This document summarizes research into electropolishing niobium cavities using an ionic liquid electrolyte without fluorine. Key findings include:
1) A mixture of choline chloride and urea was found to successfully etch niobium without fluorine and provide good results on test samples and cavities.
2) Adding 30g/L of sulfamic acid to the ionic liquid gave the best surface quality with minimal roughness and defects.
3) Tests on 6GHz cavities showed promising results, though non-uniform heating remained a challenge, and different cathode and flux configurations are still being optimized.
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Rampazzo - Electropolishing of niobium 6 GHz cavities in choline chloride – urea melt
1. V.Rampazzo Superconductivity Laboratory Legnaro National Laboratories National Institute of Nuclear Physics, Italy Electropolishing of Niobium 6GHz cavities inCholine chloride – Urea melt 2010
2. Goals Find solution/melt recipe for electropolishing Nb without using F—-ions Put this recipe on application to 6GHz cavities 2
3. Electrochemistry and properties of Nb Nb -> Nb3+ +3e- E0 = -1.1 V Nb -> Nb5+ +5e- E0 = -0,96 V Perniobates 2Nb + 5Cl2 -> 2NbCl5 NbCl5 + 4Н2О -> 5HCl + Н3NbO4 Niobium acid A(+): Niobate K(-): 3
4. IonicLiquid: Composition A Ionicliquidis a mixtureoftwosalt, thatdissolvesitself at a temperature lowerthan the fusionpointof single salt Heating the salts, those dissociate itselfintoions and assume the liquid state After the formationof IL, thisremainliquidwhencooled down 4
5. Structural formulas of mixture We have found mixture which can etch Nb without using F--ions and gives good result on application to cavities 5 Urea CholineChloride Sulfamic acid
9. Surface quality analyze with profilometer 9 where m – quantity of measurements which where taken to calculation Scan 1 Scan 6 Scan 2 Scan 5 Scan 3 Scan 4
12. Changing quantities of melt compounds we have found correct recipe Because of high relatively power of process solution gets big heat. 12
13. Ra – f(C(SA)) Roughness: classical EP versus Ionic liquid EP 13 Adding 30g/l Sulfamic acid in 4:1 Choline Chloride Urea melt gives possibility to obtain brightness surface, without spots and pitting on sides of the sample The best result of IL is comparable with the result of classical EP The back roughness is the same of the front : good current distribution around the sample
18. EP on 6GHz cavities After the goodresult on samples, we start toapply the EP on real 6GHz cavities Cathodes, flux system, concentration, newadditionwerestudiedtofind the best EP We are stillworking… 18
19. Improvement road Toimprove the EP westudy some possibilities: Alternative tosulfammicacid Differentflux inside cavity Differentorientationofcavity 19
20.
21. High activity formation of cathode gas brings to saturation of electrolyte with H2
22. IL comes from flanges and goes out from the cathode.
34. Alternative tosulfammic acid The best result on sampleswerereachedwithsulfammic acid, butcavityisquitedifferentenvironment Wechecked the performancesofvariousregulatorcontaining the group (–NHx) on samples 30
36. AmmoniumPersulfate The addidionofAmmoniumPersulfatedecreases the high initialvoltagenecessarytodisrupt the oxidefilm Butthiscompoundincreases the roughness and pitting Possibility: can Sulfammic acid and AmmoniumPersulfate work together? 32
37. AmmoniumPersulfate and Sulfammic Acid On samples, agood compromise is the proportion 30 g/L ofSulfammic acid and 2.5 g/L ofAmmoniumPersulfate On cavity, the best concentrationwerefound mixing 1.5 g/L ofAmmoniumPersulfate and 30 g/L ofSulfammic Acid 33
46. Best results obtained in current density 0,33 A/cm2. This current density provides temperature equilibrium in range 150C, and stable yellow viscose film around the anode and “protect” from oxidizing. Distribution of current is similar on both sides which may give good surface quality inside cell and cutoff parts of the cell. 42 Ra – f(i)
49. Previous slide illustrates necessity of dissolved Nb ions inside the electrolyte. 45 Ra–f(C(NbDissolved))
50. From samples to Cavities… We tried to work in two geometrical performances: horizontal and vertical Horizontal EP didn’t give content results because of very fast temperature rising, after 1 minute temperature inside the cavity was more 190C which brought to degradation of electrolyte and to formation white viscous mass. In that places was done note a polishing but oxidizing. Cutoff part of cavity has enough good view. 46
51. Cavity after EP in IL: horizontal type 47 Cutoff part Cell part Cell part
52. Dissolving speed 6,2 um/min (in our laboratory on cavities in classical EP we have 0,5 um/min) 48