some history of electropolishing of niobium 1970 1990 l.
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Some History of Electropolishing of Niobium 1970 – 1990. P. Kneisel Jefferson Lab. Siemens Process(1). Siemens Process(2). The process was developed within a government funded collaboration agreement between Siemens AG and the Kernforschungszentrum Karlsruhe (GfK)

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some history of electropolishing of niobium 1970 1990

Some History of Electropolishing of Niobium1970 – 1990

P. Kneisel

Jefferson Lab

TTC Meeting, Frascati

siemens process 1
Siemens Process(1)

TTC Meeting, Frascati

siemens process 2
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

TTC Meeting, Frascati

siemens process 3
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

TTC Meeting, Frascati

siemens process 4
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

TTC Meeting, Frascati

electropolishing anodizing
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

TTC Meeting, Frascati

application of siemens process 1
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

TTC Meeting, Frascati

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), L145-148

Application of Siemens Process(2)

TTC Meeting, Frascati

ep at kek 1 k saito et al proc 4 th srf workshop 1989 p 635 kek tsukuba
EP at KEK (1)(K. Saito et al., Proc, 4th SRF workshop(1989), p.635, KEK,Tsukuba)

TTC Meeting, Frascati

slide10

EP at KEK (2)

30< Is < 100 mA/cm2

Voltage or Current density ? This is a coupled problem.

TTC Meeting, Frascati

slide11

EP at KEK (3)

KEK

Current oscillation control is not

right EP condition

Best Finishing

Fig.1 : Current oscillation and best EP finishing surface

TTC Meeting, Frascati

ep at kek 4
EP at KEK (4)

#s in bracket: current density

TTC Meeting, Frascati

ep at kek 5
EP at KEK (5)

TTC Meeting, Frascati

ep at kek 6
EP at KEK (6)

TTC Meeting, Frascati

ep at kek 7
EP at KEK (7)

TTC Meeting, Frascati

ep at kek 8
EP at KEK (8)

TTC Meeting, Frascati

ep at kek 9
EP at KEK (9)

TTC Meeting, Frascati

ep at kek 10
EP at KEK (10)

TTC Meeting, Frascati

ep at kek 11
EP at KEK (11)

TTC Meeting, Frascati

ep at kek 12
EP at KEK (12)

TTC Meeting, Frascati

summary 1
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

TTC Meeting, Frascati

summary 2
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

TTC Meeting, Frascati

summary 3
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

TTC Meeting, Frascati