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.     

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.     

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.     

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.     

Effects of Rolling Passes on the Transport Properties of 37-Filamentary AgAu-Sheathed Bi-2223 Tapes

Thirty-seven-filamentary AgAu-sheathed Bi-2223 tapes were fabricated by a powder-in-tube (PIT) process. And, the round wires (? 1.86 mm) were rolled to 0.35-mm tapes with 12, 7, 5, and 4 rolling passes through flat rolling, respectively. The influences of different rolling passes on the core density, deformation, and transport properties of Bi-2223/AgAu tapes were systematically investigated. It was noticed that after rolling, the Vickers microhardness of the superconducting core and deform homogeneity along both the horizontal and vertical directions on the cross section of seven-pass rolled tape were better than those on the tapes with other passes, which proved the larger core density and uniform deformation with the seven-pass rolling process. Meanwhile for the wires with 12 and 7 passes, the AgAu/superconducting core interfaces were much flatter. With the rolling passes decreasing from 12 to 4, the critical current density (J_c) first increased and then decreased. Due to the better homogeneity and flatter interfaces, J_c reached the maximum value of 17.3 kA/cm² on the seven-pass sample. Meanwhile, the enhancement of current capacities in magnetic field applied parallel to the Bi-2223/AgAu tape surface could also be recognized as the evidence of improving intergrain connections due to the higher density in seven-pass rolled tapes.     

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.     

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.     

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.     

Pressing Processing Diminishes Outgrowth and Bridging for AgMg/Bi-2223 Tapes

Multifilamentary Bi-2223 tapes were fabricated by PIT, using a silver alloy sheath with 2.5% magnesium metal. Outgrowth and bridging are major disadvantages for some silver alloy sheathed Bi-2223 tapes. To solve the outgrowth problem, green tapes were pressed with different reduction rates and then sintered at high temperatures. Critical current I _c of the sintered tapes was measured at nitrogen temperature using the four-probe method. The morphology of the filament core was observed with scanning electron microscope (SEM) to investigate outgrowth of sintered tapes with different reduction rates. Outgrowth and bridging on cross and longitudinal sections of filaments were studied using TEM images. Experimental results suggest that pressing processing may diminish outgrowth and bridging. Number of outgrowths and cases of bridging are reduced when the reduction rate increases, but the slope of the reduction falls at large reduction rates. The I _c curve indicates that there is an optimum reduction rate at which I _c reaches a maximum. At the best reduction rate the amount of outgrowth and bridging is close to the lowest. Experimental results show that pressing processing can diminish outgrowth and bridging by as much as 50%. Therefore, proper pressing is an effective method for both diminishing outgrowth and bridging and enhancement of I _c.     

Ic degradation behaviors of Bi-2223 superconducting tapes under axial fatigue loading

For the endurance evaluation of High Temperature Superconductors (HTS), the mechanical and transport properties of multifilamentary Bi₂Sr₂Ca₂Cu₃O_10+x (Bi-2223) superconducting tapes with different reinforcements subjected to high-cycle axial fatigue loading were investigated at 77 K in the self-field. The mechanical fatigue limits based on the relations between the applied stress amplitude and the numbers of cyclic steps to reach failure (S-N_f curves) were obtained. The transport properties were evaluated with the increase of repeated cycles, N, at different applied stress amplitudes which eventually leads to the electric fatigue limit. The influence of reinforcement on the mechanical and transport properties of Bi-2223 tapes were discussed. Fractographic observation was performed in order to understand the I_c degradation mechanism in fatigue tested Bi-2223 tapes.     

A Quantitative Study on Temperature Dependent of Outgrowth in Bi-2223 Tapes

Outgrowths of Bi-system superconducting multifilament tapes were known as an imperfect that could increase ac-current loss. To approach this problem sintering temperature dependent of outgrowth was studied and experimental results were reported in this text. Tapes in the experiments were sheathed with pure silver and silver alloys and carefully treated. Critical current I _c of the sintered tapes was measured at 77 K. Compound phases in tapes were determined by analysis of X-ray powder diffraction. Outgrowths of the tapes were observed and studied by Scanning Electron Microscope. Experimental results showed that outgrowths changed when sintering temperatures increased. It was found that the best sintering temperature for Ag sheathed tape was 840°C, and that for Ag/Mg and Ag/Sb sheathed tapes was lower than 835°C.     

Transport Ic measurement technique with non-soldered contacts for Ag-sheathed high Tc superconductors

A transport current measurement technique suitable for the technological development of hightemperature superconducting tapes has been designed. The main advantage of the technique presented is in the application of non-soldered current and voltage contacts which allows measurement of the same element of superconducting tape at different technological states (roll-sinter or press-sinter step). Voltage-current characteristics can be measured in self field as well as in external magnetic field at 77 K or 4.2 K and at different field orientations. The technique capabilities are demonstrated by presenting I_c data obtained from Bi(2223)/Ag tape.     

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.     

Physical Properties and Diffusion-Coefficient Calculation of Iron Diffused Bi-2223 System

This study includes two parts: (I)?investigation of the effect of different annealing time (10?h, 30?h, and 60?h) on physical, superconducting, and microstructural properties of Fe-diffused Bi-2223 superconductor ceramics prepared by the conventional solid-state reaction method with the aid of the X-ray diffraction (XRD), scanning electron microscopy (SEM), dc resistivity (???CT) and transport critical current density (J _c ) measurements, and (II) determination of the diffusion coefficient and the activation energy of iron in the Bi-2223 system. In the former part, the zero-resistivity transition temperature (T _c ), phase purity, volume fraction, hole-carrier concentration, lattice parameters, surface morphology, texturing, crystallinity, grain connectivity, grain size, and room temperature resistivity values of the bulk samples are found and compared with each other. The results obtained show that both the zero resistivity transition temperature (T _c ) and transport critical current density (J _c ) regularly enhance with the increment in the diffusion-annealing time. The maximum T _c of 107±0.2 K and J _c of 50.0?A?cm^?2 are observed for the sample annealed at 830?°C for 60?h. As for the XRD investigations, according to the refinement of cell parameters done by considering the structural modulation, the enhancement in the diffusion-annealing is confirmed by both a decrease of the cell parameter a and an increase of the lattice parameter c of the samples, meaning that the greatest Bi-2223 phase fraction belongs to the sample annealed at 830?°C for 60?h. Moreover, SEM images display that the sample has the best crystallinity, grain connectivity, and largest grain size. Based on the results, the superconducting and microstructural properties improve with the increase in the diffusion-annealing time. In the latter part, Fe diffusion in the Bi-2223 system is examined in a range of 500?C830?°C by the variation of the lattice parameters evaluated from the XRD patterns. The temperature dependence of the Fe diffusion coefficient is described by the Arrhenius relation D=4.27×10^?5exp(?1.27±0.10) eV/k_BT, and the related activation energy of the iron in the Bi-2223 system is found to be about 1.27?eV. The relatively low value of activation energy obtained illustrates that the migration of the Fe ions primarily proceeds through defects such as pore surfaces and grain boundaries in the polycrystalline structure, leading to the improvement of the microstructural and superconducting properties of the samples, supported by the results of part?I. All in all, the aim of the present study is not only to analyze the role of diffusion-annealing time on superconducting and microstructural properties of Fe-diffused Bi-2223 superconductors, but also to find the diffusion coefficient and activation energy of Fe in the Bi-2223 system.     

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.     

Effects of local characteristics on the performance of full length Bi2223 multifilamentary tapes

The effects of local characteristics on the performance of full length Bi2223 multifilamentary tapes are investigated computationally and experimentally at 77 K and self-field. Generally the current-voltage characteristics of superconductors are described by the standard power law model with parameters such as critical current I_c and index n. By measuring the critical current {I_ci} and index {n_i} values of local tapes, we can get the critical current I_c and n value of full length tapes by means of statistical method. The results show that the distribution of local critical currents are non-uniform, and local critical currents have important effect on the performance of the entire tapes. The critical current of the entire tape is different from the mean value of local critical current based on Gaussian statistical distribution along the long tape.     

The Contradiction Behavior Between Mechanical Performance and Degradation Mechanism of Electric at Cryogenic Temperature in Bi-2222/Ag Tapes with Different Protective Layer

The mechanical performance at cryogenic temperature and the degradation mechanism of electric behaviors of Bi-2222/Ag tapes with different protective layers as well as their contradiction laws are discussed. A variable temperature cryostat system is constructed to provide the successive cooling environment from room temperature (RT) to the liquid nitrogen temperature (LNT), and a cryogenic-type extensometer is also used to measure strain behavior of the superconducting tapes. And, the effects of bending strain on the critical current of Bi-2222/Ag tapes with different protective layer were measured using arched abrasives with different radius. Experimented results have shown that the protective layer of the tapes could have strong positive effects on the measured mechanical performance at room temperature and cryogenic temperature. The irreversible degradation on strain indicates that the I _c reduction is caused mainly by crack formation and propagation in the brittle Bi-2222/Ag tapes. In particular, the degradation mechanism of multifilamentary sample with protective layer was also elaborated, and protective layer has some negative effect on electric behaviors. In addition, the behaviors of the n value with strains on standard Bi2222/Ag tape was also argued to predict the damage process in Bi-2222 tapes indirectly during bending tests.     

Study for AC Loss Reduction in Bi2223 Tapes by Introducing Oxide Barriers

This paper presents our recent activities for the development of low-loss Bi2223 tapes with interfilamentary oxide barriers. In order to suppress the side effect on Bi2223 phase formation during sintering process, SrZrO₃ was selected as barrier materials. Moreover, small amount of Bi2212 was mixed with SrZrO₃ to improve their ductility for cold working. By controlling coating thickness of oxide barriers before stacking, reducing a tape width below 3 mm and careful twisting of the filaments with its length below 5 mm, coupling frequency f _c exceeded 250 Hz even in an AC perpendicular magnetic field. Critical current densities J _c of tightly twisted barrier tapes were ranged in 12?C14 kA/cm² at 77 K and self-field, which was 25% lower than the nontwisted one (=18 kA/cm²). To our knowledge, this is the first result to achieve both J _c>12 kA/cm² and f _c>250 Hz simultaneously for Bi2223 tapes in an isolated state. These twisted barrier tapes showed 60?C70% lower perpendicular field losses than a conventional 4 mm-width tape with fully coupled filaments at 50 mT and 50 Hz.     

On the synthesis and structure of single-phase (Bi,Pb)2Ca2Sr2Cu3O10

We report an elegant method for the synthesis of single-phase Bi-2223 superconductor from a stoichiometric composition Bi_1.7Pb_0.3Ca₂Sr₂Cu₃O_y by a matrix reaction route. The superconducting transition temperatureT _c (R=0) of this single-phase compound is 120 K. The effect of Pb-content and sintering temperature on the formation and stability of Bi-2223 phase is described.