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
91
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.
92
S.D. Anghel et al.
3
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
4
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.
94
S.D. Anghel et al.
5
REFERENCES 1. C. Tendero, C. Tixier, P. Tristant, J. Desmaison, P. Leprince, Atmospheric pressure plasmas: a review, Spectrochim. Acta Part B 61, 2–30 (2006). 2. S.D. Anghel, A Simon, An alternative source for generating atmospheric pressure non-thermal plasmas, Plasma Sourc. Sci. and Technol. 16, B1-B4 (2007). 3. S.D. Anghel, T. Frentiu, A. Simon, Atmospheric pressure plasmas in resonant circuits, The Open Plasma Physics Journal, 2, 8–16 (2009). 4. S.D. Anghel, Generation of a low-power capacitively coupled plasma at atmospheric pressure, IEEE Trans. Plasma Sci. 30, 660 (2002). 5. A. Simon, S. D. Anghel, M. Papiu and O. Dinu, Diagnostics and active species formation in an atmospheric pressure helium sterilization plasma source, NIMB 267, 438–441 (2009). 6. A. Simon, S.D. Anghel, J. Papp, Optimum working parameters for the plasma needle used for bacterial deactivation, JOAM 10, no. 8, 2077–2081 (2008).
