Download A VERY LOW TEMPERATURE ATMOSPHERIC

A VERY LOW TEMPERATURE ATMOSPHERIC-PRESSURE PLASMA JET
IN A SINGLE ELECTRODE CONFIGURATION*
S.D. ANGHEL, A. SIMON, M.A. PAPIU, O.E. DINU
Faculty of Physics, Babes-Bolyai University, Cluj-Napoca, M. Kogalniceanu 1, RO-400084, Romania
E-mail: [email protected]
Received January 24, 2011
An atmospheric-pressure very cold helium plasma jet generated at 48 kHz in a
single-electrode configuration is characterized based on visual observation, gas
temperature monitoring and electrical and optical measurements. The plasma is nonaggressive with thermo-sensitive materials, the neutral gas temperature being slightly
higher than the ambient one.
Key words: atmospheric pressure plasma jet, low temperature, optical emission,
hydrophilicity, sterilization.
1. INTRODUCTION
Atmospheric pressure plasmas (APP) overcome the disadvantages of vacuum
operation. A general recipe for obtaining non-thermal plasmas is the reduction of
the discharge size and/or its duration, and working at low excitation frequency. The
most used devices for generating atmospheric pressure non-thermal plasmas are
atmospheric pressure plasma jet, plasma needle, plasma pencil, miniature pulsed
glow-discharge torch, one atmosphere uniform glow-discharge plasma, resistive
barrier discharge and dielectric barrier discharge [1]. Most of non-thermal plasma
sources use commercial devices as waveform generators with transfer efficiency
energy from the power source to the plasma of about 20%. In this contribution, the
principle of plasma generation at 800 kHz [2] was used to generate a non-thermal
APP at 48 kHz by a simple modification. The electrical and thermal characteristics,
optical emission and some possible applications of generated plasma are presented.
2. PLASMA GENERATION AND CHARACTERISTICS
We have demonstrated [3] that the RLC series resonant circuits could be used
for obtaining the conditions for generating APP’s. When the primary coil of a
transformer is powered with voltage pulses having the frequency equal to the
*
Paper presented at the 15th International Conference on Plasma Physics and Applications,
1–4 July 2010, Iasi, Romania.
Rom. Journ. Phys., Vol. 56, Supplement, P. 90–94, Bucharest, 2011
2
A very low temperature atmospheric-pressure plasma jet in a single electrode configuration
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resonant frequency of the secondary circuit, the secondary voltage will be
sinusoidal and Q times higher than the induced voltage (Q – the quality factor of
the secondary circuit). For generating an APP at 48 kHz, a modified fly-back
transformer was used. Its primary coil was replaced by another one having 6 turns.
The schematics of plasma generator and the voltage and current waveforms in
primary and secondary circuits are shown in Fig. 1. Both the discharge current and
voltage are predominantly sinusoidal, with the current leading the voltage, which
demonstrates the capacitive character of the discharge.
Ed = 5.1V
modified flyback
transformer
copper wire
syringe needle
48 kHz, 9 Vpk-pk
triggering signal
5
2
4
1
3
0
2
-1
1
0
PTFE
Secondary voltage [kV]
3
0.1
1.0
0.5
0.0
0.0
-0.5
-0.1
-1.0
-1.5
Drain current [A]
Gate voltage [V]
6
plasma gas
He
1.5
Secondary current [mA]
IRF540A
0.2
2.0
isolators
WAVEFORM
GENERATOR
10-1000kHz
-0.2
treated material
(non-conductive)
glass
plasma
-2.0
0
10
20
30
Time [µs]
40
50
-2
0
10
20
30
40
fiber optic
-3
50
Time, µs
Fig. 1 – Schematics of plasma generator and voltage and current waveforms in its different points.
Fig. 2 shows the plasma appearance and the plasma temperature under
different operating conditions. The length of the plasma column, the plasma power
and the plasma temperature are dependent on the diameter of the exit nozzle of the
glass tube and on the gas flow rate. The plasma power was estimated according to
the calculation algorithm presented in [4]. The axial gas temperature was measured
with a K-type thermocouple of which the hot junction was covered with a cap of
Pyrex-glass in order to prevent the arc generation between the powered electrode
and the thermocouple.
(a)
(b)
Fig. 2 – (a) plasma appearance and plasma power as function of inner diameter of the exit nozzle of
the glass tube (He flow rate, 4 l min-1), and (b) gas temperature for different helium flow-rates.
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S.D. Anghel et al.
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Fig. 3 shows a typical optical emission spectrum of the plasma, along with
the operating conditions and physical characteristics. Emission lines and bands
identification and temperature measurements were performed according to the
methodology and procedure presented in [5]. As one can see, plasma emission (He,
N2, O) might provide an important role to our plasma for possible biomedical,
surface treatment and modification targeted applications.
Trot(N2) = 396 K
Tvibr(N2) = 3802 K
Texc(He) = 1248 K
20000
He lines
+
N2 391 nm
16000
Hα
0
200
300
400
500
600
728 nm
667 nm
4000
587 nm
8000
O lines
777 nm
12000
844 nm
SPS N2
706 nm
24000
+
f = 48 kHz
P = 152 mW
He flow-rate = 3 l/min
torch i.d. = 4 mm
501 nm
Emission intensity (arb. units)
28000
700
800
900
Wavelength (nm)
Fig. 3 – Emission spectrum, operating conditions and physical properties of the very low power
atmospheric pressure He plasma jet in single electrode configuration.
Laserjet printing paper, tracing paper and PET foil were exposed to plasma
action while varying treatment time, in the range of 1 to 15 s. The treatment effect
was evaluated by measuring the contact angle of an 1 µl distilled water droplet
placed on the treated surface. Each contact angle shown in Fig. 4a is the average of
triplicate measurements. As one can see, that hydrophilicity of the treated materials
was enhanced by He plasma treatment, the effect being considerable after the first
4-5 s of treatment, than showing saturation tendencies for longer treatment times.
2.8
(a)
80
70
2.6
2.4
Absorbed water [mg]
Contact angle [degree]
(b)
printing paper
tracing paper
PET
60
50
40
30
2.2
2.0
1.8
1.6
1.4
20
-2
0
2
4
6
8
10
Treatment time [s]
12
14
16
1.2
0
2
4
6
8
10
12
Treatment time [s]
Fig. 4 – Contact angle (a) and absorbed water (b) as function of treatment time.
14
16
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A very low temperature atmospheric-pressure plasma jet in a single electrode configuration
93
Wettability changes for the printing paper, due to plasma treatment, were also
evaluated by estimating the absorbed quantity of distilled water. Paper squares of
1 cm2 were weigh in and than exposed to plasma action for different time intervals.
After treatment a droplet of 10 µl distilled water was placed on the square for a
time of 10 s, than the excess non-absorbed water was removed. A new weight
scaling was performed in order to estimate the wettability of paper by means of the
difference between the paper squares weight after and before plasma treatment.
Fig. 4b shows that the absorbed quantity of water increases with treatment time by
a factor of about 1.85. This finding is in good agreement with the conclusions
drawn from contact angle measurements.
Fig. 5 is depicting the sterilizing capability of the plasma estimated by
exposing microbial cultures, deposited on a glass plates, to the action of the plasma
as function of treatment time. The methodology of preparing and handling of the
microbiological samples was described elsewhere [6]. It can be seen that, the very
low temperature atmospheric-pressure plasma jet in a single electrode
configuration could be used succesfully to inactivate both B. Subtilis and E. Coli.
100
B. Subtilis
E. Coli
90
5
Colony forming units [10 /ml]
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
Treatment time [s]
Fig. 5 – Survival curves for B. Subtilis and E. Coli.
3. CONCLUSIONS
The characteristics of the generated atmospheric pressure plasma jet (length,
temperature, emission, etc.) are dependent on both He gas flow rate and exit nozzle
diameter and it was succesfully used to increase hydrophilicity of materials and to
inactivate microorganisms.
Acknowledgements. This study was supported by National University Research Council,
Ministry of Education, Research and Innovation, Romania, Grant PN II, code ID_2270.
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Plasma Physics Journal, 2, 8–16 (2009).
4. S.D. Anghel, Generation of a low-power capacitively coupled plasma at atmospheric pressure,
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atmospheric pressure helium sterilization plasma source, NIMB 267, 438–441 (2009).
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