Discharge-mode transition of jet-type dielectric barrier discharge
in mixed flow of argon gas and acetone vapor
Keiichiro Urabe, Keitaro Yamada and Osamu Sakai
Department of Electronic Science and Engineering, Kyoto University
A1-211 Kyotodaigaku-Katsura, Nishikyo-Ku, Kyoto 615-8510, Japan
E-mail address: [email protected]
Abstract: We introduce a discharge-mode transition in a jet-type dielectric barrier discharge
(DBD), using an Ar gas flow with an acetone vapor impurity. A small He plasma jet was
also used to ignite the DBD in the Ar/acetone flow. It was observed clearly that the
filamentary discharge was changed to the glow-like mode as increasing the acetone impurity
with a transition criterion around 0.3% condition of the impurity ratio. We also comprehend
the characteristics of each discharge mode by analyzing optical emission spectra and
deposited materials on the substrate.
Keywords: Dielectric barrier discharge, Optical emission spectroscopy
1. Introduction
Recently, material processes using atmosphericpressure plasmas (APPs) have attracted much
interest instead of that using low-pressure plasmas
[1]. In many kinds of APP-generation method,
dielectric barrier discharge (DBD) is one of the most
widely used ones, and its discharge mode is typically
distinguished in glow-like and filamentary modes
[2,3]. The glow-like discharge mode, called an
atmospheric-pressure glow discharge (APGD), has
been observed mainly in He gas. On the other hand,
the filamentary mode is in atmospheric air, pure Ar
gas, and so on. For material-process applications of
the DBD, the APGD using He dilute gas has been
widely used because of a need to make temporally
stable and spatially homogeneous APPs.
In this paper, we report about a discharge-mode
transition between filamentary and glow-like modes
in a jet-type DBD inside an Ar gas flow with an
acetone vapor impurity (Ar/acetone flow). The jettype DBD is a kind of DBD having similar electrode
structure to an atmospheric-pressure plasma jet
(APPJ) with a coaxial DBD configuration [4,5], and
we sprayed the APPJ to a conductive substrate
simulating thin-film deposition processes [6-8]. We
have investigated discharge mechanisms of the
APPJ and jet-type DBD in a He gas flow using laser
spectroscopic measurements [9-11]. For the next
step, we tried to confirm an effect of acetone
molecules on the glow-like discharge generation
previously reported by Okazaki et al. [12].
In order to characterize each discharge mode in
the jet-type DBD, we recorded discharge images and
optical emission spectra. We also analyzed deposited
material on the substrate of jet-type DBD by several
microscopic methods.
2. Experimental setup
Figure 1 shows a schematic diagram of the jet-type
DBD used in this study. A 45° tilted pure He gas
flow at a flow rate of 100 mL/min and a vertical
Ar/acetone mixed flow at 5.60 ~ 6.10 L/min were
fed into two glass tubes. The tilted He flow was used
as a small APPJ twisting a copper-ring powered
electrode, and sprayed in the vicinity of the verticaltube exit to generate the plasma in the Ar/acetone
flow. A Cu plate, connected to the ground with a
series capacitance of 50 pF limiting the discharge
current to keep the discharge’s stability [8], was
placed at 10 mm from the vertical-tube exit.
Ar/acetone
gas flow
He gas
flow
Glass
tube
Power
supply
10 mm
Electrode
Cu plate
Discharge
current
Capacitance
Figure 1: Experimental setup of jet-type DBD using two
crossed gas flows of He and Ar/acetone mixture.
Peak value and repetition rate of the applied
voltage were set at 6.0 kV and 5.0 kHz [13]. The
small ratio of acetone vapor was added into the Ar
gas flow by a bubbling method of liquid acetone. We
used the FT-IR method to measure absolute acetoneimpurity ratio in the Ar gas flow. From the
absorption spectra of acetone molecules in the
Ar/acetone flow, the absolute acetone-impurity ratio
was derived using Lambert-Beer’s law and reported
absorption coefficient of acetone vapor [14].
The temporally-averaged OES method was
performed to compare emission spectra of the jettype DBD in a wavelength range from 280 to 850
nm. The emission in the middle of discharge gap
between the vertical-tube exit and Cu plate was
inserted into an optical fiber making an image by a
lens. A small spectrometer recorded the spectra by a
2048-pixel CCD linear array. For the investigation
of materials deposited on the Cu plate by the jet-type
DBD, we prepared the material samples with 10minutes continuous discharge keeping the same
impurity ratio. An optical microscope was used to
record microscopic images of the deposited
materials. Fine structures of their surface
morphology were observed using a scanning
electron microscope (SEM).
Figure 2: Overviews of discharge emission in jet-type DBD in
Ar/acetone flow with small He APPJ, together with (a) setup
image. Acetone-impurity ratios are (b) 0.15% (in filament mode),
(c) 0.55% (in glow mode), and (d) 1.69% (in mix mode). Shutter
speed of digital camera is 1/160 s in (b), (c), and (d).
3. Results and Discussion
Figure shows the experimental setup (a) of grass
tubes and Cu plate, and the discharge emissions in
three conditions of the acetone impurity ratio. The
absolute acetone-impurity ratios in the conditions of
Figs. 4(b), (c), and (d) are 0.15%, 0.55%, and 1.69%.
In lower acetone-impurity ratio shown in Fig. 4(b),
we observed thin bright filaments moving rapidly,
and the filaments’ position was localized one side
near the crossing point of two gas flows. We call this
temporally unstable filamentary discharge mode
observed in lower acetone-impurity ratio as a
“filament mode” in followings. In an image taken
when the acetone-impurity ratio was 0.55%, the
glow-like homogeneous emission was observed
without any filament as shown in Fig. 4(c). In this
discharge mode, called as a “glow mode”, the
emission diffused to the whole region of Ar/acetone
flow near the Cu plate from the crossing point of
two flows. The green-color emissive spots having
similar color to the filaments were also observed on
the Cu plate in the glow mode. In an image taken in
1.69% condition of the acetone-impurity ratio,
shown in Fig. 4(d), it can be seen that the discharge
structure was changed from filamentary (upstream
side of the Ar/acetone flow) to glow-like
(downstream side) discharges gradually. We named
this third discharge mode as a “mix mode”.
We measured the temporally-averaged emission
spectra in the middle of the Ar/acetone flow by the
OES method in each discharge mode. Figure 3
shows the spectra, and the acetone-impurity ratios in
black, red, and blue solid lines are 0.15%, 0.55%,
and1.69%, respectively. In these spectra, the main
emission components were identified as OH A-X (at
309 nm), N2 C-B (at 337 nm), CH B-X and A-X (at
388 and 431 nm), C2 A-X (at 515 nm) molecular
bands, and Ar I atomic lines (over 690 nm).
Comparing the three spectra in each discharge mode,
it was clearly seen that the intensities of OH, N2, and
Ar emissions monotonically decreased with the
increase of acetone-impurity ratio, and the CH and
C2 mission intensities could not be observed only in
the glow mode.
OH N2
CH
Ar
Emission intensity (arb. unit)
C2
0.15%
Figure 5(a) shows a microscopic photograph of
the deposited materials on the Cu plate prepared by
10-minutes discharge using the acetone-impurity
ratios of 0.55% (in the glow mode). We found two
kinds of material surface which are half-moon
discolored region on a side near the crossing point of
two gas flows (lower side in Fig. 5(a)) and an
interference pattern on the other side (upper side).
The SEM images of the material deposited in the
glow mode, which is the same material shown in Fig.
5(a), are shown in Figs. 5(b), (c), and (d). The image
taken in the region with the interference pattern (Fig.
5(b)) suggested that the surface morphology in this
region was very similar to the bare Cu-plate’s
surface, and the film-like structure’s deposition was
occurred preserving the incident streaky structure.
On the other hand, there were randomly aligned
structures composed of nm-order small particles in
the image taken in the discolored half-moon region
(Fig. 5(d)). The streaky structure of the bare Cu plate
could be seen in the back of the gathered-particle
structure, and the back surface was also different
from the bare Cu-plate’s surface. In their interface
region (Fig. 5(c)), we could observe the gradual
change from film-like to the gathered-particle
structures with porous structures between two kinds
of materials.
From these experimental results, it was revealed
that the discharge-mode transition in the jet-type
DBD largely affects the deposition processes on the
Cu plate. The detailed relationships between the
discharge phenomena in the Ar/acetone flow and the
deposited materials on the substrate will be
discussed in the future works.
4. Concluding remarks
0.55%
1.69%
300
400
500
600
700
800
Wavelength (nm)
Figure 3: Optical emission spectra of jet-type DBD in a
wavelength range from 280 to 850 nm at 5 mm from Cu plate,
measured in acetone-impurity ratios of 0.15% (black line, in
filament mode), 0.55% (red line, in glow mode), and 1.69%
(blue line, in mix mode).
In summary, we introduced three kinds of discharge
modes in the jet-type DBD in the Ar/acetone flow,
and controlled the discharge mode by changing the
acetone-impurity ratio. The two flow configuration
composed of the tilted He APPJ and the vertical
Ar/acetone flow enabled us to perform long-running
experiments in the same discharge conditions. From
the optical emission spectra, it was revealed the C2
and CH generation does not occur in the glow mode,
although these molecular generations increased with
the increase of the acetone-impurity ratio in the
filament mode. We also found that the deposited
materials on the Cu plate were also very different in
the filament and glow discharge modes, and the morder random structures with nm-order particles
were observed in the material deposited in the glow
mode.
Acknowledgements
This study has been partially supported by a Global
COE program on “Photonic and Electronic Science and
Engineering” in Kyoto University. The authors would
like to thank Dr. Biswa Ganguly and Dr. Brian Sands
at Air Force Research Laboratory for their contribution
in trial experiments for this study.
References
Figure 4: SEM images of deposited materials on Cu plate.
Acetone-impurity ratio and peak value of applied voltage are
0.55% (in glow mode) and 6.0 kV respectively. Observation
points of enlarged images (b), (c), and (d) are indicated in
overall photograph (a).
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