Download surface discharge characteristics for different types of applied

SURFACE DISCHARGE CHARACTERISTICS FOR DIFFERENT
TYPES OF APPLIED VOLTAGE AND DIFFERENT DIELECTRIC
MATERIALS
M.V.Kozlova, M.V.Sokolovab, A.G.Temnikovb, V.V.Timatkovb, I.P.Vereshchaginb
a) Russian Joint Stock Company “Unified Energy System of Russia” Export Dept.
ul.Academica Chalomeya 5a, 117630 Moscow, RUSSIA
b) Department of Electrophysics and High Voltage Technique, Moscow Power Engineering Institute,
Krasnokazarmennaya 14, 111250 Moscow, RUSSIA
Abstract. Surface discharge characteristics from multi-striped electrodes on the surface of thin
dielectric plates of different kind of ceramics are presented. Experimental results obtained with
voltage pulses of microsecond and nanosecond duration show significant difference of discharge
structure, current impulse amplitude and intensity of ozone formation for the case when a
polyaluminosilicate ceramic is used as a dielectric barrier compared with an alumina ceramic.
Nanosecond voltage impulses give more dense and uniform discharge structure than long ones.
1. INTRODUCTION
The surface discharge as well as a barrier one can be used as plasma chemical reactor to produce
different types of species such as ozone or atomic oxygen needed for treatment of gases containing
toxic additives or in other plasma chemical technologies. Investigations of surface discharge [1,2]
show that the most convenient form of the electrode is a stripe (or a wire) on the barrier surface and
only initial stages of the discharge are of an interest when plasma-chemical processes are taken in
view. More developed discharge stages when thermal ionisation is possible lead to a heating of the gas
and of the dielectric barrier and to a decrease of the chemical species production efficiency. Although
it is known that the main factors that define plasma chemical processes in a surface discharge are the
electrode configuration, the applied voltage (its form and amplitude) and the material of the dielectric
barrier, nevertheless for most cases the discharge structure and other parameters of the discharge such
as its form (diffusive or discrete), the length, the diameter and the branching intensity of streamer
channels or their number per unit length of the high voltage electrode are not known.
The aim of the present work was to investigate surface discharge characteristics for different types of
ceramics because this material is one of the most advantageous for its use in a plasma-chemical reactor
[2], and to analyse the influence of the material composition on the discharge development, its
structure and light emission, its current characteristics and its effect on the ozone formation. The
investigation is carried out using voltage impulses of different duration (this fact being significant for
analysis of the mechanism of the discharge development) and using multi-striped configuration of
high voltage electrode characteristic for ozone generators on surface discharge [2]. The main feature of
the discharge that is analysed is its phenomenology in different conditions.
2. EXPERIMENTAL SETUP
A surface discharge developing from edges of thin metallic stripes placed on the surface of a ceramic
plate was investigated. The stripe thickness is ≈ 25 µm, its width is 2 mm and the distance between the
stripes from 2 to 8 mm. The dimensions of the plate are 24 x 36 mm for a single stripe case and
___________________________________
Electronic address: [email protected]
48 x 60 mm for a multi stripe case, the plate thickness being 1 mm. The earthed electrode is situated
at the other side of the plate. The material of the plates is high frequency alumina ceramic of two types
and polyaluminosilicate ceramic (policor). Volume (ρv) and surface (ρs) resistivity are given in table 1.
TABLE 1. Characteristics of the barrier materials
Ceramic material
Alumina, type A
Alumina, type B
Policor
ε
9.4
9.1
9.9
ρv , Ohm.m ρs for 20 OC, Ohm
1014
1015
14
10
1015
15
10
2.1015
Electric strength, kV/mm
54
50
50
tg δ
0.0008
0.0001
0.00005
The high voltage pulses are formed by two specially constructed impulse generators. One of them
forms pulses of microsecond duration (5-7µs) having form of a bell. The voltage amplitude can be (210) kV. Other generator produces rectangular pulses {full duration τp = 200-3000 ns, front duration τf
= 50-500ns, Um = (3-10) kV}. Additionally a series generator that forms nanosecond rectangular
pulses of 5, 10, 20 or 50 ns duration with 1 ns front and amplitudes from 3 to 8 kV was used. For
microsecond voltage pulses the pulse front steepness dU/dt could be made from 1 to 10 kV/µs.
Pictures of the discharge pattern were made by means of an image converter and by photo camera with
high resolution and magnification. Discharge light emission was measured by means of a
photomultiplier using filters that give ultraviolet and infrared parts of the light emission. To analyse
the discharge current a 0.224 Ohm low inductance shunt and an impulse high-speed oscilloscope were
used. Additionally ozone concentration in the gas flow was measured. The experimental procedure is
given in details in [3].
3. EXPERIMENTAL PROCEDURE AND RESULTS
3.1. Initial voltage of the discharge
The initial voltage U0 of the discharge appearance was measured for a series of subsequent high
voltage pulses (the frequency being about 10 Hz) of microsecond duration applied to the stripped
electrode. As the amplitude of the pulses is increased micro discharges appear at the edges of the
electrode and the light emission of the discharges and the corresponding current pulses are measured.
The U0 values were measured for different kinds of ceramic, different electrode configuration (one
stripe and multi-striped electrode with distances d = 4, 6 and 8 mm between the stripes) and different
thickness of the dielectric plate δ = 0.5, 1 and 1.5 mm. Less values of U0 for negative polarity of the
applied voltage pulse for all cases are obtained as a result of photo-effect at the electrode edge. The
mean values are: U0 − = 2.6 kV and U0 + = 2.8 kV for all values of d and dU/dt = 1-10 kV/µs. An
increase of dielectric thickness from 0.5 to 1.5 mm leads to a practically proportional increase of the
initial voltage whereas there is no difference in U0 values for different high voltage electrode pattern.
The most interesting feature is a slight increase of U0 for policor compared with other ceramic plates:
2.9 kV instead of 2.8 kV for positive polarity and 2.8 instead 2.6 kV for negative one.
3.2. Phenomenology of the discharge from a single stripe for different experimental conditions
Most experiments were carried out with alumina ceramic (type A) it being the one used as a rule in
ozone generators. Photographs of micro discharges from the edges of a single stripe (its width 2mm)
done by means of an image converter through a slit are given in fig.1 (a,b). The voltage range for
which the structure of the discharge corresponds to ones shown in fig.1 is 3.5-12 kV. With higher
voltages the discharge transforms into a creeping one that can further transform into a spark and
breakdown occurs between the high voltage and earthed electrodes. With an increase of the voltage
pulse an increase of the channels length and of their number per unit length of the electrode are seen.
The structure of the discharge is defined by voltage polarity. For a single stripe and positive polarity
the distribution of the channels along the stripe edge is less uniform and their number per unit length is
less than for negative polarity. In the last case a diffusive zone is clearly seen around each channel.
a)
b)
FIGURE 1. Pictures of discharge from positive (a) and negative (b) single stripes. Um = 9 kV. Alumina ceramic (type A).
For 4-6 kV the channels are straight
and their length does not exceed 1
mm. For higher voltage values the
negative channels begin to overlap
each other and on a diffusive
background there are seen separate
channels
that
develop
more
intensively and are longer. It seems
that these intensive channels screen
the development of nearby initial
channels. The long intensive channels
have very tortuous trajectory and do
not branch. In case of positive polarity
of the electrode the branching of the channels is seen even for (4-6 kV).
It is assumed on base of previous investigations that so called back discharges appear at the electrode
edge during falling part of the applied voltage pulse. They are caused by the electric field of the
charges left on the barrier surface as a result of primary discharges corresponding to the rise of the
applied voltage pulse. It means that the back discharge must have the polarity opposite to the primary
ones. One of the peculiarities of the discharges seen in the pictures for a single stripe is that up to Um =
6 kV there is no special visible traces of back discharges. For Um = 9 kV the streamer channels have
bright thick parts near the electrode which can possibly be the traces of the back discharges that go
along the first discharge channels.
3.3. Peculiarities of self-restricted discharge from a multi-stripped electrode
When a multi-stripped high voltage electrode is used the discharge pattern becomes more uniform. Its
development in the direction perpendicular to the electrode edge becomes less intensive but the
number of micro discharges per unit length of the electrode increases. For negative polarity long
intensive channels do not appear at all (fig.2a,b); the channels seen have a stable form slightly
enlarging as the channel develops. Near the electrode there is a short (0.2-0.4 mm) more bright part
connected to our opinion with back discharge. The channel length does not exceed half distance
between the stripes. The positive channels (fig.3a,b) for the same case of a multi-stripped electrode
have very tortuous paths and intensive luminescence of branches that have tortuous paths as well. The
luminescence of the channels near the electrode edge is very high and the diameters of the channels in
this part increases. The length of the positive channels is as a rule longer than the distance between the
stripes. Most channels have two or three branches.
Simultaneous development of streamers from neighbour stripes prevents the elongation of channels
when voltage increases. It is a result of an opposite electric field created by channel heads. A zone
without discharge appears between the stripes for negative polarity. The width of the zone filled by the
discharge decreases with the increase of Um. We call the discharge described a self-restricted one.
a)
b)
FIGURE 2. Discharge from multi-stripped
electrode. Microsecond voltage of negative polarity:
a) 4kV; b) 9 kV; d = 4 mm.
To analyse the individual discharge channels a
telescope with 3.5 magnifications was used. The
telescope optical axis formed an angle of 8-10O
with the barrier surface and the sharpness of the
picture corresponded to the middle of the stripe.
Observations of the discharge pictures show that
the discharge for both polarities develops along
the dielectric plate surface and the dimension of
the channels in the direction perpendicular to the
surface does not exceed 0.2 mm. The diameter of
the channels in the plane of the barrier measured
by means of an image converter with high
magnification can be evaluated as 0.15-0.2 mm.
An increase of the distance d between the stripes does not lead to significant qualitative differences in
the discharge except an increase of the brightness of the channel base. For higher d values there is a
proportional elongation of the channels.
The pictures of a self-restricted discharge for
different duration τ of the applied voltage
impulse (6 µs and 20 ns) are clearly different.
For short impulse the discharge channels lay
more closely and their length is less. The
luminescence has a more diffusive character
and there is no increased brightness of the
channels near the electrode as it is seen for
long voltage pulses. Such picture seen for
a)
b)
short voltage impulse changes and tends to be
FIGURE 3. Discharge from multi-stripped electrode. as one for long pulse if τ exceeds 500 ns with
the impulse front duration 150 ns.
Microsecond voltage of positive polarity: a) 4kV; b) 9kV.
.
An important fact is that the discharge pattern of a self-restricted discharge over policor is different
compared with the discharge over both kinds of alumina ceramic. In a whole the discharge over
policor has more bright and more infrequent channels especially for positive polarity of the applied
voltage (table2).
TABLE 2. Number of visible discharge channels per 3 cm of the edge of the electrode. Multi-stripe system. U =
9 kV; 1.5/6 µs.
Polarity of the electrode
Alumina A
Alumina B
Policor
+
16
14
5
−
26
27
8
3.4. Current characteristics and plasma-chemical processes for self-restricted discharge
The full current curve includes two long parts of displacement current corresponding to the front and
the tail of the applied voltage pulse and the micro discharge current impulses superimposed on the
displacement current (fig.5). The first group of current impulses correspond to prime discharges, the
second one - to back discharges. Current parameters
for self-restricted discharge are of a very near form
and amplitude. For alumina ceramic of both types and
interstripe distance d=4 mm the impulse current
amplitude Im = 0.1- 0.12 A, current curve front tf = 1.8
- 1.9 ns, current duration tc = 9.0 - 9.2 ns. If the
interstripe distance is increased (d = 6 mm) the current
amplitude increases Im = 0.2 A, but have practically
the same form of the curve as for d=4 mm. As for
policor the brightness of the streamer channels and the
current amplitude for d = 4 mm is slightly higher than
for alumina ceramic: Im = 0.12-0.18 A with practically
the same form of the current curve (tf = 1.9 - 2.0 ns, tc
= 9.2 - 9.3 ns). All these parameters are the same for
current pulses at the front of the voltage curve and at
FIGURE 4. Current oscillogram. a) alumina its tail. In fig.5 a) is for alumina ceramic; b) – policor.
A; b) Policor
The peculiarities of the surface discharge for different materials of the barrier are further manifested
when the discharge is used to produce chemically active products (ozone). If we range the materials
used according to the intensity of the discharge channels filling the discharge gap than on the first
place there will be alumina ceramic of type A, then the same of type B and the last - the policor
ceramic. Used as a barrier in a surface discharge ozone generator in which the high voltage electrode
is a multi-stripped one with d = 4 mm and the dielectric plate thickness is 1 mm, the above materials
have shown the same sequence of their ability to ozone formation (fig.7). The results presented are for
high frequency of the applied voltage f = 16 kHz, oxygen and different flow rates of the gas.
C, mg/l 50
1a
45
40
35
30
25
20
15
2a
1
2
3a
3
2,5
3
3,5
4
4,5
FIGURE 7. Ozone concentration for different barrier
materials. Oxygen, 0.5 l/min (dots) and 1 l/min.
1 and 2 – alumina ceramic type A and B, 3 – policor.
5
U, kV
4. CONCLUSIONS
The experimental results presented demonstrate a significant difference in the discharge characteristics
with dielectric barriers of equal electric parameters but different material composition. These
differences lay mainly in the number of microdischarges per unit length of the stripe electrode and are
more pronounced for not too high applied voltage (overvoltage in the range 1.5-3 U0). They are also
followed by a difference in the intensity of ozone formation. A specific discharge form is seen for selfrestricted discharge in a multi-stripped electrode arrangement that is different from the one developing
from a single electrode.
REFERENCES
[1] Masuda S., Kiss E. On streamer discharges in ceramic-based ozoniser using high frequency surface
discharge. In: Inst. Phys. Conf. Ser. No 85 : Poster Session 2, ed. IOP Publishing Ltd. Oxford, 1987, pp. 243-248
[2] Gibalov V.I., Pietsch G.J. J.Phys.D: Appl. Phys., 33, 2618-36 (2000)
[3] Kozlov M.V. Surface discharge investigation aimed to an increase of effectiveness of work of electrotechnological devices. Ph.D.Thesis, Moscow Power Engineering Institute, Moscow, 1994.