High-field magnetic and transport critical current density of Sm-Ba-Cu-O: Agx superconductors at low temperature

Variation of magnetic critical current density (J _mc) and transport critical current density (J _tc) in high magnetic fields at liquid helium temperature were investigated on Sm-Ba-Cu-O: Ag _x (x=0.0, 0.2, 0.4, and 0.6) superconducting samples prepared by the coprecipitation technique.T _c and crystallographic parameters remain unaffected by Ag addition. However, bothJ _mc andJ _tc were found to increase on Ag addition. The volume pinning force (F _p) also increased with increase in Ag concentration, resulting in stronger flux line sheer. Microwave-induced d.c. voltage measurement shows a significant reduction of the total number of weak links between the superconducting grains with increasing concentration of Ag, which creates stronger pinning potentials between the boundary of the superconducting intergrains. Hence, the volume pinning force is greatly enhanced by Ag addition, leading to flux line lattice movement which is responsible for highJ _mc andJ _tc.     

Magnetic and transport critical current density of laser-irradiated Sm-Ba-Cu-O superconductor at 4.2 K

The low temperature (4.2 K) magnetic and transport critical current density of laser-irradiated (Q-switched ruby laser, 694 nm, 30 ns) Sm-Ba-Cu-O ceramic superconductors prepared by the coprecipitation technique have been investigated. Laser irradiation did not significantly change the structural parameter and the critical transition temperatureT _c but caused an appreciable increase in magnetic critical current densityJ _mc and transport critical current densityJ _tc . Inverse a.c. Josephson effect studies at 77 K showed a sharp decrease of microwave-induced d.c. voltage after laser irradiation. SEM studies revealed partial melting at grain boundaries and grain growth due to sintering which improves the interconnectivity in the network of superconducting grain structures after laser irradiation. These phenomenon are attributed to physical densification and consequent reduction in the total number of weak links between the superconducting grains. The significant increase ofJ _mc andJ _tc after laser irradiation is presumably connected with the creation of irradiation-induced mobile defects which act as pinning centers and, hence, stronger Josephson current paths between the superconducting intergrains.     

Low-temperature magnetic and microwave studies of Gd0.95Ba2Cu3.05O7−δ: Agx composites

Gd_0.95Ba₂Cu_3.05O_7?δ: Ag_x (x=0.0, 0.02, 0.04, 0.06) ceramic superconducting samples were prepared by a co-precipation technique using organic carbonates in the presence of stable polymers. The room-temperature normal-state resistivity decreases with Ag addition. However,T _cand crystallographic parameters were uneffected. Low-temperature (4.2 K) magnetic and electrical transport studies in high magnetic fields (5 T) reveal that the addition of Ag into the Gd_0.95Ba₂Cu_3.05O_7?δ (GBCO) enhances the magnetic critical current density (J _mc), volume pinning force (F _p), flux explusion, and transport critical current density (J _tc). Microwave-induced dc voltage studies show the reduction in the total number of weak links between superconducting GBCO grains with increase in Ag concentration. The flux creep rate was also increased with increase in Ag concentration in GBCO resulting in stronger pinning potential. The increase inJ _mc, J_tc, and, hence,F _pwith Ag addition in the GBCO suggests the creation of an SNS-type proximity junction at the intergrannular region and stronger Josephson current paths between the superconducting intergrains, which may be due to the physical densification and reduction of the total number of weak links by Ag addition into the GBCO superconducting system.     

Critical Current Density Behaviors for Sinter-Forged Bi-2223 Bulks

Bi-2223 bulks were fabricated by a technique, a combination of cold-isostatic-press (CIP) and sinter-forging. Two batches of samples were prepared as presintering was conducted after (Route 1) and before CIP (Route 2). The effects of the deformation rate during sinter-forging, and presintering to critical current density J _c from exterior and interior sections were investigated. The results revealed that the critical current density of bulks fabricated by Route 1 was much higher than for bulks made by Route 2. The interior section of a sample with higher deformation rate has more contribution to J _c, whereas that with low and too high deformation rate has more equal contribution from exterior and interior sections. Field dependence of critical current density revealed that optimized sinter-forging could improve the critical current density J _c behavior in external fields, particularly in the low field region below 50 mT, for the samples where CIP was conducted before presintering. For the samples initially pressed with CIP, a higher sinter-forging deformation rate can improve the flux pinning force when the external field is parallel to the sample surface, but degrades the pinning force density when the external fields is perpendicular to the sample surface (H//c, 77 K).     

Determination of the Intragrain Critical Current Density and Thermogravimetric Analysis on YBCO/Ag Superconductors Irradiated with 60Co Gamma Rays

We investigated the flux pinning behavior induced by gamma irradiation in Y₁Ba₂Cu₃O_(7–x) silver-added samples. The superconductors were prepared through solid-state route and added with silver in amounts of 0-, 6.5-, and 20-wt%, following which the samples were irradiated by high-energy gamma irradiation () at doses of 0, 500, and 1500 kGy at the dosage rate of 8.2 kGy/h. We performed magnetization loops to study the flux pinning of vortex in YBCO/Ag superconductors. We found that silver addition and irradiation on superconductors may increase the width in magnetization loops, which is related to the enhancement in the critical current density, J _c . We established that an interaction between silver addition and irradiation that results in higher values in J _c for YBCO/Ag irradiated samples already exists. The J _c in samples containing 20-wt% of silver decreased with irradiation because a higher content of silver produces small crystals and secondary phases growing. On comparing the TGA analysis for both YBCO and YBCO/Ag samples, we found higher-weight oxygen losses in YBCO silver samples, which suggests that silver increases the oxygen saturation content. The higher oxygen content found on irradiated specimens it is indicative of oxygen losses from CuO chains or CuO₂ planes reached by -irradiation.On leave from     

Effect of laser irradiation on the superconducting properties of high-T c SmBa2Cu3O x

Studies of the effect of high power laser (Q-switched Ruby laser, 694 nm, 30 ns) irradiation on the critical current density (J _c ) and magnetic hysteresis at 77K and temperature variation of microwave induced d.c. voltage on SmBa₂Cu₃O _x ceramic samples have been performed. Irradiation did not substantially changeT _c but caused a strong increase inJ _c and magnetic hysteresis at 77K. The microwave-induced d.c. voltage at 77K showed appreciable decrease after irradiation. SEM studies showed grain growth due to sintering which improves the interconnectivity among the superconducting grains. These are attributed to physical densification and consequent reduction in the number of weak links. The increase of magnetic hysteresis after laser irradiation is presumably connected with the creation of defects which act as pinning centres. Thermal modelling suggests that on irradiation the surface melts up to a depth of 1μ and laser-induced evaporation occurs at energy density of 2·5 J/cm².     

Studies on magnetization and microwave properties of gamma-ray-irradiated Sm-Ba-Cu-O superconductors

Studies on the effect of gamma-ray irradiation on critical transition temperature, transport and magnetic critical current density (at 4.2 K), high-field magnetization (at 4.2 K), and microwave-induced d.c. voltage (inverse a.c. Josephson effect) have been performed on SmBa₂Cu₃O_7– ceramic superconducting samples prepared by the coprecipitation technique. Gamma-ray irradiation of the samples was carried out using a⁶⁰Co source of 10³ Ci strength for several hundreds of hours; the dose received by the samples was 80 K rad/h.-Ray irradiation was found to have no effect on its structural modification and on the critical transition temperature. However, transport and magnetic critical current density are increased. Irradiation also caused a significant increase in the high-field magnetic hysteresis, which is presumably connected with the creation of radiation-induced mobile defects. An appreciable decrease in the microwave-induced d.c. voltage at 77 K was also observed after irradiation, which suggests that the mobile defects are clustered at the major defect region and thus reduce the total number of weak links. Enhancement of transport and magnetic critical current density may be due to the stronger pinning of flux lines at the-ray-induced defect site in SBCO ceramic superconductors.     

Low-field behavior of magnetic and transport properties of laser irradiated Nd-Ba-Cu-O ceramic superconductors

The low-field behavior of magnetic and transport properties of high-power laser-irradiated (Q-switched ruby laser, 694 nm, 30 ns) Nd-Ba-Cu-O (NBCO) ceramic superconductors prepared by the coprecipitation technique have been investigated. Laser-irradiated NBCO samples contribute significantly to the flux pinning for low applied magnetic fields. A well-defined field valueH _c was observed for different laser-irradiated samples which is characteristic of a cross-over between regions dominated by the inter- and intragranular effects. The value ofH _c for a laser-irradiated sample is increased with increasing number of laser pulses. Thermal modeling of laser heating reveals that on irradiation the surface melts up to a depth of 1m and laser-induced evaporation occurs at energy density 2.5 J/cm². SEM studies showed grain growth due to laser-induced surface sintering which improves the interconnectivity among the superconducting grain structure and thus attributed to physical densification and consequent reduction in the total number of weak links. The increase of flux trapping and transport critical current density after laser irradiation is determined by the rigidity of the flux line lattice which may be due to the presence of additional pinning centers caused by the creation of laser-induced mobile defects and their clusters.     

Magnetic and microwave properties of Sm1?xBa2Cu3+xO7?δ superconductors at 4.2 K prepared by an organic carbonate coprecipitation route

Samples of Sm₁Ba₂Cu₃O_7– and Sm_0.95Ba₂Cu_3.05O_7– are prepared by a coprecipitation technique using organic carbonates in the presence of stable polymers. The electrical, magnetic, and microwave measurements have been carried out to characterize the materials. Both normal (Sm₁Ba₂Cu₃O_7– ) and copper-rich (Sm_0.95Ba₂Cu_3.05O_7– ) samples showed sharp superconducting transitions at 94 K. Lower and upper critical fields of the samples are estimated from magnetization measurements. The critical magnetization current density (J _mc ) and pinning force density (F _p ) of the samples are obtained from the magnetization hysteresis loop at 4.2 K for fields up to 5T. Superconducting parameters (H _c (O),K _GL,,) are obtained by using the values of lower and upper critical magnetic fields. Microwave-induced d.c. voltage measurements indicated a smaller number of weak links for copper-rich samples. The enhancement of critical current density and the reduction of total number of weak links for copper-rich samples may be due to the increase in hole concentration caused by the partial replacement of Sm by Cu.     

Microstructures of high-Jc melt-textured YBa2Cu3O7?x/Ag superconductors

Silver has been previously added to the melt-textured YBa₂Cu₃O_7–x in order to increase the critical current density (J _c ) of these materials. However, the effect of this addition on theJ _c is presently unclear. The purpose of this study is to investigate the effect of silver on both critical current density and the microstructure of the melt-textured YBa₂Cu₃O_7–x superconductors by means of X-ray diffraction, optical polarized microscopy, and transmission electron microscopy (TEM). TheJ _c of the MTG YBCO/Ag samples is more than 10⁴A/cm² under the 5 kOe magnetic field. It has been shown that as the concentration of silver increases, the fraction of the 211 phase dispersed within the 123 matrix decreases. Therefore, theJ _c slightly decreases. These results, together with the effect of the 211 phase, dislocations, and other structure defects on flux pinning, are described in this paper.     

The Effect of the Pinning Center Size on the Vortex Pinning by?Embedded ZrO2 Nano-particles

The influence of pinning centers size on the superconducting properties was investigated. Through the addition of three batches of ZrO₂ nano-particles with mean size of D ₁=13 nm, D ₂=21 nm, and D ₃=85 nm, we have succeeded in incorporating effective artificial pinning centers within the YBCO matrix of the bulk superconductor. An enhancement in the flux pinning and an improvement in the critical current densities (transport critical current density J _ct and magnetic critical current density J _cm) were achieved. The results indicate that slight inclusions of ZrO₂ can greatly enhance the flux pinning capability of samples. Comparative analyses of the critical current densities and the resulting pinning force F _p for the three diameters have shown that pinning centers with finer size are much more efficient than those with a size larger than the coherence length ??.     

Process Optimization for Ag-Sheathed Bi-2212 Superconductors

The powder-in-tube (PIT) process has been widely used to fabricate long lengths of superconducting wires and tapes. However, it has been noted that the performance of long lengths of superconductor is variable and difficult to replace. To help pinpoint the possible sources of variation, a systematic study of the effect of processing variables, including deformation and heat treatment procedures, on the electrical properties of the Bi-2212 tapes at cryogenic temperatures was conducted. In addition, the effect of varying powder particle sizes was examined. For tapes fabricated by different thickness reduction schedules, significant variations in critical current density (J _c) were observed. It is concluded that a combination of small roll diameter and small reduction-per-pass produces tapes with highest J _c. Moreover, the maximum J _c occurred in a narrow temperature range when melt processing was done in pure oxygen. Microstructural examination was used to correlate J _c and both the volume fractions of a nonsuperconducting second phase and the Bi-2212 grain orientation.     

Low-temperature magnetic and electromagnetic studies of Y-Ba-Cu-O: Agx Ceramic superconductor

Magnetization and magnetic relaxation measurements have been studied at 4.2 K on YBa₂Cu₃O_7–:Ag _x (x=0 to 1.2) superconducting samples prepared by a coprecipritation technique. Remanent magnetization and hysteresis loops were found to be increased on Ag addition. Transport current density (J _c) was enhanced with the addition of Ag concentration. Flux creep rate was also increased with increase in Ag addition resulting in stronger pinning potential. Microwave-induced d.c. voltage measurements show a reduction of weak links with increase in Ag concentration. The enhancement of transportT _c is attributed to the stronger Josephson current paths due to the reduction of the total number of weak links after Ag addition.     

The Jc Limit of Multifilamentary Ag/Bi-2223 Tapes

To improve on present critical current (J _c) performance, multifilamentary Ag/Bi-2223 tapes with a large range of reduction rates were manufactured. The relative core mass density D was calculated, dependent on the measured geometric dimensions of the tapes. Experimental results, D vs. J _c, D vs. maximum pinning force density F _max , and D vs. irreversible magnetic field B _irr, are quantitatively formatted. In particular, the magnetic field dependence of J _c is critically dependent on its core density. If the core density increases by 10%, J _c of the tapes in this experiment is enhanced by as much as 100%. Therefore, in the present state of the technological process for manufacturing Ag/Bi-2223 tape, increasing the core density is clearly a significant strategy in improving the electronic and magnetic properties of the tapes and enhancing the capacity for carrying current at high magnetic fields. The limit of the bulk self-field-J _c can be calculated by the relationships of J _c vs. D. The limit is estimated to be on the order of 200 kA/cm² for multifilamentary Bi-2223 tapes, which was supported by magneto-optical (MO) magnetization measurements results. It is a hard task to approach this limit with the present state of the art in manufacturing Ag/Bi-2223 tape, and it is the time to suggest some new ideals for Bi-2223 tapes to promote large-scale applications.     

Computer Simulation of Flux Pinning in the High-Temperature Superconductors Bi-2212

The magnitude of critical current density J _c (or critical magnetic induction B _c), is closely related to the flux pinning property of the high-temperature superconductors (HTSC) materials. So, the investigation on flux pinning mechanism is an important research field in the high-temperature superconductors. There are different pinning centers by analyzing the flux pinning force of the HTSC. In this study, based on Sun and Zou et al.’s previous works, we studied the flux pinning mechanism in Bi-2212 single crystals with an improved scaling function of pinning density, which is simplified and possesses determined physical meaning. Then, a computer simulation was conducted to determine the category of flux pinning in Bi-2212 according to Sun’s experimental data. It is revealed that the simulation result is consistent with the experiment.     

Improvement of Pinning Force and Critical Current Density in Thick YBa 2 Cu 3 O 7−δ Films Grown on SrTiO 3 Substrates Decorated with LaNiO 3 Nanodots

We have investigated by DC magnetization measurements and frequency-dependent AC susceptibility the critical current density (J _c), pinning force (F _p) and pinning potential in thick (1.3–1.6 μm) YBa₂Cu₃O_7−δ (YBCO) films grown by Pulsed Laser Deposition on SrTiO₃ substrates decorated with LaNiO₃ nanodots deposited by a few (5–15) laser pulses, in comparison with those of a 1 μm thick YBCO reference sample. Experiments show that the highest improvement of superconducting properties was achieved for films grown on substrates decorated with 10 laser pulses on the LaNiO₃ target, which have, at 77.3 K, a J _c of 40–125% higher than in pure YBCO in fields between 1 and 2 T, and F _p increased by 40%. These results could be important for further improvement of current-carrying capability of coated conductors for in-field power applications.     

Magnetic field orientation dependence of critical current in industrial Nb3Sn strands

In usual superconducting devices such as magnets for NMR, the magnetic field is perpendicular to the superconducting strand axis. But in some special devices, such as magnets for the toroidal field system of fusion machines, the strands can experience any field orientation. For NbTi strands, the pinning force is dependent on the field orientation because of the drawing process (Takacs, S., Polak, M. and Krempasky, L., Critical currents of NbTi tapes with differently oriented anisotropic defects, Cryogenics, 1983, 23, 153–159). In the case of Nb₃Sn strands, the draw and react process suggests that the pinning force is isotropic. In fact, preliminary experiments have shown the contrary, which is why the magnetic field orientation dependence of the critical current for two types of industrial Nb₃Sn strands has been measured. These measurements have been performed for seven field orientations at field strengths up to 20 T. A clear anisotropic effect has been observed, which cannot be explained by Kramer's pinning law. The results are in very good agreement with an empirical law proposed in a recent study by Takayasu et al. (Takayasu, M., Montgomery, D.B. and Minervini, J.V., Effect of magnetic field direction on the critical current of twisted multifilamentary superconducting wires, Inst. of Phys. Conf. Ser., 1997, 158, 917–920). The parameters to be used in this law could be specific to the manufacturing process.     

Low-temperature magnetic and electromagnetic studies of Y-Ba-Cu-O: Agx Ceramic superconductor

Magnetization and magnetic relaxation measurements have been studied at 4.2 K on YBa₂Cu₃O_7?δ:Ag _x (x=0 to 1.2) superconducting samples prepared by a coprecipritation technique. Remanent magnetization and hysteresis loops were found to be increased on Ag addition. Transport current density (J _c) was enhanced with the addition of Ag concentration. Flux creep rate was also increased with increase in Ag addition resulting in stronger pinning potential. Microwave-induced d.c. voltage measurements show a reduction of weak links with increase in Ag concentration. The enhancement of transportT _c is attributed to the stronger Josephson current paths due to the reduction of the total number of weak links after Ag addition.