The J c 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.     

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.     

Development of Bi-2223 HTS tape and its application to coil and current leads

We are developing Bi-2223/Ag tapes with a high engineering critical current density by optimizing the powder-in-tube process and are studying its application to coil and current leads. We have fabricated 250 m-long tape and investigated optimized processing conditions to enhance engineering critical current density. More bubbling was found when the tape was heat-treated with a higher heating rate. Different kinds of superconducting joints were fabricated with multi-filamentary Bi-2223/Ag tapes, and 58% of retained I_c was achieved using the insertion of Bi-2223 core between two exposed tapes. Current decay property of the persistent mode HTS coil was investigated. Rapid current decay was observed when the operating current is in a flux-flow range. We could successfully fabricate a low heat leak type HTS current lead with Bi-2223/Ag–Au tapes by employing a stepped geometry. Using this lead, safe operation of 2 kA current transport was confirmed.     

Phase and Texture Evolution of Ag/Ni Composite-Sheathed Bi-2223 Tapes with Different Thickness

The influence of green tape thickness on the Bi-2223 phase formation and texture evolution in Ag/Ni composite-sheathed tapes fabricated by the “powder-in-tube” technique has been studied. Microstructural observations by SEM as well as critical current density (J _c) measurements at 77 K, 0 T have been performed to analyze the performance of the tapes. The results show an important influence of the green tape thickness on the critical current depending on the content and texture of Bi-2223 phase. The J _c increases with decreasing thickness. Moreover, texture measured by omega scans shows that the texture of the Bi-2223 phase is significantly influenced by the thickness of the green tape after the first and final sintering processes. Alignment of Bi-2223 grains in the thin tapes is much better. Higher performance of Ag/Ni composite-sheathed Bi-2223 tapes can be obtained by controlling the thickness of the green tapes.     

Fabrication and Properties of Ag–Ni Bimetallic Sheathed (Bi,Pb)-2223 Tapes

10-meter-long Ag?CNi bimetallic sheathed (Bi,Pb)-2223 tapes with outer nickel sheath and inner silver sheath have been successfully fabricated by the ??Powder in tube?? technique. Microstructure and phase evolution studies by means of SEM and XRD, as well as critical current density (J _c ) measurements have been performed. It is found that the nickel sheath and dwell time in the first sintering process have great influences on the texture evolution, phase transformation and J _c of the Bi-2223/Ag/Ni tapes. Mono-filament (Bi,Pb)-2223 tape with a J _c of 6656?A?cm^?2 and 61-filament tape with a J _c of 12420?A?cm^?2 are obtained. Although using composite bimetallic sheaths can reduce production costs and improve mechanical properties of the Bi-2223 tapes, the Bi-2223 content and J _c of Bi-2223/Ag/Ni tapes are relatively lower than that of traditional Bi-2223/Ag tapes. Meanwhile, due to higher Bi-2223 content and better alignment of Bi-2223 grains, tapes with 61-filament have higher J _c than mono-filament tapes.     

The influence of electroplastic rolling on the mechanical deformation and phase evolution of Bi-2223/Ag tapes

Electroplastic rolling (EPR) of Bi-2223/Ag superconducting wires was performed, where pulse currents were applied during rolling to introduce an electroplastic effect. It was found that the rolling force decreased significantly compared with the traditional rolling process. Furthermore, EPR favorably minimized the sausage effect. It is revealed that the electroplastic effect can facilitate the mechanical deformation of Bi-2223/Ag composites. Segments of the Bi-2223/Ag tapes were heat treated at 830 °C for different time periods. The phase assemblies of these samples suggest that current pulses contribute to faster transformation kinetics from the Bi-2212 phase to the Bi-2223 phase. In addition, a preliminary improvement of 28% of critical current density has been achieved in a fully processed tape with EPR.     

Transport critical current density of Ag-sheathed Pr-doped (Tl,Cr)Sr2CaCu2O7 superconductor tapes in applied magnetic field

Powder-in-tube Tl-1212 (Tl,Cr)-Sr-(Ca,Pr)-Cu-O/Ag tapes were prepared and processed with different thermomechanical treatments. The processing conditions caused the variation of the transport critical current density (J_c) in the tapes. Tapes subjected to rolling showed lower J_c compared to the pressed tapes. Uniaxial pressing is more effective in densifying the tape cores by forcing the grains into closer contact and enhances the connectivity between grains. All the tapes showed a majority phase of 1212. The existence of 1201 phase considerably reduced J_c in the rolled tapes. Intermediate rolling is not favorable in improving J_c for the irregular grains structure, in contrast to Bi-2223 superconductors with plate-like grains. The in-field behavior of the tapes indicates that they are dominated by weak links as J_c falls rapidly in low applied field (H<0.06 T). No anisotropic transport properties were observed for all the tapes. SEM images showed randomly oriented irregular grains making texturing difficult to achieve. Neither the grain size nor the morphology was changed significantly by the thermomechanical treatments. The observed irregular grains imply that high-angle tilt boundaries were formed with intergranular weak links. Grain morphology is suggested to be the key factor that limits the J_c in these tapes.     

Microstructure of Ag/Bi(Pb)-2223 Tapes Prepared by Solid-State Reaction

The processing of Ag/Bi(Pb)-2223 tapes via the prevailing solid state reaction was investigated. A precursor powder of the composition Bi_1.86Pb_0.26Sr_1.96Ca_1.95Cu_2.97O₁₀₊ containing mainly Bi-2223 phase was employed. Particular attention was devoted to the microstructural properties of the tapes subjected to rolling and subsequent heat treatment in one, two, and three steps, respectively. A sharp texturing gradient exists in the superconducting cores, and the repeated treatment leads to deterioration of the superconducting properties. The latter effect is ascribed to the formation of periodic defects arising during the repeated rolling of the sintered superconducting core.     

Development of high-T c superconducting Bi-2223 tape joints

High-T _c superconducting joints between Ag-clad Bi-2223 tapes have been developed for persistent current applications. Two presintered tapes with one side of the silver stripped were lapped and then wrapped by a silver foil. The complex was uniaxially pressed followed by appropriate sintering to form a high-T _c superconducting tape joint. It was found that the ratio of critical currents through the joint to that of the tape,I _cj/I_c, depended on the uniaxial pressure and the sintering conduction. At liquid-nitrogen temperature 77 K,I _cj/I_c=99% has been achieved. Persistent current loops formed by Bi-2223 tapes have also been fabricated and tested. Joint resistance of a loop was determined to be ～4×10^?13 Ω between the decay time of 120 and 3600 sec.     

Effect of Various Intermediate Deformation Techniques on Jc and Grain Texture for Ag/Bi-2223 Tapes

Three Ag sheathed Bi-2223 multifilamentary tapes were produced by a processing method that consists of two sintering treatments with an intermediate deformation, involving sandwich rolling (SR), pressing (P), or normal rolling (NR). The magnetic field dependence of the critical current density J _c was measured with the magnetic field H applied parallel to both the ab plane (H ab) and the c-axis (H c) of the Bi-2223 grains. Experimental results show that J _c of the pressed (P) tape (J _cP) for both H ab and H c is about 1.5–1.8 times higher than that for the NR tape (J _cNR) and the SR tape, although J _cSR is always larger than J _cP. The ratio of J _cSR/J _cNR for H c increases rapidly with the applied magnetic field and reaches a maximum of about 12 at ₀ H 900 T. The calculated density of the pinning force F as a function of magnetic field shows that curves of F for SR, NR, and P tapes all have their maximum F _max at different magnetic fields and the magnitudes of F _max are also different from each other. The SR tape has the largest value of F _max, while NR has the smallest. XRD analysis shows that an intermediate deformation can destroy the grain alignment, and the larger the deformation, the worse the grain texture will be. Our experimental results, however, clearly show that J _c for Bi-2223 multifilamentary tapes is independent of grain alignment. The significant differences in J _c for tapes processed using the three different intermediate deformation procedures are dependent on the density of the pinning force and cannot be attributed to the grain alignment. Our experimental results support the view that SR processing is the best method for fabricating Ag/Bi-2223 tapes of high quality.     

Magnetic Susceptibility and High Field Magneto-transport of Silver-Added Bi-2223 Superconductor: a Revisit

The effect of Ag addition on weak link behaviour of a Bi-2223 (Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O₁₀) polycrystalline sample has been investigated in terms of AC susceptibility, critical current density (J _c), electrical resistivity ρ(T)H and upper critical field (H _c2). A series of phase pure Bi-2223-Ag _x (x = 0.0–0.3) samples are prepared by the solid-state synthesis route. The phase purity, crystal structures and surface morphology are being studied using the X-ray diffraction and scanning electron microscopy (SEM) technique, respectively. The effect of Ag addition on inter- and intragranular coupling has been investigated by means of AC susceptibility and magneto-transport ρ(T)H measurements, and the results are compared with the pure Bi-2223 sample. Enhancement in granular coupling between the grains of the 20 wt% Ag-added Bi-2223 sample has been witnessed. Critical current density (J _c) has been estimated using the AC susceptibility technique, and the results are interpreted in terms of inter- and intragranular coupling of the investigated samples. The high field magneto-transport technique has been used to estimate the upper critical field (H _c2) and thermally activation flux flow (TAFF) activation energy. The ensuing results revealed that H _c2 increases for the 20 wt% Ag-added sample along with enhancement in grain alignment and intergrain connectivity.     

The influence of the first heat-treatment on the critical current density of (Bi,Pb)-2223/Ag superconducting tapes

Optimization heat-treatments have been performed on multi-filament Bi-2223/Ag superconducting tapes under 1 bar total gas pressure, the oxygen partial pressure being 8.5%. In a first heat-treatment (HT1), the tapes were sintered within 822-838 °C for 1-50 h. After intermediate deformation, all the samples underwent the second heat-treatment (HT2) at 825 °C and 830 °C for 20 h followed by a thermal sliding procedure. The relative content of the phases present in HT1 samples was measured by XRD. It was found that the Bi-2212 phase content after HT1 strongly influences the values of J_c after HT2. There is a correlation between the amount of Bi-2212 phase after HT1 and the final J_c values after HT2. A maximum of J_c was found for a ratio of 0.15 between Bi-2212 and Bi-2223.     

Effect of starting precursor powders on microstructural development and critical current density properties of BSCCO 2223 tapes

Bi-2223/Ag tapes with different lead content (Pb=0.2–0.4) powders were fabricated. The microstructural development and J_c properties were studied with starting precursor powder prepared in different conditions. The experimental results indicate that the variations of lead content extremely influence the reactivity of precursor powders, which is closely related to the formation rate of 2223 phase, microstructure and J_c values of Bi-2223/Ag tapes. In addition, the particle size distribution of precursor powders has a large effect on the transport properties. By optimizing these powder parameters, J_c values above 60,000 A/cm² (77 K, 0 T) in short tapes were achieved.     

Development of high-Tc superconducting Bi-2223 tape joints

High-T _c superconducting joints between Ag-clad Bi-2223 tapes have been developed for persistent current applications. Two presintered tapes with one side of the silver stripped were lapped and then wrapped by a silver foil. The complex was uniaxially pressed followed by appropriate sintering to form a high-T _c superconducting tape joint. It was found that the ratio of critical currents through the joint to that of the tape,I _cj/I_c, depended on the uniaxial pressure and the sintering conduction. At liquid-nitrogen temperature 77 K,I _cj/I_c=99% has been achieved. Persistent current loops formed by Bi-2223 tapes have also been fabricated and tested. Joint resistance of a loop was determined to be 4×10^–13 between the decay time of 120 and 3600 sec.     

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.     

Comparing the Effects of Nb,Pb, Y,and La Replacement on the Structural,Electrical, and Magnetic Characteristics of Bi-Based Superconductors

In this study, the effects of Pb, Nb, La, and Y replacements were investigated on Bi-based superconducting materials. In preparing the samples, we used a method called solid-state reaction method. The patterns of the X-ray diffraction of all samples indicated presence of Bi-2212 and Bi-2223 phases. The results obtained from XRD revealed that with increase of the melting point of substation elements, the Bi-2223 phase decreased while the Bi-2212 phase and impurity phases of samples grew. From the electrical resistivity measurements using the four-probe method, it was found that sample A with Pb and sample B with La replacements had the maximum and minimum critical temperatures of 111.4 and 81.6 K, respectively. Based on hysteresis loop (M–H) measurement using Bean’s model, estimation of critical current density (J_c) showed that sample A with Pb and sample B with La substitution had the maximum and minimum values respectively. These results may be due to the melting point of these elements with values of 888, 1512, 2315, and 2425 ^°C for PbO, Nb₂O₅, La₂O₃, and Y₂O₃, respectively. These elements were replaced by Bi₂O₃ with a melting point of 817 ^° C. Further, the samples were prepared at the temperature of 845 ^°C. It seems at this temperature, these elements not only dissolve within the main matrix and participate in the formation of the Bi-2212 phase during the sintering process but they also participate in the development of the variety of the impurity phases as confirmed by XRD results.     

Instabilities above critical current region in Bi-2223/Ag superconducting coils cooled by liquid nitrogen

A sudden drop of the coil voltage and a hysteresis of I-V curve were observed in measurement of one-layer Bi-2223/Ag coils cooled by liquid nitrogen at currents well above critical current region. Their temporal behavior indicates, that the improvement of the cooling and corresponding decrease of temperature after the jump takes place. To study this phenomenon we measured I-V curves of two Bi-2223/Ag coils made from tapes with various degree of critical current homogeneity and analogical curves of two non-superconducting coils made from thin Cu tapes having various widths. In Cu coils we really observed a sudden drop of the temperature, measured in parallel with Cu resistance drop, after reaching heat flux of about 0.4 W cm⁻² during current ramping up. In spite of non-superconducting character of the tape, the hysteresis, i.e. difference between increasing branch and decreasing branch of I-V curves, was observed too! Approximately the same value of heat flux, at current corresponding to the jump, was found also in superconducting coil on segment with least value of local critical current. We conclude that observed voltage drop of the Bi-2223/Ag does not bear upon superconducting nature of the coil and, as that for Cu coil, can be explained by dynamics of heat transfer to liquid nitrogen and its history.     

Effect of refractory nitride nanoadditives on the properties of (Bi,Pb)<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10+δ</Subscript>

We have investigated the interaction between (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+δ (Bi-2223) and small additions (0.05–0.3 wt %) of nitride powders (TaN, AlN, HfN, NbN, Si₃N₄, TiN, and ZrN) with a particle size from 0.02 to above 0.5 μm and the effect of these nitrides on the microstructure, phase composition, distribution, and morphology of the resulting second-phase inclusions. The concentration and particle size of the nitrides and sintering conditions are shown to influence the superconducting transition temperature T _c, critical current density j _c, irreversible remanent magnetization, bulk density, and mechanical properties of the Bi-2223/nitride composites.     

A Statistical Model of <Emphasis Type="Italic">I</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Changes for Bending Bi-2223 Tapes

Bi-2223 tape was one of the high-temperature superconductors with commercial applications. One of the applications was bending and winding Bi-2223 tape into solenoids to produce high magnetic fields. To study bending properties, three multifilamentary of Bi-2223 tapes sheathed with silver alloys were manufactured. Bending experiments for the tapes were performed, and critical currents I _c of tapes with definite bending radius were measured. And, current transferring mechanisms in filaments were analyzed, as well. Experimental results showed that silver alloy sheathed tapes had better bending properties than pure silver-sheathed one. On the contrary, to describe bending radius dependence of I _c , a statistical model was suggested. The model expected that bending radius dependence of I _c was following an exponential law that was quantitatively expressed by mathematic expressions. Bending dependence of I _c could be calculated from the expression and calculated results agreed with experimental data very well. Therefore, the suggested model has successfully explained the experimental results.     

Multifilamentary V3Ga wires and tapes with composite cores

Results on the critical current behaviour of 19 core composite superconducting wires and tapes in magnetic fields up to 9 T are reported. The composite cores used in the present studies are vanadium tubes filled with Cu-Ga matrix. Two V₃Ga layers are formed on either side of the tubular vanadium. For a given matrix volume to vanadium surface area ratio, the inner V₃Ga layer has much faster growth rate and consequently finer grains than the corresponding outer layer. An increase of about 50% in overall critical current density compared to the 19 core wires using the same starting materials but prepared with conventional geometry has been obtained. The critical current density of V₃Ga (jc) as well as the overall critical current density (J_c) of the tape specimens with composite cores is significantly higher than the corresponding wires. Enhanced surface flux pinning seems to be responsible for the higher current density in the tapes. Addition of 6 at% gallium to vanadium and 0.5 at% Mg to CuGa (19 at%) matrix leads to enhanced critical current densities in these specimens in conformity with our earlier studies.