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.     

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.     

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.     

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.     

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.     

Development of Bi(2223) Multifilamentary Tapes with Low ac Losses

A significant reduction of ac losses in twisted Bi(2223) multifilamentary tapes with Ag sheaths has been achieved by using oxide (BaZrO₃ and SrZrO₃) barriers between filaments. These barriers have two important effects: they increase the transverse resistivity, which suppresses induced coupling currents, and they reduce filament bridging, which in pure Ag sheath tapes largely cancels the beneficial effect of filament twisting. The decoupling can be gauged by the frequency at which loss shows a maximum in a low-amplitude ac field applied perpendicular to the tape. So far, the frequency of the loss maximum, f _m, in Ag-sheathed tapes has been enhanced from 5 Hz (untwisted) to 82 Hz (11 mm in twist pitch length). Different ways to introduce oxide barriers in tapes with 19–95 filaments are presented. The critical current density in the filaments varied between 10,000 and 20,000 A/cm². Ac loss measurements as well as the electrical and mechanical characterization are discussed in detail. The variation of the critical current density with bending strain is shown to be similar to that of tapes without barriers.     

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.     

Upper Critical Field Hc2 on c Axis of Ag/Bi-2223 Polycrystal Tape

The upper critical field H _c2, based on Ginzburg–Landau theory at medial magnetic fields, can be calculated from the relationship of the magnetization intensity M(H) versus magnetic field H if M is linear with ln₀ H. For Ag/Bi-2223 tapes, the measured M(H) was found to be linear with ln₀ H. In this case, the values of H _c2(T) may in principle be determined. To do this, we will meet another problem in that the obliquity of crystal planes is not equal for different grains in tapes, and values of grain-oblique angle will appear in the calculated H _c2. Obviously, for Ag/Bi-2223 tapes is an uncertain parameter and hard to determine. To avoid the effects of , we only study the H _c2 in the c axis direction and the projections of H onto the c axis is taken as the actual applied fields. Thus, the effects of grain-oblique angle may be counteracted when measuring critical current density J, if the external magnetic fields are applied to the tape along both the parallel to and perpendicular to the c axes directions (on the narrow surfaces of the tape). On medial fields (H = 0 – 3 T), the upper critical fields H _c2(T) on c axis for the Ag/Bi-2223 tape are obtained and fitted as ₀ H _c2(T) = 830 e ^–0.07T . The average slope d[₀ H _c2(T)]/dT – 8T/K is found to be as large as that of Bi-2212. On extrapolating the relation to T = 0 K, the value of ₀ H _c2(0) 800T. The coherence length _ab is determined from H _c2(c), and _ab (0) = 0.63 nm at T = 0 K.     

高温超导Bi-2223线材宏观缺陷对载流能力的影响

本文对国内某公司高温Bi-2223超导线材的典型宏观缺陷进行了观察,用EDS对缺陷成分进行了分析,并测试了缺陷处临界电流Ic值,结果表明,点缺陷和鼓泡两种典型缺陷都使Bi-2223超导线材缺陷处临界电流值出现了衰减,在点缺陷处Zr元素出现了偏析.     

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.     

Orientation Distribution of Grain Planes in Polycrystal Ag/Bi-2223 Tape

For polycrystalline Ag/Bi-2223 tapes, the preferential orientation of grains is a very important issue. The platelike grains in the tapes are generally believed in a high-order alignment. However, microstructural observation by SEM shows that the grain alignment is far from perfect. Theoretically, upper critical field, H _c2, for H parallel to ab plane and c axis was calculated form the relation of M(H). However, the ratio of H _{c2 (H//ab)} to H _c2(H//c) depends on the angle () between the ab plane of grains and the broad surface of the tapes. Based on the ratio, the orientation distribution of grains is obtained. The results show that grains in Ag/Bi-2223 tapes can grow with ab plans at any angle between 75° > > –75°, but no grain can grow with its ab plane perpendicular to the tape broad surface. The overall distribution is that: >90% grains orient in the angle range of || < ± 75°, about 5% grains at ±75° || ±85°, and no grains at || > ±85°.     

弯曲应变对Bi2223/Ag带材临界电流的影响

本文主要研究了Bi223/Ag带材的弯曲应力-应变特征及弯曲疲劳对其在77K自场下临界电流的影响.分析临界电流Ic降低的原因是应变和热循环引起的超导陶瓷芯内部的微裂纹.实验研究发现当带材的弯曲应变超过0.3％以后,Ic显著降低;当带材受到多次弯曲时,前四次弯曲会使Ic急剧降低,然后Ic降低非常缓慢.因此,在实际应用过程中,应使Bi223/Ag带材的弯曲应变不超过0.3％,且在Bi223/Ag带材的生产和使用过程中,均应尽量减少其弯曲的次数.     

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.     

The process of a Bi-2223/Ag ribbon sintering studied by the method of acoustic emission

The secondary sintering of a Bi-2223/Ag ribbon, following the stages of primary sintering and rolling to a strain of ε∼18%, was studied by methods based on the acoustic emission, thermal expansion, and magnetic susceptibility measurements. During the second thermal treatment, a signal of acoustic emission from the ribbon was detected in a temperature range from 570 to 660°C. The process of liquid-phase healing of the rolling cracks in the ribbon material is considered as a reason of the acoustic emission. The possibility of using the acoustic emission measurements for nondestructive testing in the Bi-2223/Ag ribbon fabrication technology is considered.     

Characterization of the Elastic Properties of Bi2223/Ag/Ag Alloy Composite Tapes by the Generalized Self-Consistent Approach

The characterization of the elastic properties of whole Bi2223/Ag/Ag alloy composite tapes is a very important issue, even when these properties of each constituent are known. In this paper, the generalized self-consistent approach is used for predicting the elastic properties of inner core (Bi2223 filaments combined with Ag matrix), and the mixture rule for those of the whole composite tape (the inner core sheathed with Ag alloy). Compared with the simple estimation of the mixture rule, the prediction of the generalized self-consistent approach is not only physically rational, but also is much more accurate. Further, the theoretical prediction agreed well with the measured data. Hopefully, the generalized self-consistent approach can be adopted to predict the elastic properties of other superconducting composite structures.     

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.     

Straining of a Bi-2223/Ag ribbon studied by the method of acoustic emission

The process of cooling of a Bi-2223/Ag ribbon after secondary sintering at 1000 K is studied by the method of acoustic emission (AE). The acoustic response is observed below 500 K and increases in intensity with decreasing temperature. It is established that the AE signal variation reflects the plastic flow in the silver coating caused by a difference between the thermal expansion coefficients of silver and the ceramic core of the Bi-2223/Ag ribbon.     

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.