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Rampazzo - Electropolishing of niobium 6 GHz cavities in choline chloride – urea melt
 

Rampazzo - Electropolishing of niobium 6 GHz cavities in choline chloride – urea melt

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Electropolishing of 6GHz cavities by ionic liquids (Vanessa Rampazzo - 20')
Speaker: Vanessa Rampazzo - Legnaro National Laboratories of INFN | Duration: 20 min.
Abstract
The electropolishing of niobium using RTIL without fluorine shows good surface improvements, and the original recipe based on Urea and Choline Chloride is under study for application on 6 GHz niobium cavities. The goal is to obtain a uniform electrical field on the internal surface, despite of the big differencies in distances from the cathode inserted into the cavity. Morevoer, the electrical power injected into the cavity degrades the ionic liquid, if this isn't efficiently cooled down. All of this pratical problems are partially solved adding various special reagents in IL, that raise the uniformity in electrical field and decrease the working current of electropolishing. Another goal is the use of a continuos flux of liquid, that flows through the cavity.

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    Rampazzo - Electropolishing of niobium 6 GHz cavities in choline chloride – urea melt Rampazzo - Electropolishing of niobium 6 GHz cavities in choline chloride – urea melt Presentation Transcript

    • V.Rampazzo
      Superconductivity Laboratory
      Legnaro National Laboratories
      National Institute of Nuclear Physics, Italy
      Electropolishing of Niobium 6GHz cavities inCholine chloride – Urea melt
      2010
    • Goals
      Find solution/melt recipe for electropolishing Nb without using F—-ions
      Put this recipe on application to 6GHz cavities
      2
    • 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
    • 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
    • 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
    • Investigation samples system
      Holder
      Cathode
      Thermocouple
      Anode (Sample)
      Stirrer
      6
      Electrolyses time – 5 min
    • We were using our automatic method to find correct electrical characteristic of electropolishing
      EP plateau
      7
    • Electropolishing of samples
      Front
      Back
      8
    • 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
    • Surface quality as function ofsulfamic acid concentration
      10
    • Surface quality as function of sulfamic acid concentration
      11
    • Changing quantities of melt compounds we have found correct recipe
      Because of high relatively power of process solution gets big heat.
      12
    • 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
    • Surface quality as a function of process time
      14
    • Results in different treatment duration (min)
      15
      Front side
      5
      60
      30
      20
      10
      5
      60
      30
      20
      10
      Back side
    • Surface characterization with profilometer
      Classical EP, 10min
      Raw
      IL EP, 60min
      IL EP, 10min
      16
    • 17
      EP samples in process…
    • 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
    • Improvement road
      Toimprove the EP westudy some possibilities:
      Alternative tosulfammicacid
      Differentflux inside cavity
      Differentorientationofcavity
      19
    • Vertical EP: holedcathode
      • Vertical EP
      • High activity formation of cathode gas brings to saturation of electrolyte with H2
      • IL comes from flanges and goes out from the cathode.
      • The cathode makes from tube 8mm in diameter with holes.
      • During the pumping electrolyte goes through the holes inside the tube
      20
    • 21
      Cavity 6 GHz EP system with IL
      Holed cathode
      Output flange
      Input flange
      Solution collector
      Pump
    • Vertical EP: holedcathode
      22
      Dopo
      Prima
      1:4 CholineChloride-Urea
      Sulfammicacid: 30 g/L
      T: 150°C
      0,3 -0,4 A/cm2
    • Q-factorresult
      23
    • About electrical field distribution
      24
      • Tobalance the differentdistancebetweencavity and cathode, thiswereshaped in variousmodes
      Holedcathode
      Two part cathode
    • Differentflux
      Brokencathodes: the shapeofcathodeschangedtoget more uniformity on IL flux
      25
    • Twopossibilitiesofflux
      26
      OUT
      OUT
      IN
      IN
      Fromcathodestoflanges
      Fromflangestocathodes
    • New shapedcathodes
      27
    • Sulfamic Acid: fromflangestocathodes
      28
      Solution: 1:4 CholineChloride – Urea,
      Sulfammic Acid :30 g/L
      T:120-160°C
      The best surface quality appeared on bottom cutoff part.
    • Sulfammic Acid:fromcathodestoflanges
      The oppositeconfigurationbrings a lotofbubbles and the cavityweren’t electropolished
      29
    • Alternative tosulfammic acid
      The best result on sampleswerereachedwithsulfammic acid, butcavityisquitedifferentenvironment
      Wechecked the performancesofvariousregulatorcontaining the group (–NHx) on samples
      30
    • Comparisonbetweenregulator (–NH4)
      31
    • AmmoniumPersulfate
      The addidionofAmmoniumPersulfatedecreases the high initialvoltagenecessarytodisrupt the oxidefilm
      Butthiscompoundincreases the roughness and pitting
      Possibility: can Sulfammic acid and AmmoniumPersulfate work together?
      32
    • 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
    • Vertical EP: PA+SA
      Vertical
      ChCl:Urea 1:4
      c(SA) = 30 g/l
      c(PS) = 1.5 g/l
      Distancebetweencathodes: 10 mm
      Bright, withfluxline
      34
    • Vertical EP: PA+SA
      Vertical
      ChCl:Urea 1:4
      c(SA) = 30 g/l
      c(PS) = 1.5 g/l
      Some irregularsurface
      Distance: 5 mm
      35
    • Horizontal EP: PA+SA
      Horizontal
      ChCl:Urea 1:4
      c(SA) = 30 g/l
      c(PS) = 1.5 g/l
      Distance from cathodes: 4 mm
      Some passivation zone
      36
    • In future
      Set up offluxsistemof EP
      CalculationofQ-factorofILsEpcavities
      Test toget the result on 1.5 /1.3 GHz
      37
    • Advantages and disadvantages
      38
    • Thankstoattention!
      39
    • Surface quality as function of current density
      40
    • Surface quality as function of current density
      41
    • Best results obtained in current density 0,33 A/cm2.
      This current density provides temperature equilibrium in range 150C, 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)
    • Surface quality as a function of quantity Nb-ions in the melt
      43
    • Surface quality as a function of quantity Nb-ions in the melt
      44
    • Previous slide illustrates necessity of dissolved Nb ions inside the electrolyte.
      45
      Ra–f(C(NbDissolved))
    • 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 190C 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
    • Cavity after EP in IL: horizontal type
      47
      Cutoff part
      Cell part
      Cell part
    • Dissolving speed 6,2 um/min
      (in our laboratory on cavities in classical EP we have 0,5 um/min)
      48
    • Conventions
      49