Nb3Sn film deposition
Transcript of Nb3Sn film deposition
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Physical Vapor Deposition of Bronze-Route Nb3Sn for SRF Cavities
Wenura K. Withanage1, Andre Juliao1, Shreyas Balachandran1, Christopher Reis1, Shengzhi Zhang1, Wan Kyu Park1, Yi-Feng Su1, John Buttles2, Choong-Un Kim3, Peter J. Lee1,
Lance D. Cooley1
1 Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University
2 Bailey Tool & Manufacturing, Lancaster, TX3 Materials Science & Engineering Department, University of Texas – Arlington, Arlington, TX
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Why investigate bronze routes?
• Move reaction window from ~1100°C to ~700°C
– Avoid the Nb6Sn5 and NbSn2 phases by exploiting ternary Cu-Sn-Nb system
• Make the reaction compatible with cavity bodies made from copper; avoid bulk niobium (and)
– Possible cost and formability advantages
– Potential direct application with conduction cooling
• Genesis of ideas traces back to founding of IARC at Fermilab
• Avoid Sn vapor, chlorides of Sn and Nb polishing chemistry (corrosive, toxicity).
• Tap into wealth of processing knowledge from composite wires.
• If desired, can push grain size to < 50 nm; very low roughness.
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Cu-Sn-Nb pathways for SRF
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• Apply Nb to bronze and convert entirely to Nb3Sn
– Also works for bronzed copper → utilize Cu cavities
– More complicated geometries are possible, e.g. with diffusion barriers
• Apply bronze to Nb and react to form Nb3Sn, then remove bronze post-process
– Utilize Nb/Cu bimetal, e.g. hydroformedNb/Cu cavities
– Use Nb cavities
Phase diagram from Xingchen Xu, 2017 topical review of prospects for Nb3Sn conductors, SuST 30 093001
T ~ 650 °C
Avoid this region:Cu-Sn dissolves Nb
Nb3Sn
“Bronze” need not be bulk bronze (although bulk bronze is an easy starting point for studies) and can be bronzed Cu
CuSn
Clean Cu
Bronze surface
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First studies: Nb on a-bronze
• Substrates: Mechanically polished zero phosphorous Cu-15%wt Sn and Cu-15%wt Sn- 0.3%wt Ti bronzes.
• Commercial Cu and bronze usually has P as a de-oxidizing agent. Ti was explored here as a getter.
• Substrates were cold-rolled and homogenized at 525°Cprior to polishing.
• Nb film deposition conditions.
• Background pressure ~5E-9 Torr
• Processing gas 8 mTorr Ar
• Deposition rate 22.4 nm per minute
• Niobium film Tc 9.2 K, DTc 0.1 K
• Both process routes took place in the depositionchamber; no vacuum break or exposure to ambient
High Sn regions
Before
After
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100 µm 100 µm
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Nb deposited on hot bronze achieves 10x faster reaction
14-16 K Tc transition consistent with CTE mismatch between Nb3Sn and bronze.Also seen by CERN for direct deposition of Nb3Sn on copper
14 16 18
-0.0008
-0.0006
-0.0004
-0.0002
0.0000
700 °C/Cu-Sn
700 °C/Cu-Sn-Ti
725 °C/Cu-Sn
725 °C/Cu-Sn-Ti
775 °C/Cu-Sn
775 °C/Cu-Sn-Ti
Norm
aliz
ed m
om
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T (K)
4 6 8 10 12 14 16 18 20
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
No
rma
lize
d m
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t
T (K)
775 °C/Cu-Sn
775 °C/Cu-Sn-Ti
725 °C/Cu-Sn-Ti
725 °C/Cu-Sn
700 °C/Cu-Sn
700 °C/Cu-Sn-Ti
ZFC 10 Oe ZFC 10 Oe
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650 nm film produced in 20-minute process!
Addition of Ti is consistent with known alloying effects.
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4 6 8 10 12 14 16 18 20
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
No
rma
lize
d m
om
en
t
T (K)
650 °C/Cu-Sn
650 °C/Cu-Sn-Ti
700 °C/Cu-Sn
700 °C/Cu-Sn-Ti
775 °C/Cu-Sn
775 °C/Cu-Sn-Ti
ZFC 10 Oe
14 16 18 20
-0.0004
-0.0003
-0.0002
-0.0001
0.0000
650 °C/Cu-Sn
650 °C/Cu-Sn-Ti
700 °C/Cu-Sn
700 °C/Cu-Sn-Ti
775 °C/Cu-Sn
775 °C/Cu-Sn-Ti
No
rma
lize
d m
om
en
t
T (K)
ZFC 10 Oe
Nb on bronze + post-reaction behaves like processes in wires
6-hour heat treatments at > 700 °C were required to get full conversion of 500 nm
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Similar effect of CTE mismatch between Nb3Sn and bronze as for route 1
Addition of Ti seems to accelerate diffusion, which is also noted for wires.
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Columnar grain structure produced by route 1 was never observed in bronze route
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• Columnar grains through entire thickness
• Pipeline diffusion at bronze GBs → larger Nb3Sn grains
• Bronze twin boundaries are evident beneath Nb3Sn
• Very low roughness RA ~ 7–10 nm in 100 µm2.
• Sn content of Nb3Sn layer is measured (EDX) at 25% or higher in both substrates.
Protective silver
Protective silver
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Post-deposition reaction produces structures like those seen in wires
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• Tendency toward high-angle equiaxed grains
• Evidence for pipeline diffusion along Ti alloyed bronze GBs, with enriched Sn and Ti.
• Very low roughness RA ~ 10 - 15 nm in 100 µm2.
• Sn content of Nb3Sn layer
• 22–23.5 % no Ti
• 24.5–25.5 % with Ti
• Large Ti6Sn5 grains in films on Tialloyed bronze.
Protective silver
Protective silver
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Discussion and summary
• Address the CTE mismatch issues, e.g. by engineering the copper
– But 15 K is already sufficient to use conduction cooling for low RBCS
• The hot bronze method may be better
– Full tin content in Nb3Sn
• What is the significance of the unique grain structure obtained by deposition of Nb onto hot bronze?
– Hot bronze in vacuum has a tin-rich surface → high tin activity. Can this be exploited further?
– Can Nb deposition properties give microstructure control?
→ HIPIMS and biasing schemes are available and not yet fully exploited
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Discussion and summary
• While roughness is extremely low inside the underlying bronze grains, twin boundaries and grain boundaries affect the Nb3Sn coating. Is it important to suppress twins and GB effects?
– Alloying with Ti and other elements increases stacking fault energy, reduces twinning
• What is the significance of the very low roughness? Does this trade off with very high GB density?
• RF measurements are needed.
• Copper in GBs studies are needed.
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Acknowledgment
• Many thanks to Akihiro Kikuchi (NIMS, Japan) for providing Ti alloyed bronze.
• Alexander Wozny and Jonathan Wozny for their support with bronze substrate polishing.
• This work at ASC, NHMFL-FSU was funded by U.S. Department of Energy, Office of Science, Office of High Energy Physics under Award No. DE-SC 0018379.
• A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida.
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Backup Slides
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Bronze route Nb3Sn films for SRF cavities
• Low temperature deposition allowsbronze or Cu base cavity.
• Simplest case, Nb3Sn coated bronzecavities.
• Significant material cost saving fromswitching Nb host cavity to a bronzeor Cu host cavity.
• Easy fabrication and scaling.
• Cu at grain boundaries, Effect on RF?
• Low thermal conductivity, Engineeringsolutions? Reaction heat
treatment
Bronze cavityNb3Sn coating
Nb coating
Cavity heaterNb magnetron source
Bronze cavity
Nb3Sn coating
Hot bronze cavity
Route 1 for cavities
Route 2 for cavities13
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Proposed deposition chamber
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cryopump
3 position gate valve
20” Z shift mechanism
resistive heater
magnetron source
cylindrical vacuum chamber 14” OD, 20” height
water-cooled heat shield
extension for pre-sputtering
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More introduction
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J. P. Charlesworth et al J. Mat. Sci.5, 580 (1970)
P.J. Lee, D.C. Larbalestier, IEEE Transactions 11 (2001)
Nb – Sn phase diagram
Bronze process
Fractographs of a bronze route Nb3Sn filament• To synthesis pure Nb3Sn by reacting Nb
and Sn directly require temperaturesabove 930 °C.
• Eliminates the use of Cu substrate cavityin Nb – Sn direct reaction -> Significantmaterial costs
• Require sophisticated furnace systemsfor Nb – Sn reaction.
• Bronze route is a well-establishedtechnique in Nb3Sn wire fabrication.
• Bronze route guarantees only the pureNb3Sn phase at much lowertemperatures (~600 – 800 °C).
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Cu-Sn phase digram
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CTE mismatch
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CTE mismatch between Nb3Sn and common substratesD. O. Welch, Adv. Cryo. Eng., 26, pp. 48-65, 1980