1. Some History of Electropolishing of Niobium 1970 – 1990
P. Kneisel
Jefferson Lab
Dec. 5-7, 2005
TTC Meeting, Frascati

2. Siemens Process(1)
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3. Siemens Process(2)
The process was developed within a government funded collaboration agreement between Siemens AG and the Kernforschungszentrum Karlsruhe (GfK)
It was based on a proprietary process used at Siemens AG for the processing of Ta for capacitors
The process was subsequently used for the surface treatment of cavities ( rf separator, helix,
R&D) at GfK
It was subsequently “exported” to HEPL(P.Kneisel, C.Lyneis,J.P.Turneaure, IEEE Trans,Nucl,Sci NS-22,1197(1975)) , Cornell and KEK, modified by K. Saito in ~ 1980 for Tristan cavities
Dec. 5-7, 2005
TTC Meeting, Frascati

4. Siemens Process(3) Acid mixture:
HF(40%):H2SO4(95-97%) = 10 : 85 by volume
Constant voltage 9 – 15 V
depending on bath temperature( 25C-35C), solution concentration and electrode geometry
The optimium polishing conditions are not in the plateau region of the polarization curve, but are characterized by damped current oscillations
Oscillations reflect the generation of the viscous layer at the anode, which builds up and partially dissolves in the HF
Voltage drop in anode layer is about 90%, no matching of cathode geometry to anode geometry necessary.
Mean current density in oscillation
~ 100 mA/cm2
Dec. 5-7, 2005
TTC Meeting, Frascati

5. Siemens Process(4)
EP generates an extremely smooth surface, but not necessarily clean surface because of residual oxides on the surface
Anodizing/oxipolishing with ammonium-hydroxide solution
Fully oxidizing of suboxides
Interface is shifted from activated surface into clean material
Ammonium-hydroxide has cleaning capacity
EP samples show contamination of sulfur in the form of sulfate and fluorine (M.Grunder, Dissertation, Karlsruhe 1977)
Oxipolished samples show no sulfur anymore,less fluorine;
boiling in water for 5 min reduces sulfur by factor of 10 and sulfate is converted to sulfid; fluorine concentration is not affected
Dec. 5-7, 2005
TTC Meeting, Frascati

6. Electropolishing/Anodizing
Multi-mode (2 -4 GHz) pill box cavity, electropolished and anodized (20 V and 60 V)
TE011 cavity, ep + anodized, 9.5 GHz(H.Diepers et al., Phys. Lett. 37A, 139 (1971)
K.Saito, PAC 2003
Dec. 5-7, 2005
TTC Meeting, Frascati

7. Application of Siemens Process(1)
Karlsruhe-Cern Superconducting RF Separator
( A. Citron et al.;Nucl.Instr.& Meth. 164(1979).p.31
30 micron ep (horizontal)
Anodizing to ~ 50 V ( 0.1 micron Nb2O5)
1850 C UHV annealing for 24 hrs
70 micron ep (horizontal)
1850 C UHV annealing for 2 hrs
Intermittent EP
Dec. 5-7, 2005
TTC Meeting, Frascati

8. Application of Siemens Process(2)
First results on a 500 MHz SC
Test Cavity for TRISTAN
Spinning of half cell with 3 intermediate annealing steps
Ebw of stiffening ribs
Individual EP ( 80 micron) of cavity halfes and beam pipes
EBW of parts with inside/outside welds
900 C stress annealing at 900 C
Vertical EP , 30 micron
2 x oxipolishing at 80 V
For 3-cell cavity
Poor EP at equator
Hydrogen absorption
T.Furuya et al, Jap. Journ.Appl.
Physics, 20(1981), L
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9. EP at KEK (1)(K. Saito et al. , Proc, 4th SRF workshop(1989), p
EP at KEK (1)(K. Saito et al., Proc, 4th SRF workshop(1989), p.635, KEK,Tsukuba)
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10. Voltage or Current density ? This is a coupled problem.
EP at KEK (2)
30< Is < 100 mA/cm2
Voltage or Current density ? This is a coupled problem.
Dec. 5-7, 2005
TTC Meeting, Frascati

11. Current oscillation control is not
EP at KEK (3)
KEK
Current oscillation control is not
right EP condition
Best Finishing
Fig.1 : Current oscillation and best EP finishing surface
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12. EP at KEK (4) #s in bracket: current density Dec. 5-7, 2005
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13. EP at KEK (5)
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14. EP at KEK (6)
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15. EP at KEK (7)
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16. EP at KEK (8)
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17. EP at KEK (9)
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18. EP at KEK (10)
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19. EP at KEK (11)
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20. EP at KEK (12)
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21. Summary(1) Most important parameters:
Current density: 30 – 100 mA/cm^2
Voltage: 8 – 16 V
Bath temperature: 25 – 35 C
Optimal HF concentration: 60 – 90 cc/L
based on brightness, if concentration smaller, increase in voltage necessary, outside range of micropolishing
Rotational speed:0.7 rpm for 508 MHz
Acid flow rate: 60 l/min
Viscous layer needs to be preserved
Dec. 5-7, 2005
TTC Meeting, Frascati

22. Summary(2)
For good results, initial roughness is important: mechanical polishing
After ep the surfaces are still contaminated even after some rinsing: H2O2 at 50C+ultrasonic for 40 min; excessive rinsing afterwards
Choice of materials in contact with acid mixture:
PTFE,PVDF,PE
Sulfur contamination: sulfur is generated during ep by reaction of hydrogen with sulfuric acid at a rate of 4 mg/l for 80 micron
Sulfur can be dissolved in CCl4,CS2 or acetone
some improvement with active carbon filtering
Dec. 5-7, 2005
TTC Meeting, Frascati

23. Summary(3) Remaining issues: QA of acid: HF concentration
sulfur contamination
QA of work: how to avoid human mistakes
how to achieve an absolute clean
surface (HPR, dry ice,….)
how to avoid recontamination
Dec. 5-7, 2005
TTC Meeting, Frascati