Diagnostics of Diaphragm Discharge in Electrolytes by Optical Emission
Spectroscopy
V. Sázavská1, Z. Kozáková1, L. Hlochová1, F. Krčma1, P. Slavíček2 and V. Mazánková1
1
Faculty of Chemistry, Brno University of Technology, Purkyňova 118, 612 00 Brno, Czech Republic
2
Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic
Abstract: The paper deals with diagnostics of the diaphragm discharge generated in
electrolyte solutions using DC non-pulsed high voltage up to 2 kV. Based on
emission spectra recorded over the UV-VIS region (250−700 nm), plasma
parameters such as rotational temperature, electron temperature, and electron density
have been determined. Obtained results are compared with respect to the input
power, electrode polarity, and initial conductivity of NaCl solution. Both rotational
and electron temperature decrease with the increasing input power. Electron
temperature and electron density are higher on the cathode side of the diaphragm.
Keywords: discharge in electrolytes, optical emission spectroscopy, rotational
temperature, electron temperature, electron density.
1. Introduction
Electrical discharges generated in water solutions
have been intensively studied during the last three
decades in order to utilize them in numerous
applications such as water treatment [1, 2], plasma
sterilization [3], surface treatment [4], lithotripsy [5],
nanotube synthesis [6], etc. Various electrode
configurations, voltage regimes as well as reactor
constructions have been investigated by different
researchers. A common feature of the discharge
ignition in water is the initiation of physical and
chemical processes, e.g. UV irradiation, shockwave
formation, and production of reactive species
(radicals, ions, and molecules with high oxidation
potential). They can be subsequently utilized in the
previously mentioned applications. However,
detailed diagnostics of the formed plasma is
necessary for a proper control of the required
process.
This work is focused on the diaphragm configuration
using a dielectric barrier separating two electrode
parts of the reactor [2]. The discharge is ignited by
the application of DC non-pulsed high voltage in a
small orifice in the barrier. Due to a relatively long
distance between HV electrodes and the orifice,
plasma channels propagated from the orifice do not
reach electrode surface and thus electrode lifetime is
significantly prolonged. The main advantage of the
applied DC non-pulsed voltage is the usage of a
relatively simple HV source construction and lower
total power consumption. Diagnostics of plasma
generated by the diaphragm discharge is carried out
by the optical emission spectroscopy in order to
determine rotational temperature, electron density,
and electron temperature in the dependence on
experimental parameters.
2. Experimental
The batch discharge reactor was divided in two
electrode parts by a dielectric barrier made of PET
(thickness of 0.25 mm) with one central pin-hole
(initial diameter of 0.4 mm). Planar electrodes were
made of platinum. The distance between each
electrode and the dielectric diaphragm was
adjustable from 1 to 10 cm. For most of
experiments, the electrode distance was fixed at
2 cm from the diaphragm. The DC high voltage
source giving non-pulsed voltage up to 4 kV, current
up to 300 mA, and total discharge power up to
400 W was used for the discharge ignition.
Water solutions containing NaCl electrolyte in
concentrations, which provided initial solution
conductivity in the range from 200 to 1000 µS/cm,
were used in experiments. Total volume of the
electrolyte solution was 4 liters (2 liters in each
reactor part).
Diagnostics of the discharge ignition was carried out
by the optical emission spectroscopy in order to
record both overview spectra as well as detailed
spectra of selected species formed by the discharge
(OH radicals, H atoms, etc.). The overview spectra
were recorded over the near UV and visible region
(250−700 nm) for various discharge conditions, and
electron temperature was calculated from these
spectra. The detailed spectra of selected species were
used for the calculation of plasma parameters such
as rotational temperature (OH A-X 0-0 band at
310 nm), and electron density (Hβ line at 486 nm).
3. Results
The discharge was generated by DC non-pulsed
voltage in the orifice of the dielectric barrier
separating two electrode parts of the batch
diaphragm reactor. Discharge emission was recorded
by the optical fiber maintained as close to the orifice
as possible. Due to the different polarity of HV
electrodes, emission spectra were recorded from
both sides of the diaphragm, perpendicularly to the
axis of the discharge. A typical emission spectrum
obtained in NaCl electrolyte is shown in Figure 1.
In general, electric discharges generated in water
solutions produce various chemical species by the
dissociation and ionization of water molecules, such
as hydroxyl and hydrogen radicals, hydrogen
peroxide, etc. [7]. The presented spectrum confirms
the formation of OH radicals (emission bands
around 310 nm) as well as atomic hydrogen
(emission lines Hα at 656 nm, Hβ at 486 nm, and Hγ
at 434 nm). Additionally, spectral lines of elements
dissolved in the treated solutions can be detected,
too. The spectrum contains a strong emission
doublet of sodium atom (589 nm) coming from the
supporting NaCl electrolyte.
Figure 1. Emission spectrum of the diaphragm discharge in
NaCl electrolyte without second order removal; initial
conductivity of 400 µS/cm, input power of 120 W.
Based on the emission of OH radicals at 310 nm,
rotational temperature at low rotational levels of the
diaphragm discharge was calculated in the
dependence on various experimental conditions. The
dependence of the discharge rotational temperature
on the applied power is presented in Figure 2. The
data were obtained from the emission spectrum
recorded from the cathode side of the diaphragm.
Rotational temperature varied from 750 to 1000 K,
and it decreased with the increasing input power. An
explanation of this progress could be found in an
intensive dissipation of higher supplied energy into
other processes initiated by the discharge in water
solution. Moreover, higher input power induces
formation of bigger bubbles and wall evaporation
which leads to lower heating of the solution.
Figure 2. OH rotational temperature as a function of the
applied power; NaCl electrolyte, initial conductivity of
750 µS/cm, cathode side of the diaphragm.
Comparing rotational temperature obtained from the
emission spectra recorded from the anode and
cathode side of the diaphragm, there is no obvious
influence of the electrode polarity on the rotational
temperature (Figure 3). Graphs in Figure 3 show
rotational temperature as a function of input power
in NaCl solution with different initial conductivity.
For better transparency, error bars are not included
in these graphs. Values of rotational temperature
were obtained in the range of 800−1000 K, and they
decrease with the enhanced power. This trend as
well as rotational temperature values were achieved
more or less similar from both anode and cathode
side.
energy and propagation velocity in the DC
diaphragm discharge (106 cm/s on the cathode side,
105 cm/s on the anode side [9]).
Figure 3. OH rotational temperature from the anode and
cathode side of the diaphragm; NaCl electrolyte, initial
conductivity of 750 µS/cm (upper) and 980 µS/cm (bottom).
Electron temperature was calculated from the
emission spectra of the diaphragm discharge, too.
Obtained data reached values from 2500 to 4000 K
(Figure 4). Graphs in Figure 4 compare electron
temperature of the diaphragm discharge in NaCl
electrolyte at two initial conductivities with respect
to the electrode polarity, and as a function of applied
power. Contrary to rotational temperature, electron
temperature seems to be independent on the applied
power (Figure 4, bottom for 750 µS/cm) or only
slightly decreasing with the enhanced power
(Figure 4, upper for 550 µS/cm). On the other hand,
a substantial difference of electron temperatures can
be observed between the values obtained from the
anode and cathode side of the diaphragm. Electron
temperature is higher when achieved from the
cathode side (3000−3500 K) comparing to the anode
side (2500−3000 K). Obtained data corresponds to
the formation of plasma streamers with different
Figure 4. Electron temperature calculated from the emission
spectra taken from the anode and cathode side of the
diaphragm; NaCl electrolyte, initial conductivity of 550 µS/cm
(upper) and 750 µS/cm (bottom).
Electron density was calculated from the profile of
Hβ line at 486 nm with respect to the electrode
polarity on each side of the diaphragm. Results
obtained in NaCl electrolyte at conductivity of
750 µS/cm are presented in Figure 5. Significantly
higher electron density in the range of
2.5−5.0⋅1020 m-3 was determined on the cathode side
of the diaphragm. On the anode side, lower electron
density up to 2.0⋅1020 m-3 was achieved. These
results probably also correspond to the formation of
plasma streamers with different energy, as it has
been already mentioned above. Moreover, the
dependence of electron density on the applied power
has a different course in each electrode part. While
this parameter substantially decreases with the
increasing input power on the cathode side, it is
enhanced by the higher power on the anode side.
Figure 5. Electron density calculated from the profile of Hβ line
(486 nm) taken from the anode and cathode side of the
diaphragm; NaCl electrolyte, initial conductivity of 750 µS/cm.
4. Conclusions
Diagnostics of the DC non-pulsed diaphragm
discharge was carried out by the optical emission
spectroscopy in order to determine rotational and
electron temperature and electron density of the
forming non-thermal plasma. Rotational temperature
was determined in the range from 750 to 1000 K,
depending on experimental conditions. It decreased
with the increasing input power, and it was more or
less independent on the electrode polarity. Electron
temperature was achieved in the range of
2500−4000 K. Higher values were obtained on the
cathode side of the diaphragm, and they slightly
decreased with the increasing applied power.
Determined electron density was significantly
dependent on the electrode polarity. It reached
values of 2.5−5.0⋅1020 m-3 on the cathode side while
it was only up to 2.0⋅1020 m-3 on the anode side. It is
assumed that the main reason of this difference is the
formation of plasma streamers with different energy
distribution.
Acknowledgements
This work has been supported by the Czech Science
Foundation, contracts No. 202/07/P371, and
No. 104/09/H080.
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