Download Ongoing: Ion-Enhanced Field Emission

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