Development and Characterisation of a Microwave-heated Atmospheric Plasma Torch

Full Paper

Development and Characterisation of aMicrowave-heated Atmospheric Plasma Torch

Martina Leins,* Lukas Alberts, Mathias Kaiser, Matthias Walker,Andreas Schulz, Uwe Schumacher, Ulrich Stroth

Among other applications, microwave plasma sources at atmospheric pressure are used for thedecomposition of halogenated volatile organic compounds (VOC). The presented microwaveplasma torch is based on an axially symmetric resonator. Simulations of the electric fielddistribution, Eigen frequency analyses and measurements of the resonant frequency resultedin an improved design. Microwaves of 2.45GHz are fed into this cavity resulting in asufficiently high electric field for ignition andmaintaining stable plasma. The characterisationof the plasma is made with optical emission spectroscopy. The decomposition of VOC wasanalysed with FTIR-spectroscopy, a mass spectrometer and an FID.

Introduction

Microwave plasma sources at atmospheric pressure have

applications inmanyareas.Ontheotherhand thesesources

canbeused for theactivationof surfaces, e.g. to increase the

adhesion of lacquer and glue, and on the other hand, they

can be applied in many areas of treatment and conversion

of different kinds of gases. One example is the cleaning of

pollutants,which is amandatory task in small andmedium

enterprises due to various environmental regulations.

Especially, the decomposition of halogenated volatile

organic compounds (VOC), which are produced, e.g. by

semiconductor industries, has become more and more

important in thepast fewyears, since theycontribute to the

greenhouse effect.[1] Microwave plasma sources at atmo-

spheric pressure provide a promising alternative to

conventional thermal combustion processes, since they

have no electrodes, are easy to handle and offer the

advantage of high electron, ion and radical densities.

Research on the decomposition of halogenated VOC with

microwave plasma sources has already been performed.[2]

M. Leins, M. Walker, A. Schulz, U. Schumacher, U. StrothInstitut fur Plasmaforschung, Universitat Stuttgart,Pfaffenwaldring 31, D-70569 Stuttgart, GermanyFax: þ49 711 685 6 3102; E-mail: [email protected]. Alberts, M. KaiserFraunhofer-Institut fur Chemische Technologie, Joseph-von-Fraunhofer-Straße 7, D-76237 Pfinztal, Germany

Plasma Process. Polym. 2009, 6, S227–S232

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The presented plasma torch is based on an axially

symmetric resonator. Microwaves of a frequency of

2.45GHz are fed into the cavity. To ensure that the plasma

is ignited without any additional igniters, detailed infor-

mation about the electric field distribution is needed.

Therefore, finite element simulations of the electric field

distribution, as well as Eigen frequency analyses of the

whole configuration were performed using the simulation

software ComsolMultiphysics1. These simulations

revealed that there are two different kinds of possible

modes a coaxial one and a resonator one. Experimental

measurements of the resonant frequencies of different

configurations were in excellent agreement with the

simulations and resulted in an improved design of the

resonator device, which enabled ignition of plasma at

atmospheric pressure without any igniters and maintain

stable plasma operation. To characterise the discharge,

optical emission spectroscopy was carried out. The A2Sþ–

X2Pg-transition in theUV-region of the freeOH-radicalwas

used to obtain rotational gas temperature. As few atomic

oxygen lines were present in the IR-region of the spectrum,

these lines were used to determine an excitation tempera-

ture with a Boltzmann-plot. Furthermore, studies for the

decomposition of VOC and their halogenated derivates

were carriedout. Therefore, propanecontainingair plasmas

and CF4 containing nitrogen plasmas were investigated,

and clean and raw gases were analysed by using quadru-

polemass spectroscopy, Fourier transform infrared spectro-

scopy (FTIR), a flame ionisation detector (FID) and a

gas-phase chromatograph.

DOI: 10.1002/ppap.200930604 S227

M. Leins et al.

S228

Experimental Part

Figure 1 shows a schematic view of the experimental setup. The

microwave power was coupled into the resonator (r¼0.05m,

h¼0.0485m) from themagnetron via a rectangular wave guide. A

three-stub tuner, not explicitly shown in Figure 1, was used to

maximisethepowerabsorbedbytheplasma.Thegasescouldbefed

into the cavity, either tangentially via four inlets or into the centre

through a metallic nozzle. The tangential gas supply leads to a

vortex, which stabilises the plasma and avoids an attachment of

theplasma to the surroundingquartz tube. Themetallic nozzlewas

alsoused toadjust the resonant frequency. Thiswill beexplained in

the section ‘Simulationof theelectricfielddistributionand ignition

of plasma’. The plasma itself is, as mentioned above, confined in a

quartz tube acting as a reaction chamber. Optical emission

spectroscopy was carried out using two different spectrometers:

an overview spectrometer (Mechelle 7500), whichwas sensitive in

the visible and IR-region and a second spectrometer (Acton,

SpectraPro-750i)whichwas sensitive in theUV- and visible region.

The Mechelle spectrometer has a focal length of 190mm and a

resolution of Dl/l¼7 500. The acton spectrometer has a focal

length of 750mmand amaximumresolution ofDl/l¼ 20000. The

spectral analysis could be performed with three different gratings

(150, 600, and 1800 grooves�mm�1). At the exit slit, an image-

amplified CCD camera was mounted. This spectrometer was used

toget overviewspectra (grating: 150grooves�mm�1) in theUV-and

visible region and was also used to have a close look (grating:

1 800grooves�mm�1) at the rotational bands of the free OH-radical

to determine the rotational gas temperature. The Mechelle

spectrometer was used to obtain overview spectra in the visible

and IR-region and for themeasurement of the atomic oxygen lines.

For the characterisation of the plasma inside the resonator, the

cavity was furnished with a slit on the front, which did not affect

theelectricalpropertiesof theresonator.Thecharacterisationof the

clean and raw gases was performed with the shown quadrupole

mass spectrometer, the Fourier transform infrared spectrometer

(FTIR), a Multicomponent FTIR Gas Analyzer GASMET DX4000N

from Temet, Finland, an FID and an Agilent GC/MSD 6890/5970

gas-phase chromatograph. Gas-flows of 10–70 sl�min�1 airwith an

Figure 1. Experimental setup for optical emission spectroscopy ofthe plasma and diagnostics for the characterisation of clean andraw gases.



addition of 21.25–70.8mg propane and 25–50 sl�min�1 N2 with an

addition of 50.19–250.94mg CF4 were explored. The magnetron

power was varied from 1 to 3 kW.

Results and Discussion

Simulations of the Electric Field Distribution andIgnition of Plasma

Simple geometries, such as an axially symmetric resonator,

can easily be solved analytically by solving Maxwell’s

equations. The resonant frequency vmnl of a cylindrical

resonator with radius r and height h is given by,

vmnl ¼1

2prffiffiffiffiffiffi"m

p

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffix2mn þ

lpr

h

� �2s

(1)

with m, n and l the azimuthal, axial and radial wave

numbers, respectively, and xmn is zero of the Bessel

function. When only Emnl¼ 0-modes are regarded, Equation

(1) can be resolved to:

rmn0 ¼ xmn

2pvffiffiffiffiffiffi"m

p (2)

so that the radius for a given frequency can be calculated.

This is necessary, since the magnetron frequency is fixed

and the resonator’s radius must be adapted to the

frequency. In the presented case, the magnetron’s fre-

quency isfixed to2.45GHzand thecorresponding resonator

radius is r¼ 0.0468m. Having the dimensions of a

rectangular wave guide with 0.864m� 0.432m for micro-

waves of 2.45GHz in mind, one can easily notice that the

resonatorhasnearly thesamedimensionsso the thequality

of the resonator ispoor, resulting ina lowelectricfield in the

resonator.[3] A higher quality and hence a higher electric

field in the cavity can be achievedwhen either the coupling

is improved or the dimension of the resonator is enlarged.

An improvementof thecouplingcanbeobtained ifa taper is

used. An enlargement of the resonator leads to higher

modes for which the radius can be calculated by Equation

(2).[3] Regarding a real resonator configuration, a quartz

tube to confine the plasma and ametallic nozzle for the gas

inlet are necessary. Therefore, simulations of the electric

field distribution as well as Eigen frequency calculations

have been performed with the simulation software

ComsolMultiphysics. This revealed that the quartz tube

causes a shift of the resonant frequency to lower

frequencies by enlarging the resonator virtually. When a

metallic nozzle is inserted, the whole configuration is

changed dramatically since a coaxial structure below the

cylindrical resonator supervenes. This coaxial structure, in

DOI: 10.1002/ppap.200930604

Development and Characterisation of a Plasma Torch

Figure 2. (a) z-component of the electric field for two differentnozzle positions and the two different modes: the coaxial and theresonator mode. Variation of the resonant frequencies in depen-dence of (b) the nozzle position for a resonator radius of 0.05 mand (c) the resonator radius for one fixed nozzle position athnozzle¼8 mm.

Figure 3. (a) Overview spectra in the UV- and visible region ofhumid air plasma at different axial positions taken with the acton(grating: 150 grooves�mm�1) spectrometer. (b) Gas temperatureprofile spatially resolved in axial direction for 1 kW microwavepower and an air-flow of 10 sl�min�1. Trot from G0/Gref and G1/Grefwere performed in collaboration with the INP Greifswald.(c) Overview spectrum of air plasma in the visible and IR regionperformed with the Mechelle spectrometer. (d) Excitationtemperature Tex of an air plasma spatially resolved in axialdirection at an air-flow of 10 sl�min�1 and microwave powersof 1 and 2 kW.

addition to the common resonator mode, causes a second

coaxial mode to appear. In Figure 2a, the z-components of

the electric field for the two modes are shown for two

different nozzle positions. The common E010-resonator

mode has a high z-field component in the centre of the

resonator and no z-field components at the nozzle. The

coaxial mode has high z-field components at the nozzle tip,

which extends a little bit into the resonator centre. This

result is found for all explored nozzle positions and

resonator radii. Furthermore, the resonant frequency is

influenced by the nozzle’s position. Figure 2b shows the

dependence of the resonant frequency for a resonator

radius of r¼ 0.05m in dependence of the nozzle position. It

can be seen clearly, that the resonant frequency, which

belongs to the coaxial mode, decreases when the nozzle is

moved into the resonator. In contrast, the resonant

frequency of the resonator mode is not affected by the

nozzle. Additionally, the analytically calculated resonant

frequency for an undisturbed cylindrical resonator is

shown in the diagram. The calculated resonant frequency

for an undisturbed resonator is in good agreementwith the

simulated one for the resonator with the nozzle inside. In

Figure 2c, the variation of the resonant frequency for the

twomodes is shown in dependence of the resonator radius

for one fixed nozzle position (hnozzle¼ 8mm). Here, the

resonant frequency of the coaxial mode is not affected by

the variation of the resonator radius. The resonator mode

frequency decreases with increasing resonator, radius and

is in good agreement with the analytically calculated one

for an undisturbed resonator which is also plotted in

Figure 2c. Experimental measurements with a network

analyser of the resonant frequency in dependence of the

nozzle’s position were performed and are in excellent

agreement with the simulations.[3] This resulted in an



improved resonator configuration with a moveable nozzle

so that the resonant frequency can be adjusted to the

magnetron frequency. After measuring the magnetron’s

frequency, the resonant frequency of the resonator config-

uration can be adjusted to the magnetron frequency by

adapting the nozzle’s position. Then, the forward powers

are maximised using a three stub tuner, and 1 kW of

microwave power is sufficient to ignite plasma at atmo-

spheric pressure.

Characterisation of the Plasma Using OpticalEmission Spectroscopy

Optical emission spectroscopy was carried out to char-

acterise the plasma, to get information about what kind of

species are present in the plasma and to get knowledge

about gas and excitation temperatures. Figure 3a shows

overview spectra of humid air plasma at different axial

positions performedwith the Acton spectrometer (grating:

150 grooves�mm�1) in the wavelength range from 175 to

700nm. The whole spectrum is dominated by NO- and

OH-bands in theUV-region. This indicates that nitric oxides

are produced in the plasma torch. In the visible and IR-

region, two atomic oxygen lines at 777.34 nm and

844.65nm can be found. This shows the spectrum

(performed with the Mechelle spectrometer) in Figure 3c.

However, no atomic nitrogen lines or ionic oxygen or

nitrogen lines could be observed. The transition A2Sþ,

n0 ¼ 0!X2Pg, n00 ¼ 0 between 306–310nm of the free OH

www.plasma-polymers.org S229

M. Leins et al.

Figure 4. Degradation rate for propane in air plasma in depen-dence of (a) the gas-flow and for a microwave power of 1 kW and(b) the microwave power and for a gas-flow of 75.0 sl�min�1.Degradation rate of CF4 in N2-plasma in dependence of (c) thegas-flow and for a microwave power of 3 kW and (d) the micro-wave power of 3 kW and for a gas-flow of 30 sl�min�1.

S230

radicalwasused todeterminea rotational gas temperature.

Thiswasperformed in twodifferentways.On theonehand,

a simple method proposed by C. Izzara, which is based on

the comparison of three band heads at 306.50, 306.91 and

309.16nm, which are sensitive to temperature variation

was used.[4] On the other hand, the whole spectrum from

306 to 309.5 nmwas simulated for different temperatures,

compared to the measured spectrum and then the

temperature with the best agreement of the two spectra

was taken.[5] In Figure 3b, the gas temperature profile

spatially resolved in axial direction for 1 kW microwave

power and a gas-flow of 10 sl�min�1 humid air is shown.

The maximum gas temperature of about 3 600–4 000K is

reached in the resonator. Outside the resonator the gas

temperature decreases to 2 500K at a distance of about

110mmabove the resonator. A variation of themicrowave

power showed that the gas temperature is not increasing

with increasing microwave power but the plasma is

extended.[5] The two atomic oxygen transitions5P0! 5S at 777.34 nm and 3S0! 3P at 844.65 nmwere used

to determine an excitation temperature Tex assuming a

Boltzmann population of the energy levels. The excitation

temperature Tex gives an estimation of the electron

temperature Te. Figure 3d shows Tex profiles spatially

resolved in axial direction obtained from a Boltzmann-plot

of the two atomic oxygen lines for a gas-flowof 10 sl�min�1

air and microwave powers of 1 and 2 kW. For a microwave

power of 1 kWamaximumTex of about 5 000K is reached in

the resonator. Outside the resonator no Tex can be

measured. An increase in the microwave power to 2 kW

leads to an increase inTex to almost 6 500K in the resonator.

In addition Tex can be measured until 15mm above the

resonator. So, an increase in themicrowave power does not

result in an increase of the rotational gas temperature but

leads to an enlargement of the plasma column aswell as to

an increase in the excitation temperature Tex.[4]

Figure 5. FTIR-spectra of clean gases: (a) an FTIR-spectrum of theclean gas where no VOC was added: intense bands of nitric oxidesare visible, (b) an FTIR-spectrum of the clean gas with an admix-ture of 14.80 mg�sl�1 toluene: additionally CO, CO2 and H2O bandsappear.

Decomposition of (halogenated) Volatile OrganicCompounds (VOC)

One possible application of the plasma torch is the

decomposition of (halogenated) VOC. The clean and raw

gases were analysed by using quadrupole mass spectro-

metry, Fourier transform infrared spectrometry (FTIR), an

FIDandagas-phasechromatograph. Experimentsaimingat

the decomposition of VOC were performed with 10–

75 sl�min�1 air and an addition of 20–70mg propane as

VOCandmicrowavepowersupto2 kW.Figure4ashowsthe

degradation rate measured with an FID in dependence of

the gas-flow for an applied microwave power of 1 kW. At

low gas-flows a degradation rate of 100% can be reached.

When the gas-flow is increased the plasma diminishes and

so the dwell time is reducedwhich leads to a decrease of the

degradation rate to 55% at an air flow of 75 sl�min�1. In



Figure4bthemicrowavepower foragas-flowof75 sl�min�1

is increased from1 to2 kWwhich results in an enlargement

of the plasma. Since, the plasma is enlarged the dwell time

increases leading toadegradation rate of 100% for 2 kW. For

detailed analyses of the clean gas and reaction products

samples were assayed and absorbed on Tenax1. The

desorption and sample feeding occurred thermally in a

helium gas flow with a Gerstel TDS 2-system. The gas

chromatic analyses showed that no by-products are

generated during the degradation of propane. Comparable

results concerning the degradation rate were obtained

when toluene instead of propane was used. Further

analyses of the raw and clean gases were performed using

FTIR spectroscopy. Figure 5 shows two FTIR spectra, both of

the clean gases behind the plasma torch. Spectrum (a) is a

DOI: 10.1002/ppap.200930604

Development and Characterisation of a Plasma Torch

spectrum of the clean gas where no VOC was added. The

spectrumshows intensebands ofNO,NO2andevenofN2O4

what displays that already pure air plasma produces a high

amount of nitric oxides. This is already indicated in the

optical emission spectrum which is dominated by NO-

bands. Spectrum (b) shows a spectrumof the clean gaswith

anadmixtureof14.80mg�sl�1 toluene.Thespectrumshows

additionally to the NOx-bands intense CO, CO2

and H2O bands but no toluene bands are visible what

shows that toluene is decomposed to carbon oxides and

water. However, gas chromatic analyses revealed that

critical reaction products such as benzol or benzoquinone

are produced.[6] Summarising these results it can be said

that the degradation of VOC, here propane and toluene,

with the presented microwave plasma torch of 100% is

possible but critical reaction products and ahigh amount of

nitric oxides are generated. Thus, the decomposition of

halogenated VOC which have distinct higher climate

harmfulness and are much more difficult to decompose

seems to be a more promising application. Therefore,

experiments to the decomposition of halogenated VOC

with 50–250mg CF4 as halogenated VOC were performed.

During these experiments, the exhaust gas system was

completed by awet vent gaswasher that converted the F to

calcium fluoride. Besides, nitrogen instead of air was used

as carrier gas since CF4 is used in etching processes and only

comes along with N2. Additionally, the gas-flow was

moistened to offer the C and F atoms reaction partners.

The N2-flow was varied from 25–50 sl�min�1. Measure-

ments before the vent gas washer with the mass spectro-

meter showed that CF4 molecules are converted to SiF3when no water is present and to HF and CO2 when the gas

flow ismoistened.[7] The degradation rate in dependence of

the microwave power measured with the mass spectro-

meter is shown in Figure 4d. For low powers of 1 kW the

degradation rate reaches only 65% but with increasing

power the plasma column is increased what leads to a

longer dwell time and results in a degradation rate of 100%

for 3 kW. Nantel-Valiquette et al. could observe for a

nitrogen flow of 30 sl�min�1 and 5 000ppmv CF4 and an

oxygen admixture to offer the C and F reaction partners a

degradation rate of about 50% and they also showed that

the degradation rate can be increasedwhen themicrowave

power is increased.[2a] In Figure 4c the dependence of the

degradation rate of the nitrogen flow is given. It shows that

for gas-flows of 40 and 50 sl�min�1 the degradation rate is

decreasing. This can be explained by the decrease in the

plasma column at high gas flows. Nantel-Valiquette et al.

also observed a decrease in the degradation rate when the

gas flowwas in increased.[2a] FITR analyses of the clean gas

behind the vent gas washer showed that only CO,

CO2, H2O and little amounts of NOx could be measured.[7]

So the abatement of halogenated VOC in contrast to the

abatement of VOC seems to be a promising alternative to



conventional thermal combustions since only little or no

nitric oxides are produced and high degradation rates can

be achieved.

Conclusion

In conclusion the simulations of the electric field distribu-

tion showed that there are two different kinds of possible

modes when a cylindrical resonator with a metallic nozzle

is regarded. The resonant frequency of the resonator mode

changes when the resonator radius is varied. The resonant

frequency of the coaxial mode stays undisturbedwhen the

radius is varied but varies when the metallic nozzle is

moved into the resonator. This could be verified with

experimental measurements and resulted in an improved

configuration with an adjustable nozzle which is able to

ignite plasma without any additional igniters. The char-

acterisationof theplasmabyoptical emission spectroscopy

revealed a gas temperature of about 3 600–4 000K in the

resonator which is independent of the applied microwave

power. An increase in the microwave power leads to an

enlarged plasma and to an increase of the excitation

temperature Tex from max. 5 000K at 1 kW microwave

power tonearly6 500Kat2 kWatanair-flowof10 sl�min�1.

Experiments concerning the abatement of (halogenated)

VOC revealed that the decomposition of propane of 100% is

possible but much NOx is produced. In contrast, the

decomposition of CF4 in nitrogen also has degradation rate

of 100%, but no nitric oxides are produced, since oxygen is

missing. Thus, the decomposition of halogenated VOC

seems to be one promising application for the presented

plasma torch.

Acknowledgements: The authors wish to thank the Arbeitsge-meinschaft industrielle Forschungsvereinigungen (AiF) for partlyfunding this research (contract no 14248).

Received: September 12, 2008; Accepted: March 26, 2009; DOI:10.1002/ppap.200930604

Keywords: abatement of VOC; atmospheric; microwave; opticalemission spectroscopy; plasma torch

[1] Y. Ko, D. P. Y. Chang, I. M. Kennedy, J. Air and Waste Manage.Assoc. 2003, 53(5), 580.

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[3] [3a] M. Leins, K.-M. Baumgartner, M. Walker, A. Schulz, U.Schumacher, U. Stroth, Plasma Process. Polym. 2007, 4, S493;[3b] M. Leins, K.-M. Baumgartner, M. Walker, A. Schulz, U.



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DOI: 10.1002/ppap.200930604

Documents

Development and Characterisation of a Microwave-heated Atmospheric Plasma Torch