The Mechanism of Electropolishing of Nb in Hydrofluoric-Sulfuric Acid (HF+H 2 SO 4 ) Electrolyte

Transcription

1 The Mechanism of Electropolishing of Nb in Hydrofluoric-Sulfuric Acid (HF+H 2 SO 4 ) 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-FG2-6ER41434

2 Introduction Electropolishing 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.

3 Classical Two-Electrodes Set-up Is Not Enough to Identify the Local Effect on Nb EP Current density ( ma/cm 2 ) Nb I-V pwrsup curve T = /-.5 o C Reactive area = 5.72cm 2 Current limited plateau V pwsup : 12 ~25 V 25~5 ma/ cm 2 Power supply Nb Cavity is anode 1 r/min Al is cathode HF: H 2 SO 4 =1:9 (volume) Voltage ( power supply) V pwsup = V Nb +V eletrolyte +V Al I-V pwsup only is impossible to clearly identify the local effect on Nb EP, such as electrolyte temperature, acid concentrations, viscosity and stirring.

4 Three-Electrode-Setup Improved Electrochemical Characterization of EP 4 Cathode: Al I-V Example: V PwrSup = 15 V V cathode : ~ 4 V V electrolyte : ~ 2 V V anode :~ 9V Current density ( ma/cm 2 ) T = /-.5 o C Ref electrode is nearby Nb. Reactive area = 5.72 cm Voltage ( vs. MSE ) Anode: Nb I-V Not Power Supply Voltage MSE : Mercury / Mercurous Sulfate Reference Electrode Separating impacts of individual components in EP system. Enables enlightening study of temperature, flow, and composition dependent effects (electrolyte) in detail.

5 Anode Current Density Strongly Depends on Local Temperature Anode Current Density ( ma/cm 2 ) o C; o C o C; o C o C 54.6 o C 45.6 o C 33.5 o C 26.3 o C 21.3 o C V Nb ( V ) (vs. MSE ) Anode Current Density ( ma/cm 2 ) S Nb / S Al = 1 : 1 Past studies identified ºC for best EP gloss on Nb surface Temperature ( o C )

6 The output flow temperature is in within specs (~35ºC), but the actual cavity wall temperature is out of control (4~56ºC). 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 expected. Non-uniform polishing is C. Reece et. al, SRF 7

7 Anode Current Density Varies Linearly with HF Acid Volume Concentration Anode Current Density ( ma/cm 2 ) Area ratio of Nb/Al =1:1 T = 31.5+/- 1.5 o C 1:9.8:9.2.6:9.4.4:9.6.2: Voltage ( V ) ( vs. MSE) M HF : decreases 8% ; M H2O : decreases 32% M H2SO4 : increases 8.8% 5.27 ma/cm ma/cm 2 decreases 78% 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.

8 Additional Volume H 2 O slightly Increases Anode Current Density Current density ( ma/cm 2 ) T = 14 +/- 1 o C HF: H 2 SO 4 : H 2 O (by volume) 1 : 9 -fresh mixed acid 1 : 9 :.5 1 : 9 : 2 1 : 9 : 3 1 : 9 : 5 M HF : decreases 33.6% M H2SO4 : decreases 33.4%. M H2O : increases 235% Voltage (V) vs. MSE Anode Plateau Current Density ( ma/cm 2 ) 1 V vs. MSE T = 14 +/- 1 o C 1: Fresh mixed 1:9 eletrolyte + V H2 O 2: * Used 1:9 eletrolyte+ V H2 O * Fresh mixture exposured under chemical hood for 5 hrs) Different volume ratio of H 2 O Anode current density slightly increases with additional volume of H 2 O (38~42%); Compare curve 1 and curve 2( after 5 hrs exposure under chemical hood), current density decreases 3% -HF evaporation

9 Anode Current Density Does Not Depend on the Relative Area of Anode and Cathode Anode current density ( ma/cm 2 ) Area ratio of Nb/ Al a T =.5 +/- 1.3 o C Cathode area (Al) kept unchanged ( 2.6cm 2 ) 1 : 1 ; 2 : 1 ; 4 : 1 6 : 1 ; 8 : 1 ; 1 : 1 Anode current Density ( ma/cm 2 ) 28 Area ratio of Nb/ Al T = /- 1.7 o C b 24 Anode area(nb) kept unchanged ( 2.6 cm 2 ) 1 : 1 ; 1: 2 ; 1 : 4 1: 6 ; 1 : 8 ; 1 : V Nb (V) vs. MSE V Nb ( V ) vs. MSE Anode current density remain constant

10 Current-Limited Plateau of Nb EP: Mass Transport Mechanism Has Been Unknown Nernst-Planck equation Ci ( x) zif φ( x) Ji( x) = Di 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) I) Metal Ions (M n+ aq ) II) Acceptor anion (A- ) III) H 2 O Concentration C st C st C st M aq M aq diffusion layer MA y diffusion layer diffusion layer Understanding of mass transport mechanism requires further electrochemical study-eis.

11 What is Electrochemical Impedance Spectroscopy? -ImZ High Frequency Nyquist Plot Z = ReZ -ImZ 2 R p ωcdl R p + R s j ω Cdl R p 1 + ω Cdl R p ReZ R s : electrolyte, temperature. R p : temperature, concentration of reaction products, potential. R w : the frequency of potential perturbation C dl : electrolyte, temperature, potential, oxide layer electrode roughness R p : polarization/charger transfer resistor R warburg : diffusion resistor Possible equivalent circuit of Nb-acid interface during EP Nb Ref. Elec. R s : solution resistor C dl : double layer capacitor of electrode surface

12 EIS Study of Constant Current Density 25 Area Ratio of Nb/Al = 1 : 1 Anode Current Density ( ma/cm 2 ) ( Nb : cm 2 ; Al : cm 2 ) Ref electrode & Thermal Couple nearby Nb ( < 5 mm ) T = 21.5 o C Hz.2 Hz Anode Potential ( V ) 1.1 KHz =ω max =1/R p C dl -Z imag ( Ω) KHz R p increases with the potential R s remains constant Z real (Ω)

13 EIS Study of Different Flow Rates Static (triangle) vs. Agitation (dot) flow rate ~ 4 ~ 5 cm/sec T = 9.2±.1 ºC.2 Hz.2 Hz.2 Hz -Z imag ( Ω).2 Hz Flow Static Z real (Ω) KHz 3V 6V R at different flow condition remains as constant R p decreases with increasing flow

14 What We have Learned from EIS Studies? Polarization resistance (ohms) 24 T = 9. +/-.2 o C 22 Without agitation with agitation ( flow rate : v ~ 4 ~ 5 cm/sec) Potential ( V ) Capacitance ( μf ) T = 9. +/-.2 o C Without agitation with agitation ( flow rate : v ~ 4 ~ 5 cm/sec) Potential ( V ) The change of R s, R p and C dl at the different potential regions and flow condition are used to identify the possible mass transport mechanism

15 EIS Indicates Compact Salt Film Model Constant R s at different potential regions and flow condition rules out the porous salt film model. R p increase at different potential regions is inconsistent with the adsorbates acceptor model. C dl decrease at different potential regions & C dl increase at different flow conditions are consistent with the compact salt film model.

16 Nb The Diffusion-Limited Access of F - To the Salt Film Produces Best Polishing Diffusion Layer (~ um) F - % Bulk Electrolyte Sulfuric acid tends to anodize the Nb under polarization potential producing the "compact salt film - Nb 2 O 5. HF acid tends to dissolve the Nb oxide under kinetic control with the "at the surface" concentration of F -. Distance F - concentration at the surface is limited by how fast it diffuses through the electrolyte ( ~diffusion layer). Compact Salt Film (~ nm) Nb 2 O 5 F - % Distance The local gradient in F - concentration produces the desired polishing action. Local temperature, flow (stirring) & electrolyte composition affect the local F - gradient.

17 EIS Study for Implications Surface Optimization & Quality Control Mechanical ground sample Light BCP ( 3 μm removal) 3 mins 31ºC Preliminary studies show that there is a signature difference in EIS response between rough & smooth surfaces. Potentially useful for on-line process feedback

18 Summary and Future Work The first use of three-electrode-setup reveals that Nb EP strongly depends on the local electrolyte temperature and HF/H 2 SO 4 volume ratio. High frequency impedance data provide strong evidence for the presence of a compact salt film (Nb 2 O 5 ) 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. 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.

19 Thank You