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.     

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.     

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.     

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.     

Preparation and critical current density of hot pressed Bi2Sr2Ca2Cu3Ox/Ag tapes

Bi2Sr2Ca2Cu3Ox(Bi-2223)/Ag tapes have been prepared by hot pressing performed at 800–840 °C for 12–96 h under 6 or 12 MPa in air. The highest transport critical current density, Jc, is 3900 A cm–2 at 77 K and 8800 A cm–2 at 65 K under zero magnetic field, which is observed in tape hot pressed at 820 °C under 12 MPa for 24 h twice. The tape has undergone a cold pressing under 260 MPa between hot pressings. Jc is limited to 120 A cm–2 in tape hot pressed for 48 h continuously, in spite of total hot pressing time, temperature and pressure all being the same as for tape hot pressed for 24 h twice. It is found that alternate hot pressing and cold pressing is effective in the preparation of Bi-2223 tapes with high Jc, which is determined by the strength of grain coupling. Grain coupling is strengthened in tape hot pressed with an intermediate cold pressing. © 1998 Chapman & Hall     

Experimental Investigations on the Critical Current of the Jointed Bi-2223/Ag HTS Tapes

This paper summarizes the experimental investigations on the critical current of two jointed Bi-2223/Ag superconducting tapes connected by Sn63Ag2Pb solder. Different lap lengths of contact surface were studied. The joint resistance was measured to be in the range of 0.059??0.76????, and the critical current of the jointed Bi-2223/Ag HTS tapes was measured with different charging rates using standard four-point-method in a zero-applied magnetic field applying 1 ??V/cm criterion. The experimental results showed that the longer the lap length was, the smaller the joint resistance was and the nearer the critical current approached that of Bi-2223/Ag superconducting short-sample. On the other hand, the critical current decreased with the increasing of the charging rate.     

Abnormal Stress–Strain Properties of Bi-2223 Tapes

The stress–strain properties of Bi-2223 tapes directly relate to its applications, but have not been systematically studied yet. Three tapes sheathed by Ag and Ag alloys were manufactured for the study of stress–strain. X-ray diffraction analyses were used to determine the amorphous and Bi-2223 phases for the three tapes before and after sintering. Tensile experiments were performed to study the stress–strain properties as well. Micro-morphologies of the three tapes were observed and recorded by scanning electron microscopy. The experimental results show green tapes with a linear stress–strain relation. It is suggested that this relation comes from the sheathed metal’s properties. The mechanical properties of Ag/Mg sheathed tapes are like those of a rigid body, which do not present plasticity and elasticity. The phenomena of outgrowth and bridges were observed in Ag and Ag/Sb sheathed tapes. Also, Ag and Ag/Sb sheathed tapes showed abnormal stress–strain properties, which were subjected to micro-cracks existing in the sheathed metals and imperfections of Bi-2223 crystals.     

Conductor design with high-Tc superconducting Bi(Pb)—2223/Ag multifilamentary tapes

With high upper critical fields, high temperature superconductors (HTSC) have been recognised as candidate materials for coil and magnet applications. High field devices, at one time or another, when operated close to their rated limits are often faced with instability problems which normally are electrical, magnetic or mechanical in nature. The determination of stability parameters, therefore, is of interest to the conductor designer which very much assists in the processing aspect of long length production of wires and tapes. Due to the morphology of the superconductor most HTSC devices made with Bi(Pb)—2223 precursor exist in tape form. Here the determination of stability parameters (for flux-jump and cryogenic stability) for multifilamentary Bi(Pb)—2223/Ag tapes are presented. Processing parameters such as intermediate deformation and filling factors have been found to play a crucial role, not only on the critical current density of the tapes but on the stability aspect in conductor design as well.     

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.     

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.     

Bi系2223高温超导带材力学性质

对4种Bi系2223高温超导带材77 K下的力学性质进行了实验研究.通过选择合适的合金包套替代传统的银包套,带材的机械性能能有所提高,但电学性质有所降低,尤其是内包套采用合金、外包套采用银,其机械性能反而低于纯银包套带材.通过X射线衍射图谱发现,当包套采用合金材料,尤其是内包套采用合金、外包套仍采用纯银,会大大降低样品中2223相的体积含量.相比较临界电流密度,n值对材料的破坏更为敏感.利用X光同步辐射技术初步探讨了银包套带材在室温和77 K下的损伤破坏过程.     

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 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.     

Numerical Studies of Thermally Induced Residual Strain/Stress in Bi2Sr2Ca2Cu3O x /Ag/Ag Alloy Composite Tapes and the Dependence of Material Properties on the Temperature

Thermally induced residual strain/stress in Bi₂Sr₂Ca₂Cu₃O _x (Bi2223)/Ag/Ag alloy composite tapes and the dependence of material properties on the temperature have been studied numerically. Based on both the straight and bending 3D tape models, and with the temperature dependence on material properties (especially the coefficient of thermal expansion) among the constituents (Bi2223, Ag and Ag alloy sheath) of Bi2223 multifilament composite tapes, the residual strain accumulation and the distribution of the residual stress have been obtained. We found that by taking account of the temperature dependence on material properties of Bi2223 composite tapes the residual strain in the current transportation direction is up to 15 % larger than that without taking temperature dependence into account. Furthermore, by considering the distribution of the stress induced from the changing temperature, we analyzed the mechanical strength of Bi2223 composite tapes and concluded that the initial mechanical failure due to large temperature circle (intrinsically induced from the mismatch of the coefficient of thermal expansion of each constituent in composite tapes) comes from the following aspects: (i) the tensile fracture in the Bi2223 filaments occurring at the center of the tape and (ii) the delamination most likely arising at the interface between the Bi2223 filaments and Ag matrix near both edges of the cross-section of the tape, which originates at the Bi2223 side of the interface.     

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.     

Thermal stability in high-Tc coil and magnet design by process control of Bi(Pb)-2223/Ag multifilamentary tapes

It has been found that the degree of thermal stability of Bi(Pb)-2223/Ag pancake-shaped coils at 77 K can be determined by controlling the amount of matrix and superconducting materials during processing. The intermediate deformation step between sintering stages has been found to be crucial in optimizing the performance of the processed composite tapes as well as governing the thermal stability of the subsequently made pancake-shaped coils. Results obtained from numerical analysis of the finite element method has shown that monolayer coils produced from Bi(Pb)-2223/Ag composite tapes are thermally very stable with high values of the fill factor. However, increasing the number of co-wound tapes would require either a reduction in the fill factor or an increase in cooling rate for thermal stability to be sustained as would otherwise be achieved with the metallurgically same single tape.     

Thermal stability in high-T c coil and magnet design by process control of Bi(Pb)-2223/Ag multifilamentary tapes

It has been found that the degree of thermal stability of Bi(Pb)-2223/Ag pancake-shaped coils at 77 K can be determined by controlling the amount of matrix and superconducting materials during processing. The intermediate deformation step between sintering stages has been found to be crucial in optimizing the performance of the processed composite tapes as well as governing the thermal stability of the subsequently made pancake-shaped coils. Results obtained from numerical analysis of the finite element method has shown that monolayer coils produced from Bi(Pb)-2223/Ag composite tapes are thermally very stable with high values of the fill factor. However, increasing the number of co-wound tapes would require either a reduction in the fill factor or an increase in cooling rate for thermal stability to be sustained as would otherwise be achieved with the metallurgically same single tape.     

Development of Bi-oxide superconducting tapes and coils

We have developed Bi-2212 and 2223 tapes. For Bi-2212, two double stacked pancake type coils were fabricated using Bi-2212/Ag tapes prepared by a combination of the continuous dip-coating process and melt-solidification. A small coil (13 mm inner bore, 46.5 mm outer diameter) was inserted in a conventional superconducting magnet system. In a bias field of 20.9 T, the generated field of the coil was 0.9 T, at an I_c of 310 A (criterion 10⁻¹³ Ωm) at 1.8 K. Thus, the superconducting magnet system achieved the generation of a field of 21.8 T in the full superconducting state. A large coil (20 mm inner bore, 94 mm outer diameter) generated a field of 2.6 T (I_c = 385 A (10⁻¹³ Ωm)) at 4.2 K and 1.53 T (I_c = 225 A (10⁻¹³Ωm)) at 20 K in self-field. For Bi-2223, tapes were prepared by the powder-in-tube technique using Ag-10% Cu-x%M (x = 0–1.0, M = Ti, Zr, Hf or Au) alloy sheaths. The high J_c values of 5–7 × 10⁴ A cm⁻² at 4.2 K and 14 T were obtained for the tapes doped with x = 0.03–0.1 at.% Ti, 0.1 at.% Zr, 0.1 at.% Hf or 0.3% Au. These tapes have a modified Bi-2223 grain structure at the sheath/core interface and also a dense and more aligned microstructure, resulting in higher J_c values.