48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition
4 - 7 January 2010, Orlando, Florida
AIAA 2010-1588
Page |1
Cracking of Selected Hydrocarbon Gases in Low-Power, Low-Pressure rf Plasma
Charles Q. Jiao a, Biswa N. Ganguly b and Alan Garscadden b
a
b
UES, Dayton, OH 45432-1894, USA
Air Force Research Laboratory, Wright-Patterson AFB, OH 45433-7251, USA
Abstract
The cracking of four hydrocarbons, C3H8, C2H6, C2H4 and CH4, respectively, in an Ar
plasma has been investigated. Ions observed in electron impact ionization mass spectra of the
plasma are used to probe the neutral components of the plasma. A matrix of data of ion
intensities under varying rf powers (0-30W) and gas pressures (100-400 mTorr), for the four
hydrocarbon gases, respectively, has been collected, and the rates of the depletion of the
hydrocarbons in the plasma and the formation of H2 from the cracking of the hydrocarbons,
under the varying experimental conditions, have been evaluated. Among the four hydrocarbons,
C2H4 undergoes cracking most readily and CH4 yields the most amount of H2 by the cracking.
I. Introduction
In recent years, plasma assisted ignition and combustion (PAI and PAC) have gained
increased attention. A great amount of experimental data has been accumulated during the past
decade, providing grounds for using the low temperature plasma of nonequilibrium gas
discharges for a number of applications at conditions including high speed flows.1 The
interaction between the fuel and the plasma is a complex phenomenon, an understanding of
which requires a wide-range of studies involving, e.g., optical methods, electrical probes, mass
spectrometry measurements, etc. In our previous study,2 we examined the cracking of JP-10
(exo-tetrahydrodicyclopentadiene, a synthetic fuel) in an Ar rf plasma under low power and low
pressure conditions, using a quadrupole mass spectrometer to detect both ionic and neutral
species. We found that the cracking of JP-10 is readily achieved under power densities as low as
0.7 W/cm3 in which each JP-10 molecule experiences ~30 collisions (estimated) with electrons,
producing a mixture of hydrocarbon fragments enriched with H2. Our current study extends the
investigated hydrocarbon gases to propane (C3H8), ethane (C2H6), ethylene (C2H4) and methane
(CH4), using a similar mass spectroscopic method as in the study of JP-10.2 In this paper the
This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.
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amounts of the hydrocarbon depletion and the H2 production in an Ar rf plasma under varying rf
powers and gas pressures are reported.
II. Experimental
The experimental arrangement is shown in figure 1. A capacitively coupled rf generator
applied to an aluminum round disk of 3/4” diameter (a) and a screen electrode of ~ 1.8 squareinch cross section (b) is used to create an Ar discharge. A matching network with automatic
tuning is used to couple the rf (13.56 MHz) power (CESAR 1310, 0–1000W) that was varied in
the range of 3-30 W. The discharge chamber (A) is pumped by a 380 l/s turbopump to a base
pressure of ~ 3 mTorr. Ar is introduced into chamber A with a flow rate in a range of 5.5-22.0
SCCM (standard cubic centimeter per minute). The hydrocarbon gas, namely, C3H8, C2H6, C2H4
and CH4, respectively, is brought into chamber A separately with a flow rate in a range of 2.112.2 SCCM. The flow rates of the hydrocarbon and Ar gases are chosen such that the pressure
ratio of the hydrocarbon to Ar is ~ 1:5.5, as determined from mass spectrometry measurements
calibrated by the ionization cross section data of Ar and the hydrocarbons from the literature.3-7
Figure 1. Schematic diagram (not to the scale) of the experimental setup.
The total pressure of chamber A is maintained at 100, 200, 300 and 400 mTorr, respectively, for
different experiments. Gaseous species from the plasma are analyzed by a quadrupole mass
spectrometer (Stanford Research Systems, RGA-200) located in chamber B that is separated
from chamber A by a 100 m skimmer and pumped by a 200 l/s compound turbomolecular
pump. The typical working pressure of the mass spectrometer is in the range of 10-6 Torr.
The mass discrimination of the quadrupole mass spectrometer is determined using a method
similar to that described by Wood et al.8 and the calibration gases used are H2, N2, Ar, Kr, and
Xe along with the electron impact ionization cross sections for these gases from the literature.9-11
The ion intensities reported in this paper have been calibrated against these measured mass
discrimination factors. The electron gun of the mass spectrometry is operated at electron energy
of 70 eV. The rf power stated in this paper is recorded from the power meter of the rf generator,
which includes the power to the plasma and the power dissipated in the matching network.
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III. Results and Discussion
When the electron gun is on, the ion signals in the mass spectrum are typically 2 to 3
orders higher than the ion signals when the electron gun is off, and therefore represent essentially
the densities of neutral species in the plasma since the contribution of ionic species from the
plasma to the mass spectrum is then negligible. A matrix of data of ion intensities at varying rf
powers ranging from 3 to 30 W and varying total gas pressures ranging from 100 to 400 mTorr,
for the four hydrocarbon gases respectively, has been collected. The ions observed include Ar+,
hydrocarbon ions arising from the precursor hydrocarbon gas and its cracking products, and H2+
whose formation will be discussed later. Ar+ intensity decreases slightly as the rf power increases
under a fixed total pressure, which is attributed to the increased gas temperature and numbers of
particles due to the increased molecular dissociation at elevated rf powers, resulting in the
decreased density of Ar gas because the total pressure of the plasma chamber is kept constant by
an adjustable valve aperture to the pump. A similar effect on the sum of densities of the reactant
hydrocarbon and its cracking products is expected. To eliminate this effect, as well as the effect
of the shot-to-shot variation of the electron current of the ionizer, we have normalized all ion
intensities to that of Ar+. A typical set of data is shown in figure 2 for C3H8 under 100 mTorr
total pressure with the pressure ratio of C3H8:Ar=1:5.5. Presented in the figure are rf-power
dependencies of intensities of major ions observed in mass spectra, namely, Ar+, H2+ and
hydrocarbon ions of CHa+ (a=2,3), C2Hb+ (b=1-6) and C3Hc+ (c=5-8).
C2H5
+
C3H8
+
Normalized Ion Intensity (to Ar+)
C2H4
10
+
H2
+
-2
C2H2
+
+
+
C3H5 /ArH
+
+
C2H3 CH + CH3
4
10
C3H6
10
C3H2
+
-3
-4
CH2
0
+
C3H7
+
C2H6
5
+
10
15
20
+
+
C2H
25
30
Power (W)
Figure 2. Intensities (normalized to Ar+) of selected ions as functions of the rf power in C3H8/Ar
experiments. The total pressure is 100 mTorr with a pressure ratio of C3H8:Ar = 1:5.5. C3H5+
and ArH+ are isobar ions, whose individual intensities cannot be resolved. There are no data
between 0 to 3 W. The Lines are the fittings to the data points and serve only as a guide to the
eye.
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Ions larger than C3H8+ have been observed, such as C4Hd+ (d=1-4), which arise from the
ionization of polymerization products that are formed in the plasma, but their intensities are
negligibly small, suggesting insignificant amounts of the polymerization products produced
under low pressures in our measurements.
In figure 2 we see that intensities of all ions, except H2+, CH3,4+ and C2H1,2+, decrease
monotonically as the rf power changes from 0 to 30 W. The C3H8+ is considered to arise mainly
from the ionization of C3H8 only, since molecules larger than C3H8 are insignificant. The rapid
decay of C3H8+ around 3 W rf power indicates that the depletion of C3H8 occurs significantly
even at low rf powers. The H2+, CH3,4+ and C2H1,2+ are the few ions whose intensities rise from 0
to 3W, which suggests that these ions arise from precursors that are the products of the cracking
of C3H8. CH3+ and CH4+ may be due to the cracking product CH4, especially at high rf powers;
these two ions are the major peaks in the CH4 mass spectrum12 and their intensity ratio at high rf
powers matches well with the literature mass spectrum data for CH4.12 At low rf powers, the
branching ratio CH3+/CH4+ is greater compared to at high rf powers, because an additional
amount of CH3+ is formed by the ionization of the small amount of un-cracked C3H8 at low rf
powers. With similar arguments, we assign ions C2H+ and C2H2+ to be due to another possible
cracking product C2H2. Data in figure 2 indicate that even the cracking products such as CH4 and
C2H2 further react away in the Ar plasma at elevated rf powers, as indicating by the continuing
decreasing intensities of the hydrocarbon ions as the rf power is raised above 4 W.
Figure 3, shown below, displays the rf power dependency of the total normalized hydrocarbon
ion intensity, which changes from 0.29 to 0.015. The decrease of the total hydrocarbon ion
intensity may be caused by (1) the change in the composition of the ion population causing the
change of the averaged ionization cross-section, and/or (2) the decrease of the total density of
hydrocarbon species including C3H8 and its fragments.
Total Hydrocarbon Ion Intensity (Normalized)
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
5
10
15
20
25
30
Power (W)
Figure 3. RF power dependency of the total hydrocarbon ion intensity normalized to Ar+, in
C3H8/Ar experiments. The total pressure is 100 mTorr with a pressure ratio of
C3H8:Ar = 1:5.5.
Page |5
The change in the average ionization cross-section cannot account for the nearly 20 fold
decrease of the total ion current, and therefore it is possible that a considerable amount of
hydrocarbon species in the gas phase is consumed in certain reactions at the electrode surface
resulting in the film deposition. The formation of films on electrodes is evidenced by visual
examination of the electrode surface after experiments. Having noticed the formation of the film
on the electrodes, we checked the probability of the time-dependent mass spectrum. After ~60
minutes from the initiation of the plasma, the mass spectra were found to be reproducible within
experimental error, across the whole experimental time span, indicating the absence of timedependent factors affecting the plasma cracking kinetics in our measurements.
H2+ is the only ion which intensity increases with the rf power over the power range
studied. It arises most probably from the ionization of the precursor H2, which is a component
product in the C3H8 cracking process. The possibility of H2+ arising from the precursor C3H8 or
its hydrocarbon fragments are small, because H2+ has an ionization energy of 15.4 eV,13
considerably greater than most of the hydrocarbon ions, and according to Stevenson’s rule the
positive charge will remain on the fragment of lower ionization potential during the
fragmentation of a molecular ion.14 The argument that H2+ is unlikely to be the product ion of the
ionization of the hydrocarbon precursors is supported by mass spectrum data of over 900
hydrocarbons ranging from C1 to C10, published in the NIST Chemistry WebBook,12 which show
little existence of H2+. Therefore, the intensity of H2+ can be used to represent the density of H2,
which is seen in figure 2 to rise sharply from 0 to 3 W and then slowly increase.
Figure 4a displays the rf power dependencies of the yield of H2 and the depletion of C3H8,
derived from data in figure 2, using the intensities of H2+ and C3H8+ to calculate the densities of
H2 and C3H8, respectively, via normalization of these ion intensities to C3H8+ intensity at 0 W
where the original partial pressure of C3H8 is known. To derive the H2 density, H2+ intensity is
calibrated by the ratio of the ionization cross-sections of C3H8+/C3H8 and H2+/H2.4,15 The amount
of C3H8 depletion is derived by subtracting the C3H8+ intensity at a given rf power from that at 0
W where no C3H8 cracking occurs. Data from experiments under different total pressures are
shown in figure 4a: 100, 200, 300 and 400 mTorr, in which, with a constant pressure ratio of
C3H8 and Ar (1:5.5), the partial pressures of C3H8 are: 15, 31, 45, 62 mTorr, respectively. The
flow rates of C3H8 and Ar are changed with the same scale as the pressure is changed, so that the
residence time of the gas in the plasma is constant independent of the pressure changes. Also
shown in figure 4 are the data from experiments using other hydrocarbons, namely, C2H6, C2H4
and CH4. Similar to the C3H8 experiment, no significant amounts of polymerization products are
observed in the gas phase of the plasma volume, and therefore intensities of the parent ions,
C2H6+ and CH4+ are used to calculate the densities of precursor gases C2H6 and CH4, respectively.
To calculate the density of the precursor gas C2H4, the intensity of C2H3+ rather than C2H4+ is
used because of the finite contribution to this mass peak by N2+ from the background.
From figure 4 one can see that at 100 mTorr, the depletion of hydrocarbons appraches
completion even at rf powers as low as 3 W. At higher pressures, the depletion shows more rf
power dependence and is farther from reaching completion even at 30 W. While C3H8, C2H6, and
CH4 show depletion similar to each other, C2H4 shows noticeably easier depletion compared to
the three other gases. Since C2H4 is the only unsaturated compound among the four
hydrocarbons, the easier depletion of C2H4 appears to be due to the double bond in this molecule
that may facilitate its fragmentation and its surface reaction. The formation of H2 at 100 mTorr
Page |6
also approaches saturation at low rf powers, and at higher pressures, increased dependence on the
rf power is seen, with C2H4 seemingly being an exception. Among the four gases, CH4 produces
the most amount of H2, although the number of H atoms per molecule is not the greatest in CH4.
60
60
(a)
(b)
C3H8 (IV)
C2H6 (IV)
50
Absolute Pressure (mTorr)
Absolute Pressure (mTorr)
50
C3H8 (III)
40
C3H8 (II)
30
H2 (IV)
H2 (III)
20
C3H8 (I)
H2 (II) H (I)
2
C2H6 (III)
40
C2H6 (II)
30
H2 (IV)
H2 (III)
20
C2H6 (I)
H2 (II)
H2 (I)
10
10
0
0
0
5
10
15
20
25
30
0
5
10
Power (W)
60
60
(c)
C2H4 (III)
25
30
40
C2H4 (II)
30
H2 (IV)
H2 (III)
CH4 (IV)
50
Absolute Pressure (mTorr)
50
Absolute Pressure (mTorr)
20
(d)
C2H4 (IV)
C2H4 (I)
H2 (II) H (I)
2
20
15
Power (W)
CH4 (III)
40
CH4 (II)
30
H2 (IV)
20
CH4 (I)
H2 (III)
H2 (II)
10
10
H2 (I)
0
0
0
5
10
15
Power (W)
20
25
30
0
5
10
15
20
25
30
Power (W)
Figure 4. The depletion of hydrocarbon molecules and the respective yields of H2, expressed in
absolute pressure, in the plasmas of the mixtures of Ar with C3H8 (a), C2H6 (b), C2H4
(c) and CH4 (d), respectively. The Roman numbers in parentheses in the labels
indicate the experimental conditions with different total pressures: (I) 100, (II) 200,
(III) 300 and (IV) 400 mTorr. The corresponding original partial pressures of the
precursor hydrocarbons are 15, 31, 45, and 62 mTorr, respectively, which are
indicated in the figure by horizontal dashed lines for visual comparison to the
depletion extents of the hydrocarbons.
Page |7
The formation of H2 may involve more complex multi-channel mechanisms. We notice in
the experiments that after the hydrocarbon gas flow is shut off, a small amount of H2+ is
observed for an extended length of time, as shown in figure 5. The time dependency of H2+
intensity in C3H8 experiments under two different rf powers is presented, with time set to 0 when
the C3H8 gas flow to the plasma is stopped and time for each data point of H2+ intensities is
recorded when the mass spectrum scans to the peak of H2+. For comparison, C3H8+ data at 0 W rf
power (the plasma is off) are also included in this figure.
Normalized (to Ar+) Ion Intensity
+
8x10
H2 , 5W
-3
+
H2 , 30W
+
C3H8 , 0W (x0.25)
6
+
H2 , exp. fitting, 5W
+
H2 , dblexp. fitting, 5W
4
+
H2 , exp. fitting, 30W
+
H2 , dblexp. fitting, 30W
2
+
C3H8 , exp. fitting, 0W
0
0
100
200
300
400
500
Time (sec)
Figure 5. Decay of intensities of H2+ and C3H8+ (normalized to Ar+) after C3H8 gas supply to the
plasma is shut off. Symbols are experimental data and lines are fittings using single
exponential (exp.) and double exponential (dblexp.) fitting functions, respectively.
When there is no plasma, the decay of C3H8+ is purely due to the pumping away of the C3H8 gas,
in a form of single exponential decay as shown by the fitting curve, with a decay time constant of
6.8 seconds. The decay of H2+, on the other hand, cannot be properly fitted by a single
exponential function but a double exponential function. Both types of fitting curves made by
computer programs are included in the figure for assessment. We explain the double exponential
decay by two possible sources of H2, one as the product from reactions of gas-phases
hydrocarbon species, another as the product from the etching process of the films on electrodes.
Table 1 summarizes the fitting parameters for the data from all four gases, with the fitting
function in the form of:
𝑓 𝑡 = 𝐴 + 𝐵𝑒 −𝑡/𝜏 1 + 𝐶𝑒 −𝑡/𝜏 2
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The faster decays have time constants (2) in the range of 3.6 to 5.6 seconds, comparable to that
of the C3H8+ decay, and therefore are expected to be associated with the pumping away of H2
formed from the reactions of gas-phases hydrocarbon species, while the slower decays, with time
constants (1) ranging from 40 to 104 seconds, appear associated with the film etching processes
producing H2. The ratio of C/(A+B+C) therefore represents the branching ratio of the reactions
of gas-phases hydrocarbon species over all channels for the H2 formation, which is in a range of
76% to 90%, shown in table 1.
Table1. Fitting parameters to the decay of the H2 density as a function of the rf power.
rf power
A
(x10-4)
B
(x10-4)
1
(sec.)
C
(x10-3)
2
(sec.)
C/(A+B+C)
C3H8
5W
30W
3.5
8.0
8.7
15.3
59.5
92.6
8.3
7.5
4.6
5.4
0.87
0.76
C2H6
5W
30W
4.6
7.7
11.5
11.3
74.1
96.2
10.5
6.4
3.6
5.6
0.87
0.77
C2H4
5W
30W
3.2
4.1
5.2
7.3
52.9
103.8
4.3
3.9
3.5
4.1
0.84
0.77
CH4
5W
30W
3.2
7.5
8.0
20.0
67.1
39.7
9.9
10.1
3.6
3.6
0.90
0.79
Hydrocarbon
In general the ratio of C/(A+B+C) has lower values at 30W than at 5W, which we ascribe to the
fact that at 30W the etching proceeds at a higher rate and therefore contributes more to the
formation of H2. With regard to the formation of H2 from the hydrocarbon species, whether it
occurs dominantly in the volume of the gas phase or at the electrode surface remains to be
answered.
Although the etching of the films on electrodes contributes to the formation of H2 to a certain
extent, the data in figure 4 correctly represent the net yields of H2 in the hydrocarbon containing
plasmas, because we detect the H2+ signal in steady state plasma.
IV. Summary
A matrix of data of ion intensities in the electron impact ionization mass spectra of the Ar
plasma containing C3H8, C2H6, C2H4 and CH4, respectively, has been measured under varying rf
powers and gas pressures. The densities of the hydrocarbons and the major product from the
cracking, H2, are derived using the measured ion intensities combined with ionization cross
section data from the literature. The rates of the depletion of the hydrocarbons in the plasma and
the formation of H2 from the cracking of the hydrocarbons, under varying experimental
conditions, have been evaluated. With the pressure ratio of hydrocarbon to Ar as 1:5.5, at 100
mTorr total pressure, the rates of hydrocarbon depletion and H2 formation approaches saturation
at rf powers as low as 3W. At higher pressures, the rates show increased dependences on rf
power. Among the four hydrocarbons, C2H4 undergoes cracking most readily and CH4 yields the
most amount of H2 by the cracking.
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The total hydrocarbon density is found to drop considerably as rf power changes from 0
to 30 W, which is believed to be due to the surface reactions at the electrodes that consume the
hydrocarbon molecules and their cracking products, resulting in film deposition. A small portion
of H2 formed from the plasma is contributed by the plasma etching of the film at adjacent
surfaces.
Acknowledgment
The authors thank the Air Force Office of Scientific Research for support.
References:
(1) S. M.Starikovskaia, J. Phys. D: Appl. Phys. 39 (2006) R265.
(2) C.Q. Jiao, B.N. Ganguly, A. Garscadden, J. Appl. Phys., 105 (2009) 33305.
(3) H.C. Straub, P. Renault, B.G. Lindsay, K.A. Smith, R.F. Stebbings, Phys. Rev. A 52 (1995)
1115.
(4) V. Grill, G. Walder, D. Margreiter, T. Rauth, H.U. Poll, P. Scheier, T.D. Mark, Z. Phys. D
25 (1993) 217.
(5) V. Grill, G. Walder, P. Scheier, M. Kurdel, T.D. Mark, Int. J. Mass Spectrom. Ion
Processes, 129 (1993) 31.
(6) C. Tian, C.R. Vidal, Chem. Phys. Lett. 288 (1998) 499V. Grill, G. Walder, P. Scheier, M.
Kurdel, T.D. Mark, Int. J. Mass Spectrom. Ion Processes, 129 (1993) 31.
(7) K. Gluch, P. Scheier, W. Schustereder, T. Tepnual, L. Feketeova, C. Mair, S. Matt-Leubner,
A. Stamatovic, T.D. Mark, Int. J. Mass Spectrom. 228 (2003) 307.
(8) K.V. Wood, A.H. Grange, J.W. Taylor, Anal. Chem. 50 (1978) 1652.
(9) H.C. Straub, P. Renault, B.G. Lindsay, K.A. Smith, R.F. Stebbings, Phys. Rev. A 54 (1996)
2146.
(10) H.C. Straub, P. Renault, B.G. Lindsay, K.A. Smith, R.F. Stebbings, Phys. Rev. A 52 (1995)
1115.
(11) R.C. Wetzel, F.A. Baiocchi, T.R. Hayes, R. S. Freund, Phys. Rev. A 35 (1987) 559.
(12) http://webbook.nist.gov/chemistry/.
(13) CRC Handbook of Chemistry and Physics, 81st ed.; D. R. Kide, Ed.; CRC Press, Boca
Raton, FL, 2000.
(14) D.H. Williams, I. Howe, Principles of Organic Mass Spectrometry, McGraw-Hill, London
(1972).
(15) R. Riahi, Ph. Teulet, Z.B. Lakhdar, A. Gleizes, Eru. Phys. J. D, 40 (2006) 223.