Characterization of a surface dielectric barrier discharge

A.J.M. Pemen1, F.J.C.M. Beckers

1, E.J.M. van Heesch

1

1Eindhoven University of Technology, Department of Electrical Engineering,

P.O.Box 513, 5600 MB Eindhoven, The Netherlands, email: a.j.m.pemen@tue.nl

Abstract: A surface dielectric barrier discharge (SDBD) reactor provides a homogeneous plasma

over a large surface area. This allows surface treatments of foils, textiles or fibers. Here we present

results of a study to characterize the AC and pulsed performance of SDBD with regard to ozone

production, equivalent electrical model, and fast imaging of the plasma development.

Keywords: surface-dielectric barrier discharge, ozone yields, fast imaging.

1. Introduction

Surface Dielectric Barrier Discharges (SDBD) are gen-

erated in a thin layer along along a dielectric surface, at

room temperature and atmospheric pressure [1-4]. SDBD

has a streamer like nature on macroscopic scale. These

streamers move rapidly and have a high density along the

surface, resulting in homogeneous surface treatment.

SDBD reactors are constructed of a dielectric material with

parallel strips (high voltage electrode) at one side, and a

ground plane at the backside (Fig.1).

Fig.1 Example of a SDBD plasma reactor (10x20 cm).

Pulsed and AC operation is possible because any applied

voltage variation dU/dt will result in a displacement cur-

rent in the dielectric barrier which enables discharge cur-

rent to flow. The AC excited plasma consists of small local

discharges which extinguish almost immediately due to

charge accumulation on the dielectric barrier which re-

duces the gap voltage.

For uni-polar ns pulses the SDBD reactor behaves dif-

ferently. The gap voltage is able to exceed the discharge

inception voltage which results in a higher initial electric

field in the gap. The high dU/dt of the pulse slopes results

in a larger discharge current, and in one discharge on the

rising edge and a discharge on the falling edge of the pulse.

Here we present results of a study to characterize the AC

and pulsed performance of SDBD with regard to ozone

production, equivalent electrical model, and fast imaging

of the plasma development.

2. Experimental set-up

The SDBD reactor is constructed of a 1 mm thick Al2O3

plate (10x20 cm). The HV electrodes consist of 20 parallel

platinum strips, printed on top of the dielectric (1 mm wide,

180 mm long, mutual distance 3 mm. A ground plane is

printed at the back. Total capacitance between ground

plane and electrodes is ≈700pF. AC operation was done by

a 22 kHz resonator, with a voltage up to 4 kVrms. Pulsed

operation [5], is performed with pulse voltages up to 7.2

kV (30 ns duration, 20 ns risetime).

We applied ozone measurements to compare chemical

activity of pulsed and AC excited plasma. Uniformly dis-

tributed ambient air is flushed along the SDBD, O3 was

measured by UV absorption (230-290 nm) in the exhaust

of the reactor [6]. An ICCD camera was used to study the

pulsed and AC excited plasma optically (180-800nm,

576x384 pixels, shutter time >5 ns).

Fig.2 Equivalent electrical model for AC SDBD

3. Verification of AC equivalent model

An equivalent electrical model for SDBD is shown in

Fig.2. If an AC voltage (Ue) is applied over the electrodes,

upon reaching the inception voltage, a small, transient

discharge will occur which accumulates charge on the

dielectric surface. The charge causes a voltage drop over

the discharge region, resulting in the quenching of the

discharge so that the electric field drops below the critical

value to maintain the discharge. The voltage over the dis-

charge region Udis increases again when the electrode

Plasma off

Plasma on

Cd

Cg

Ce

Udis

Cd

Cg Udis

Ce

voltage increases, resulting in a next discharge. This

process will repeat itself, resulting in a burst of micro

discharges and a nearly constant voltage Udis over the

discharge zone. So the electric charge accumulated on the

dielectric combined with the applied voltage to the elec-

trodes will determine when the voltage in the gap is high

enough to initiate the discharges.

The SDBD model (Fig.2) consists of capacitance Cd ,

which represents the capacitance of the plasma layer to the

ground plane, and Cg is the capacitance of the discharge

region. During discharges the voltage Udis over the dis-

charge region remains more or less constant as explained

before, here represented by two zener diodes, with a zener

voltage that is equal to the discharge inception voltage. The

electrodes have a constant capacitance Ce to the ground

plane. Note that a single discharge occurs in the model

instead of several micro discharges. The energy dissipation

of the plasma takes place in the zener diodes.

Fig.3 Measured voltage-charge plot for SDBD (integrated reactor

current [total charge q] vs applied rms-electrode voltage).

The voltage-charge Lissajous plot for a SDBD (Fig.3)

can be corrected for capacitance Ce by substracting its

charge from the total electrode charge. This gives curve

1-2-3-4. At point 1 the displacement current through the

reactor will be zero because dUe/dt is zero. The zero

crossing causes an interruption of the previous negative

discharges because the voltage in the discharges (Ug) de-

creases due to the negative displacement current. Voltage

Ug has to reach the positive inception voltage during period

interval 1→2. Now a conductive channel is introduced by

the discharges, during interval 2→3 and 4→1. These

slopes will correspond therefore to capacitance Cd. The

external current will increase substantially during the

discharge periodes. Between 3 and 4 the voltage Ug has to

swing over again and the slope corresponds to the series

connection of Cd and Cg. Negative discharges will occur

during interval 4→1. The width of the plot corresponds to

twice the magnitude of the required minimum external

applied electrode voltage Ue which is necessary to reach

the discharge inception voltage. The voltage Ug (which is

close to the inception voltage) can be calculated by multi-

plying Ue with ratio Cd/(Cd+Cg), since Cg and Cd act as

capacitive divider during interval 1→2 and 3→4.

Cd depends on the amount of discharges covering the

SDBD surface. Fig.4 shows Cd versus the applied rms

voltage. Capacitance Cd has a lower value for lower volt-

ages, because the surface of the SDBD isn’t completely

covered with discharges. For voltages >3kV (≈225W) the

value of Cd stabilizes. At this point the plasma is visually

homogeneous. Second observation is the lower value of Cd

for negative discharges.

Fig. 4 Capacitance Cd vs. applied rms voltage

Fig.5 (a) Results of ozone measurements. (b) Zoom of plot (a)

4. Results of ozone measurements

Figure 5 shows the measured ozone concentrations for

pulsed and AC excitation as function of the applied energy

density of the plasma (J/L). The plasma power for AC

2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6

200

300

400

500

600

Cd (

pF

)

RMS voltage (kV)

positive cycle (2>3)

negative cycle (4>1)

0 500 1000 1500 20000

50

100

150

200

250

300

30 ns pulses, 2.2-7.2 kV

25 kHz AC, 2-4 kV

Ozo

ne c

once

ntr

ation

(p

pm

)

Plasma energy density (J/L)

0 50 100 150 200 250 3000

25

50

75

100

Ozo

ne

con

ce

ntr

atio

n (

pp

m)

Plasma energy density (J/L)

30 ns pulses, 2.2-7.2 kV

25 kHz AC, 2-4 kV

operation is varied by the reactor rms voltage, while for

pulsed operation the pulse repetition rate is used to adjust

the power. Only relative low power levels could be reached

for pulsed operation, because of limited repetition rate and

energy per pulse.

The AC plasma produces little O3 for energy densities

3

kV. In this case, the entire SDBD surface is covered with

discharges. Also, the intensity at 3.8 kV is almost similar to

the intensity at 3.47 kV. Apparently, the energy cannot be

converted into a larger plasma surface coverage, resulting

into more intense or faster repetitive discharges. For pulsed

26 mm

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2100

200

300

400

500

600

700

800

900

1000

1100

Distance (mm)

Avera

ge

Inte

nsity

(au)

2.56kV

3.06kV

3.47kV

3.84kV

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2100

200

300

400

500

600

700

800

900

1000

1100

Distance (mm)

Avera

ge

Inte

nsity

(au)

2.56kV

3.06kV

3.47kV

3.84kV

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

200

400

600

800

1000

1200

1400

Distance (mm)

Avera

ge

In

ten

sity

(au)

3.57kV

4.21kV

5.90kV

7.19kV

9.80kV

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

200

400

600

800

1000

1200

1400

Distance (mm)

Avera

ge

In

ten

sity

(au)

3.57kV

4.21kV

5.90kV

7.19kV

9.80kV

operation, the intensity increases with increasing pulse

voltage.

Scaling and normalizing the various profiles shows that

they are nearly similar for AC and pulsed operation, and

for the various voltage levels (Fig.9). No significant dif-

ference in streamer length or scaled intensity near the high

voltage electrode strip is observed. A persistent mechanism

controls the expansion of the streamers. Presumably the

interacting E field around the strips and the electron fronts

in the head of the streamers from opposite side determine

the expansion of the discharges. Note that Fig.9 gives the

scaled profile for only a limited set of voltage levels.

However, profiles at other voltages are similar.

Fig. 9 Scaled and normalized intensity profiles for both AC and

pulsed operation

Conclusions

An equivalent electrical model is introduced to describe

the AC behavior of a SDBD. The parameters of the model

can be determined from a lissajous plot of the total charge

applied to the SDBD (integral of the measured current) and

the applied rms-voltage. The value for Cd (dielectric sur-

face capacitance) depends on the plasma coverage of the

reactor. The value stabilizes if the applied voltage is large

enough to create homogeneous plasma coverage.

The chemical efficiency of a SDBD plasma for AC or

pulsed operation was compared by measuring ozone con-

centrations in the exhaust of the reactor. Although com-

parison of the pulsed and AC excited plasmas ability to

produce ozone proved to be difficult, it is evident that a

pulsed exited plasma is more efficient in producing ozone.

High speed images showed that, for pulsed operation, the

plasma is already very homogeneous during one single

shot. The plasma profile along the dielectric surface of the

SDBD appeared to be similar for pulsed and AC excited

plasma at different voltage/energy levels. A persistent

mechanism controls the expansion of the streamers.

References

[1] M. Simor et.al. Atmosperic-pressure plasma treatment

of polyester nonwoven fabrics for electrodeless plating,

Surface and Coatings Techn. 172 (2003) 1-6.

[2] L. Cernakova et.al., Surface modification of polypropy

lene non-woven fabrics by atmospheric-pressure

plasma activation followed by acrylic acid grafting,

Plasma Chem.& Plasma Proc., Vol.25, No.4, August

2005

[3] D. Korzec et.al., Insulated surface discharge for

metastables driven processing at atmospheric pressure,

Surface and Coatings Techn. 169-170, (2003),

228-232.

[4] T. Hoder et.al., Investigation of the coplanar barrier

discharge in synthetic air at atmosperic pressure by

cross-correlation spectroscopy, J.Phys.D, Appl.Phys,

41 (2008) 035212.

[5] A. J. M. Pemen, I. V. Grekhov, E. J. M. van Heesch, K.

Yan, S. A. Nair, S. V. Korotkov. Pulsed corona

generation using a diode-based pulsed power generator.

Review of Scientific Instruments, Vol. 74, No.74, 2003

[6] E.J.M. van Heesch, G.J.J. Winands, A.J.M. Pemen,

Evaluation of pulsed streamer corona experiments to

determine the O radical yield, J.Phys.D, Appl.Phys 41

(2008) 234015.

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Distance (mm)

Scale

d A

ve

rag

e I

nte

nsity

AC: 2.56kV

AC: 3.47kV

Pulse: 5.90kV

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Distance (mm)

Scale

d A

ve

rag

e I

nte

nsity

AC: 2.56kV

AC: 3.47kV

Pulse: 5.90kV