LABORATORI NAZIONALI DI LEGNARO

Progress on Fluorine-free

ElectropolishingVlada Pastushenko,

F. Stivanello, V. Palmieri6th Workshop on "Thin Films and New Ideas for SRF", Legnaro (PD) Italy, October 6-8 2014

Outline Ionic liquids, their properties and application.

Electropolishing of Niobium in Fluorine-free solutions (first steps).

Study the parameters of Electropolishing in Ionic Liquids.

Procedure of standard electropolishing 6 GHz cavities.

Development the systems for Electropolishing in Ionic Liquids: Different shapes of the cathode; Vertical system for EP 6 GHz cavity; Horizontal system for EP 6 GHz cavity.

Q-factor result.

DES is a type of ionic solvent with special properties composed of a mixture which forms a eutectic with a melting point much lower than either of the individual components.

The deep eutectic phenomenon was first described in 2003 for a mixture of choline chloride and urea in a 1:2 mole ratio, respectively.

Choline chloride has a melting point of 302 °C and that of urea is 133 °C. The eutectic mixture melts lower than 100 ˚C.

Deep Eutectic Solvent or DES*

*Andrew Abbott; John Barron, Karl Ryder and David Wilson (2007). "Eutectic-Based Ionic Liquids with Metal-Containing Anions and Cations". Chem. Eur. J. 13: 6495– 6501. doi:10.1002/chem.200601738.

Type I Eutectic metal salt + organic salt (e.g. ZnCl2 + choline chloride)

Type II Eutectic metal salt hydrate + organic salt (e.g. CoCl2*6H2O + choline chloride)

Type III Eutectic organic salt + hydrogen bond donor (e.g. choline chloride + urea)

Type IV Eutectic metal salt (hydrate) + hydrogen bond donor (e.g. ZnCl2 + urea)

Four types of eutectic solvents:

Room Temperature 120˚C

Choline Chloride – based Ionic liquids

Conductivity and viscosity of a Urea : Choline chloride mixture (2:1) as a function of temperature*

*A. P. Abbott, G. Capper, D. L. Davies, R. Rasheed and V. Tambyrajah “Novel Solvent Properties of Choline Chloride/ Urea Mixtures” Chem. Commun, 2003, 70-71.

Electropolishing of Niobium in

Fluorine-free solutions(first steps)

Acetonitrile tetramethylammonium perchlorate tetrabutylammonium perchlorate potassium perchlorate

Dimethylformamide tetrabutylammonium perchlorate potassium perchlorate Urea

Organic solvents Ionic Liquids Choline Chloride : Urea (1:2) tetramethylammonium perchlorate tartaric acid oxalic acid ammonium chloride

Choline Chloride : Urea (1:3) ammonium chloride potassium tartrate

Choline Chloride : Urea (1:4) ammonium chloride

Choline Chloride: Ethylene Glycol (1:2) ammonium chloride

M. Ceccato, “Elettropulitura del Niobio con Elettroliti a base di Liquidi Ionici”, Master Thesis (2007) University of Padua.

Optical microscopy analysis

SEM analysis

Sulfamic acid – first success*

*Rupp V. "Electropolishing of Niobium 6 GHz RF cavities in fluorine-free electrolyte", Master Thesis, 2009.

Influence of S and N containing compounds

Surface maps of different treatments

raw Nb

10 min IL

10 min classical EP

60 min IL

Choline Chloride and Urea (1:4) with Sulfamic Acid (SA) and Ammonium Sulfamate (AmS)

+30 g/l SA

+50 g/l SA +75 g/l AmS +114 g/L AmS

+ 97 g/l SA

The Best parameters for EP Nb samples

Choline Chloride : Urea 1 : 4

Sulfamic acid, g/l 97

Material cathode Nb

Temperature, °C Higher then 120

Current density, A/cm2 0,3

Procedure of standard electropolishing

6 GHz cavities

Cavity kit appearance

1 – cavity; 2 – cathode; 3 – up flange; 4 – bottom flange

1

4

2

3

1 – cavity;

2 – anode contacts;

3 – cathode contact;

Blue indicator – direct flow,

red – indirect.

Stand for BCP and EP

EP result obtained in 6GHz cavity

Schematic diagram of an anodic polarization

Automatic software for controlling potential.

System for Electropolishing

in Ionic Liquids

Electropolishing in Ionic Liquid (preliminary tests)

Different cathode

performance

Cut-offCell

“Full holed” cathode

Cut-offCell

“Center holed” cathode

“Two parts” cathode

1 – cathode; 2 – flange; 3 – surface of cathode which has to be

covered with Teflon ribbon; 4 – cavity; 5 – tube junction

1

23

45

Vertical system for EP 6GHz

cavities in Ionic liquids

Scheme of flux direction ”inside cathode” 6GHz cavities treatment procedure IL EP process

Cavity after Vertical EP

Horizontal system for EP

6GHz cavities in Ionic liquids

SEM Analysis

Half cavity polished in horizontal position

Solution - 1:4 Choline :Urea,Sulfammic Acid - 1M/l;

T- 120°C

Horizontal system for EP 6 GHz cavity*

*O. Malkova, “Study of the Electropolishing of 6GHz Niobium cavity in ionic liquids”, Master Thesis, 2013.

Surface inside cavity after EP

Modified system for EP in Ionic liquids

Iris

Cell

Electropolishing of the Niobium: result after first try (60min)

Electropolishing of the Niobium: result after 6 hours

Q-factor result

Choline Chloride - urea based Ionic liquids are a possible alternative to the hazardous standard electropolishing electrolyte based on hydrofluoric and sulphuric acid.

The best solution that we found for Electropolishing of Niobium is Choline Chloride with Urea in ratio 1 to 4 with 1 M/l Sulfamic acid.

The electropolishing in horizontal system give us shiny surface without pitting and damages and the result is reproducible.

Summary

Thank you for attention!