ISTITUTO NAZIONALE DI FISICA NUCLEARE

Legnaro National Laboratories

HF free solutions for Nb Electropolishing: Something Unusual !!

M. Ceccato, G. Mondin, V. Rampazzo, A.A. Rossi and V. Palmieri

PADUA UNIVERSITYScience Faculty

Material Science Dept.

Standard Nb Etching :

• Buffered Chemical Polishing: HF : HNO3 : H3PO4

(ratio 1:1:1 or 1:1:2)

• ElectroPolishing: HF : H2SO4 (ratio 1:9)

Composition HF : H2SO4 : Lactic acid

in the ratio 18 : 21 : 61

Current Density = 90-110 mA/cm2

Voltage = 15-18 VTemperature = 50-60 °C

Note: Lactic acid solutions can be explosive

Improved Methods for Electrochemical Polishing Nb Superconducting cavitiesby V.M. Efremov, L.M. Sevryukova, M. Hein, L. Ponto, 2° TESLA Workshop, DESY, Aug 1991

1. Alternatives for EP: lactic acid

Japanese Patent officePublication number 60092500 ADate of filing 26.10.83Date of Application Publication 24.05.85Applicant Mitsubishi heavy Ind LTD., Nomura Tokin: KKInventor : Yoshisuke Keisuke; Nomura Hirotoshi

PATENT PURPOSE: To obtain a polishing solution which prevents HF evaporation to form a mirror finished surface

See also: K. Saito, Proceedings of the 4th workshop on RFSC, Tsukuba 1989

2. Alternatives for EP: fluosulfuric acid

Note: Fluosulfuric acid is suspected cancerousgenic

Working point (I 0, V0) automatically found by locking the minimum of dI/dV

(Palmieri et al., Tsukuba SRF Workshop 2001)

3. Alternatives for EP: getting rid of sulphuric

Working point (I 0, V0) automatically found by locking the minimum of dI/dV

This approach has permitted the authors to easily find other electrolytes for Nb, i.e.:

HF + OXALIC ACID + BORIC ACID + H3PO4

30% HF, 15% H3PO4, 30 gr/lt Oxalic acid, 10 gr/lt Boric acid

(Palmieri et al., Tsukuba SRF Workshop 2001)

3. Alternatives for EP: getting rid of sulphuric

The electrolyte is 0.05 mole/liter Mg(ClO4)2 in CH3OH

EP Voltage = 50-70 V, Temperature = -5°C

by Schober and Sorajic, extrapolating what proposed by Epelboin [Rev. Met. 49 (1952) 863.]

The electrolyte does not contain Hydrogen radicals, so no Hydrogen intake is possible

4. Alternatives for EP: perchlorate salts

for its remarkable solubility of numerous metallic perchlorates in non aqueous media, …

An EP mixture of • perchloric acid, • ethylic acid, • acetic anhydride

would be the solution!However, in the technical literature, many explosions caused by perchloric acid solutions have been reported.

Some authors claim that ethyl-perchlorate is as dangerous as Nitroglycerin

If Percloric acid gives rise to explosive mixtures, under other aspects Hydrofluoric acid is not less dangerous :

(refineries using HF acid have a plethora of devastating histories)

• On contact, HF passes through skin and tissue.

• Because its action can be delayed for many hours, it can distribute throughout the body, causing the erosion of bone.

• F- ions bind to Ca++ and Mg++ ions to form insoluble salts (CaF2 and MgF2 salts form some natural gemstones).

• Contact with HF does not cause immediate pain, so systemic poisoning begins before the person is aware

• In the body, Ca and Mg ions are used to mediate a variety of physiological processes, such as muscle movement.

• Cardiac arrests have been reported from concentrated acid burns to as little as 2.5% of exposed body surface area

5. Alternatives for EP: molten salts

Anodic dissolution of Nb is possible

in

(NaCl-KCl-NbCln) - AlCl3 melts at 710°C

In molten salts, electrical conductance is due to ionic mobility, but high temperatures are necessary for salt melting

6. Alternatives for EP: Ionic liquids

‘‘Ionic liquids’’ are molten salts

• having a melting point less than 100 °C

• which solely consist of cations and anions

For Niobium:

One such system that could be explored is that formed from 1-ethyl-3-methylimidazolium (EMIC) and

anhydrous AlCl3

However Anhydrous AlCl3 can give rise to unpleasent exothermic reactions!

Glow box is mandatory!!

….. But Choline Chloride is an ingredient of chicken feed !?!

Urea

Choline Chloride

Campione 47 - Densità di corrente = 0,18 A/cm2

Campione 51 - Densità di corrente = 0,10 A/cm2

Campione 52 - Densità di corrente = 0,05 A/cm2

Choline Chloride-Urea mixture melting point

4020 60 80 1000

0

50

100

150

200

300

350

250

mol % Urea

Tm

°C

We first succeeded in electropolishing Nb by a mixture of Choline Cloride, Urea, NH4Cl at 80°C

Urea/Choline Chloride 3/1 + 10g/L Ammonium Chloride

(190 °C; 200 mA/cm2)

Untreated Surface

Optical Microscope

80 μm

80 μm

Bare Surface

Urea/Choline Cloride 3/1 + Ammonium Chloride 10g/L (190 °C; 200 mA/cm2).

Standard Electropolishing

Sample 71

•Current Density = 0,46 A/cm2

•Temperature = 165°C

•Time = 25 min

Mirror-like finishing

Application to 6 GHz Cavities still in progress

6 GHz seamless monocells

Choline Chloride Drink

Dosage and Use_

Take 1 to 3 teaspoons daily.(best if mixed with fruit-juice)

The brain has a voracious appetite for Choline.

Choline is required for synthesis of the key neurotransmitter acetylcholine, and it is used for the building and maintenance of brain cell membranes.

Conclusions

Work supported by the European Community ResearchInfrastructure Activity under the FP6 "Structuring theEuropean Research Area" programme (CARE, contractnumber RII3 CT-2003-506395)

Best Recipe at the moment:

Urea : Choline Chloride in ratio 3 : 1

+

Ammonium Chloride 10g/L (165 °C; 460 mA/cm2)