Energy of Reactions in Atmospheric-Pressure Plasma Polymerization with Inert Carrier Gas Bernard Nisol, Herve Gagnon, Sophie Lerouge, Michael R. Wertheimer* A large reactor for performing dielectric barrier discharges (DBD) experiments at atmospheric pressure (AP) has been built and tested. The area of electrodes is more than 40 times greater than that of a small DBD cell, in which we have perfected a method formeasuring E g , the energy dissipated per cycle of the applied a.c. high voltage, V a (f). This methodology has been successfully applied to plasma polymerization experiments on the larger system, using volatile organic precursors (dopants) at ‰ concentrations in 10 standard liters per minute of argon (Ar). We measured DE g , the energy difference with and without dopant, for V a (f) 3 kV rms (20 f 40 kHz). From DE g we then derived E tot /N, the energy per molecule, and observed surprisingly good agreement with data published in the literature relating to low- pressure (LP) plasmas. 1. Introduction The literature regarding deposition of thin organic films for practical uses by plasma-enhanced chemical vapor depo- sition (PECVD), also known as plasma polymerization (PP), goes back at least to the early 1960s. [1] During intervening decades, literally thousands of articles devoted to this subject have been published worldwide, as well as numerous monographs. [2,3] While earlier literature almost exclusively dealt with high-frequency (h.f.: radiofrequency, r.f., or microwave) glow-discharge plasmas sustained at reduced pressure, typically near 100 mTorr (13.3 Pa), there has more recently been growing interest in PP based on gas discharges at atmospheric pressure (AP). [4–12] Dielectric barrier discharges (DBD) constitute the main approach that enables scale-up for industrial processing; of course, AP plasmas obviate the need for expensive vacuum systems, and they can, thereby, potentially reduce costs very significantly. DBD plasmas may be obtained in gaps between two electrode surfaces at least one of which is covered by a dielectric. They are non-equilibrium (cold) plasmas, useful in numerous plasma-chemical reactions beside PECVD, such as ozone synthesis, surface modifica- tion of polymers, abatement of pollutants, excimer lamps, and others. [13] In the PP literature, there has long been an interest in correlating deposition kinetics, physico-chemical and structural properties of films with energy absorbed by the organic precursor (so-called monomer) molecules in the plasma. Indeed, this often controversial subject has been the object of a series of debates in this journal; [14] Hegemann and coworkers [15–17] developed an original approach toward the macroscopic phenomenology of PP, one which leads to an unifying dependence of the mass deposition rate per unit of monomer flow, R m /F, on the macroscopic reaction parameter W/F (W being power input), by way of a quasi-Arrhenius expression R m =F ¼ G exp½E a =ðW=FÞ ð1Þ where E a is an apparent activation energy, and G a reactor- and process-dependent factor related to the maximum B. Nisol, H. Gagnon, M. R. Wertheimer Groupe des Couches Minces (GCM) and Department of Engineering Physics, Polytechnique Montreal, Box 6079, Station Centre-Ville, Montreal, QC, Canada H3C 3A7 E-mail: [email protected]S. Lerouge Research Centre, Centre Hospitalier de l’Universite de Montreal (CRCHUM), and Department of Mechanical Engineering, Ecole de technologie superieure ( ETS), Montreal, QC, Canada Full Paper Plasma Process. Polym. 2015, DOI: 10.1002/ppap.201500068 ß 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 DOI: 10.1002/ppap.201500068 wileyonlinelibrary.com Early View Publication; these are NOT the final page numbers, use DOI for citation !! R
Transcript
Full Paper
Energy of Reactions in Atmospheric-PressurePlasma Polymerization with Inert Carrier Gas
Bernard Nisol, Herv�e Gagnon, Sophie Lerouge, Michael R. Wertheimer*
A large reactor for performing dielectric barrier dis
charges (DBD) experiments at atmosphericpressure (AP) has been built and tested. The area of electrodes is more than 40 times greaterthan that of a small DBD cell, in which we have perfected a method formeasuring Eg, theenergy dissipated per cycle of the applied a.c. high voltage, Va(f). This methodology has beensuccessfully applied to plasma polymerization experiments on the larger system, usingvolatile organic precursors (dopants) at ‰ concentrations in 10 standard liters per minuteof argon (Ar). We measured DEg, the energy difference with and without dopant, for Va(f)� 3kVrms (20� f� 40 kHz). From DEg we thenderived Etot/N, the energy per molecule, andobserved surprisingly good agreement with datapublished in the literature relating to low-pressure (LP) plasmas.
B. Nisol, H. Gagnon, M. R. WertheimerGroupe des Couches Minces (GCM) and Department ofEngineering Physics, Polytechnique Montr�eal, Box 6079, StationCentre-Ville, Montreal, QC, Canada H3C 3A7E-mail: [email protected]. LerougeResearch Centre, Centre Hospitalier de l’Universit�e de Montr�eal(CRCHUM), and Department of Mechanical Engineering, �Ecole detechnologie sup�erieure (�ETS), Montr�eal, QC, Canada
r.f., or microwave) glow-discharge plasmas sustained at
reduced pressure, typically near 100mTorr (13.3 Pa), there
hasmore recently been growing interest in PP based on gas
discharges at atmospheric pressure (AP).[4–12] Dielectric
barrier discharges (DBD) constitute themain approach that
enables scale-up for industrial processing; of course, AP
plas
and they can, thereby, potentially reduce costs very
significantly. DBD plasmas may be obtained in gaps
between two electrode surfaces at least one of which is
covered by a dielectric. They are non-equilibrium (cold)
plasmas, useful in numerous plasma-chemical reactions
beside PECVD, such as ozone synthesis, surface modifica-
tion of polymers, abatement of pollutants, excimer lamps,
and others.[13]
In the PP literature, there has long been an interest in
correlating deposition kinetics, physico-chemical and
structural properties of films with energy absorbed by
the organic precursor (so-called monomer) molecules in
the plasma. Indeed, this often controversial subject has
been the object of a series of debates in this journal;[14]
Hegemann and coworkers[15–17] developed an original
approach toward the macroscopic phenomenology of PP,
one which leads to an unifying dependence of the mass
deposition rate per unit of monomer flow, Rm/F, on the
macroscopic reaction parameter W/F (W being power
input), by way of a quasi-Arrhenius expression
o
e
Rm=F ¼ G exp½�Ea=ðW=FÞ� ð1Þ
where Ea is an apparent activation energy, and G a reactor-
and process-dependent factor related to the maximum
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B. Nisol, H. Gagnon, S. Lerouge, M. R. Wertheimer
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monomerconversion intofilmgrowth.W/F, proportional tothe average energy transferred per monomer molecule
during its travel through the active plasma zone, governs
the formation of reactive intermediates. It was originally
knownas theBeckerparameter,[18] or since the late1970sas
the Yasuda parameter,[2,19] W/FM (energy per mass of
monomer), M being molecular mass of the monomer. The
use of Equation (1) in plots ofRm/F versusW/F allows one to
identify different regimes in the PP process, usually leading
to films with differing structures and characteristics. An
important motivation of Hegemann’s work has been to
develop their macroscopic approach to facilitate reactor
scale-up, and to permit comparison of data from different
laboratories.
Implementation of the concepts just described is
relatively straightforward in the case of h.f. low-
pressure PP, for the following reasons; (i) usually, the
monomer is introduced into the plasma reactor undi-
luted, although Hegemann and coworkers have shown
how their method can be used also for mixtures
including a second (reactive, but non-polymerizing)
gas;[15,16] (ii) while not simple and unambiguous, it is
often possible to measure power absorbed in h.f.
plasmas with reasonable precision. Now, in the case
of AP discharges, particularly for PP reactions, neither of
these generally apply:
(i)
Plasm
� 20
rly
First, AP PP processes necessarily use a flow of carrier
gas, wherein that of the monomer is generally
highly diluted (typically in the parts per thousand,
‰, range);
(ii)
regarding power measurements in AP plasmas, the
present authors have strong reasons to believe that
such measurements are far from being simple,
certainly not those involving AP DBDs. For example,
many workers use Q–V plots, also known as Lissajous
figures;[8,9,20–22] first introduced by Manley in
1943,[21,23] this technique is now known to require
considerable caution.
Figure 1. (a) Scale drawing of the DBD reactor and (b) photographic image of the overallsystem. The two high-voltage electrodes are encased in poly(dimethylsiloxane) (PDMS).In thecenter,onenotes thethree injectionportsof the feed-gasdiffuser.Theglass-coveredgrounded electrode platen is mounted on a back-and-forth conveyor mechanism.
Although various authors have pro-
posed and attempted to use the
Yasuda parameter in the context of AP
DBD PP,[8–11,24] combined complications
(i) and (ii) above have so far presented
major obstacles, often apparently unbe-
knownst to those authors. Recently,
this laboratory has developed a precise
method for measuring the energy, Eg,dissipated per cycle of the applied
(multi-kHz) a.c. voltage in noble gas
AP DBD.[25,26] Resulting data were
found to be trustworthy, based on
objective assessment criteria discussed
a Process. Polym. 2015, DOI: 10.1002/ppap.201500068
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elsewhere[26] and later in this text. The objectives of
research reported here have been twofold:
1.
e
to investigate whether the methodology[25,26] can be
transferred from a small DBD reactor, designed primar-
ily for plasma diagnostics, to a much larger (pilot-scale)
reactor designed for PECVD, particularly for PP;
2.
to investigate whether Eg measurements can yield
specific information about PP reactions, even when
monomer flow constitutes only a few‰ (or less) of total
feed gas flow into the DBD reaction zone, the major
portion being many standard liters per minute (slm) of
inert (argon, Ar) carrier gas.
We start by describing the experimental apparatus and
methodology for measuring Eg, then present and discuss
experimental results.
2. Experimental Section
2.1. DBD Plasma Reactor
Figure 1a presents a scale drawing of the DBD reactor, namely the
high-voltage (HV) electrodes, consisting of polished aluminum
plates (180�60�1.75mm3) (A), the top (3.50�0.05mm thick)
Macor, (B) and bottom (3.00� 0.02mm thick) glass, (C) dielectric
barriers; the lower, moveable grounded electrode platen (D); the
discharge gap, (E), between (B) and (C), typically 2mm; from the
above data, the plasma volume can be evaluated at v¼ 43.2 cm3.
The feed gas injector, also fabricated from machinable ceramic
(Macor, F). This 240�30�12.7mm3 gas diffuser, is provided with
ؼ 6.35mm channels to allow injection from the top as well as
from the sides. On the bottom, 22 holes (ؼ3mm) permit
homogeneous distribution of the feed gas across the ca. 20 cm
width of the moving substrate.
This assembly is new, but other parts of the overall system
(see Figure 1b) have been described earlier;[7,27] for the sake of
brevity, the reader is, therefore, kindly referred to those articles
for details.
DOI: 10.1002/ppap.201500068
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Figure 2. Equivalent electrical circuit diagram; the portion in the dashed rectanglerepresents the discharge cell. Rm¼ 50Ω; see text for further details.
Energy of Reactions in Atmospheric-Pressure Plasma Polymerization with Inert Carrier Gas
It is important, however, to briefly describe the
electrical system:
Theupperelectrodes (A)wereconnectedtoaHV
power supply, comprising a variable-frequency
a.c. generator (1Hz to >50kHz, Hewlett-Packard
3310A), a power amplifier (QSC Ltd., Model
RMX2450), an HV transformer (Enercon, Model
LM2727-03), and an impedance matching unit.
The a.c. power supply voltage, Vps(t), was moni-
toredbymeansofanHVprobe (TektronixP6015A).
The lower electrode (D) was connected to ground
via a 50Ω precision resistor, which served to
measure the discharge current pulse amplitude
and shape. The discharge behavior was exam-
ined at applied HV a.c. frequencies, f, rangingbetween ca. 1 and 50 kHz, voltage and current
signals all being synchronously displayed using
a digital storage oscilloscope (Instek GDS-2204A,
200MHz). In turn, these data were transmitted and acquired in
real time by USB link on a PC, where they were post processed by
MATLAB code (see below) in order to calculate the true voltage
across the gas gap, Vgap, the discharge current, Id, and the energy
per cycle, Eg.[26]
2.2. Equivalent Circuit Model and Computation of
Energy, Eg
Figure 2 presents the equivalent electrical circuit model for the
entire experimental set-up, while the portion inside the dashed
rectangle corresponds only to the DBD cell.
Figure 3. Measured characteristics (amplitude and phase) of the DBD cell, as afunction of frequency, f, of the applied a.c. voltage in the absence of discharges. Thecontinuous curve through the data points represents the best fit to the presentedequivalent circuit model. Notice that deviations in phase angle are at most 18.
Vps and Vm correspond to the voltage signals,
respectively, measured by the aforementioned
high- and low-voltage probes. Rm is a precision
50Ω resistor. Zd is a non-linear variable impe-
dance; its value, although unknown with preci-
sion, tends toward zero during discharges and
toward infinity between discharges. Parameters
C1,C2,Cdie,Cgap, andRwerederivedfromaseparate
set of nominally identical Vps and Vm measure-
ments, but obtained when the cell was open to
atmospheric air and, therefore,nodischargeswere
present. From these Vps and Vm measurements,
Im¼Vm/Rm and Zeq¼Vps/Im were calculated.
These sets of Vps and Vm measurements in air
without dischargeswere repeated for frequencies,
f, varyingfrom0.5upto40kHz.Theamplitudeand
phase of Zeq, when plotted as a function of f for atypical scenario, follow the theoretical response
corresponding to the model described in Figure 2
over the entire range of f. Figure 3 shows a
graphical comparison with experimental data
points, the best fit obtained from the equivalent
C1, C2, Cdie, Cgap, and R model being quite
satisfactory (notice that deviations in phase angle
are at most 18).Since in this model, capacitors C2, Cdie, and
et al.[28] In the present Va range, Eg values are seen to have
risen from ca. 500 to 2 200mJ per period. For comparison, in
our much smaller (A¼ 5.1 cm2, gap width d¼ 0.5mm;
v¼ 0.25 cm3) DBD cell,[26] of 173� smaller volume com-
pared with the inter-electrode space shown in Figure 1a,
we found Eg¼ 9mJ per period at Va¼ 955Vrms. Calculating
and comparing values of Eg/v, the energy density, in
the two DBD cells at comparable electric field values,
Va/d� 1.9� 104V cm�1, we find values Eg/v¼ 36 and
56mJ cm�3, respectively, for the small and large DBD cells.
Considering that they were equipped with different sets
of dielectric barrier materials[26] (alumina/alumina and
Macor/glass, respectively), the agreement is considered
satisfactory. This demonstrates the feasibility of trans-
ferring information from a small laboratory-scale to a
much larger reactor, for example when planning scale-up
to an industrial reactor system.
In Figure 5a and b, we respectively reproduce the
curves for 20 and 40 kHz from Figure 4, but now with
further sets of data (lower curves) corresponding to the
addition of 20 sccm (2‰) of acetylene into the 10 slm flow
of Ar carrier gas. For sufficiently high values of Va
[�2 400Vrms in (a)], one notices near-constant gaps, DEg,between the upper (pure) and lower (doped) branches.
Before further investigating the physical interpretation
of DEg, it is useful to calculate the average residence
time of a particle in the plasma zone, t0 ¼ v/F� 0.26 s
(v, the plasma volume¼ 43.2 cm3, and F the 10 slm
flow¼ 167 cm3 � s�1); since the duration of one cycle at
f¼ 20 kHz is t¼ 5� 10�5 s, the average particle experi-
ences t0/t� 5.2� 103 applied voltage cycles during its
residence time in the plasma. Of course, the total energy
DOI: 10.1002/ppap.201500068
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Figure 5. Comparison of Eg versus Va (rms) between pure Ar (F¼ 10 slm, upper curve)and acetylene-doped Ar (Fd¼ 20 sccm, lower curve) DBD plasmas at 20 kHz (a) and40 kHz (b).
Energy of Reactions in Atmospheric-Pressure Plasma Polymerization with Inert Carrier Gas
absorbed by the plasma during that time interval is
Etot¼ 5.2� 103 Eg.In order to clarify the significance of DEg, we next
measured Eg at fixedVa¼ 2.8 kVrms, and f¼ 20 kHz, but now
defined critical values, (Fd)crit. Our tentativehypothesis is the following: DEg is the
amount given up by the ‘‘reservoir’’ of
energy stored in the flow of (excited) Ar
atoms to the added dopant molecules,
the presumed dominant mechanism being
Penning transfer during collisions. The
lowest energy of metastable [Ar(4s3P2)] Ar
atoms is 11.55 eV. The observed monotonic
rise in DEg for O2 and N2, therefore, appears
logical. It is useful to briefly justify the
term ‘‘reservoir’’ used above, in view of
the very short (ca. 100 ns) lifetime of Ar: theduration of the current peaks in Ar DBD is
quite long, a sizeable fraction (ca. 65%) of the
a.c. half-period. Therefore, we believe that
Ar are constantly replenished and that a
quasi-steady-state concentration of Ar
exists during the two current peaks of each
voltage cycle.
Tentative interpretation for the different
hydrocarbon curves is the following: for
Fd< (Fd)crit, all molecules undergo reaction
during their residence time in the plasma
zone. (Fd)crit corresponds to the concentra-
tion in Ar at which Etot¼ 5.2� 103 Eg still
suffices for total reaction of all reagent
molecules: at higher Fd, this is no longer
the case, and a certain fraction may escape
without reacting (completely), or by con-
suming energy via a different set of
channels.
To further test that hypothetical inter-
pretation, we now plot (Figure 7) values
of DEg, the energy difference (per cycle)
between the case of pure Ar (�1 600mJ, see
Figure 6b) and Ar with Fd of C2H2 additive
(dopant), versus Fd, corresponding to f¼ 20, 30, and 40 kHz. We calculate the
energy consumed per molecule under the presumed
condition where all C2H2 molecules just appear to have
been converted in the plasma, namely at the very
beginning of the plateau regions in Figure 7, correspond-
ing to (Fd)crit. For the case of 20 kHz, and during a 1 s
time interval, the total energy absorbed by the C2H2
molecules, Etot, is
Etot ¼ DEg � f ffi 600mJ
cycle� 20� 103 cycles
Etot ¼ 12J� 6:24� 1018eV
J¼ 7:49� 1019eV
ð3Þ
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Figure 6. Plots of Eg versus dopant gas flow, Fd, for Ar (F¼ 10 slm)DBD plasmas doped with (a) permanent diatomic gases, O2 andN2; and (b) hydrocarbons. The frequency was f¼ 20 kHz and theapplied voltage, Va¼ 2.8 kVrms.
Figure 7. Plots of absorbed energy difference, DEg, versus C2H2dopant flow, Fd, for Ar (F¼ 10 slm) DBD plasmas at 20, 30, and40 kHz. The applied voltage, Va, was 2.8 kVrms.
B. Nisol, H. Gagnon, S. Lerouge, M. R. Wertheimer
6
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Figure 7 shows that the plateau is reached at (Fd)crit¼20 sccm; the corresponding number of molecules, N,entering into the discharge zone is, at 295K
Table 1. Calculated values of energy-per-molecule forhydrocarbons at their respective (Fd)crit values. For the cases ofO2 and N2, calculations correspond to Fd¼ 100 sccm.
Dopant DEg Fd Etot/N
Plasma
� 2015
rly V
N ¼ 20� 10�3 L
min� 1mol
24L� 6:022� 1023
molec
mol
¼ 5� 1020molec
min
gas (eV) (sccm) (eV/molec)
or, for the 1 s time interval
CH4 250 7 10.7
C2H6 350 10 10.5
N ¼ 8:3� 1018
molec
sð4Þ
C2H4 450 10 13.5
C2H2 600 20 9.0
Therefore, energy consumed per molecule is, from (3)
and (4):
O2 600 100 1.8
N2 400 100 1.2
EtotN ¼ 7:49� 1019=8:3� 1018 ¼ 9:0
eV
molecð5Þ
Process. Polym. 2015, DOI: 10.1002/ppap.201500068
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From Figure 7, we note that for f¼ 30 and 40 kHz,
(Fd)crit�30 and�40 sccm, respectively; therefore, the value
Etot/N¼9.0 eVmolec�1 appears to have remained constant
for all three conditions. This tends to confirm the totalconversion hypothesis proposed above.
Similar calculations have been carried out for the other
hydrocarbons, also at f¼ 20 kHz (see Figure 6b), and for
O2 and N2. The results are listed in Table 1.
The relatively high energies for the hydrocarbon
molecules, close to that of the lowest Ar metastable state,
bear witness to substantial fragmentation, considering
that C–C (�3.6 eV), C55C (�6.2 eV), and C������C (�8.7 eV) bond-
breakages all occur below those values. Figure 6a shows
that, unlike the hydrocarbons, the two permanent gases,
O2 and N2, displayed no plateaus. At Fd¼ 100 sccm, the
corresponding respective values of Etot/N were 1.8 and
1.2 eV. Since dissociation energies of O2 and N2 molecules
are respectively 9.80 and 5.04 eV, the values in Table 1
DOI: 10.1002/ppap.201500068
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Energy of Reactions in Atmospheric-Pressure Plasma Polymerization with Inert Carrier Gas
suggest that these additive gases underwent primarily ro-
vibrational excitation in the plasma.
Further evidence that confirms the total conversioninterpretation above comes from the following experi-
ment that was performed with Fb¼ 20 sccm C2H2 in
F¼ 10 slm Ar (i.e., 2 ‰ concentration) in the reactor
under static conditions, for total durations of 10min:
Figure 8a and b shows layers of plasma-polymerized
acetylene, PPA (whitish rectangular regions just left-of-
center in the photographs, evidently after the substrate
platen had been moved to the left) deposited at f¼ 40
and 20 kHz, respectively. It should be stressed that these
PPA deposits were not high-quality thin films, but porous
and powdery in nature.[29] From Figure 7, we note that
Fd (¼20 sccm)< (Fd)crit at 40 kHz, but � (Fd)crit at 20 kHz.
We further note that C2H2 is well-known to be the most
reactive of the light hydrocarbon monomers; and the
deposit width in Figure 6a is significantly less than that of
Figure 8. Thick PPA deposits generated in Ar (F¼ 10 slm)/acetylene (Fd¼ 20 sccm) DBD plasmas at 40 (a) and 20 kHz (b).The applied voltage, Va, was 2.8 kVrms and the deposition time10min. The substrates were stationary during deposition, thegrounded electrode platen only being moved afterward foracquisition of these photographs.
the electrode (6 cm), while that in 6b well matches the
electrode width.
Theobvious conclusion is that theaddedC2H2was totallyconsumed in the leftward flow of feed gas (to form PPA
deposit) in Figure 6a, between the center of the gas injector
(extreme right-hand side of the whitish deposit) and its
irregularly shaped left-hand extremity. Of course, the
mirror image situation occurred on the right-hand side of
the DBD reactor.
Using 5 cm� 5 cm, 125mm-thick Kapton polyimide
substrates, we performed a long-duration (40min) depo-
sition of PPA at 20 kHz with 40 sccm of C2H2. The samples
were then weighed, the weight of bare Kapton being
subtracted, we found close to 16.7mg of solid deposit.
Normalizing this with respect to the total coated area, the
total mass of deposit (288mg) and of C2H2 monomer
injected (ca. 1.7 g), we deduced a gas-to-solid conversion
efficiency close to 17%. However, this was clearly an
underestimate, because fine powder deposits, entrained
in the exiting 10 slm gas flow, were later encountered in
substantial quantities elsewhere in thePlexiglass enclosure
(see Figure 1b). The deposit thickness was found to be
ca. 40mm, from which the approximate density of the PPA
was calculated. The value, r¼ 0.17 g cm�3, confirms the
porous, powdery nature of the coating.
Discussion and Conclusions
Evidently, this DBD reactor systemwas not intended for
the purpose of depositing poor-quality PECVD coatings like
the PPA layers discussed above, quite the contrary: To
repeat theobjectives stated in the Introduction, the researchwe report here has served
1.
T t
to investigate whether methodology developed ear-
lier[25,26] can be transferred from the small DBD cell,
designed for plasma diagnostics, to this much larger
PECVD reactor;
2.
to investigate whether Eg measurements can yield
useful information aboutAP plasma-chemical reactions
when dopant concentrations are ca. 1‰ in theAr carrier
flow.
Clearly, affirmative answers can be given to both
questions raised in (1) and (2): We have found that the
equivalent circuit model of the small DBD cell applies
equally well to the present (ca. 50-fold) larger reactor,
and that our highly-perfected MATLAB code-based
methodology for evaluating Eg has led to exquisitely
reproducible and meaningful results. Just like in ref.[26]
where absolute validation of experimentally determined
Eg could be demonstrated through an energy balance
experiment, values presented here in Table 1 led to very
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CH4
C2H2
C2H4
B. Nisol, H. Gagnon, S. Lerouge, M. R. Wertheimer
8
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credible Etot/Ndata (see last column). Let us nowattempt to
compare these with data from the literature. To the best
of our knowledge, this is the first publication in which
absolute reaction energy values, DEg, are being reported.
Several of the authors cited above[8–11,24] published AP
DBD studies designed to correlate PP deposition with
W/FM, the energy values often being based on the
Lissajous figure method. For example, Kakaroglou et al.[24]
entitled their recent article ‘‘Evaluation of the Yasuda
parameter on atmospheric plasma deposition of allyl
methacrylate’’: Using a DBD reactor of similar design as
the present one, also with Ar carrier gas, they did not
distinguish between energy absorption with and withoutmonomer, which can also be said for all of the other
published research known to us. Therefore, based on the
present work, we can state with confidence that W/FMwas so far always overestimated, and that correlations
could be achieved only in a qualitative manner. To
illustrate this more clearly, let us consider Figure 6b as
an example: most workers use monomer concentrations
in the sub-‰ range, sometimes merely a few ppm,[8]
so that their determinations of ‘‘W’’ correspond to the
region near the top (Fd� 0 sccm, Eg¼ 1600mJ) of Figure 6b.
Taking our C2H2 example at Fd¼ 10 sccm (1‰), for which
Eg¼ 1200mJ but DEg¼ 400mJ, the overestimation would
be by a factor of 3. It is now instructive, however, to
examine the literature on low-pressure h.f. plasma
experiments, which have important advantages already
cited in the Introduction:
1.
Plas
� 2
rly
relatively straightforward measurement of h.f. power,
with adequate precision;
2.
use of pure reagent gases (e.g., monomer vapors),
without carrier gas.
In their well-known, frequently cited article published
in 1978, now probably mostly of historic interest,
Yasuda and Hirotsu measured and compiled many values
of the parameter W/FM, including for gases investigated
here.[19] We have converted their data, reported in
units of kWhg�1, into the ones used here for Etot/N,eV/molecule, and found the following: by way of
examples, their reported values for Ar, N2, CH4, and
C2H2 were, respectively, 22, 39, 30, and 48 eV/molecule,
compared with our respective values of 2.5, 1.2, 10.7,
and 9.0 eV/molecule (see Table 1). Needless to say, the
current values appear to be far more realistic in terms
of true reaction energies. Turning to more recent work
than Yasuda’s 1978 article, let us compare the present
data with those of Hegemann et al. in ref.[15] and
Hegemann in ref.[17] In Table 1 (page 234) of ref.[15] the
authors compile generalized activation energy values,
Ea [in eV, see Equation (1)], for different monomers,
along with their correlation to the possible PP growth
ma Process. Polym. 2015, DOI: 10.1002/ppap.201500068
015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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mechanism. Three results are of particular interest here,
namely:
e nu
: Ea ¼ 5:3� 0:5 eV ðassumedbond breakage :C�HÞ
: Ea ¼ 9:0� 0:6 eV ðassumedbond breakage :C� CÞ
: Ea ¼ 12� 1:2 eV ðassumedbond breakage: C¼ C; C�HÞ
In ref.,[17] Hegemann states ‘‘Ea of C2H6 seems to be
comparable to that of CH4, also pointing to hydrogen
abstraction.’’ The last two values of Ea above are
remarkably close to corresponding ones in our Table 1,
namely Etot/N¼ 9.0 and 13.5 eV/molecule, respectively.
Only our value for methane, 10.7 eV/molecule, differs
significantly from theirs. Considering the fundamental
differences in fragmentation-initiating mechanisms,
primarily electron-impact and Penning transfer in the
low- and atmospheric-pressure (AP) cases, respectively,
these agreements are quite astonishing. The overall
higher energies per molecule observed in AP DBD for
the saturated hydrocarbons might thus be related to the
Penning transfer, while the excitation of unsaturated
hydrocarbons appears to be similar. Moreover, despite
higher energies per molecule reported by Yasuda, the
ratio between C2H2 and CH4 (�1.6) agrees well with
Hegemann’s findings (�1.7). Therefore, the reported
energies per molecule might indeed be interpreted as
due to an activation barrier opposing the chemical
reaction pathway. The observed transition in energy
difference between the monomer-deficient and the
energy-deficient regime (to use Yasuda’s terminology)
when reaching the plateau, further supports this view.
Comparison of plasma polymer coatings prepared by AP
DBD with their LP-PECVD counterparts is highly desirable,
but not entirely new: This has been reported for the case of
organosilicones (tetraethoxysilane – TEOS) and hexame-
thyldisiloxane (HMDSO) by Sawada et al.[30] While those
authors did not report reaction energies, they did assume
that plasma polymerization (at least of organosilicones) in
AP and LP PECVD proceeds via the same chemical reaction
pathway. The task of confirming this by reaction energy
measurements is now in progress in our laboratory.
Obviously, there is much justification and need to pursue
this line of investigation further.
The research we report here has evidently gone a long
way toward characterizing this new reactor’s overall
performance with inert (Ar) carrier gas, to measure energy
consumption in its DBD plasma with and without the
addition of gaseous reagents. Although PPA deposits
examined to date were powdery, deposition experiments
have been conducted with acrylic acid (AA) vapor, a
monomer used in numerous other published studies.[31,32]
DOI: 10.1002/ppap.201500068
mbers, use DOI for citation !!
Energy of Reactions in Atmospheric-Pressure Plasma Polymerization with Inert Carrier Gas
We found that very smooth films could be deposited,
again on Kapton substrates, using 0.3‰ AA vapor in Ar
(F¼ 10 slm); f¼ 20kHz; 2.1�Va� 3.5 kVrms, but presenting
further details is beyond the scope of this article. Detailed
results about high-quality deposition experiments using
various monomers will be presented subsequently.
Acknowledgements: The authors are grateful for financial sup-port from the Natural Sciences and Engineering Research Councilof Canada (NSERC). Financial support was also provided by theFonds de recherche du Qu�ebec – Nature et technologies (FRQNT) viaPlasma Qu�ebec. We thank Ms. Myl�ene Archambault-Caron andMr. SeanWatson for participating in some of the experiments, andYves Leblanc and Francis Boutet for skilled technical help.
Supporting Information is available at Wiley Online Library orfrom the author.
Received: April 23, 2015; Revised: June 2, 2015; Accepted: June 23,2015;DOI: 10.1002/ppap.201500068
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