Recent issues in fabrication of Ag-clad BSCCO superconductors

Long lengths of mono-and multifilament Ag-clad BSCCO superconductors were fabricated by the powder-in-tube technique. Critical current density (J_c) up to 12,000 A/cm² has been achieved in an 850 m long multicore conductor. Long length conductors were formed into pancake-shaped coils by the wind-and-react approach. Test magnets were then fabricated by stacking the pancake coils and connecting them in series. The magnets were characterized as a function of applied magnetic field at various temperatures. A test magnet, fabricated with ≈770 m of BSCCO tape, generated fields of ≈1 T at 4.2K and ≈ 0.6 T at 27K, both in an applied background field of 20 T. Additionally, the strain tolerance of both mono-and multifilament conductors at 77K in 0.5 T applied field has been studied. We observed that multifilament conductors have better strain tolerance than monofilament tapes, retaining more than 90% of the initial critical current (at 0.5 T) with strain ≥1%.     

Effect of final annealing temperature on critical current density of Ag/Bi-(Pb)-Sr-Ca-Cu-O tapes

A two-stage annealing procedure in the final thermal cycle has been tested for processing Ag/Bi:2223 tapes. The effect of various annealing temperatures in the final stage of heat treatment on phase transition, microstructure and critical current (I_c) was investigated for Ag/Bi:2223 tapes. It was found that annealing at higher temperatures (835–845°C) leads to a relatively purer Bi:2223 phase but a small fraction of Bi:2201 always coexists with Bi:2223. Whereas annealing at lower temperatures (800–825°C) results in conversion of Bi:2201 to Bi:2212 and possibly partial decomposition of Bi:2223. I_c was reduced to half for tapes annealed at 835–845°C due to the presence of Bi:2201, compared with tapes annealed at low temperatures. Scanning electron microscopy (SEM) studies show that, although the amount of Bi:2201 is small, it resides along the Bi:2223 colony boundaries, blocking the current flow between colonies. The drastic weak link effect of Bi:2201 is attributable to the combination of the elongated plate-like morphology and insulating property.     

Long lengths of silver-clad Bi2223 superconducting tapes with high current-carrying capacity

Long lengths of silver-clad (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) high-T_c multifilamentary tapes were produced using the powder-in-tube (PIT) technique followed by a thermomechanical process. The relationships between microstructure and electrical, magnetic and mechanical properties of the heat treated tape were evaluated from the critical current density measurements, irreversibility magnetic field determination and mechanical bending tests. Emphasis was stressed on the J_c behavior in magnetic fields at different temperatures. A J_c of 10,000 A/cm² at 77 K in a zero field for a 10 m tape and 75,000 A/cm² at 23 K in a field of 3 T for a short tape was achieved. The results obtained showed that Bi2223/Ag high-T_c composite tapes are a potential alternative to conventional low-T_c superconductors in magnetic levitation (MAGLEV) applications.     

Processing and transport properties of high-Jc silver-clad Bi-2223 tapes and coils

The powder-in-tube process has been used to fabricate long lengths of flexible, high-J_c, silver-clad Bi-2223 HTS conductors. By improving thermomechanical processing and precursor powder preparation, we have succeeded in achieving J_c values of≥4×10⁴ A/cm² at liquid nitrogen (77K) temperature and >10⁵ A/cm² at liquid helium (4.2K) and liquid neon (27K) temperatures in short tape samples. Detailed measurements with high applied magnetic fields are reported. Several long tapes up to 10 m in length have also been fabricated and cowound into small superconducting pancake coils by the “wind-and react” approach. Transport measurements at 77 and 4.2K for these coils are also reported.     

On the role of Bi2Sr2Ca1Cu2O8 and Bi2Sr2Cu1O6 on the weak links and critical currents in (Bi,Pb)2Sr2Ca2Cu2O10/Ag superconducting tapes

Silver-sheated (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) superconducting tapes with different Bi₂Sr₂CaCu₂O₈ (Bi2212) and Bi₂Sr₂Cu₁O₆ (Bi2201) concentrations, were prepared by using a two-step sintering processing and by varying cooling rates in the fabrication of the superconductors. The effect of residual Bi2212 and Bi2201 phases on weak links and critical currents of the Bi2223/Ag tapes was investigated. It was found that residual Bi2201 caused weak links at grain boundaries and limited the current-carrying capacity of the tapes. Comparatively, the residual Bi2212 phase had much less influence on both weak links and critical currents. Elimination of Bi2201 by sintering tapes at a low temperature in the final thermal cycle, or by cooling the tapes slowly, increased critical current by a factor of two. Flux pinning property was also improved by removing the residual Bi2201 phase.     

Fabrication and characteristics of tapes and test magnets made from Ag-Clad Bi-2223 superconductors

Pb_0.4Bi_1.8Sr₂Ca_2.2Cu₃O_x (Bi-2223) precursor powder was prepared by a solid-state reaction of carbonates and oxides of lead, bismuth, strontium, calcium, and copper, and the powder was then used to fabricate silver-clad tapes by the powder-in-tube technique. Transport critical current density (J_c) values>4×10⁴ A/cm² at 77K and 2×10⁵ A/cm² at 4.2 and 27K have been achieved in short tape samples. Long lengths of tape were tested by winding them into pancake coils. Recently, we fabricated a test magnet by stacking ten pancake coils, each containing three 16m lengths of rolled tape, and tested it at 4.2, 27 and 77K. A maximum generated field of 2.6 T was measured in zero applied field at 4.2K and the test magnet generated significant self-field in background fields up to 20 T. The results are discussed in this paper.     

Fabrication of long length Bi-2223 superconductor tape using continuous electrophoretic deposition on round and flat substrates

We have developed a continuous fabrication process for producing long lengths of Bi-2223 superconductor tapes. The process involves sequentially electrophoretically depositing and sintering superconductor and then silver layers on a substrate, followed by rolling and thermal processing. Both round and flat silver substrates have been used. Bi-2223 tapes made using flat silver substrates require only a few processing steps. Transport critical current densities at 77K in zero applied magnetic field exceeding 20000 A/cm² have been obtained.     

Superconducting magnet system containing Bi-2212/Ag Coil Generates 21.8 T at 1.8K

We fabricated a Bi-2212/Ag double stacked pancake coil of 13 mmØ) in inner bore and of 46.5 mmØ in outer diameter, by using Bi-2212/Ag tapes prepared by the combination of continuous dip-coating process and melt-solidification technique. This small superconducting magnet was used as an insert magnet of a conventional superconducting magnet system and tested at saturated superfluid helium temperature (～ 1.8K) in various bias fields. The generated field of Bi2212/Ag coil was 0.9 T, with I_c of 310 A(criterion 10^-13Ω·m), in the bias field of 20.9 T. Thus, this superconducting magnet system achieved generation of magnetic field of 21.8 T in the full superconducting state.     

Physical Effects of Mechanical Processes on Bi-2223 Tapes

The effects of intermediate mechanical deformation （IMD） and bending processing on Bi-2223 tapes were studied. Bi-2223 tapes were manufactured by powder-in-tube process with an IMD. Normal rolling （NR）, pressing （P） and sandwich rolling （SR） with different reduction rate were used in the IMD. And there were an optimum reduction rate existing for the three MID techniques, at which critical current reached maximum. Critical current densities Jc of Bi-2223 crystals were measured with an applied magnetic field B respectively parallel to ab face and c axis. Relations of Jc dependences of reduction rate and superconducting materials density D were respectively studied and showed a Gaussian distribution law. Maximum pinning force density Fmax and irreversible magnetic field Birr were introduced to describe the effects of mechanical processing. Analysis of experimental results showed that Jcs Fmax and Birr were linear dependence on D. Obviously, increasing D was a vital way to enhance Jc Bending experiments were performed for SR tapes sheathed by Ag and Ag/Sb and Ag/Mg alloy, respectively. Silver alloy sheathed tapes showed better bending properties than pure silver sheathed one. Therefore, silver alloy sheathed, optimum reduction rate of IMD, and increasing D for Bi-2223 tapes＇ applications were important technical strategies to enhance their mechanical, electrical, and magnetic properties.     

Sandwich Roiling,Jc, and pinning energy in Ag-sheathed BPSCCO superconducting tapes

A “sandwich rolling” process was developed to prevent the formation of sausaging and cracks in the longitudinal direction. The stress-strain state of the tape in “sandwich” rolling is the same as that of uniaxial pressed tape because the deformation of steel sheets is negligible in comparison to that of Ag-clad tape. Critical current densities of 3.2 × 10⁴ A/cm² at 77K and 2.7 × 105 A/cm² at 4.2K and zero field Ag-sheathed Bi-based 2212 tapes have been achieved using a melt and atmosphere-controlled process. The comparison of pinning potential U₀(B) < U(T-0, B) for Bi-2212 tape and Bi-2223 tapes consisting of a different fraction of 2212 phase as well as Bi-2212 and Bi-2223 thin films shows that for the same fields, the U_o for good quality 2223 tapes is at least 1.3 times that for the best 2212 tape and epitaxial thin films after taking into account the difference of the T_c between 2223 tape and 2212 tape, indicating that in BSCCO compounds, in addition to anisotropy, the specific pinning centers such as dislocations, introduced during processing, affect the flux motion at lower B.     

Conceptual design of an electrical machine with both low and high Tc superconductors

The emergence of high T_c superconducting (HTS) wires with large critical current densities allows the design and construction of many applications, for example, rotating electrical machines. Nevertheless, the current “state of the art” of Bi “powder in tube” wires, the most advanced HTS conductor, is not adequate for an ac operation. We thus propose a fully superconducting synchronous machine with an NbTi stationary 4 K armature and a rotating Bi-2223 field winding at 20 or 40 K. Thanks to the armature cryogenic environment, the cooling of the Bi coils is very simple. The conceptual design of a typical machine is suggested and several electromagnetic designs are presented. The first one deals with a Bi field winding adapted to an existing NbTi armature of a tenth of kYA. Afterwards, large torque motors are theoretically designed comparing the high (at 20 and 40 K) and low T_c field winding solutions. The results are comparable for NbTi and Bi at 20 K. But with a Bi field winding operating at 40 K, the performances are degraded. Otherwise, at 4 or 20 K fully superconducting motors show high performances in terms of weight, volume, and efficiency. Nevertheless, the Bi field windings require a large amount of material indicating that improvements in critical current density are needed     

Microstructures and superconducting properties of V doped Bi2223 tapes

We prepared Ag-sheathed tapes by the powder-in-tube method using V doped Bi2223 powder. We found that V doping into Bi oxide superconductors greatly enhances the Bi2223 phase formation of the tapes without any degradation of the superconducting critical temperature T_c. The doped V is not included in the Bi2223 (or Bi2212) grains, but instead exists as Sr₆V₂O₁₁ particles with diameters of 0.1 to several micrometers, in which the larger plate-like particles play an important role to promote the formation of the Bi2223 phase. With V doping, the optimum critical current density J_c value is obtained for a short time heat treatment.     

Influences of the precursor powder on the microstructure of HTSC-tapes

Wires and tapes are the most promising application of high temperature superconductor material for the future. For their preparation both the doctor-blade process (DBP) and the powder-in-tube((PIT) method with silver as a matrix material are mainly used. To obtain good electrical properties, the preparation had to start from a precursor material instead of fully reacted, phase pure superconducting powder. Tapes of Bi(Pb)−Sr−Ca−Cu−O 2212 and 2223 were fabricated by the DBP or PIT methods, respectively. We compared the suitability of precalcined precursor powders prepared by different routes and investigated their microstructure in the reacted wire or tape. Homogeneity, phase purity, and carbon content were investigated. The precursor powder prepared by a modified coprecipitation exhibited improved properties in comparison to other routes.     

Fatigue behavior of multifilamentary BSCCO 2223/Ag Superconducting tapes

The mechanical properties and the critical current were studied in three commercial multifilamentary Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+x//Ag tapes subjected to monotonic and fatigue tension at 77 K in the longitudinal direction. It was found that transport properties were not compromised under monotonic tension if the maximum tensile stress remained below the conventional 0.2% yield strength. This magnitude was reduced by 10% to 20% in the case of fatigue loading, and the service life of the tape was dictated by the mechanical fatigue life.     

Aspects of the current understanding of the supercurrent transport in (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed Tapes—the “railway-switch” model

The “railway-switch” model describes the superconducting current transport in (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ silver-sheathed tapes under the basic assumption that small-angle c axis tilt grain boundaries (“railway-switch”) constitute strong intergrain links for the supercurrent in the textured filament [B. Hensel, J.-C. Grivel, A. Jeremie, A. Perin, A. Pollini, and R. Flükiger,Physica C 205, 329 (1993)]. We give an overview of the model and some recent experimental results with the objective to identify the mechanisms that limit the critical current density. The measurements have been performed on monofilamentary “powderin-tube” samples [J_c(T < 77K, B < 0 T) < 20..30 kA/cm²] that were prepared in long lengths by rolling as the only tape-forming process. We conclude that the low intragrain critical current density j_c ^c along the c axis (or the even lower critical current density j_c ^t across twist boundaries or intergrowths) is the dominant limitation for the transport critical current in high-quality tapes. We discuss possible starting points for a performance improvement of the (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ silver-sheathed tapes for applications. On leave from Consorzio INFM, Universitá di Genova, Italy.     

Fabrication of Ag-Sheathed Bi-2223 tapes using powders produced by aerosol spray pyrolysis

Bi-2223 tapes were manufactured from a fine “two-powder” product produced by using an aerosol spray pyrolysis technique. Critical current density of 22000 A/ cm2 at 77K and 0 T was achieved. Nondestructive transmission x-ray diffraction study indicated good alignment of the superconducting grains. The texturing process of the superconducting phase was found to be nearly complete after the first 24 h of heat treatment for the samples studied. Pressing was found to play little role in the texturing process. The texturing can be enhanced by Ag-doping. J_c, however, was not found to be improved significantly, presumably due to the reduced effective cross-sectional area. A new phase, Bi-4435, was identified which may play a significant role in the formation of 2223. On leave from Northeastern University, Shenyang, P.R.C. On leave from Kobe Steel Ltd., Kobe, Japan     

Fabrication of Bi(Pb)-Sr-Ca-Cu-O superconducting Thin films by rapid thermal annealing

Superconducting Bi(Pb)-Sr-Ca-Cu-O thin films were successfully fabricated on MgO substrates by dc sputtering and rapid thermal annealing. No furnace annealing was used in the process. The superconducting thin films, deposited by dc sputtering for 8 hr and rapid thermally annealed at 845°C for 60 sec, showed a superconducting transition onset temperature of 110 K and a zero resistance transition temperature of 86 K. Highly oriented films with the c-axis normal to the substrate surface were obtained. To our knowledge, this is the first time that Bi(Pb)-Sr-Ca-Cu-O superconducting thin films have been fabricated using rapid thermal annealing.     

Numerical Study on AC Loss Characteristics of HTS Coils With Various Cross Sections and Methods of AC Loss Reduction

AC loss characteristics of high temperature superconducting coils wound by Bi2223/Ag tape with various cross sections are numerically investigated. The numerical results show that the smaller coil aspect ratio, the smaller AC losses in the HTS coil. It was considered from the numerical results that the coil aspect ratio giving the minimum coil volume is the better shape judging from the many aspects such as the required length of tape, the stored energy, the central magnetic field and the critical current together with the AC loss. It was also shown by the numerical study that the optimal coil design giving the reduced AC loss can be done by employing the stepped cross section coil and the turn number graded coil.     

Electromechanical characterization of silver-clad BSCCO tapes

During the fabrication of silver-clad BSCCO tapes they are subjected to stresses which could lead to degradation in their current transport property. In the present investigation, studies were made to evaluate the electromechanical characteristics of silver-clad BSCCO conductors. The tensile strain tolerance characteristics of the monofilament, multifilament and composite (15 and 30% of Ag powder by volume) tapes were evaluated at 77 K. The average irreversible strain of monofilament and composite tapes were 0.19 and 0.47%, respectively. No noticeable improvement in strain tolerance was observed with the multifilament tapes. Detailed phase and microstructural analysis have been conducted using scanning electron microscopy.     

磁流体包覆薄包层光纤光栅磁场传感研究

利用磁流体替代光纤布喇格光栅(FBG)的部分二氧化硅包层,制作了一种磁流体封装薄包层FBG结构的磁场传感器,研究了传感器对磁场和温度的响应特性。结果表明,在5.0～20.0mT的磁场范围内,传感器的波长灵敏度和功率灵敏度分别为34.9pm/mT和-1.063dBm/mT,波长线性响应度达到了99.2%。封装工艺未改变FBG波长随温度线性变化的特性,但受磁流体磁光效应影响,其温度灵敏度减小到9.2pm/℃。该传感器可实现磁场测量中的温度补偿,方法简单、易于实现。