21 Search Results

High purity niobium (Nb) is a technologically important material for large-scale accelerator and nano-scale quantum computing applications in microwave frequency range. The high thermal conductivity and low resistivity of Nb are critical to the high performance at temperature range of 0.01–4.0 K. The presence of interstitials such as O, N, H, and C act as scattering centers and alter the mean free path, reducing resistivity and thermal conductivity, and contributing significantly to Nb's thermal performance for temperatures of 2.0 K and above. The residual resistivity ratio (RRR), defined as the ratio of the normal state resistivity at 300 K tomore » that at 4.2 K (Nb, T_c = 9.2 K), is an accepted direct estimate of the impurity content of fully recrystallized Nb. Complete re-crystallization of Nb is challenging unless very high temperatures are employed, which is often impractical, hence, in practice, dislocation and dislocation structures impact the thermal performance of Nb due to strong phonon scattering contributions. Furthermore, this paper reports on the degradation of thermal conductivity and RRR of high purity Nb large grain, single crystal with fixed impurity, varying strain, and dislocation content levels. Experimental thermal conductivity data fits the Boltzmann transport equation incorporating dislocation density.« less

Absmore » tract Nb is an elemental superconductor with a critical temperature of 9.3 K and is widely used to fabricate superconducting radiofrequency (SRF) cavities for particle accelerators. However, microstructural defects in Nb, such as grain boundaries (GBs) and dislocations, can act as pinning centers for magnetic flux that can degrade SRF cavity performance. Hydrogen contamination is also detrimental to SRF cavity performance due to the formation of normal conducting hydrides during cool down. In this study, disc shaped Nb bi-crystals extracted from high-purity large-grain Nb slices were investigated to study the effects of GBs, hydrogen, and dislocations on superconducting properties. Grain orientation and GB misorientation were measured using Laue x-ray diffraction and electron backscattered diffraction (EBSD) analyses. Cryogenic magneto-optical imaging was used to directly observe magnetic flux penetration below $T_c$ = 9.3 K. Damage caused by low temperature precipitation of hydrides and their dissolution upon reheating after cryogenic cycles was examined using electron channeling contrast imaging, and EBSD. The relationships between hydride formation, dislocation content, GBs, cryo-cooling, heat treatment (HT), and flux penetration indicate that both GB character and hydrogen content affect magnetic flux penetration. Such flux penetration could be facilitated by dislocation structures and low angle GBs resulting from hydride precipitation and HT.« less

We report the results from the measurements of high purity Nb samples and superconducting radio-frequency (SRF) cavities doped with nitrogen and followed by either electropolishing (EP) or buffered chemical polishing (BCP), in order to understand the role of the postdoping treatment on the performance of SRF cavities. The samples characterization via scanning electron microscope, x-ray photoelectron spectroscopy and secondary ion mass spectroscopy showed topographical differences on the surface of the samples after EP versus BCP treatment, but similar surface composition. Radio-frequency measurements were done on single cell cavities made from fine-grain and large-grain Nb treated by nitrogen doping followed bymore » BCP and showed that improved Q₀ in the medium field in both fine-grain and large-grain cavities is possible with BCP postprocessing. However, there are differences between performances of large-grain versus fine-grain cavities after BCP. A cavity made from large-grain Nb showed a larger increase in Q₀ and a lower quench field compared to cavities made from fine-grain Nb.« less

The increasing demand for improving the high-field (16-22 T) performance of Nb₃Sn conductors requires a better understanding of the properties of modern wires much closer to irreversibility field, H _Irr. In this study we investigated the impact of Ta, Ti and Hf doping on the high-field pinning properties, the upper critical field, H _c2, and H _Irr. We found that the pinning force curves of commercial Ti and Ta doped wires at different temperatures do not scale and that the Kramer extrapolation, typically used by magnet designers to estimate high-field critical current density and magnet operational margins from lower fieldmore » data, is not reliable and significantly overestimates the actual H _Irr. In contrast, new laboratory scale conductors made with Nb-Ta-Hf alloy have improved high-field J _c performance and, despite contributions by both grain boundary and point defect pinning mechanisms, have more predictable high-field behavior. Using Extended X-ray Absorption Fine Structure spectroscopy, EXAFS, we found that for the commercial Ta and Ti doped conductors, the Ta site occupancy in the A15 structure gradually changes with the heat treatment temperature whereas Ti is always located on the Nb site with clear consequences for H _c2. Finally, this work reveals the still limited understanding of what determines H _c2, H _Irr and the high-field J _c performance of Nb₃Sn and the complexity of optimizing these conductors so that they can reach their full potential for high-field applications.« less

For broader acceptance, the next generation of superconductors for high magnetic field appli-cations will need to be both higher performance and lower cost. In this work, we contrast two super-conductors that are candidates for high field magnets; Nb₃Sn, which has a long history yet has seen renewed interest in recent years because of remarkable advances in its properties, and Fe-based superconductors that offer potentially low-costs and very high fields. TEM techniques are essential to understand how to engineer the desired micro-/nanostructure that ultimately defines the properties for these superconductors.

In this paper we show that addition of Hf to Nb4Ta can significantly improve the high field performance of Nb₃Sn, making it suitable for dipole magnets for a machine like the 100 TeV Future Circular Collider (FCC). A big challenge of the FCC is that the desired non-Cu critical current density (J _c ) target of 1500 A mm^–2 (16 T, 4.2 K) is substantially above the best present Nb₃Sn conductors doped with Ti or Ta (~1300 A mm^–2 in the very best sample of the very best commercial wire). Recent success with internal oxidation of Nb–Zr precursor has shownmore » significant improvement in the layer J _c of Nb₃Sn wires, albeit with the complication of providing an internal oxygen diffusion pathway and avoiding degradation of the irreversibility field H _Irr. We here extend the Nb1Zr oxidation approach by comparing Zr and Hf additions to the standard Nb4Ta alloy of maximum H _c2 and H _Irr. Nb4Ta rods with 1Zr or 1Hf were made into monofilament wires with and without SnO₂ and their properties measured over the entire superconducting range at fields up to 31 T. We found that Group IV alloying of Nb4Ta does raise H _Irr, though O₂ addition still slightly degrades it. As noted in earlier Nb1Zr work with an O source, the pinning force density F _p is strongly enhanced and its peak value shifted to higher field by internal oxidation. A surprising result of this work is that we found better properties in Nb4Ta1Hf without SnO₂, F _pMax achieving 2.35 times that of the standard Nb4Ta alloy, while the oxidized Nb4Ta1Zr alloy achieved 1.54 times that of the Nb4Ta alloy. The highest layer J _c (16 T, 4.2 K) of 3700 A mm^–2 was found in the SnO₂-free wire made with Nb4Ta1Hf alloy. Using a standard A15 cross-section fraction of 60% for modern powder-in-tube and rod restack process wires, we estimated that a non-Cu J _c of 2200 A mm^–2 is obtainable in modern conductors, well above the 1500 A mm^–2 FCC specification. Moreover, since the best properties were obtained without SnO₂, the Nb4Ta1Hf alloy appears to open a straightforward route to enhanced properties in Nb₃Sn wires manufactured by virtually all the presently used commercial routes employed today.« less

Niobium provides the basis for all superconducting radio frequency (SRF) cavities in use, however, hydrogen is readily absorbed by niobium during cavity fabrication and subsequent niobium hydride precipitation when cooled to cryogenic temperatures degrades its superconducting properties. In the last few years the addition of dopant elements such as nitrogen has been experimentally shown to significantly improve the quality factor of niobium SRF cavities. One of the contributors to Q degradation can be presence of hydrides; however, the underlying mechanisms associated with the kinetics of hydrogen and the thermodynamic stability of hydride precipitates in the presence of dopants are notmore » well known. Using first principles calculations, the effects of nitrogen on the energetic preference for hydrogen to occupy interstitial sites and hydride stability are examined. In particular, the presence of nitrogen significantly increased the energy barrier for hydrogen diffusion from one tetrahedral site to another interstitial site. Furthermore, the beta niobium hydride precipitate became energetically unstable upon addition of nitrogen in the niobium matrix. Through electronic density of states and valence charge transfer calculations, nitrogen showed a strong tendency to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen atoms leading to a very unstable state for hydrogen and hydrides. These calculations show that the presence of nitrogen during processing plays a critical role in controlling hydride precipitation and subsequent SRF properties.« less

The question of whether grain boundaries (GBs) in niobium can be responsible for lowered operating field (B RF) or quality factor (Q 0) in superconducting radio frequency (SRF) cavities is still controversial. Here, we show by direct DC transport across planar GBs isolated from a slice of very large-grain SRF-quality Nb that vortices can preferentially flow along the grain boundary when the external magnetic field lies in the GB plane. However, increasing the misalignment between the GB plane and the external magnetic field vector markedly reduces preferential flux flow along the GB. Importantly, we find that preferential GB flux flowmore » is more prominent for a buffered chemical polished than for an electropolished bi-crystal. The voltage–current characteristics of GBs are similar to those seen in low angle grain boundaries of high temperature superconductors where there is clear evidence of suppression of the superconducting order parameter at the GB. While local weakening of superconductivity at GBs in cuprates and pnictides is intrinsic, deterioration of current transparency of GBs in Nb appears to be extrinsic, since the polishing method clearly affect the local GB degradation. The dependence of preferential GB flux flow on important cavity preparation and experimental variables, particularly the final chemical treatment and the angle between the magnetic field and the GB plane, suggests two more reasons why real cavity performance can be so variable.« less

The next generation of superconducting accelerator magnets for the Large Hadron Collider at CERN will require large amounts of Nb₃Sn superconducting wires and the Powder-In-Tube (PIT) process, which utilizes a NbSn₂-rich powder core within tubes of Nb(7.5wt%Ta) contained in a stabilizing Cu matrix, is a potential candidate. But, the critical current density, J _c , is limited by the formation of a large grain (LG) A15 layer which does not contribute to transport current, but occupies 25-30% of the total A15 area. Thus it is important to understand how this layer forms, and if it can be minimized in favormore » of the beneficial small grain (SG) A15 morphology which carries the supercurrent. The ratio of SG/LG A15 is our metric here, where an increase signals improvement in the wires A15 morphology distribution. We have made a critical new observation that the initiation of the LG A15 formation can be controlled at a wide range of temperatures relative to the formation of the small grain (SG) A15. The LG A15 can be uniquely identified as a decomposition product of the Nb6Sn5(Cu x ), surrounded by a layer of rejected Cu, thus the LG A15 is not only of low pin density, but is not continuous grain to grain. We have found that in single stage reactions limited to 630 °C - 690 °C, the maximum SG A15 layer thickness prior to LG A15 formation is very sensitive to temperature, with a maximum around 670 °C. This result led to the design of four novel heat treatments which all included a short, high temperature stage early in the reaction, followed by a slow cooling to a more typical reaction temperature of 630 °C. We also found that this heat treatment (HT) modification increased the SG A15 layer thickness while simultaneously suppressing LG A15 morphology, with no additional consumption of the diffusion barrier. In the best heat treatment the SG/LG A15 ratio improved by 30%. Unfortunately, J _c values suffered slightly, however further exploration of this high temperature reaction region is required to understand the limits to A15 formation in Nb3Sn PIT conductors.« less

Dipole magnets for the proposed Future Circular Collider (FCC) demand specifications significantly beyond the limits of all existing Nb₃Sn wires, in particular a critical current density (J_c) of more than 1500 A mm^-2 at 16 T and 4.2 K with an effective filament diameter (D_eff) of less than 20 μm. The restacked-rod-process (RRP^®) is the technology closest to meeting these demands, with a J_c(16 T) of up to 1400 A mm^-2, residual resistivity ratio > 100, for a sub-element size D_s of 58 μm (which in RRP^® wires is essentially the same as D_eff). An important present limitation of RRP^®more » is that reducing the sub-element size degrades J_c to as low as 900 A mm^-2 at 16 T for D_s= 35 μm. To gain an understanding of the sources of this J_c degradation, we have made a detailed study of the phase evolution during the Cu-Sn 'mixing' stages of the wire heat treatment that occur prior to Nb₃Sn formation. Using extensive microstructural quantification, we have identified the critical role that the Sn-Nb-Cu ternary phase (Nausite) can play. The Nausite forms as a well-defined ring between the Sn source and the Cu/Nb filament pack, and acts as an osmotic membrane in the 300 °C-400 °C range—greatly inhibiting Sn diffusion into the Cu/Nb filament pack while supporting a strong Cu counter-diffusion from the filament pack into the Sn core. This converts the Sn core into a mixture of the low melting point (408 °C) η phase (Cu₆Sn₅) and the more desirable ϵ phase (Cu₃Sn), which decomposes at 676 °C. After the mixing stages, when heated above 408 °C towards the Nb₃Sn reaction, any residual η liquefies to form additional irregular Nausite on the inside of the membrane. All Nausite decomposes into NbSn₂ on further heating, and ultimately transforms into coarse-grain (and often disconnected) Nb₃Sn which has little contribution to current transport. Understanding this critical Nausite reaction pathway has allowed us to simplify the mixing heat treatment to only one stage at 350 °C for 400 h which minimizes Nausite formation while encouraging the formation of the higher melting point ϵ phase through better Cu-Sn mixing. At a D_s of 41 μm, the Nausite control heat treatment increases the J_c at 16 T by 36%, reaching 1300 A mm^-2 (i.e. 2980 A mm^-2 at 12 T), and moving RRP^® closer to the FCC targets.« less