Long-term stability of Bi(2223) and Dy(123) superconducting tapes in the direct current circuit

The stability of critical parameters T _c and I _c of commercial high-temperature superconducting wires upon long-term passage of transport current (about 0.7I _c) in liquid nitrogen (77 K) is studied. Voltage-current characteristics U(I), as well as the critical current and critical temperature, are investigated for the case of Bi(2223) hermetic multifilament wires and Dy(123) superconducting tapes covered by a thin Ag layer. In the former case, a considerable decrease in the critical current (by ～30%) and swelling of the wires after passage of the current for 323 h are observed. The same is true for a reference sample, which does not experience the action of current and stays in liquid nitrogen for 700 h. The decrease in the critical current in the Bi(2223) sample is likely to be associated with penetration of a liquid coolant into the composite conductor: evaporating and expanding as a result of heating, it severely deforms the material. The Dy(123) sample grown epitaxially demonstrates high stability of the critical current after it has experienced the action of current for 400 h and been kept in liquid nitrogen for 1000 h.     

The effect of sintering temperature on the intergranular properties of Bi2223 superconductors

A systematic study of the intergranular properties of (Bi,Pb)₂Sr₂Ca₂Cu₃O_y (Bi2223) polycrystalline samples has been done using the electrical resistivity and Ac susceptibility techniques. In this project, we have prepared a series of Bi2223 samples with different sintering temperature. The XRD results show that by increasing sintering temperature up to 865 °C the Bi2212 phase fraction decrease. It was found that the Bi2212 phase on the grain boundaries is likely to play the role of weak links and consequently reduces the intergranular critical current densities. Analysis of the temperature dependence of the Ac susceptibility near the transition temperature (T_c) has been done employing Bean's Critical State Model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the weak links are changed from superconductor–normal metal–superconductor (SNS) for well-coupled sample to superconductor–insulator–superconductor (SIS) type of junctions for the sample with high Bi2212 phase fraction.     

Discrete temperatures in high-temperature superconductors

The low-amplitude AC susceptibility on intact and deformed Bi2223/Ag tapes has been measured as a function of temperature, frequency, AC amplitude and DC magnetic field. The deformation resulted in the splitting of the χ″(T) peak into three peaks situated near 30, 58 and 90 K. In zero magnetic field, these temperatures were identified as the Kosterlitz–Thouless transition temperatures of low number stacks of superconducting layers. An external magnetic field redistributed the dissipation among the peaks, and moved them to lower temperatures (and suppressed the highest temperature peak). In a finite field, each peak corresponds to the stack melting temperature T_m. The melting temperature in each stack was found to be a field-dependent parameter, with a minimum value=T_KT of a stack of thickness that is less by one layer. The T_m decreases exponentially with the field, and the rate of decrease depends on the interstack Josephson and magnetic interactions. With a universal set of T_KT, the vortex melting line of a tape is a linear combination of the T_m(H) for the low-number stacks.     

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical-chemical vapor deposition

The MgB₂ coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB₂/Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (T_c) ranging between 36 and 38 K with superconducting transition width (ΔT_c) of about 0.3-0.6 K. The highest critical current density (J_c) of 1.34 × 10⁵ A/cm² at 5 K under 3 T is obtained for the MgB₂/Cu tape grown at 460 °C. To further improve the flux pinning property of MgB₂ tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB₂/Cu tapes, the MgB₂ on SiC-coated Cu tapes exhibited opposite trend in the dependence of J_c with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB₂ grain boundaries lead to strong improvement in J_c for the MgB₂/SiC/Cu tapes. The MgB₂/Hastelloy superconducting tapes fabricated at a temperature of 520 °C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher J_c values over the wide field range for MgB₂/Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The J_c values of J_c(20 K, 0 T) ∼5.8 × 10⁶ A/cm² and J_c(20 K, 1.5 T) ∼2.4 × 10⁵ A/cm² is obtained for the 2-μm-thick MgB₂/Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB₂ conductors with high T_c and J_c values which are useful for large scale applications.     

Study of air heat treatment on Bi-2223/Ag superconducting tapes

Heat treatment has been carried out on commercially available Bi-2223/Ag superconducting tape under various temperature conditions in air. The critical currents (I_C) of tape after heat treatment at different annealing temperatures were tested at the liquid-nitrogen temperature. The microstructures and phase constituents of the tapes were investigated by SEM and XRD, respectively. The results indicate that annealing temperature has a very important influence on the superconducting properties of the tapes. When the temperature is above 840 °C, I_C value drops dramatically. Furthermore, when the temperature reaches up to 860 °C, the tape loses most of its superconducting property because of the decomposition of the superconducting phase Bi-2223.     

Interfilamentary coupling properties of Bi2223/Ag tapes with oxide barriers subjected to AC perpendicular magnetic field

In this paper, we fabricated Bi2223 tapes with interfilamentary oxide barriers and evaluated interfilamentary coupling properties under an AC perpendicular magnetic field at 77 K. To avoid the side effect on Bi2223 phase formation during sintering process, SrZrO₃ was selected for barrier materials. Moreover, 20 wt.% Bi2212 was mixed with SrZrO₃ to improve its ductility for cold working. Monocore Ag-sheathed rods were coated by the oxide barriers with slurry before stacking with a honeycomb structure. By twisting the filament with twist pitch length below 10 mm and introducing interfilamentary barriers, the coupling frequency (f_c) under an AC perpendicular field, which is inversely proportional to the decay time constant (τ_c) of coupling current, exceeded 100 Hz. At perpendicular field amplitude above full-penetration field, the magnetization losses of the twisted barrier tape were reduced by 30–40% around power-grid frequency, compared with analytical values for fully-coupled filaments. However, the loss values were still considerably higher than the prediction of the hysteresis loss (Q_h) for the completely decoupled filaments. From the frequency dependence of losses, it was suggested that the loss reduction of twisted barrier tape around power-grid frequency were limited by not only the contribution of coupling current loss (Q_c) but also the insufficient Q_h reduction due to the presence of physical connection among the filaments positioned near the center of a tape section.     

Stoichiometry dependence of the critical current density in pure and nano-SiC doped MgB2/Fe tapes

The stoichiometry dependence of the microstructure and superconducting properties of pure and nano-SiC doped MgB₂/Fe tapes was systematically investigated. The tapes prepared with the composition of a slight deficiency of Mg (Mg_0.9B₂ and Mg_xB₂(SiC)_0.1 (x = 0.9 and 1.0)) showed the best transport J_c. Adding a slight excess of Mg like 5%, as being done by many researchers, was not beneficial for the J_c improvement as expected. The onset T_c was not significantly changed in both doped and undoped tapes by adding excess of boron or magnesium, but the transition widths were broadened due to the induced impurities. The slightly-Mg-deficient pure samples show smaller grain sizes, which corresponds to a better J_c–B performance at high magnetic field due to the enhanced grain boundary pinning. The field dependence of J_c in nano-SiC doped tapes was almost not influenced by varying the starting Mg content although microstructural difference can still be seen, suggesting that the flux pinning ability was mainly controlled by the carbon substitution effect for boron.     

Tunneling conductance of a Bi2−xPbxSr2Ca2Cu3O10−y-SnO2 junction

Fine structures were observed in the tunneling conductance of a sintered Bi 2223-SnO₂ junction. The structures are weaker than those of a single crystalline Bi2212. They correspond to the Raman spectrum of Bi2223 and approximately to the phonon density of states of Bi2212. The structures must therefore be phonon structures, and phonons may contribute substantially to highT _c superconductivity. Multiphonon structures are scarcely discernible. Hence we propose a new model, in place of the prior multiphonon model, to explain the rapid increase inT _c with the CuO₂ layer number. 2 (21 K)=68±4 meV inT _c=98±5 K. 2 (0)/k _B T _c is 8.1±0.9. The temperature dependence of the gap was also observed and discussed.     

Low-field AC susceptibility study of critical current density in Eu:123 and Bi:2223 superconductors

Low-field AC susceptibility measurements have been made, at different values of AC field amplitude H_m (26–600 A/m), on Eu:123 and Bi:2223 samples. Analysis of the temperature dependence of the AC susceptibility has been done employing Beans’ critical state model. The intergranular critical current density (J_c) is calculated as a function of both H_m and temperature for both samples, respectively. It is found that J_c increases with increasing H_m for both samples and its values are generally higher in Bi sample than in Eu sample. Our results are discussed in terms of zero-temperature pinning potential βI(0) in both systems.     

Critical current degradation behaviors of ReBCO coated conductor tapes under pressurized liquid nitrogen

In the case of 2G coated conductor (CC) tapes, it has been reported that thin–thick CC tapes with IBAD substrate showed a superior electromechanical property even at smaller bending radius compared with the cases of 1G BSCCO tapes. Considering the application of CC tapes it is significant to evaluate the transport property under operating environment, because CC tapes might experience a change in operating pressure that can affect its current carrying capacity due to temperature variation and deformation. This study was focused on the I_c degradation behavior in bent CC tapes under pressurized liquid nitrogen. Differently processed YBCO and SmBCO CC tapes with IBAD substrate are used as samples. The bending strain characteristics at elevated pressure levels were evaluated by using the ρ-shaped sample holder which can induce different bending strain values at pressured state. Depressurization and thermal cycling were performed to check the reversibility of I_c in CC tapes. Vacuuming tests were also carried out to investigate the characteristics of I_c at different LN₂ temperature levels.     

Reactivity and oxygen diffusion property of resistive barriers for Bi-2223/Ag tapes

Reactivity of several oxide materials (OM) with BSCCO powder and oxygen diffusion through OM layer has been tested at temperature ≈840 °C in air. The OM (e.g.: BaZrO₃, SrCO₃, MgO and ZrO₂) showing the low or no reactivity with BSCCO have been mixed (10 wt.%) with precursor powder and used for single-core tapes. Bi-2223/Ag/OM/Ag single-core tapes with oxide barriers made of BaZrO₃, SrCO₃, ZrO₂ and Al₂O₃ have been also prepared by a standard powder-in-tube technique. The used OM in the direct contact with BSCCO influences the electrical properties of Bi-2223 phase differently. These is because the oxides react with BSCCO during the heat treatment and simultaneously affect the 2212→2223 phase transformation, the Bi-2223 grain growth and so also grain connectivity. SrCO₃ powder has been evaluated as the best material from the point of no destructive effect on 2223 phase transport current property. The oxide barrier controls the oxygen diffusion during the tape heat treatment and simultaneously the HTS phase formation kinetics, its purity and content within the superconducting core. For single-core Bi-2223/Ag/OM/Ag tapes, the highest current density was measured for Al₂O₃ due to only slightly reduced oxygen diffusion through the barrier.     

The impact of Y substitution on the 110 K high Tc phase in a Bi (Pb):2223 superconductor

The structural and superconducting properties of Bi_1.7Pb_0.3Sr₂Ca_2−xY_xCu₃O_y superconducting samples are investigated by X-ray diffraction (XRD), resistivity and thermoelectric power (TEP) measurements. XRD results reveal that the volume percentage of the 2223 high T_c phase decreases with an increase in Y content. The replacement of the Ca²⁺ ion by the Y³⁺ ion does not influence the tetragonal structure of the pure Bi (Pb): 2223 system and the lattice parameters vary with Y content. The results of resistivity indicate that the critical temperatures T_c of the samples decrease monotonically with an increase in Y content. Further, the critical concentration of Y to completely suppress superconductivity in the Y-doped Bi (Pb):2223 system is higher (0.60) than that reported (0.20) for the other rare-earth elements. On the other hand, the values of TEP at room temperature are found to be negative for Y=0.00 and 0.10 samples, and it changed to positive with further increase in Y content. The hole-carrier concentration per Cu ion (P) is deduced by using two different ways: the first in terms of T_c values in the superconducting state and the other in terms of TEP values in the normal state. Interestingly, it is found that the values of P deduced from the formal way are not consistent with the reported parabolic behavior for superconducting systems in the under-doped region, and consequently disagree with the general roles of substitution. However, the vice versa is recorded for the values of P deduced from the latter way. The results are discussed in terms of the possible reasons for the suppression of superconductivity in the considered system.     

Effect of fluorine doping on phase formation and properties of Bi(Pb)-2223 ceramics

Superconducting ceramics of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_yF_x (x = 0–0.6) are prepared in air by conventional solid state reaction and characterized. The study shows that the melting point of the samples decreases as fluorine content increases. As a consequence, the grain size increases with the doping level and for x = 0.6, the sample is completely deformed and presents a concave shape making impossible the measurements on it. The Vickers microhardness reaches its maximum for x = 0.2. The analysis of the X-ray diffraction results reveals that all the samples are composed of only Bi(Pb)-2212 and Bi(Pb)-2223 phases. The highest proportion of the high T_c phase (Bi(Pb)-2223) is also observed for x = 0.2 and is about 67.32%. The refinement of cell parameters is done by considering the structural modulation. The results show that the doping leads to a reduction of cell volume as well as the a axis component of modulation. From resistivity versus temperature measurements, it is shown that the doped phases exhibit higher onset critical transition temperatures than the undoped one. The residual resistivity increases with fluorine content suggesting that the doping introduces structural defects and disorder into the samples. The obtained critical current density at 77 K under zero magnetic field also increases with fluorine doping.     

Transport properties of long monofilamentary Bi(2223) tapes

Summary Monofilamentary Bi(2223) tapes withJ _c(77 K, 0 tesla) up to 30 000 A/cm² have been prepared by cold rolling using the powder-in-tube method. An optimization of the precursor powders has led to a higher phase purity after the reaction heat treatment. The deformation process has been optimized in order to increase the oxide density and to reduce sausaging effects on the oxide thickness. The transport properties of these tapes have been studied in a wide range of temperature (4.2K-T _c) and magnetic fields (up to 28 tesla). The critical-current values at 77 K fields of 0.5 T and 1 T parallel to the tape surface are 10 000 A/cm² and 5400 A/cm², respectively. At 4.2 K theJ _c value decreases from 1.6·10⁵ A/cm² at 0 T to 6·10⁴ A/cm² at 15 T. At fields higher than 15 tesla a very low field dependence ofJ _c has been found, regardless of the tape orientation. Transport properties have also been studied by cutting small sections of the tape in order to investigate the local critical-current distribution. It has been found that, even in rolled tapes of good quality (J _c (77 K, 0 T)>20000 A/cm²), theJ _c distribution is homogeneous: the critical current density increases gradually from the centre of the tape to the sides, the latter exhibiting much higherJ _c (46000 A/cm²) than in the centre (18000 A/cm²). Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

On some consequences of an external magnetic field applied to HTS coils

A computational model which enables to evaluate the distribution of the critical currents, electric fields and the voltage in the winding of a solenoidal high temperature superconducting (HTS) magnets subjected to an external magnetic field parallel with the magnet axis, was developed. The model comes out from the well-known power law between the electric field and the transport current of the HTS tape short sample. It allows to predict the voltage–current V(I) characteristics of both the pancake coils and the complete magnet. The model was applied to the magnet system consisting of 22 pancake coils made of multifilamentary Bi(2223)/Ag tape at 20 K, which is subjected to an external uniform magnetic field parallel with the coil axis. A rather unexpected behavior of the magnet at different operating conditions (operating current and external magnetic field strength) is predicted, analyzed and reported together with a theoretical explanation. On one hand, the external uniform magnetic field parallel with the coil axis increases the resulting magnetic field strength, however, on the other hand it simultaneously decreases the angle between the resulting magnetic field and the tape surface. Thus, the effect of higher magnetic loading caused by the presence of an external magnetic field strength which is acting on individual turns located close to the coil’s flanges is compensated by more favorable orientation of the tape with respect to the resulting magnetic field. As a result, increase in the critical currents of these turns is expected. Further, the results indicate, that in case of the high field HTS insert coils the anisotropy in the I_c(B) characteristic does not play a substantial role. As a consequence, the technology of the production of the tapes for high field insert HTS coils should concentrate rather on the tapes having the current carrying capacity as high as possible, than on the attempt how to decrease the anisotropy in the I_c(B) by changing the architecture of the filaments in the tape.     

(Bi,Pb)2Sr2Ca2Cu3Oy单相样品的超导电性与载流子浓度

对(Bi,Pb)₂Sr₂Ca₂Cu₃O_y单相样品进行不同条件下的热处理,通过X射线衍射、电阻-温度关系、交流磁化率,以及Hall系数等测量,发现样品均具有较好的单相性,随着热处理条件的变化,其超导转变中点温度(T_c)有规律地分布在100—110K之间,T_c随载流子浓度(n_H)的增加而升高。实验结果表明,热处理条件对样品的相结构、超导 关键词：     

Critical current density in MgB2 bulk samples after co-doping with nano-SiC and poly zinc acrylate complexes

The co-doped MgB₂ bulk samples have been synthesized using an in situ reaction processing. The additives is 8 wt.% SiC nano powders and 10 wt.% [(CH₂CHCOO)₂Zn]_n poly zinc acrylate complexes (PZA). A systematic study was performed on samples doped with SiC or PZA and samples co-doped with both of them. The effects of doping and co-doping on phase formation, microstructure, and the variation of lattice parameters were studied. The amount of substituted carbon, the critical temperature (T_c) and the critical current density (J_c) were determined. The calculated lattice parameters show the decrease of the a-axis, while no obvious change was detected for c-axis parameter in co-doped samples. This indicates that the carbon was substituted by boron in MgB₂. The amount of substituted carbon for the co-doped sample shows an enhancement compared to that of the both single doped samples. The co-doped samples perform the highest J_c values, which reaches 3.3 × 10⁴ A/cm² at 5 K and 7 T. It is shown that co-doping with SiC and organic compound is an effective way to further improve the superconducting properties of MgB₂.     

Formation of high-T c phases of the system Bi-Pb-Sr-Ca-Cu-O in a fluxed KCl melt

The influence of the conditions of synthesis and annealing on the ratio of the volumes and the temperature T _c ⁿ at which transition to the superconducting state begins is investigated on the basis of diamagnetic susceptibility measurements and x-ray phase analysis for bismuth 2212 and 2223 phases obtained from solution in a KCl melt. It is found that the value of T _c ⁿ for the 2212 phase decreases as the temperature and the holding time are increased in synthesis. The 2212–2223 transition in the fluxed KCl melt takes place in the presence of an oxygen deficiency, and the width of the transition interval to the superconducting state of the 2223 phase depends on the cooling rate of the fluxed melt after isothermal holding and also on subsequent annealing in air. Temperatures T _c ⁿ =107 K, 90 K, and 20 K are observed for unannealed crystals of the 2223 phase with average dimensions 50×50 μm. Air annealing leads to oxygen saturation of their lattice, and T _c ⁿ =107 K throughout the entire structure of the crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1761–1763 (October 1997)     

A study of Fe-doped Bi-Sr-Ca-Cu superconductors

We have substituted 1.5% of Fe for Cu in several “2212” and “2223” Bi-Sr-Ca-Cu superconductors. All of the samples show a reduction ofT _c about 13 K due to the Fe impurities. Mössbauer measurements at room temperature reveal the structural characteristics such as stacking faults and intergrowth of different phases in these Bi-based compounds on the microscopic scale. The susceptibility ofT _c to Fe-doping in the Bi-“2212” or “2223” system is comparable to that of the “123” system but much smaller than that of the “214” system. The interplanar correlation existing in the “123” and the Bi-“2212” and “2223” systems seems to play an important role in sustaining the high temperature superconductivity and weakening the detrimental effect of impurity elements on superconductivity in these two systems.     

Specific heat and thermal expansion of the Bi and Tl high temperature superconductors nearT c

We present measurements of the specific heat and of the thermal expansion of the most prominent phases of the Bi and Tl high temperature superconductors (Bi-2212, Bi-2223, Tl-2212, Tl-2223) in the neighbourhood ofT _c . In all these systems we observe small but sharp anomalies which have very little similarity with the mean field jump found in the conventional superconductors. The analysis of these anomalies shows clear evidence for the presence of strong fluctuations of the order parameter. For temperatures more than 5 K away fromT _c ,2D Gaussian fluctuations are found, while within ±5 K ofT _c the fluctuation contribution is best fitted by critical fluctuations. The shape of the thermal expansion anomalies is similar to that of the specific heat. Combination of both measurements predicts dT _c /dp+0.2 K/kbar for Bi- and Tl-2223 systems.