1. Alternative materials and coating techniques for cavities
E. A. Ilyina1, G. Rosaz1, W. Vollenberg1, J. B. Descarrega1, A. Lunt1, A. Gerardin1, F. Leaux1, S. Calatroni1, A. Sublet1, M. Taborelli, 1 W. Venturini-Delsolaro1, M. Bonura2 & C. Senatore2
1. CERN, Route de Meyrin, CH-1211 Geneva 23, Switzerland
2. Univeristy of Geneva, 24 rue du Général-Dufour, CH-1211, Geneva 4, Switzerland

2. Outline of the presentation
1. Motivation of the work
2. Elaboration method
3.1 Crystallinity and morphology of the films
3.2 Critical temperature measurements
3.4 Substrate influence on A15 properties
4. Achievements and future actions
3. Results
3.3 Microstructural properties of the films

3. Outline of the presentation
1. Motivation of the work
2. Elaboration method
3.1 Crystallinity and morphology of the films
3.2 Critical temperature measurements
3.4 Substrate influence on A15 properties
4. Achievements and future actions
3. Results
3.3 Microstructural properties of the films

4. Motivation
… to investigate new materials for superconducting accelerating cavities capable of providing the desired characteristics, in addition to operational cost reduction.
Proposed solutions:
Replace expensive Nb bulk cavities with coated copper ones
Copper cavities offer high thermal conductivity at low temperature, which should greatly help to increase the stability of the cavity against breakdown.
Nb coated copper cavities are successfully used in CERN ( LEP, LHC and HIE-ISOLDE machines).
Replace Nb thin films with superconductor with superior superconducting properties (A15 intermetallic compound)

5. Choice of Nb3Sn Advantages and challenges
Why is it attractive?
High critical temperature Tc = 18.3 K (Nb ~ 9.3 K)
Small BCS surface resistance RBCS ~ K and 400MHz (Nb ~ 29 nΩ)
Challenges
Stoichiometry control. The only superconducting A15 phase is reachable only in very narrow region of Sn % (marked with blue colour)
Formation of superconducting A15 phase possible only after high temperature treatment
Temperature should be high enough to promote SC but not too high to not destroy the cavity (brazing of the flanges and do not melt Cu substrate)
Copper substrate
Nb3Sn
Binary phase diagram of the Nb-Sn system [1]
[1] J. Charlesworth, I. MacPhail, and P. Madsen, J. Mater. Sci. 5, 580 (1970)

6. Outline of the presentation
1. Motivation of the work
2. Elaboration method
3.1 Crystallinity and morphology of the films
3.2 Critical temperature measurements
3.4 Substrate influence on A15 properties
4. Achievements and future actions
3. Results
3.3 Microstructural properties of the films

7. With/without additional annealing in the same chamber
“Room-temperature” coatings
High-temperature coatings
Characterisation
as deposited +
Nb:Sn 3:1
Annealing
Annealing temperatures
600oC - 800oC
Annealing time
24H…72H
With/without additional annealing in the same chamber
Characterisation
after annealing + +
Characterisation

8. Outline of the presentation
1. Motivation of the work
2. Elaboration method
3.1 Crystallinity and morphology of the films
3.2 Critical temperature measurements
3.4 Substrate influence on A15 properties
4. Achievements and future actions
3. Results
3.3 Microstructural properties of the films

9. Presence of A15 phase. XRD analysis & morphology
“Room-temperature” coatings
as-deposited
100 nm
Confirming presence of superconducting A15 phase (crystallite size  70 – 200 nm)
Absence of non-superconductive phases - NbSn2, Nb6Sn5, as well as Nb
after annealing
100 nm

10. after annealing Presence of A15 phase. XRD analysis & morphology
“Room-temperature” coatings
after annealing
Cracks after annealing
20 µm
All “room temperature” samples are evincing cracks after annealing.
Now coating recipe to overcome this problem is found !

11. Analysis of high temperature coatings
XRD
Confirming presence of superconducting A15 phase
With crystallite size  nm
Coating pressure 1*10-3 mbar
Coating temperature 710oC
SEM top view
FIB cross section
100 nm
200 nm Cu
Nb3Sn Pt
Dense, crack free grainy pattern
K. Ilyina

12. Outline of the presentation
1. Motivation of the work
2. Elaboration method
3.1 Crystallinity and morphology of the films
3.2 Critical temperature measurements
3.4 Substrate influence on A15 properties
4. Achievements and future actions
3. Results
3.3 Microstructural properties of the films

13. Magnetic moment vs. temperature at B = 12 Oe.
Tc measurements
Measurements are done using SQUID-VSM in collaboration with University of Genève.
Tconset
Best achieved Tc value is 16.5 K
Magnetic moment vs. temperature at B = 12 Oe.
The inset shows the temperature derivative of the magnetic moment as a function of the temperature

14. Tc vs Composition
Room temperature coatings process lead to higher critical temperature values no matter coating parameters
High temperature coating + annealing post coating
Could be a good combination by modulating temperature and duration
[2] A. Godeke. Supercond. Sci. Technol.,  19 (2006) R68-R80 
Tc values constantly lower for the films coated on copper substrates with respect to the bulk Nb3Sn values
Copper substrate influence? Stresses in the film??

15. Outline of the presentation
1. Motivation of the work
2. Elaboration method
3.1 Crystallinity and morphology of the films
3.2 Critical temperature measurements
3.4 Substrate influence on A15 properties
4. Achievements and future actions
3. Results
3.3 Microstructural properties of the films

16. Micro-strain sensitivity of the critical temperature
“Room-temperature” coatings+annealing
While uniform residual stress is released, micro strain remains significant.
Increase in micro strain in the films leads to the critical temperature depression, while dependence from tin content is not obvious
Calculated values from broadening of the diffraction line using Rietveld analysis.

17. Outline of the presentation
1. Motivation of the work
2. Elaboration method
3.1 Crystallinity and morphology of the films
3.2 Critical temperature measurements
3.4 Substrate influence on A15 properties
4. Achievements and future actions
3. Results
3.3 Microstructural properties of the films

18. Copper substrate could be a reason of Tc depression!!!
[3] R. Kampwirth ; J. Hafstrom ; C. Wu IEEE Transactions on Magnetics ( Volume: 13, Issue: 1, Jan 1977 )
Difference in thermal expansion coefficient between substrate
and Nb3Sn contribute to disordering effect.
Element
α (x 10-6) K-1 Cu
16.8 Ta
7.64 Nb
7.02
Nb3Sn
6.3
Nb buffer layer could improve Tc value

19. “Room temperature” coatings + annealing
Copper interdiffusion
“Room temperature” coatings + annealing Cu Nb Sn Pt
Nb3Sn
STRONG Cu interdiffusion within the coated layer
Not RF compatible Cu
1 µm
1 µm
1 µm
1 µm
Accelerating voltage of 30 kV and imaging current of 2.6 nA
1 µm
Nb3Sn Cu Pt Nb Sn
High temperature coatings
0.1 At%
Possibly RF compatible
XPS confirmed the presence of Cu in the layer

20. Possible solutions against copper and stress influence
Thin buffer/diffusion barrier layer (Ta, Nb)
Reducing disordering effect, preventing copper diffusion
Development of optimized annealing protocols
Stress reduction, Tc improvement

21. Outline of the presentation
1. Motivation of the work
2. Elaboration method
3.1 Crystallinity and morphology of the films
3.2 Critical temperature measurements
3.4 Substrate influence on A15 properties
4. Achievements and future actions
3. Results
3.3 Microstructural properties of the films

22. Target surface resistance 50 nOhm
Conclusions
Achievements:
Presented technology allow to synthesise films with desired composition
Presence of superconducting A15 phase is confirmed
Best Tc  16.5 K
There are recipes for both “room temperature” and high temperature coatings, allowing to produce crack-free surfaces
Established dependence of critical temperature on microstructural properties
Future action:
Get rid of copper in Nb3Sn layer (buffer layer)
Improvement of the superconducting properties of the films
Continue study for another A15 material – V3Si
Test of the RF properties of the film
Target surface resistance 50 nOhm
By courtesy of Sarah Aull

23. Thank you for your attention!