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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Effect of magnetic field, thermal cycling and bending strain on critical current density of multifilamentary Bi-2223/Ag tape and fabrication of a pancake coil

Multifilamentary HTSC tapes are important for their applications in various electrical devices. Powder-in-tube technique with improved optimized synthesis parameters is regarded as one of the most promising ways to prepare long-length multifilamentary Bi-2223/Ag tapes. Nevertheless, usefulness of such tapes depends on their electrical and mechanical properties. Critical current density of a Bi-2223/Ag tape with 37 filaments has been studied at 77 K with field, field orientation, thermal cycling and bending strain as parameters. Results have been discussed in light of various mechanisms and models. A small pancake coil has been fabricated out of the same tape and the test results presented.     

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.     

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.     

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.     

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.     

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.     

Pinning Energy of High-Tc (Bi, Pb)2Sr2Ca2Cu3Oy

The flux pinning potential energy of silver (Ag) sheathed high-T _c Bi-2223 tapes determined from measured current densities, J(T,H), is reported in this paper. Measurements of the transport current density, J, of the Ag sheathed Bi-2223 tapes were performed with an applied magnetic field directions parallel (H‖ab) and perpendicular (H‖c) to the tape’s board surface from 0 to 3 T, at temperatures from 23 to 77 K. To obtain the pinning potential energy of the Bi-2223 superconductor, an empirical scaling of J(T,H) is introduced in this study. Under the flux dynamics, the effective pinning energy, U _eff(T,H,J), can be formulated using the measured current density, J(T,H). Furthermore, the pinning potential energies, U(T,H), for both H‖ab and H‖c were quantitatively determined by the scaling of J(T,H). The two energies, U(T,H), exhibit as a function that is inversely proportional to both temperature and logarithmic magnetic field. The magnitude of U(T,H) for H‖ab is three or more times larger than that for H‖c under the considered conditions. The U(T,H) is sensitive to the temperature, but insensitive to the magnetic field. This characteristic of insensitiveness towards magnetic field is an advantage for applications on the conditions of high current density, high magnetic field, and low temperature.     

Improvement of Jc and Grain Alignment of Bi-2212/Ag Multilayer Tapes by Pre-Annealing and Intermediate Rolling Process

The high transport critical current density (J _c ) > 500 kA/cm ² at 4.2 K, 10 T is obtained in the Bi-2212/Ag multilayer tapes fabricated by using PAIR (Pre-Annealing and Intermediate Rolling) and melt-solidification process. This J _c value is twice higher than existing high-quality Bi-2212/Ag tapes (250 kA/cm ² ). By applying PAIR process to Bi-2212/Ag tapes, Bi-2212 grain alignment is much improved and a large J _c enhancement is achieved. J _c has been increased strongly by performing pre-annealing at 840°C in oxygen (1 atm) and intermediate rolling with 25% deformation.     

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.     

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.     

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 of BSCCO Films on SrTiO3 Substrates Using Ultrasonic Spray Pyrolysis (USP) Technique

In this study, the BiSrCaCuO (Bi-2212) films on SrTiO₃ substrates were fabricated using an ultrasonic spray pyrolysis technique (USP). Structural, electrical, magnetic, and critical current density, J _c, properties of the films fabricated were investigated under different heat treatment conditions. XRD analysis showed that the films mainly consisted of the Bi-2212 phase, but the Bi-2223 phase was also detected. T _c values of the films were found between 81 K and 88 K, depending on the heat treatment conditions. J _c values of the films were calculated using the Beans’ equation. Highest J _c value was found to be 2.93×10⁵ A?cm^?2 at 5 K and 0 T for Film C. The results obviously showed that USP method is a very effective technique for fabrication of the HT _c films having high J _c values as well as its simplicity, low cost, and easily coating.