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

Bi-2212 is the only high field, high temperature superconductor (HTS) available in the macroscopically isotropic, multifilament high Jc round wire (RW) form capable of generating high uniformity fields with minimum screening current errors. However, the heat treatment that enables impressively high Jc(4.2 K, 30T) values that can attain ~5000 Amm-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 by 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 Jc 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 Jc values. Although it has been reported that twisting can reduce wire Jc by damaging filaments, we found no reduction in transport Jc, even for nominal twist pitches of 12 mm in 0.8 mm diameter wires. Evaluation of more-recent, higher Jc Engi-Mat powder wires showed that their reduced filament bridging and improved longitudinal connectivity significantly improved transport Jc and reduced the Jc normalized losses, signaling that Jc 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.

Section: Resultssupporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

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

Davis

Jiang

et al. 2022

“…As shown in figure 2, in the best case d eff (4.2 K) mildly decreases with increasing field (35%-37% of d Bundle ), whereas stronger changes are observed in the other two cases (55%-62% and 51%-59% of d Bundle ). A similar decreasing d eff trend was previously found in a different type of Bi-2212 wires in [35], where the investigation was performed in a wider temperature range. The authors found a strong difference in d eff depending on the wire quality and architectures (d eff (10 K, 4-7 T) ∼70%-80% d Bundle in the best case), but also observed a clear temperature dependence with d eff approaching the filament size at higher temperature (>20-25 K) and increasing toward d Bundle with decreasing temperature.…”

Section: Effective Filament Diametersupporting

confidence: 85%

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

Tarantini,

Barua,

Oloye

et al. 2023

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.

“…As a matter of fact, the magnetic moment of HB samples is close to that of the LB samples in the low magnetic field, while the distinction was intensified with the increase of magnetic fields, as shown in figure 4(a). The effective diameters of HB and LB samples were calculated based on equation (1) [15]. The corresponding results are given in figure 4(b).…”

Section: Resultsmentioning

confidence: 99%

“…However, high-temperature heat treatment result in intense atom diffusion, which also leads to the appearance of bridges between filaments in the Bi-2212 wires [14]. According to previous research, the formation of the bridges could lead to changes in the effective diameter on the basis of the Bean model [15]. The bridges were thought to be hyper channels for current conducting which could be beneficial to increase the critical current.…”

Section: Introductionmentioning

confidence: 99%

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

Zhang,

Yu,

Yang

et al. 2024

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.