Mechanism of Nb EP H-tian of Nb EP_H-tian.pdf3thth international workshop on thin film & new ideas...

33thth international workshop on thin film & new ideas for pushing thinternational workshop on thin film & new ideas for pushing the limits of RF Superconductivitye limits of RF Superconductivity

JJ--lab, Newport News, VA July 22lab, Newport News, VA July 22--25, 200825, 2008

The Mechanism of Electropolishingof Nb in Hydrofluoric-Sulfuric Acid

(HF+H2SO4) Electrolyte

Hui Tian*+, Charles E. Reece+, Michael J. Kelley*+

Applied Science Department, College of William and Mary, Williamsburg, VA + Thomas Jefferson National Accelerator Facility, Newport News, VA

Sean G. Corcoran## MSE Department, Virginia Tech, Blacksburg, VA

This work is supported by the US Dept. of Energy under grant DE-FG02-06ER41434



IntroductionElectropolishing seems to be a superior technique to treat

niobium cavity surfaces for achieving high SRF performance SRF and is selected to replace BCP for the highest gradient applications (TESLA, ILC).

The large variation of cavity performance results from the present empirical EP processes. A microscopic understanding of the basic Nb EP mechanism is yet lacking.

The application of electrochemical techniques is needed for the development of a clear picture of the exact role of each parameter involved during the EP process.



0 2 4 6 8 10 12 14 16 18 200

5

10

15

20

25

30

35

40

Cur

rent

den

sity

( m

A/c

m2 )

Voltage ( power supply)

T = 31.5 +/- 0.5oC Reactive area = 5.72cm2

Classical “Two-Electrodes” Set-up Is Not Enough to Identify the Local Effect on Nb EP

Nb I-Vpwrsup curve

Vpwsup: 12 ~25 V

25~5

0 m

A/ c

m2

Vpwsup= VNb+Veletrolyte+VAl

I-Vpwsup only is impossible to clearly identify the local effect on NbNb EPEP , such as electrolyte temperature, acid concentrations, viscosity and stirring.

HF: H2SO4=1:9 (volume)

Al is cathode

Nb Cavity is anode

1 r/min

Power supply

Current limited plateau



-4 -2 0 2 4 6 8 10 12 14 16 18 200

5

10

15

20

25

30

35

40

Cur

rent

den

sity

( m

A/c

m2 )

Voltage ( vs. MSE )

T = 31.5 +/- 0.5 oC Ref electrode is nearby Nb .Reactive area = 5.72 cm2

Three-Electrode-Setup Improved Electrochemical Characterization of EP

Enables enlightening study of temperature, flow, and composition dependent effects (electrolyte) in detail.

Separating impacts of individual components in EP system.

Cathode: Al I-V

Anode: Nb I-VExample:VPwrSup = 15 V Vcathode : ~ 4 V Velectrolyte: ~ 2 V Vanode:~ 9V

Not Power Supply Voltage

MSE : Mercury / MercurousSulfate Reference Electrode



0 2 4 6 8 10 12 14 16 180

20

40

60

80

100

21.3 oC26.3 oC

33.5 oC

45.6 oC

54.6 oC

1 54.6 o C; 3 33.5 o C 2 45.6 o C; 4 26.3 o C 5 21.3 o C

Ano

de C

urre

nt D

ensi

ty (

mA

/cm

2 )

VNb( V ) (vs. MSE )20 30 40 50 60

0

20

40

60

80

100

Ano

de C

urre

nt D

ensi

ty (m

A/c

m2 )

Temperature ( oC )

SNb/ SAl = 10 : 1

Anode Current Density Strongly Depends on Local Temperature

Past studies identified 25-35 ºC for best EP gloss on Nb surface



For cavity EP,For cavity EP, electrolyte also serves as the process coolant. Unstable temperatures is expected and particularly hot in no-flow condition and higher heat flux where flow rate is high. NonNon--uniform polishing is uniform polishing is expected.expected.

The output flow temperature is in “within specs”(~35ºC), but the actual cavity wall temperature is out of control (40~56ºC).

R. Geng et. al, internal talk

C. Reece et. al, SRF 2007



0.2 0.4 0.6 0.8 1.00

5

10

15

20

25

23.44 mA/cm2

Concentration of HF ( by volume )

5.27 mA/cm2

Anode Current Density Varies Linearly with HF Acid Volume Concentration

0 2 4 6 8 10 12 14 16 180

5

10

15

20

25

30

35

40

0.2:9.8

0.4:9.6

0.6:9.4

0.8:9.2

Ano

de C

urre

nt D

ensi

ty (

mA

/cm

2 )

Voltage ( V ) (vs. MSE)

Area ratio of Nb/Al =10:1T = 31.5+/- 1.5 oC

1:9

HF acid loss may be expected due to evaporation and chemical reaction during process. The understanding of the detailed role of HF involved during the EP process requires further electrochemical studies.

MHF: decreases 80% ;

MH2O: decreases 32%

MH2SO4: increases 8.8%

decreases 78%



-2 0 2 4 6 8 10 12 14 16 18 200

10

20

30

40

50

60T = 14 +/- 1oCHF: H2SO4: H2O (by volume)1 : 9 -fresh mixed acid1 : 9 : 0.51 : 9 : 21 : 9 : 31 : 9 : 5

Cur

rent

den

sity

( mA

/cm2 )

Voltage (V) vs. MSE

MHF: decreases 33.6%

MH2SO4: decreases 33.4%.

MH2O: increases 235%0 1 2 3 4 5

0

5

10

15

20

25

1

T = 14 +/- 1oC1: Fresh mixed 1:9 eletrolyte + VH2O

2: * Used 1:9 eletrolyte+ VH2O

* Fresh mixture exposured under chemical hood for 5 hrs)

Ano

de P

late

au C

urre

nt D

ensi

ty (

mA

/cm

2 )10

V v

s. M

SEDifferent volume ratio of H2O

2

Additional Volume H2O slightly Increases Anode Current Density

Anode current density slightly increases with additional volume of H2O (38~42%); Compare curve 1 and curve 2( after 5 hrs exposure under chemical hood), current density decreases 30% -HF evaporation



0 2 4 6 8 10 12 14 16 18 200

4

8

12

16

20

24

28

32

Area ratio of Nb/ AlT = 20.5 +/- 1.3 o C Cathode area (Al) kept unchanged ( 2.6cm 2 )1 : 1 ; 2 : 1 ; 4 : 16 : 1 ; 8 : 1 ; 10 : 1

Ano

de c

urre

nt d

ensi

ty (m

A/c

m2 )

VNb (V) vs. MSE

a

0 2 4 6 8 10 12 14 16 18 200

4

8

12

16

20

24

28

Area ratio of Nb/ AlT = 19.7 +/- 1.7 o C Anode area(Nb) kept unchanged ( 2.6 cm 2 )1 : 1 ; 1: 2 ; 1 : 41: 6 ; 1 : 8 ; 1 : 10

A

node

cur

rent

Den

sity

( m

A/c

m2 )

VNb( V ) vs. MSE

b

Anode Current Density Does Not Depend on the Relative Area of Anode and Cathode

Anode current density remain constant



Current-Limited Plateau of Nb EP:Mass Transport Mechanism Has Been Unknown

Mass transport limitation for anodic dissolution are generally believed to be responsible for electropolishing, but micro-polishing (brightening) only occurs when the plateau is the result of diffusion-limitation alone.

Possible mass transport limiting species proposed -D. Landolt, Electrochemica Acta, Vol . 32(1)

I) Metal Ions (Mn+aq) II) Acceptor anion (A-) III) H2O

CstCst

Cst

Maq

Maq

MAy

Con

cent

ratio

n

diffusion layerdiffusion layer diffusion layer

)()()()( xCxxCD

RTFz

xxCDxJ iii

iiii υφ

+∂

∂−

∂∂

−=Nernst-Planck equation

Understanding of mass transport mechanism requires further electrochemical study-EIS.



Rs: electrolyte, temperature.

What is Electrochemical Impedance Spectroscopy?

Rp: polarization/charger transfer resistor

Cdl : double layer capacitor of electrode surface

Rwarburg: diffusion resistor

Rs: solution resistor

Nb Ref. Elec.Possible equivalent circuit of Nb-acid interface during EP

High Frequency Nyquist Plot

222

2

222 11 pdl

pdls

pdl

p

RCRC

jRRC

RZ

ωω

ω +−+

+=

ReZ

-ImZReZ -ImZ

Rp: temperature, concentration of reaction products, potential.

Rw: the frequency of potential perturbation

Cdl: electrolyte, temperature, potential, oxide layer

electrode roughness



Zreal (Ω)Rs remains constant

-Zim

ag(Ω

)EIS Study of Constant Current Density

0 2 4 6 8 10 12 14 16 180

5

10

15

20

25Area Ratio of Nb/Al = 10 : 1 (Nb : 26.035 cm 2; Al : 2.6035 cm 2)Ref electrode & Thermal Couple nearby Nb ( < 5 mm ) T = 21.5 o C

Ano

de C

urre

nt D

ensi

ty (

mA

/cm

2 )

Anode Potential ( V )

Rp increases with the potential

0.2 Hz

200 KHz

1.01 KHz =ωmax=1/RpCdl

0.2 Hz



EIS Study of Different Flow Rates

Zreal (Ω)

-Zim

ag(Ω

)

Static (triangle) vs. Agitation ((dot)dot)flow rate ~ 4 ~ 5 cm/sec

T = 9.2± 0.1 ºC

Rs @ at different flow condition remains as constant

3V6V

Static

Flow

Rp decreases with increasing flow

200 KHz

0.2 Hz

0.2 Hz0.2 Hz

0.2 Hz



3 4 5 6 71.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Cap

acita

nce (μ

F )

Potential ( V )

T = 9.0 +/- 0.2 oC Without agitationwith agitation ( flow rate : v ~ 4 ~ 5 cm/sec)

What We have Learned from EIS Studies?

3 4 5 6 76

8

10

12

14

16

18

20

22

24

T = 9.0 +/- 0.2 oC Without agitationwith agitation ( flow rate : v ~ 4 ~ 5 cm/sec)

Pola

rizat

ion

resi

stan

ce (o

hms)

Potential ( V )

The change of Rs, Rp and Cdl at the different potential regions and flow condition are used to identify the possible mass transport mechanism



Constant Rs at different potential regions and flow condition rules out the “porous salt film” model.

Rp increase at different potential regions is inconsistent with the “adsorbatesacceptor” model.

Cdl decrease at different potential regions & Cdl increase at different flow conditions are consistent with the “compact salt film” model.

EIS Indicates “Compact Salt Film” Model



The Diffusion-Limited Access of F- To the Salt Film Produces Best Polishing

Sulfuric acid tends to anodize the Nbunder polarization potential producing the "compact salt film”- “Nb2O5”.

HF acid tends to dissolve the Nb oxide under kinetic control with the "at the surface" concentration of F-

.

F- concentration “at the surface” is limited by how fast it diffuses through the electrolyte ( ~diffusion layer).

The local gradient in F-

concentration produces the desired polishing action.

“Nb2O5”

NbBulk Electrolyte

Diffusion Layer(~ um)

Compact Salt Film(~ nm)

F -

%

F -

%

Distance

DistanceLocal temperature, flow (stirring) & electrolyte composition affect the local F- gradient.



EIS Study for ImplicationsSurface Optimization & Quality Control

Preliminary studies show that there is a signature difference in EIS responsebetween rough & smooth surfaces. Potentially useful for on-line process feedback

Mechanical ground sample

Light BCP ( 30 μm removal) 30 mins EP @ 31ºC



Summary and Future Work

Quantify the scale-dependant smoothing effects of niobium EP as a function of temperature, electrolyte composition, and flow conditions. Specify optimum processing protocol. Build electrochemical understanding to better appreciate conditions that contribute to smoothing.Identify useful on-line process monitors for process assurance and progress tracking – EIS and etc?Guide the evolution of cavity treatment protocols toward well engineered and controlled conditions for cavity productions.

The first use of “three-electrode-setup” reveals that Nb EP strongly depends on the local electrolyte temperature and HF/H2SO4 volume ratio.

High frequency impedance data provide strong evidence for the presence of a compact salt film (Nb2O5) in the current-limited plateau region.

The results suggest that the diffusion-limited access of the F- anion to the salt film surface limits the local reaction rate, creates the plateau and yields the micropolishing.



Thank YouThank You

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Mechanism of Nb EP H-tian of Nb EP_H-tian.pdf3thth international workshop on thin film & new ideas...

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