Why Do Subtle Changes in srf cavity Treatments Produce Profound Changes in Performance?



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Why Do Subtle Changes in SRF Cavity Treatments Produce Profound Changes in Performance?

  • Niobium cavities (melting point 2000 °C) – a bake at 150°C for a few hours can transform a good cavity into an ILC-qualifier or even a record-setter – why??

    • Any changes must be subtle
    • Any changes must happen at the surface
    • Oxygen is the most likely actor
  • The Sibener Group has great expertise in surface and interfacial chemistry, including a detailed understanding of the atomic-level aspects of metallic oxidation



Review of Structure

  • Crystal structures of Nb and some of its oxides

    • Nb: body centered cubic
    • NbO: rock salt structure
    • Nb2O5: many polymorphs exist; most have octahedral Nb-O coordination
      • an well-known feature of metal-oxide magnets such as manganates


BACKDROP: IIT/ANL/FNAL Discovery! Signatures of magnetism in SRF niobium at ~2K

  • We suspect defects in the Nb2O5-x since pure samples (e.g. Nb12O29) are known to have various magnetic phases at 2-5 K (Cava 1991)

  • Magnetism is anti-superconductivity, hence this is a (the first?) plausible reason why subtle changes in the oxide would have profound effects on performance

    • Thomas Proslier et al., IIT, Appl. Phys. Lett. 92, 212505 (2008)
  • That work cross-fertilized to ANL work led by Pellin to coat cavities with aluminum oxide. Pellin’s work has been successful! We now have a recipe for ameliorating the oxide layer entirely!

    • Proslier et al, APL submitted
    • Pellin, Norem et al., report on coated-cavity tests to FNAL 28 May 2008


Year 1 UC/FNAL result: An explanation of the baking effect?

  • FNAL asked UC to look at “real” niobium first, instead of crystals as planned, in view of these developments

  • Cabot Microelectronics – niobium polished to atomic flatness

  • The pentoxide DEGRADES EASILY when attacked by a mild ion beam. Baking the native oxide TRANSFORMS it IRREVERSIBLY into a TOUGH layer with different bonding configuration (NbO).

  • Tentative conclusion: The oxide that forms at room temperature (e.g. after etching) is not desirable because it contains defects that spawn magnetism. Fortunately baking heals these defects (but for how long?).

  • Details are not known; now (year 2) we need to explore more ideal systems (e.g. crystals) to understand



Growth Possibilities Enabled by This Seed

  • Project submitted to University-Based Research for the ILC (George Gollin, UIUC – lead) but proposal to NSF cancelled by Omnibus

    • Nonetheless, it alerted us to possible synergisms
  • Regional center opportunity (FNAL/ANL/UC/IIT/NWU):

    • Themes revolve around niobium oxidation science
    • Full-scale non-acid polishing of cavities in new Fermilab tumbling machine using Cabot slurries
    • Capping of post-tumbled surfaces using ALD to prevent magnetic oxide
    • Understand mechanism of polishing (which involves embrittlement by oxygen injection) to optimize slurry
    • Understand structure-property relationships of surfaces
    • Long term: consider new accelerator possibilities of coatings, other materials topics (carbides…), mitigation of field emission
      • Possible new members: FNAL-APC, NIU-CADD, UIC




Year 1 Accomplishments

  • Focus: Polished polycrystalline Nb from SRF cavity stock; also an unpolished piece cut from a single Nb grain

  • Surface and interface chemistry using new X-ray Photoemission Spectroscopy (XPS) tools

  • Ion sputtering & annealing in UHV to simulate cavity etching and baking under controlled conditions

  • Heating in the “air of the day” for studies of the “baking effect”

  • Instrumentation development for infrared and STM studies





Initial XPS -- Survey UC Grad Student: Miki Nakayama



Overview - Nb XPS Spectroscopy

  • Nb 3d peak positions from prior literature

    • oxidation state (3d3/2, 3d5/2) (units in eV)
    • Nb+5 (210.0, 207.3) Nb+4 (208.8, 206.0)
    • Nb+2 (206.8, 204.0) Nb0 (205.0, 202.2)


XPS of Pristine Sample -- Nb 3d

  • Reveals Nb2O5-dominated layer on top of metallic Nb for both samples



Heating Effects Mild Changes at Modest Temperatures Oxygen diffusion into Selvedge and Interstitials

  • Broadening of the Nb 3d peaks with initial heating (522K) clearly indicates that oxygen is mobile, even at this temperature



Sputtering Effects on Single Grain Sample

  • Nb 3d peaks gradually convert to NbO, suggesting massive rearrangement of oxygen. But why NbO?



Heating Effects + Sputtering = NbO

  • After 790 K heating, Nb2O5 is gone, NbO remains and is thick enough to almost mask Nb underneath.

  • Nb2O5 apparently gives up its oxygen easily, either by annealing or by sputtering. This is not as true for NbO…



Order of Sputtering & Heating: Notable Stability of NbO

  • For Nb 3d and O 1s, we see that the end result is the same both ways, but heating after a sputter has no effect

  • That is, once the transformation to NbO happens, it remains stable



Exposure to Air: Return of Other Oxides

  • Nb 3d peaks show that NbO is still the dominant oxide species after air exposure, but higher oxides are forming

  • The third peak does not match Nb+4 nor Nb+5, indicating a mixture of the higher oxides





1 hr 140 C

  • Nb 3d peaks show that more of the Nb are converting from NbO to higher oxides; O being added to sample (in UHV, oxygen tends to be subtracted from the surface and move into niobium metal)



Carbon and NbC: Sputtering Induced Chemistry

  • C 1s peak shows that almost all of the graphite converted to NbC after the lower energy sputters



Key Questions!

  • What is the nature of the clean Nb interface?

  • What is the oxidation mechanism? Kinetics? Stability?

  • How do other species, e.g. H2O in “air of the day”, affect the oxides ? H2 ?

  • How do crystal faces (100, 110, 111) and polycrystallinity (which includes grain boundaries) affect oxides?

  • What is the role of Carbon and NbC?



Year 1 Summary and Year 2 Plans

  • Year 1: A Good Start! Strong Interactions Between UChicago and FNAL Emphasize the Strength of Joint Efforts on this Project

  • Real effects observed on SRF cavity niobium, and correlation of effects with changes in gap (through IIT/ANL work) were found

  • FNAL Supplied and Cabot Polished Samples used at UC for Oxide Studies

  • We have begun to explore the efficacy that different heating, sputtering, and polishing preparations have on the quality of the interface



Backup slides





Interfacial Dynamics & Oxygen Driven Reconstruction of a Stepped Metallic Surface Via Time-Lapse Scanning Tunneling Microscopy

  • T.P. Pearl and S.J. Sibener

  • J. Phys. Chem. B105, 6300-6306 (2001)

  • J. Chem. Phys. 115, 1916-1927 (2001)

  • Surf. Sci. Lett. 496, L29-L34 (2002)





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