Annealing to mitigate pitting in electropolished niobium coupons and SRF cavities

Cooley, L. D.; Hahn, Eric N.; Hicks, D. G.; Romanenko, Alexander; Schuessler, R.; Thompson, C.

doi:10.1063/1.4712108

Cited by 3 publications

(5 citation statements)

References 8 publications

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“…It would although make it less intense [50]. Impurities or inclusions introduced in the EBW region of SRF cavities also present similar problems of pitting for EP of niobium [48]. Sulphide inclusions in nickel have been found to be the site where pits nucleate during anodic dissolution of nickel [49].…”

Section: Other Factors Resulting In Pitting During Electropolishingmentioning

confidence: 99%

“…The application of current and voltage on the polishing plateau is a necessary condition to obtain a pit free surface but not a sufficient condition. A multitude of reasons have been attributed to be responsible for pitting some of which include (i) oxygen evolution at the surface getting polished, (ii) hydrogen evolved on the cathode, (iii) inclusions on the surface, (iv) stresses on the surface, (v) excess agitation in the polishing bath, and (vi) external treatments given to the surface [3,4,28,[48][49][50][51][52][53][54].…”

Section: Phenomenon Of Pitting In Anodic Dissolution Of Metals and Al...mentioning

confidence: 99%

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On the Mechanism of Niobium Electropolishing

Chandra

2012

J. Electrochem. Soc.

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The mechanism of Nb electropolishing was studied. In particular, the effects of acid concentration, stirring, temperature and amount of material removed on the surface finish were investigated using electrochemical polarization measurement, galvanostatic and potentiostatic techniques. A current density plateau in the niobium polarization curves, characteristic of mass transport control, was observed. Fluoride ion was shown to be the transport-limited species. Galvanostatic results deviated from a perfect semi infinite linear diffusion model, indicating the effects of diffusion and temperature gradients.

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Section: Other Factors Resulting In Pitting During Electropolishingmentioning

confidence: 99%

Section: Phenomenon Of Pitting In Anodic Dissolution Of Metals and Al...mentioning

confidence: 99%

On the Mechanism of Niobium Electropolishing

Chandra

2012

J. Electrochem. Soc.

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Section: Introductionmentioning

confidence: 99%

“…Increased chemical activity due to hydrogen and hydrogen bubbles were plausible causes of pits. A follow-up experiment [12] showed that high-vacuum annealing to recover cold work prior to the EP experiment improved this sensitivity. Reference [12] recommended changing the cavity processing sequence to put annealing ahead of chemical polishing as a preventative measure against pitting.…”

Section: Introductionmentioning

confidence: 99%

“…A follow-up experiment [12] showed that high-vacuum annealing to recover cold work prior to the EP experiment improved this sensitivity. Reference [12] recommended changing the cavity processing sequence to put annealing ahead of chemical polishing as a preventative measure against pitting. Unfortunately, coupon experiments such as these can only hint at benefits for SRF cavities, because only RF cavity tests give effective data about Q(E acc ).…”

Section: Introductionmentioning

confidence: 99%

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Combined effects of cold work and chemical polishing on the absorption and release of hydrogen from SRF cavities inferred from resistance measurements of cavity-grade niobium bars

Dzyuba¹,

Cooley²

2014

Supercond. Sci. Technol.

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A series of small fine-grained and single-crystal bars, with strain from 0% (recrystallized) to 50%, were given different amounts of chemical polishing. Four-point resistivity () data was used to characterize the electron scattering from dislocations, hydrogen, and any other trace contaminants. As noted by previous studies, annealed Nb displayed a weak linear increase of (11 K) with polishing time due to hydrogen absorption, and bulk hydrogen concentration did not exceed 15% for 200 µm metal removed. Cold-worked samples displayed steeper slopes with polishing time (after subtracting resistivity due to strain alone), suggesting that dislocations assist the absorption of hydrogen during polishing. Absorption accelerated above 30% strain and 100 µm material removal, with room-temperature hydrogen concentration rising rapidly from 2% up to 5%. This threshold is significant, since superconducting radio-frequency (SRF) cavities are usually polished as-formed, with >35% strain, and polishing removes >150 µm of metal. Resistance jumps between 40 and 150 K, which signal the formation of hydride precipitates, were stronger in cold-worked samples, suggesting that dislocations also assist precipitate nucleation. High-vacuum anneals at 800 °C for 2 hours, which are known to fully recrystallize cavity-grade niobium and de-gas hydrogen, removed the 40-150 K jumps and recovered the resistivity increase due to chemical polishing entirely. But, about 30% of the resistivity increase due to cold work remained, possibly due to residual dislocation clusters. Continued annealing only facilitated the diffusion of surface impurities into the bulk and did not recover the initial 0% state. Strain, polishing, and annealing thus appear to combine as irreversible paths that change the material. Bearing this in mind, the significant difference in hydrogen uptake between annealed and coldworked samples suggests that annealing SRF cavities prior to chemical polishing could greatly reduce hydrogen uptake and storage in the metal, reducing risk of quality-factor loss. This inverts key steps of the present widely-used cavity processing sequence.

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Evidence of incomplete annealing at 800 °C and the effects of 120 °C baking on the crystal orientation and the surface superconducting properties of cold-worked and chemically polished Nb

Sung

Dzyuba

Lee

et al. 2015

Supercond. Sci. Technol.

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High-purity niobium rods were cold-worked by wire-drawing, followed by various combinations of chemical polishing and high-vacuum baking at 120 °C or annealing at 800 °C in order to better understand changes to the surface superconducting properties resulting from typical superconducting radio-frequency cavity processing. AC susceptibility measurements revealed an enhanced upper transition Tc at ∼ 9.3–9.4 K in all samples that was stable through all annealing steps, a value significantly above the accepted Tc of 9.23 K for pure annealed niobium. Corresponding elevations were seen in the critical fields, the ratio of the surface critical field Hc3 to the bulk upper critical field Hc2 rising to 2.3, well above the Ginzburg–Landau value of 1.695. Orientation imaging revealed an extensive dislocation rich sub-grain structure in the as-drawn rods, a small reduction of the surface strain after baking at 120 °C, and a substantial but incomplete recrystallization near the surface after annealing at 800 °C. We interpret these changes in surface superconducting and structural properties to extensive changes in the near-surface interstitial contamination produced by baking and annealing and to synergistic interactions between H and surface O introduced during electropolishing and buffered chemical polishing.

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Annealing to mitigate pitting in electropolished niobium coupons and SRF cavities

Cited by 3 publications

References 8 publications

On the Mechanism of Niobium Electropolishing

On the Mechanism of Niobium Electropolishing

Combined effects of cold work and chemical polishing on the absorption and release of hydrogen from SRF cavities inferred from resistance measurements of cavity-grade niobium bars

Evidence of incomplete annealing at 800 °C and the effects of 120 °C baking on the crystal orientation and the surface superconducting properties of cold-worked and chemically polished Nb

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