Hui Tian Charles Reece Jefferson Lab TTC April 2010 Tutorial in - - PowerPoint PPT Presentation

Hui Tian Charles Reece Jefferson Lab TTC April 2010

Tutorial in spirit, see published literature for more precise use of language.

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We continue to deepen our understanding of what “EP” does to niobium surfaces and apply that knowledge to

• ptimize the process.

We want to understand the scale-specific details of surface leveling. We pursue a reliable, cost-minimized process for JLab, ILC and other applications.

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Nb

Bulk Electrolyte Diffusion Layer (~ um)

Compact Salt Film (~ nm) F - % F - %

Distance

Distance

Diffusion-limited access of F- to the surface produces “best” polishing

Local temperature, flow and electrolyte composition affect the local F- gradient

• Anodization of Nb in H2SO4 forces growth of

Nb2O5.

• F- dissolves Nb2O5.
• These competing processes result in current flow

and material removal.

• Above a certain anodization potential, the

reaction rate plateaus, limited by how fast fresh F-can arrive at the surface. (diffusion-limited)

• In this steady-state case, this Nb2O5 layer is a

“compact salt film” with specific resistivity.

• The thickness of the salt film increases with

applied potential, although the steady-state current does not change (plateau).

• In the true diffusion-limited circumstance,

material removal is blind to crystallography (avoids crystallographic etching).

• The diffusion coefficient sets a scale for the most

effective leveling Summary

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Nb

Bulk Electrolyte Diffusion Layer (~ um)

Compact Salt Film (~ nm) F - % F - %

Distance

Distance

Diffusion-limited access of F- to the surface produces “best” polishing

Local temperature, flow and electrolyte composition affect the local F- gradient

• Anodization of Nb in H2SO4 forces growth of

Nb2O5.

• F- dissolves Nb2O5.
• These competing processes result in current flow

and material removal.

• Above a certain anodization potential, the

reaction rate plateaus, limited by how fast fresh F-can arrive at the surface. (diffusion-limited)

• In this steady-state case, this Nb2O5 layer is a

“compact salt film” with specific resistivity.

• The thickness of the salt film increases with

applied potential, although the steady-state current does not change (plateau).

• In the true diffusion-limited circumstance,

material removal is blind to crystallography (avoids crystallographic etching).

• The diffusion coefficient sets a scale for the most

effective leveling Summary So we want to understand this diffusion coefficient

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We have successfully characterized the

• temperature-dependent viscosity of the EP fluid
• diffusion constant of F- in the fluid

This allows us to calculate the scale of most effective leveling. We have also clearly identified that a parallel etching process is present at higher temperatures – this works against obtaining the smoothest surfaces, yielding a reaction rate that is spatially varying with local chemical potential – grain orientations and lattice stresses.

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I-V curves of Nb electropolishing at different temperatures with RDE

2 4 6 8 10 12 14 16 18 20 22 0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010 0.011 0.012 0.013 0.014 0.015

4000 RPM 3600 RPM 3200 RPM 2800 RPM 2400 RPM 2000 RPM 1600 RPM 1200 RPM 800 RPM 400 RPM

Current (mA) Voltage (vs. MSE)

HF(49% ):H

2SO 4 (96%

)=1:10 T=10

• C

0 RPM

2 4 6 8 10 12 14 16 18 20 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

4000 RPM 3600 RPM 3200 RPM 2800 RPM 2400 RPM 2000 RPM 1600 RPM 1200 RPM 0 RPM 800 RPM

Current (mA) Voltage( vs.MSE)

HF(49% ):H

2SO 4(96%

)=1:10 T=19

• C

400 RPM

2 4 6 8 10 12 14 16 18 20 25 50 75 100 125 150 175 200 225 250

4000 RPM 3600 RPM 3200 RPM 2400 RPM 2800 RPM 2000 RPM 1600 RPM 1200 RPM 800 RPM 400 RPM HF(49% ):H

2SO 4 (96%

)=1:10 T=30

• C

Current Density (mA/cm

2)

Voltage (V) (vs. MSE)

0 RPM 2 4 6 8 10 12 14 16 18 20 50 100 150 200 250 300 350 400

2400 RPM 3600 RPM 3200 RPM 4000 RPM 2000 RPM 1600 RPM 1200 RPM 800 RPM 400 RPM

Current Density (mA/cm

2)

Voltage(V) (vs. MSE)

HF(49%):H

2SO 4(96%)=1:10

T=41oC

0 RPM

10 °C 19 °C 30 °C 41 °C 0 – 4000 rpm

Note only ~20% current rise with 400 rpm, at constant temperature

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Strong evidence for temperature-dependent electrochemical etching in parallel with the diffusion-limited process. For analysis, we must separate these current contributions. 2 4 6 8 10 12 14 16 18 20 50 100 150 200 250 300 350

slope=10.14 slope=9.82 slope=5.68 slope=4.23 slope=2.87 slope=1.79

Current density(mA/cm

2)

ω

1/2 (rad.s

• 1)

1/2

T= 1 +/- 0.5

• C

T= 9.0 +/- 0.5

• C

T= 19.0 +/- 0.5

• C

T= 30.0 +/- 0.5

• C

T= 41.0 +/- 0.5

• C

T= 50.0 +/- 0.5

• C

0.5 0.67 0.166

( . ) 0.62 slope J vs nFD c ω υ − =

Excellent linear fit provides definitive evidence of a diffusion-limited process. Knowing ν and c yields D.

RDE measurements

cF = 2.67×10-3 mol/cm3

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0.00 0.05 0.10 0.15 0.20 0.25 0.30 10 20 30 40 50 ν (cm2/s) T (°C)

Kinematic Viscosity of 1:10 HF/H2SO4 Electrolyte

• H. Tian, JLab

Measured using a Brookfield DV-II pro viscometer

0.0E+00 5.0E-08 1.0E-07 1.5E-07 2.0E-07 2.5E-07 3.0E-07 3.5E-07 10 20 30 40 50 D (cm2/s) T (°C)

Diffusion Coefficient of 1:10 HF/H2SO4 Electrolyte

RDE measurements + viscosity measurements + concentration determine the Diffusion coefficient cF = 2.67×10-3 mol/cm3

Determining Electrolyte Physical Properties

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10 20 30 40 50 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 Temperature (

• C)

Diffusion layer thickness (µm)

1 δ 2 δ (0, )

F

C t

−

≈

Bulk F

C

−

c J n F D δ = × × ×

Estimation of diffusion layer thickness in 1:10 HF/H2SO4 Electrolyte at different temperatures

cF = 2.67×10-3 mol/cm3 There exists a F- concentration gradient within the 10-20 µm away from the surface. On this scale, peaks are dissolved much faster than valleys. 2 µm scale structure should vanish much faster that 40 µm structure

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10

• 2

10

• 1

10 10

1

10

• 2

10

• 1

10 10

1

10

2

10

3

10

4

10

5

10

6

10

7

Spatial frequency (µm-1) PSD(nm2) PSD of Fine CBP Nb Surface Before/ After EP

KEK fine CBP fine grain sample 2 KEK fine CBP large grain sample 9 KEK fine CBP single crystal sample 13 KEK fine CBP large grain sample 9 after EP KEK fine CBP single crystal sample 13 after EP KEK fine CBP fine grain sample 2 after EP

RMS~200nm AFM Measurement ( 50µm*50µm) RMS~7nm RMS~40nm

With “standard”1:10 HF/H2SO4 Electrolyte at 30°C Nb crystallography affects the polishing effectiveness. With identical starting topography from CBP, given identical 100 min “EP” at 30°C, single-crystal material was significantly smoother. Evidence for a significant etching activity at 30°C, consistent with RDE analysis and visual experience.

Not all Nb “EPs” the same

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Avoid sulfur at the cathode

• Most commercial electropolishing applications attempt to maximize the surface

area of the cathode to avoid process complications (power cost and chemistry).

• In contrast to this, typical horizontal cavity EP circumstances have a

cathode:anode active area ratio of 1:10. (Worse if “masking” is applied.)

• Result is high current density on the cathode and necessary high overpotential

to drive the current. This risks driving other chemistry, such as S reduction.

• ~5.5 V polarization potential @ cathode

to drive 300 mA/cm2 =~30 mA/cm2 at anode. SO4

2- + 8 H+ + 6 e- → S + 4 H2O

may proceed if cathode overpotential is too high

• 8
• 6
• 4
• 2

2 4 6 8 10 12 14 16 18 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Current ( A ) Voltage ( V )

Area ratio of Nb/ Al T = 20.5 +/- 1.3

• C

Al area kept unchanged ( 2.6 cm

2 )

10 : 1 ; 8 : 1 ; 6 : 1 4 : 1 ; 2 : 1 ; 1 : 1

• 8
• 6
• 4
• 2

2 4 6 8 10 12 14 16 18 20 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340

• 8
• 6
• 4
• 2

2 4 6 8 10 12 14 16 18 20 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340

Anode Current Density ( mA/cm

2)

Area ratio of Nb/ Al T = 20.5 +/- 1.3

• C

Al area kept unchanged ( 2.6 cm

2 )

10 : 1 ; 8 : 1 ; 6 : 1 4 : 1 ; 2 : 1 ; 1 : 1

Cathode Current Density ( mA/cm

2)

Voltage ( V )

Power supply voltage Power supply voltage

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Implications:

• We should expect the best micropolishing for topographic

features smaller than ~ 10 µm, so start with surfaces that are consistently smooth to this scale: e.g. CBP.

• This process we call “EP” also has a temperature-dependent

etching process present. So, minimize the temperature as much as is practical (and minimize lattice strains).

• Reduce or eliminate sulfur production at the cathode by

minimizing cathode current density and improving the reaction kinetics for hydrolysis at the cathode.

(1:10 HF/H2SO4 Electrolyte with Nb)

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If the objective is maximally smooth surfaces without sulfur particles :

Contributors

• H. Tian

JLab, (W&M)

• O. Trofimova JLab
• M. J. Kelley

JLab, W&M, VT

• L. Zhao

JLab, W&M

• S. Corcoran VT
• G. Ribeill

JLab (DOE SULI)

Key References:

• A. J. Bard and L. R. Faulkner, Electrochemical Methods, Wiley: New York, 1980.
• H. Tian, S. G. Corcoran, C. E. Reece and M. J. Kelley, J. Electrochem. Soc. 155(2008), p. D563.

F F. Éozénou, S. Berry, Y. Gasser, and J-P. Charrier, SRF2009, Berlin, Germany (2009), THPPO069. V.G. Levich, Physicochemical Hydrodynamics, Prentice-Hall, New York, 1962.

• H. Tian, Ph.D. Dissertation, Dept. of Applied Science, College of William and Mary, (2008).

“A Novel Approach to Characterizing the Surface Topography of Niobium Superconducting Radio Frequency (SRF) Accelerator Cavities,” H. Tian, C. E. Reece, and M. J. Kelley, Appli. Surf. Sci., (submitted) (2010). “Evaluation of the diffusion coefficient of fluorine during the electropolishing of niobium” H.Tian and C. E. Reece, PRST-AB, (submitted) (2010). Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.

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