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.     

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.     

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.     

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.     

Low-field behavior of critical current in Sm1−x −Ba2−Cu3+x −O7−δ ceramic superconductor

The transport critical currentI _c carried by ceramic Sm_1?x Ba₂?Cu_3+x ?O_7?δ (SBCO) superconducting samples (prepared by the coprecipitation technique using organic carbonate in the presence of stable polymer) in low magnetic fields (0–100 Oe) have been investigated at different temperatures close to the critical transition temperatureT _c . A significant reduction of the transport current-carrying capacity has been observed for magnetic fields smaller than the lower critical fieldH _cl . A semilog plot of the field dependence of the transport critical current densityJ _ct reveals a well-defined field valueH′ _c characteristic of a crossover between regions dominated by the inter- and intragranular effects. Microwave measurements show evidence of a reduction in the total number of weak links between the network of the superconducting grain structure for Cu-rich SBCO samples. The enhancement ofJ _tc for Cu-rich SBCO samples is due to the increase in hole concentration caused by partial replacement of Sm by Cu which caused physical densification between the network of superconducting grain structure in SBCO.     

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.     

Magnetic field dependence of shielding current density in Y-Ba-Cu-O rings at 77 K

A method for contactless measurement of the shielding critical current density and its dependence on the external magnetic field is described and analyzed. The obtained values are compared with those measured resistively on two different samples. It is shown that the shielding critical current densityJ _cs and the intergranular transport current densityJ _cr are identical if the measurement conditions are similar. A degradation ofJ _cs measured in the external field with AC ripple has been observed.     

Gamma Ray Irradiation Effects on Electrical Transport and Microwave Properties of BISCO Superconducting Films

High-T _c Superconducting films of Bi–Sr–Ca–Cu–O (2:2:1:2) have been synthesized by spray pyrolysis of nitrate precursors onto yttria stabilized zirconia (YSZ). -ray irradiation of the sample, was carried out using a ⁶⁰Co source of 10³ Ci strength for several hundreds of hours; the dose received by the samples was 80 K rad/hr. Superconducting properties such as critical transition temperature (T _c), resistivity (R), critical current density (J _tc). Voltage-time relaxation (V _t) and microwave induced dc voltage were investigated as a function of temperature down to 77 K after -ray irradiation. -ray irradiation was found to have practically no effect on its structural modification and on the critical transition temperature. However, transport critical current density (J _tc) increased. The increase of pinning energies with irradiation suggests that these changes in properties are dominated by radiation-induced randomly distributed mobile oxygen defects in the films. An appreciable decrease in the microwave-induced dc voltage at 77 K was also observed after irradiation which suggests that the mobile defects are clustered at the major defect region and reduce the total number of weak links. These results suggests that oxygen defects induced by -ray irradiation of BISCO films act as important and major pinning centers which is responsible for the enhancement of J _tc and reduction of microwave induced dc voltage.     

Long multifilament Bi-2223 Ag-sheathed superconducting tapes and solenoids

Multifilament Ag-sheathed BiPbSrCaCuO (2223) superconducting tapes containing 49 filaments were fabricated by the powder-in-tube route and the roll-anneal process. The transport critical current densityJ _c was 1.3×10⁴ A cm^–2 at 77 K and 7×10⁴ A cm^–2 at 4.2 K in self-field. A 12-m-long tape was used to construct superconducting solenoids (50, 28, and 14 mm internal diameters) generating dc fields 380–1070 G at 4.2 K. Measurements of the variation ofJ _c with field (0–1.6 T) and bend strain (0–5%) are used to explain the performance of the solenoids. The critical bend strain of tapes was about 1.5%.     

Dissipation in current-carrying high-Tcsuperconducting ceramics in applied magnetic field

The dissipation mechanism in high-T _csuperconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.     

Flux dynamics of YBCO/Ag composite

DC magnetization measurements have been carried out on bulk YBCO/Ag composites with silver content up to 20wt per cent. DC fields in the range 0·5 mT to 200 mT have been used to investigate the inter- and intragranular properties at 77K. The AC susceptibility as a function of temperature at different AC fields (0·026–0·30 mT) has also been studied. Under small DC fields (≈ 4 mT), depending on the Ag content andH _max, the M-H loop shows a complicated behaviour. This behaviour can be explained on the basis of effect of strong field dependence of transport critical current, grain size and intragrain critical current densityJ _cgm on low-field M-H loop. The estimation of intergranular critical current densityJ _cjm from these loops does not remain a simple function of ΔM/d. The AC susceptibility measurements show a small increase inJ _c(T) with silver content under low AC fields only, consistent with the transportJ _c data; beyond thatJ _c(T) decreases. This improvement inJ _c(T) and transportJ _c with silver can be ascribed to the improved coupling between grains but not to the pinning. Also at higher field (H _max>20 mT) the addition of Ag decreases the intragrain critical current density. The upper critical field of intergranular regionH _c2j and lower critical field of intragrain regionH _c1g also decrease with silver content.     

Dissipation in current-carrying high-T c superconducting ceramics in applied magnetic field

The dissipation mechanism in high-T _csuperconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.     

Effect of post-annealing on thin YBCO films deposited from a nanocrystalline target

Pulsed laser deposition is used to ablate thin superconducting YBCO films on SrTiO ₃ substrates. The most important parameters of thin superconducting films are high critical current density, ability to stand magnetic fields and smoothness of surfaces. Smoothness is important in fabrication of layered structures and for research of basic properties of thin superconducting structures. The target sintered from YBCO nanopowder is a promising material for making films which meet most of the requirements above. Investigations by AFM show that our target has grains about one order of magnitude smaller than usual grain size of commercial targets. At optimal deposition parameters, the oxygen pressure of 0.4 torr in the chamber and the substrate temperature 725°C, films with T _c = 90 K, J _c =8 × 10⁶ A/cm ² (77 K) and RMS surface roughness = 1.5 nm are obtained. Thermal annealing of the deposited films for 18 h at 900°C further increases the value of J _c .     

Influence of preparation conditions on superconducting properties of Bi-2223 thin films

We report electrical transport properties of Bi₂Sr₂Ca₂Cu₃O_10+x (Bi-2223) superconducting thin films fabricated by pulsed-laser deposition on SrTiO₃ substrate. The aim of the study was to investigate the influence of preparation conditions such as deposition temperature (T _S), annealing time (t _A) and deposition rate (r). A critical temperature (T _c) as high as 110 K and critical current density (J _c) of 6·2 × 10⁶ A/cm² at 20 K were obtained for T _S = 760°C, t _A = 4 h and r = 1·5 Å/s. We also investigated the effect of Li doping on Bi-2223 thin films. Li intercalation results in high resistive onset transition temperature and the resistivity shows broadening in magnetic field that increases with field. The large broadening of resistivity curve in magnetic field suggests that this phenomenon is directly related to the intrinsic superconducting properties of the copper oxide superconductors. The sudden drop in J _c at relatively low magnetic field (H < 0·5 tesla) is due to the effect of Josephson weak-links at the grain boundaries.     

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.     

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.     

Progress in Nanoengineered Microstructures for Tunable High‐Current,High‐Temperature Superconducting Wires

High critical current densities (J_c) in thick films of the Y₁Ba₂Cu₃O_7–δ (YBCO, T_c ≈ 92 K) superconductor directly depend upon the types of nanoscale defects and their densities within the films. A major challenge for developing a viable wire technology is to introduce nanoscale defect structures into the YBCO grains of the thick film suitable for flux pinning and the tailoring of the superconducting properties to specific, application‐dependent, temperature and magnetic field conditions. Concurrently, the YBCO film needs to be integrated into a macroscopically defect‐free conductor in which the grain‐to‐grain connectivity maintains levels of inter‐grain J_c that are comparable to the intra‐grain J_c. That is, high critical current (I_c) YBCO coated conductors must contain engineered inhomogeneities on the nanoscale, while being homogeneous on the macroscale. An analysis is presented of the advances in high‐performance YBCO coated‐conductors using chemical solution deposition (CSD) based on metal trifluoroacetates and the subsequent processing to nano‐engineer the microstructure for tuneable superconducting wires. Multi‐scale structural, chemical, and electrical investigations of the CSD film processes, thick film development, key microstructural features, and wire properties are presented. Prospects for further development of much higher I_c wires for large‐scale, commercial application are discussed within the context of these recent advances.     

Critical current in YBa2Cu3O7 ceramics

Measurements of critical currents using a four-point direct current (dc) method and an alternating magnetic field method have been performed on several superconducting YBa₂Cu₃O₇ ceramics at 77 K. In the presence of a constant magnetic field, the critical currents obtained with the alternating field method are several orders of magnitude larger than the critical currents measured by the dc method. Also, we observed a minimum in the dc critical current as a function of applied transverse magnetic field. Several authors have suggested that these ceramics behave as individual superconducting grains coupled by Josephson junctions. In this paper, we explain the two observations above using that model.