22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Microdischarges: a chemical process and particle density diagnostics
O. Sakai1,2, Y. Hiraoka2, N. Kihara2, Ella Blanquet1,2 and K. Urabe2,3
1
Department of Electronic Systems Engineering, The University of Shiga Prefecture, Hikone, Shiga, Japan
2
Department of Electronic Science and Engineering, Kyoto University, Kyoto, Japan
3
Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba, Japan
Abstract: Due to their smallness, microdischarges or microplasmas can play different
roles on various chemical processes and in future electronic devices from large-volume
discharges. Here we review their chemical and electric properties investigated so far; for
instance, microplasmas are easy to be installed in a conventional gas tubing system, and
useful to work for gas reformation. However, their smallness also leads to their
disadvantage in diagnostics of plasma parameters inside them. We developed several
diagnostic methods for microplasmas based on electromagnetic and optical techniques,
which are also reviewed in this report.
Keywords: microdischarge, microplasma, gas reformation, metamaterial
1. Introduction
Microdischarges have attracted much attention in this
decade, and several advantages have been pointed out in
potential chemical processes and electronic devices partly
because they are free for installation due to their
smallness in various configurations and in many reaction
phases [1-3]. Examples of the configuration proposed so
far are hollow cathodes [4], micro jets [5, 6], and
integrated array [7, 8]. Examples of the reaction phases
are in atmospheric-pressure gasses, liquids [9],
supercritical fluids [10], internal area of gas capillary [3,
11] and microwave metamaterials [12].
Here we report one example of a novel chemical
process
and
electronic
devices
achieved
in
microdischarges with their diagnostics. In the chemical
process we successfully detected N 2 H 4 which was
atmospheric-pressure
generated
in
Ar-NH 3
microdischarges [13], and also observed formation of
metallic nanoparticle patterns in the downstream region.
Another example demonstrated here is “plasma
metamaterial,” [12] which is a novel concept of media
with microplasmas for controlling electromagnetic waves.
Its electromagnetic properties are useful when we
diagnose microplasmas using microwaves to understand
electron-induced
chemical
processes
inside
microdischarges.
2. N 2 H 4 generation by microdischarges
Usual gas supply system includes tubing parts with
several millimetre diameter.
Conventional plasma
reactors are installed after the gas supply systems, and
materials processes such as deposition or etching by ions
and/or excited species are performed in (at least) severalcm space. However, microdischarges are so small, with
less than several millimetres, that they don’t need large
chamber system. Instead, they can be installed, for
instance, in a tubing part, and the total system becomes
quite simple; the gas supply and the abatement system are
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directly connected, and inserted microplasmas in the tube
with small reactor or T-branch connector for output are
sufficient components.
We replaced a part of metal tube just before the
abatement chamber by a glass capillary with the same
outer diameter and 1.5 mm inner diameter, and configured
microdischarge space
with external electrodes
surrounding the outer diameter of the capillary in which
Ar-NH 3 gas flowed [13], as shown in Fig. 1a. That is,
microplasmas were generated in dielectric barrier
discharges with discharge cross section of 1.5 mm and
total length of ~10 cm. Discharge voltage at 10-30 kHz
ignited microdischarges. NH 3 gas was diluted by Ar, and
we performed absorption spectroscopy in the downstream
gas tube; N 2 H 4 has significant absorption coefficient in
the ultraviolet ray region [14]. Fig. 1b shows a typical
absorption spectrum; from the comparison with the
reported cross section, estimated absolute number density
of N 2 H 4 was 2.3x1015 cm-3). Since the concentration of
NH 3 in the mixed gas at atmospheric pressure was < 5%,
the efficiency of N 2 H 4 generation was about several
percent.
Using this N 2 H 4 , which is a powerful reductant agent,
we induced a reduction reaction for formation of metal
particles in aqueous solution of AgNO 3 . In the downflow
region of the generated microplasmas, we set a 50 µl
droplet of AgNO 3 with 0.02-0.2 mol/L on glass/Si
substrates. In the gas flow of Ar/NH 3 gas with produced
N 2 H 4 , after the droplet being dried up, Ag particles,
Ag-particle network, or AgNO 3 solutes were observed in
various experimental conditions.
The size of the
Ag particles was in the range of several hundreds of nm,
and the sufficient number of them aggregated to form the
Ag network.
Consequently, the created structure
exhibited very wide range of sheet resistance (from that of
insulator to conductor) when we assume that the
structures are equivalent to one layer. In addition, the
created structures showed different spectra in the
1
mid-infrared ray region since the outlook in the cases of
Ag-particle network was in a fractal-like shape in the
µm range.
a)
(c)
b)
c)
Fig. 1. a) Schematic view of experimental setup for N 2 H 4
generation and reduction process. b) Absorption of deep
ultraviolet ray observed in experiment with reported cross
section of N 2 H 4 [14]. c) Ag pattern observed after
reduction treatment.
2
3. Plasma metamaterials and their application to
diagnostics of internal electron density
Roles of electrons in plasma chemistry are quite
essential to decompose molecules and to generate radical
species. Electron density in a microdischarge is unusual
in comparison with the cases in conventional
large-volume
low-pressure
discharges,
and
its
identification is quite significant to evaluate efficiency of
chemical processes. So far we have investigated plasma
metamaterials, which are electromagnetic-wave media
with extraordinary responses [12], and their microwave
responses include quite useful data of information, one of
which is electron density.
That is, one of the
disadvantages of microdischarges is difficulty of
diagnostics of their internal plasma parameters, and
plasma array structure realized in plasma photonic
crystals and plasma metamaterials which includes
microdischarges is so effective to identify absolute value
of electron density when we can assume homogeneous
microdischarge ignition throughout the array.
A simple structure suitable for evaluation of electron
density in microdischarges is a two-dimensional array of
microdischarges, as shown in Fig. 2 [15]. This is
equivalent to dynamic photonic crystals in microwave and
millimetre range when we assume their individual size
ranges from sub-millimetre to millimetre. In addition to
wave damping via electron-neutral collisions, if the
condition is equal to that of band-gap formation, we can
observe very sensitive response of electron densities in
microwave signals; in a band of forbidden propagation,
waves are evanescent although microscopic refractive
index and permittivity are always positive, and the
decaying length which strongly depends on electron
density is much smaller than the case of the collisional
damping. In the case of Fig. 2b, for different two rows,
we can confirm consistency of electron density
(0.9-1.1x1013 cm-3).
4. Diagnostics of microdischarges based on optical
methods
When we increase the frequency of the electromagnetic
waves up to the light range, the waves suffer other
attenuations. Since atoms, molecules and radical species
have their own absorption frequencies, we can estimate
their densities from the attenuation rates of the lights via
the Beer Lambert law.
In the case of N 2 H 4 , the broad absorption spectra
(220-250 nm) are in the deep ultra-violet ray regions [14],
as we mentioned in Section 2. We can use conventional
optical continuum deuterium (D 2 ) lamp for this
measurement. We also monitored Ar metastable atoms
whose absorption wavelengths are around 811.5 nm by a
novel laser spectroscopic method [16, 17]. In this case,
one absorption spectrum has only 3 GHz width, and laser
spectroscopy is inevitable. To detect absorption spectra,
conventional method is to scan wavelength of the laser
light. To eliminate this scanning time, we developed new
laser spectroscopic method “frequency-comb interference
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a)
b)
Fig. 2. a) Schematic view of experimental setup for
electron density evaluation of microplasmas.
b)
Transmittance of microwave observed in experiment with
numerically calculated data [15].
spectroscopy,” in which a frequency-comb laser and a
single-frequency laser are superposed and absorption
spectra are detected by microwave spectrum analyser.
That is, we can measure various species in
microdischarges using optical absorption methods.
7. References
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5. Summary
In this report, we demonstrate novel chemical processes
inside and in the downstream of microdischarges and
diagnostics on active species generated inside of them.
Their smallness allows us to install them at various spatial
positions, and we set them inside the gas tubing system in
our experiment. We also show various methods of
density evaluations for electrons, atoms, and molecules.
Through these diagnostics, we can elucidate chemical
processes inside of microdischarges.
6. Acknowledgements
This study was partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
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