preliminary work on microplasmas and apgds [email protected] paul bryant department of electrical...
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Preliminary work on microplasmas and APGDs
Paul Bryant
Department of Electrical Engineering and Electronics, Brownlow Hill, University of
Liverpool, L69 3GJ.
Recent technological and scientific advances in atmospheric pressure discharges have opened up an exciting area of research with opportunities for novel applications. These can be broadly classified into:
• Large area atmospheric pressure discharges
• AC and RF excited barrier and barrier free discharges e.g. DBDs (Dielectric barrier discharges) and bare electrode discharges. (Textiles treatment)
• Packed bead reactors (de-toxification of car gas exhausts NOx etc)
• Corona and Arc discharges (Textile treatment, environmental detoxification of gas plumes etc)
and …
• Micron - sized discharges (microplasmas)
• PDPs Plasma display panels (micro DBDs)
• Si based microplasma arrays (displays, UV sources)
• Plasma needles (dentistry)
• Micro hollow cathodes
Introduction
1) Si-based Microplasmas (with J. G. Eden and S. J. Park, University of Illinois, USA)
2500 (50 x 50) plasma pixel array based on pSi.
Each pixel is an inverted pyramid 50 m square by 30 m deep.
AC
pSi
Polyimide3 m
Ni
Si3N4
1.8 m
1.7 m
Plasma ball
Distance between pixels approx. 50 m.
Experimental set- up< 1 kVpk-pk, < 20 mA
5, 10, 15 and 20 kHz
AC Pwr
supply
Vacuum chamber and rotary pump
IdVd Active
strain gauge
Ar Ne
SRS Delay Gen.
Computer
Osc. ICCD camera
Plasma on!
Micro-Langmuir probe
Filters: 810nm, 750nm
700 V Ne
-12
-6
0
6
12
0 0.02 0.04 0.06 0.08 0.1
t / ms
I d /
mA
-2.4
-1.2
0
1.2
2.4
Ip /
mA NOPLASMA
PLASMA
diff
5 and 10kHz 500Torr Ar (B)
0
200
400
600
800
1000
0 5 10 15 20 25
Ip-p / mA
Vp
-p /
V 5 kHz
10kHz
Preliminary Results
AC
Polyimide capacitor
Zs Za<<
plasma
array
Id ip
IV independent of pressure, gas etc ! plasma is a parasitic load with Id >> ip
C10kHz = 0.46 nF
Ip per pixel approx 1.20mA / 2500 = 0.48 A!
plasma current peaks (green) increase with Vd and move towards voltage peak.
C5Khz0.57nF
C = 0.4 nF (measured independently)
Imaging results: (with Dr Greg Clarke)
700Torr Ne
7
Matching unit
2) APGD Atmospheric Pressure Glow Discharge
Dielectric Barrier Discharge
d = 1 – 2 mmAC
Filamentary plasmaHomogeneous
plasmaunstable !
Glass, quartz
rf DBD MHz
13.56MHz
Stable! With and modes like in low pressure rf CCP discharges
AC
Barrier free discharge
10 -100 kHz
Flow: 5 slm
He
50Hz, kHz
Ar, He, Ne,
O2
Collaborative Project withIn high pressure plasmas excimers (excited molecular complexes) are readily formed via three body collisions.
The resulting excimer Rg2* is unstable and rapidly disintegrates (within a few ns) typically emitting photons via spontaneous emission.
Table 1: Rare gas excimer peak radiation wavelength in nm
He2* Ne2
* Ar2* Kr2
* Xe2*
74 83 126 146 172
Glass
Stack
TiO2
ZnO2
Ag
ZnO2
ZnSnOx
18-20 nm
5 nm
10-12 nm
3-5 nm
40 nm
Figure 2. Schematic of glass sample stack structure
UV wavelengths good for photon-induced bond-breaking.
E.g. Modifying properties of glass stacks.
Need large area, stable & homogeneous plasma source.
Rg* + Rg + Rg Rg2* + Rg.
Experimental set-upExperimental set-up
1-2 mm
Current probe
High voltage probeHH audio
amp.
Audio Osc.
Oscillo.
Figure 1. Schematic of the existing APGD source set-up.
Gas in
2800 : 54
Active strain gauge
He Ne Ar
Vacuum chamber & Rotary pump
1st stage: Ignite a barrier-free discharge (simpler!)
2nd stage: rfDBD – need RF power feedthrough, ceramic plates
< 5KV, < 2A
Barrier free arrangement with 2 cm diam. Stainless steel electrodes.
Preliminary results:
Ne 10 kHz 700Torr
-300-200-100
0100200300
0 0.02 0.04 0.06 0.08 0.1
t / ms
Va
/ V
-100
-50
0
50
100
Ia /
mA Va
Ia
At ignition (approx 1 kVp-p) observe a discharge column:
Stable Ne, Ar glow
High voltage, low current
No electrode damage
Ne 700 Torr 10kHz
Not an arc !
In rfDBD by decreasing Va discharge column should fill electrode gap with uniform plasma glow …
But in our case it extinguishes!
Possible reason:
Gas flow – reduces breakdown voltage, easier to ignite volume
Summary• Microplasma array:
• plasma array ignited only at several points during the driving voltage cycle, and seem to be related to the plasma current spikes and power maximums. Various other studies are underway (i.e. Paschen curves, micro-Langmuir probe, filtered ICCDs, material treatment) to shed further light on plasmas ignition process.
• APGD:
• Upon ignition a stable discharge column (filamentary discharge?) appears. However, unlike in rfDBDs and other barrier free discharges, reducing the applied voltage does not lead to a uniform glow filling the inter-electrode space. Reason might be due to absence of flow of approx 5slm!
• Is there room for dust in microplasmas / APGDs? Yes!
• See “Observation of individual particles and Coulombic solids in a microdischarge” John, P.C., et al IEEE Trans. Plasma Science, 27, 199.
• APGDs? Reactive gases could produce dust disrupting processes, coating of dust particles, dust crystals at high pressure new phenomena!