Superconducting RF Materials University Collaboration

STATUS

Lance Cooley

June 07, 2010

Status of the Collaboration

June 09, 2010

ILC ART Review at Fermilab 2

• FNAL, UChicago, IIT, Northwestern, Florida State– UIC not directly funded but is

involved– IIT work occurs partly at ANL

• FNAL – Cooley– Peoples Fellow A. Romanenko

(SRF surface science)– NW Ph.D. student Denise Ford

(computational chemistry)• UChicago – Sibener

– M. Nakayama, T. Wang (oxidation fundamentals, STM)

• IIT – J. Zasadzinski– T. Proslier (ANL) & TBD (PCT,

ESR)

• $1.5M / 3 years, started 9/1/09– POs issued to collaborators on

separate schedules

• Northwestern – D. Seidman– McCormick Appointment – Yoon-

Jun Kim

(LEAP microscopy of Cottrell atmospheres)

• Florida State – P. Lee– Z. Sung (FIB isolation and

electron microscopy)• UIC – R. Klie

– Atomic resolution EELS

Recent achievements

Subtle effects of contaminants• Oxidation

– Thick, stressed oxidation (stuck HPR) initiates quench (JLab result)

– A grain boundary in the hot spot is also a source of magnetic scattering (IIT – at right)

– Other locations seem benign – the oxide itself is not the origin of quench. Defects in the oxide continue to be plausible sources

• ESR data, too – Nb 4+ (IIT)

• Implications– Avoid stress to the natural oxide– Avoid “flash” oxidation

• Acid trapped in pits hit with rinse water– Hunt for the oxide defects

June 09, 2010

ILC ART Review at Fermilab 3

Point-contact tunneling:Normal electrons at

oxide hot spot

Ohm’s law

In hot spot

Typical superconductor response at cold areas

(red, green)

2D

Ciovati – Jlab, Proslier – ANL, Zasadsinzki – IIT,Cooley, Romanenko – FNAL

Recent achievements

Subtle effects of contaminants• Hydrogen

– During EP, sulfate anodization (oxidation) of niobium may produce threading dislocations (A.Romanenko)

– EP also loads metal with hydrogen (unlike BCP)– Hydrogen binds to Nb vacancies, prevents

removal of threading dislocations (so-called Cottrell atmosphere)

– Mild baking (120°C) releases vacancies and thereby restores dislocation climb, which improves surface resistance

• Implications– 800 °C bake to remove surface hydrogen– Final EP must be cold– Re-tool surface science to find H (e.g. LEAP)– Positron annihilation, µ-SR

June 09, 2010

ILC ART Review at Fermilab 4

Plot: Visentin – SRF09

Cooley, Romanenko – FNAL Seidman - NWU

H

Vac.

Baked – mobile H

No bake – bound H

Materials achievements – past 12 months

Understanding electrochemistry• Do things go wrong?

– Coupon EP is glossy, with no grain boundary contrast– Cavity EP is less glossy, with visible grain boundaries– Agitation and stirring circulate fluorine to coupon

surface, producing grain-boundary contrast• High temperature reduces viscosity, promotes circulation

(H. Tian Ph.D. thesis)• Some grains may etch quickly, leaving faceted pits

• Implications– Keep EP cold and don’t agitate

• Do not use acid flow as the coolant! Instead, apply external cold water spray to EP tool, and turn back flow.

– Final EP will then be slow – use alternate process (tumbling) to make up processing time

• Process MUST pre-condition surface to a roughness comparable with the thickness of fluorine diffusion layer (~40 µm at 20 °C – Reece et al.)

June 09, 2010

ILC ART Review at Fermilab 5

Non-typical EP coupon showing grains (FNAL)

Summary – Good materials science AND implications for SRF

• Collaboration PIs are advancing basic materials topics– Cottrell atmospheres– Spin-resonance and positron annihilation spectroscopy– Oxidation kinetics

• Collaboration work is stimulating new tool development– Low-T STM at UChicago– Electron di-chroism in the STEM at UIC– LSCM and EBSD at FNAL

• Implications of work have direct impact on SRF process– Avoid excessive oxidation, “flash” oxides, and mechanical damage to

oxide layer– Take better care of hydrogen– Don’t heat or agitate the electrolyte if it can be helped – not only does

this make surfaces rougher, it also promotes the subtle complications of oxide and hydride chemistries

– New question: Is contamination more important than roughness?

June 09, 2010

ILC ART Review at Fermilab 6