Ag Composite Tapes Research Articles

Modification of critical current in HTSC tape conductors by a ferromagnetic layer

In some applications of tape conductors from high temperature superconductors (HTSC) the magnetic field is created by the transported current itself. This is e.g. the case of power transmission cables or current leads. Quite complex distribution of local magnetic field determines then the ability of the superconducting element to carry electrical current. We have investigated how much the critical current of a tape conductor can be changed by putting a ferromagnetic layer in the vicinity of the HTSC material. Numerical procedure has been developed to resolve the current and field distribution in such superconductor-ferromagnet composite tape. Theoretical predictions have been confirmed by experiments on sample made from Bi-2223/Ag composite tape. The critical current of such tape can be improved by placing a soft ferromagnetic material at the tape's edges. On the other hand, the calculations show that the ferromagnetic substrate of YBCO coated tape reduces its self-field critical current.

Critical Current Degradation of Bi-2223 Composite Tapes Induced byCyclic Deformation and Fatigue-Related Effects

Application of high temperature superconductor Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (Bi-2223) compound embedded in an Ag matrix requires the knowledge of critical current as a function of mechanical properties. Commercial tapes available in different types have been developed in industrial production scale in which a combination of small diameter filaments, long tape lengths and a ductile matrix results in a conductor with low crack formation and good tolerance against strain. The measurement of critical current and the evaluation of n-index from V-I characteristic curves of Bi-2223/Ag composite tapes subjected to an initial bending strain as a function of number of thermal cycles were done for two types of Bi-2223/Ag composite tapes: with and without steel tape reinforcement. The results showed that tapes with reinforcement presented small critical current degradation as a function of the number of thermal cycles whereas tapes without reinforcement exhibited steadily critical current degradation caused by the propagation of cracks. The n-index followed the same critical current behavior

Continuous deposition of YBCO film on Ag composite tape by PLD

A 100-m-long clad tape consisting of Ag–0.1%Cu alloy and Ag–10%Ni core metal was prepared uniformly by the cold roll method. The recrystallization of the clad tape and the deposition of YBa 2Cu 3O 7 film by PLD were performed continuously in the reel-to-reel vacuum system using as-rolled clad tape. A uniform superconducting tape which had a J c value of 1.5×10 5 A/cm 2 ( I c=7.5 A) was obtained in 5-m lengths at a deposition tape speed of 2 m/h.

Beneficial effect of solid nitrogen on a BSCCO-2223/Ag composite tape subjected to local heating

This paper presents results of a quench/recovery experiment for a BSCCO-2223/Ag composite tape in the presence or absence of a thin layer of solid nitrogen on each side of the tape. Voltage and temperature data were recorded for a 20-cm long BSCCO-2223/Ag tape operating in the range 20–55 K and subjected to a heat pulse of a 10–600 s duration applied over a short distance at its midpoint. The data clearly show that solid nitrogen is beneficial to the stability of high-temperature superconductors operating in this temperature range and subjected to transient heating disturbances.

Bi-2212/Ag線材における高&lt;I&gt;J&lt;/I&gt;&lt;SUB&gt;C&lt;/SUB&gt;化のための組織制御

The relationships between a critical current density (JC) and microstructure of Bi-2212/Ag composite tapes prepared by the melt-solidification under a temperature gradient have been investigated. We used a tube furnace with a temperature gradient for the heat treatment. The critical current density (JC) of the order of 4×109 A/m2 at 4.2 K under 10 T were obtained for the tape fabricated under a temperature gradient. We optimized the tape width, temperature gradient and cooling rate in melt-solidification process. The optimum tape width, temperature gradient and cooling rate were 2-4 mm, 200 K/m, and 2 K/h, respectively. JC enhancement of these tapes fabricated by melt-solidification under temperature gradient was attributed the improvement of the c-axis grain alignment, the in-plane texturing near the oxide/silver interface, and large grain size.

Direct atomic scale imaging of grain boundaries and defects in Bi-2223/Ag high- Tc superconducting tapes

Grain boundaries and defects in (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+ δ (Bi-2223)/Ag composite tapes are investigated by the atomic resolution Z-contrast imaging technique in the scanning transmission electron microscope. One of the most prevalent types of boundary between superconductor grain colonies is the c-axis twist boundary that is located in the middle of the double BiO layers. Although no amorphous layers are found to exist at these boundaries, local phase variations at/near the boundaries are frequently observed. Another typical grain boundary is the small-angle asymmetrical tilt boundary (ATB), which is parallel to the basal planes of one grain. At the small-angle ATB, different types of phases with various numbers of CuO 2 layers in their unit cell are formed periodically along the boundary plane. This means that this type of the boundary also occurs in the middle of the BiO double layers. These local phase variations exist in close proximity to the Ag interface and may give rise to weak link behavior. Additionally, many dislocations and stacking faults are observed within the filaments. Although these defects may provide a benefit in the pinning of magnetic flux, they also give rise to local variations of the superconducting properties.

Effect of Pb-, La- and Ti-substitutions on the microstructure and superconducting properties of Bi-2212/Ag composite tapes

We investigated the effect of Pb-, La- and Ti-substitutions on the microstructure and superconducting properties of Bi-2212/Ag composite tapes. Ti-substituted tapes were more poorly aligned than Pb- and La-substituted tapes. Transition temperature (T/sub c/) could be achieved above 80 K by Pb- and La-substitutions, while T/sub c/ decreased below 80 K by Ti-substitution. Transport critical current density (J/sub c/) values were below 10 kA/cm/sup 2/ for the Pb- and Ti-substituted tapes, while the La-substituted tapes showed highest J/sub c/ values of 150 kA/cm/sup 2/. The magnetically determined irreversibility field (B/sub irr/) was not improved by substitution. This was independent of elements substituting.

Transient electrical and thermal responses of a 2-section BSCCO-2223 coil under overcurrent pulses

This paper presents experimental and numerical simulation results of 2-section coil subjected to overcurrent pulses in the temperature range 20-60 K. The experiment simulates a high-temperature superconducting (HTS) magnet for electric devices such as fault-current limiters, transformers, motors, and power lines under fault-mode overcurrent pulses. Each section of the test coil is layer-wound with Bi-2223/Ag composite tape, 3.5 mm wide and 0.23 mm thick, with the outer section wound directly over the inner section. A pulse current exceeds the critical current of each conductor and may drive each section normal. A constant current that follows the pulse current leaves each section in three possible conditions: completely superconducting, recovering, and quenching. Simulation agrees reasonably well with experiment.

Stand-alone solid nitrogen cooled "permanent" high-temperature superconducting magnet system

This paper describes a stand-alone solid nitrogen cooled "permanent" high-temperature superconducting magnet system and presents its performance data. The system's cold body includes the magnet, shunted with a superconducting switch and comprised of six double pancakes, each wound with BSCCO-2223/Ag composite tape, and a volume of solid nitrogen and operates within a cyclic temperature range between 20 K and 40 K. At an operating current of 50 A the coil generates a central field of 0.5 T in a 12-mm diameter room temperature bore; the system's 2 liters of solid nitrogen is intended to give a cyclic operating period, from 20 K to 40 K, of about 70 hours. Included also in the paper is a preliminary design code that includes magnetic and thermal requirements of the system.

Formation and growth mechanisms of Bi-2212/Ag composite tapes

The microstructure changes during the solidification process of Bi-2212 tapes with several nominal compositions of Sr and Ca were investigated using an oil-quenching technique. The formation mechanism was almost the same in all composition tapes. Non-superconducting phases such as Bi-free and Cu-free solid phases exist in the liquid at T/sub max/. The quantity of the non-superconducting phases of Sr/Ca=1.8/1.2 and 2.2/0.8 tapes was larger than those of the 2.0/1.0 tape. With the solidification of the Bi-2212 phase near the oxide/silver interface, the non-superconducting phases melted into the liquid phase and shrank. Bi-2212 grains grew along the silver interfaces. The high J/sub c/ enhancement of the Bi-2212/Ag tape was explained by the excellent grain alignments at the oxide/silver interfaces. Locally, biaxially aligned Bi-2212 was observed at the oxide/silver interface for tapes fabricated using melt-solidification under a temperature gradient.

Conduction-cooled Bi 2Sr 2Ca 2Cu 3O x (Bi-2223) magnet for magnetic separation

A prototype of a conduction-cooled Bi 2Sr 2Ca 2Cu 3O x (Bi-2223) magnet system for magnetic separation was constructed. The magnet system has a 200 mm room temperature bore and generates fields higher than 1 T in an 11-liter room temperature space. The magnet axis of the system was designed to be horizontal in order to attain effective magnetic separation. The magnet consisted of 42 double-pancake coils. Each double-pancake coil was fabricated by the react and wind method using 61 multifilamentary Bi-2223/Ag composite tapes. Three Bi-2223/Ag tapes, each 66.9 m long, were bundled and wound into a double-pancake coil. A stainless steel tape was also co-wound to reinforce the coil. Eighteen Cu disks were inserted between the pancakes and thermally connected to the cryocooler in order to enhance the conduction cooling. The magnet was cooled down to 13 K using a Gifford–McMahon-type cryocooler, and it was tested at various excitation speeds. In a magnetic separation system, it is important to attain rapid energizing and de-energizing of the magnet in order to obtain high separation efficiency. We cyclically excited the magnet up to 1.7 T at a speed of 1.7 T/min. The temperature gradually increased with the number of excitations; however, the temperature saturated at 35–38 K, and stable operation of the magnet was obtained at these temperatures. This result indicates that the Bi-2223 magnet is promising for magnetic separation.

Microstructure and superconducting properties of Bi-2212/Ag tapes fabricated using melt-solidification under a temperature gradient

We fabricated Bi-2212/Ag composite tapes by applying a melt-solidification process under a temperature gradient. We used a tube furnace with a temperature gradient for the heat treatment. The critical current density ( J c) of the tape fabricated under a temperature gradient of 1.5°C/cm was 4.4×10 5 A/cm 2 at 4.2 K under 10 T. This J c value was much higher than the J c value of the tapes fabricated by using the conventional melt-solidification process. We clearly observed quadruple rotation symmetry peaks in the (1 1 5) pole figure of Bi-2212 near the Ag substrate. These results show that the in-plane textured Bi-2212 oxide was obtained near the oxide/silver interface of the Bi-2212/Ag tape. J c enhancement was achieved due to this in-plane texturing together with the improvement of the c-axis grain alignment.

Chemistry of uranium compound doping in (Bi,Pb) 2Sr 2Ca 2Cu 3O x/silver superconducting tapes

Equilibrium stable compounds U(Ca,Sr) 2O 5 and U(Ca,Sr) 3O 6, have been identified in the U Bi Sr Ca Cu O system. These compounds have been synthesised and added to Bi-2223/Ag composite tapes in progressively greater amounts. Bi-2223/Ag doped with these compounds using compositions closer to the equilibrium composition has been shown to have superior superconducting properties to that doped with Uranium oxide compounds. The effects of chemical degradation by Uranium compound dopants via Strontium and Calcium removal from the Bi-2223 structure have been indirectly determined by I c measurements. Losses in I c can be minimised by stabilising Uranium into appropriate compounds.

The changes of microstructure during solidification in Bi-2212/Ag tapes

We investigated the microstructure changes in Bi 2Sr 2CaCu 2O y /Ag composite tape during cooling from partially molten state by oil-quenching techniques. At maximum processing temperature T max, Bi-free phase and Cu-free phase exist in the liquid phase. The solidification of Bi-2212 phase starts near the free surface and oxide/silver interface. Bi-2212 grains grow along silver interface grow into the inside of molten oxide region randomly. No relation can be observed between Bi-2212 grain growth and grain boundaries of silver substrates.

The Study of Grain Boundaries in Ag-Sheathed Bscco Tapes by Scanning Transmission Electron Microscopy

Abstract Highly textured (Bi,Pb)2Sr2Ca2Cu3O10(Bi-2223)/Ag composite tapes have received considerable attention as high-Tc superconducting materials for electric power and high-magnetic-field applications because of their relatively high-critical current densities and their flexibility. In order to improve the performance of these tapes for many commercial applications, it is essential that the mechanisms that limit the critical current density are fully understood. Previous microscopical studies of these tapes have revealed that interfaces such as grain boundaries strongly influence the transport of large currents1. From a morphological point of view, several models have been proposed to describe potential current transport mechanisms. However, as these models consider mainly the large-scale configuration of the boundaries, the underlying mechanism controlling the properties is still unclear. In order to elucidate the exact role of grain boundaries, systematical studies on the effect of the atomic structure as well as any chemistry change that occurs at the interface are required.

Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x//Ag multilayer tapes with J/sub c/ (4.2 K, 10 T) of 500,000 A/cm/sup 2/ by using PAIR process

We developed PAIR (pre-annealing and intermediate rolling) process, which is a fabrication procedure for Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/(Bi-2212)/Ag composite tapes, in order to improve their superconducting properties. PAIR process is the combination of pre-annealing (PA) and subsequent intermediate rolling (IR) processes and is performed prior to a melt-solidification process. By performing the PAIR process, Bi-2212 grain alignment and intergrain connectivity are much improved and a large J/sub c/ enhancement can be expected for Bi-2212/Ag conductors. We study the effects of pre-annealing and intermediate rolling processes on microstructure and superconducting properties of the Bi-2212/Ag multilayer tapes fabricated by the oxide-painting and lamination method. The results show that J/sub c/ has been increased strongly by performing pre-annealing at 1133 K in oxygen (1 atm) and intermediate rolling with 25% deformation. PAIR processed samples have transport J/sub c/-oxide (4.2 K, 10 T) exceeding 5.0/spl times/10/sup 5/ A/cm/sup 2/. Whereas J/sub c/-oxide for the samples melt-solidified without PAIR process remains 3.0/spl times/10/sup 5/ A/cm/sup 2/. The enhancement of J/sub c/ is assigned to the microstructure with high grain alignment and uniformity, as well as the improvement in grain connectivity achieved by the PAIR process.

Effect of strain on ac power loss of Bi-2223/Ag superconducting tapes

The ac power losses of monofilament and multifilament Bi-2223/Ag composite tapes were investigated to determine the effect of mechanical strain on the loss and to identify the loss mechanisms. Measurement of the self-field losses was performed at 77 K and 60 Hz as a function of ac current amplitude (0–100 A) for tapes in their as-prepared or virgin state and after being subjected to applied strain by temperature cycling or bending to small radii of curvature ( R=1–50 mm). For good quality virgin tapes the experimental data are well described by the Norris equation P∼ I m n for the dependence of the power loss P on the amplitude I m of the ac transport current, with n=3 for monofilament and n=4 for multifilament tapes. Applied strain causes the loss to increase by several orders of magnitude and the exponent n to decrease below the Norris values. At strains very much greater than the irreversible strain limit, the loss becomes purely ohmic (i.e., I 2 R loss) and n=2. Examination by SEM of the transverse cross-section of tapes reveals a variety of strain-induced structural defects including transverse cracks that sever the filaments and extended regions where the superconductor core has delaminated from the silver sheath.

Effects of PAIR (pre-annealing and intermediate rolling) process on superconducting properties of Bi 2Sr 2CaCu 2O x/Ag multilayer tapes

PAIR process, which is the combination of pre-annealing (PA) and intermediate rolling (IR) processes and is performed prior to melt-solidification, enhances J c of the Bi-2212/Ag composite tapes. The effects of pre-annealing and intermediate rolling process on microstructure and J c of the Bi-2212/Ag multilayer tapes fabricated by the oxide-painting method have been studied. The results show that J c has been increased strongly by performing pre-annealing at 800–880°C and intermediate rolling with >25% deformation. Transport J c of 3.3×10 5 A/cm 2 in magnetic field (4.2 K, 10 T) is obtained. The enhancement of J c is assigned to the microstructure with high grain alignment and uniformity, as well as the improvement of grain coupling by the PAIR process.

Kinetics of the alignment and the formation of the Bi(2223) platelets in the powder-in-tube processed Bi(2223)/Ag composite tapes

The alignment and the formation of (Bi, Pb)2Sr2Ca2Cu3O10 inside an Ag sheath were investigated for tapes from four different precursor powders. Microstructural characterization along with in situ transmission x-ray diffraction experiments revealed that the kinetics depended strongly on the processing technique and the phase assemblage of the precursors. The alignment process was governed by the preferential grain growth of the Bi-cuprates along the a-b plane and the constraint applied by the sheath. The formation mechanism of (Bi, Pb)2Sr2Ca2Cu3O10 was either the intercalation or the nucleation and growth, depending on whether excessive liquid is adjacent to the platelets.

J c vs. strain characteristic of Bi-2223/Ag composite tapes in applied magnetic field

In operative conditions of the most important transport applications of Bi-2223/Ag superconducting tapes, like cables for power transmission lines and superconducting windings, the tapes will be subjected to mechanical stresses and magnetic fields causing the degradation of their transport properties. In this work we investigated the simultaneous effects of mechanical tensile stress and magnetic fields, up to 140 mT, applied parallel and perpendicular to the tape surface on the critical current density Jc at 77 K of Bi-2223/Ag composite tapes. The Jc vs. strain measurements showed an abrupt Jc decay above a critical strain value, ɛcr.