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Ion-Enhanced Field Emission for Control
of Atmospheric Pressure Discharges
(Grant FA9550-11-1-0020)
Prof. David B. [email protected]
http://www.nd.edu/~sst
Aerospace and Mechanical Engineering
01/07/2013
2013 Plasma and Electro-energetic Physics Program Review
slide 2 D B. Go 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
slide 3 D B. Go 01/07/2013
Effects of Confinement*• decreased electrode spacing
affects charge density distribution & Debye length
• increased surface-to-volume ratio affects energy balance and distribution
MicrodischargesMicrodischarges• 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
Lighting
http://www.edenpark.com/ http://www.plasmainstitute.org/
Medical and DentalEnvironmental 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
slide 4 D B. Go 01/07/2013
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.
slide 5 D B. Go 01/07/2013
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
slide 6 D B. Go 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
slide 7 D B. Go 01/07/2013
PIC/MCC SimulationsPrior analytical models illustrate role of ion-enhanced field emission on breakdown, but do not articulate exact interaction. Can we dissect the discharge/field emission coupling and its role during breakdown?
particle-in-cell / Monte Carlo collision simulations (NDPIC1D)
anode
d ~ 2-10 μm
seco
ndar
y em
issi
on (
γ)io
n-en
hanc
ed F
-N b
ound
ary
cath
ode
PIC/MCC Parameters
• 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 Fowler-
Nordheim equation based on PIC-calculated surface field
Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013
slide 8 D B. Go 01/07/2013
PIC/MCC: Microscale Breakdown
Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013
below breakdown voltage
above breakdown voltage
inflection point = breakdown voltage
Defined PIC/MCC breakdown voltage (simulation method) that is consistent with theoretical basis for breakdown of gas gaps
4 μm gap, 760 torr
slide 9 D B. Go 01/07/2013
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
slide 10 D B. Go 01/07/2013
PIC/MCC: Positive Space ChargeIn the pre-breakdown regime, transport is dominated by drift such that there is a net ion accumulation (103 – 104 cm-3 greater number density)
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
5 μm gap, applied voltage is 98% of breakdown voltage
slide 11 D B. Go 01/07/2013
PIC/MCC: Ion Concentrations
emitted electron
ionization
ions enhance electric field
e–
e–
Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013
100 torr 760 torr
5 μm gap, applied voltage is 98% of breakdown voltage
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
slide 12 D B. Go 01/07/2013
PIC/MCC: Cathode Processes
Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013
10 μm gap, 760 torr applied voltage is breakdown voltage
3 μm gap, 760 torrapplied 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.
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.
slide 13 D B. Go 01/07/2013
Fluid ModelPIC/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 incorporates field emission into Townsend-type equations that describe steady state behavior below the breakdown threshold Townsend discharge regime
Fluid Model
1/α
e
e
e
e
e
+
+
+
+
+
Anode
e
e
+
+
Cathode
j+
positive ion current
j+
electron current
Rumbach, and Go, J. Appl. Phys., 2012
• system of coupled, non-linear ODEs• pseudo-analytical and numerical solution• constant secondary emission coefficient γ
and field enhancement β• field emission modeled with Fowler-
Nordheim equation based on self-consistently determined surface field
slide 14 D B. Go 01/07/2013
Fluid Model: Governing Equations
Maxwell’s equation:
electron conservation
ion conservation
incorporate field emission in the boundary condition
Boundary Conditions
Coupled by drift and
≈ 0
Governing ODEs
Pseudo-analytical assume E ≈ V/d to decouple
Rumbach, and Go, J. Appl. Phys., 2012
slide 15 D B. Go 01/07/2013
Fluid Model: Scaling Laws
where
total current density
electric field due to ions (space charge)
ion concentration
Rumbach, and Go, J. Appl. Phys., 2012
Pseudo-analytical solution leads to scaling laws• assumes space charge field is much smaller than applied field
All parameters scale directly with field emission current which implies
scaling with ~exp(d/V)
slide 16 D B. Go 01/07/2013
Fluid Model: Ion Concentrations
Field-emitted electrons create an abundance of ions in the gap consistent with PIC/MCC simulations
Rumbach, and Go, J. Appl. Phys., 2012
argon, 760 torr, applied voltages below the breakdown threshold
slide 17 D B. Go 01/07/2013
Fluid Model: Ion ConcentrationWhy are ion concentrations so large at the microscale?
non-dimensionalized Poisson’s equation:
implying
Abundantfield emitted electrons produce large number of ions sufficient to distort electric field and enhance ionization
ion density necessary to significantly distort applied electric field
Rumbach, and Go, J. Appl. Phys., 2012
φ = Φ/VA
X = x/d
space charge is negligible if
in smaller gaps, more space charge is needed to distort the applied field to cause breakdown
slide 18 D B. Go 01/07/2013
Fluid Model: Numeric ResultsNumerically solved fully coupled system
Rumbach, and Go, J. Appl. Phys., 2012
argon, 3 μm gap, 760 torr
divergence when space charge influences numerical solution
breakdown voltage
slide 19 D B. Go 01/07/2013
Fluid Model: Critical Space Charge
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
argon, 3 μm gap, 760 torr
breakdown voltage
slide 20 D B. Go 01/07/2013
Fluid Model: BreakdownBoundary condition at the cathode produces a transcendental equation for the integration constant c1
Rumbach, and Go, J. Appl. Phys., 2012
A solution does not exist for large enough applied potentials model predicts breakdown voltage due to field emission
argon, 3 μm gap, 760 torr
LHS RHS
slide 21 D B. Go 01/07/2013
Fluid Model: Modified Paschen’s Curve
Rumbach, and Go, J. Appl. Phys., 2012
Fluid model produces multiple paths to predict the modified Paschen’s curve that are consistent with earlier theory, PIC/MCC simulations, and experiments
slide 22 D B. Go 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
slide 23 D B. Go 01/07/2013
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
slide 24 D B. Go 01/07/2013
Experiments: I-V Measurements
Consistently observe field emission at pressure consistent with field emission-driven Townsend discharge
slide 25 D B. Go 01/07/2013
Experiments: Pressure Scaling
Theory implies that current should scale with pressure and experiments support this conclusion (preliminarily).
j ≈ exp(αd)×jFE
N2, 4 μm gap, tungsten electrodes
fluid model predicts scaling of
where α scales with pressure
slide 26 D B. Go 01/07/2013
Experiments: Exotic Cathode MaterialsSilver nanoparticles synthesized in our lab via microhollow cathode plasma jet-induced electrochemistry and dropcast on titanium substrates
Nanoparticles increase current 3 orders of magnitude at less than 50 V due to local field enhancement
slide 27 D B. Go 01/07/2013
Experiments: Exotic Cathodes Materials
Ar, 3 μm gap size
Carbon nanotubes (CNTs) synthesized at Georgia Tech via plasma-enhanced chemical vapor deposition
CNTs increase current 5 orders of magnitude at less than 100 V due to field enhancement and show similar pressure scaling
repeatability a challenge
slide 28 D B. Go 01/07/2013
Experiments: Post-breakdownWhat happens after breakdown? Depends on the active field emission area (sites)
Large Active Area → High current → Power supply current limitedbefore breakdown occurs
for this data, a stable glow was visually observed through the ITO anode.
Small Active Area → High current density j → Large flux j ~ 109 A/m2 causes explosive field emission and arcing
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
N2, 3 μm gap size, 630 torr, tungsten electrodes
slide 29 D B. Go 01/07/2013
Experiments: Practical ConsiderationsVarying surface properties make repeatability an issue
Active sitesLarge variability in βWork function and oxide layersGas adsorption
TEM image ofoxide layer oncobalt nanowire.
Xavier et al. Nanotechnology, 2008.
G.N. Fursey, App. Surf. Sci., 2003.
active sites
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
slide 30 D B. Go 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
slide 31 D B. Go 01/07/2013
Ongoing: Glow Discharge Simulations
total current potential distribution
ion distribution electron distribution
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
(Macroscale) DC glow discharge using NDPIC1DElectron Energy Distribution (95% of breakdown voltage)
10 μm gap
3 μm gap
Note: high energy tail
slide 32 D B. Go 01/07/2013
Ongoing: Ion-Enhanced Field Emission
Emission current
1D time-independent Schrodinger equation
Preliminary calculations of pure Fowler-Nordheim 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
slide 33 D B. Go 01/07/2013
Ongoing: Electron/Surface Interactions
*Collaboration with R. Mohan Sankaran, Case Western Reserve University
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
e-
100 μm
Microhollow cathode plasma jet impinging on liquid surface
Silver nanoparticles synthesized by reduction of Ag+
slide 34 D B. Go 01/07/2013
Ongoing: Electron/Surface Interactions
Cathode 2H+ + 2e- → H2(g)
Anode 2H2O → O2(g) + 4H+ + 4e-
Becomes more basic
Becomes more acidic
Witzke, Rumbach, Go, and Sankaran, J. Phys. D: Appl. Phys., 2012
Production and measurement of hydrogen gas and pH changes by plasma jet confirms gaseous electrons reduce protons in liquid
chromatograph of H2 production
*Collaboration with R. Mohan Sankaran, Case Western Reserve University
slide 35 D B. Go 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
slide 36 D B. Go 01/07/2013
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
slide 37 D B. Go 01/07/2013
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
slide 38 D B. Go 01/07/2013
Relevant PublicationsJournal 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)
4. 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.
5. P. Rumbach, D. B. Go, “Properties of a Field Emission-Driven Townsend Discharge” Gaseous Electronics Conference, Austin, TX, 2012.
6. 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.
slide 39 D B. Go 01/07/2013
AcknowledgementsStudents
• 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 AwardGrant FA9550-11-1-0020