of intragranular and intergranular critical currents differ by about one and a half orders of magnitude, the demagnetization factor neff is mainly determined by the geometry of a sample rather than by the energy of Josephson coupling between grains. The experimental results are in agreement with the results of calculations performed within the proposed model of isolated crystallites (grains).

MO observation of flux penetration into high- Tc superconductors with grain boundaries

Effect of grain boundaries on the magnetic field distributions in YBCO was studied by use of magneto-optical (MO) imaging technique. The grain boundary was artificially introduced with a multi-seeded melt-growth process. The current distribution in the sample was obtained from the MO image using an inversion scheme based on the inverse-matrix method. With this technique, we could obtain intergranular and intragranular critical current densities. Large depression of the intergranular critical current was observed in the sample with large misorientation angles.

Improved grain boundary currents in textured YBa2Cu3Oδ thin films on bicrystalline Ni substrates

Using magneto optical imaging a comparative study of YBa2Cu3Oδ thin films grown on Gd-10at% doped CeO2 buffered Ni and SrTiO3 bicrystals with tilt angles ranging from 3° to 16° has been made. The Ni substrates have been chosen as a model system for the RABiTS coated conductor technology. It is found that the YBa2Cu3Oδ grain boundaries on Ni exhibit improved superconducting transport properties, while for the intra-granular critical currents a moderate decrease is observed. The improved Jgb/Jc ratio could be detected for all studied grain boundary angles as well as over the whole accessible temperature range from 8 to 60 K. Extensive x-ray measurements on the in- and out-of-plane texture of all layers have been performed to explain the change in the intra- and inter-granular critical currents.

Enhancement of orthorhombicity, Tc, shielding and irreversibility line in argon preheated Sm(SrBa)Cu 3O 6+ z

We have studied the structural and superconducting properties of the superconductor SmSrBaCu 3O 6+ z . This compound when annealed in oxygen at 450 °C showed a tetragonal structure and a T c of 79 K. When the same sample was heated in argon followed by oxygen annealing; we observed an orthorhombic structure and an increase of T c by 6 K. Further, there was an enhancement in the shielding and the intergranular critical current. A combination of several factors such as decrease in the Cu(1)–apical oxygen distance, increase in phase purity, in hole density and chain oxygen ordering for the [AO] sample may qualitatively account for the observed data.

AC and DC magnetic studies of critical currents in ceramics of the Ho 1− xPr xBa 2Cu 3O 7− δ system

AC magnetic susceptibility measurements were performed for ceramics of the Ho 1− x Pr x Ba 2Cu 3O 7− δ system as a function of temperature, frequency, field amplitude and static magnetic field. From the measurements, temperature dependences of intergranular critical currents and corresponding pinning energies at grain boundaries were determined for different Pr concentrations. These critical currents strongly decrease with Pr doping. They are limited by superconductor–insulator–superconductor or superconductor–normal metal–superconductor junctions and for higher Pr concentrations additionally suppressed by flux creep. Also intragranular critical current at 4.3 K was determined as a function of Pr concentration from magnetic hysteresis loops. It is a monotonically decreasing function of Pr doping.

The effect of Bi2201 phase on the intergranular critical field and current density in Bi2223 superconductors

The phase equilibria between Bi2201, Bi2212 and Bi2223 were established relative to the sintering temperature. By improving the sintering conditions (temperature, intermediate grindings), a procedure was determined to prevent the formation of Bi2201 phase and also to obtain nearly pure Bi2223 powder. It was found that the residual Bi2201 phase is an important limiting factor of superconducting properties—samples rich in the Bi2201 phase exhibited almost no intergranular critical currents and fields, while the highest superconducting values were found in the samples free of the phase. Magnetic and transport measurements showed that the transport behaviour of the grain boundaries becomes semiconducting in samples sintered at high temperature, in which an increase of the Bi2201 phase content is observed.

The influence of lithium halides on the superconducting properties of YB2Cu3O7-x[YBa2Cu3O7-δ

The effects of addition to YBa2Cu3O7−x of lithium halides (YBa2Cu3O7−x (LiF)y, and YBa2Cu3O7−x (LiCl)y) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to y= 0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48K. for y= 0.02 in YBa2Cu3O7−x (LiF)y and 91.30 K in YBa2Cu3O7−x (LiCl)y), but for higher concentration of Li it rapidly decreases (81.68K for y= 0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law:jc∝B−α, with a lithium-concentration-dependent α. The voltage–current characteristics follow a law typical for granular superconductors, V∝(I−Ic(B,T))n(B,T). The dependence of the intergranular critical current, Ic, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.

Influence of the mass density on the critical current density of the bulk Tl 2Ba 2Ca 2Cu 3O 10 material

The influence of the mass density of bulk Tl 2Ba 2Ca 2Cu 3O 10 sample on its critical current density is investigated. An empirical relationship between the sample density and the transport critical current density was found and this relationship can be explained by assuming that the intergranular critical current is approximately inversely proportional to the average linear separation of the grains.

Intergranular critical currents and excess conductivity in the Fe-substituted Bi-2223 high Tc superconductor

In this paper me present the effect of iron substitution in the (Bi1.6Pb0,4)(Sr1.88Ba0.2)Ca2(Cu1−xFex)3.1Oy system. From the resistivity and AC susceptibility data we observed that a monotonie decrease of Tc occurs with increasing Fe concentration. A strong reduction of Jc (calculated from AC susceptibility data performed at low magnetic fields in the range 0.4A/m-800A/m) is observed in the doped samples. The crossover temperature from the Anderson-Kim thermally activated flux creep to the thermally activated phase slippage behaviour is depending on the doping concentration. The excess conductivity and 2D-3D crossover is well described by considering Δσ/σ0 as a product of a Gaussian centered at Tc with a Student distribution centered at 2Tc. The alteration of 2D dimensionality above Tc by doping results in the decrease of the number of freedom degrees of the Student distribution.

A theoretically based critical-state model for granular high- Tc superconductors applied to YBCO

A critical-state model, based on a critical current density dependence on internal field J c ( H), which has been modeled for a random 3D network of weak-link Josephson junctions assumming spatial distribution of current in an individual grain- boundary junction (GBJ), is presented. The model is used in an analysis of isothermal magnetisation and pulse transport critical current measurements on sintered 123-YBCO superconducting material in the low-field “Josephson” regime. The analysis yields self-consistent agreement with experiment for both types of measurement. It also gives predictions for the superconducting parameters of the inter- and intra-granular material and yields some physical insights into the behaviour of the junctions. In particular, there exists a field-independent component of the inter-granular critical current which depends on the GBJ microstructure. In the present specimen material it generally increases with decreasing temperature but shows a marked increase at about 60 K corresponding to the onset of superconductivity in the 60 K oxygen-deficient phase in the GBJ.

Enhancement of orthorhombicity and superconductivity in argon preheated EuSrBaCu 3O 6+ z

We have studied the structural and superconducting properties of the superconductor EuSrBaCu 3O 6+ z . This compound when annealed in oxygen at 450°C showed and orthorhombic structure in contradiction to the earlier published data. When the same sample was heated in argon followed by oxygen annealing, the orthorhombicity ( b− a)/( b+ a) increased from 2.86X10 −3 to 6.96X10 −3 and T c from 80 to 87 K. Further, there was an enhancement in the shielding and the intergranular critical current. A combination of several factors such as changes in the Cul-apical oxygen distance, chain oxygen ordering and hole density may qualitatively account for the observed data.

Sample size dependence of the AC-susceptibility of sintered YBa 2Cu 3O 7−δ superconductors

The field dependence of both the intergranular peak temperature, T m and the intragranular peak temperature, T g, of the imaginary component, χ″, of the AC-susceptibility has been studied for sintered YBaCuO samples having orthorhombic shape and different thicknesses. A strong systematic influence of sample thickness on the field dependence of T m was found. The conclusion has been drawn that the “irreversibility line” defined as a relation between the χ″-peak temperature and the AC-field amplitude being sample size dependent is not an “intrinsic” property of the material alone. Following Chen's approach, the temperature dependence of the full penetration field, H p ( T), and the average intergranular critical current, J c( T), have been deduced from the field dependence of T m. The relation between the intergranular χ″-peak temperature T g and the AC-field amplitude does not show a sample thickness dependence.

Current-activated residual surface resistance in high temperature superconductors

A prominent and unusual feature of the microwave surface resistance of high temperature superconductors is a comparatively large “residual” resistance observed from a few degrees below the transition temperature, to the lowest temperatures. This resistance, which varies slowly with temperature, is also observed to vary with the rf field amplitude, sample quality, and frequency. Using a classical treatment for the rf field, we show that a surface resistance ∼ mohms arises from current-activated phase-slip resistivity. At low temperatures, this contribution to the surface resistance varies approximately as ωH rf/J c0, where ω is the angular frequency, H rf is the rf field amplitude, and J c0 is the inter-granular critical current.

Inductive measurements of intergranular critical currents in ceramic rings

Abstract A comparative study of superconducting properties at T =77 K and T =4.2 K was made between ceramics prepared by conventional sintering techniques and ceramics which were melt processed. The field dependence of the critical current was determined by means of an inductive method. Using ring shaped samples the intergranular currents can be unambiguously separated from the intragranular currents since the magnetic flux shielded by the screening current circulating in the ring is monitored instead of the magnetization. Values for j c obtained this way are in good agreement with results from resistive measurements of the same sample. Except for Bi 2 Sr 2 CaCu 2 O 8+δ all our sintered materials show weak link behaviour. Melt processing was favourable for Tl 2 Sr 2 Ca 2 Cu 3 O 10+δ and especially for Bi 2 Sr 2 CaCu 2 O 8+δ , but led to poor intergrain current densities in YBCO.

A contactless method for measurement of the critical current density and critical temperature of superconducting films

A new method is described for measuring the critical current density and transition temperature of a superconducting film without making contact to it or modifying it in any way. This technique is particularly well suited for use with high transition temperature oxide films which are notoriously irreproducible and sensitive to patterning. It consists of positioning a flat, multiturn coil near the film surface and driving the coil with an audio frequency sine-wave current. Induced shielding currents flow in the film. We have calculated the radial dependence of the induced currents and show that the induced current density is zero at the coil center, rises to a maximum near the mean radius of the drive coil, and then falls off rapidly as the radius continues to increase. A measurement of the critical current per length can be obtained by monitoring the development of odd harmonic voltage components across the coil as the drive current is increased. We find that this measure of nonlinearity in the coil–film system increases abruptly when the maximum induced current equals the critical current. The critical current obtained by this inductive measurement has been demonstrated to give nearly the same value as that obtained by a transport measurement. Other advantages to this inductive measurement approach include: sensitivity only to intergranular critical current, and an ability to accommodate large substrates. Finally, with only minor modification to the electronics used to measure Jc, the superconducting transition temperature of the sample may be measured as well.