1. S.M. Deambrosis*^, G. Keppel*, N. Pretto^, V. Rampazzo*, R.G. Sharma°, D. Tonini * and V. Palmieri*^ Padova University, Material Science Dept * INFN - Legnaro National Labs ^ Padua University, Science faculty, Material Science Dept ° Interuniversity Accelerator Center, New Delhi Nb 3 Sn by Liquid Tin Diffusion

2. Nb 3 Sn by liquid Sn diffusion 1) Theory 2) Literature review 3) Technique choice reasons 4) Method 5) Work in progress 6) Conclusions Vapor Sn diffusion Liquid Sn diffusion Used system “1 Step” process “2 Steps” process “Hybrid” process

3. R BCS If T < T c / 2 Empirically, R res is found to be dependent on  n too. Theory For low rf losses, a high T C value is not sufficient A metallic behaviour in the normal state is mandatory

4. Nomogram Theory RBCS Ideal R BCS ~ 1 nΩ At T = 4.2 K, f = 500 MHz, s = 4, R BCS depends on Δ and ρ n ~ 10 μΩcm

5. Literature review Literature Many papers of different authors with different aims: Nb 3 Sn by CVD and PVD techniques to compare bulk and film properties Nb 3 Sn by bronze process for high field Superconducting Magnets Nb 3 Sn RF application: Wuppertal

6. Wuppertal: Nb 3 Sn cavity (1.5 GHz) obtained trough Sn vapour phase diffusion (’90s) Literature

7. Vapor Sn Diffusion Technique Choice Reasons 1)Cavity manufactoring 2)Formation of nucleation centers of Nb 3 Sn (Nb Surface Anodization + SnCl 2 Treatment) 3)Nb 3 Sn film growth in a Sn atmosphere (T = 1050-1250°C, t = dozens of h, p(Sn) ~ 10 -3 mbar) 4)Cool down and unwanted phases Chemical removal (anodizaton + HF 48%) Laboratory Procedure Heating system Accelerating structure Sn source Sn source heater Pumping system

8. Liquid solute diffusion technique Technique Choice Reasons

9. Liquid Sn Diffusion? Bulk Nb substrate dipping in a liquid Tin bath Sample Annealing No nucleation sites on Nb are required Fast growth of Nb 3 Sn layer Deasirable uniform thickness Technique Choice Reasons

10. Used System Method: used system

11. Nb 3 Sn: Phase Diagram Nb 3 Sn <T c phases 930°C Method: used system

12. To Summarise ●Liquid solute diffusion technique choice ●Working T > 930 °C

13. 1)Bulk Nb Substrate chemical cleaning (10 min in a 1:1:2 solution) 2)Substarte fixing to feedtrough, vacuum and T reaching (1 day) 3)Substrate thermalization (30 min - 1 h) 4)Dipping (few min - 2 h) 5)Annealing above the Sn bath without opening the chamber (some h) 6)Residual Sn Chemical removal trials Laboratory Procedure “1 Step” Process Method: “1 step” process

14. Nb 3 Sn photo Residual Sn Sn drop Method: “1 step” process

15. SEM Image Nb Nb 3 Sn Method: “1 step” process Process T = 1000°C Dipping t = 120’ Annealing t = 14h Post annealing: 5h at 500°C

16. XRD spectrum Method: “1 step” process Process T = 1000°C, Dipping t = 30’, Annealing t = 10h

17. EMPA Analysis Nb 3 Sn n°16 Dipping T = 1000 °C Dipping t = 120 min Annealing T = 1000 °C Annealing t = 14 h Post annealing T = 500 °C Post annelaing t = 5 h Method: “1 step” process

18. A Superconductive Transition Curve Nb 3 Sn n°16: 1000°C; 120’+14h+post annealing 500°Cx5h Method: “1 step” process

19. Mechanical polishing Chemical polishing 3. Q Factor Measurement Nb 3 Sn film obtainment 6 GHz Cavities 1. Spinning Technique Spinning Technique 2. Surface Treatments Method: “1 step” process

20. T = 1025°C t dipping = 15 min, t annealing = 15 h HCl 37%, t = 10 min, T = 85°C Nb 3 Sn film obtainment Film production: Chemical treatment: Method: “1 step” process

21. Q Factor Measurement Nb 3 Sn Method: “1 step” process

22. A Nb 3 Sn 6 GHz Cavity Method: “1 step” process

23. Nb 3 Sn with good superconductive properties Residual Sn traces on the sample surface To Summarise + T c = 16,9 K  T c = 0,2 K - Sn rich Phases Presence - Method: “1 step” process

24. “2 Steps” Process Method: “2 steps” process 1)Bulk Nb Substrate chemical cleaning (10 min in a 1:1:2 solution) 2)Substarte fixing to the feedtrough, vacuum and T reaching (1 day) 3)Substrate thermalization (30 min - 1 h) 4)Dipping (few min - 2 h) 5)System opening to remove Sn bath, vacuum and T reaching (1 day) 6)Sample annealing without Sn vapor (some h) 7)Growth film chemical treatment Laboratory Procedure

25. Nb 3 Sn photo Method: “2 steps” process

26. SEM Images Method: “2 steps” process Proc T = 1025°C, Dipp t = 15’, Ann t = 15h Proc T = 1025°C, Dipp t = 5’, Ann t = 20h

27. XRD spectra Method: “2 steps” process Proc T = 1025°C, Dipp t = 5’, Ann t = 20h Proc T = 1025°C, Dipp t = 5’, Ann t = 10h

28. A Superconductive Transition Curve Method: “2 steps” process Proc T = 1025°C, Dipp t = 5’, Ann t = 20h T c (Nb 3 Sn) = 14,9 K  T c (Nb 3 Sn) = 0,43 K

29. Growth film chemical treatment (HCl) Method: “2 steps” process

30. T c and  T c vs T HCl Method: “2 steps” process

31. Worst Nb 3 Sn film superconductive properties + - To Summarise No Residual Sn traces on the sample surface T c = 15,2 K,  T c = 0,5 K Sn rich Phases Presence - Method: “2 steps” process HCl chemical treatment deteriorates the growth film -

32. “Hybrid” Process Method: “Hybrid” process Laboratory Procedure 1)Bulk Nb Substrate chemical cleaning (10 min in a 1:1:2 solution) 2)Substarte fixing to the feedtrough, vacuum and T reaching (1 day) 3)Substrate thermalization (30 min - 1 h) 4)Dipping (few min - 2 h) 5)Sample annealing with Sn vapor (some h) 6)System opening to remove Sn bath, vacuum and T reaching (1 day) 7)Sample annealing without Sn vapor (some h)

33. Method: “Hybrid” process Nb 3 Sn photo

34. XRD spectrum Method: “Hybrid” process Proc T = 975°C, Dipp t = 30’, Ann (Sn) t = 2h, Ann t = 5h

35. A Superconductive Transition Curve Method: “Hybrid” process Proc T = 975°C, Dipp t = 30’, Ann (Sn) t = 2h, Ann t = 5h T c (Nb 3 Sn) = 16,6 K  T c (Nb 3 Sn) = 0,28 K

36. Good Nb 3 Sn film superconductive properties + No Residual Sn traces on the sample surface T c = 16,5 K,  T c = 0,3 K No Sn rich Phases To Summarise + + Method: “Hybrid” process

37. Work in progress Two furnaces system to avoid air contamination of the superconducting thin film while opening the vacuum system Use of the best results to coat 6 GHz Nb cavities for a Nb 3 Sn RF properties sistematic testing Use of a different experimental method to prepare Nb 3 Sn: multilayer obtained altermatively depositing Nb and Sn

38. Conclusions Liquid solute diffusion technique (working T > 930 °C) Three different processes: “1 step” “2 steps” “Hybrid” trying to optimize T and t Finally: ◊ Good superconducting properties ◊ No Sn ◊ No Sn rich Phases

39. End