Intergrain Critical Current Research Articles

Thermal properties, texture, second-order stress, and magnetic susceptibility of the Bi1.8Pb0.3Sr2Ca2Cu3.3Ox

Mono-phase high critical temperature (Bi,Pb)-2223 superconductor from the off-stoichiometric Bi1.8Pb0.3Sr2Ca2Cu3.3Ox belonging to the tetragonal system, has been obtained. We studied the crystalline structure, the stress of second order, and the texture of the surface pellet, in the framework of XRD diffractometry. Results of the thermal measurements made in the nitrogen, with a heating rate of 10 K min−1, from room temperature (RT) to 1273.15 K, show a slow mass decrease of 1.25 % from RT to 1003.15 K probably due to the elimination of water, oxygen, and of the other gases accumulated on the crystallite surface through chemical adsorption. Two endothermic processes were evidenced on DSC, and DTG curves: the first in the range of 1135.15–1193.15 K (melting and slow decomposition), and another after 1217.45 K (decomposition). The contribution of crystal lattice to the estimated specific heat capacity was in conformity with the Einstein model, giving for the Einstein temperature a value of 1297.5 K. In conformity with the Muller critical state model, we found linear dependences for intragrain critical transition temperature, Tg, and intergrain coupling temperature, Tp, versus the amplitude of the external AC magnetic fields. There was evidenced that intergrain critical currents are five orders smaller than the intragrain critical currents.

Intergrain Effects in the AC Susceptibility of Polycrystalline LaFeAsO0.94F0.06

The AC susceptibility, χ, at zero DC magnetic field of a polycrystalline sample of LaFeAsO0.94F0.06 (T c ≈24 K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range H ac =0.003 Oe÷4 Oe) and the frequency (in the range f=10 kHz÷100 kHz). The χ(T) curve exhibits the typical two-step transition arising from the combined response of superconducting grains and intergranular weak-coupled medium. The intergranular part of χ strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T c down to T=4.2 K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.

Revealing the Anisotropy in MgB2 Superconductor and Investigation of Critical Current near Critical Temperature by Using Hall Probe Susceptibility Technique

We report the results of an investigation on critical current measurements in bulk MgB2 superconductor by using Hall probe ac susceptibility. The temperature versus ac susceptibility has been measured as a function of external ac magnetic field with no dc part and magnitude in the range 240–1200 A/m by using the Hall probe method. The real part of the Hall probe susceptibility showed two-step transitions near critical temperature. We attributed this behavior to the anisotropic nature of the MgB2 compound. Close to the critical temperature the estimated anisotropy parameter is γ≈2.5, a value that is in fair agreement to the ones reported in the literature for polycrystalline and single crystals MgB2. We have also analyzed the susceptibility data within the framework of a critical state model (using the real part of the susceptibility) and an anisotropic nature of MgB2 compound. The inter-grain critical current is found to be 4×106 A/m2 at 39.59 K by using the critical state model. The temperature dependence of the critical current density was found to be in the form of J c (T)∝(1−T/T c ) β , where β was calculated as 2.30 by using critical state model, respectively. Scanning electron microscopy was used for grain size estimation.

Microstructure and superconducting properties of the DyBaCuO ceramic doped with Na2CO3, NaCl, and KClO3

This paper reports on a study of the effect of doping with Na2CO3, NaCl, and KClO3 salts on the microstructure and superconducting characteristics of ceramics with nominal compositions Dy1 − xMxBa2Cu3 − yO7 − δ and DyBa2Cu3 − yO7 − δ: Mx for M = Na, K; x = 0.2, 0.3, and y = 0, 0.2. The microstructure was characterized by transmission electron microscopy with local energy-dispersive x-ray in situ analysis (probe size ∼1 nm). An analysis shows that none of the doping elements (Na, K, or Cl) enters into 123 grains in sizable amounts and that, as a result, the critical temperature of the superconducting transition remains practically constant in the range 90.0–93.5 K. Potassium and chlorine segregate at grain boundaries. It is shown that grain-boundary segregation of chlorine leads to a substantial increase in the superconducting critical current (by a factor 3–5 at 70 K) as compared to the undoped sample. The possible mechanisms accounting for the effect of Cl on intergrain critical current are discussed.

Magnetic interaction between multiple seeded YBCO bulks and the permanent magnet guideway

Magnetic interaction between multiple seeded melt growth (MSMG) bulk YBCO and the permanent magnet guideway (PMG) has been investigated. There are two modes: parallel and perpendicular between the length directions of the PMG and MSMG bulks. We employ one piece of MSMG bulk and a stacked array consisting of three separated bulks in our experiments. They have similar dimensions and their maximum levitation forces are almost the same except for a slight difference in parallel mode. It is found that the MSMG bulk may be regarded as single larger grain bulk in the perpendicular mode due to inter-grain critical currents and it has much larger levitation force than the stacked bulk array. On the other hand guidance force of the MSMG bulk is larger than the stacked array in both parallel and perpendicular modes. In addition, its hysteresis loss becomes less in perpendicular mode. These indicate that MSMG bulk YBCO is preferable to optimize the present superconductor-PMG levitation system.

Microwave Josephson absorption of high-temperature superconductors

Magnetic field dependences of the direct and the magnetically modulated microwave absorption of a number of Y-, Bi- and Tl-based high-temperature superconductors were measured at 4.2 and 77 K by X-band microwave spectrometer. Correlation between value of the low field maximum of the absorption and the inter-grain critical currents of Josephson junctions was found. Results were analyzed and compared to theoretical models of microwave absorption available in the literature yielding good agreement.

Influence of the grain boundary network on the critical current ofYBa2Cu3O7films grown on biaxially textured metallic substrates

YBa 2 Cu 3 O 7 /YSZ/CeO 2 heterostructures have been grown epitaxially on biaxially textured Ni substrates by pulsed laser deposition. The texture of the film was determined by electron backscattering diffraction, providing information on the propagation of the grain boundary network from the Ni substrate to the YBa 2 Cu 3 O 7 film via the epitaxial growth. The grain boundary network limits the critical current density to 0.3 MA/cm 2 (77 K, 0 T), compared with 1.3 MA/cm 2 (77 K, 0 T) for a film grown on a single crystalline Ni substrate. Transport measurements on the coated conductor sample at different temperatures and magnetic fields show that there is a crossover field between intergrain and intragrain critical current that is shifted to higher magnetic fields as the temperature is reduced.

Influence of heat treatment at 550-810°C on inter- and intragrain critical currents of Bi, Pb-2223 ceramics

The degradation of the superconducting phase Bi1.65Pb0.35Sr1.8Ca2.2Cu3Oy and its characteristics: electrical resistivity R, critical temperature Tc (R=0), critical current density jc, and volume fraction of Meissner phase Cm, have been investigated at 550–810°C and PO2=2·101·1·105 Pa by X-ray diffraction, resistance, inductance and magnetic methods. It was established that at PO2≥103 Pa degradation of the ceramic samples runs through the solid solutions decomposition. As a result, the intergrain critical current is decreased 10–20 times while the intragrain one is increased 1.5–2 times. At PO2=101−102 Pa the inter- and intragrain critical currents are decreased by 5–10 times and by 1.5–2 times respectively as a result of oxygen extraction without visible destruction of the Bi,Pb-2223 structure.

Intergrain magnetic properties and critical currents of T1-1223 bulk and tape superconductors

The magnetic field and the temperature dependencies of the dispersive and the absorption components of the low frequency ac magnetic susceptibility for Pb and Bi single doped bulk superconductors as well as for Pb and Bi co-doped Ag-sheathed superconducting T1-1223 tapes have been measured and analyzed. The studies were accompanied by optical and electron microscopic observations of the microstructure. The ac susceptibility measured at various ac magnetic field amplitudes showed both intergrain and intragrain features. The temperature dependencies of the intergrain critical current densities were extracted from the absorption susceptibility via a modified Bean model. They were at relevant temperatures one order of magnitude larger for the tape than for the bulk specimens.

Low field modulated microwave absorption and electrical properties in degraded and annealed granular YBa 2Cu 3O x

Modulated microwave absorption was examined on granular YBa 2Cu 3O x. The hysteric signal can be explain by three factors related to surface flux distributions, and by intergrain Meissner and critical current.

Modified dislocation model of intergrain tilt boundaries in HTSC

Stress fields at the intergrain boundaries of three different types (symmetric and asymmetric tilt boundaries with a periodic system of edge dislocations, and a symmetric system with randomized edge dislocations) are calculated. Based on these calculations the angular dependence of intergrain critical currents in superconductors is found. It turns out that an adequate description of the superconductive features of the boundaries could only be obtained in the framework of randomly distributed dislocations.

Modulation of HTSC characteristics induced by proton irradiation

Bulk slabs of sintered YBCO were irradiated with monoenergetic protons at different energies (3.5 and 6.5 MeV) and different fluences, ranging from 1.8·1016 to 8.6·1016 p/cm2. The effects on intergrain critical currents of protons crossing the sample and the effects of protons implanting in the specimen, has been studied by means of a.c. susceptibility characterizations. D.c. hysteresis cycles were measured in low fields in order to investigate the modifications in the intergrain critical current density due to proton implantation. The main result is the evidence that the intergrain J c response to the applied field can be modulated by means of the proton irradiation. Resistive measurements were also performed in order to investigate the complex transport phenomena in polycrystalline materials. Irradiation-induced effects on both superconducting- and normal-state resistivity are studied. The addition of extrinsic defects can lead to enhanced transport properties, emerging especially when a magnetic field is applied.

配向性ＹＢＣＯ焼結体上の磁場分布と粒内臨界電流密度

The magnetic field distributions at the surface of field-oriented polycrystalline YBa2Cu3Ox disks were observed in a flux penetration process by a magneto optical method using magnetic films. The observed magnetic field has a minimum near the edge and was nearly constant in the inner region. It was found that such a distribution is characteristic of superconductors with weak links at the grain boundaries and that the inter-grain critical current is negligible in comparison with the intra-grain critical currents. We proved that the intra-grain critical current can be estimated from the difference between the applied field and the magnetic field in the inner region.

Magnetic field distribution at the surface of field-oriented YBa 2Cu 3O x polycrystals and the critical current

Magnetic field distributions at the surface of field-oriented polycrystalline YBa 2Cu 3O x disks were observed in a flux penetration process by a magneto-optical method using magnetic films, and intergrain and intragrain critical currents ( J ct and J cg) were investigated. Three garnet films with various saturation fields were used as magnetic films. The value of J ct was investigated using the garnet film with low H s, from the applied field at which the flux penetrates to the center of a specimen. Using the garnet film with high H s, the magnetic field distribution was observed. The magnetic field has a minimum near the edge and is nearly constant ( H 0) at the inner region. It turned out that such a distribution is characteristic of superconductors with weak links at the grain boundary. We prove for the first time that the J cg can be estimated from the difference between the applied field and H 0.

Effect of intergrain junctions and flux pinning on transport critical currents in YBa2Cu3O7- delta granular superconductors.

Studies of the dependence of the intergrain critical currents on temperature (60--90 K) and magnetic field (0--100 G) were performed for samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ ceramics (YBCO) and of a 2-wt.% Ag-doped ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ composite (YBCO-Ag). The measurements were done using ceramic rings in a persistent mode and a scanning cryogenic Hall probe. This technique allowed us to satisfy conditions not available in the studies of critical currents before. A self-sustaining supercurrent (sensitive to any resistive dissipation) was used to detect the intergrain critical current ${\mathit{I}}_{\mathit{c}}$. Its magnitude and relaxation were measured using a contactless method (a Hall probe). The results of the measurements of ${\mathit{I}}_{\mathit{c}}$ on temperature close to the intergrain ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$ confirmed the presence of the superconductor-insulator-superconductor tunnel intergrain junctions in YBCO and the superconductor--normal-metal--superconductor proximity junctions in YBCO-Ag.For YBCO, the ${\mathit{I}}_{\mathit{c}}$(T)=const(1-T/${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$) dependence is not affected by the applied magnetic field, however, for YBCO-Ag the ${\mathit{I}}_{\mathit{c}}$(T)=const(1-T/${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$${)}^{2}$ dependence is not preserved, suggesting a strong effect of magnetic flux creep on critical currents. The intergrain ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$ is about 5 K lower than the intragrain ${\mathit{T}}_{\mathit{c}}$ and about 4 K lower than the ``zero-resistance ${\mathit{T}}_{\mathit{c}}$'' measured using a conventional I-V technique. Dissipation of a persistent current measured in the YBCO-Ag ring provided strong evidence that the transport current is controlled by an intergrain flux creep with the energy barrier proportional to ${\mathit{I}}_{\mathit{c}}$. The results revealed that the intergrain critical current density ${\mathit{J}}_{\mathit{c}\mathit{T}}$ in YBCO-Ag has lower values than ${\mathit{J}}_{\mathit{c}\mathit{T}}$ of YBCO, and the observed higher total critical currents are due to a larger surface area of grain-boundary conduction in this composite.

Determination of the intergrain critical-current distribution for polycrystalline (Tl,Pb)(Ba,Sr)2Ca2Cu3Oy superconductors from ac-susceptibility data.

The intergrain critical-current-density (${\mathit{J}}_{\mathit{c}\mathit{i}}$) distribution and intergranular volume fraction of ceramic superconductors were determined from ac-susceptibility measurements. The sample was melt-treated polycrystalline (Tl,Pb)(Ba,Sr${)}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ [Tl (1:2:2:3)] high-${\mathit{T}}_{\mathit{c}}$ (\ensuremath{\sim}119 K in zero field) superconductor for fields up to 80 K Oe and temperatures between 30 K and ${\mathit{T}}_{\mathit{c}}$. Experimental conditions were chosen so that the data could be analyzed with Bean's critical-state model associated with an extended version of the effective-medium model for the material's magnetic-shielding properties. Within this context equations relating the statistical distribution of intergrain critical currents in the sample with the measured susceptibility were derived.

The effect of oxygen on intergrain critical currents of YBa2Cu4O8 and Y(Ba1.6Sr0.4)Cu4O8 ceramic samples

The authors have investigated the effect of quenching on the intergrain critical current, Jc(inter), of YBa2Cu4O8 and Y(Ba1.6Sr0.4)Cu4O8 bulk samples. The quenching process was performed by quenching the samples from different annealing temperatures into liquid nitrogen. The critical current was deduced by AC inductive measurements. It was found that Jc(inter) was progressively lowered with increasing quenching temperature. For Sr-doped samples the suppression of the Jc(inter) by oxygen removal was more appreciable than for the undoped samples. The results suggest that weak links at grain boundaries are sensitive to quenching treatment although the grains themselves are thermally stable.

On weak-link behavior of silver-doped high-Tc granular superconductors

A rather detailed analytical treatment of the influence of silver addition on weak-link properties of high- T c ceramics is presented within the superconductive glass model. It is shown that for small concentration of silver p (< p c ) the coherent properties of the Josephson junction (JJ) network are improved resulting in essential enhancement of intergrain superconducting parameters (such as T c , I c , and H c ). Further increase of silver leads to their degradation. The critical content of silver, p c , is found to depend on a competition between tunneling and proximity characteristics of the initially undoped ceramics. It is shown also that the compression effect increases with silver doping resulting in a weaker field dependence of the intergrain critical current. The obtained results are in qualitative agreement with the experimental data for high- T c superconductors.

Critical currents of granular YBa2Cu3O7- delta superconductors

The H-T diagram describing the critical current behaviour of granular superconductors in a magnetic field is proposed. The process of trapping magnetic field by a ceramic sample and the influence of the trapped field on the transport critical current have been studied. The temperature and magnetic field dependences of intergrain and intragrain critical currents have been analysed.

Studies of intergrain critical currents in YBCO using superconducting ceramic rings

We report on the measurement of intergrain critical currents in YBCO ceramics using persistent currents circulating in a superconducting ring. This method is very sensitive to any resistive dissipation and allows one to distinguish between very low resistance nad superconductivity. We measured profiles of magnetic field generated by the persistent current in the ring and determined saturation value of that field, which is proportional to I c. We performed measurements of I c as a function of temperature and applied magnetic field. Close to T c, I c behaves like const ( T c− T) α where α = 1 for YBCO ceramics (Josephson SIS junctions) or α = 2 for YBCO/Ag ceramic composites (proximity SNS junctions).