Intragrain Currents Research Articles

Geometrical Effects on the Levitation Capability of Multiseeded Y–Ba–Cu–O Blocks

Using a well-established theoretical model to calculate the levitation behavior of multiple high-temperature superconductors, the levitation capability of multiseeded Y-Ba-Cu-O blocks over two promising permanent-magnet guideways (PMGs) was evaluated and its dependence on the number of seeds was clearly displayed. The multiseeded Y-Ba-Cu-O blocks are ideally supposed to be composed of a set of single domains that are magnetically coupled but electrically insulated, which means we only consider the intragrain current. This assumption is a prerequisite for avoiding the complexity of assessing the morphology of the intergrain current, which is actually weaker than the intragrain current due to the unsolved obstacles in material process such as the weak link of grain boundary that impedes the current transfer across the grains. The insights attained by this numerical study, being inaccessible from experiments, tell us that, given the width of the Y-Ba-Cu-O block, the levitation force is found to be mostly degraded as the number of the seeds increases or the width of each seed decreases, whereas the lateral force exhibits a more complex behavior.

Evaluation of anisotropy of Jc in silver-sheathed Bi-2223 tape

The magnetic field angle dependence of Jc was measured by four probe method and DC magnetization method. The latter measurement was used for the specimen before and after bending to break the intergrain current in order to investigate only the intragrain current. It was found that the intragrain critical current density could be increased by a factor 2.7 in a parallel magnetic field when the c-axis misorientation could be improved. From the field angle dependence of the intragrain critical current density, the anisotropy parameter was estimated as 20.4.

Magnetic characterization of welded YBaCuO-bulk samples

The welding of individually grown pieces of high temperature superconductor bulk material is the most promising way to obtain larger single domain tiles which are required for cryomagnetic applications. In this paper we present a method for the characterization of this kind of material. The method is based on the inductive measurement of the total current in ring samples using a Rogowski coil and an integrator circuit. The advantage of this technique is that it allows a direct measurement of the current crossing the artificial grain boundary without being influenced by the intergrain current and is independent of the sample size and geometry. We present the principle of the measurement and the calibrations of the system. The independence of the output signal on intragrain currents is demonstrated. The application of the method for the characterization of a welded sample is shown.

Microwave surface resistance of superconductors with grain boundaries

Microwave-field distribution, dissipation, and surface impedance are theoretically investigated for superconductors with laminar grain boundaries (GBs). In the present theory we adopt the two-fluid model for intragrain transport current in the grains, and the Josephson-junction model for intergrain tunneling current across GBs. Results show that the surface resistance ${R}_{s}$ nonmonotonically depends on the critical current density ${J}_{cj}$ at GB junctions, and ${R}_{s}$ for superconductors with GBs can be smaller than the surface resistance ${R}_{s0}$ for ideal homogeneous superconductors without GBs.

Intergrain and intragrain currents in bulk melt-grownYBa2Cu3O7−δrings

A simple contactless method suitable for discerning between the intergrain (circular) current, which flows in the thin superconducting ring, and the intragrain current, which does not cross the weakest link, has been proposed. At first, we show that the intergrain current may directly be estimated from the magnetic flux density $B(\ifmmode\pm\else\textpm\fi{}{z}_{0})$ measured by the Hall sensor positioned in the special points $\ifmmode\pm\else\textpm\fi{}{z}_{0}$ above or below the ring center. The experimental and numerical techniques to determine the value ${z}_{0}$ are discussed. Being very promising for the characterization of a current flowing across the joints in welded YBaCuO rings (its dependences on the temperature and external magnetic field as well as the time dissipation), the approach has been applied to study the corresponding properties of the intragrain and intergrain currents flowing across the a-twisted grain boundaries which are frequent in bulk melt-textured (MT) YBaCuO samples. We present experimental data related to the flux penetration inside a bore of MT YBaCuO rings both in the nonmagnetized, virgin state, and during the field reversal. The shielding properties and their dependence on external magnetic fields are also studied. Besides, we consider flux creep effects and their influence on the current redistribution during a dwell.

Critical current densities and irreversibility fields of MgB 2 bulks

We prepared two MgB 2 bulks by applying conventional sintering and high-pressure sintering methods, and compared the current carrying properties. J c obtained by the resistive method was larger than that obtained by the magnetic method. J c– B curves obtained by the resistive method showed no history effect. These results indicate that most of the superconducting currents flowing in the MgB 2 bulks were intergrain (transport) currents and intragrain currents were negligibly small. The high-pressure sintered sample with smaller grain size showed smaller field dependence of J c and higher B irr than the conventionally sintered sample with larger grain size. This behavior can be explained by the grain boundary flux pinning.

Investigation of inter- and intragrain currents in K 3C 60 single crystals

K 3C 60 single crystals were prepared under different conditions by doping pure C 60 single crystals in potassium vapour. They were characterised by ac susceptibility and dc magnetisation measurements. In the best crystals, the bulk nature of supercurrent flow was confirmed. In this case, the results for the critical current density determined from ac and dc measurements agree very well. Furthermore, we find that the bulk dissipation peak shifts to higher temperatures with increasing frequency. This shift obeys a scaling law, in agreement with the prediction of the thermally activated flux creep model. Other crystals with a weak link structure show a more complicated pattern in their ac response, which is related to intergrain currents and strongly dependent on the ac field amplitude.

Effect of intragrain current on low-field magnetic-flux distributions of zero-field-cooled polycrystalline YBa2Cu3O7−δ

Magnetic-flux distributions B(r) below the lower critical field of the grains in a zero-field-cooled disk and a ring of polycrystalline YBa2Cu3O7−δ are measured at 77 K with a Hall probe. The measured distribution in the disk had a steep gradient on the sample surface, and the distribution in the ring, over the applied magnetic-field range of 1.5–4 mT, was ‘‘W’’ shaped. These results can only be explained consistently by considering the intergrain current, the intragrain currents, and their interaction. It is shown that the magnetic-flux density measured by a Hall probe with an active area much larger than the grain size is not the intergranular magnetic-flux density Binter but μBinter, where μ is effective magnetic permeability (intergranular volume fraction), and dB(r)/dr gives μ times the intergrain (transport) critical current density Jct and not Jct as is assumed by many authors.

Intragrain pinning strength depth dependence of 2223 (Bi,Pb)-based high critical T c superconducting ceramics made by a vitreous route

Campbell’s method for measuring the critical current in superconductors has been used to obtain the critical current density and the pinning strength in Bi1.7Pb0.3Sr2Ca2Cu3O10−y ceramics synthesized by a vitreous route. The intragrain critical current is much higher than 105 A/cm2 at 40 K in zero dc magnetic field. A large increase of the pinning strength is observed near the grain surface. The decrease with depth is hyperbolic. The role of the precursors in the synthesis route is emphasized for introducing specific pinning centers. The analysis takes into consideration the ceramics granular nature, i.e., the existence of intergrain and intragrain currents.

ＣｕＯ中のＢｉ系２２１２分散析出相の合成と超伝導特性

A thermally precipitated polycrystalline bulk system of the HTC Bi-2212 phase in CuO was successfully synthesized at the volume fraction ƒ=0-1.0. The XRD, electrical resistivity, and ac susceptibility were measured, and discussed in relation with the percolation theory. The resistivity in the normal state showed the metal-tosemiconductor transition at ƒcn=0.36 as the percolation threshold. The resistivity in the superconducting state showed the superconductor-to-semiconductor trancition at ƒcs=0.28 as the percolation threshold. The critical exponent tin the normal state and superconducting state was also obtained to be 1.3 and 2.7-3.2, respectively. The obtained percolation thresholds, ƒcn and ƒcs, are close to those of the theoretically predicted value for the two dimensions system. This seems to reflect the mechanism of conductivity and superconductivity through the Cu-O2 plane in the Bi-2212 phase. The small percolation threshold in the superconducting state, ƒcn-ƒcs=0.08, suggests an effect of the weak link through the superconductive grain. The ac susceptibility measurements show that the intra-grain current is not affected by the excited ac field Hac but is proportional to ƒ0.55, and that the inter-grain current closely depends on the Hac and the temperature, reflecting the percolation state.

New method for measuring pinning strength inside grains of HTCS ceramics

We have designed a new setup and analysis route based on the Campbell's methodfor measuring the critical current in superconductors. Our model takes consideration of the ceramics granular nature, the existence ofintergrain and intragrain currents. We obtain the variation of the pinning strength Pv as a function of p g, which is the radial position inside an average grain. We apply this scheme to Bi 1.77Pb 0.3Sr 2Ca 2Cu 3O10 -y ceramics synthesized by a vitreous route. The intragrain critical current is 10 5A/cm 2 at 40K with zero DC magnetic field. Some important increase of the pinning strength is observed near the grain surface.

Influence of additions and radiation damage on the superconducting properties of sintered YBa 2Cu 3O 7 − gd

Using the high-resolution Faraday (HRF) technique, domain patterns of sintered YBa 2Cu 3O 7 − δ samples containing additions of Ag 2O and Y 2BaCuO 5 are obtained. Similar observations are carried out on sintered YBa 2Cu 3O 7− δ samples with reduced oxygen content and on electron-irradiated samples. The penetration of flux into sintered high- T c superconductors is found to be totally different from single crystalline materials (single crystals and epitaxial thin films) as the Abrikosov vortices penetrate the sample not as a whole thus forming a so-called flux front. Only a flux penetration into single superconducting grains is observed. From the measured flux density profiles, the intragrain critical current densities are determined locally. It is found that the chemical impurity phases do not affect the intragranular flux density distribution and the intragranular critical current density, but the decomposition of the Ag 2O leads to an improved oxygen stoichiometry in these samples. Electron irradiation is found to enhance the intragrain critical current densities, whereas the oxygen reduction leads to reduced intragrain currents. To determine the influence of the additions also on the intergrain critical current densities, standard four-point transport current measurements are carried out on the same samples. It is found that a maximum enhancement of the critical current density is obtained by the addition of 10 wt% silver oxide; however, this critical current density is drastically reduced by applying small external magnetic fields. The lower critical field of the matrix and the corresponding Josephson penetration depth are determined from transport current measurements. The Ag 2O additions and the green phase particles are found to separate the superconducting YBa 2Cu 3O 7− δ grains. The resulting larger widths of the barriers between the superconducting grains are found to cause a stronger influence of external magnetic fields on the intergrain critical current densities.

Transport critical currents in field-oriented Y1Ba2Cu3Oy polycrystals

Field-orientedY1Ba2Cu3Oy samples with various values of the transport critical current density, Jc, have been prepared with various heat treatments. The Jc increased rapidly with an increase in relative sintered density of the oriented samples. The field-oriented samples with higher Jc had grains of larger sizes and better connections between the neighboring grains than the low Jc samples. The maximum values of Jc at a magnetic field of 7 T were 370 A/cm2 at 4.2 K and 24 A/cm2 at 77 K. The hysteresis in the Jc-H curves at 4.2 K decreased with an increase in Jc both for an applied field H∥ab plane and for H∥c axis. The critical currents of the samples with higher Jc were less degraded by aging in the air for 1 year than those of the low Jc samples. The relation between the field-cooled Jc-H curves and the zero-field-cooled ones for an oriented sample with low Jc is qualitatively interpreted by the effect of the intragrain currents on the intergrain fields.

Magnetic flux distribution and magnetic relaxation in polycrystalline Bi,PbSrCaCuO superconductors

We have studied magnetic flux distribution and magnetic relaxation in polycrystalline (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+ x superconductors at 77 K using a movable miniature Hall sensor. Flux distribution was studied by measuring magnetic field profiles as a function of external field and time. The effects of inter- and intragrain shielding currents on magnetic flux distribution were distinguished using these measurements. The intergrain critical current density can be calculated from the field profile also in the case where intragrain currents are present. The relaxation of inter- and intragrain currents was distinguished and studied. The relaxation was logarithmic in both current systems. The relaxation rates of intergrain currents were remarkably higher than the rates of intragrain currents. When neither internor intragrain currents dominated, the total relaxation was non-logarithmic.

Interaction between intra-grain currents and transport current in bulk YBaCuO

The hysteric magnetization curves of transport and grain currents are measured for a ring shaped sample by applying an a.c. magnetic field and using pickup coils. The dependence of the magnetization curves on the amplitude of the applied a.c. magnetic field can be explained by a simple model. This model takes into account the time dependence of the transport and the intra-grain current. We will show that the mutual interaction of the grain and transport currents cannot be neglected in our samples. At first the influence of an additional d.c. background field on the transport current seems quite unexpected, but this d.c. field dependence can be interpreted too within our model.

Selected area measurements of magnetization in high-Tc YBCO superconductor

Selected area measurements of magnetization and remanent magnetization fields in sintered specimens of high-Tc YBCO superconductors are reported. The measurements show that the field dependence of the intergranular currents can be expressed in the exponential form, while the intra-granular currents at higher fields by a power-law. Temperature dependence of the critical currents, both of inter- and intra-grain currents, shows the behavior expected for the Josephson junctions in the dirty-limit of the BCS theory.

Hysteretic critical intergrain transport current in superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ as a basis for new switching and data storage effects

In sintered polycrystalline high-T/sub c/ superconductors, intergrain I/sub c/ depends on the magnetic history of the sample because the intragrain screening currents contribute a hysteretic component to the total B field at the grain interfaces. Experimental results on hysteretic intergrain I/sub c/ are presented, and application-relevant aspects of this behavior are investigated. Programmable switching and data storage effects were experimentally verified on the basis of this hysteretic I/sub c/ using the remanent state of intragrain magnetization. In the remanent state, information can be stored in the grains and read out by the transport current. Different I/sub c/-values were found at the same B/sub a/ and the same intragrain M but different shapes of the intragrain flux density profiles. The time relaxation behavior of intergrain I/sub c/ was studied, and a large reduction of relaxation rate was found under certain field cycling conditions. This could be important for future electronic applications.

Effects of intergrain and intragrain currents on flux profile in granular superconducting ceramics

We present generalizations of Bean critical state model to take into account granular effects as in high critical temperature superconductor ceramics. Both intragrain and intergrain critical current effect are discussed as they modify the “flux profile” determined by AC susceptibility measurements. We also examine the effect of Josephson weak links on the flux profile. We show how to obtain these electrical currents from the data together with the “granular fraction” and the flux penetration depth. Data on Bi 0.85Pb 0.15SrCa 1.2Cu 2O 6 sample is shown to be fitted to our theoretical expressions.

Investigation of inter- and intragrain critical currents in high Tc ceramic superconductors

Polycrystalline sintered bulk samples of REBa2Cu3O7 with RE = Y, Gd were studied by a.c. susceptibility and an inductive measurement technique which allows the separation of intergrain (transport) from intragrain critical current density. Field and temperature dependence of the intergrain current are compared with theoretical predictions for a weakly coupled grain structure. Up to fields 0.3 T the weak link character of the intergrain current predominates. At higher fields it changes and points to a percolation current via superconducting grain boundaries. A quantitative correlation between intragrain current and twin spacing is hidden, probably by oxygen deficiency in the grain interior. No variation of the intragrain critical current density with temperature is observed up to 85 K in a textured specimen.

AC Inductive Measurement of Intergrain and Intragrain Currents in High-Tc Oxide Superconductors

Bulk intergrain current and closed intragrain current in sintered Y-Ba-Cu-O superconductors were measured at 77 K by using an ac inductive method. These currents can be separated because of the large difference in penetrating rates of the magnetic flux into specimens with respect to the ac field amplitude. The obtained intergrain current density agreed approximately with critical current density measured resistively. The closed intragrain current density amounted to 4.3×108 A/m2 at B=0.5 T and decreased gradually with increasing magnetic field. The present results show that the ac inductive measurement is one of the available nondestructive methods to characterize sintered oxide specimens.