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Some History of Electropolishing of Niobium 1970 – 1990

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

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  1. Some History of Electropolishing of Niobium1970 – 1990 P. Kneisel Jefferson Lab TTC Meeting, Frascati

  2. Siemens Process(1) TTC Meeting, Frascati

  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 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 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 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 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) • Anodizing to ~ 50 V ( 0.1 micron Nb2O5) • 1850 C UHV annealing for 2 hrs Intermittent EP TTC Meeting, Frascati

  8. 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

  9. EP at KEK (1)(K. Saito et al., Proc, 4th SRF workshop(1989), p.635, KEK,Tsukuba) TTC Meeting, Frascati

  10. EP at KEK (2) 30< Is < 100 mA/cm2 Voltage or Current density ? This is a coupled problem. TTC Meeting, Frascati

  11. 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

  12. EP at KEK (4) #s in bracket: current density TTC Meeting, Frascati

  13. EP at KEK (5) TTC Meeting, Frascati

  14. EP at KEK (6) TTC Meeting, Frascati

  15. EP at KEK (7) TTC Meeting, Frascati

  16. EP at KEK (8) TTC Meeting, Frascati

  17. EP at KEK (9) TTC Meeting, Frascati

  18. EP at KEK (10) TTC Meeting, Frascati

  19. EP at KEK (11) TTC Meeting, Frascati

  20. EP at KEK (12) TTC Meeting, Frascati

  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 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 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 TTC Meeting, Frascati

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