Ag-sheathed Tapes Research Articles

On the mechanisms of J c increment and degradation in high-J c Ba122 tapes made by different processing methods

We compared the grain and grain boundary (GB) nanostructures in two Ba122 tapes with similarly high J c. The Ag-sheathed tape made by hot pressing has larger, more plate-like grains with better c-axis alignment but has more GBs blocked by FeAs and Ba–O. In contrast, the tape made by cold pressing with an Ag-Sn/stainless steel sheath possesses fewer plate-like grains and weaker grain alignment but has more continuous current paths with clean physically well-connected GBs. Our nanostructural comparison emphasizes the strong need to achieve both good grain alignment and clean GBs for further J c improvement of Ba122 tapes.

In-Field Thermal Conductivity of Ag and Cu/Ag Ba1-xKxFe2As2 Composite Conductors

This work reports the longitudinal thermal conductivity, κ, of Ag- and Cu/Ag-sheathed Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> conductors. Our investigation has been carried out between 4 K and 80 K with magnetic fields up to 19 T. In the case of Ag-sheathed tapes, the thermal conductivity reaches maximum values of ~80 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and is weakly dependent on the field. The use of Cu/Ag in the place of Ag allows increasing κ up to ~250 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Zero-field κ values are about one order of magnitude lower than those achieved in other technical superconductors that employ a silver matrix, as 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> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8+x</sub> (Bi2212) and Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-x</sub> Pb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</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">10-x</sub> (Bi2223). This is a consequence of the contamination of the metal sheaths because of elemental diffusion occurring during the tape sintering. The differences in κ between the Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tapes and the other technical superconductors with an Ag matrix become smaller at high field. We suggest that performing the sintering at high pressure could represent a way to reduce the elemental contamination and thus improving the thermal conductivity.

Effect of Co&lt;sub&gt;0.5&lt;/sub&gt;Ni&lt;sub&gt;0.5&lt;/sub&gt;Fe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; Nanoparticles Addition in Ag-Sheathed Bi-2223 Superconductor Tapes Prepared by Co-Precipitation Method

Effect of complex magnetic oxide Co0.5Ni0.5Fe2O4 (CNFO) nanoparticles addition in (Bi1.6Pb0.4)Sr2Ca2Cu3O10 (Bi-2223) superconductor tapes was investigated. Ultrafine Bi-2223 powder precursor was prepared via co-precipitation method and was added with 0.01 – 0.05 wt.% Co0.5Ni0.5Fe2O4 nanoparticles during the final heating stage. The sample with 0.01 wt.% addition, Bi-2223(CNFO)0.01 was found to have the highest critical current density, Jc. This sample were then chosen to be fabricated into Ag-sheathed superconductor tapes using the powder-in-tube (PIT) method. The tapes were sintered for 50 and 100 h at 845 °C. The phase, microstructure and Jc of the samples were determined by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and four point probe, respectively. Jc of Ag-sheathed Bi-2223(CNFO)0.01 tapes sintered for 100 h was 19830 A/cm2 at 30 K and 3970 A/cm2 at 77 K compared to tapes without addition which showed a much lower Jc(6370 A/cm2 at 30 K). This study showed that CNFO nanoparticles could act as an effective flux pinning centers to enhance the critical current density in the Bi-2223 superconductor.

Optimization of a Novel Melt-Growth Heat Treatment of YbBa2Cu3O7-δ /Ag Tapes

A novel melt-growth heat treatment of YbBa 2 Cu 3 O 7-δ (Yb123) powder in Ag-sheathed tape has been established as a necessary first step to form aligned Yb123 grains in a powder-in-tube (PIT) conductor. The heat treatment parameters including the rate at which the atmosphere was changed from Ar to pure O 2, the heat treatment temperature, and the time during which the melt transformed back to Yb123 were investigated to find the optimum parameters for reforming Yb123 in Ag-sheathed tape. The Yb123 purity after being reformed is insensitive to the rate at which the atmosphere was changed. The optimized parameters are 910 °C for 48 h to reform Yb123.

Development of high Jc Bi2223/Ag thick film materials prepared by heat treatment under low PO2

In general, a dense and c-axis grain-oriented microstructure is desirable in order to achieve the high critical current properties of Bi2223 polycrystalline materials. On the other hand, our recent studies have shown that precise control of the chemical compositions of Bi2223 is also effective for the enhancement of intergrain Jc. In this study, the development of Bi2223 thick film materials with high critical current properties was attempted by controlling both the microstructure and the chemical compositions. A high intergrain Jc of ∼8 kA cm−2 at 77 K of a film with ∼40 μmt was achieved by increasing the Pb substitution level for the Bi site and controlling the nonstoichiometric chemical compositions. Furthermore, it was revealed that an increase in the thickness enabled us to obtain high Ic films suitable for practical applications. In contrast, there are still issues, especially in controlling the grain alignment at the inner part of the film, which suggests that the Jc properties of thick film materials could be further improved by forming a more ideal microstructure, as realized in the Bi2223 filaments of multi-filamentary Ag-sheathed tapes.

Quench Property of Twisted-Pair &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$\hbox{MgB}_{2}$&lt;/tex-math&gt;&lt;/inline-formula&gt; Superconducting Cables in Helium Gas

CERN's twisted-pair superconducting cable is a novel design which offers filament transposition, low cable inductance and is particularly suited for tape conductors such as 2G YBCO coated conductors, Ag-sheathed Bi2223 tapes and Ni/Monel-sheathed MgB 2 tapes. A typical design of such twistedpair cables consists of multiple superconducting tapes intercalated with thin copper tapes as additional stabilizers. The copper tapes are typically not soldered to the superconducting tapes so that sufficient flexibility is retained for the twisting of the tape assembly. The electrical and thermal contacts between the copper and superconducting tapes are an important parameter for current sharing, cryogenic stability and quench propagation. Using an MgB 2 twisted-pair cable assembly manufactured at CERN, we have carried out minimum quench energy (MQE) and propagation velocity (v p ) measurements with point-like heat deposition localized within a tape. Furthermore, different contacts between the copper and superconductor around the hot spot have also been studied, including the co-twisted assembly in Kapton wrapping and locally separated tapes. The measurements have been performed in cooling helium gas at temperatures between 20 K and 35 K, with different current fractions with respect to the thermal runaway current. The results suggest a potential optimization strategy, compromising between: a higher stabilization with better contact between the copper and superconducting layers; and a faster propagation velocity and easier quench detection with a higher contact resistance.

The Study on the Overcurrent Pulse Testing of Bi-2223/Ag HTS Winding and HTS Transformer

This paper summarizes the experimental investigations on a small-scale air-core high-temperature superconducting (HTS) transformer operated in liquid nitrogen. The designed HTS transformer is being used as part of a current pulsed-power supply. First, the HTS winding was tested to experience a short overcurrent pulse. The results indicated that a 170-A critical current Bi-2223/Ag HTS winding can bear a short overcurrent pulse to at least 1 kA without being damaged electromechanically. The transformer consists of an air-core 48-turn primary winding, which is enclosed by a 6-turn secondary winding. The primary and secondary windings are one- and three-strand parallel conductors of Bi-2223 Ag-sheathed multifilamentary tapes, respectively. The experimental results show that the output current of the HTS transformer can reach up to 1154 A, which is several times than the critical current of the secondary HTS winding.

Transport AC Loss Properties of a Bi-2223 Superconducting Coil From 0.1 Hz to 10 Hz

We propose a method to evaluate transport AC losses in Bi-2223 superconducting coils by integrating the local external-field losses of the stacked taped wires over the coil's windings. We measured the transport AC losses of a 16 double-pancake coil of twisted Bi-2223 Ag-sheathed tapes, in the frequency range from 0.1 to 10 Hz at liquid nitrogen temperature. We also measured the external-field losses of short specimens of 6-stack Bi-2223 wires both in perpendicular and parallel field configurations through a saddle-shaped pickup coil method. The external losses are mainly due to hysteresis losses in the superconducting filaments and coupling losses among the filaments. The local external-field loss in the test coil is approximately calculated as the sum of perpendicular- and parallel-field losses of the short specimen. In the calculation, geometrical effects because of the neighboring turns in the coil are also considered, in addition to the magnitude and direction distributions of the local magnetic field. We compared the numerically evaluated external-field losses to the measured transport AC losses, and discussed the applicability of the proposed method.

Effect of Ca Content on the Critical Current Density and Flux Pinning Properties of Ag Sheathed Bi-2212 Tapes

Abstract Single filament Ag sheathed Bi2.1Sr1.96Ca x Cu2O8+δ (Bi-2212) tapes with different Ca content of x =0.90, 0.95, 1.00, and 1.05, were prepared by powder in tube (PIT) process. The critical current density and flux pinning properties have been systematically studied with the variation of Ca content. X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been performed for the characterization of the tapes. Although the maximum critical current density, J c, was obtained from the stoichiometric sample under zero field, the nonstoichiometric Ca content are beneficial for the improvement of J c under magnetic field, due to the enhancement of flux pinning. The J c value of 950 A/cm2 has been obtained at 0.15 T, which is over 30% higher than that from x =1.00 tape.

5T級 Bi-2223 超伝導コイルの通電損失の評価

This study considers a method to evaluate AC transport current losses in Bi-2223 superconducting coils through integrating local external field losses of stacked, taped wires over the winding part of the coils. We measured the transport current losses of two kinds of pancake superconducting coils of Bi-2223 Ag-sheathed tapes, which were composed of 8 or 16 doublepancake (DP) coils, at 0.008 and 0.04 Hz. We also measured the AC external field losses of short specimens of six-layer Bi-2223 wires in both perpendicular and parallel field configurations using a method based on saddle-shaped pickup coils. The approximate local external field loss was calculated as the sum of perpendicular and parallel field losses of the short specimen. In the calculation, a demagnetization effect was considered in order to introduce the geometrical situation of winding in the coil, in addition to the distributions of the perpendicular and parallel magnetic-field components. The comparison between the experimental results of the transport current loss and the numerical results revealed that there is good agreement in the lower amplitude region of the transport current, and deviation in the higher amplitude region. The former finding indicates that this method is applicable as a general means to estimate the transport current losses in superconducting coils. The latter finding suggests that the effects of the transport current may cause significant changes to the AC loss properties of the superconducting coils.

Development of a Compact Measurement System of Transport Characteristics in HTS Conductors by Using an HTS Current

In order to realize HTS power machines and devices with high performance, it is important to evaluate the characteristics of HTS conductors quantitatively. We developed a compact system to measure the transport characteristics of HTS conductors under various experimental conditions. This system enables us to measure the transport characteristics of straight HTS conductors up to a current capacity of 1 kA class at 77 K, 66 K, and 4.2 K. The measurement of transport current losses is done by the Poynting vector method. The distribution of Poynting vectors around the sample conductor can be obtained by using many sets of both potential leads and pickup coils. An HTS current transformer (HTS-CT) is used to feed current to the sample conductor for the measurement. A secondary current of about 1 kA at 77 K can be induced when the primary current is 40 A. The sample conductor is connected to the secondary coil of this transformer. As a sample conductor, we used a conductor consisting of 16 parallel Ag-sheathed Bi-2223 multifilamentary tapes. The joint method between the HTS-CT and sample conductors is also examined. Based on these results, the usefulness of this measurement system is discussed.

Influence of the initial Bi2223 phase content on microstructure development in Bi2223/Ag tapes

(Bi,Pb)2Sr2Ca2Cu3O10 (Bi2223) Ag-sheathed tapes were produced from precursors containing various amounts of pre-reacted Bi2223 phase obtained by means of controlled calcinations of the starting powder mixture. The evolution of the critical current density during the first 20h of the first heat treatment follows closely that of the Bi2223 phase content. A maximum rate of increase is observed for precursor powders initially containing a small amount of Bi2223. The crystallinity of the Bi2223 phase along its c-axis improves with sintering time and is better in the tapes manufactured with precursor powders containing the largest amount of prereacted Bi2223. While the degree of preferential orientation of the Bi2223 phase at the Ag/ceramic core interface is not influenced by the initial Bi2223 phase content, the average texture in the ceramic core worsens with the increase of initial Bi2223 phase content. This effect is likely to play a role in the limited performance of Bi2223-rich precursors.

Effect of filament twist on the structure and properties of Ag-sheathed Bi2223 tape

Ag-sheathed Bi2223 tapes show good performances and are ready for many prototype applications. However, drastic reduction of AC loss is strongly required for practical AC power applications. To reduce the AC loss under AC external magnetic field, the introduction of filament twisting with high resistive barrier is effective, but the filament twisting tends to decrease the J c value. This decrease in J c is a problem to balance low AC loss with high I c value. In this study, the effect of filament twisting on the microstructure and J c property was investigated to overcome the J c reduction. The Bi2223 tapes sheathed with Ag–Cu alloy were fabricated with a standard PIT method. Although the formation of Bi2223 phase was not affected by filament twisting from the XRD analysis and SEM observation, the J c values of the twisted tapes decreased about 30% compared to that of non-twisted tape. In the case of the same twist pitch, the sample rolled under large pass reduction ratio showed relatively high J c value. This high J c would be caused by the high aspect ratio of the filament of those tapes, in which the grain alignment along the filament interface would be well. On the other hand, a reason for the J c reduction of twist tapes is proposed to be the misorientation at grain boundaries of Bi2223 grains in the transference region near the edges of the tape, where the filaments move up and down in the thickness direction.

A round robin test for pre-standardization of a saddle-shaped pickup coil method to measure AC losses in Bi-2223 Ag-sheathed tapes

We performed a round robin test to measure AC losses in Bi-2223 Ag-sheathed tapes with saddle-shaped pickup coils in an alternating perpendicular magnetic field at liquid nitrogen temperature. Two laboratories participated in the round robin test with each system of the pickup coil and the measurement circuit. Short specimens of a Bi-2223 tape and a copper magnet to generate the alternating magnetic field were used for common conditions in the test. The saddle-shaped pickup coils were made under recommended conditions for the structure and the sizes that give a calibration coefficient around unity. The signals of the magnetic moment from the pickup coils were calibrated by using a standard hairpin copper tape. AC losses of the common specimen of the Bi-2223 tape were measured in the round robin test in the ranges of frequency up to 60 Hz and the amplitude up to 0.2 T of perpendicular magnetic field. While we had only two participants in the test, we obtained the following results in a pre-standardization process of the AC loss measurement with the saddle-shaped pickup coil (i) it was suggested from the calibration procedure using hairpin copper tape that the deviations from numerical simulation for an ideal pickup coil and a sufficiently long specimen is within 3% on an average, (ii) the values of COV for the AC losses measured at 1 Hz and 10 Hz are less than about 3% in the range of the field amplitude between 0.02 T and 0.2 T.

Synthesis and superconducting properties of Nd0.33Eu0.08Gd0.58Ba2Cu3Oz materials

Abstract We have studied the effect of the ratio of different rare-earth element on the superconducting properties and phase formation of (Nd0.33Eu0.08Gd0.58)Ba2Cu3Oz ceramic. Bulk NEG samples were prepared using the solid-state reaction process. The superconducting transition for Nd0.33Eu0.08Gd0.58Ba2Cu3Oz is T c ≈90 K and the value of the critical current density (J c) is 13.9 A/cm2 at 77 K under zero magnetic fields. This value is twice as high when compared with the (J c) value of YBCO systems (J c = 7.31 A/cm2). The obtained bulk sample was used for the production of superconducting Ag-sheathed tapes by OPIT method including hot rolling. The critical current density of the obtained tape (337 A/cm2) is one order higher than the one of the bulk sample.

Fabrication and Properties in Ag/Bi2223 Tapes With Ca-Cu-O as Interfilamentary Resistive Barriers

In this study, we fabricated Ag-sheathed Bi2223 tapes with interfilamentary resistive barriers by using standard powder-in-tube (PIT) method and evaluated their AC loss characteristics at 77 K. The mixture of Ca2CuO3 and 30 wt.% Bi2212 are used as new barrier materials for tape fabrication. Although some broken parts in barrier layers were observed in a final tape section, it was confirmed that the barriers were almost arranged around each Bi2223 filament. In addition, XRD results indicated that the barrier introduction among the filaments has little side effect for the formation of Bi2223 phase inside the filaments. By introducing the barrier layers, the Jc values at 77 K and self-field are degraded about 15%. From the results of the measurements for AC magnetization loss under parallel transverse field, it was suggested that introducing the mixture of Ca2CuO3 and Bi2212 as barriers is effective to increase the transverse resistivity, and also to suppress the electromagnetic coupling among the filaments.

Study on Thermal Stability of Conduction-Cooled HTS Coil

One of the features of an HTS coil is its high thermal stability against thermal disturbances and the minimum quench energy of an HTS coil is several orders higher than that of an LTS coil. However, thermal runaways are observed in several conduction-cooled HTS coils. As thermal runaway causes degradation of the HTS coil, it is important to understand how it occurs. We evaluated the thermal stability of a conduction-cooled HTS coil. The HTS coil was composed of four single-pancake coils wound with Ag-sheathed Bi2223 tapes and the inductance of the HTS coil was approximately 3.8 H. The thermal runaway currents of the HTS coil at 15-30 K were measured. The experimental results were found to be in good agreement with the calculated results at 15-30 K.

Current sharing among filaments for Bi-2223 Ag-sheathed tapes with inter-filament barrier

Current distribution in Bi-2223 multifilamentary tapes with inter-filament barrier is numerically evaluated for the application of an alternating transport current. In the case of barrier materials with an infinite resistivity, the current distribution can be determined with a lumped parameter circuit model including the inductances and resistances of all the filaments. The mutual inductance between a pair of filaments is obtained by estimating their geometric mean distance. The power-law model is also assumed to take into account the effect of resistance in each filament. The influences of the silver ratio and filament number in the tape wire on the current distribution are discussed systematically as well as the power index and frequency.

Development of Ag-Sheathed Bi2223 Tapes with Interfilamentary Resistive Barriers for Reducing AC Losses

Ag-sheathed Bi2223 composite tapes with interfilamentary resistive barriers were successfully fabricated by using a powder-in-tube (PIT) method and their AC loss properties at 77 K are evaluated. The mixture of Ca2CuO3 and 30 wt% Bi2212 was used as the barrier material for tape fabrication to make an electromagnetic decoupling between the filaments. The barrier layers formed on all surfaces of hexagonal monocore wires by a dip-coating method, and several pieces of the coated wires were stacked and inserted into an Ag tube. Subsequently, the composites deformed into tape shape and subjected to a standard process consisting of a flat rolling and sintering. Degradations in the Jc values at 77 K and self-field by introducing the barriers were estimated to be at most 15%. The transverse resistivity and AC loss properties under AC external magnetic fields for the barrier tapes were examined and compared with those for the tape without barriers. The results showed that an introduction of Ca2CuO3 barriers was effective to suppress the electromagnetic coupling among the filaments and also to reduce the magnetization losses under parallel transverse field. The fabrication of the barrier tapes on the order of several meters, together with the uniformity of superconducting properties along a length direction has been also presented.

Influences of Preparation Conditions on Melting and Reformation Behaviors of (Bi,Pb)-2223 Core in the Ag-Sheathed Tape

Three kinds of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3O10 ((Bi,Pb)-2223) monofilamentary tapes were fabricated by powder-in-tube (PIT) process under different conditions. The phase evolutions during melting and reformation of (Bi,Pb)-2223 core in them have been investigated by ex-situ X-ray diffraction (XRD) analysis. The short tapes were melted at 855°C, 858°C in 7.5% O2 balanced with argon, and then slowly cooled to 801°C and 804°C at 1.5°C/h, respectively. The results show that the (Bi,Pb)-2223 phase can reform directly from the melt in all three tapes if the appropriate melting temperatures are used. However, the ceramic cores underneath the silver sheath exhibit different melting and recrystalization behaviors, as the differences of phase compositions in the original tapes before melted.