Download Generation of 3-Dimensionally Integrated Micro Solution Plasmas and Its Application to Decomposition of Organic Contaminants in Water

st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Three-Dimensional Integration of Micro Solution Plasmas and
Its Application to Material Processing in Large Volume Liquid
T. Shirafuji1
1
Dept. Physical Electronics and Informatics, Osaka City University, Osaka, 558-8585 Japan
Abstract: Three-dimensionally integrated micro solution plasma (3D IMSP), in which
microplasmas are generated in Ar gas bubbles in aqueous solution being held in a porous dielectric material, has been proposed. Electric field in the bubbles is calculated with numerical
simulation and is confirmed to be high enough for igniting electrical discharge in the bubbles
even if the bubbles are surrounded with electrically conductive liquid. A proto type reactor for
generating the 3D IMSPs has capability of generating large volume plasma in liquid. The 3D
IMSP reactor is applied to material processing, in which the 3D IMSP is found to be able to
synthesize gold nanoparticles and to decompose organic substance in water.
Keywords: Microplasma, Solution Plasma, Liquid, 3D Integration
2. Numerical Study of 3D IMSP
2. 1 Model Description
Figure 1 shows cross-sectional view of the model geometry of an ideal 3D IMSP reactor. The reactor is a glass
tube containing porous dielectric material. The electrode
is located at the central axis of the porous dielectric material and outer surface of the glass tube. Pores in the dielectric material employed in the 3D IMSP reactor should
not be isolated but connected like the structure of a trabecular-bone. This is a very important feature that the
porous dielectric material should have, because the
gas-liquid mixed medium can flow in the reactor and
plasma in the gas bubble can interact with the liquid.
In this geometry, however, we must pay attention to the
fact that all the bubbles are surrounded by the liquid medium which may be electrically conductive. If the electriModel geometry
Grounded
2 mm
Porous dielectric
filled with gas/liquid
mixed medium
2 mm
Outer
electrode
Dielectric
tube
Inner
electrode
Tube wall
εr = 4 - 10000
Dielectric
frame
εr = 4
φ 0.8 mm
Gas
bubble
εr = 1
1 mm x
1 mm
10 mm
1. Introduction
Plasmas in and in contact with liquid have attracted
much attention because of their possible application fields
such as nano-materials synthesis, surface modification,
water treatment, sterilization, recycling of rare materials,
and decomposition of toxic compounds [1]. In our previous work, we have successfully obtained glow discharges
in aqueous solution, which have been named as "solution
plasmas", and applied this technique to nanoparticles
synthesis and modification of the surface of
nano-materials [2].
Our solution plasma, however, is ignited in a small
volume between two stylus electrodes, actual treatment
area or volume should be enlarged for practical industrial
applications. In the case of gas phase processes, large area
processing is realized by producing large area plasmas. In
the case of solution plasma processing, however, large
volume plasma in liquid is meaningless, because the most
important region is gas-liquid or plasma-liquid interface.
Thus, preparation of large number of tiny plasmas
(microplasmas [3]), which may be called as "integrated
micro solution plasmas", is rather important in the case of
solution plasma processing, because we can expect much
larger area of the interface between gas (or plasma) and a
liquid medium.
The preliminary experiments for the integration of the
micro solution plasmas were performed in two dimensions [4]. Feasibility study concerning possibility of
three-dimensionally integrated micro solution plasmas
(3D IMSP) was already performed and reported [5]. An
actual 3D IMSP reactor was also constructed and its material processing capability was also confirmed through
the experiments of synthesizing gold nanoparticles [6]
and decomposing methylene blue molecules [7].
In this paper, I briefly describe how the 3D IMSP reactor works, and also describe how to enhance its performance together with its material processing capability.
Liquid
εr = 80
σ = 1-1000
uS/cm
Probe point
X-boundary
= Periodic
Vapp.
Fig. 1 The model geometry for numerical calculation
of electric field in the micro bubbles in the 3D IMSP
reactor.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Electric field (Y-dir)
in a bubble
1.5
Liquid
1000 uS/cm
Tube eps.
10000
6
Electric field (x10 V/m)
2.0
1.0
1000
100
0.5
10
0.0
-11
10
-9
10
-7
10
-5
10
-3
10
Time (s)
Fig. 3 Electric field in the bubble in the reactor being
composed of a dielectric tube with various dielectric
constants.
Fig. 2 (a) Applied voltage between the electrodes, and
(b) electric field at the center of the bubble indicated
by “Probe point” in Fig. 1.
cal conductivity is infinite, we cannot expect any electric
field in the bubbles because the surface of the bubbles
works as a short circuit to extinguish potential difference
on the bubble surface.
However, the electrical conductivity of the liquid is finite. For example, typical electrical conductivity of tap
water is 100 - 200 uS/cm. In addition to this, the resistance of liquid surrounding the bubble becomes higher
than that of bulk liquid because of narrow liquid channel
in the porous dielectric material. This means that we can
expect potential drop on the surface of a bubble even
though the bubble is surrounded by electrically conductive liquid.
In order to investigate how much electric field can be
obtained and to verify whether we can ignite electrical
discharge with the electric field obtained, we have performed numerical calculation of electric field in the model
geometry shown in Fig. 1.
The dc 5 kV is applied between the central axis and
outer electrode. The waveform of the applied voltage is
shown in Fig. 2 (a). Electrical conductivity of the liquid
has been varied in the range of 1 - 1000 uS/cm. Relative
dielectric constant of the liquid is 80, which corresponds
to that of water. The framework of the porous dielectric
material and the tube is insulator with relative dielectric
constant of 4, which corresponds to that of SiO2. The gas
bubbles are assumed to be insulator with relative dielectric constant of unity. The shape and size of these compo-
nents are shown in Fig. 1.
For calculating electric field in such system, we cannot
apply simple Poisson’s equation, because the medium is
composed of mixture of electrically conductive and dielectric materials. Thus, we have employed an electro-quasi static method [8] for solving Maxwell’s equations in our system by using the commercially available
finite-element-method solver, COMSOL multiphysics.
Two dimensional spatial profile of the electric field is
obtained as a function of time by the numerical calculation, which has been reported in our previous report [5].
In this work, we focus on the dependence on electrical
conductivity of liquid.
2.1 Dependence on electrical conductivity of liquid
Figure 2(b) shows electric field in the bubble indicated
by “Probe point” in Fig. 1. The electric field in the bubble
reaches its maximum value around 10-7 - 10-6 s after applying the ramp voltage. Since the dielectric framework
and gas bubbles are arranged regularly, all the bubbles
have the same electric field. This feature can be seen in
our previous report [5]. The formation of high electric
field in the bubble is a transient phenomenon as can be
understood from Fig. 2(b). After 1 us or longer time, most
of the voltage is applied between outer and inner surface
of the glass tube, because the equivalent circuit of the 3D
IMSP is a series circuit composed of capacitance (glass
tube wall) and lossy dielectric (porous dielectric filled
with gas and conductive liquid). Because of that, we have
employed bipolar pulse voltage source for the purpose of
continuous treatment of liquid medium as described in the
experimental section of this paper.
The maximum value of the electric field depends on
electrical conductivity of liquid and varies from 5×105
down to 0.7×105 V/m. The threshold electric field for igniting atmospheric pressure discharge is 3×105 V/m [8].
Thus, according to the Fig. 2(b), we can expect generation
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
30 mm
3 cm
1 mm
Fig. 5 Optical emission from the 3D IMSP.
Fig.4 Overview of the 3D IMSP reactor.
of plasma if the electrical conductivity of the liquid is less
than 100 uS/cm. This will be confirmed in the experimental section.
2.2 Enhancement of electric field in the bubble
In our system, applied voltage is divided into two parts.
One is the wall of glass tube and the rest is the porous
dielectric filled with gas-liquid mixed medium. The voltage on the glass tube wall can be reduced by employing
the dielectric tube with thinner wall and/or higher dielectric constant. In this work, we have examined effect of
utilizing the dielectric tube with higher dielectric constant.
Figure 3 shows electric field in the bubble in the system
being composed of the dielectric tube with dielectric constant of 10, 100, 1000 and 10000. These values are not
absurd since the PZT has dielectric constant of 1700.
Electrical conductivity of the liquid is 1000, for which
electric field in the bubble is as low as 7×104 V/m in the
case of glass tube. As can be understood from Fig. 3, if
we employ the dielectric tube with dielectric constant of
1000, electric field in the bubble can be enhanced up to
approximately 1×106 V/m even if the electrical conductivity of liquid is as high as 1000 uS/cm. Since the electrical conductivity of 1000 is an extreme case, the reactor
with high-dielectric-constant tube is considered to have
potential to handle wide range of liquid media.
3. Experimental
According to the results of numerical calculation, we
have constructed actual reactor. At this moment, we do
not have a dielectric tube with high dielectric constant.
Thus, we have employed glass tube for construction of the
3D IMSP reactor. Figure 4 shows the overview of the 3D
IMSP reactor. The inset photograph in Fig. 4 is the magnified view of the porous dielectric material. The porous
dielectric material is a rod-shaped silica pumice (20 mm
diam.), which is inserted in a glass tube. Averaged size of
the pores is approximately 0.5 mm in diameter. The pores
are not isolated but connected. A metal electrode (SUS
304, outer diam. 1/4 inch), where high voltage pulses are
applied, is inserted in the central axis of the pumice rod. It
has five holes (diam. 5 mm) to feed gas into the porous
dielectric material filled with liquid to be treated. The
grounded electrode (metal mesh, SUS 304, wire diam.
0.29 mm, aperture 0.98 mm) is attached on the outer surface of the glass tube. Since we cannot complete contact
between the metal mesh and glass tube, we may have atmospheric pressure discharge on the glass tube surrounded with the metal mesh. The voltage is supplied from a
bipolar high voltage power source (Haiden,
SBP-5K-HF2). The voltage waveform is square-wave
pulses with amplitude of 5 kV, frequency of 20 kHz and
pulse width of 2.3 us. The gas supplied through the central electrode is Ar, and its flow rate is 1.1 L/min. Liquid
temperature is controlled to be 30oC by circulating
through the cooling device.
3.1 Confirmation of plasma generation
Figure 5 shows the photograph of the reactor during
operation using water with electrical conductivity of 200
uS/cm. According to the prediction by numerical calculation, electric field formed in gas bubbles are not enough
for igniting electrical discharge if the electrical conductivity of liquid is 200 uS/cm, which can be understood from
Fig. 2(b). Underestimation of the threshold electrical
conductivity for igniting electrical discharge may be due
to the fact that actual size and position of pore sizes and
width of liquid flow channel are not regulated in our present 3D IMSP reactor.
3.2 Gold nanoparticle synthesis
In order to demonstrate material synthesis capability of
the 3D IMSP system, we have performed gold nanoparticle synthesis using HAuCl4 aqueous solution. Figure 6 (a)
shows photographs of the HAuCl4 aqueous solution after
the 3D IMSP treatment for 0 to 60 min. We can confirm
that color of the aqueous solution changes from yellowish
transparent toward wine-red color, which is result of
plasmon resonance absorption by gold nanoparticles. The
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
(a)
0
AuHCl4 0.15 mM
10
20
30
40
50
60
Time (min)
(b)
(c)
Fig. 6 (a) Aqueous solution of HAuCl4 after 3D IMSP
treatment for 0 to 60 min, (b) STEM image of the
synthesized particles, and (c) EDS image of the
STEM-observation area.
0
10
20
30
40
50
60 min
5 cm
Fig. 7 Photographs indicating decolorization due to
decomposition of methylene blue molecules in methylene blue aqueous solution treated with the 3D
IMSP.
size of the particles is 6 ± 4 nm as seen in the STEM image shown in Fig. 6 (b). The EDS mapping image shown
in Fig. 6 (c) indicates that composition of the particle is
surely gold. Details on the experimental procedure and
particle analysis are reported in elsewhere [6].
3.3 Methylene blue decomposition
Figure 7 shows photographs indicating decolorization
of methylene blue (MB) aqueous solution due to decomposition of MB molecules in the aqueous solution through
the treatment by the 3D IMSP. The decolorization of the
MB is known to occur also through a simple reduction
reaction to form leuco MB [9], which is known to be easily oxidized through oxidation procedure such as agitation
in the air. However, the transparent aqueous solution after
the 3D IMSP is not colored again, which means that the
MB molecules have been decomposed by the 3D IMSP
treatment. Through the comparison of decomposition efficiency of the 3D IMSP to that of conventional method,
in which the plasma is ignited between two stylus electrodes in aqueous solution, we have found that the 3D
IMSP has 16 times higher decomposition efficiency than
that of conventional method. Details are reported in elsewhere [7].
4. Conclusion
3D IMSP, in which a large number of micro solution
plasmas are generated in a porous dielectric material filled
with gas-liquid mixed medium, has been proposed and
constructed for treatment of large volume liquid medium.
Through numerical simulations and experiments using the
proto-type reactor, we have confirmed that the 3D IMSP
can be generated. The numerical simulation has also predicted that the performance can be enhanced by using a
high-dielectric-constant material as the wall of reactor
tube.
Capability of material synthesis has been demonstrated
through gold nanoparticle synthesis using HAuCl4 aqueous solution. Decomposition of an organic substance in
water has also been demonstrated through MB
decolorization experiments. Its energy efficiency has been
found to be 16 times higher than conventional method in
which the plasma is ignited between two stylus electrodes
in aqueous solution.
5. Acknowledgments
This work has been partly supported by the
Grant-in-Aid for Scientific Research on Priority Area
"Frontier science of interactions between plasmas and
nano-interfaces" by MEXT, Japan.
6. References
[1] Eds. V. I. Parvulescu, M. Magureanu and P. Lukes,
Plasma Chemistry and Catalysis in Gases and Liquids
(Wiley-VCH, 2012).
[2] O. Takai, Pure Appl. Chem. 80 (2008) 2003.
[3] K. Tachibana, IEEJ Trans. Elec. Electron. Eng. 1
(2006) 145.
[4] T. Shirafuji, J. Hieda, N. Saito and O. Takai, Proc.
20th Int. Symp. Plasma Chem., 20 (2011) 640.
[5] T. Shirafuji and A. Nakamura, Trans. Mater. Res. Soc.
Jpn. (2013) in press.
[6] T. Shirafuji, J. Ueda, A. Nakamura, S.-P. Cho, N.
Saito and O. Takai, Jpn. J. Appl. Phys. submitted.
[7] T. Shirafuji and Y. Himeno, Jpn. J. Appl. Phys.
submitted.
[8] A. Fridman, A. Chirokov and A. Gustol, J. Phys. D:
Appl. Phys., 38, R1 (2005).
[9] O. Impert, A. Katafias, P. Kita, A. Mills, A.
Pietkiewicz-Graczyk and G. Wrezeszcz, Dalton.
Trans. (2003) 348.