V.Rampazzo Superconductivity Laboratory Legnaro National Laboratories National Institute of Nuclear Physics, Italy Electropolishing of Niobium 6GHz cavities in Choline chloride – Urea melt 2010
Rampazzo - Electropolishing of niobium 6 GHz cavities in choline chloride – urea melt
Jun 10, 2015
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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.
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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Transcript
V.RampazzoSuperconductivity LaboratoryLegnaro National LaboratoriesNational Institute of Nuclear Physics, Italy
Electropolishing of Niobium 6GHz cavities in
Choline 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
Perniobates
Niobate
Niobium acid
2Nb + 5Cl2 → 2NbCl5
Nb → Nb3+ +3e- E0 = -1.1 V
NbCl5 + 4Н2О → 5HCl + Н3NbO4
A(+): E0, V Reaction
+0,8 4OH-→2H2O+O2+4e
+1,35 2Cl-→Cl2+2e
Organics oxidation
K(-): E0, V Reaction
0,0 ROH+e→RO-+H
-0,4 2H2O+O2+4e→4OH-
-1,1 Nb3+ +3e- → Nb
RNH2+e→RNH-+H
Nb → Nb5+ +5e- E0 = -0,96 V3
Ionic Liquid: Composition
A Ionic liquid is a mixture of two salt, that dissolves itself at a temperature lower than the fusion point of single salt
Heating the salts, those dissociate itself into ions and assume the liquid state
After the formation of IL, this remain liquid when cooled 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
Choline Chloride
Sulfamic acid
Urea
Investigation samples system
Thermocouple
Cathode
Anode (Sample)
Holder
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
Scan 6Scan 5Scan 4
Scan 1Scan 2Scan 3
m
ScanMedium i
i
where m – quantity of measurements which where taken to calculation
Surface quality as function ofsulfamic acid concentration
10
0 10 20 30 400
100
200
300
400
500
600
700
800Front
Raw Classical EP C(SA), g/l
Ra,
nm
Surface quality as function of sulfamic acid concentration
11
0 10 20 30 400
100
200
300
400
500
600
700
800 Back
Raw Classical EP C(SA), g/l
Ra,
nm
Changing quantities of melt compounds we have found correct
recipe
Because of high relatively power of process solution gets big heat.
12
Choline chloride : Urea ratio 1:4
Sulfamic acid, g/l 30
t, C 120
i, A/cm2 0,33
Potential range, V 20-60
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
0 10 20 30 40 50 600
100
200
300
400
500
600
700
800 Front
Raw Nb Classical EP
t, min
Ra,
nm
Results in different treatment duration (min)
15
Front side
Back side
5 60302010
5 60302010
Raw
Classical EP, 10min
IL EP, 60min
IL EP, 10min
Surface characterization with profilometer
16
17
EP samples in process…
EP on 6GHz cavities
After the good result on samples, we start to apply the EP on real 6GHz cavities
Cathodes, flux system, concentration, new addition were studied to find the best EP
We are still working…
18
Improvement road
To improve the EP we study some possibilities:
Alternative to sulfammic acid
Different flux inside cavity
Different orientation of cavity
19
Vertical EP: holed cathode
•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
Pump
Solution collector
Input flange
Output flange
Holed cathode
Cavity 6 GHz EP system with IL
Vertical EP: holed cathode22
DopoPrima
1:4 Choline Chloride-Urea
Sulfammic acid: 30 g/L
T: 150°C0,3 -0,4
A/cm2
Q-factor result23
About electrical field distribution24
•To balance the different distance between cavity and cathode, this were shaped in various modes
Holed cathode
Two part cathode
Different flux
Broken cathodes: the shape of cathodes changed to get more uniformity on IL flux
25
Two possibilities of flux26
IN IN
OUT
OUT
From cathodes to flanges From flanges to cathodes
New shaped cathodes27
Sulfamic Acid: from flanges to cathodes
28
The best surface quality appeared on bottom cutoff part.
Solution: 1:4 Choline Chloride – Urea,Sulfammic Acid :30 g/LT:120-160°C
Sulfammic Acid:from cathodes to flanges
The opposite configuration brings a lot of bubbles and the cavity weren’t electropolished
29
Alternative to sulfammic acid
The best result on samples were reached with sulfammic acid, but cavity is quite different environment
We checked the performances of various regulator containing the group (–NHx) on samples
30
Comparison between regulator (–NH4)
Sulfammic Acid
Ammonium Sulfate
Ammonium Sulfamate
Ammonium persulfate
c : g/L
30 40 40 10
31
Ammonium Persulfate
The addidion of Ammonium Persulfate decreases the high initial voltage necessary to disrupt the oxide film
But this compound increases the roughness and pitting
Possibility: can Sulfammic acid and Ammonium Persulfate work together?
32
Ammonium Persulfate and Sulfammic Acid
On samples, a good compromise is the proportion 30 g/L of Sulfammic acid and 2.5 g/L of Ammonium Persulfate
On cavity, the best concentration were found mixing 1.5 g/L of Ammonium Persulfate and 30 g/L of Sulfammic Acid
33
Vertical EP: PA+SA
VerticalChCl:Urea
1:4 c(SA) =
30 g/l c(PS) =
1.5 g/lDistance
between cathodes: 10 mm
Bright, with flux line
34
Vertical EP: PA+SA
VerticalChCl:Urea
1:4 c(SA) = 30 g/l c(PS) = 1.5 g/l
Some irregular surface
Distance: 5 mm
35
Horizontal EP: PA+SA
HorizontalChCl:Urea
1:4 c(SA) = 30
g/l c(PS) = 1.5
g/lDistance
from cathodes: 4 mm
Some passivation zone
36
In future
Set up of flux sistem of EP
Calculation of Q-factor of ILs Ep cavities
Test to get the result on 1.5 /1.3 GHz
37
Advantages and disadvantages38
Classical EP ILs EP
Composition H2SO4 98% HF 49% - corrosive, dangerous and volatile
Choline Chloride, Urea sulfamic acid and others- No dangerous elements (salt form)
Process speed 30-80mA/cm2 (30 m/hour)
300-500 mA/cm2 (350 m/hour)
Pre-treatment BCP -
t, C room temperature 120-160
Thanks to attention!
39
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