Direct Observation of Magnetic Structure in Niobium Single Crystals with Hydride Precipitate Pinning Centres

Abstract. Niobium hydride is suspected to be a major contributor to degradation of the quality factor of niobium superconducting radio-frequency (SRF) cavities. In this study, we connect the fundamental properties of hydrogen in niobium to SRF cavity performance and processing. We modeled several of the niobium hydride phases relevant to SRF cavities and present their thermodynamic, electronic, and geometric properties determined from calculations based on density-functional theory. We find that the absorption of hydrogen from the gas phase into niobium is exothermic and hydrogen becomes somewhat anionic. The absorption of hydrogen by niobium lattice vacancies is strongly preferred over absorption into interstitial sites. A single vacancy can accommodate six hydrogen atoms in the symmetrically equivalent lowest-energy sites and additional hydrogen in the nearby interstitial sites affected by the strain field: this indicates that a vacancy can serve as a nucleation center for hydride phase formation. Small hydride precipitates may then occur near lattice vacancies upon cooling. Vacancy clusters and extended defects should also be enriched in hydrogen, potentially resulting in extended hydride phase regions upon cooling. We also assess the phase changes in the niobium-hydrogen system based on charge transfer between niobium and hydrogen, the strain field associated with interstitial hydrogen, and the geometry of the hydride phases. The results of this study stress the importance of not only the hydrogen content in niobium, but also the recovery state of niobium for the performance of SRF cavities.

Section: Introductionmentioning

confidence: 99%

First-principles calculations of niobium hydride formation in superconducting radio-frequency cavities

Ford

Cooley

Seidman

2013

“…It was clear over 40 years ago [1] that the chemical polishing of a niobium surface results in a complicated interplay between hydrogen uptake, surface topographical features, and other trace atomic contaminants, such as oxygen, nitrogen, and carbon atoms [2]. The effect of these atomic contaminants, and in particular hydride precipitates, on niobium superconductivity has been demonstrated [3][4][5][6][7][8][9]; and numerous reports, see the review by Knobloch [10], describe how SRF cavities tested immediately after heavy chemical polishing developed a rapid drop of Q starting with zero accelerating gradient, a phenomena called 'Q disease'. Knobloch explains that a comparison of the phase diagrams, concentrations, diffusivities, and ability to enter or exit niobium during processing of these common processing impurities and cavity cool down rate versus performance statistics led to the belief that hydrogen is responsible for Q disease.…”

Section: Introductionmentioning

confidence: 99%

Suppression of hydride precipitates in niobium superconducting radio-frequency cavities

Ford

Cooley

Seidman

2013

Niobium hydride is a suspected contributor to degraded niobium superconducting radio-frequency (SRF) cavity performance by Q slope and Q disease. The concentration and distribution of hydrogen atoms in niobium can be strongly affected by the cavity processing treatments. This study provides guidance for cavity processing based on density functional theory calculations of the properties of common processing impurity species-hydrogen, oxygen, nitrogen, and carbon-in the body-centered cubic (bcc) niobium lattice. We demonstrate that some fundamental properties are shared between the impurity atoms, such as anionic character in niobium. The strain field produced, however, by hydrogen atoms is both geometrically different and substantially weaker than the strain field produced by the other impurities. We focus on the interaction between oxygen and hydrogen atoms in the lattice, and demonstrate that the elastic interactions between these species and the bcc niobium lattice cause trapping of hydrogen and oxygen atoms by bcc niobium lattice vacancies. We also show that the attraction of oxygen to a lattice vacancy is substantially stronger than the attraction of hydrogen to the vacancy. Additionally hydrogen dissolved in niobium tetrahedral interstitial sites can be trapped by oxygen, nitrogen, and possibly carbon atoms dissolved in octahedral interstitial sites. These results indicate that the concentration of oxygen in the bcc lattice can have a strong impact on the ability of hydrogen to form detrimental phases. Based on our results and a literature survey, we propose a mechanism for the success of the low-temperature annealing step applied to niobium SRF cavities. We also recommend further examination of nitrogen and carbon in bcc niobium, and particularly the role that nitrogen can play in preventing detrimental hydride phase formation.

“…Note that spatially resolved vortex pinning on hydrides was studied by Bitter decoration forty years ago [43]. Interestingly, in that study the researchers could not determine whether the hydrides were normal or superconducting.…”

Section: Polarized Optical and Mo Imagingmentioning

confidence: 99%

“…Hydrogen may come from various hydrogenrich sources, for example, during liquid-assisted cutting and polishing, and even from ambient moisture. Since all applications related to the superconducting state of niobium require cooling to low temperatures, it is important to understand the formation of various phases, in particular, niobium hydrides and their effect on the superconducting properties [35,36,39,43,44]. In addition, hydrogen is a possible source for twolevel systems (TLS), e.g.…”

Section: Introductionmentioning

confidence: 99%

“…In an unannealed sample with a high load of hydrogen, niobium hydrides of various shapes form below 190 K as very large precipitates, up to hundreds of micrometers in size, easily visible in an optical microscope [36,43,44]. If the hydrogen load is not large, round or square precipitates of sub-micrometer size are formed, leading to local plastic deformations and nucleation of interstitial prismatic dislocation loops [39].…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

The nontrivial effects of annealing on superconducting properties of Nb single crystals

Datta,

Joshi,

Berti

et al. 2024

The effect of annealing on the superconducting properties of niobium single crystals was studied using optical, magnetic, and scanning tunneling microscopy methods. Pieces of the same crystal boule were studied before and after the annealing at 800 C, 1400 C, and near the melting point of niobium (2477 C). The initial samples had a high hydrogen content and low-temperature imaging revealed large hydrides (hundreds of micrometers) appearing below 190 K. The formation of these large precipitates is already completely suppressed by annealing at 800 C. However, the overall superconducting properties of the annealed samples did not improve and, in fact, worsened. In particular, the superconducting transition temperature decreased, the upper critical field increased, and the pinning strength increased. In the STM study, the sample was annealed initially at 400 C, measured, annealed at 1700 C, and measured again. The STM revealed a ``dirty'' superconducting gap with a significant spatial variation in tunneling conductance after annealing at 400 C. The clean gap was recovered after annealing at 1700 C. This is likely due to oxygen redistribution near the surface, which is always covered by oxide layers in as-grown crystals. Our results indicate that vacuum annealing at least up to 1400 C, while removing a large percentage of hydrogen, introduces additional nanosized defects, likely hydride precipitates, that act as efficient pair-breaking and pinning centers. The dimensionless scattering rate is estimated to have increased from $\Gamma=0.2$ to about $\Gamma=0.4$ after annealing at 1400 C. These results on single crystals differ drastically from those obtained in polycrystalline bulk niobium (i.e. cut from superconducting radio-frequency (SRF) cavities), where annealing is known to have a significant positive effect that is attributed to the improvement of the crystalline structure masking the more subtle influence of the hydrides.