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Phase Field Modeling of Electrochemistry J. E. Guyer, W. J. Boettinger, J. A. Warren & G. B. McFadden Features common to electrochemistry & melt growth Review of – Sharp interface model of electroplating – Interface structure Phase field model – 1-D Equilibrium Results Bob Sekerka NASA

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Cu from aqueous solution: Barkey, Oberholtzer & Wu, PRL 75(1995) 2980 Mullins-Sekerka widely quoted and adapted to predict roughening of plated surface; e.g. Aogaki et al.(1980-1995) “A theory of dendritic growth in electrolytes”, D. R Hamilton, Electrochimica Acta 8 (1963) 731. Ivantsov / maximum growth rate hypothesis to fit velocity vs. overpotential data of Barton & Bockris (Ag dendrites growing from AgNO 3 in NaNO 3 -KNO 3 molten salt eutectic). current~velocity; undercooling~overpotential

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D. Josell, B. Baker, C. Witt, D. Wheeler and T.P. Moffat, Via Filling by Electrodeposition: Superconformal Silver and Copper and Conformal Nickel, Journal of the Electrochemical Society, in press D. Josell, D. Wheeler, W.H. Huber and T.P. Moffat, Superconformal Electrodeposition in Submicron Features Physical Review Letters 87, 016102 (2001). Plating in Narrow Trenches and Vias for Electronic Applications pores

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Some possible uses of phase-field modeling in electrochemistry Plating in submicron features – High curvatures, high electric field gradients – Avoid approximations Pulse plating Alloy Plating Alloy Dissolution Reveal new insight into relationship between interface charge distribution / adsorption and kinetics

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Electrochemistry

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Double Layer +++++ - - electrodeelectrolyte + + + +

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Length Scales in Electrochemistry a)Thickness of Electrode-Electrolyte interface b)Charge separation distance (Debye layer); related to concentrations and dielectric constant c)Long range concentration decay length due to diffusion/convection in electrolyte Helmholtz model Gouy-Chapman model Gouy-Chapman-Stern model

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Models of Interface Charge Distribution Helmholtz Model – Sharp Electrode-Electrolyte interface – No Debye layer ( = 0) – Voltage jump at interface (“dipole layer”) – Constant differential capacitance Gouy-Chapman Model – Sharp Electrode-Electrolyte interface – Finite Debye length (“double layer”) – Voltage continuous across interface – “Parabolic” differential capacitance Gouy-Chapman-Stern Model – Linear voltage adjacent to electrode –“Parabolic” differential capacitance with “wings”

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Interface Properties Surface energy depends on the voltage Surface charge Differential capacitance Electrocapillary equation

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experimental data Ag electrode aqueous NaF electrolyte G. Valette, J. Electroanal. Chem. 138 (1982) 37 Differential Capacitance increasing NaF concentration Comparison of our results with sharp interface models C d / (F/m 2 ) C d / (µF/cm 2 ) °/V

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Phase Field Model Add a new phase variable, and equation Solve over entire domain: Phase field equation Poisson's equation Transport equations No boundary conditions at interface Treat complex interface shape / topology changes Avoid approximations Phase-Field Model – Diffuse Electrode-Electrolyte interface – Finite Debye length – Differential capacitance appears realistic – Long-range diffusion possible ElectrodeElectrolyte Diffuse Interface

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Example of Components in Phases Mole Fractions Molar Volume Concentrations Assume Constraint Ion Charge z i Electrode is solid solution of Cu +2 and interstitial e -. Electrolyte is aqueous solution of Cu +2, SO 4 -2 and H 2 O.

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Free Energy

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Equilibrium Minimization of free energy subject to: – Solute conservation – Poisson's equation –

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Choice of Thermodynamics Ideal solution Interpolation and double-well functions p( ) g( )

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Phase Equilibria: zero charge plane X i L,Ref, X i S,Ref chosen to obtain equilibrium between a liquid solution of CuSO 4 in H 2 O & metal (Cu +2 + 2e -1 ) Ref chosen to set interface charge distribution, for Ref X’s

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Interface Properties Surface Energy Surface Charge definitions Differential Capacitance Can also define adsorptions (a result, for our model)

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Choice of other Parameters Equilibrium Double Well Heights, Gradient Energy Coefficient, Dielectric constant Related to: thickness of transition surface energy Numerical Calculations (First cut…) Finite difference scheme Evolution of dynamical equations to steady state Insensitive to initial guess (but slow)

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Equilibrium Profiles

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Concentration Profiles

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Voltage Decay Length in Electrolyte Reproduces Gouy-Chapman Result Dilute electrolyte ~ Exponential Decay Exponential fits

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Traditional Double-Layer Theory (Gouy-Chapman) Boltzmannn Distribution Poisson Equation (more generally, there is a first integral …)

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Electrolyte voltage profile is the voltage/concentration decay length (Debye length) Surface energy, surface charge, differential capacitance, etc. all related to voltage across interface, i.e., Nernst relation Double Layer (cont.)

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Numerical Technique – Spectral Element Method

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Spectral Resolution and Adaptive Strategy In each panel, monitor a N & a N-1 Fix N = 16 If max(|a N |,|a N-1 |) > bisect panel and repeat until function is well-resolved on all panels

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Numerical Method

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Resolution of Double Layer Charge distribution with uniform panels Phase field with uniform panels Chebyshev coefficients with uniform panels

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Adaptivity Charge distribution with uniform panels Chebyshev coefficients with uniform panels Charge distribution with two refinement levels Chebyshev coefficients with two refinement levels

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Spectral Computation surface energy surface charge differential capacitance

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Electrocapillarity Bard & Faulkner, Electrochemical Methods 2nd Ed., Wiley & Sons, New York (2001) after D.C. Grahame, Chem. Rev. 41 (1947) 441

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Differential Capacitance experimental data Ag electrode aqueous NaF electrolyte G. Valette, J. Electroanal. Chem. 138 (1982) 37 increasing NaF concentration Our results C d / (F/m 2 ) C d / (µF/cm 2 ) °/V

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Sharp Interface Limit

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Numerical Solution in Outer Variables Interface width: = 0.1 /2, /4, /8, /16

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Numerical Solution in Inner Variables Interface width: , /2, /4, /8, /16

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Outer and Inner Solutions

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Matched Asymptotic Expansion

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Surface Charge

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Sharp Interface Limit Interface width: , /2, /4, /8, /16 = 0 (sharp)

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Conclusions Equilibrium 1-D solutions of the model exhibit double layer behavior….Consistent with Gouy-Chapman model, and incorporate –Decay length of electrostatic potential –Interface energy (“electrocapillary curves”), surface charge and differential capacitance all look reasonable

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Current and Future Work Continue study of sharp-interface limits Kinetic studies underway Explore behavior for W e 0, non- constant ( ) Explore effects of curvature (cylindrical electrode) Adaptive Mesh in 2-D Alloy Plating/Corroding Additives, Adsorption and inhibitors

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