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2013 Plasma and Electro-energetic Physics Program Review Ion-Enhanced Field Emission for Control of Atmospheric Pressure Discharges (Grant FA9550-11-1-0020) Prof. David B. Go [email protected] http://www.nd.edu/~sst Aerospace and Mechanical Engineering 01/07/2013 Outline • Overview / Motivation for Microdischarge Research – The importance of field emission • 2012 Accomplishments – Modeling and Theory – Experiments • Ongoing work – Glow discharge simulations – Quantum modeling of ion-enhanced field emission – Electron/surface interactions • Conclusions and Outlook D B. Go 01/07/2013 slide 2 Microdischarges Microdischarges Effects of Confinement* • gas discharges with a characteristic dimension less than 1 mm • advantageous pd scaling enables stable operation at high p (1 atm) • high pressure leads to new chemical pathways new applications • decreased electrode spacing affects charge density distribution & Debye length • increased surface-to-volume ratio affects energy balance and distribution Lighting http://www.edenpark.com/ Medical and Dental http://www.plasmainstitute.org/ Environmental and Chemical Analysis Nanomaterials Harper et al., Anal. Chem., 2009 *Mariotti & Sankaran, J. Phys D: Appl. Phys., 2010 As surfaces begin to play a dominant role, it is necessary to establish a better understanding of plasma/surface interactions D B. Go 01/07/2013 slide 3 Plasma/Surface Interactions • Plasma/surface interactions important for applications – liquid/flesh/sputtering/cells/etc. – electrochemistry/biological/environmental • Plasma/electrode damage important for device development – device lifetime/robustness/design … • Plasma/electrode coupling important for fundamental understanding – emission processes (secondary/photo/thermionic/field) – charging processes (dielectric barriers) At the microscale (< 10 μm), field emission becomes an important emission process that is not important at larger scales. D B. Go 01/07/2013 slide 4 Summary of Research Program • At the microscale, the electric field can be very high (~10-100 V/μm) such that electrons tunnel from the cathode – electron field emission acts as an additional charge source – mechanism responsible for deviation from Paschen’s curve in microscale gaps – at pressure, ions in the electrode gap affect the electric field ion-enhanced field emission • Prior work established theories for the modified Paschen’s curve that included field emission (Go and Pohlman, J. Appl. Phys., 2010; Tirumala and Go, Appl. Phys. Lett., 2010) • Research Overview – explore relationship between field emission and discharge using combination of theoretical modeling and computational simulations – experimentally determine impact of ions on field emission and establish controllable field emission-driven Townsend discharges – new direction: explore how electrons produced in atmospheric-pressure discharges interact with surfaces and liquids D B. Go 01/07/2013 slide 5 Outline • Overview / Motivation for Microdischarge Research – The importance of field emission • 2012 Accomplishments – Modeling and Theory – Experiments • Ongoing work – Glow discharge simulations – Quantum modeling of ion-enhanced field emission – Electron/surface interactions • Conclusions and Outlook D B. Go 01/07/2013 slide 6 PIC/MCC Simulations particle-in-cell / Monte Carlo collision simulations (NDPIC1D) cathode PIC/MCC Parameters anode secondary emission (γ) ion-enhanced F-N boundary Prior analytical models illustrate role of ionenhanced field emission on breakdown, but do not articulate exact interaction. Can we dissect the discharge/field emission coupling and its role during breakdown? • 1d/3v • argon • 100-760 torr • non-uniform grid to capture near cathode effects • constant secondary emission coefficient γ and field enhancement β • field emission modeled with FowlerNordheim equation based on PICcalculated surface field d ~ 2-10 μm Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013 D B. Go 01/07/2013 slide 7 PIC/MCC: Microscale Breakdown Defined PIC/MCC breakdown voltage (simulation method) that is consistent with theoretical basis for breakdown of gas gaps 4 μm gap, 760 torr below breakdown voltage above breakdown voltage inflection point = breakdown voltage Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013 D B. Go 01/07/2013 slide 8 PIC/MCC: Microscale Breakdown PIC/MCC simulations and a detailed comparison to theory confirm field emission-driven breakdown. Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013 D B. Go 01/07/2013 slide 9 PIC/MCC: Positive Space Charge In the pre-breakdown regime, transport is dominated by drift such that there is a net ion accumulation (103 – 104 cm-3 greater number density) 5 μm gap, applied voltage is 98% of breakdown voltage 100 torr 760 torr • Net positive space charge enhances the electric field in the domain, increasing field emission • Appreciable build-up of positive ions (~1014 –1015 cm-3) suggests significant discharge below breakdown threshold Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013 D B. Go 01/07/2013 slide 10 PIC/MCC: Ion Concentrations Because of the sensitivity to electric field, field emission closely tracks the ionic space charge creating a positive feedback mechanism leading to high ion densities ionization e– ions enhance electric field emitted electron e– 5 μm gap, applied voltage is 98% of breakdown voltage 100 torr Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013 760 torr D B. Go 01/07/2013 slide 11 PIC/MCC: Cathode Processes 10 μm gap, 760 torr applied voltage is breakdown voltage In larger gaps, secondary emission leads to breakdown before the electric field (or ionization) is sufficient to induce field emission. Secondary emission quickly becomes the dominant cathode process. 3 μm gap, 760 torr applied voltage is breakdown voltage In small gaps, the electric field and ionization is sufficient to induce field emission and it grows comparably to secondary emission. Both phenomena are important cathode processes. Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013 D B. Go 01/07/2013 slide 12 Fluid Model PIC/MCC modeling useful for short transient effects and to get particle statistics, but not useful to parametrically explore impact of field emission and to get basic scaling laws. Can we understand the field emission-driven Townsend discharge regime? 1-D Steady State Fluid Model Fluid Model e 1/α + e + e + e + e e e Cathode + positive ion current j+ electron current j+ + + Anode Fluid model incorporates field emission into Townsend-type equations that describe steady state behavior below the breakdown threshold Townsend discharge regime • system of coupled, non-linear ODEs • pseudo-analytical and numerical solution • constant secondary emission coefficient γ and field enhancement β • field emission modeled with FowlerNordheim equation based on selfconsistently determined surface field Rumbach, and Go, J. Appl. Phys., 2012 D B. Go 01/07/2013 slide 13 Fluid Model: Governing Equations Governing ODEs electron conservation ion conservation dj e = aj e dx dj + dj e =dx dx Maxwell’s equation: Coupled by drift Pseudo-analytical assume E ≈ V/d to decouple ≈0 j = rmE and æ è ö ø a = p × f çE p÷ Boundary Conditions d j + (d) = 0 V = je (x = 0) = jFE (E0 ) + gj + (x = 0) + j 0 ò -E(x)dx = F(d) - F(0) 0 incorporate field emission in the boundary condition Rumbach, and Go, J. Appl. Phys., 2012 D B. Go 01/07/2013 slide 14 Fluid Model: Scaling Laws Pseudo-analytical solution leads to scaling laws • assumes space charge field is much smaller than applied field total current density ead éë jFE ( bV d ) + j0 ùû jtot = ad 1- g (e -1) jFE ( bV d ) + j0 A (V, d, p) electric field due to ions (space charge) Eions = ion concentration ad ax e e ( ) é j ( bV ) + j ù n+ (x) = 0û ë FE d eb+ (V d ) éë1- g (ea d -1)ùû 1- g (e -1) ad where A(V, P, d) = 1 Ve 0 b+ [ 1 a 2 ad (1 - e ) + 2 d 2 ad e + ad ] All parameters scale directly with field emission current which implies scaling with ~exp(d/V) Rumbach, and Go, J. Appl. Phys., 2012 D B. Go 01/07/2013 slide 15 ion concentration near cathode, n+,0 (cm-3) Fluid Model: Ion Concentrations argon, 760 torr, applied voltages below the breakdown threshold 1015 1mm 3mm 5mm 1013 7mm 10 11 9mm 109 107 105 11mm 103 101 10-1 0 50 100 150 200 250 300 350 applied potential, VA (V) Field-emitted electrons create an abundance of ions in the gap consistent with PIC/MCC simulations Rumbach, and Go, J. Appl. Phys., 2012 D B. Go 01/07/2013 slide 16 Fluid Model: Ion Concentration Why are ion concentrations so large at the microscale? non-dimensionalized Poisson’s equation: ¶ 2j qd 2 = n+ 2 ¶ X VAe 0 φ = Φ/VA X = x/d space charge is negligible if ¶ 2j »0 2 ¶ X ion density necessary to significantly distort applied electric field n+ = qd 2 n + << 1 implying VAe 0 VAe 0 10qd 2 in smaller gaps, more space charge is needed to distort the applied field to cause breakdown Abundantfield emitted electrons produce large number of ions sufficient to distort electric field and enhance ionization Rumbach, and Go, J. Appl. Phys., 2012 D B. Go 01/07/2013 slide 17 Fluid Model: Numeric Results Numerically solved fully coupled system argon, 3 μm gap, 760 torr 3.5x104 Numeric Approximate Native field emission 3.0x104 2.5x104 2.0x104 divergence when space charge influences numerical solution breakdown voltage current density, j (A/m2) 4.0x104 1.5x104 1.0x104 5.0x103 0.0 -5.0x103 140 150 160 170 180 applied potential, VA (V) Rumbach, and Go, J. Appl. Phys., 2012 D B. Go 01/07/2013 slide 18 argon, 3 μm gap, 760 torr 8x1013 7x1013 Numeric Approximate 6x1013 5x1013 4x1013 breakdown voltage ion concentration near cathode, n+,0 (cm-3) Fluid Model: Critical Space Charge 3x1013 2x1013 1x1013 0 -1x1013 140 150 160 170 180 applied potential, VA (V) Assumption that space charge is negligible (ESC << V/d ) fails when n ~ 1013 cm-3 consistent with scaling! Rumbach, and Go, J. Appl. Phys., 2012 D B. Go 01/07/2013 slide 19 Fluid Model: Breakdown (a) 8000 [ ( )] c1 1- g e -1 = C FN [ ad LHS bV Current Density, A/m2 Boundary condition at the cathode produces a transcendental equation for RHS, 173V 6000 LHS, 173V the integration constant c1 é ù -DFN ,d, p)] expê bV ú + j0 d + c1 A(V 2000 ë d + c1 A(V ,d, p) û 4000 2 0RHS 0 2000 argon, 3 μm gap, 760 torr (a) RHS, 173V LHS, 173V 6000 4000 2000 0 0 2000 Current Density, A/m2 6000 8000 8000 6000 8000 RHS, 174V LHS, 174V 4000 2000 0 0 2000 4000 C1 (b) 8000 6000 4000 C1 6000 6000 (b) 8000 Current Density, A/m2 Current Density, A/m2 8000 4000 C1 RHS, 174V LHS, 174V A solution does not exist for large enough applied potentials 4000 model predicts breakdown voltage due to field emission 2000 0 Rumbach, and Go, J. Appl. Phys., 2012 D B. Go 01/07/2013 slide 20 Fluid Model: Modified Paschen’s Curve Fluid model produces multiple paths to predict the modified Paschen’s curve that are consistent with earlier theory, PIC/MCC simulations, and experiments Rumbach, and Go, J. Appl. Phys., 2012 D B. Go 01/07/2013 slide 21 Outline • Overview / Motivation for Microdischarge Research – The importance of field emission • 2012 Accomplishments – Modeling and Theory – Experiments • Ongoing work – Glow discharge simulations – Quantum modeling of ion-enhanced field emission – Electron/surface interactions • Conclusions and Outlook D B. Go 01/07/2013 slide 22 Experiments: Experimental Setup • tungsten electrodes for durability and good field emission properties • thin spacers made of photoresist enable varying the electrode gap • transparent ITO anodes enable imaging of the discharge D B. Go 01/07/2013 slide 23 Experiments: I-V Measurements Consistently observe field emission at pressure consistent with field emission-driven Townsend discharge Tungsten, 4mm, 750 torr, Ar 2.5x10-9 -31.0 -31.2 ln(i / V2) current, i (A) 2.0x10-9 Tungsten, 4mm, 750 torr, Ar 1.5x10-9 1.0x10-9 -31.4 -31.6 5.0x10-10 -31.8 0.0 0 50 100 150 200 applied potential, VA (V) 250 300 -32.0 0.002 0.004 0.006 0.008 1/V D B. Go 01/07/2013 slide 24 Experiments: Pressure Scaling 6.0x10 N2N2, , 4 μm gap,Tungsten tungsten electrodes 4mm, -9 -30.5 -30.6 LN(i / V2 ) current, i (A) -30.7 4.0x10-9 -30.8 fluid model predicts scaling of -30.9 -31.0 j ≈ exp(αd)×jFE -31.1 -31.2 0.00 2.50x10-3 5.00x10-3 1/V 7.50x10-3 1.00x10-2 where α scales with pressure 2.0x10-9 110 torr 320torr 630 torr 0.0 0 50 100 150 200 250 300 350 applied potential, VAP (V) Theory implies that current should scale with pressure and experiments support this conclusion (preliminarily). D B. Go 01/07/2013 slide 25 Experiments: Exotic Cathode Materials Silver nanoparticles synthesized in our lab via microhollow cathode plasma jet-induced electrochemistry and dropcast on titanium substrates Ag nanoparticles, Ar, p = 100torr, d = 5mm -20 3.28x10 -6 2.46x10-6 LN(i / V2 ) current, i (A) -21 -22 -23 1.64x10 -6 -24 1.0x10-2 2.0x10-2 3.0x10-2 1/V 4.0x10-2 5.0x10-2 8.20x10-7 0.00 0 10 20 30 40 50 applied potential, VAP (V) Nanoparticles increase current 3 orders of magnitude at less than 50 V due to local field enhancement D B. Go 01/07/2013 slide 26 Experiments: Exotic Cathodes Materials Carbon nanotubes (CNTs) synthesized at Georgia Tech via plasmaenhanced chemical vapor deposition Ar, 3 μm gap size -16 Intercept = -15.00971, Slope = -134.04668 7.0x10-4 current, i (A) 5.0x10-4 ln(i / V2) 760torr 300torr ~150torr 10torr 6.0x10-4 4.0x10-4 760torr 300torr ~150torr 10torr -17 3.0x10-4 -18 2.0x10 -4 1.0x10-4 0.0 30 40 50 60 70 80 applied potential, VA (V) 90 100 -19 0.00 0.01 0.02 0.03 0.04 0.05 1/V CNTs increase current 5 orders of magnitude at less than 100 V due to field enhancement and show similar pressure scaling repeatability a challenge D B. Go 01/07/2013 slide 27 Experiments: Post-breakdown What happens after breakdown? Depends on the active field emission area (sites) Large Active Area → High current → Power supply current limited before breakdown occurs 400 Small Active Area → High current density j → Large flux j ~ 109 A/m2 causes explosive field emission and arcing Intercept = -30.24097, Slope = -153.87224 -30.0 -30.2 200 -30.4 -30.6 LN(i / V2) applied potential, VAP (V) N2, 3 μm gap4mm, size,N2, 630 torr, tungsten 630torr, FE to glowelectrodes 0 -30.8 -31.0 -31.2 -31.4 0.00 10-11 10-10 10-9 10-8 10-7 2.50x10-3 5.00x10-3 1/V 10-6 10-5 7.50x10-3 10-4 1.00x10-2 10-3 10-2 Moderate Active Area → If the area is just right, a transition from field emission to a glow regime can be observed → difficult to control active emission area current, i (A) for this data, a stable glow was visually observed through the ITO anode. D B. Go 01/07/2013 slide 28 Experiments: Practical Considerations Varying surface properties make repeatability an issue Active sites Large variability in β Work function and oxide layers Gas adsorption TEM image of oxide layer on cobalt nanowire. Xavier et al. Nanotechnology, 2008. active sites G.N. Fursey, App. Surf. Sci., 2003. E. V. Nefyodtsev et al., IEEE Trans., 2011. Field emission experiments at pressure especially challenging to due gas environment and ion bombardment continued area of study D B. Go 01/07/2013 slide 29 Outline • Overview / Motivation for Microdischarge Research – The importance of field emission • 2012 Accomplishments – Modeling and Theory – Experiments • Ongoing work – Glow discharge simulations – Quantum modeling of ion-enhanced field emission – Electron/surface interactions • Conclusions and Outlook D B. Go 01/07/2013 slide 30 Ongoing: Glow Discharge Simulations Glow discharge (PIC/MCC) simulations: • reveal thermodynamics of field emission-driven Townsend discharge (below breakdown) • understand impact of field emission on DC glow discharges at microscale Electron Energy Distribution (95% of breakdown voltage) (Macroscale) DC glow discharge using NDPIC1D total current potential distribution 10 μm gap 3 μm gap ion distribution electron distribution Note: high energy tail D B. Go 01/07/2013 slide 31 Ongoing: Ion-Enhanced Field Emission Quantum modeling of ion-enhanced field emission: • understand quantum effects more completely • develop model that can be incorporated into PIC/MCC simulations for more accurate discharge modeling 1D time-independent Schrodinger equation Emission current Preliminary calculations of pure Fowler-Nordheim field emission D B. Go 01/07/2013 slide 32 Ongoing: Electron/Surface Interactions Electrons produced by atmospheric discharges (either microhollow cathode or field emission-driven Townsend) can be used to drive electrochemical (reduction) reactions at both solid and liquid surfaces for applications such as materials synthesis Microhollow cathode plasma jet impinging on liquid surface Silver nanoparticles synthesized by reduction of Ag+ 100 μm e- *Collaboration with R. Mohan Sankaran, Case Western Reserve University D B. Go 01/07/2013 slide 33 Ongoing: Electron/Surface Interactions Production and measurement of hydrogen gas and pH changes by plasma jet confirms gaseous electrons reduce protons in liquid chromatograph of H2 production 225 150 Intensity (a.u.) 300 (v) 75 (iv) 45 min (iii) (ii) (i) 20 min 10 min 5 min No plasma 200 250 300 Time (s) Cathode 2H+ + 2e- → H2(g) Anode 2H2O → O2(g) + 4H+ + 4e- Becomes more basic Becomes more acidic *Collaboration with R. Mohan Sankaran, Case Western Reserve University Witzke, Rumbach, Go, and Sankaran, J. Phys. D: Appl. Phys., 2012 D B. Go 01/07/2013 slide 34 Outline • Overview / Motivation for Microdischarge Research – The importance of field emission • 2012 Accomplishments – Modeling and Theory – Experiments • Ongoing work – Glow discharge simulations – Quantum modeling of ion-enhanced field emission – Electron/surface interactions • Conclusions and Outlook D B. Go 01/07/2013 slide 35 Conclusions • Clarified interaction between discharge and field emission that leads to breakdown • Established theoretical basis and basic scaling parameters for field emission-driven Townsend discharges • Experimentally confirmed field emission-driven Townsend discharge regime and preliminarily confirmed pressure scaling – Showed nanomaterials can produce high field emission currents at pressure under 100 V – Showed that it is possible to form a DC glow discharge in a microgap D B. Go 01/07/2013 slide 36 Outlook • Areas of future work – Clarifying the thermodynamics and properties of field emission-driven Townsend discharges and microscale glow discharges – Understanding ion-enhanced field emission at a more fundamental level and incorporating more accurate physics into PIC/MCC – Experimentally confirming the pressure scaling of field emission – Exploring electron/surface interactions • Field emission-driven reduction reactions at solid surfaces • Microhollow cathode plasma jets at liquid surfaces D B. Go 01/07/2013 slide 37 Relevant Publications Journal Articles 1. Y. Li, R. Tirumala, P. Rumbach, D. B. Go, “The coupling of ion-enhanced field emission and the discharge during microscale breakdown at moderately high pressures,” IEEE Transactions on Plasma Science – in press. 2. P. Rumbach, D. B. Go, “Fundamental properties of field emission-driven DC microdischarges,” Journal of Applied Physics, vol. 112, art. no. 103302, 2012. 3. M. Witzke, P. Rumbach, D. B. Go, R. M. Sankaran, “Evidence for the electrolysis of water by plasmas formed at the surface of aqueous solutions,” Journal of Physics D: Applied Physics, vol. 45, art. no. 442001, 2012. Conferences (2012 only) 1. M. Witzke, P. Rumbach, D. B. Go, R. M. Sankaran, “Reactions at the Interface of Plasmas and Aqueous Electrodes: Identifying the Role of Electrons,” AVS International Symposium and Exhibition, Tampa Bay, FL, 2012. 2. P. Rumbach, D. B. Go, “Properties of a Field Emission-Driven Townsend Discharge” Gaseous Electronics Conference, Austin, TX, 2012. 3. P. Rumbach, J. Li, R. Tirumala, D. B. Go, “The Influence of Field Emission on Breakdown and Townsend Discharges in Microscale Gaps,” International Workshop on Mechanisms of Vacuum Arcs, Albuquerque, NM, 2012. D B. Go 01/07/2013 slide 38 Acknowledgements Students • • • • • • • • • Rakshit Tirumala Jay Li Paul Rumbach Danny Taller Michael Johnson Sara Dale (ug) Matt Goedke (ug) Zack Woodruff (ug) Adam Talbot (ug) Air Force Young Investigator Award Grant FA9550-11-1-0020 D B. Go 01/07/2013 slide 39