22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Mass spectrometric investigation of the ionic species in a dielectric barrier
discharge operating in He-H 2 O and N2 -C 2 H 2 mixtures
Z. Abd-Allah1, D.A.G. Sawtell2, G.T. West2, P.J. Kelly2 and J.W. Bradley1
1
University of Liverpool, Dept. of Electrical Engineering and Electronics, Brownlow Hill, Liverpool L69 3GJ, U.K.
2
Manchester Metropolitan University, Surface Engineering Group, Manchester M1 5GD, U.K.
Abstract: Using advanced mass spectrometry the chemistry of ionic species present in an
atmospheric-pressure parallel plate dielectric barrier discharge (DBD) with one or two
dielectrics on the powered and grounded electrodes have been identified. Both negative
and positive ions were identified in He-H 2 O and N 2 -C 2 H 2 discharges. The influence of
several operating parameters on the chemistry of negative and positive ions was studied.
Keywords: mass spectrometry, dielectric barrier discharge (DBD), water clusters,
non-thermal plasma
1. Introduction
Dielectric barrier discharge (DBD) plasma sources are
used in numerous applications including areas such as
ozone generation, plasma medicine, pollution control and
surface modification [1]. Dielectric barrier discharges are
low temperature non-equilibrium plasmas typically
obtained between two parallel electrodes, one or both of
which is covered with a dielectric barrier, separated by a
gap ranging from 0.1 mm to several centimetres. The
discharge properties of DBD’s have been studied
previously using a variety of diagnostic methods such as
optical emission, Fourier Transform Infra-red (FTIR),
infra-red laser absorption and electrical diagnostics [2, 3].
To date, however, very little detailed analysis of the ion
chemistry produced in kHz DBDs has been conducted.
Here, we present a mass spectrometric study of negative
and positive ions produced in a kHz dielectric barrier
discharge under various operating conditions.
2. Experimental-set-up
The atmospheric pressure discharge was generated in a
parallel plate reactor, consisting of two metal electrodes
with a dielectric disc of alumina (0.5 mm thick, 99.6%
alumina and dielectric constant 9.9).
The mass
spectrometer extraction orifice plate acted as the ground
electrode allowing for in-situ measurements of the species
arriving at the boundary. One dielectric barrier was
present on the driven electrode with helium plasma, while
two dielectrics covering both electrodes were used with
nitrogen discharge. The dielectric covering the extraction
orifice has a centre hole of 25 µm diameter which was
aligned with the 200 µm diameter hole of the extraction
orifice plate of the spectrometer. The discharge was
driven in helium with controllable concentrations of water
vapour using an excitation frequency of 10 kHz and an
applied voltage of 1.2 kV [4]. A voltage of about 11 kV
was required for nitrogen and acetylene experiments. The
power source consists of a digital function generator,
driving a commercial audio amplifier connected to a
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purpose built voltage step-up transformer to generate the
required high voltages. The mass spectrometer employed
here was a quadrupole-based molecular beam mass
spectrometer, HPR-60, manufactured by Hiden Analytical
Ltd.
The instrument consists of a three stage
differentially pumped inlet system separated by aligned
skimmer cones and turbo-molecular pumps.
The
pressures in these stages were as follow:
P 1 = 3.3 ×10-1 Pa, P 2 = 9.8 ×10-3 Pa and P 3 = 7.7 ×10-5
Pa.
3. Results
Mass spectrometry measurements were carried out to
characterize the neutral and ionic species in the gas
mixture. Figs. 1 and 2 show negative ions for helium
with 3000 ppm of water vapour and nitrogen with 75 ppm
acetylene respectively. The most abundant negative ions
for helium and water mixtures were of the family
OH-(H 2 O) n , while CN-, CH 2 CN- and C 2 H 4 CN- were the
dominant ions with nitrogen and acetylene discharge.
Positive ions were also measured for both discharges. For
helium and water vapour, positive ions of the form of
H+(H 2 O) n with n up to 9 were identified. N+ and N 2 +
were the dominant ions in nitrogen and acetylene plasma.
The influence of several operating parameters such as
water and acetylene concentration, residence time and
applied power on the intensity of ionic species in the
discharge was examined.
1
5. References
[1] U. Kogelschatz. Plasma Chem. Plasma Process.,
23, 1-46 (2003)
[2] I.S. Vinogradov, et al. Plasma Process. Polymers,
4, 797-800 (2007)
[3] A.S. Chiper, et.al. in: Proc. 19th Int. Symp. on
Plasma Chemistry. (2009)
[4] Z. Abd-Allah, et.al. J. Phys. D: Appl. Phys.,
(accepted, JPhysD-104022.R1) (2015)
Fig. 1. Experimental set-up.
Fig. 2. Negative ion mass spectrum for He + 3000 ppm
H 2 O. Total flow rate of about 5 slm, electrode separation
0.5 mm and frequency of 10 kHz were used.
Fig. 3. Negative ion mass spectrum for N 2 + 75 ppm
C 2 H 2 . Total flow rate of about 2 slm, electrode
separation 0.5 mm and frequency of 10 kHz were used.
4. Conclusions
Negative and positive ions generated in the active
plasma of atmospheric-pressure parallel plate DBD
reactor have been identified. This study demonstrates the
effectiveness of mass spectrometry for detailed in-situ
analysis of the ionic chemistry produced in DBD plasma.
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