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Dielectric-Barrier Discharges. Principle and
Applications
U. Kogelschatz, B. Eliasson, W. Egli
To cite this version:
U. Kogelschatz, B. Eliasson, W. Egli. Dielectric-Barrier Discharges. Principle and Applications. Jour-
nal de Physique IV Proceedings, EDP Sciences, 1997, 07 (C4), pp.C4-47-C4-66. �10.1051/jp4:1997405�.
�jpa-00255561�
J.
PHYS
IV
FRANCE
7
(1 997)
Colloque C4, Supplement
au Journal de Physique
I11
d'octobre 1997
Dielectric-Barrier Discharges. Principle and Applications
U.
Konelschatz,
B.
Eliasson and
W.
Egli
ABB Corporate Research Ltd., Baden, Switzerland
Abstract:
Dielectric-barrier discharges (silent discharges) are non-equilibrium discharges that can be conveniently
operated over a wide temperature and pressure range. At about atmospheric pressure electrical breakdown occurs in
many independent thin current filaments. These short-lived microdischarges have properties of transient high
pressure glow discharges with electron energies ideally suited for exciting or dissociating background gas atoms and
molecules. The traditional application for large-scale ozone generation is discussed together with novel applications
in excimer
UV
lamps, high power
CO,
lasers and plasma display panels. Additional applications for surface
treatment and pollution control are rapidly emerging technologies. Recent results on greenhouse gas recycling and
utilisation in dielectric-barrier discharges are also discussed.
1.
INTRODUCTION
Dielectric-barrier discharges (DBDs), also referred to as barrier discharges or silent discharges have for a
long time been exclusively related to ozone generation. Detailed investigations into the properties of this
non-equilibrium discharge which can be conveniently operated at about atmospheric pressure led to a
number of new applications: The generation of powerful coherent infrared radiation in
CO, lasers and of
incoherent
VUV
or
UV
excimer radiation in excimer lamps are two examples that became commercially
available within a few years. Other processes like pollution control or surface treatment with DBDs show
great promise for the future. As far as market potential is concerned the most important use of DBDs will
be in ac plasma display panels. In 1996 multi-billion dollar investments in production facilities for
large-
area flat television screens in Japan and South Korea started a new age of large-scale industrial
dielectric-barrier applications.
The most important characteristic of dielectric-barrier discharges is that non-equilibrium plasma
conditions can be provided in a much simpler way than with other alternatives like low pressure
discharges, fast pulsed high pressure discharges or electron beam injection. Its flexibility with respect to
geometrical configuration, operating medium and operating parameters is unprecedented. Conditions
optimised in laboratory experiments can easily be scaled up to large industrials installations. Efficient low
cost power supplies are available up to very large powers.
First introduced by
W.
Siemens in 1857 [I] for the purpose of "ozonizing" air DBDs have for a long
time been regarded as the ozonizer discharge. Important new insight into the structure of the discharge
was gained by high voltage engineers studying gas breakdown. In 1932 Buss [2] observed that in a plane
parallel gap with insulated electrodes air breakdown occurs in a number of individual tiny breakdown
channels. More detailed information about these current channels was collected by
Klemenc et al. in 1937
[3], by Honda and Naito in 1955 [4] and later by Gobrecht et al. [S], by Bagirov et al. [6], by Tanaka et al.
[7], Hirth [8] and by Heuser [9].
More recently it was realised that the plasma parameters in these breakdown channels, now
frequently referred to as microdischarges, can be influenced and modelled and consequently can be
optimised for a given application
[8,10-171. Advanced plasma diagnostics and computer modelling has
put us in a position to understand and tailor microdischarge properties for an intended purpose. Our
present understanding of dielectric-barrier discharges is described starting with an overview of overall
discharge characteristics and microdischarge properties. Ways of influencing microdischarge properties
are discussed. Models of different degrees of sophistication are presented for computing plasma physical
details about microdischarge formation during breakdown, plasma chemical reactions following
breakdown, or both.
A
number of novel applications based on DBDs are described.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1997405
C4-48
JOURNAL
DE
PHYSIQUE
IV
2.
DISCHARGE STRUCTURE AND MICRODISCHARGE PROPERTIES
Dielectric-barrier discharges are characterised by the presence of one or more insulating layers in the
current path between metal electrodes in addition to the discharge space. Different planar or cylindrical
configurations are common (Fig. 1). Closely related are surface discharge configurations in which
discharges are initiated at a dielectric surface due to strong electric fields generated by imbedded metal
electrodes. The presence of the
dielectric(s) precludes dc operation. Although
DBD
configurations can be
operated between line frequency and microwave frequencies the typical operating range for most
technical
DBD
applications lies between 500 Hz and 500 kHz.
High Voltage
/
Electrode
High
Voltage
Discharge Gap
Ground
Electrode
?ctric Discharge
rrier\
-
/
Fig.
1:
Common dielectric-barrier discharge configurations
Gas spaces bounded by one or two dielectrics have practically the same breakdown voltage as if they
were between metal electrodes. Typical clearances vary from less than 100
prn
to several cm. For
atmospheric pressure discharges gap spacings of a few mm are common thus requiring alternating driving
voltages with amplitudes of typically 10
kV. Low loss dielectrics of high breakdown strength such as
glass, quartz or ceramic plates or tubes are used to which metal electrode coatings can be applied. On the
other hand, also metal electrodes with dielectric coatings, e. g. steel tubes with enamel layers can be used.
2.1
Microdischarge Properties
The most interesting property
of
DBDs is
that
in most gases at about atmospheric pressure breakdown is
initiated in a large number of independent current filaments or
rnicrodischarges. Fig.
2
shows a snapshot
of such microdischarges in a
1
rnrn
air gap photographed through a transparent electrode.
Fig. 2: End-on view of microdischarges
Original size:
6
x
6
cm, exposure time:
20
ms
Fig.
3:
Lichtenberg figure showing footprints of individual
microdischarges, original size:
7
x
10
cm
Fig. 3 shows footprints of individual microdischarges left on a photographic plate with the emulsion
facing the discharge gap and the glass plate serving as the dielectric. After applying a high voltage
halfwave the plate was developed in the dark room. This photographic technique had already been used
by Buss [2] in 1932 who found that we can expect about 10 microdischarges of submicrosecond duration
per cm2 and that the channel diameter is roughly 0.1 mm. These findings are in accordance with later
more sophisticated measuring and modelling techniques.
At the dielectric surface a microdischarge channel spreads into a surface discharge covering a
region much larger than the original channel diameter. Typical microdischarge properties (order of
magnitude) for a 1 mm air gap at 1 bar can be summarised as follows:
Table
1:
Characteristic Microdischarge Properties.
The microdischarge filaments can be characterised as weakly ionised plasma channels with properties
resembling those of transient high pressure glow discharges. They ignite when the breakdown field is
reached and extinguish not far below the same field value when electron attachment and recombination
reduce plasma conductivity. Due to charge build up on the dielectric the field at the location of a
microdischarge is reduced within a few ns after breakdown thus terminating the current flow at this
location. Detailed measurements about microdischarge properties have been performed
18-10,14,18-211.
The transported charge is proportional to the gap spacing and the permittivity of the dielectric but does
not depend on pressure. The current density in a microdischarge channel can reach 100 to
1000 ~cm-~.
Due to the short duration this normally results in very little transient gas heating in the remaining channel.
Humidity tends to increase the strength of a microdischarge while irradiating the cathode with UV
photons tends to decrease it. The dielectric barrier limits the amount of charge and energy deposited in a
microdischarge and distributes the microdischarges over the entire electrode surface. As long as the
external voltage is rising additional microdischarges will occur at new positions because the presence of
residual charges on the dielectric has reduced the electric fields at positions where microdischarges have
already occurred. When the voltage is reversed, however, the next microdischarges will form in the old
microdischarge locations. So, high voltage low frequency operation tends to spread the microdischarges,
while low voltage high frequency operation tends to reignite the old microdischarge channels every half
period. This memory effect due to charge deposition on the dielectrics is extensively used in ac plasma
displays.
Over a wide range of operating frequencies and voltage shapes microdischarge properties do not
depend on the external driving circuit. They are determined by the gas properties, the pressure and the
electrode configuration. Raising the powkr for a given configuration means generating more
microdischarges per unit of time
andlor per unit of electrode surface area. This characteristic of DBDs is
very important because it allows us to investigate and optimise microdischarge properties for a given
application in a fairly small laboratory set-up. Scale-up even to very large electrode surfaces in industrial
applications normally does not present a problem if gap spacing and power density is not changed.
Individual microdischarge properties can however be changed for a given configuration when
extremely fast rising voltages are applied. When the rise time of the voltage becomes comparable to the
duration of a microdischarge a large number of microdischarges may be started simultaneously. In this
case there may not be enough surface area available on the dielectric to accommodate all surface
discharges of the initiated microdischarges. As a consequence weaker rnicrodischarges result. The pulsed
DBD mode offers additional flexibility by synchronising the microdischarges, by overshooting the
stationary breakdown voltage and by adapting the pauses between pulses in accordance with the process
under investigation.
Duration: 1-10 ns
Filament Radius: about 0.1
mm
Peak Current 0.1 A
Current Density: 100
-
1000 A/cm2
2.2
Overall Discharge Characteristics
Total Charge: 0.1
-
1 nC
Electron Density:
1014
-
1015 cm-3
Electron Energy: 1-10 eV
Gas Temperature: close to average gap
temperature
The described uniformity of individual microdischarges leads to a well defined overall discharge
behaviour. This can be shown, for example, by recording
voltagelcharge Lissajous figures (Fig. 4).
Suppose we apply a sinusoidal voltage. Initially, without discharge, the total capacitance
C,,
is charged
with rising voltage. When the breakdown voltage of the gas space is reached microdischarges occur that
start to charge the capacitance of the dielectric
C,while the discharge voltage
U,,
,
the average voltage
C450
JOURNAL DE PHYSIQUE
IV
across the gas space, remains constant. This stops when the maximum of the voltage
0
is reached and the
reverse situation occurs in the second half wave. In many DBD applications the voltagelcharge Lissajous
figure is an almost ideal parallelogram
(oscill9gram in Fig. 4). From the slopes the effective capacitances
C,,, and C, can be derived, and U,,, and U can be immediately determined [22]. The form of the
voltagelcharge Lissajous figure is independent of the
form of the applied voltage aslong as the notion of
a constant discharge voltage holds. The enclosed area is always proportional to the power independent of
such assumptions. According to Miiller and Zahn
[23] DBDs can exhibit different modes depending on
the gas and the operating conditions.
I
m
B
Y)
Mi"
F
8
Sf
.-
u
g
I
a b
c
Fig.
4:
Schematic diagram of applied voltage and microdischarge activity (a) and recorded
(b)
and schematic representation (c)
of
voltagelcharge Lissajous figure
From voltagelcharge diagrams Manley in 1943 [24] derived the power formula for ozonizers which
applies to many DBDs. The enclosed area corresponds to the energy dissipated during one period of the
applied voltage. According to
Manley the total power is given by:
P
=
4 f C, U,,, {
0
-
c;'
(CD
+
C,) UDi,
1,
6
2
c;'
(c,
+
c,)
uDiS
(1)
where f is the frequency and C the capacitance of the discharge gap. Since the fictitious discharge
voltage
U,, cannot be measure% directly, it is sometimes advantageous to express the power
P
by
measurable quantities
[22].
where UMi,,is the minimum external voltage required to maintain a discharge. The voltages are related by
u,is
=
C,(c,+C,)-' UMi,.
The applied voltagelcharge Lissajous figure and corresponding gap voltagelcurrent presentations
are very useful instruments for characterising the discharge
behaviour and designing power supplies and
matching circuits.
Tanaka and co-workers have extensively used these tools to characterise different
modes of dielectric-barrier discharges
[25,
261.
At
very high operating frequencies, e. g. a 10 MHz sine
wave, or also for wide gaps at reduced pressure, the parallelogram of Fig. 4 turns into an ellipse,
indicating that plasma conductivity no longer decays between successive halfwaves. The voltagelcharge
Lissajous figure of a plasma display panel driven by a 100 kHz square wave, on the other hand, shows a
well defined jump of the charge whenever an applied voltage
off 200
V
is reached [26].
3.
MODELLING DIELECTRIC-BARRIER DISCHARGES
3.1
Modelling of the Reaction Kinetics
Different aspects of dielectric barrier discharge modelling can be addressed. For a rough determination of
the different time constants involved it is often convenient to disregard spatial gradients in the beginning
and treat a homogeneous plasma as a first step. To simulate the action of a short-lived microdischarge
either a short high voltage pulse is applied
[lo, 121 or an electron beam is injected [27,
281.
In both cases
it is necessary to derive the rate coefficients for electron impact collisions in the gas mixture under
consideration by solving the Boltzmann equation. This requires a reliable set of electron collision cross
sections. For DBD calculations the local field approximation
is normally used, assuming that the
electron energy distribution is in equilibrium with the electric field and that all rate coefficients can be
tabulated as a sole function of the mean electron energy or the reduced field Eln.
Although these assumptions appear rather crude they have been used extensively and rather
successfully in optimising ozone generators. The physical reason for this success is as follows. At
atmospheric pressure electrons accelerated in the electric field perform so many collisions with the
background gas that they approach equilibrium values within about 10 ps. Appreciable voltage changes