Bi-2212 Wires Research Articles

Influence of pre-overpressure heat treatment on micro-structure and related properties of Bi-2212 round wire

The Bi2Sr2CaCu2O8+x (Bi-2212) material, a high-temperature superconductor, holds significant promise for future high-field applications because of its multifilamentary, twistable production capabilities, coupled with a high upper critical field. Bi-2212 round wire (RW) is made by the powder-in-tube (PIT) technique, and Bi-2212 coils are usually being developed using the wind-and-react method and a high temperature and over-pressure (OP) heat treatment (50 atm, 890℃) is then required to achieve the high current performance. However, over-pressure (OP) heat treatment can lead to a reduction in wire diameter, compromising the stability of the coil structure. Addressing this issue, the pre-overpressure (pre-OP) heat treatment is used and proves effective in mitigating the reduction in Bi-2212 wire diameter. In this paper, to comprehensively investigate the impact of pre-OP heat treatment on the microstructure of Bi-2212 wire, a metallographic analysis of two kinds of Bi-2212 wires was carried out by the scanning electron microscope (SEM) and the ImageJ software. It was found that larger-area filaments experienced a greater reduction in area after pre-OP heat treatment, that pre-OP heat treatment increases the aspect ratio of filaments, and polygonal bundle structure helps maintain filament structure stability during pre-OP heat treatment. Additionally, the impact of pre-OP heat treatment on the performance of Bi-2212 wires was analyzed through critical current testing under high fields.

Improvement of plastic deformation property and current carrying capacity of Bi2212 wires through low oxygen partial pressure post-annealing of precursor

The performance of superconducting magnets is greatly dependent on the size and uniformity of the superconducting filaments within the wire. However, in the fabrication of Bi2Sr2CaCu2Ox (Bi2212) wires using ceramic oxide as the superconducting filaments, we face multiple challenges including filament fragility, poor uniformity, and non-smooth Ag/superconductor interfaces. These challenges primarily stem from the significantly lower plastic deformation capability of ceramic oxide powder filaments compared to metal matrices. To address these challenges, this study focuses on the modification of ceramic oxide precursor powders. Through the utilization of low-temperature, low oxygen partial pressure (pO2) heat treatment, we have observed significant changes in the Cu chemical states and lattice parameters within the Bi2212 grains. These alterations have proven to be highly effective in reducing the energy required for grain sliding and, in turn, enhancing the plastic deformation properties of the Bi2212 oxide filament. Furthermore, post-annealing the oxide powder under low pO2 conditions narrows the gap in plastic deformability between the Ag sheath and the oxide filaments, resulting in improved filament uniformity and a smoother Ag/oxide interface in Bi2212 wires. Notably, the Bi2212 grains exhibit an orderly arrangement within the wires. As a result of these improvements, the critical current density (Jc) of the final wires increases by an impressive 70 %. The outcomes of this study are not only crucial for producing Bi2212 magnets with higher magnetic field strength, improved magnetic field uniformity, and enhanced operational stability, but also offer fresh insights and methodologies for advancing the field of superconducting materials.

Analysis of the Flattening on Bi-2212 Wires Due to the Cabling Process and Its Effect on Their Performances

Analysis of the Flattening on Bi-2212 Wires Due to the Cabling Process and Its Effect on Their Performances

Development of persistent joints for superconducting Bi-2212 coils

Bi2Sr2CaCu2O8+x (Bi-2212) multifilamentary wire is the only high-temperature superconductor manufactured in the form of an isotropic round wire, and so offers a number of advantages for the designers of high field magnets. However, for high-field (>25 T), high-stability magnet applications, ultra-low resistance superconducting joints (R < 10−12 Ω) will be needed to take advantage of the excellent properties of the Bi-2212 wire. This study focuses on the fabrication of compact melt processed joints in small coils of Bi-2212/Ag multifilamentary round wires and the testing of their superconducting performance by inductive resistance measurements. Microstructural analysis is carried out to correlate the microstructure to the superconducting performance of the joints. Our optimized technique led to a reliable process for the preparation of small coils with melt processed joints that occupy very small volumes but can still carry the highest persistent currents reported so far for Bi-2212.

Impact of hot isostatic pressing on the microstructure and current-carrying capacity of Bi-2212 wires

Impact of hot isostatic pressing on the microstructure and current-carrying capacity of Bi-2212 wires

Revealing the effect of bridges in the multi-filamentary Bi-2212 wires

As the excellent current carrying capacity in ultra-high magnetic fields, Bi-2212(Bi2Sr2CaCu2O8+x ) has attracted much attention. In view of the unique nature of Bi-2212, high-temperature heat treatment is highly needed for the formation of the continuous textured structure in the multi-filamentary Bi-2212 wire. In the meanwhile, bridges between filaments can also be produced in the heat treatment process, which brings structural changes in the wires. However, the mechanism of the effect of the bridges is yet to be understood. In this work, systematical research was carried out on the multi-filamentary Bi-2212 wire for the effect mechanism. Based on the results of the four-probe method and the magnetic moment results, inferiority in Ic combined with a larger magnetic moment at high magnetic fields was detected in samples that have a large number of bridges. Further analysis indicates the inferiority in Ic can be attributed to the reduced cross-sectional area of the filaments and the inferiority in the quality of texture on the basis of results of a series of structural characterizations.

Variation of effective filament diameter, irreversibility field, anisotropy, and pinning efficiency in Bi-2212 round wires

In recent years Bi2Sr2CaCu2O x (Bi-2212) received increasing attention due to its round wire multifilamentary architecture, a unique feature in high-Tc superconductor. In fact, round wires are preferable for magnet designs, including solenoids for nuclear magnetic resonance (NMR) or research purpose and accelerator magnets. However, due to the narrow over-pressure heat treatment conditions necessary to obtain high Jc and to the peculiar microstructure of Bi-2212 wires, a full understanding of the correlations between the different properties has not yet been developed. In this paper we investigate the effect of a vital part of Bi-2212 optimization, the maximum heat-treatment temperature T max in the range of 885 °C–896 °C, on the variations of Jc , effective filament diameter d eff, anisotropy γ, INTER- and intra-grain irreversibility fields and pinning energies U 0, all critical parameters in unravelling the complex mix of vortex pinning and connectivity that ultimately determines the critical current density. We found that d eff of the higher Jc wires heat-treated at lower temperature is much smaller than for the lower Jc wires. Moreover, a systematic increase of the irreversibility field and a decrease of the intrinsic Bi-2212 anisotropy underpins the higher Jc . The analysis of the pinning energies reveals that there is little sample-to-sample variation in the INTER-grain pinning, whereas in all samples the intra-grain pinning has an enhancement below ∼40–45 K becoming more and more evident with increasing Jc . These results suggest that the overall Jc performance are not only related to the wire microstructure and connectivity, which obviously affect the INTER-grain properties, but they are also intimately related to the intrinsic and intra-grain properties such as γ and U 0.

The research on the influence mechanism of internal deformation for the performance of Bi-2212

Cable-in-conduit conductor (CICC), one of the most promising conductors for manufacturing magnets in magnetic confinement fusion devices, has attracted lots of attention. In the production of CICC, porosity control is necessary for its stability. The porosity control is usually realized by the diameter-reducing process, which would also lead to indentation damages to the elements of CICC-superconducting wires. In this article, systematic research of indentation damages was carried out on the next generation of high-temperature superconducting materials–Bi-2212 wires. The results indicate that the current carrying capacity of the indentation-damaged wire would first keep steady and then show exponential decline with the increase of indentation depth. The wires subjected to pre-overpressure (pre-OP) treatment exhibit slightly improved resistance against indentation damage at shallow indentation depths. However, their critical current (I c) deteriorates more rapidly at greater indentation depths when compared to wires that have not undergone the per-OP process. The following structural characterization analyzed the reasons for the property changes with the help of scanning electron microscopy, energy dispersive x-ray spectroscopy, computed tomography, and hardness tester. Misalignment between the Bi-2212 grains and shaped filaments was found in the indentation-damaged wires, with which the degradation in I c of the wires and the differences in properties of the two kinds of wires were further discussed.

Correction to: Preparation of high-performance high temperature superconducting Bi-2212 wires by the spray pyrolysis powder

Correction to: Preparation of high-performance high temperature superconducting Bi-2212 wires by the spray pyrolysis powder

Dominant effect of residual secondary phase of powders on Jc and microstructure of Bi-2212 superconducting wires

Bi2Sr2CaCu2O8+δ (Bi-2212) superconducting round wires exhibited great potential for use in high-field applications. The purity of the precursor powders is critical for the transport current of the wires. However, the role of the residual secondary phase in the precursor powders is not fully understood. Here, the origin of the secondary phase was investigated in precursor powders that were prepared using ultrasonic spray pyrolysis (USP) and calcination processing. The microstructure and phase evolution of the precursor powders during the crystallization process were analyzed. Moreover, the effects that the residual secondary phase has on melting behavior, morphology properties, and the supercurrent flow of Bi-2212 multi-filamentary wires are systematically discussed. The residual secondary phase in the filament caused further crystallization, and this led to the formation of more and larger Bi-2201 grains at the onset of the melting process. The poor microstructure and low critical current of the final Bi-2212 wires can be attributed to the presence of the residual copper-rich phase. Bi-2212 wires that were prepared with fully crystallized powders had a high critical current density (Jc) of 6773 A/mm2 at 4.2 K, self-field. It was revealed that control of the secondary phases in precursor powders is greatly significant for achieving superior values of Jc.

Preparation of high-performance high temperature superconducting Bi-2212 wires by the spray pyrolysis powder

Preparation of high-performance high temperature superconducting Bi-2212 wires by the spray pyrolysis powder

High-Speed Spin Testing of Bi-2212 Superconducting Wires for High-Field NMR Magnet Qualification: Final Progress Report

This paper summarizes an NIH-funded research project designed to study the strength of high temperature superconductors under high circumferential hoop stress, in order to qualify these materials for high-field (> 1 GHz-class) NMR magnets. The unique approach presented here is to spin test coils at high rotational speeds, approaching 50,000 rpm, in order to induce mechanical hoop stresses which are equivalent to the magnetic Lorentz stresses experienced by a high-field solenoid insert coil. Bi-2212 wires and coils were tested in sub-cooled nitrogen to determine if any mechanical damage was imparted by each spin test. These preliminary experimental results demonstrate that, with further development, this test could prove to be a simple method for qualifying superconducting wires for high-field insert magnets.

Influence of the Precursor Powder on the Processing Window and Current-Carrying Property of Bi-2212 Wires

Influence of the Precursor Powder on the Processing Window and Current-Carrying Property of Bi-2212 Wires

Ac susceptibility studies of intra- and intergrain properties of high- Jc Bi-2212 wires

${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{CaCu}{\mathrm{O}}_{x}$ (Bi-2212) is the only high-${T}_{c}$ superconductor (HTS) available as a multifilamentary round wire with multiple architectures and it is a very promising conductor for the realization of high-field applications. Despite their relatively simple wire fabrication by the powder-in-tube technique, Bi-2212 wires require a tightly controlled overpressure heat treatment (HT) with a multiparameter time-temperature schedule to achieve high critical current density, ${J}_{c}$. The variation of these HT parameters, changes in the wire design, wire diameter, and powder quality can lead to variations in both the microstructure and the superconducting performance. Particularly noticeable are variations in ${J}_{c}$ performance and degree of filament bridging. In this work, we focus on the use of different magnetic characterization techniques to estimate the bridging level and assess the balance of intergrain and intragrain superconducting properties including the irreversibility field (${H}_{\mathrm{irr}}$) and the pinning energy (${U}_{o}$) in differently processed wires. Regardless of the actual bridging level, we find that the supercurrent flows at the filament bundle level, not just at the individual filament level. Moreover, using ac susceptibility we identify two distinct supercurrent contributions, one related to the intragrain and one to the intergrain properties, whose irreversibility fields are different but without large sample-to-sample variation. Moreover, an additional component of intragrain pinning mechanism becomes effective at low temperatures with positive effects also on the intergrain performance. The work clearly shows that detailed magnetic characterizations can become valuable tools to investigate the performance of differently processed Bi-2212 wires, correlating their microstructure and overall transport ${J}_{c}$, to obtain a deeper understanding of the causes of performance variation and paths to achieve further improvement.

Examination of commercial ceramic coatings for the electrical insulation of Bi-2212 wire

In the last twenty years, a number of studies have been dedicated to the development of thin ceramic coating for the electrical insulation of Ag-sheathed Bi-2212 wires. Instead of preparing a coating by mixing different compounds in house, this technical note examines if commercial acrylic coatings containing TiO2 particles (white paints) could be used to form TiO2 coating on Ag-sheathed Bi-2212 wires. The compatibility of the acrylic coating with the heat treatment of Ag-sheathed Bi-2212 wire was verified by measuring the effect of coating on the critical current. It was found that the critical current is almost unaffected by the presence of the coating. The acrylic coating characteristics (for example drying time versus temperature and coating thickness at different dip coating speed) were studied and used to prepare a reel-to-reel system for coating long wires.

Development of a Small-Aperture Cos-Theta Dipole Insert Coil Based on Bi2212 Rutherford Cable and Stress Management Structure

The U.S. Magnet Development Program (US-MDP) is developing high-field accelerator magnets with magnetic fields beyond the limits of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn technology based on 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> 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). However, Bi2212 wires and cables are sensitive to transverse stresses and strains, which are substantial in high-field accelerator magnets. To prevent large degradation of the Bi2212 coils and achieve the required field quality, an innovative design which provides turn positioning during coil fabrication and operation and manage azimuthal and radial strains/stresses in the coil has been proposed at FNAL. This paper describes the development of a small-aperture two-layer Bi2212 dipole coil with stress management. The main parameters of the Bi2212 wire and Rutherford cable, the design of coil stress management structure and its plastic model, the coil mechanical analysis in the dipole mirror configuration are presented and discussed.

Influence of the precursor powder composition on the microstructure and the critical current density of Bi2212 wires

Influence of the precursor powder composition on the microstructure and the critical current density of Bi2212 wires

Improving Bi-2212 multi-filament wire quality and superconductivity by cryogenic drawing

Bi2Sr2CaCu2O8+δ (Bi-2212)/Ag round wires were fabricated by cryogenic drawing under liquid nitrogen temperature (77 K). The influences of cryogenic drawing on the metallic sheath mechanical properties, filament densification, filament size uniformity, and superconducting properties have been systematically studied. Significant improvement of both strength and plasticity of Ag sheath was achieved under low temperature. Higher wire uniformity and densification state have been obtained by applying cryogenic drawing under 77 K, leading to the critical current density promotion by nearly 40 %. It is believed that cryogenic drawing of Bi-2212 wire is a feasible and effective way to improve the wire quality due to the mechanical reinforcement of metal sheath.

Influence of twist pitch on hysteretic losses and transport J c in overpressure processed high J c Bi-2212 round wires

Bi-2212 is the only high field, high-temperature superconductor (HTS) available in the macroscopically isotropic, multifilament high J c round wire (RW) form capable of generating high uniformity fields with minimum-screening current errors. However, the heat treatment that enables impressively high J c (4.2 K, 30 T) values that can attain ∼5000 A mm−2 also produces significant filament bonding (bridging). Filament bridging appears to significantly enhance hysteretic losses of the filaments themselves by coupling neighboring, nominally independent filaments, enabling shielding currents to flow across multiple filaments as though they were one filament of much larger diameter. Wire twisting can be employed to reduce filament-to-filament eddy current coupling losses due to induced currents flowing across the matrix, but twisting is less effective in reducing increased losses from bridging. Here, we compare the twist-pitch dependence of the losses of overpressure processed (OP) high J c Bi-2212 RWs with partially bridged filaments to those found in OP Bi-2212 RWs with discrete, not-bridged filaments. We show that filament sub-bundles in standard, partially-bridged wires that have some superconducting connections between filaments can exhibit significant coupling (much larger effective filament diameter), but twisting still reduces their hysteretic losses to values close to or below the ITER Nb3Sn wire loss specification, even though Bi-2212 wires have significantly larger J c values. Although it has been reported that twisting can reduce wire J c by damaging filaments, we found no reduction in transport J c , even for nominal twist pitches of 12 mm in 0.8 mm diameter wires. Evaluation of more-recent, higher J c Engi-Mat powder wires showed that their reduced filament bridging and improved longitudinal connectivity significantly improved transport J c and reduced the J c normalized losses, signaling that J c can be further improved without commensurate increase in losses. This important result strengthens the argument for production of high field, low loss HTS magnets made with Bi-2212 RWs.

Investigation of transport mechanisms induced by filament-coupling bridges-network in Bi-2212 wires

One of the features unique in Bi-2212/Ag wires is the network of bridges between the filaments formed by grains grown through the Ag matrix during the partial-melt heat treatment process. Although these interconnections favor a redistribution of the current among the filaments allowing high critical current density, they represent a strong electrical coupling between the filaments themselves. Such a coupling increases the AC losses, present also in the case of the charge and discharge of DC magnets, and principal applications of this kind of superconductor. In this work, through transport and magnetic measurements and their comparison, we study the behavior of these bridges as a function of applied magnetic field and temperature and the implications they have on the electrical coupling. The experiment has been performed on two multifilamentary wires prepared by Groove-Die-Groove Powder In Tube process starting from two commercial Bi-2212 precursor powders: Nexans and Engi-Mat. The reported results show that the effective length scale on which the filaments are coupled is dependent on the field and temperature, passing from the filaments-bundle diameter at low field and temperature to single filament diameter at high field and temperature. Although the two samples have different Jc values, such filaments coupling behavior is common to both. We believe that these findings are very useful in magnet design.