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Plasma-spectroscopic studies on alternating current arc discharges in methane
Spectrochmnca Acta, Vol. 33B, pp. 635 to 647 Pergamon PressLtd.1978. PrInted in Great Britain
Plasma-spectroscopic studies on alternating current arc discharges in methane* G. HEINRICH,
H. NICKEL, M. MAZURKIEWICZ
Kernforschiingsanlage Jiilich GmbH, Institut fiir Reaktorwerkstoffe, Federal Republic of Germany
and
R. AVNI N.R.C.-Negev, (Received
POB 9001, Beer Sheva, Israel
18 November
1977; revised 7
Abstract-Plasma
Murch 1978)
spectroscopic studies on alternating current arcs in methane were pyrolytical processes leading to the formation of pyrolytic carbon. Operating parameters electrode gap and methane filler gas pressure were varied. Excitation temperatures rotational levels of CH and CZ radicals. Temperature determinations were carried out and in different zones in the plasma column. The validity of the Boltzmann equation number density was determined from the Stark broadening of the H,-line.
performed with a view to such as current intensity, were measured from the for various arcing periods was proved. The electron
1. INTRODUCTION PARTICULARinterest is attached to the formation of pyrolytic carbon by pyrolysis of gaseous hydrocarbons. A more thorough knowledge of this process is desirable in view of the extensive application of this material, and the possibility of affecting its properties. For this reason extensive investigations were already conducted using different methods of thermal decomposition of hydrocarbon gas. In this connection the efforts were concentrated predominantly on the product formation, the influence of additional radical traps or the admixture of organic compounds, which may also influence the pyrolysis process. Furthermore, the formation rate and properties of pyrolytic carbon were studied. It could be proved in particular by means of the “cold-wall” investigations of the actual deposition mechanism, that carbon formation is purely a plasma process [l, 21. Further information about the nature of the process of pyrolysis should therefore become available chiefly by investigating hydrocarbon plasmas. Research was performed in which hydrocarbon plasmas were produced by known methods and which permitted non-contacting measurement techniques. In this context, microwave-induced plasmas have been studied by AVNI et al. [3]. The study of hydrocarbon plasmas in an a.c. arc, which is the subject of this paper, represents a further contribution. These experimental conditions make it possible to adjust the pressure range so that it corresponds to the partial pressures in fluidized beds used for coating nuclear fuel with pyrolytic carbon, which is one of the most interesting applications of carbon pyrolysis. Using spectroscopic methods, various plasma characteristics were measured under different discharge conditions. The following discharge parameters were varied : (i) (ii) (iii) (iv)
the the the the
* Dedicated [l] [2] [3]
methane filler gas pressure ; duration of discharge ; electrode gap ; current intensity. to the memory
of Professor
HEINRICH KAISER.
L. S~TTERLIN, KFA Report. Jill-735-RW(1971). K. KOIZLIK. J. LINKE, H. LUHLEICH, H. NICKEL and P. PFLAUM. KFA Reporr Jill-112%RW(1975). R. AVNI, H. NICKEL and J. D. WINEFORDNER, KFA Report Jill-1188-R W (1975). 635
636
G. HEINRICH et al
In addition, the influence of argon on the plasma characteristics was examined. Axial and radial distributions were determined to obtain a picture of the spatial variation of the plasma parameters. Temperature determinations in the methane plasma were carried out with the rotational lines of individual vibrational bands of the free radicals of C2 and CH. The results were compared with those in literature [4]. For comparison and to facilitate further investigations of possible excitation mechanisms, the temperature of excited atoms introduced into the plasma was also measured with the aid of Zn lines using the two-line method. The various results were compared to establish whether local thermal equilibrium (LTE) exists. Thermal excitation giving the possibility of defining a gas temperature, in addition to the question of energetic equilibrium in the arc and, possibly, detailed chemical equilibria, is the fundamental process describing a plasma by means of a small number of parameters. Besides the temperatures, we measured the electron number density. Great significance must be attributed to the free electrons due to their function as charge carriers for maintaining the plasma, and, possibly, also as reaction partners. The electron number density was determined from the Stark broadening of the HP-line. 2. 2.1.
Temperature
THEORETICAL
measurement
Hydrocarbon plasmas are regarded as chemical plasmas, where pyrolytical reactions take place during which methane molecules are split while simultaneously unsaturated hydrocarbons of a higher atomic order are built up [3,5]. It may be assumed that species activated for reaction exhibit a temperature different from that of the products formed immediately after an endothermic or exothermic reaction. Owing to the fact that neither the carbon nor the hydrogen atom have adequate line pairs for temperature determination, the latter can be carried out in hydrocarbon plasmas only via the two-line method by introducing suitable elements, such as Zn used here. Since, however, this will first of all enable statements to be derived only on possible excitation mechanisms, specific temperature measurements on pyrolysis products must be carried out to obtain results relevant to the reaction mechanisms. Suited for this purpose are molecular spectra. Since, as a rule, temperature equilibrium exists among the rotational degrees of freedom, reliable temperatures can be derived from rotational spectra. 2.1.1. Temperature determination by means of rotational lines. Temperature measurements from rotational lines are possible if the rotational levels of the individual vibrational levels follow a Boltzmann distribution. Then the intensity of a rotational line due to a transition from an excited to a ground state can be derived; the results are given in literature (e.g. [6,7]). Regarding the ratio of two rotational lines (marked by the indices 1 and 2) belonging to the same branch of a vibrational band, a convenient expression can be derived for the temperature
El
*
E2 =
B,D = molecular constants; Wi, W2 = intensity factors; frequencies.
BJl(J1 + 1) - DJ$Jr BJ2(J2
+
+ 1)2,
1) - DJj(J2 + 1)2.
J1,J2 = rotational quantum numbers; h = Planck constant; k = Boltzmann
Ii, I2 = intensities; constant; vl, v2 =
[4] R. A. DURIE, Proc. Phys. Sot. 65, 125 (1952). [5]
W. F. LIBBY, J. Chem. Phys. 35, 1714 (1961).
[6] [7]
J. A. SMIT, Ph.D. Thesis, University of Utrecht (1951). P. W. J. M. BOUMANS, Theory qf Spectrochemical Excitation.
Hilger & Watts, London
(1966).
Plasma-spectroscopic
Fig. 1. Emission
spectrum
studies on alternating
of methane
current
plasmas produced 380-515 nm.
arc discharges
in an a.c. arc.
in methane
Wavelength
637
range:
It is thus demonstrated that the rotational temperature can be determined from the slope of In I - In Pv4, plotted against the rotational energy (8). 2.1.2. Temperature determination by means of C2- and CH-rotational spectra. The rotational lines oYCz and CH are suited for temperature determinations in the a.c. arc plasma in methane. As may be observed from the emission spectrum in Fig. 1, the &-bands occur in the singlet (‘II + ‘II) and triplet (311 + 311) state, whereas the CH-bands appear as a (2A + ‘II) transition. The CH+-bands (‘II + ‘C) are very faint and not suitable for temperature measurements. For the sake of simplicity, the rotational lines of the singlet state were selected for C2 from the various vibrational bands. In particular, the lines of the (0,O) and (0,l) transitions are fairly easy to measure. From the CH rotational lines, the P and Q lines of the (0,O) transition were used.* The pertinent intensity factors W for the temperature determination are specified in Table 1. In a computer programme, a straight line was fitted through approximately 18 points. 2.1.3. Temperature measurements by means of Zn lines. Temperature measurements using the two line method were carried out by means of the Zn lines 307.6 and 328.2 nm, as described in [7]. 2.2. Determination
of the electron number density
The electron number density can be determined spectroscopically, e.g. with the aid of the broadened line profile of the Ho-line. Such broadening is to be ascribed to the influence of the microfields of charged particles which, due to their statistical distribution, do not cause any discrete Stark-effect splittings, but line broadening. The collision
* The rotational lines of the Cs(‘lI + ‘lT) (0,O) transition were determined up to J 6 85 for the R and P branch: those of the C2(‘fI + ‘ff) (0,l) transition up to J = 65. The P and Q rotational lines of the CH(‘A + ‘ff) (0,O) transition were determined up to J = 17. The wavelength of the individual rotational lines were predetermined by computer programmes and subsequently measured on the photoplate. The wavelength values of the rotational lines mentioned may be obtained from Prof. Dr H. NICKEL, Institut fur Reaktorwerkstoffe der Kernforschungsanlage Jiilich; P.O. Box 1913, D 5170 Jiilich, West Germany.
638
G. HEINRICH et 01.
Table 1. Intensity Radical
factors
of CH and C2 bands
Branch
Intensity
J’ - R2
P(J 1
J (J+
R(J) CH
factor
l)‘-R2
J+l (25 + l)(J + Cl)(J - R + 1)
Q(J1
2J(J + 1) (J - Cl)(J - R + 1)
P(J)
25
J = Quantum number of the angular momentum; Quantum number of spin-angular momentum coupling; P-branch ; Q = Q-branch : R = R-branch.
n = P=
broadening is theoretically described most comprehensively by GRIEM [8] and PENNER [9]. Assuming the “classical path approximation” (particles move in a straight line between collisions; total number of collisions > number of collisions leading to excitation) and the “collision approximation” (De Broglie wavelength of the interfering particle < all of the collision parameters contributing to the line broadening), a wave-mechanical solution of the system by methods of perturbation computation is performed. This results in a relation for the line profile, in which solely the Hamilton operator remains to be interpreted. In connection with the linear Stark effect the ion movement is ignored. Consequently, the energy operator depends only on the electric field caused by the ions. Since the field strength F of the point charge of an ion is proportional to e/r2, and since the effective distance r of the emitting particle from the next ion is again proportional to applies the ion distance ni- ‘I3 , the following proportionality F z eOn2’3,
(2)
where e. is the electron charge and ni the number density of the ions. The statistical fluctuations of F are taken into account by a distribution function W(F). If the electron contribution, due to the short interaction time, and the ionion interactions are ignored, the profile may be determined by the Holtsmark distribution function W(F/F,), in which the so-called normal field strength F. is defined by FO = 2.61e,&‘”
z
e0.
Values for W(F/Fo) are tabulated by e.g. GRIEM [8] for the H, line at various temperatures. The probability distribution W(F/Fo) may be transformed into a frequency distribution, corresponding to the intensity distribution I(o) of the line. It is then appropriate to introduce (4) in which AA and Aw, respectively, denote the distance the line centre io. The profile may then be defined by S(a) = Z(0)
II
of wavelength
and frequency
ds= 27ccFo /22 I(w)
(5)
0
[8] H. R. GRIEM, Plasma Spectroscopy. McGraw-Hill, New York (1964). [9] S. S. PENNER, Quantitative Molecular Spectroscopy ad Gus Emissivities. Addison-Wesley, (1959).
from
New
York
Plasma-spectroscopic
studies on alternating
current
arc discharges
in methane
639
Fig. 2. Schematic drawing of the experimental equipment, Top: side view. Bottom: top view. 1 -discharge chamber; Z-arc generator; 3-oscilloscope; 4-recorder; 5-diffusion pump; 6-vacuum measuring apparatus; 7-vacuum tube gauge; 8-Hg-manometer; 9-calibration lamps; lo-rotation sector; 1 l-condenser lenses; 12-diaphragms; 13-deviating mirror; 14-spectrograph entrance slit.
involving the normalizing condition
s ‘x
S(cc)dt = 1. -UZ
(6)’
MAECKER [lo] specifies a semi-graphic method by means of which the electron number density can be determined from the measured line profile on the basis of the described derivation. It is thus possible to determine the ion number density and, owing to the quasi-neutrality, also the electron number density (n,). The procedure is well described in MAECKER’S paper and will not be repeated here. .
3. 3.1. Apparatus
and operating
EXPERIMENTAL
conditions
The essential apparatus components are shown in Fig. 2. The discharge chamber contains a rotating electrode system accommodating up to 5 pairs of electrodes. Prior to each discharge, the chamber was evacuated to lo-* torr and then flushed twice with argon and once with methane. A 2-m plane grating Ebert spectrograph (RSV) was used: grating: 216Ogroves/mm; slit height: 2- 16 mm ; slit width : 30pm. As photographic material Kodak SA-1 and SA-3 plates were used. They were developed in Kodak D-19 for 4 min at 20°C. The entrance slit was parallel to the arc’s axis. Therefore the image had to be rotated by 90” by means of two mirrors, when the radial distribution was investigated. The geometric-optical imaging conditions, however, were identical for the spectra in axial and radial direction (source, lens system and diaphragm were shifted by the same distance). Spectra were taken during the following time intervals after ignition of the arc: 5-15, 25-35 and 55-65s. Therefore for each spectrum 3-7 individual exposures were necessary. The arc generator was an FES 270/GTT 550 RSV-generator. The electrode material used was RW-0 (Ringsdorff Company). Blackening measurements and line profile recordings were made with either a twin projector (Steinheil) or a “Schnellphotometer” (Jenoptik). [lo]
H. MAECKER, 2. Physik 136, 119 (1953)
SA(B) 33p-C
640
G. HEINRICH et al. U
-tB-~~ .~ Fig. 3. Arc voltage
3.2. Discharge
T
(U) characteristic.
Us = k25 V
; rs = 8 ms, T = 0.02 s
conditions
The arc discharge could be ignited up to a maximum intensity, and electrode gap were varied as follows:
methane
pressure
of 250 torr.
Pressure,
current
(i) methane filler pressure: 25, 50, 100, 170 and 250 torr ; (ii) pulse current of the arc: 8 and 12A; (iii) electrode gap: 2 and 4mm. The a.c. arc discharge is characterized as follows: a sinusoidal voltage with the mains frequency (50Hz) was applied to the upper electrode featuring a peak value of k 150V against the lower grounded electrode. A superimposed high-voltage spark could be shifted stepwise with respect to the phase of the sinusoidal voltage using a delay unit. A maximum phase reduction was achieved if the ignition was effected at a phase value of 5n/l2 and 17~/12. As shown in Fig. 3, the high-voltage spark was applied to every half-wave, resulting in an operating voltage to ground of LIB = +25V after ignition of the arc during an operating time of ts = 8 ms. The discharge was quenched prior to each zero passage (pulsating arc discharge). The voltage signal could be picked up in the generator via a voltage divider.
4.
RESULTS
AND DISCUSSION
4.1. Temperature 4.1.1. Population of the rotational levels of CH and Cz. In Fig. 4, the results of the measured intensities, divided by the theoretical intensity values and v4, are plotted against J(J + 1) at methane pressures between 25 and 250 torr. Evidently for the Q lines of CH in Fig. 4a, straight lines can be fitted for rotational quantum numbers 2 3. The points for smaller quantum numbers deviate increasingly in proportion to the rise in methane filler pressure. Figure 4b shows the relevant diagram’ for the C2 band of the ‘I7 - ‘II (0,l) transition. Since this band is P-edged, the line values of the lower intensity R-branch were plotted to enable observation of the behaviour even at lower rotational quantum numbers. In this connection it may be stated that the intensity values between R15 and R6.5 can be fitted by a straight line for all of the methane filler pressures, and that for reduced methane pressures even values for smaller rotational quantum numbers are located on the straight line. It could be also proved for the P-lines that the intensity values can be approximated by a straight line between P20 and P66. It has thus been demonstrated that a Boltzmann energy distribution occurs in the a.c. arc for rotations up to high rotational levels in the case of Cz. This is, for instance, not true for discharge tubes [ 111, where deviations were found for higher rotations. On the basis of these results it is possible to define a rotational temperature for both Cz and CH. 4.1.2. Behaviour of rotational temperatures in the plasma column. In Fig. 5, the rotational temperatures of Cz and CH in axial direction at 12-A pulse current and 4-mm electrode gap are plotted for methane filler pressures of 25, 50 and 100 torr. A striking feature is the temperature difference indicated by the two radicals. While the temperature for Cz lies between 5500 and 63OOK, the temperature for CH ranges from 3000 to 4200K. Apart from measuring points immediately in front of the electrodes, the difference of 2000K does not significantly change along the axis. An appreciable [ 1 l]
W. LOCHTE-HOLTGREVEN, Z. Physik 64,443
(1930); 67,590
(1931)
Plasma-spectroscopic
t 0
.
studies on alternating
current
in methane
arc discharges
641
I
20
CO
80
60
100
120 J lJ*l)-
P
_ -
25T CHI, 50T CHL +1OOT CHI, ~-6 170T CHL -250T CHb
- J 0
500
1000
1500
2000
2500
3000
3500
.. Fig. 4. Semi-logarithmic
plot of I/Pv4
of CH (a) and J(J + 1).
C2 (b) rotational
Lo00 4500 JIJ*l)-
-I
lines as a function
of
axial temperature gradient cannot be detected. This also applies to higher methane pressures. The rotational temperatures change only slightly in proportion to the discharge time. Consequently, the radial distributions were only determined for the time interval between TIKl A
25 torr
CHL
TIKII
100 torr
CHL
3000-
?OOO;* 0 +-•+5-15s
1 x-x
2 25-35s
3
(b
1
2
3
(
o--055-65s
Fig. 5. Axial distribution of rotational temperatures for CH and Cz in an a.c. arc plasma methane. G-ground; E-electrode; 12-A pulse current ; 4-mm electrode gap.
of
3
G. HEINRICH et al.
642
+-+25 torr O--O 50 torr x--x100 torr
2000;
CHb CHL CHL
/-‘1’ 1
2
3
C r[mml
Fig. 6. Radial distribution of rotational temperatures of CH and Cz in an ax. methane. G-ground; E-electrode: 12-A pulse current; 4-mm electrode
arc plasma gap.
of
5 and 15 s ; the curves for various methane filler pressures (determined after Abel inversion) are plotted in Fig. 6. They suggest that high rotational temperatures occur for Cz even in the vicinity of the plasma boundary. The error for results referring to the interior of the plasma lies in the range of f 300 K for C2 and f250 K for CH. At the plasma boundary this error is &-400K. 4.1.3. Dependence of rotational temperatures on discharge conditions. Since the relative variation of rotational temperatures in space is essentially maintained when the electrical parameters are varied, it is only necessary to observe the change in absolute values. In Fig. 7, the rotational temperatures of the central section of the arc are plotted as a function of methane filler pressure for various current intensities and electrode gaps and the time interval between 5 and 15 s. A pronounced maximum can be observed in particular for the CH rotational temperatures at a methane pressure of 50 torr.
7000 -
c2 /-
x--x----.x 0-.0lx
/ J -+6ooo: 5000
t----+1
6CH
2000
o-o
Pulse
current
+-+
Pulse
currentBA,
8A,
Electrode
gap
2mm
Electrode
gap
L mm
X-X
Pulse
currentl2A.Electrode
gap
Lmm
* 50
100
150
200
250
300 P ltorr
Fig. 7. Rotational
1
temperatures at different discharge conditions in an a.c. arc operating methane as a function of the filler pressure. Time interval: 5-15 s.
in
Plasma-spectroscopic
studies on alternating
150
Fig. 8. Rotational
temperatures
200
250
300
current
350
arc discharges
in methane
643
ll.I[Wattsl
of Cz and CH in an a.c. arc operating of energy input.
in methane
as a function
For Cz radicals the drop in temperature is lower at higher pressures ; at a pulse current of 8 A and 4-mm electrode gap the changes are within the margin of error. The temperature difference for the two radicals is slightly reduced at a shorter electrode gap. For a methane pressure of 25 torr it may be assumed (by considering the drift velocity) that excitations due to electron collisions acquire an additional significance. The cross section for such collisions reaches its highest value for small vibrational quantum numbers (cf., e.g. HASTED [12]) to which our investigations apply. On the other hand, the cross section decreases with increasing rotational quantum numbers. Therefore low-energy levels will be excited comparatively more intensely so that a lower rotational temperature will be simulated. The behaviour at increasing methane pressure could be partially explained by the excitation mechanism. It is most likely that the radicals receive their high rotational energy already during the formation process, since a major energy transition due to collision, in particular in connection with molecules of higher atomic order, is improbable in view of the angular momentum conservation for the entire system. For the purpose of interpreting the excitation mechanism, GAYDON [ 131 assumes high-energy collision partners in the plasma. The radical under examination could then loose part of its rotational energy due to collisions with low-energy particles before a collision with a high-energy partner leads to excitation. With increasing methane pressure the probability of collision with a high-energy partner decreases at constant energy supply to the plasma. Several collisions involving a reduction in rotational energy would thus be possible prior to excitation, which would explain the reduction in temperature. The influence of the arc parameters of methane filler pressure and electrode gap on rotational temperatures is illustrated in Fig. 8. The rotational temperature was plotted against the plasma power input (lowest input at 2-mm electrode gap, maximum at 12-A pulse current, 4-mm electrode gap). With the exception of 25 torr, a temperature increase may be observed for the two radicals in the case of both rising arc current intensity and reduced electrode gap. It is remarkable that the effect caused by a current increase becomes lower for CH rotational temperatures with increasing methane filler [12] J. B. HASTED, Physics of AtomicCollisions. Butterworths, London (1964). 1131 A. G. GAYD~N and H. G. WOLFHARD, Proc. Roy. SM. (London) 199, 89 (1949).
644
G. HEINRICH et al.
6000. -statlonory ---fluid 5000.
Ll
81 Ar CH,,
Fig. 9. Dependence of the rotational temperatures of CH and Cz in a d.c. arc in methane on various argon additions. Methane partial’ pressure: 50 torr ; current intensity : 8 A ; electrode gap: 4 mm ; time interval: 5- 15 s.
pressure as compared to that due to a reduction in electrode gap. On the other hand, a current increase to 12A always causes a higher temperature increase for Cz radicals as compared to that resulting from a reduction in electrode gap to 2 mm. 4.1.4. Temperature measurements by means of Zn lines. The excitation temperature for Zn atoms in the plasma was determined by evaporating minor amounts of Zn as additive. It is 7200 K in the case of a methane pressure of 50 torr. Investigations into the axial distribution of Zn temperatures did not reveal any appreciable change along the arc axis either. Observations similar to those described for Cz and CH were made also in radial direction: the drop in temperature is relatively small towards the plasma boundary where excitation still takes place. The high excitation temperatures of Zn atoms substantiates the assumption discussed above as to the presence of high-energy collision partners in the plasma. 4.15. Temperatures in the presence of inert gases. Measurements of the CH and Cz rotational temperatures in the presence of an inert gas were to provide a further understanding of the different behaviour of the two radicals. However, a direct current arc was used for these experiments, since also measurements by a probe were performed for which the spark had to be suppressed. In Fig. 9, the distribution of rotational temperatures of the two radicals at a constant methane partial pressure of 50 torr and varying argon to methane pressure ratios is plotted for the time interval between 5 and 15 s. The experiment was conducted for a stationary system as well as in a flowing system. As can be seen, the temperature difference between CH and Cz is higher than in the a.c. arc plasma without argon. Whereas the temperature of C2 remains virtually unaffected by the addition of argon, that of the CH radical is reduced. This behaviour is not easy to explain in terms of collision kinetics. It appears possible, however, that the transfer of energy to higher rotational levels might be enhanced during discharge in the d.c. arc. Apparently, this energy distribution is not significantly influenced by the presence of argon in the case of Cz. For the CH radical it can be assumed that an increased transfer of rotational energy to levels of higher quantum number increases predissociation, which, in a way, acts as a barrier. The influence exerted on the energy distribution of the upper rotational level by the presence of argon due to kinetic interaction cannot be checked using simple means. However, the drop in temperature, as well as a reduction in intensity observed suggests increasing pre-dissociation effects, resulting in a decreasing probability of energy transfer to higher rotational levels.
Plasma-spectroscopic
studies on alternating
current
j>,
arc discharges
, 0,lo
0.20
0.30
645
in methane
,~ O.LO AhInml
Fig. 10. Line profile of the long wave section of the HB line in, an a.c. arc in methane various filler pressures. 12-A pulse current; 4-mm electrode gap.
for
4.2. Electron number density Because there is no LTE in the plasma under investigation, the determination of the electron density by the Saha equation was evaded and the method described by MAECKER [lo], mentioned in Section 2, was applied to the HP line profile. Unfortunately, only the long-wave wing of this line is not overlapped in the spectrum at methane pressures of 25 and 50 torr. It has furthermore been observed that the H, line emerges most distinctly in this low range of pressure, whereas it appears only faintly-at increased pressures (in conjunction with a simultaneous strong enhancement of the background). This behaviour is illustrated by Fig. 10, in which the intensity curves determined after background correction are plotted. At the same time, it becomes evident that evaluation becomes highly inaccurate at elevated pressures. Investigations into the axial distribution of the electron number density have shown that excess negative charges are built up in front of the non-ground electrode, causing a transient in field strength (Fig. 11). The dependence of electron density on methane filler pressure is shown in Fig. 12, in which the values for 12-A pulse current and 4-mm electrode gap are plotted for the time interval 5-15 s. The values plotted are the mean values determined over the entire arc but excluding the zones in the electrode proximity. The substantial increase in electron number density with methane pressure is likely to have various causes. On the one hand, gas density is increased, so that more particles can be ionized. Moreover, the change in gas composition caused by simultaneous decomposition and pyrolysis processes in the plasma must be taken into account. As will be reported in detail later on, the detectable radicals become less apparent in proportion to increasing methane pressures. It could be proved that these must continue to react immediately, pyrolysing to form unsaturated hydrocarbons of a higher atomic order [3]. As a result of such enhanced formation of radicals of higher atomic order in the plasma, these will increasingly contribute to the electron number density due to ionization, the more so, since the ionization potential or thermal energy required for
G. HEINRICHet al.
646
x-x O-0 +-+
5-15s 25-35s so- 70s 1
1
2
I
3
electrode
gap [mm] -
Fig. 11. Axial
pressures
distribution of the electron number density in an a.c. arc in methane at filler of 25 and 50 torr. 12-A pulse current; 4-mm electrode gap. G-ground, E-electrode.
ion formation is significantly reduced in proportion to growing molecular weight [14]. In a plasma, an equilibrium is established in most instances between the processes of ionization and recombination. The number of recombinations ought to become more frequent as a result of the increase in collision frequency with rising methane pressure. Since, as already mentioned, there is a simultaneous strong enhancement of the background, the following scheme must be assumed in particular for processes under radiant emission : e+X+-+X
+hv,
e+X++X*+hv.
0
k
0
c” 1o16-
10'5:
10’1’
25
50
75
100
125
150
C
175 P (torr)
Fig. 12. Dependence of the electron number density on the filler pressure in an operating in methane. Current intensity: 12 A ; electrode gap: 4 mm. [14]
G. FRANCIS,Ionisation
Phenomena
in Gases. Butterworths,
London
(1966).
a.c.
arc
Plasma-spectroscopic
studies on alternating current arc discharges in methane
j
647
Pulse current0 A Electr.gapZmm
I
‘use-current81 ledr
gap Lmm
Ike-current 121 ectrgop Lmm
Fig. 13. Dependence of electron density in an a.c. arc operating in methane 50-torr filler pressures under different discharge conditions.
for 25-torr and
Since the electrons have a continuous energy distribution, the radiation emitted appears as continuum radiation. It should be mentioned that the influence of microfields on the ionization energy may be neglected. An assessment according to the theory specified by ECKER and KRGLL [15] resulted in a reduction by less than 0.1 eV. As may be clearly observed on Fig. 13, the electron density is influenced to a larger amount by external discharge conditions. Reducing the electrode gap from 4 to 2mm causes a reduction of IZ, to nearly half its amount. It may be assumed that, owing to a higher field strength at a reduced electrode gap, the electrons have a shorter residence time in the plasma. However, the arc current intensity can be kept constant in the shortened plasma column and with reduced electron number density. On the other hand, the electron number density changes almost proportionally to the arc pulse current in the case of constant electrode gap. 5. CONCLUSIONS Measurements of the rotational temperatures in ac. arc discharges in methane have shown that there is no volume element in the plasma in which local thermal equilibrium exists. No gas temperature for the plasma can be defined. In view of the lacking thermal equilibrium, any attempt to equate a translation temperature in terms of order of magnitude to the temperature of the CH radical is not allowed. An essential cause of the lack of thermal equilibrium may be seen in chemical reactions taking place in the gaseous zone. The radicals formed by methane decomposition continue to react chemically before being capable of causing a temperature equilibration as a result of a sufficient number of collisions. However, more in-depth investigations relating to reaction kinetics can only be carried out by means of additional measurement techniques, in particular by mass spectrometry. As will be shown in a further contribution, the reactions take place irreversibly, leading to the formation of carbon. At any rate, a model-type description of the plasma is likely to be extremely difficult due to the irreversibility. Further investigations into electrically charged plasma components as possible reaction carriers should be envisaged to obtain additional clarification. [ 151 H. ECKERand P. KRGLL,Phys. Fhids 6, 62 (1963).
Plasma-spectroscopic studies on alternating current arc discharges in methane* G. HEINRICH,
H. NICKEL, M. MAZURKIEWICZ
Kernforschiingsanlage Jiilich GmbH, Institut fiir Reaktorwerkstoffe, Federal Republic of Germany
and
R. AVNI N.R.C.-Negev, (Received
POB 9001, Beer Sheva, Israel
18 November
1977; revised 7
Abstract-Plasma
Murch 1978)
spectroscopic studies on alternating current arcs in methane were pyrolytical processes leading to the formation of pyrolytic carbon. Operating parameters electrode gap and methane filler gas pressure were varied. Excitation temperatures rotational levels of CH and CZ radicals. Temperature determinations were carried out and in different zones in the plasma column. The validity of the Boltzmann equation number density was determined from the Stark broadening of the H,-line.
performed with a view to such as current intensity, were measured from the for various arcing periods was proved. The electron
1. INTRODUCTION PARTICULARinterest is attached to the formation of pyrolytic carbon by pyrolysis of gaseous hydrocarbons. A more thorough knowledge of this process is desirable in view of the extensive application of this material, and the possibility of affecting its properties. For this reason extensive investigations were already conducted using different methods of thermal decomposition of hydrocarbon gas. In this connection the efforts were concentrated predominantly on the product formation, the influence of additional radical traps or the admixture of organic compounds, which may also influence the pyrolysis process. Furthermore, the formation rate and properties of pyrolytic carbon were studied. It could be proved in particular by means of the “cold-wall” investigations of the actual deposition mechanism, that carbon formation is purely a plasma process [l, 21. Further information about the nature of the process of pyrolysis should therefore become available chiefly by investigating hydrocarbon plasmas. Research was performed in which hydrocarbon plasmas were produced by known methods and which permitted non-contacting measurement techniques. In this context, microwave-induced plasmas have been studied by AVNI et al. [3]. The study of hydrocarbon plasmas in an a.c. arc, which is the subject of this paper, represents a further contribution. These experimental conditions make it possible to adjust the pressure range so that it corresponds to the partial pressures in fluidized beds used for coating nuclear fuel with pyrolytic carbon, which is one of the most interesting applications of carbon pyrolysis. Using spectroscopic methods, various plasma characteristics were measured under different discharge conditions. The following discharge parameters were varied : (i) (ii) (iii) (iv)
the the the the
* Dedicated [l] [2] [3]
methane filler gas pressure ; duration of discharge ; electrode gap ; current intensity. to the memory
of Professor
HEINRICH KAISER.
L. S~TTERLIN, KFA Report. Jill-735-RW(1971). K. KOIZLIK. J. LINKE, H. LUHLEICH, H. NICKEL and P. PFLAUM. KFA Reporr Jill-112%RW(1975). R. AVNI, H. NICKEL and J. D. WINEFORDNER, KFA Report Jill-1188-R W (1975). 635
636
G. HEINRICH et al
In addition, the influence of argon on the plasma characteristics was examined. Axial and radial distributions were determined to obtain a picture of the spatial variation of the plasma parameters. Temperature determinations in the methane plasma were carried out with the rotational lines of individual vibrational bands of the free radicals of C2 and CH. The results were compared with those in literature [4]. For comparison and to facilitate further investigations of possible excitation mechanisms, the temperature of excited atoms introduced into the plasma was also measured with the aid of Zn lines using the two-line method. The various results were compared to establish whether local thermal equilibrium (LTE) exists. Thermal excitation giving the possibility of defining a gas temperature, in addition to the question of energetic equilibrium in the arc and, possibly, detailed chemical equilibria, is the fundamental process describing a plasma by means of a small number of parameters. Besides the temperatures, we measured the electron number density. Great significance must be attributed to the free electrons due to their function as charge carriers for maintaining the plasma, and, possibly, also as reaction partners. The electron number density was determined from the Stark broadening of the HP-line. 2. 2.1.
Temperature
THEORETICAL
measurement
Hydrocarbon plasmas are regarded as chemical plasmas, where pyrolytical reactions take place during which methane molecules are split while simultaneously unsaturated hydrocarbons of a higher atomic order are built up [3,5]. It may be assumed that species activated for reaction exhibit a temperature different from that of the products formed immediately after an endothermic or exothermic reaction. Owing to the fact that neither the carbon nor the hydrogen atom have adequate line pairs for temperature determination, the latter can be carried out in hydrocarbon plasmas only via the two-line method by introducing suitable elements, such as Zn used here. Since, however, this will first of all enable statements to be derived only on possible excitation mechanisms, specific temperature measurements on pyrolysis products must be carried out to obtain results relevant to the reaction mechanisms. Suited for this purpose are molecular spectra. Since, as a rule, temperature equilibrium exists among the rotational degrees of freedom, reliable temperatures can be derived from rotational spectra. 2.1.1. Temperature determination by means of rotational lines. Temperature measurements from rotational lines are possible if the rotational levels of the individual vibrational levels follow a Boltzmann distribution. Then the intensity of a rotational line due to a transition from an excited to a ground state can be derived; the results are given in literature (e.g. [6,7]). Regarding the ratio of two rotational lines (marked by the indices 1 and 2) belonging to the same branch of a vibrational band, a convenient expression can be derived for the temperature
El
*
E2 =
B,D = molecular constants; Wi, W2 = intensity factors; frequencies.
BJl(J1 + 1) - DJ$Jr BJ2(J2
+
+ 1)2,
1) - DJj(J2 + 1)2.
J1,J2 = rotational quantum numbers; h = Planck constant; k = Boltzmann
Ii, I2 = intensities; constant; vl, v2 =
[4] R. A. DURIE, Proc. Phys. Sot. 65, 125 (1952). [5]
W. F. LIBBY, J. Chem. Phys. 35, 1714 (1961).
[6] [7]
J. A. SMIT, Ph.D. Thesis, University of Utrecht (1951). P. W. J. M. BOUMANS, Theory qf Spectrochemical Excitation.
Hilger & Watts, London
(1966).
Plasma-spectroscopic
Fig. 1. Emission
spectrum
studies on alternating
of methane
current
plasmas produced 380-515 nm.
arc discharges
in an a.c. arc.
in methane
Wavelength
637
range:
It is thus demonstrated that the rotational temperature can be determined from the slope of In I - In Pv4, plotted against the rotational energy (8). 2.1.2. Temperature determination by means of C2- and CH-rotational spectra. The rotational lines oYCz and CH are suited for temperature determinations in the a.c. arc plasma in methane. As may be observed from the emission spectrum in Fig. 1, the &-bands occur in the singlet (‘II + ‘II) and triplet (311 + 311) state, whereas the CH-bands appear as a (2A + ‘II) transition. The CH+-bands (‘II + ‘C) are very faint and not suitable for temperature measurements. For the sake of simplicity, the rotational lines of the singlet state were selected for C2 from the various vibrational bands. In particular, the lines of the (0,O) and (0,l) transitions are fairly easy to measure. From the CH rotational lines, the P and Q lines of the (0,O) transition were used.* The pertinent intensity factors W for the temperature determination are specified in Table 1. In a computer programme, a straight line was fitted through approximately 18 points. 2.1.3. Temperature measurements by means of Zn lines. Temperature measurements using the two line method were carried out by means of the Zn lines 307.6 and 328.2 nm, as described in [7]. 2.2. Determination
of the electron number density
The electron number density can be determined spectroscopically, e.g. with the aid of the broadened line profile of the Ho-line. Such broadening is to be ascribed to the influence of the microfields of charged particles which, due to their statistical distribution, do not cause any discrete Stark-effect splittings, but line broadening. The collision
* The rotational lines of the Cs(‘lI + ‘lT) (0,O) transition were determined up to J 6 85 for the R and P branch: those of the C2(‘fI + ‘ff) (0,l) transition up to J = 65. The P and Q rotational lines of the CH(‘A + ‘ff) (0,O) transition were determined up to J = 17. The wavelength of the individual rotational lines were predetermined by computer programmes and subsequently measured on the photoplate. The wavelength values of the rotational lines mentioned may be obtained from Prof. Dr H. NICKEL, Institut fur Reaktorwerkstoffe der Kernforschungsanlage Jiilich; P.O. Box 1913, D 5170 Jiilich, West Germany.
638
G. HEINRICH et 01.
Table 1. Intensity Radical
factors
of CH and C2 bands
Branch
Intensity
J’ - R2
P(J 1
J (J+
R(J) CH
factor
l)‘-R2
J+l (25 + l)(J + Cl)(J - R + 1)
Q(J1
2J(J + 1) (J - Cl)(J - R + 1)
P(J)
25
J = Quantum number of the angular momentum; Quantum number of spin-angular momentum coupling; P-branch ; Q = Q-branch : R = R-branch.
n = P=
broadening is theoretically described most comprehensively by GRIEM [8] and PENNER [9]. Assuming the “classical path approximation” (particles move in a straight line between collisions; total number of collisions > number of collisions leading to excitation) and the “collision approximation” (De Broglie wavelength of the interfering particle < all of the collision parameters contributing to the line broadening), a wave-mechanical solution of the system by methods of perturbation computation is performed. This results in a relation for the line profile, in which solely the Hamilton operator remains to be interpreted. In connection with the linear Stark effect the ion movement is ignored. Consequently, the energy operator depends only on the electric field caused by the ions. Since the field strength F of the point charge of an ion is proportional to e/r2, and since the effective distance r of the emitting particle from the next ion is again proportional to applies the ion distance ni- ‘I3 , the following proportionality F z eOn2’3,
(2)
where e. is the electron charge and ni the number density of the ions. The statistical fluctuations of F are taken into account by a distribution function W(F). If the electron contribution, due to the short interaction time, and the ionion interactions are ignored, the profile may be determined by the Holtsmark distribution function W(F/F,), in which the so-called normal field strength F. is defined by FO = 2.61e,&‘”
z
e0.
Values for W(F/Fo) are tabulated by e.g. GRIEM [8] for the H, line at various temperatures. The probability distribution W(F/Fo) may be transformed into a frequency distribution, corresponding to the intensity distribution I(o) of the line. It is then appropriate to introduce (4) in which AA and Aw, respectively, denote the distance the line centre io. The profile may then be defined by S(a) = Z(0)
II
of wavelength
and frequency
ds= 27ccFo /22 I(w)
(5)
0
[8] H. R. GRIEM, Plasma Spectroscopy. McGraw-Hill, New York (1964). [9] S. S. PENNER, Quantitative Molecular Spectroscopy ad Gus Emissivities. Addison-Wesley, (1959).
from
New
York
Plasma-spectroscopic
studies on alternating
current
arc discharges
in methane
639
Fig. 2. Schematic drawing of the experimental equipment, Top: side view. Bottom: top view. 1 -discharge chamber; Z-arc generator; 3-oscilloscope; 4-recorder; 5-diffusion pump; 6-vacuum measuring apparatus; 7-vacuum tube gauge; 8-Hg-manometer; 9-calibration lamps; lo-rotation sector; 1 l-condenser lenses; 12-diaphragms; 13-deviating mirror; 14-spectrograph entrance slit.
involving the normalizing condition
s ‘x
S(cc)dt = 1. -UZ
(6)’
MAECKER [lo] specifies a semi-graphic method by means of which the electron number density can be determined from the measured line profile on the basis of the described derivation. It is thus possible to determine the ion number density and, owing to the quasi-neutrality, also the electron number density (n,). The procedure is well described in MAECKER’S paper and will not be repeated here. .
3. 3.1. Apparatus
and operating
EXPERIMENTAL
conditions
The essential apparatus components are shown in Fig. 2. The discharge chamber contains a rotating electrode system accommodating up to 5 pairs of electrodes. Prior to each discharge, the chamber was evacuated to lo-* torr and then flushed twice with argon and once with methane. A 2-m plane grating Ebert spectrograph (RSV) was used: grating: 216Ogroves/mm; slit height: 2- 16 mm ; slit width : 30pm. As photographic material Kodak SA-1 and SA-3 plates were used. They were developed in Kodak D-19 for 4 min at 20°C. The entrance slit was parallel to the arc’s axis. Therefore the image had to be rotated by 90” by means of two mirrors, when the radial distribution was investigated. The geometric-optical imaging conditions, however, were identical for the spectra in axial and radial direction (source, lens system and diaphragm were shifted by the same distance). Spectra were taken during the following time intervals after ignition of the arc: 5-15, 25-35 and 55-65s. Therefore for each spectrum 3-7 individual exposures were necessary. The arc generator was an FES 270/GTT 550 RSV-generator. The electrode material used was RW-0 (Ringsdorff Company). Blackening measurements and line profile recordings were made with either a twin projector (Steinheil) or a “Schnellphotometer” (Jenoptik). [lo]
H. MAECKER, 2. Physik 136, 119 (1953)
SA(B) 33p-C
640
G. HEINRICH et al. U
-tB-~~ .~ Fig. 3. Arc voltage
3.2. Discharge
T
(U) characteristic.
Us = k25 V
; rs = 8 ms, T = 0.02 s
conditions
The arc discharge could be ignited up to a maximum intensity, and electrode gap were varied as follows:
methane
pressure
of 250 torr.
Pressure,
current
(i) methane filler pressure: 25, 50, 100, 170 and 250 torr ; (ii) pulse current of the arc: 8 and 12A; (iii) electrode gap: 2 and 4mm. The a.c. arc discharge is characterized as follows: a sinusoidal voltage with the mains frequency (50Hz) was applied to the upper electrode featuring a peak value of k 150V against the lower grounded electrode. A superimposed high-voltage spark could be shifted stepwise with respect to the phase of the sinusoidal voltage using a delay unit. A maximum phase reduction was achieved if the ignition was effected at a phase value of 5n/l2 and 17~/12. As shown in Fig. 3, the high-voltage spark was applied to every half-wave, resulting in an operating voltage to ground of LIB = +25V after ignition of the arc during an operating time of ts = 8 ms. The discharge was quenched prior to each zero passage (pulsating arc discharge). The voltage signal could be picked up in the generator via a voltage divider.
4.
RESULTS
AND DISCUSSION
4.1. Temperature 4.1.1. Population of the rotational levels of CH and Cz. In Fig. 4, the results of the measured intensities, divided by the theoretical intensity values and v4, are plotted against J(J + 1) at methane pressures between 25 and 250 torr. Evidently for the Q lines of CH in Fig. 4a, straight lines can be fitted for rotational quantum numbers 2 3. The points for smaller quantum numbers deviate increasingly in proportion to the rise in methane filler pressure. Figure 4b shows the relevant diagram’ for the C2 band of the ‘I7 - ‘II (0,l) transition. Since this band is P-edged, the line values of the lower intensity R-branch were plotted to enable observation of the behaviour even at lower rotational quantum numbers. In this connection it may be stated that the intensity values between R15 and R6.5 can be fitted by a straight line for all of the methane filler pressures, and that for reduced methane pressures even values for smaller rotational quantum numbers are located on the straight line. It could be also proved for the P-lines that the intensity values can be approximated by a straight line between P20 and P66. It has thus been demonstrated that a Boltzmann energy distribution occurs in the a.c. arc for rotations up to high rotational levels in the case of Cz. This is, for instance, not true for discharge tubes [ 111, where deviations were found for higher rotations. On the basis of these results it is possible to define a rotational temperature for both Cz and CH. 4.1.2. Behaviour of rotational temperatures in the plasma column. In Fig. 5, the rotational temperatures of Cz and CH in axial direction at 12-A pulse current and 4-mm electrode gap are plotted for methane filler pressures of 25, 50 and 100 torr. A striking feature is the temperature difference indicated by the two radicals. While the temperature for Cz lies between 5500 and 63OOK, the temperature for CH ranges from 3000 to 4200K. Apart from measuring points immediately in front of the electrodes, the difference of 2000K does not significantly change along the axis. An appreciable [ 1 l]
W. LOCHTE-HOLTGREVEN, Z. Physik 64,443
(1930); 67,590
(1931)
Plasma-spectroscopic
t 0
.
studies on alternating
current
in methane
arc discharges
641
I
20
CO
80
60
100
120 J lJ*l)-
P
_ -
25T CHI, 50T CHL +1OOT CHI, ~-6 170T CHL -250T CHb
- J 0
500
1000
1500
2000
2500
3000
3500
.. Fig. 4. Semi-logarithmic
plot of I/Pv4
of CH (a) and J(J + 1).
C2 (b) rotational
Lo00 4500 JIJ*l)-
-I
lines as a function
of
axial temperature gradient cannot be detected. This also applies to higher methane pressures. The rotational temperatures change only slightly in proportion to the discharge time. Consequently, the radial distributions were only determined for the time interval between TIKl A
25 torr
CHL
TIKII
100 torr
CHL
3000-
?OOO;* 0 +-•+5-15s
1 x-x
2 25-35s
3
(b
1
2
3
(
o--055-65s
Fig. 5. Axial distribution of rotational temperatures for CH and Cz in an a.c. arc plasma methane. G-ground; E-electrode; 12-A pulse current ; 4-mm electrode gap.
of
3
G. HEINRICH et al.
642
+-+25 torr O--O 50 torr x--x100 torr
2000;
CHb CHL CHL
/-‘1’ 1
2
3
C r[mml
Fig. 6. Radial distribution of rotational temperatures of CH and Cz in an ax. methane. G-ground; E-electrode: 12-A pulse current; 4-mm electrode
arc plasma gap.
of
5 and 15 s ; the curves for various methane filler pressures (determined after Abel inversion) are plotted in Fig. 6. They suggest that high rotational temperatures occur for Cz even in the vicinity of the plasma boundary. The error for results referring to the interior of the plasma lies in the range of f 300 K for C2 and f250 K for CH. At the plasma boundary this error is &-400K. 4.1.3. Dependence of rotational temperatures on discharge conditions. Since the relative variation of rotational temperatures in space is essentially maintained when the electrical parameters are varied, it is only necessary to observe the change in absolute values. In Fig. 7, the rotational temperatures of the central section of the arc are plotted as a function of methane filler pressure for various current intensities and electrode gaps and the time interval between 5 and 15 s. A pronounced maximum can be observed in particular for the CH rotational temperatures at a methane pressure of 50 torr.
7000 -
c2 /-
x--x----.x 0-.0lx
/ J -+6ooo: 5000
t----+1
6CH
2000
o-o
Pulse
current
+-+
Pulse
currentBA,
8A,
Electrode
gap
2mm
Electrode
gap
L mm
X-X
Pulse
currentl2A.Electrode
gap
Lmm
* 50
100
150
200
250
300 P ltorr
Fig. 7. Rotational
1
temperatures at different discharge conditions in an a.c. arc operating methane as a function of the filler pressure. Time interval: 5-15 s.
in
Plasma-spectroscopic
studies on alternating
150
Fig. 8. Rotational
temperatures
200
250
300
current
350
arc discharges
in methane
643
ll.I[Wattsl
of Cz and CH in an a.c. arc operating of energy input.
in methane
as a function
For Cz radicals the drop in temperature is lower at higher pressures ; at a pulse current of 8 A and 4-mm electrode gap the changes are within the margin of error. The temperature difference for the two radicals is slightly reduced at a shorter electrode gap. For a methane pressure of 25 torr it may be assumed (by considering the drift velocity) that excitations due to electron collisions acquire an additional significance. The cross section for such collisions reaches its highest value for small vibrational quantum numbers (cf., e.g. HASTED [12]) to which our investigations apply. On the other hand, the cross section decreases with increasing rotational quantum numbers. Therefore low-energy levels will be excited comparatively more intensely so that a lower rotational temperature will be simulated. The behaviour at increasing methane pressure could be partially explained by the excitation mechanism. It is most likely that the radicals receive their high rotational energy already during the formation process, since a major energy transition due to collision, in particular in connection with molecules of higher atomic order, is improbable in view of the angular momentum conservation for the entire system. For the purpose of interpreting the excitation mechanism, GAYDON [ 131 assumes high-energy collision partners in the plasma. The radical under examination could then loose part of its rotational energy due to collisions with low-energy particles before a collision with a high-energy partner leads to excitation. With increasing methane pressure the probability of collision with a high-energy partner decreases at constant energy supply to the plasma. Several collisions involving a reduction in rotational energy would thus be possible prior to excitation, which would explain the reduction in temperature. The influence of the arc parameters of methane filler pressure and electrode gap on rotational temperatures is illustrated in Fig. 8. The rotational temperature was plotted against the plasma power input (lowest input at 2-mm electrode gap, maximum at 12-A pulse current, 4-mm electrode gap). With the exception of 25 torr, a temperature increase may be observed for the two radicals in the case of both rising arc current intensity and reduced electrode gap. It is remarkable that the effect caused by a current increase becomes lower for CH rotational temperatures with increasing methane filler [12] J. B. HASTED, Physics of AtomicCollisions. Butterworths, London (1964). 1131 A. G. GAYD~N and H. G. WOLFHARD, Proc. Roy. SM. (London) 199, 89 (1949).
644
G. HEINRICH et al.
6000. -statlonory ---fluid 5000.
Ll
81 Ar CH,,
Fig. 9. Dependence of the rotational temperatures of CH and Cz in a d.c. arc in methane on various argon additions. Methane partial’ pressure: 50 torr ; current intensity : 8 A ; electrode gap: 4 mm ; time interval: 5- 15 s.
pressure as compared to that due to a reduction in electrode gap. On the other hand, a current increase to 12A always causes a higher temperature increase for Cz radicals as compared to that resulting from a reduction in electrode gap to 2 mm. 4.1.4. Temperature measurements by means of Zn lines. The excitation temperature for Zn atoms in the plasma was determined by evaporating minor amounts of Zn as additive. It is 7200 K in the case of a methane pressure of 50 torr. Investigations into the axial distribution of Zn temperatures did not reveal any appreciable change along the arc axis either. Observations similar to those described for Cz and CH were made also in radial direction: the drop in temperature is relatively small towards the plasma boundary where excitation still takes place. The high excitation temperatures of Zn atoms substantiates the assumption discussed above as to the presence of high-energy collision partners in the plasma. 4.15. Temperatures in the presence of inert gases. Measurements of the CH and Cz rotational temperatures in the presence of an inert gas were to provide a further understanding of the different behaviour of the two radicals. However, a direct current arc was used for these experiments, since also measurements by a probe were performed for which the spark had to be suppressed. In Fig. 9, the distribution of rotational temperatures of the two radicals at a constant methane partial pressure of 50 torr and varying argon to methane pressure ratios is plotted for the time interval between 5 and 15 s. The experiment was conducted for a stationary system as well as in a flowing system. As can be seen, the temperature difference between CH and Cz is higher than in the a.c. arc plasma without argon. Whereas the temperature of C2 remains virtually unaffected by the addition of argon, that of the CH radical is reduced. This behaviour is not easy to explain in terms of collision kinetics. It appears possible, however, that the transfer of energy to higher rotational levels might be enhanced during discharge in the d.c. arc. Apparently, this energy distribution is not significantly influenced by the presence of argon in the case of Cz. For the CH radical it can be assumed that an increased transfer of rotational energy to levels of higher quantum number increases predissociation, which, in a way, acts as a barrier. The influence exerted on the energy distribution of the upper rotational level by the presence of argon due to kinetic interaction cannot be checked using simple means. However, the drop in temperature, as well as a reduction in intensity observed suggests increasing pre-dissociation effects, resulting in a decreasing probability of energy transfer to higher rotational levels.
Plasma-spectroscopic
studies on alternating
current
j>,
arc discharges
, 0,lo
0.20
0.30
645
in methane
,~ O.LO AhInml
Fig. 10. Line profile of the long wave section of the HB line in, an a.c. arc in methane various filler pressures. 12-A pulse current; 4-mm electrode gap.
for
4.2. Electron number density Because there is no LTE in the plasma under investigation, the determination of the electron density by the Saha equation was evaded and the method described by MAECKER [lo], mentioned in Section 2, was applied to the HP line profile. Unfortunately, only the long-wave wing of this line is not overlapped in the spectrum at methane pressures of 25 and 50 torr. It has furthermore been observed that the H, line emerges most distinctly in this low range of pressure, whereas it appears only faintly-at increased pressures (in conjunction with a simultaneous strong enhancement of the background). This behaviour is illustrated by Fig. 10, in which the intensity curves determined after background correction are plotted. At the same time, it becomes evident that evaluation becomes highly inaccurate at elevated pressures. Investigations into the axial distribution of the electron number density have shown that excess negative charges are built up in front of the non-ground electrode, causing a transient in field strength (Fig. 11). The dependence of electron density on methane filler pressure is shown in Fig. 12, in which the values for 12-A pulse current and 4-mm electrode gap are plotted for the time interval 5-15 s. The values plotted are the mean values determined over the entire arc but excluding the zones in the electrode proximity. The substantial increase in electron number density with methane pressure is likely to have various causes. On the one hand, gas density is increased, so that more particles can be ionized. Moreover, the change in gas composition caused by simultaneous decomposition and pyrolysis processes in the plasma must be taken into account. As will be reported in detail later on, the detectable radicals become less apparent in proportion to increasing methane pressures. It could be proved that these must continue to react immediately, pyrolysing to form unsaturated hydrocarbons of a higher atomic order [3]. As a result of such enhanced formation of radicals of higher atomic order in the plasma, these will increasingly contribute to the electron number density due to ionization, the more so, since the ionization potential or thermal energy required for
G. HEINRICHet al.
646
x-x O-0 +-+
5-15s 25-35s so- 70s 1
1
2
I
3
electrode
gap [mm] -
Fig. 11. Axial
pressures
distribution of the electron number density in an a.c. arc in methane at filler of 25 and 50 torr. 12-A pulse current; 4-mm electrode gap. G-ground, E-electrode.
ion formation is significantly reduced in proportion to growing molecular weight [14]. In a plasma, an equilibrium is established in most instances between the processes of ionization and recombination. The number of recombinations ought to become more frequent as a result of the increase in collision frequency with rising methane pressure. Since, as already mentioned, there is a simultaneous strong enhancement of the background, the following scheme must be assumed in particular for processes under radiant emission : e+X+-+X
+hv,
e+X++X*+hv.
0
k
0
c” 1o16-
10'5:
10’1’
25
50
75
100
125
150
C
175 P (torr)
Fig. 12. Dependence of the electron number density on the filler pressure in an operating in methane. Current intensity: 12 A ; electrode gap: 4 mm. [14]
G. FRANCIS,Ionisation
Phenomena
in Gases. Butterworths,
London
(1966).
a.c.
arc
Plasma-spectroscopic
studies on alternating current arc discharges in methane
j
647
Pulse current0 A Electr.gapZmm
I
‘use-current81 ledr
gap Lmm
Ike-current 121 ectrgop Lmm
Fig. 13. Dependence of electron density in an a.c. arc operating in methane 50-torr filler pressures under different discharge conditions.
for 25-torr and
Since the electrons have a continuous energy distribution, the radiation emitted appears as continuum radiation. It should be mentioned that the influence of microfields on the ionization energy may be neglected. An assessment according to the theory specified by ECKER and KRGLL [15] resulted in a reduction by less than 0.1 eV. As may be clearly observed on Fig. 13, the electron density is influenced to a larger amount by external discharge conditions. Reducing the electrode gap from 4 to 2mm causes a reduction of IZ, to nearly half its amount. It may be assumed that, owing to a higher field strength at a reduced electrode gap, the electrons have a shorter residence time in the plasma. However, the arc current intensity can be kept constant in the shortened plasma column and with reduced electron number density. On the other hand, the electron number density changes almost proportionally to the arc pulse current in the case of constant electrode gap. 5. CONCLUSIONS Measurements of the rotational temperatures in ac. arc discharges in methane have shown that there is no volume element in the plasma in which local thermal equilibrium exists. No gas temperature for the plasma can be defined. In view of the lacking thermal equilibrium, any attempt to equate a translation temperature in terms of order of magnitude to the temperature of the CH radical is not allowed. An essential cause of the lack of thermal equilibrium may be seen in chemical reactions taking place in the gaseous zone. The radicals formed by methane decomposition continue to react chemically before being capable of causing a temperature equilibration as a result of a sufficient number of collisions. However, more in-depth investigations relating to reaction kinetics can only be carried out by means of additional measurement techniques, in particular by mass spectrometry. As will be shown in a further contribution, the reactions take place irreversibly, leading to the formation of carbon. At any rate, a model-type description of the plasma is likely to be extremely difficult due to the irreversibility. Further investigations into electrically charged plasma components as possible reaction carriers should be envisaged to obtain additional clarification. [ 151 H. ECKERand P. KRGLL,Phys. Fhids 6, 62 (1963).