1. Lance Cooley Monday morning meeting 27 September 2010
Review of 1st International Workshop on Superconducting Science and Technology of Ingot Niobium
Lance Cooley
Monday morning meeting
27 September 2010

2. Workshop Overview 2 Days plus tutorial 3rd day About 40 attendees
CBMM, Tokyo Denkai, Plansee, Cabot , Niowave were represented
MSU, NC State, FSU, ODU
JLab, Fermilab, IHEP, BARC, DESY, KEK, TRIUMF, NIST
1 World View session
3 Science and Technology sessions
1 Panel Discussion
Motivation: Large-grain (LG) niobium may:
Be cheaper to use
Reduce tendencies for interstitial atom uptake (esp. H)
Provide superior thermal conductivity (esp. phonon peak).
September 27, 2010
Review SSTIN

3. Improvement of LG Ingot at Tokyo Denkai
Ingot No.1
Ingot No.2
Ingot No.3
Ingot No.3
Single Crystal
Improvement of crystallization at Tokyo Denkai
Grain grow column like in EBM
Make a seed plate for single crystal ingot
Ultrasonic image by H.Umezawa
Tokyo Denkai

4. Miscellaneous
LG sheets – are we convinced that slicing is a proven technology?
September 27, 2010
Review SSTIN

5. Comments / Take-Aways – Thermal conductivity
The correlation between RRR and thermal conductivity is not strong (Chandrasekaran, MSU)
High RRR measures a conduction pathway that does not include phonon scattering terms.
Is a magnetic pair-breaker important? Phonons scatter off of normal fluid component
Wiedemann-Franz
“Phonon peak”
September 27, 2010
Review SSTIN

6. MSU Thermal conductivity studies
Factor of 5 reduction in b4 RT vs 800 °C
Factor of 2 reduction 600 vs 800 °C
b1 increases only 2x between RRR 150 and 400
Comment: b3 is sensitive to normal fluid component

7. Important comments / take-aways - Hydrogen
Usually LG cavities have less Q-slope than FG cavities
Do we agree?
Less hydrogen in LG?
Evidence for magnetic transitions in niobium hydride at ~120 K, in particular when Nb is tetrahedrally coordinated with H
“NbH4”
Hydrogen electromigration: electron beam attracts H to weld (HAZ is hydrogen rich)
Different probes and different labs’ data now converge: 10 to 40 percent hydrogen at Nb surface
Recoil spectroscopy
Induction spectroscopy
September 27, 2010
Review SSTIN

8. Comments / Take-Aways
Niobium Pentoxide is a good diffusion barrier for hydrogen
Evidence: implantation experiments
Implication: must heat above 600 C to dissolve oxide and let hydrogen out from surface (explains why 800 C bake works better)
Implication: oxide makes a good passivation layer against hydrogen
Implication: niobium is “naked” to hydrogen uptake whenever active fluroine or chlorine is around
Electropolishing should repel hydrogen
Nb anode is (+)
Hydrogen uptake might then come after current is off and HF is present (again, Nb is “naked”)
September 27, 2010
Review SSTIN

9. Why is hydrogen such a big deal? My observations…
IIT (not this workshop): electron spin resonance indicates Nb4+ as the active spin leading to magnetic scattering in point-contact tunneling experiments
So far we have thought this is associated with a defect in the oxide
Is this spin in the metal, especially at niobium (IV) hydride? Nb4+(H-)4
Plausible: Nb = [Kr] 4d4 5s1; Nb4+ = [Kr] 4d1 magnetic
Implication: magnetic niobium hydride precipitates
Magnetism breaks cooper pairs; dislocations and normal precipitates typically do not. Precipitates therefore increase the normal fluid component
Does everything boil down to presence or absence of a certain niobium hydride?
Q disease: the whole metal is full of precipitates
Q slope: certain regions, e.g. cores of dislocations, have precipitates
Thermal conductivity: phonon peak appears when precipitate is absent
Implication: removal of hydrogen leads to NO Q drop!
Is this what has been seen in ALD coated cavities?
Implication: removal of hydrogen may improve Q0 too
September 27, 2010
Review SSTIN

10. Coherent hydride precipitates – further observations
A coherent precipitate shares an arrangement of one sub-lattice of atoms with the other phase across its boundaries
Several hydrides are possible with very little lattice strain
1/2 cell plane
Base 100 plane
Tetrahedral 1/4 cell
A15 structure
NbH3 ½ filling
NbHx with x = 2,3,4,6,12 should be possible
110 planes
Nb lattice = 3.3 Å; Nb dia = 2.8 Å
Tetrahedral void ~ 0.6 Å
September 27, 2010
Review SSTIN

11. Why then does 120 °C bake work? More observations…
Romanenko: internal misorientation is reduced after bake, implying that dislocations are healed.
Nb vacancies may become mobile to assist dislocations
Vacancy mobility is due to breaking of H-vac binding
Tetrahedral void is not quite large enough at room temperature to accommodate H
Thus, H has to reside in vacancies, dislocation cores, grain boundaries => binding of vacancies
H can hop over a limited distance => high enough local concentration to form magnetic precipitates
Upon heating:
Thermal expansion opens Nb lattice
Attempt frequency exp (-E/kT) changes by orders of magnitude
Result: very fast turn-on of hydrogen mobility out from vacancies at a modest temperature (~200 C is plausible)
Implication: vacancies, dislocations are reservoirs of hydrogen and precipitate formation is likely in nearby regions unless mild baking permits H to move away.
September 27, 2010
Review SSTIN

12. To recap… and plan what we should do about this
Nb oxide is a hydrogen diffusion barrier
Good if H is outside: prevents penetration
Bad if H is inside: traps H underneath skin
Concentrated H may form into magnetic coherent precipitates
Q disease: precipitates everywhere
Q slope: precipitates localized
Possible loss of phonon peak
Grain boundaries and dislocations are gaps in barrier, permit entry / exit of H (but in local region)
Forged sheet has many more opportunities for hydrogen uptake than LG slices
New strategy?
Form
Anneal (recrystallize)
Weld
Anneal (drive H away from weld)
Bulk EP / BCP / tumble
Passivation rinse? Fluorine taper?
Final EP
passivation? Or dry fluorine?
Bake 800 C
Dry passivation?
HPR & Assy
120 bake should not be needed
Result: higher Q0 and no Q slope
September 27, 2010
Review SSTIN