Download Nb Electropolishing

The Mechanism of Electropolishing
of 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
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
Introduction
¾Electropolishing 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).
¾The large variation of cavity performance results from the
present empirical EP processes. A microscopic understanding of
the basic Nb EP mechanism is yet lacking.
¾The 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.
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
25~50 mA/ cm2
Classical “Two-Electrodes” Set-up Is Not Enough
to Identify the Local Effect on Nb EP
Nb I-Vpwrsup curve
40
o
2
Current density ( mA/cm )
T = 31.5 +/- 0.5 C
35 Reactive area = 5.72cm2
30
Current limited plateau
25
Power supply
Nb Cavity is anode
1 r/min
20
Al is cathode
15
Vpwsup: 12 ~25 V
10
5
HF: H2SO4=1:9 (volume)
Vpwsup= VNb+Veletrolyte+VAl
0
0
2
4
6
8
10
12
14
16
18
20
Voltage ( power supply)
I-Vpwsup only is impossible to clearly identify the local
effect on Nb EP , such as electrolyte temperature, acid
concentrations, viscosity and stirring.
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
Three-Electrode-Setup Improved
Electrochemical Characterization of EP
40
2
Current density ( mA/cm )
Cathode: Al I-V
Example:
VPwrSup = 15 V
Vcathode : ~ 4 V
Velectrolyte: ~ 2 V
Vanode:~ 9V
o
35
T = 31.5 +/- 0.5 C
Ref electrode is nearby Nb .
2
Reactive area = 5.72 cm
30
25
Anode: Nb I-V
20
15
Not Power Supply Voltage
10
5
0
-4
-2
0
2
4
6
8
10
12
14
16
18
20
Voltage ( vs. MSE ) MSE : Mercury / Mercurous
Sulfate Reference Electrode
Separating impacts of individual Enables enlightening study of
temperature, flow, and composition
components in EP system.
dependent effects (electrolyte) in detail.
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
Anode Current Density Strongly
Depends on Local Temperature
80
o
o
1 54.6 C; 3 33.5 C
o
o
2 45.6 C; 4 26.3 C
o
5 21.3 C
100
o
54.6 C
o
45.6 C
40
o
33.5 C
20
o
26.3 C
o
21.3 C
0
2
4
6
8
SNb/ SAl = 10 : 1
2
60
0
Anode Current Density ( mA/cm )
2
Anode Current Density ( mA/cm )
100
10
VNb( V ) ( vs. MSE )
12
14
16
18
80
60
Past studies identified
25-35 ºC for best EP
gloss on Nb surface
40
20
0
20
30
40
50
o
Temperature ( C )
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
60
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
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. Non-uniform polishing is
expected.
C. Reece et. al, SRF 2007
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
Anode Current Density Varies Linearly
with HF Acid Volume Concentration
25
40
2
Anode Current Density ( mA/cm )
2
35
23.44 mA/cm
Area ratio of Nb/Al =10:1
o
T = 31.5+/- 1.5 C
20
30
MH2O: decreases 32%
1:9
25
15
0.8:9.2
20
0.6:9.4
15
MHF: decreases 80% ;
MH2SO4: increases 8.8%
10
0.4:9.6
10
5
0.2:9.8
5
2
5.27 mA/cm
decreases 78%
0
0
0
2
4
6
8
10
12
Voltage ( V ) ( vs. MSE)
14
16
18
0.2
0.4
0.6
0.8
1.0
Concentration of HF ( by volume )
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.
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
Additional Volume H2O slightly Increases
Anode Current Density
2
o
T = 14 +/- 1C
HF: H2SO4: H2O (by volume)
1 : 9 -fresh mixed acid
1 : 9 : 0.5
1:9:2
1:9:3
1:9:5
2
Current density( mA/cm )
50
40
30
25
Anode Plateau Current Density ( mA/cm )
10 V vs. MSE
60
1
20
2
15
o
T = 14 +/- 1 C
1: Fresh mixed 1:9 eletrolyte + VH O
10
20
MHF: decreases 33.6%
10
MH2SO4: decreases 33.4%.
MH2O: increases 235%
0
-2
0
2
4
6
8
10 12 14 16 18 20
Voltage (V)vs. MSE
2
2: * Used 1:9 eletrolyte+ VH O
5
2
* Fresh mixture exposured under chemical
hood for 5 hrs)
0
0
1
2
3
4
Different volume ratio of H2O
5
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
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
Anode Current Density Does Not Depend on
the Relative Area of Anode and Cathode
32
Area ratio of Nb/ Al
a
o
T = 20.5 +/- 1.3 C
2
Cathode area (Al) kept unchanged ( 2.6cm )
1:1;2:1;4:1
6 : 1 ; 8 : 1 ; 10 : 1
24
20
Anode current Density ( mA/cm2)
2
Anode current density ( mA/cm )
28
Area ratio of Nb/ Al
b
o
T = 19.7 +/- 1.7 C
2
Anode area(Nb) kept unchanged ( 2.6 cm )
1 : 1 ; 1: 2 ; 1 : 4
1: 6 ; 1 : 8 ; 1 : 10
28
16
12
8
4
24
20
16
12
8
4
0
0
2
4
6
8
10
VNb (V) vs. MSE
12
14
16
18
20
0
0
2
4
6
8 10 12
VNb( V ) vs. MSE
Anode current density remain constant
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
14
16
18
20
Current-Limited Plateau of Nb EP:
Mass Transport Mechanism Has Been Unknown
Nernst-Planck equation
J i ( x ) = − Di
∂Ci ( x) zi F
∂φ ( x)
−
DiCi
+ Ciυ ( x )
∂x
RT
∂x
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)
Concentration
I) Metal Ions (Mn+aq)
II) Acceptor anion (A-)
Cst
Cst
Cst
III) H2O
Maq
Maq
diffusion layer
MAy
diffusion layer
diffusion layer
Understanding of mass transport mechanism
requires further electrochemical study-EIS.
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
What is Electrochemical Impedance Spectroscopy?
High Frequency Nyquist Plot
ReZ
-ImZ
Z =
-ImZ
Rp
1 + ω 2C dl R p
2
2
ω C dl R p 2
+ Rs − j
2
2
1 + ω 2C dl R p
Rs: electrolyte, temperature.
Rp: temperature, concentration of reaction products,
potential.
Rw: the frequency of potential perturbation
Cdl: electrolyte, temperature, potential, oxide layer
ReZ
electrode roughness
Rp: polarization/charger transfer resistor
Possible equivalent
circuit of Nb-acid
interface during EP
Rwarburg: diffusion resistor
Ref. Elec.
Nb
Rs: solution resistor
Cdl : double layer capacitor of electrode surface
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
EIS Study of Constant Current Density
2
Anode Current Density ( mA/cm )
25
20
Area Ratio of Nb/Al = 10 : 1
2
2
( Nb : 26.035 cm ; Al : 2.6035 cm )
Ref electrode & Thermal Couple nearby Nb ( < 5 mm )
o
T = 21.5 C
0.2 Hz
15
10
0.2 Hz
5
0
0
2
4
6
8
10
12
14
16
Anode Potential ( V )
18
1.01 KHz =ωmax=1/RpCdl
-Zimag(
)
Ω
200 KHz
Rp increases with the potential
Rs remains constant
3th
Zreal (Ω)
international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
EIS Study of Different Flow Rates
Static (triangle) vs. Agitation (dot)
flow rate ~ 4 ~ 5 cm/sec
0.2 Hz
T = 9.2± 0.1 ºC
0.2 Hz
-Zimag(
)
Ω
0.2 Hz
0.2 Hz
Static
Flow
200 KHz
Zreal (Ω)
3V
6V
Rs @ at different flow condition
remains as constant
Rp decreases with increasing flow
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
What We have Learned from EIS Studies?
5.0
22
o
T = 9.0 +/- 0.2 C
Without agitation
with agitation ( flow rate : v ~ 4 ~ 5 cm/sec)
o
4.5
T = 9.0 +/- 0.2 C
Without agitation
with agitation ( flow rate : v ~ 4 ~ 5 cm/sec)
4.0
20
Capacitance ( μF )
Polarization resistance (ohms)
24
18
16
14
12
3.5
3.0
2.5
2.0
10
1.5
8
1.0
6
3
4
5
Potential ( V )
6
7
3
4
5
6
7
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
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
EIS Indicates “Compact Salt Film” Model
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 “adsorbates
acceptor” model.
Cdl decrease at different potential regions & Cdl increase at different flow
conditions are consistent with the “compact salt film” model.
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
The Diffusion-Limited Access of F- To the
Salt Film Produces Best Polishing
¾ Sulfuric acid tends to anodize the Nb
under polarization potential producing
the "compact salt film”- “Nb2O5”.
Diffusion Layer
(~ um)
Nb
Bulk Electrolyte
F- %
¾ HF acid tends to dissolve the Nb oxide
under kinetic control with the "at the
surface" concentration of F- .
Distance
¾The local gradient in Fconcentration produces the desired
polishing action.
F-%
Compact Salt Film
(~ nm)
“Nb2O5”
¾ F- concentration “at the surface” is
limited by how fast it diffuses through the
electrolyte ( ~diffusion layer).
Distance
Local temperature, flow (stirring) & electrolyte
composition affect the local F- gradient.
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
EIS Study for Implications
Surface Optimization & Quality Control
Mechanical ground sample
Light BCP ( 30 μm removal)
30 mins EP @ 31ºC
Preliminary studies show that there is a signature difference in EIS response
between rough & smooth surfaces. Potentially useful for on-line process feedback
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
Summary and Future Work
9The first use of “three-electrode-setup” reveals that Nb EP strongly depends on
the local electrolyte temperature and HF/H2SO4 volume ratio.
9High frequency impedance data provide strong evidence for the presence of a
compact salt film (Nb2O5) in the current-limited plateau region.
9The 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.
9 Quantify the scale-dependant smoothing effects of niobium EP as a function of
temperature, electrolyte composition, and flow conditions. Specify optimum
processing protocol.
9 Build electrochemical understanding to better appreciate conditions that
contribute to smoothing.
9 Identify useful on-line process monitors for process assurance and progress
tracking – EIS and etc?
9 Guide the evolution of cavity treatment protocols toward well engineered and
controlled conditions for cavity productions.
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008
Thank You
3th international workshop on thin film & new ideas for pushing the
the limits of RF Superconductivity
J-lab, Newport News, VA July 2222-25, 2008