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Some History of Electropolishing of Niobium
1970 – 1990
P. Kneisel
Jefferson Lab
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Siemens Process(1)
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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
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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
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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
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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
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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)
• Anodizing to ~ 50 V ( 0.1 micron Nb2O5)
• 1850 C UHV annealing for 2 hrs
Intermittent EP
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Application of Siemens Process(2)First results on a 500 MHz SCTest 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 VFor 3-cell cavity• Poor EP at equator• Hydrogen absorption
T.Furuya et al, Jap. Journ.Appl.Physics, 20(1981), L145-148
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EP at KEK (1)(K. Saito et al., Proc, 4th SRF workshop(1989), p.635, KEK,Tsukuba)
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Voltage or Current density ? This is a coupled problem.
30< Is < 100 mA/cm2
EP at KEK (2)
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Ro
Best Finishing
Current oscillation control is not right EP condition
KEK
Fig.1 : Current oscillation and best EP finishing surface
EP at KEK (3)
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EP at KEK (4)
#s in bracket: current density
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EP at KEK (5)
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EP at KEK (6)
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EP at KEK (7)
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EP at KEK (8)
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EP at KEK (9)
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EP at KEK (10)
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EP at KEK (11)
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EP at KEK (12)
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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/minViscous layer needs to be preserved
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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 acetonesome improvement with active carbon filtering
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Summary(3)Remaining issues:
QA of acid: HF concentration sulfur contamination
QA of work: how to avoid human mistakeshow to achieve an
absolute cleansurface (HPR, dry ice,….)how to avoid
recontamination