To explore AC loss characteristics of commercial Bi-2212 conductors made in the USA, a small ohmic heating (OH) prototype coil is being developed for compact tokamaks using high performance Bi-2212 wires newly manufactured by Bruker EST. The goal is to build and test Bi-2212 prototype coils for fast ramp central solenoid (CS) operation of spherical tokamaks (ST). For a ST pilot plant, one of the key challenges is the tight space available for CS in a center column where in-board legs of the toroidal field (TF) and OH coils are assembled into a cryostat with a few millimeters clearance. A high current density Rutherford cable consisting of 17 legacy Bi-2212 wires, a contribution of the US Magnet Development Program (MDP), was fabricated at the Lawrence Berkeley National Laboratory (LBNL) for the development of fast ramp CS magnets in the ST Advanced Reactor (STAR) designed by PPPL. Characterization of the new and legacy HTS wires is presented, along with the coil design wound directly with new wire, and a Rutherford cable-wound solenoid using the legacy wire. A comparison of AC losses is made based on expected performance for a fast ramp ST operation. The HTS cable and cabled coil fabrication issues such as the heat treatment, coil mandrel design, assembly and 2212 quench characteristics are discussed, along with a test plan for CS design validation.

Iron-based superconductors have attractive properties for high-field applications, but there is a lack of understanding of the effect of grain boundary chemistry on the in-field performance. The near atomic-scale resolution, ppm sensitivity and 3D analysis offered by atom probe tomography make it a powerful tool to investigate the nanoscale structure and chemistry of these defects in fine-grained K-doped BaFe2As2 samples. A computational method to systematically extract and compare the Gibbsian interfacial excess of chemical species across grain boundaries has been explored in this work. The robustness of the method has been tested by evaluating the effects of selected variables on simulated APT datasets. The accuracy and precision of the calculated Gibbsian interfacial excess were found to be stable over a range of analysis conditions: varying grain boundary widths and detection efficiencies, spatial precisions below 1.5 nm, and bin widths between 1.2 and 1.6 nm. For the K-doped BaFe2As2 samples studied, segregation of As, Ba, K and impurities of O, Na, and Sb were found at grain boundaries. The Gibbsian excess values were found to vary widely between different boundaries, showing the complexity of the grain boundary chemistry in this material. Possible links between the observed critical current density (Jc) of these samples and their nano-and micro-structure have also been investigated and discussed. (c) 2026 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Bi₂Sr₂CaCu₂O8-x (Bi-2212) multi-filament round wire is a high-temperature superconductor (HTS) capable of carrying high transport currents, which makes it suitable for high-field magnet applications. However, its weak Ag-Mg sheath leaves it vulnerable to mechanical stress, posing challenges for high-field magnet design. To better understand and improve mechanical stress management in Bi-2212 winding packs, we conducted an experimental study evaluating the axial stress-strain behavior of five winding pack configurations with varying insulation materials, reinforcement strategies, and construction quality. Using uniaxial tensile testing at 77 K, we measured Young's modulus and Poisson's ratio for each composition. Our results show that pure alumina braid insulation and co-wind reinforcements significantly enhance stiffness compared to aluminosilicate braids, with more than 2.5 times increased winding pack Young's modulus. Rule of mixtures analysis further quantified the contribution of non-wire composite components to overall stiffness. These findings highlight the critical role of insulation material selection and reinforcement design in optimizing Bi-2212 coil performance under stress, providing a foundation for improved mechanical models and more reliable high-field HTS magnet designs.

Bi-2212 Rutherford cables have been fabricated into flat racetrack coils and canted-cosine-theta dipole magnets. The performance gap between the magnets made with Rutherford cables and the "short-sample-limit" is about 30%. To better understand the influence of Rutherford cable processing on the strand performance, we studied three Bi-2212 wires with filament architectures of 37 x 18 and 55 x 18 and diameters of 0.8 and 1.0 mm. To simulate the deformation caused by cabling process, the three wires were rolled with thickness reductions ranging from 10% to 30%. The aspect ratios of rolled strands are between 1.29 and 2.05. The low aspect-ratio wire is also an interesting form for fabricating solenoid coils with higher packing density. The round and rolled strands were heat-treated under 50 bar and with maximum heat treatment temperatures of 885.5 degrees C and 890.5 degrees C. The rolling deformation reduced filament size uniformity, resulting in filament merging in fully heat-treated wires. It was found that rolling reduction reduced wire critical current density (J(E)) by 16 to 18%, but the J(E) decrease saturated at 15 to 20% of the thickness reduction. It is believed that the reduced J(E) results from the filament merging caused by rolling and non-uniform shrinking during overpressure heat treatment.

It is still largely elusive whether K-doped BaFe 2 As 2 (K-Ba122) is granular due to the intrinsic blocking effects at the grain boundaries (GBs). We investigated the remnant magnetization characteristics of a K-Ba122 polycrystalline bulk. Remnant magnetization is effective to evaluate the contribution of magnetization from multi-scale current loops particularly if electromagnetic granularity is present due to weakly coupled GBs. The derivative of remnant magnetization of the K-Ba122 sample showed only a single peak, which was markedly dependent on the specimen size and shifted toward lower field as the specimen size decreases, strongly indicating that the current loop is intergrain. However, the high angle annular dark field scanning transmission electron microscope (HAADF-STEM) analysis revealed that this sample still has nano-cracks at GBs, degrading the continuous network of strong intergrain connectivity. Such extrinsically degraded network of intergrain connectivity can also be seen as a size-dependence of remnant magnetization J c , which indicated the presence of strongly localized regions with much higher J c than the bulk J c . Our study suggested that the extrinsic GB nano-cracks are still the major cause of connectivity degradation, and the intrinsic current suppression might not be practically detectable at clean, fully connected GBs.

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 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 degrees C-896 degrees C, on the variations of Jc , effective filament diameter d eff, anisotropy gamma, 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 similar to 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 gamma and U 0.

Bi-2212 (Bi 2 Sr 2 CaCu 2 O 8+x ) multifilamentary round wires possess manifold superior superconducting properties compared to other high temperature superconductors suitable for very high field magnet applications; however, vortex pinning investigation has so far been absent, principally due to the complicated wire microstructures. Taking advantages of the simpler film-like microstructure of Bi-2212 tapes, we studied the flux pinning dependence as a function of Sr content, and correlated the tape performance to that of wires. We observed that, with increasing Sr content from 1.34 to 2.13, critical temperature Tc , and Kramer field Hk at 20 K increase respectively, from 70.5 to 80.4 K, and from 3.5 to 6.7 T, whereas α , which is the power law dependence of critical current density Jc at 4.2 K ( Jc ∝ H -α ), decreases from 0.4 to 0.3. Our work shows that flux pinning in Bi-2212 wires can be further improved by optimizing the Sr content, and flux pinning on Bi-2212 tape is a versatile tool not only for wire pinning study but also for powder quality assurance.

Optimization of intrinsic and extrinsic properties of MgB 2 superconducting material is extremely important for practical application in cables, wires, and tapes. The main mechanisms used to obtain this optimization are through the synthesis process, improvement in the grain connectivity, densification, pinning, doping, and limiting MgO formation. Many groups around the world use elements such as Ti, Zr, Hf, Al, Mn, Si and others, as dopants. Defects or any other inhomogeneity in the superconducting matrix can improve the flux pinning behavior and, hence, the transport properties. In this work MgB 2 superconducting bulks with additions of AlB 2 powder were prepared and analyzed in an attempt to enhance the critical current density of MgB 2 and to understand the effect of this addition on the intrinsic and extrinsic characteristics of the material. Crystallographic, microstructural, optical, and superconducting characterization were performed and analyzed. AlB 2 additions modified the superconducting properties of MgB 2 increasing its critical current density and irreversibility field compared to pure MgB 2 prepared using the same procedures.

The nonlinear optical dynamics and structural transformation of Co-doped BaFe2As2 superconducting films demonstrate complex behavior within a broad range of temperatures. The angle-resolved light scattering conoscopy reveals the temperature-dependent structural transformation of the optimally doped BaFe2As2. Photoinduced excited states dynamics demonstrate the instantaneous formation of the nonequilibrium state of quasiparticles with its subsequent multi-step thermalization within several picoseconds. These transient processes show noticeable temperature dependence below superconducting transition point Tc, where photoinduced dynamics correlate with the surface morphology.

Bi2Sr2CaCuOx (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-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 Jc, to obtain a deeper understanding of the causes of performance variation and paths to achieve further improvement.

We obtain the through-thickness elastic stiffness coefficient (C33) in nominal 9 nm and 60 nm BaFe2As2 (Ba-122) thin films by using picosecond ultrasonics. Particularly, we reveal the increase in elastic stiffness as film thickness decreases from bulk value down to 9 nm, which we attribute to the increase in intrinsic strain near the film-substrate interface. Our density functional theory (DFT) calculations reproduce the observed acoustic oscillation frequencies well. In addition, temperature dependence of longitudinal acoustic (LA) phonon mode frequency for 9 nm Ba-122 thin film is reported. The frequency change is attributed to the change in Ba-122 orthorhombicity (a−b)/(a+b). This conclusion can be corroborated by our previous ultrafast ellipticity measurements in 9 nm Ba-122 thin film, which exhibit strong temperature dependence and indicate the structural phase transition temperature Ts.

Bi-2212 is the only high temperature superconducting wire with the round geometry. It is multifilamentary, available in a wide range of fine filaments and twisted filament architectures and can be made into Rutherford and other cables. The properties of Bi-2212 conductors depend on powder quality, conductor fabrication and heat treatment. The heat treatment is still complex but much better understood, particularly the vital parameters of the maximum heat treatment temperature ( T max ), time-in-the-melt ( t melt ) and the cooling rate as Bi-2212 reforms on cooling. Here we report on the performance and microstructure variation with heat treatments for more than a dozen wires made with powders produced by Engi-Mat in recent years. Wire architectures include 37 × 18, 55 × 18 and 85x18 and wire diameters range from 0.8 to 1.0 mm. T max was varied between 884 and 897 °C. Wires with smaller filament diameter showed a peak JE at the low end of T ma x and also a JE that was more sensitive to T max . JE ( T max ) plots for all recent wires show a plateau between T max of 886 and 894 °C, where JE (4.2 K, 5 T) is 1100–1400 A/mm 2 . Some wires with filament size of 13–15 μm showed a 10 °C heat treatment window ( Δ T max ) with a plateau JE (4.2 K, 5 T) of about 1100 A/mm 2 .

Femtosecond photoexcitation of Ba(Fe 0.92 Co 0.08 ) 2 As 2 superconductors reveals distinct dynamics of laser fluence-dependent ultrafast processes. The modified two-temperature model shows the complex interplay between thermalization time constants, electron-phonon coupling parameters, and level of optical excitation.

Since 2008, the discovery of superconductivity in pnictide compounds with transition temperatures (Tc) above 50 K and upper critical fields (Hc2) exceeding 100 T promoted a considerable research effort to synthesize and process these materials in forms that may be suitable for conductor applications (Kamihara et al. 2008; Putti et al. 2010). Of the dozens of pnictide superconductors discovered, the AEFe2As2 (122) and REFeAs(O,F) (1111) compounds (where AE and RE stand for alkaline earth and rare earth, respectively) listed in Table E3.12.1 have attracted the most attention due to their intermediate transition temperatures between 20 K and 58 K and very high upper critical fields (Putti et al. 2010). Early experiments showed grain boundaries intrinsically block current transport in Co-doped 122 bicrystals, but this problem is less pronounced than in the cuprates (Lee et al. 2009; Katase et al. 2011). Wires and tapes have since been produced by the powder-in-tube (PIT) technique that have critical current densities (Jc) around 104–105 Acm−2 at 4.2 K and 10 T (Gao et al. 2015; Weiss et al. 2012; Zhang et al. 2014). The weak field dependence of Jc above 10 K and 10 T suggests these wires may be suitable for applications in operating fields and temperatures inaccessible by low temperature superconductors without the technical challenges inherent in coated conductors.

In 1989, Bi-2212 (Bi2Sr2CaCu2O8) was the first high-temperature superconductor made into a wire [Heine et al. 1989]. But it was only in 2014 that researchers developed the Bi-2212 processing technology that transformed Bi-2212 into a viable conductor for high-field (> 30 T) magnet applications [Larbalestier et al. 2014]. Bi-2212 is ideally suited for the niche application of high-field magnets because it is the only high-temperature superconductor that can be made as a round wire, which is the wire geometry magnet designers and builders prefer. The Bi-2212 round wire (RW) can be made with a variety of multifilamentary architectures, it can be twisted, it can be easily cabled, and it has macroscopically isotropic electromagnetic properties along its length. In contrast, the other two cuprate HTS conductors, YBCO (YBa2Cu3O7) and Bi-2223 (Bi2Sr2Ca2Cu3O10), are only available as flat tapes that cannot be cabled easily and have very anisotropic electromagnetic properties.

Emergent superconductivity is strongly correlated with the symmetry of local atomic configuration in the parent compounds of iron-based superconductors. While chemical doping or hydrostatic pressure can change the local geometry, conventional approaches do not provide a clear pathway in predictably tuning the detailed atomic arrangement due to the parent compound's complicated structural deformation in the presence of the tetragonal-to-orthorhombic phase transition. Here, we demonstrate a systematic approach to manipulate local structural configurations in BaFe2As2 epitaxial thin films by controlling two independent structural factors, orthorhombicity (in-plane anisotropy) and tetragonality (out-of-plane/in-plane balance), from lattice parameters. We tune superconductivity without doping utilizing both structural factors separately and controlling local tetrahedral coordination in the designed thin film heterostructures with substrate clamping and biaxial strain. We further show this allows quantitative control of the structural phase transition, the associated magnetism, and superconductivity in parent material BaFe2As2. This approach will advance the development of tunable thin film superconductors in a reduced dimension.

The use of high-field superconducting magnets has furthered the development of medical diagnosis, fusion research, accelerators, and particle physics. High-temperature superconductors enable magnets more powerful than those possible with Nb-Ti (superconducting transition temperature Tc of 9.2 K) and Nb3Sn (Tc of 18.4 K) conductors due to their very high critical field Bc2 of greater than 100 T near 4.2 K. However, the development of high-field accelerator magnets using high-temperature superconductors is still at its early stage. We report the construction of the world's first high-temperature superconducting Bi2Sr2CaCu2Ox (Bi-2212 with Tc of similar to 82 K) accelerator dipole magnet. The magnet is based on a canted-cosine-theta design with Bi-2212 Rutherford cables. A high critical current was achieved by an overpressure processing heat treatment. The magnet was constructed from a nine-strand Rutherford cable made from industrial 0.8 mm wires. At 4.2 K, it reached a quench current of 3600 A and a dipole field of 1.64 T in a bore of 31 mm. The magnet did not exhibit the undesirable quench training common in Nb-Ti and Nb3Sn accelerator magnets. It quenched a dozen times without degradation. The magnet exhibited low magnetic field hysteresis (<0.1%) as measured by a cryogenic Hall sensor. It was fast cycled to 1.47 T at 0.54 T/s without quenches. This work validates the canted-cosine-theta Bi-2212 dipole magnet design, illustrates the fabrication scheme, and establishes an initial performance benchmark.