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EP - Tutorial

EP - Tutorial. Up to now, Electropolishing is the surface treatment that allows to reach the highest accelerating gradients in SC RF cavities E acc > 42 MV/m on 1.3 GHz Tesla shape cavity E acc ~ 47 MV/m on 1.3 GHz Low Loss cavity E acc ~ 52 MV/m on 1.3 GHz Re-entrant cavity.

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EP - Tutorial

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  1. EP - Tutorial Up to now, Electropolishing is the surface treatment that allows to reach the highest accelerating gradients in SC RF cavities Eacc > 42 MV/m on 1.3 GHz Tesla shape cavity Eacc ~ 47 MV/m on 1.3 GHz Low Loss cavity Eacc ~ 52 MV/m on 1.3 GHz Re-entrant cavity

  2. INTRODUCTION1) MACROLEVELLING2) MICROLEVELLING/INTERFACE LAYER/BRIGHTENINGINTERMISSION3) AGING4) SULFUR ISSUES5) MODELING OF EPCONCLUSION OUTLINES

  3. EP : LARGE DISPERSION OF ACHIEVED Eacc • WHY ??? … => R&D on samples + cavities Lutz Lilje

  4. e- Nb5+ e- Nb5+ NO3- Nb0 NbF5 e- NbF5 Nb0 2H+ + 2e- H2 Nb5+ A bit of (electro)polishing Etching = oxidation / dissolution of Nb: Chemical polishing (NO3) vs Electropolishing  6 Nb + 10 HNO33 Nb2O5 + 10 NO + 5 H2O Nb2O5 + 10 HF  2 NbF5 + 5 H2O   Nb Nb5+ + 5 e-Nb2O5 Nb2O5 + 10 HF  2 NbF5 + 5 H2O

  5. EP = MACROPOLISHING + MICROPOLISHING Electrolyte • Macropolishing => defects ≥ ~ 1 µm • Origin = viscous layer (Wand/or diffusion limitation) • Micropolishing/brightening => defects some 0.1 µm • Origin =metal/solution interface, several pos. mechanisms • Nanopolishing (nm)? => N.A. 1500 nm 1 is BCP, 1 is EP, which one ? 

  6. PURITY & CRYSTALLOGRAPHIC STATE • Opt. Microscopy ≠ BCPs

  7. Electropolishing Active dissolution (no plateau) I Passivation IPass<< IEP J~ 50-100 mA/cm2 V FIRST AID KIT : I=f(V) curves • Active dissolution : the metal is soluble (Tafel Law) • Passivition : an insulating oxide or compound builds up on the surface builds up in dilute acid solutions • EP : there is a surface high resistivity layer, but it is more conducting (ions) compare to passivation builds up in concentrated solutions

  8. I=f(V) curves… 1 • AB : active dissolution • BC : instability (resistive layer builds up) • CD : plateau = polishing (diffusion limited) • DE : slow O2 evolution + pitting • EF : high O2 evolution + polishing (pitting mechanism) (NB: low faradic yield in this case) j mA/cm2 F B E 50-100 A D C V

  9. I=f(V) curves… 2 • Plateau => diffusion limitation • Necessary but not sufficient Optimum V ?

  10. I Vopt Vopt V FROM SAMPLES TO CAVITY The I=f(V) curve depends on the characteristics of the «Circuit» (Geometry, field repartition, stirring…) Increasing cathode-anode distance ↑ => The I=f(V) curve must be determined in the real situation !!!!!!

  11. Samples Aluminum cathode Rotanode set-up 0,3 EP 4V-9V mixture 1.5 Volt 1,2 0,2 Nbdiss (mg) ~10 V niobium removal rate µm/min 0,8 ~10 V 0,1 0,4 ~16 V 0 30 50 70 90 0 20 40 60 80 100 120 110 distance mm distance mm Influence of distance/etching rate Ohmic losses (soln) Cell impedance What is the voltage here? How was the surface roughness? no plateau V @ plateau

  12. 1st part : MACROLEVELING

  13. I V V/I Vopt V VOLTAGE OPTIMIZATION EPolishing = plateau in the I=f(V) curve + diffusion limitation Literature says* : • “optimum V”  = @ the highest cell impedance ~ max V/I (1st order criteria) *e.g. [Teggart]

  14. MACROPOLISHING : EP Cell impedance Ni in HClO4 / CH3COOH • V/I ~ l Z l [Teggart] • l Z l = f (composition) => Optimum V changes with composition and/or time !!!!!! (See V. Palmieri papers)

  15. MACROPOLISHING : Cell impedance and Wagner number • Primary current distribution = geometry • Secondary current distribution = geometry + charge transfer overpotential • Tertiary cur.dist.: + Concentration overpotential…. (Nernst diffn) • 4ry : Hydrodynamics… The current distribution is not only influenced by geometry… […] tends to reduce the rate of anodic levelling […]. A measure of the relative importance of charge transfer overvoltage is the Wagner number, Wa = (d/di)/ree0) where d/di is the slope of the current voltage curve, re the electrolyte resistivity, e0 the initial profile height. D. Landolt, “Fundamentals of electropolishing” (~ usually negligible in EP…) (~ negligible in usual EP, not in our conditions…)

  16. 2nd part : MICROLEVELINGINTERFACE LAYERBRIGHTENING

  17. I) Metal Ions (Mn+aq) II) Acceptor anion (A) III) H2O PLATEAU AND DIFFUSION LIMITATION… • Microlevelling : only possible under transport control • At the limiting current the current distribution is governed solely by mass transport. Transport limiting species : 3 cases Concentration Cs Cs [Landolt] Distance from electrode

  18. SALT FILM PRECIPITATION (case I) Metal A- Mn+ Salt film formation Mn+ diffusion toward solution Equilibrium : Dissolution rate = formation rate Salt film [Landolt] N.B. larger plateaus than in case II

  19. H OH2 OSO3H O H O M ELECTROLYTE O OSO3 M O OH2 OH2 F F H OH2 F Metal O M External part of the film,  fluid OH2 F Compact film. Very high [A-/O2-] Anions Cations selectiveselective CONTAMINATED OXIDE SCENARIO/ brightening [T.P Hoar] Low electronic conduction High ionic conduction Analogous to a bipolar membrane

  20. NbO2 NbO3- PO43- SO42- Anion incorporation in Nb oxide F- always found in oxides… Same geometry (Td) Found after BCP Found after EP • Slow growing rate of oxide (better packed ?) • Better subsequent corrosion resistance

  21. POROUS OXYDE AND OSCILLATIONS ←Nb2O5 observed upon anodization and in presence of HF and H2SO4 Si/HF→ oxide: ~ 30 nm porous + ~ 5nm dense [ Oscillations = porous oxide + synchronization of pores nucleation

  22. OSCILLATIONS ARE NOT MANDATORY Current oscillation and best EP finishing surface… Best Finishing [KEK, Kneisel]

  23. I (A) 1 0 5 Volts 15 10 I(V) CURVES in 1:9 MIXTURES •  Plateau • Viscous layer : visible • Oscillations… => porous oxide (?) => should work ! Actually it works… > 40 mv/M !

  24. Intermission

  25. I (A) 0.7 1:9:1 20°C EP 1:9:1 0.4 0 5 15 Volts 10 I PLATEAU :necessary but not sufficient… Adding H20 : • J↓ , gloss ↓ ↓ … • H2O is not the limiting species ! : Opt : x 1000 SEM ↑ Nb/HF(+ H2SO4)

  26. [HF] EFFECT HF ↑ I (A) 0.9 2 0.5 1 0 0 5 15 10 Volts Volts 5 10 15 -  plateau disappear progressively with [HF] ↑. 2 - I, etching rate ↑. 1 - EP seems to be controlled by F-…!? 0 15 Volts 5 10

  27. 3th part : aging

  28. Aging effect on samples’ surface EP 1-9 14 V 4mm 4mm ~ 7 g/l of Nb inside solution 4mm

  29. Gloss vs time 1Vol-9Vol , 8 Volts : 500 450 Brillance B60 maxi 400 350 mesurées 300 60 et B 250 20 200 Brillances B 150 100 50 0 0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000 6600 7200 Durée t d'électropolissage ( min ) Same mixture, ≠ voltage Not the same time, ± the same mass of dissolved Nb (~ 7g/l) • Does the Nb content of the solution matters ? • (EP bath should be changed more often ?)

  30. EP Mixture : 0.5-9 • Gloss can be further improved with a fresh bath • S produced into low [HF] solutions

  31. High HF content Nb content ~ 28g/l !!! Nb content ~ 23g/l (New Nb Sample) Fresh sample • Aging = dramatically changed with [HF]↑ • Brightening still efficient @ low etching rate • S not found • But… not polishing, active dissolution ! (no plateau !)

  32. EVOLUTIONof F- with time Ionic Chromatography : 3 ≠ species . (NMR works too) HF + H2SO4 ↔ HSO3F + H2O N.B. fluorosulfonic acid was used @ KeK to refresh mixture.

  33. F- reservoir mecanism ? HSO3F ~> F- “reservoir” ? HF Evaporation

  34. Mechanism issues • => find out the Fluorine role • Is it the limiting species ? (porous film => no !) • Does it improve Nb5+ solubility ? • => find out a way to maintain fluorine content • NaF, low temperature… • => Find out a way to monitor F- • Chromatography (diluted samples, all ions) • NMR (samples, no dilution, only F- or H+) • Abs Spectroscopy UV/Vis (effective on HF + H2SO4 ,but bubbles issues) • Resistivity measurement (expensive, but effective on HF + H2SO4 ) • ISE (dilution or works only with free F-) • …?

  35. 4th part : Sulfur issues

  36. Hints about Sulfur • ↓[H2SO4] => ↓ sulfur production but ↑ Al corrosion

  37. Hints about Sulfur S=21 g S=114 g • [S] with V >> [S] without V • ↓ ↓ [HF] => ↑ ↑ sulfur production • no observed variation in [S] with V (V~ 4=> 20 Volts)

  38. Sulfur issues • => Improve rinsing ! • Ethanol, organic solvents • Surfactant solution + US • Keep [F-] high • Lower [H2SO4] • viscosity/ acidity issues • Other viscous buffer…

  39. 5th part : Modeling

  40. Modeling DP=0 FEMLAB 2D ρ fluid ν fluid D_Nb V V= 0 Volt C_Nb=0 Considered case: Coupling between - Nernst-Planck (Convection, Diffusion, Electromigration) - Navier-Stokes (incompressible fluid)

  41.  a viscous layer ? • dab=2 cm, • V = 5 Volts • Viscous layer forms only if DNb is high • Electromigration is negligible • Fluid flux ~ 5 mm.s-1 => viscous layer thickness ↓ (~10%) • N.B. : consistent with a porous interfacial film mechanism

  42. Let’s make it more complicate… Cavity geometry + gravity … • If you want to get a uniform viscous layer • Density must remain low • Viscosity must be high • N.B.: be careful with physics…

  43. Modeling issues • => find out what conditions favor viscous layer • => find out what disturbs viscous layer • => play with parameters like viscosity, composition, EXm reactions • => getting into more complex situation : geometry, motion, hydrodynamics => Intuitions ! • Eventually : correlate with experimental facts

  44. Conclusion • R&D on EP is necessary ! • It can be done with relatively low cost on small samples. • It will save a lot of time and money compare to the same experiments conducted on a 9 cell facility…

  45. What can be done quickly on samples • Correlate degradation and actual [F-] ? • Add Nb5+ in the 1-9 EP soln • If I ↓ => limitation = [Nb]sat • Add F- Salt (NaF) • viscosity/plateau • lifetime • Other viscous buffer • ≠ temperature • Impedance measurements (Saclay)

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