23 Search Results

Abstract In recent years Bi ₂ Sr ₂ CaCu ₂ O _x (Bi-2212) received increasing attention due to its round wire multifilamentary architecture, a unique feature in high- T _c 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 J _c 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 amore » 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 J _c , 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 J _c wires heat-treated at lower temperature is much smaller than for the lower J _c wires. Moreover, a systematic increase of the irreversibility field and a decrease of the intrinsic Bi-2212 anisotropy underpins the higher J _c . 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 J _c . These results suggest that the overall J _c 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 .« less

Stakeholders representing concerns of national and global leadership, industries that use superconducting magnets in products, manufacturers of superconducting wires and tapes that supply to industries, and innovation generators from small businesses and universities came together to address stewardship of superconducting magnet technology and assurance of supply of advanced superconductors to the accelerator sector. This report outlines potential public-private partnerships that develop and enhance domestic capabilities to meet the needs of science facilities in the accelerator systems sector and in the broader commercial ecosystem.

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 T_c of 9.2 K) and Nb₃Sn (T_c of 18.4 K) conductors due to their very high critical field B_c2 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 Bi₂Sr₂CaCu₂O_x (Bi-2212 with T_c of ~82 K) accelerator dipole magnet. The magnet is basedmore » 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.« less

Bi₂Sr₂CaCu₂O_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 multi-parameter time-temperature schedule to achieve high critical current density, $$J_c$$. Further, 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 degreemore » 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 INTER-grain and intra-grain superconducting properties including the irreversibility field ($$H_{\text{irr}}$$) and the pinning energy ($$U_0$$) 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 intra-grain and one to the INTER-grain properties, whose irreversibility fields are different but without large sample-to-sample variation. Moreover, an additional component of intra-grain pinning mechanism becomes effective at low temperatures with positive effects also on the INTER-grain 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.« less

Abstract 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.more » 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 Nb ₃ Sn 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.« less

As the only high-temperature superconducting (HTS) material available as an isotropic, twisted, multifilamentary round wire, Bi-2212 is very promising for expanding the high-field superconducting accelerator magnet toolbox beyond the round-wire, isotropic Nb-Ti and Nb3Sn conductors used by HEP so far. In this paper, we describe the important roles that Bi-2212 might play for future high energy circular colliders including both high energy proton and muon colliders. We describe its present technology status (conductor development, magnet design concepts, prototype magnet status) and then provide a ten-year plan for >15 T accelerator magnets and >25 T solenoid magnets within an integrated strategymore » to engage industry and the entire US and international scientific enterprise interested in HTS magnet applications.« less

Bi₂Sr₂Ca₁Cu₂O_x (Bi-2212) is the only high-temperature superconductor (HTS) available as a round wire with high critical current density J_c, which makes it a very compelling candidate for ultrahigh-field magnet applications. By contrast, other copper oxide HTS conductors like RBa₂Cu₃O_7–δ (where R stands for rare earth) and (Bi,Pb)₂ Sr₂Ca₂Cu₃O_x (Bi-2223) must be made in tape form to minimize the density of current blocking high-angle grain boundaries. Understanding the mechanism enabling high J_c in round wire Bi-2212 is important intellectually because it breaks the paradigm that forces HTS conductors into tape geometries that reproduce their strong crystalline anisotropy. The biaxial growth texturemore » of Bi-2212 developed during a partial melt heat treatment should favor high J_c, even though its ~ 15° full width at half maximum (FWHM) grain-to-grain misorientation is well beyond the commonly accepted strong-coupling range of ≤ 5° misorientation. Its ability to be strongly overdoped should be valuable too since underdoped cuprate grain boundaries are widely believed to be weakly linked. Accordingly, we here study property changes after oxygen underdoping the optimized, overdoped wire. While J_c and vortex pinning diminish significantly in underdoped wires, we were not able to develop the prominent weak-link signature [a hysteretic J_c(H) characteristic] evident in even the very best Bi-2223 tapes with an ~ 15° FWHM uniaxial texture. We attribute the high J_c and lack of weak-link signature in our Bi-2212 round wires to the high-aspect ratio, large-grain, basal-plane-faced grain morphology produced by partial-melt processing of Bi-2212. These features enable c-axis brick wall current flow when ab-plane transport is blocked. We conclude that the presently optimized biaxial texture of Bi-2212 intrinsically constitutes a strongly coupled current path, regardless of its oxygen doping state.« less

Bi₂Sr₂CaCu₂O_8+x (Bi-2212) is the only high-field, high-temperature superconductor (HTS) capable of reaching a critical current density J_c(16 T, 4.2 K) of 6500 A·mm^–2 in the highly desirable round wire (RW) form. However, state-of-the-art Bi-2212 conductors still have a critical current density (J_c) to depairing current density (J_d) ratio around 20 to 30 times lower than that of state-of-the-art Nb–Ti or REBCO. Previously, we have shown that recent improvements in Bi-2212 RW J_c are due to improved connectivity associated with optimization of the heat treatment process, and most recently due to a transition to a finer and more uniform powder manufacturedmore » by Engi-Mat. One quantitative measure of connectivity may be the critical current (I_c) distribution, since the local I_c in a wire can vary along the length due to variable vortex-microstructure interactions and to factors such as filament shape variations, grain-to-grain connectivity variations and blocking secondary phase distributions. Modeling the experimental V-I transition measured on a low resistance shunt as a complex sum of voltage contributions of individual filament and wire sub-sections allows a numerical extraction of the I_c distribution from the d²V/dI² treatment of the V-I curves. Here we compare ~0.1 m length I_c distributions of Bi-2212 RWs with recent state-of-the-art very high-J_c Engi-Mat powder and lower J_c and older Nexans granulate powder. We do find that the I_c spread for Bi-2212 wires is about twice the relative standard of high-J_c Nb–Ti well below H_irr. We do not yet see any obvious contribution of the Bi-2212 anisotropy to the I_c distribution and are rather encouraged that these Bi-2212 round wires show relative I_c distributions not too far from high-J_c Nb–Ti wires.« less

The distinctive quasi-biaxial texture of Bi₂Sr₂CaCu₂O_x (Bi-2212) plays an important role in enabling high critical current density ($$J_c$$) in Bi-2212 round wires (RWs). Here we studied three over pressure heat treated wires with $$J_c$$ varying by a factor of ~10, all being fully dense. Using electron backscatter diffraction, we observed the differences in biaxial texture in these three wires. Transmission electron microscopy also revealed differences in grain boundary (GB) cleanliness and connectivity. These analyses showed that high $$J_c$$ is unambiguously correlated to the best biaxial texture, which is in turn correlated to slow cooling from the liquid melt into solidmore » Bi-2212. However, at 4.2 K, there is a negligible difference in intragrain pinning in the three wires, suggesting that the $$J_c$$ variation by a factor of ~10 is primarily due to variable filament and intergrain connectivity. In this work, the principal determinants of intergrain connectivity is the quasi-biaxial texture and GB cleanliness. Overall, $$J_c$$ optimization of the Bi-2212 RW is a complex multi-variable process, but this study shows that maximizing the biaxial texture quality is an important first step in such an optimization process.« less

High engineering critical current density (JE of 1300 A/mm 2 at 4.2 K and 15 T) in Bi-2212 round wire has been achieved through a partial melt, overpressure heat treatment process. JE varies strongly with processing conditions, particularly the maximum heat treatment temperature (Tmax). Increasing Tmax results in longer time in the melt (defined as the time between when Bi-2212 melts on heating and when Bi-2212 begins to form on cooling), more bridging between the filaments, lower JE, and higher ac losses. A wide processing window with a large range of Tmax that has a nearly constant JE is desiredmore » for processing large coils with large thermal mass and significant thermal time constants that may make precise control over the desired temperature - time profiles uncertain. Accordingly, we wanted to explore broadening the Tmax window by controlling the Bi-2212 powder melting or wire architecture design. Here we report on studies of the performance variation with Tmax for two production wires with a filling factor of about 20% and 85 × 18 filaments where filament size was varied by changing the wire diameter, a process which also shortens the distance between filaments. We found that wires with smaller filament diameter (9 to 11 μm) showed a peak JE at the low end of Tmax and also a JE that was more sensitive to Tmax. A JE - Tmax plot showed a plateau JE(4.2 K, 5 T) of ~1100 A/mm 2 between Tmax of 886 and 894 °C for 1.0 and 1.2 mm wires, where JE is less sensitive to the wire diameter and Tmax. This JE plateau range is a preferred processing window for achieving high JE in coils.« less