High purity niobium (Nb) is used to make superconducting radio frequency (SRF) cavities. To improve the cavity performance it is necessary to reduce the surface resistance as much as possible. To achieve this goal each Nb cavity undergoes to a series of successive treatments (mechanical polishing, buffered chemical polishing, electropolishing, annealing). Choline Chloride-based ionic liquids are considered as a new class of electrolytes. Comparing to traditional ionic liquids, choline chloride-based ionic liquids are low toxic, biodegradable, and the price is generally low, which make them more and more attractive in green chemistry and industrial chemistry. Since it was found that, the mixture of Choline Chloride and Urea can polish Niobium every year, this solution has been improved. Was investigated the optimal ratio of Choline Chloride and Urea, studied the effect of additives, temperature and current density. When it was found suitable parameters for the polishing of planar samples, we have started to implement it to the polishing of 6 GHz Niobium cavities.

1. LABORATORI NAZIONALI DI LEGNARO
Progress on Fluorine-free
Electropolishing
Vlada Pastushenko,
F. Stivanello, V. Palmieri
6thWorkshop on "Thin Films and New Ideas for SRF", Legnaro (PD) Italy, October 6-8 2014

2. Outline
 Ionic liquids, their properties and application.
 Electropolishing of Niobium in Fluorine-free solutions (first steps).
 Study the parameters of Electropolishing in Ionic Liquids.
 Procedure of standard electropolishing 6 GHz cavities.
 Development the systems for Electropolishing in Ionic Liquids:
 Different shapes of the cathode;
 Vertical system for EP 6 GHz cavity;
 Horizontal system for EP 6 GHz cavity.
 Q-factor result.

3. Deep Eutectic Solvent or DES*
 DES is a type of ionic solvent with special properties composed of
a mixture which forms a eutectic with a melting point much lower
than either of the individual components.
 The deep eutectic phenomenon was first described in 2003 for a
mixture of choline chloride and urea in a 1:2 mole ratio,
respectively.
 Choline chloride has a melting point of 302 °C and that of urea is
133 °C. The eutectic mixture melts lower than 100 ˚C.
*Andrew Abbott; John Barron, Karl Ryder and David Wilson (2007). "Eutectic-Based Ionic Liquids with Metal-Containing
Anions and Cations". Chem. Eur. J. 13: 6495– 6501. doi:10.1002/chem.200601738.

4. Four types of eutectic solvents:
Type I Eutectic metal salt + organic salt (e.g. ZnCl2 + choline chloride)
Type II Eutectic metal salt hydrate + organic salt (e.g. CoCl2*6H2O + choline chloride)
Type III Eutectic organic salt + hydrogen bond donor (e.g. choline chloride + urea)
Type IV Eutectic metal salt (hydrate) + hydrogen bond donor (e.g. ZnCl2 + urea)

5. Choline Chloride – based Ionic liquids
Room
Temperature 120˚C

6. Conductivity and viscosity of a Urea : Choline chloride
mixture (2:1) as a function of temperature*
*A. P. Abbott, G. Capper, D. L. Davies, R. Rasheed and V. Tambyrajah “Novel Solvent Properties of Choline Chloride/
Urea Mixtures” Chem. Commun, 2003, 70-71.

7. Electropolishing of Niobium
in
Fluorine-free solutions
(first steps)

8. Organic solvents Ionic Liquids
Acetonitrile
 tetramethylammonium perchlorate
 tetrabutylammonium perchlorate
 potassium perchlorate
Dimethylformamide
 tetrabutylammonium perchlorate
 potassium perchlorate
 Urea
 Choline Chloride : Urea (1:2)
 tetramethylammonium perchlorate
 tartaric acid
 oxalic acid
 ammonium chloride
Choline Chloride : Urea (1:3)
 ammonium chloride
 potassium tartrate
 Choline Chloride : Urea (1:4)
 ammonium chloride
Choline Chloride: Ethylene Glycol (1:2)
 ammonium chloride
M. Ceccato, “Elettropulitura del Niobio con Elettroliti a
base di Liquidi Ionici”, Master Thesis (2007) University
of Padua.

9. Optical microscopy analysis

10. SEM analysis

11. Sulfamic acid – first success*
*Rupp V. "Electropolishing of Niobium 6 GHz RF cavities in fluorine-free electrolyte", Master Thesis, 2009.

12. Influence of S and N containing compounds

13. Surface maps of different treatments
raw Nb
10 min IL
10 min classical EP
60 min IL

14. Choline Chloride and Urea (1:4)
with Sulfamic Acid (SA) and Ammonium Sulfamate (AmS)
+30 g/l SA
+50 g/l SA +75 g/l AmS
+114 g/L AmS
+ 97 g/l SA

15. The Best parameters for EP Nb samples
Choline Chloride : Urea 1 : 4
Sulfamic acid, g/l 97
Material cathode Nb
Temperature, °C Higher then 120
Current density, A/cm2 0,3

16. Procedure of standard electropolishing
6 GHz cavities

17. Cavity kit appearance
1
4
2
3
1 – cavity; 2 – cathode; 3 – up flange; 4 – bottom flange

18. Stand for BCP and EP
1 – cavity;
2 – anode contacts;
3 – cathode contact;
Blue indicator – direct flow,
red – indirect.

19. EP result obtained in 6GHz cavity

20. Schematic diagram of an anodic polarization

21. Automatic software for controlling potential.

22. System for Electropolishing
in Ionic Liquids

23. Electropolishing in Ionic Liquid (preliminary tests)

24. Different cathode
performance

25. “Full holed” cathode
Cut-off
Cell

26. “Center holed” cathode
Cut-off
Cell

27. “Two parts” cathode
1
2
3
4
5
1 – cathode; 2 – flange; 3 – surface of cathode which has to be
covered with Teflon ribbon; 4 – cavity; 5 – tube junction

28. Vertical system for EP 6GHz
cavities in Ionic liquids

29. Scheme of flux direction ”inside cathode” 6GHz cavities treatment
procedure IL EP process

30. Cavity after Vertical EP

31. Horizontal system for EP
6GHz cavities in Ionic liquids

32. Half cavity polished in horizontal position
SEM Analysis
Solution - 1:4 Choline :Urea,
Sulfammic Acid - 1M/l;
T- 120°C

33. Horizontal system for EP 6 GHz cavity*
*O. Malkova, “Study of the Electropolishing of 6GHz Niobium cavity in ionic liquids”, Master Thesis, 2013.

34. Surface inside cavity after EP

35. Modified system for EP in Ionic liquids

36. Electropolishing of the Niobium: result after first try
(60min)
Iris
Cell

37. Electropolishing of the Niobium: result after 6 hours

38. Q-factor result

40. Summary
Choline Chloride - urea based Ionic liquids are a possible
alternative to the hazardous standard electropolishing electrolyte
based on hydrofluoric and sulphuric acid.
The best solution that we found for Electropolishing of Niobium is
Choline Chloride with Urea in ratio 1 to 4 with 1 M/l Sulfamic acid.
The electropolishing in horizontal system give us shiny surface
without pitting and damages and the result is reproducible.

41. Thank you for attention!