Dec. 5-7, 2005 TTC Meeting, Frascati 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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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

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

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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

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 mistakeshow to achieve an

absolute cleansurface (HPR, dry ice,….)how to avoid

recontamination