Ion and electron profiles in flames

ION AND ELECTRON PROFILES IN FLAMES H . F . CALCOTE I t is shown how Langmulr probes can be used in flames to obtain not only the positive ion concentration but electron concentrations and electron temperatures. An internal method is presented for checking the resnlt~ by comparing the wall potential calculated from the above three quantities with the observed wall potential. Satisfactory agreement between these two values in hydrocarbon air/or oxygen flames from 1.5 to 760 mm Hg increases the confidence in the use of Langmuir probes to obtain plasma profile properties in flames. The accuracy of positive ion and electron collision cross sections appears to be the major factor limiting the accuracy of the probe. Detailed results are presented for a number of flames. The positive ion concentration always excoeds the electron concentration indicating the formation of negative ions. Electron temperatures exceed the gas temperature and do not decay as rapidly as might be expected. A mass spectrometric technique for obtaining ion profiles of good spatial resolution is outlined and detailed profiles are presented for an acetylene-oxygen flame at 2.5 mm Hg. The ion CHO + peaks ahead of CzH: + which precedes HzO +. Many other ions are observed to peak at about the same position in the flame as CsHz+. There are still problems, however, with respect to interpreting the results in terms Of the first ion produced from neutral species and the sequence of ion molecule reactions which follows.

Introduction I t is generally agreed today t h a t chemiionization~-~ is the dominant ion producing mechanism in hydrocarbon-air or oxygen flames. There is even some accord that an important ion forming reaction iss,7,9 CIt + 0 --* CHO + + eHowever, evidence for this reaction is still not as conclusive as we would like. From flame ionization detectors in gas chromatography, we have learned that about one ion is produced for about every million carbon atoms in the combustion gaseS; this is consistent with the rate of ion formation obtained b y Langmuir probe studies? Ionization flame detector studies for gas chromotography have also shown s t h a t the rate of ion formation in a hydrogen-air flame is directly proportional to the hydrocarbon concentration over a concentration range of roughly 1 to liP. Detailed studies of ion precursors do not make clear why this should be so. Although mass spectrometric studies of ions in flames have done much to increase our understanding of ionization in flames, because of rapid ion-molecule reactions, they have still left doubts as to the primary ions produced from neutral species. Recombination measurements of flame ions are also vague on the point of whether the negative species is an ion or an electron.

There is therefore a need in hydrocarbon flames for more detailed flame profile studies of total positive ions, electrons, and individual positive and negative ions. In addition, the gas temperature and stable species profiles should be obtained in the same flame. We have embarked upon such a program, and this contribution represents a progress report. In previous work in this field either positive ion or electron concentrations have been measured. Positive ion concentrations have been obtained with Langmuir probes which give a high degree of spatial resolution but suffer from a bad reputation. ~~ Microwave techniques have been used by others 6,~,2s for determining electron concentrations, b u t such experiments have the basic limitation of poor spatial resolution. We will show in this paper t h a t Langmuir probes can be reliably used for obtaining both positive ion and electron concentrations. Electron temperature measurements in flames will also be presented. Results of detailed mass spectrometer profiles will be described for low pressure flames where, because of the flame thickness and the ion sampling technique employed, the spatial resolution is greatly improved. Consistent with the invitation to present work in progress, it is not the intent of this paper to present a final piece of work with arguments for interpreting the data in terms of theoretical concepts but to outline the results of experiments in progress with the hope of stimulating discussion.

622

ION AND ELECTRON

PROFILES

623

IN FLAMES

40

L a n g m u i r P r o b e Studies

In the last symposium9 we outlined the use of i0 Langmuir probes to obtain positive ion concenI t-J trations; we will now discuss the use of the probe ~ 6 : :rto obtain electron concentrations and electron ~ ~ , #i r. temperatures and some of the problems involved. ~ 2 , ~ i [ >. As our use of probes has continued and we have ~1.0 ' 4 ',,~ gained more confidence in them, it has become " 8 o :6 more and more evident that one of the main limi- ~ .4 tations is our lack of knowledge of collision cross bI 1 sections of both positive ions and electrons. Inci- ~ .2 dentally, the electron collision cross section is also ~ .1 I I I I I I I I necessary for interpreting microwave techniques -4 -2 0 2 4 6 8 in terms of electron concentrations--it appears PROBE VOLTAGE as collision frequency. We will therefore discuss how this problem can be handled in treating Fzo. 2. Typical Langmuir probe curve for obtaining probe data and at the same time show how a self- electron concentration and electron temperature (from Fig. 1). consistency test can be applied to the probe data to increase confidence in the results obtained.

yr/

40 L

t U

~ 2o ~ ~o 0 w I~

0 , I

I

-15

-10

I

-5

I '

0

I

I

I

I

5

10

15

PROBE VOLTAGE

Fro. 1. Typical Langmuir probe curve in ethyleneoxygen flame, p = 2.6 mm Hg. (See Table 6.)

Small Probes In our previous work relatively large probes were employed, frequently stretching across the flame, so that it was difficult to obtain saturation electron currents on the probe without saturating the other electrode, usually a screen over the entire flame. This difficulty arises because of the large mobility of free electrons with respect to positive ions. The currents (electron) to a positive probe are always much larger than currents (positive ions) to a negative probe (See Figs. 1 and 2). With large probes the large electron currents also produce excess drainage on the flame plasma, which affects the plasma being studied. We have been thwarted previously in attempts to build small probes because all electrical insulators become either semiconductors or thermal emitters at the elevated

temperatures of flames; considerable attention was therefore directed to the solution of this problem, and after many attempts a small water-oooled probe was developed and is shown in Fig. 3. The insulating member, Fig. 3, is cooled by being in contact with a tube of material of good thermal conductivity, which is in turn cooled b y circulating water. The cooling tube extends beyond the insulator to keep the tip of the insulator cool and, hence, a stagnant gas forms at the end. Several difficulties still must be recognized. The probe length is now not accurately known, but this can be checked by testing probes of varying lengths. The cooling effect on the gas and the effect on the ion sheath are dlmeult to assess, although the visual disturbance in the low pressure flames is negligible. They must also be evaluated by varying the probe length and diameter.

Interpretation of Probe Curves In the last symposium9 the equation was given for an ellipsoidal approximation to the equations COPPER COOLINGCOILS Z32cm

SILVERCOOLINGJACKET

/

o

..NE P SE.,RE

cFRCmT : ....................... ~.......... /

,,NE'T ~

"ARTZ

~,~_t

\

T,S,NG\O..-

0.,,.

PROBE LENGT. COPPER TUBING FOR STRENGTH

Fzo. 3. Electrically and thermally insulated probe.

624

FUNDAMENTAL

of Bohm, Burhop, and Massey ~3 for calculating positive ion Concentrations from probe currents, i.e, the cylindrical probe was approximated by an ellipsoid. A similar approximation can also be given for an infinite cylinder. These two equations can be written for either positive ions or electrons by substituting the appropriate quantities. For electrons they would read for a positive probe: (1) ellipsoidal approximation; ' ' 2 7rme~ ,89/ yd

0.75Ld

ke d L X B

je = nee ~

[-e ]

exp (--eV/kTe)

(4)

On taking logarithms of both sides: ]n/e -- In [-n~e(k Te/27rme)89

-

( eV /k Te)

(s)

lnfl = B - - (e/kTe)y.

(6)

Thus, a plot of lnfi against the probe voltage, as in Fig. 2, yields through the slope, the electron temperature, Te. Because the total current to the negative probe is:

where:

e k me T~

The electron temperature can be obtained by writing either Eq. 1 pr 2 forje to a negative probe:

Neglecting the weak temperature dependence of the first term on the right-hand side, this is the equation of a straight line:

( X + B~I

(2) infinite cylinder approximation;

J~

FLAME PROCESSES

= electron current density at the plasma potential; = charge on electron; = Boltzman constant; = mass of electron; = electron temperature, obtained from the Langmuir probe data; = electron mean free path; = probe diameter; = probe length; = L+2X,;and = [-X 2 - (d + 2X~)2]89

For a negative probe Eqs. 1 and 2 must be multiplied by exp (--eV/kT,) where V is the potential difference between the probe and the plasma potential and represents the barrier over which the electrons must diffuse. The plasma potential and the procedure for obtaining it are presented in Fig. 2. The wall potential, V~, is the potential, with respect to the plasma potential, at which the probe current is zero, Fig. 1. At this potential the positive ions reaching the wall are just balanced by the electrons and negative ions reaching the wall. It can be demonstrated that, unless the electron concentration is very much less than the negative ion concentration, the contribution of negative ions to the wall potential can be neglected. Then by equating 3'+ = fl and letting V - Vw, the wall potential is given by: m = krelo ~- L \ ~ + / \ ~ / F jI-+ll j

(3)

where [ + J and I-e-J, sometimes called correction factors, are the respective bracket terms for positive ions and electrons from Eqs. (1) or (2).

jtot,, = j+ -4- je

(7)

with appropriate signs the electron current may be obtained by extrapolating the approximately linear leg of the current to the negative probe in order to obtain the positive ion contribution at any particular voltage. The electron current is then obtained by subtracting the positive ion current from the meter or probe current. Thus it is possible to extract from probe data: 1. The electron concentration from Eq. (1) or (2); 2. The positive ion concentration from Eq. (1) or (2) substituting positive ion values; 3. The electron temperature from the slope of lnje vs. V, Eq. (6); 4. The wall potential by the voltage difference between the plasma potential and the probe voltage (meter) at which the probe current (meter) is zero. An internal check on the results is afforded by the observation that the first three quantities can be used in Eq. (3) to calculate the fourth. Thus the values of n~, n+, and T, are used to calculate Vw by Eq. (3) and this value compared with the observed value of Vw. With the degree of independence of the various quantities, the restrictions imposed by the theory and the complexity of the experimental curves, agreement can hardly be considered as fortuitous. This agreement over a range of pressures and a range of the parameters involved justifies considerable confidence in the results. Experimental results will be presented in a later section. One major stumbling block to the satisfactory interpretation of Langmuir probe data is the need for accurate knowledge of the mean free paths of positive ions and electrons, or more

ION

AND

ELECTRON

basically, the need for accurate collision cross section data. As our work has progressed it has become more and more ~pparent that the differences between observed and calculated wall potentials were well within the choice of collision cross sections available in the literature. A brief discussion of this problem is thus warranted.

Mean Free Paths and Collision Cross Sections The general equation for mean free path is14:

~, = {~" ~., [-n~Si2-][1 -[- ( m i / m , ) ~ } -1 s

= {~., n~QisE1 Jr- (mjm,)~-]} -1

(8)

PROFILES

625

IN FLAMES

is H30 +, so the collision cross section of this ion with the various neutral components such as H20, COs, and N2 is required. These data are not readily available and hence were estimated b y assuming the additivity of molecular and ionic radii. The radii of neutral molecular species were taken from Hirschfelder, Curtiss, and Bird ~5 except for H20, which was estimated to be 1.7 /~ by comparison with other molecules. No value for H30 + nor H20 +, which might be expected to be close, has been found. This value was therefore estimated from Pauling's data '6 to be 1.8 ~. Thus the collision diameters and cross sections for H30+ with the products of combustion were estimated to be:

s

where:

Collision

Si, = distance between centers of colliding species i and s, i.e., collision diameter for positive ions, Qi, = collision cross section of species i colliding with species s; rni, m, = masses of colliding particles i and s; and n, = number density of s species. Because for electron collisions me/m~ << 1:

X, = (~., n~q~,) "~.

(9)

a

Because for positive ions rnJm, ~ 1

= (~ Z n,q,)-'.

(10)

s

The dominant ion in uncontaminated flames

H30 + H30 + H30 + H30 +

+ + % %

N~ O~ CO2 H20

S 3.6/~ 3.5 3.8 3.5

Q+ 4.1 X 10-15 cm~ 3.9 4.5 3.9

It is unfortunate that better values are not available. In previous work I'9 we assumed S = 2.6 •. There are several sources of electron collision cross sections including actual measurement of the collision frequency in flames. Most of the electron collision cross sections are strongly electron temperature dependent and thus must be obtained as a function of electron temperature

TABLE 1 Measurements of Electron Collision Cross Sections in Flames

Conditions

Reference No.

Collision frequency (sec-l)

Collision cross section (10-'5 cm2)

Acetylene-air flame 760 mm Hg, 2480~

28

2.6 X 10-11

Acetylene-oxygen flame 7.5 mm Hg, 2300~ 6-40 mm Hg, 2200~

28

3.7 X 109

18

--

4

Acetylene-oxygen detonation 76 mm Hg, 3500-4000~

29

--

2.5

Coalgas-air flame 760 mm Hg, 2200~

30

8.8 X 10'~

8.4

Propane-oxygen flame 760 mm Hg, 1800-2400~

31

--

1.0

2.6 3.9

FUNDAMENTAL FLAME PROCESSES

626

TABLE 2

3

~o

Electron Collision Cross Sections Used in Calculations

-. 7 15.0

0 l-U

~-i':.~ 10.0

U 0N .1 .J 8'X

ALTSHUL.ER QV2=s.gcm4sec "2

Electron temperature

.u.w,cz

\

::~ii!i.' ~

Collision cross sections (10 -~s cm~)

(~

TAKEDA AND DOUGLAS

CO 2~

CO a

1.7 1.4 1.1

1.1 1.1 1.1

N

2b

R.M. ,,LL.

5.0

2,000 3,000 4,000

(TOWNSEND) 0

1000

2000 3000

4000

5000

0.85 0.95 1.0

6 0 0 0 7000

EL.ECTROH TEMPERATURE, ~

FIG. 4. Electron collision cross sections for water (see references 17 to 21). in the flame. Some literature results in flames are summarized in T a b l e 1. The collision cross sections when not given in the references h a v e been cMeulated from: Q, = v/~ao

(11)

a Data from reference 21, chap. 3 (Ramsauer and Townsend values), and reference 32. b Data from reference 33. appropriate electron collision cross section would be to compute it from the components of the mixture. W a t e r is usually a major p r o d u c t and has the largest cross section of the products of C, H, 0, N flames. Literature values for H20 are presented in Fig. 4. Values for C02, CO, and N~ which h a v e been used in reducing our d a t a are presented in Table 2.

where:

Plasma Properties of Flames

v = measured collision frequency; no = total n u m b e r of molecules; and 6 = m e a n electron velocity. A more sophisticated means of obtaining the

The techniques of the previous section h a v e been applied to a n u m b e r of different flames, and the results are presented in this section.

TABLE 3 Plasma Properties of a Propane-Air Flame at 1 Atmosphere Probe: Platinum Length = 1.5 cm Diam. = 0.063 cm

Equivalence ratio = 1.0 Gas temperature = 2270~ Electron temperature = 2850~ no -- 3.23 X 101S/cc

Cylindrical probe approximation used j+ -Q+ = x+ = n+ =

3.94 4.15 5.26 1.42

j, -- 5.05 X 10-6 amperes Qe = 1 . 6 3 X 1O-Is cm' x, = 1.90 )< 10-4 cm n, = 2.42 X 109 electrons/cc

X 10 -s amperes X 10-15 cm 2 X 10-5 cm X 10 l~ ions/cc V~ (obs.) = 1.2 volts V , (talc.) = 1.20 volts Sheath thickness (at - 3 . 0 volts) Parallel plate theory Cylindrical probe theory Simple current increase

0.036 cm 0.035 cm 0.031 cm

ION AND ELECTRON PROFILES IN FLAMES

627

TABLE 4 Plasma Properties of a Propane-Air Flame at 33 mm Hg Probe: Pt-40% Rh Diam. = 0.025 cm Length = 0.50 cm Q+ = 4.11 X 10-15 cm2

Equivalence ratio -- 0.88 Gas temperature -- 2100~ Flow velocity ffi 182 cc/see no --- 1.52 X 1017 molecules/cc

Ellipsoidal probe approximation used

Distance from burner (cm) 0.34 0.56 1.02 1.48 2.40 3.32 4.01

Probe currents (10 -s amperes)

Conc. (10 -9 mole fraction)

i+

i,

T~ (~

0.77 3.90 1.95 0.90 0.52 0.27 0.22

150 155 142 138 133 85 74

2060 2400 2500 2700 2350 2350 2600

V, (volts)

Q, (10 -is cm2)

n+/no

n,/no

Obs.

Calc.

1.8 1.7 1.6 1.6 1.7 1.7 1.6

6.4 32 16 7.4 4.3 2.3 1.8

2.1 2.0 1.7 1.5 1.6 1.1 0.85

0.89 0.68 0.85 1.1 1.1 1.1 1.2

0.93 0.76 0.92 1.2 1.1 1.2 1.3

For a stoiehiometric propane-air flame at 1 atmosphere the results are summarized in Table 3. The internal check via the wall potential is better than could be expected. Note that the electron temperature exceeds the adiabatic gas temperature, and the positive ion concentration is more than five times the electron concentration. If the ion sheath thickness around the negative probe is calculated by the usual theory described

in Loeb l~ the two theoretical results reported in Table 3 are obtained. According to the simple picture upon which the Langmuir probe theory is based, the increase in current to the probe as it is made more negative is due to an increase in the positive ion sheath thickness, so that the area into which ions are diffusing is being increased. If the sheath thickness is zero at the plasma potential, then the sheath thickness at any voltage is

TABLE 5 Comparison of Data Reduced by Several Alternatives~ Positive ion b Collision diameter (10 -s cm)

0.34

1.02

Distance from burner (era)

Probe theory

Correction factor

Positive ion conct (10S/cc)

3.6 3.6 2.6 2.6

Cylindrical Ellipsoidal Cylindrical Ellipsoidal

41 31 21 16

0.96 0.97 0.50 0.50

3.6 3.6 2.6 2.0

Cylindrical Ellipsoidal Cylindrical Ellipsoidal

41 31 21 16

2.4 2.5 1.3 1.3

o See Table 4 for experimental conditions. b The collision diameter a~umed prior to this report was 2.6 X 10-s cm. The weighted average of the species involved gives 3.6 X 10 - s era.

628

FUNDAMENTAL FLAME PROCESSES

TABLE 6 Plasma Properties of an Ethylenc~)xygen Flame at 2.6 mm Hg Varying composition Probe: Pt-40% Rh Diam. = 0.015 cm Length = 0.159 cm

T+ (assumed) = 2000~ no = 1.22 X 10xe

Maximum probe currents (10-6 amperes) Equivalence ratio 0.583 0.708 0.759 0.831 1.00 1.03

Max. cone. (10 -7 mole fraction)

V~ (volts)

7', ~ i+

i,

(~

nt/no

0.16 0.19 0.22 0.23 0.55 0.48

15 16 22 20 26 28

7800 8500 7800 9900 4330 3880

3.0 3.6 4.2 4.4 10.4 9.0

n,/no 0.74 0.76 1.1 0.88 1.7 2.0

Obs.

Calc.

2.7 2.8 2.7 3.2 1.5 1.3

3.0 3.2 3.0 3.8 1.4 ] .3

a At the distance of maximum ion mole fraction. obtained simply from the ratio of current at the particular voltage to the positive ion current at the plasma potential. The value from this calculation is given as "simple current increase,." The agreements are remarkable. The results for a fuel lean propane-air flame at 33 mm Hg arc summarized in Table 4. Again the electron temperature exceeds the adiabatic gas temperature, and the positive ion concentration is greater than the electron concentration. The internal check via the wall potential is saris-

factory. The cylindrical probe approximation was used for the experiment in Table 3 and the ellipsoidal approximation for the experiment in Table 4 b e c a u ~ of the relative probe lengths to diameters. Table 5 summarizes some results using combinations of the two different approximations in the probe theory, and the collision diameter assumed in our previous work 9 as well as that obtained by a weighted average of the major product species. The choice of approximation in the probe theory would appear to make little

TABLE 7 Plasma Properties of an Acetylene-Oxygen Flame at 1.5 mm Hg Varying composition Probe: Pt-40% Rh Diam. = 0.015 cm Length = 0.159 cm Maximum probe currents (10-6 amperes)

T+ (assumed) = 2,000~ no = 7.29 X 10t*

Max. conc. (10-7 mole fraction)

V,, (volts)

Equivalence ratio

i+

i,

(~

n+/no

n,/no

Obs.

Calc.

0.418 0.519 0.550 0.640

0.26 0.30 0.48 0.50

40 42 52 21

3070 4030 4030 4530

9.5 9.5 15. 16.

1.9 4.7 4.9 7.1

1.I 1.9 1.6 1.7

1.0 1.9 1.6 1.7

Ve

ION AND ELECTRON PROFILES IN FLAMES

629

TABLE 8 Plasma Properties of an Acetylene-Oxygen Flame at 1.5 mm Hg 9 Probe currents (10-6 amperes) Distance b (cm)

Conc. (10 -~ mole fraction)

Vw(vol~)

T,

0.00 0.788 1.48 2.42 3.07 3-74 4.76

i+

i,

(~

n+/no

n~/na

Obs.

Calc.

0.12 0.30 0.26 0.22 0.19 0.21 0.17

12 24 21 18 18 16 11

2320 3070 3380 3100 2650 3020 2260

4.0 9.5 8.4 7.0 6.1 6.9 5.4

1.9 1.9 2.7 2.0 2.1 2.1 1.8

0.97 1.1 1.1

0.91 1.0 1.3

1.2

1.1

0.98 1.2 0.97

0.98 1.1 0.82

a Equivalence ratio = 0.418. See Table 7 for experimental conditions. b Measured from beginning of luminous zone. difference in the results. However, the different choice of collision diameter leads to almost a factor of two differences in the results. The results for an ethylene~xygen flame at 2.6 mm Hg with varying equivalence ratios are recorded in Table 6. In the flame the electron temperatures are far in excess of the adiabatic flame temperature, and the positive ion concentration exceeds the electron concentration. At such low pressures the correction term in the probe theory reduces to 1. The same comments can be made for the acetylene-oxygen flame described in Tables 7 and 8.

t sUPPLY

i ///////)

The internal check through the wall potential certainly adds confidence to the validity of Langmuir probe data. The excess electron temperatures in the combustion zone, where electrons are being created, are no problem to explain. In chemi-ionization there may certainly be sufficient available energy to "kick" the electron out with an excess of kinetic energy. The persistence of these temperatures downstream of the combustion zone, where presumably electrons are no longer being created does, however, represent a problem. Although electron temperatures would be expected to decay relatively slowly

r"OE,LE T,O', rl

II I

|

~!lj. ,~l.~

II

VOLTAGES

L,J

DEFLECTIOHELECTRODE

I

I "OIq-ELEC~TROH I 9 ~ I MIJ~TIPLIERTUBE[I ~'~t

( / ~ANALdYZER. I ~..~ I IELECTROMETERIISECTION/I ~ l / ~ [I ~

~ )

VA ( "' CUUM FREQUEHC~I ('~ PUMPSI ~/METER "~ I ~OSCILLATORI~ / ~J

IP~2wSC/'sLuAp'~COSI TpO~(I I'CTARNoL I IJ

I

/

STRIP ~

1,1

,n.~l

L. ! ! .,...t

A,oL ' i r ACCELEP.ATIOH \/4---

.

I,ONFOCUSIHG] VOLTAGES

ENTRANCEORIFICE

FIG. 5. Mass spectrometer circuit elements

~30

FUNDAMENTAL FLAME PROCESSES

because of inefficient momentum transfer to heavy molecules, the theoretical decay times~ are expected to be measured in microseconds and not milli~conds. The explanation may lie in the high diffusion velocity of elec~ons, but this remains to be demonstrated. The persistent excess of the positive ion concentration over the electron concentration indicates the presence of negative ions. Page2~ and Sugden ~ from microwave studies of electrons in flames deduced the presence of OH-. This has, however, not yet been identified in mass spectrometric studies of flames.

/ON DETECTOR

['7///////~

I/H/"I

•(•

-

-

M a s s Spectrometer Studies Although the Langmuir probe gives reasonably accurate data on total ion concentrations, this is insufficient to formulate a complete picture of the ion processes occurring in flames. The identity of the individual ions and knowledge of how they vary through the combustion wave are required. It has been shown by Knewstubb and Sugden~ and by Deckers and Van Tiggelen = that many different ion species exist. Neither of their experimental systems was capable, how-ever, of obtaining ion profiles with any spatial resolution

~J

~DEFLECTION ELECTRODE ANALYZER --SECTION

VACUUM I I j ~

w~

GLASS INSULATED Z~ ~.~LECTRIC LEAD-INS VACUUM

GAUGE ION FOCUSING LENS ELECTRIC FEED THROUGH

VACUUM CHAMBER

TEFLON

PYREX LIQUID IITROGE TRAP

COPPER PLATES ON BASE OF NITROGEN CONTAINER

ELECTRICALLY INSULATED BOLTS

WATER COO'LIN FOR PROBE

CEC 4 INCH TYPE PMC 721

VACUUMP ~ FLOW SMOOTHING GRIDS

•J

VAC VACUUM GAUGE TO DISTILLATION PRODUCTS MCF-5OO DIFFUSION PUMP

DIFFUSION PUMP

..

' ,V ~

t,

PILOT FEED I INCH

,.~

I~

BURNER POSITION

TABLE TOP

FIo. 6. Mass spectrometer system

631

ION AND E L E C T R O N P R O F I L E S IN FLAMES

because the sampling orifice was in a flat plate against which the flame played and they operated at relatively high pressures, 1 atm, and 10 to 40 mm Hg, respectively. We have developed techniques at lower pressures, I to 10 mm Hg, ~4which allows reasonably good spatial resolution. The techniques will be briefly described with some typical data, and the questions these detailed profiles raise will be discussed.

mean free path of the ions in the flame, and thus ions pass through the orifice without colliding with the wall. Ion profiles are obtained by sweeping the analyzer frequency and recording the ion current (measure of ion concentration) against the frequency (measure of ion mass). The frequency scale is calibrated in terms of mass by several means involving alkali metal ions and isotopes.

Mass Spectrometer

Ion Profiles

The instrument is shown in Figs. 5 and 6. It consists of a low pressure burner, a coneshaped ion sampling probe, two vacuum systems for the mass spectrometer, an ion focusing section to focus the ions from the entrance orifice onto the first slit of the radio-frequency mass spectrometer, an ion multiplier tube to detect the ions, and an electrometer recording system. Ion profiles arc obtained by moving the flame across the orifice by adjusting the position of the 10 cm diameter flat flame burner. The coneshaped sampling probe represents a compromise to give a minimum disturbance to the flame while allowing a maximum pumping speed in the mass spectrometer. The pressure inside the orifice is about 10-4 mm Hg, and in the analyzer section it is about 10-6 mm Hg. The entrance orifice is about 0.25 mm in diameter and 0.2 ram long. These dimensions are approximately equal to the

Ion profiles are shown for a lean acetyleneoxygen flame at 2.5 mm Hg in Fig. 7. The C3H3+ ion was identified by the addition of deuterated acetylene to the input gas; the other ions are probable identifications from the possibilities open to the system C, H, O. The general features of these results are consistent with other experiments with both acetylene-oxygen and ethyleneoxygen flames over a range of equivalence ratios. ~ A number of ionic species other than those noted in the figure also reach maximum concentrations at about 2 cm from the burner. These are:

i

i

i

l

i

i

Mass 42 53

55 27 26

Probable ion

Maximum current

C , H z O + or Carte + 80 X 10 -12 amperes HaO+(OH)2 15 H30+(H,O)2 7

C ~I-Is+ C~H2+

6 5

I000

l"

g / o~

i0

32 CH40+or 02+

~

-

29 CHO+or C2H5+

1.0

I

2

I 3

11 4

| 5

t 6

I 7

DISTAHCE ABOVE BURHER, cm

FIG. 7. Mass protiles for an acetylene-oxygen flame; pressure = 2.5 mm Hg; equivalence ratio = 0.66; total flow = 61 ce/sec.

Mass numbers: 15, 16, 21, 23, 28, 30, 31, 41, and 54 were also observed in very small concentrations. The ion CHO + (mass 29), often considered as the primary ion produced from neutral species, appears in only small concentrations early in the flame. The ion has been observed in other flames but always in small concentrations. Van Tiggelen's group also observed mass 29 but only in small concentrations. This is as expected due to rapid proton charge transfer, but is unsatisfactory with regard to obtaining experimental verificacation of the first ion produced, because the concentrations are too low to accurately plot the CHO + profile. We are increasing the sensitivity of our equipment by a factor of about 100 and plan to study dilute hydrocarbon systems, various fuel systems, and the effect of additives as a means of seeking out the primary ion production mechanisms. One of the most interesting results from this work has been the persistent appearance in large concentrations of C3H3+ (mass 39) and the

632

FUNDAMENTAL FLAME PROCESSES

consistency with which many ions, particularly masses 39 and 43, reach their maximum values at the same position in the flame--and ahead of Ha0 +. M a n y mechanisms for ion formation have been discussed (e.g., references 4 and 9), so it does not seem worthwhile belaboring the point here--more information is required before anything of consequence can be added. The problems associated with most of the previous proposals for the formation of C3H3+ have already been presented, u The fact t h a t the ion concentrations all reach a maximum at about the same position in the flame indicates either that they are all formed from the same precursor or that they are all produced very rapidly after the generation of some single ionic specie. The appearance of C3H3+ in the flame front ahead of HaO+ does not necessarily mean that it is the precursor of HaO+, although it would be nice if the interpretation were so simple. Suppose, for example, t h a t C3H~+ were produced by the sequence of reactions: CHO + -~ H~O -~ H30 + ~- CO HaO + -t- C~H, ~ C3Ha+ -[- H20 The concentration of C3Ha+ would then be strongly dependent upon the concentration of CaH~, which must certainly be decreasing rapidly downstream from the flame front while the concentration of H:O is steadily increasing. In fact, in a lean flame such as described in Fig. 7 it is difficult to understand the formation of any reasonable quantity of Call2 or other hydrocarbon fragments of greater than two carbon atoms. Nevertheless, the relative concentrations of C~H3+ and HaO+ differ very little in rich and lean flames. This again focuses attention on the need for more detailed profiles of as many facets, e.g., stable species, free radicals, and flame temperature, of the combustion wave as possible in order to choose between the possible explanations of ion production and subsequent reactions. With present information one is unable to make a unique choice. ACKNOWLEDGMENTS

Much of the experimental work was performed under Contract AF 04(647)-157 and interpreted under Contract NOw-62-4)540-c. The work is continuing under Contract Nonr-3809(00). The participation of James L. Reuter and Richard L. Revolinski in collecting the experimental results, and of Allan H. Schell who reduced the data, is gratefully acknowledged.

REFERENCES 1. CALCOTE,H. •.: Combustion and Flame 1,385 (1957). 3. DE JAEGERE, J., DECKERS, J., and VAN WIGGELEN, A.: Eighth Symposium (International) on Combustion, p. 155. Williams and Wilkins, 1962. 3. KNEWSTUBB,P. F. and SUGDEN,T. M.: Seventh Symposium (Internatio~ml) on Combustion, p. 247. Butterworth, 1959. 4. MUKHERJEE, N. R., FUENO, T., EYRING, H., and REE, W.: Eighth Symposium (International) on Combustion, p. 1. Williams and Wilkins, 1962. 5. KINBARA,T., NAKAMURA,J., and IKEGAMI,H.: Distribution of Ions in Low-Pressure Flames, Seventh Symposium (International) on Combuslion, p. 263. Butterworth, 1959. 6. BULEWICZ,E. M. and PADLEY, P. J.: Combustion and Flame 5, 331 (1961). 7. HAND, C. W. and KISTIAKOWSKY,G. B.: The Ionization Accompanying the Acetylene-Oxygen Reaction in Shock Waves. Gibbs Chemical Laboratory, Harvard University, Cambridge, March 1962. (To appear in J. Chem. Phys.) 8. STERNBERG, J. C., GALLOWAY, W. S., and JONES, T. L.: The Mechanics of Response of Flame Ionization Detectors. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 1961. 9. CALCOTE, H. F.: Eighth Symposium (International) on Combustion, p. 184. Williams and Wilkins, 1962. 10. LOEB, L. B.: Basic Processes of Gaseous Electronics, p. 201. University of California Press, Los Angeles, 1955. 11. VAN TIGGELEN, A.: Experimental Investigation of Ionization Processes in Flames. University of Louvain, ARL Report 68, May 1961. 12. SMITH,F. T. and GATZ, C. R.: Ionization in Afterburning Rocket Exhausts. Stanford Research Institute, Technical Report BSD-TR-61-76, November 1961. 13. BOHM, D., BVRHOP, E. H. S., and MASSEY, H. S. W.: The Characteristics of Electrical Discharges in Magnetic Fields, Chap. 2, edited by A. Guthrie and R. K. Wakerling, McGrawHill, 1949. 14. JEANS, J.: An Introduction to the Kinetic Theory of Gases. University Press, 1959. 15. HIRSCHFELDER, J. O., CURTmS, C. F., and BIRD, R. B.: Molecular Theory of Gases and Liquids. J. Wiley and Sons, 1954. 16. PAUL~NG,L.: The Nature of the Chemical Bond. Cornell University Press, 1944. 17. ALTSHELER, S.: Phys. Rev. 107, 114 (1957). 18. BULEWICZ,E. N.: J. Chem. Phys. 36, 385 (1962). 19. TAKEDA,S. and DOVGAL.A. A.: J- Appl. Phys. 31. 412 (1960).

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ION AND ELECTRON PROFILES IN FLAMES 20. HILL, R. M.: Abstracts Eleventh Gaseous Electronics Conference, N.Y., October 1958. 21. BURHOP, E. H. S. and M^SSEY, H. S. W.: Electronic and Ionic Impact Phenomena. Oxford University Press, 1956. 22. KNEWSTI."BB,P. F. and SUGDI~;N,T. i . : Proc. Roy. Soc. (London) A255, 520 (1960). 23. DECKERS, J. and VAN TmGELE~, A.: Seventh Symposium (International) on Combustion, p. 254. Butterworth, 1959. 24. CALCOTE, H. F. and REUTER, J. L.: Mass Spectrometric Study of Ion Profiles in Low Pressure Flames, presented at the Fall Instrument-Automation Conference and Exhibit, ]/)s Angeles, California, September 11-15, 1961, ISA Preprint No. 69-LA-61. 25. CALCOTE,H. F.: Dynamics of Conducting Gases,

Proceedings of the Third Bienneal Gas Dynamics Symposium, p. 36. Northwestern University Press, Evanston, Illinois, March 1960. 26. PAGE, F. M.: Discussions Faraday Soc. 19, 87 (1955).

27. SU(Jl}EN,T. M. : Fifth Symposium (International) on Combustion, p. 406. Reinhold, 1955. 28. •CIINEIDER, J. and t|()FI*MAN, F. W.: Phys. Rev. 116, 244 (1959). 2(.). Bxsu, S. : Phys. Fluids 3, 456 (1960). 30. BELCHER, H. and SUGDES, T. M.: Proc. Roy. Soc. (London) A201, 480 (1950). 31. 1)InELIUS, M. R., LUEAKE, E. A., and MULLANEY, G. J.: Second Symposium on Engineering Aspects of M H D , March 9, 10, 1961. 32. BRows, S. C. and ALLIS, W. P.: American Institute of Physics Handbook, sec. 5. McGraw-Hill Book Co., 1957. 33. SHKAROFSKY, I. P., BXCHYNSKI, M. P., and JOHNSTON, T. W.: Collision Frequency Associated with High Temperature Air and Scattering Cross-Sections of the Constituents, R C A Victor Co. Ltd., Montreal, Canada, Report No. 7--801; presented at AFCRC Symposium on Plasma Sheath, Dec. 1959 (from compilation of literature values).

Discussion DR. I. R. KING (Texaco Experiment, Inc.): Dr. Calcote is to be commended on the excellent work he has done and is still doing in the field of "Ions in Flames." Ilis many papers, including the present one, have added much to our knowledge in this area. Calcote was, I believe, one of the first to postulate the CHO + ion as the parent or initial ion created in the combustion of hydrocarbon-air mixtures. This idea is rapidly gaining wide support, as evidenced at the present meeting. I was particularly interested in Calcote's findings concerning the electron deficiency in flames. At Texaco Experiment, Incorporated, I have been interested in recombination processes in flames fnr some time and have been using both a probe and an electromagnetic attenuation technique in these studies. The probe measures positive ion concentrations while the attenuation technique measures the concentration of free electrons. Our results also show a deficiency of electrons. As Calcote has suggested, this is indicative of negative ion formation. Indeed, if we assume charge balance in the flame and obtain positive ion and electron concentration profiles downstream of the combustion zone, then by subtracting the two, we may obtain a negative ion profile. Second order recombination rates determined from the positive ion profih, and from the negative ion profile give almost identical values. This is strongly suggestive of an ion-ion type recombination. The order of magnitude of the recombination coefficient is also indicative of an ionion process. Furthermore, the fact t h a t recombina-

tion coefficients determined in a number of different hydrocarbon-air flames give almost identical values suggests t h a t not only the same process is active in all these flames b u t the same species must be involved. Some typical results are shown in Table 1. TABLE 1 Recombination in Flames of Several Fuels a

Fuel

Equivalence ratio b

Ambient pressure (atm)

Methane Propane Acetylene

1.08 1.18 1.15

0.087 0.072 0.026

Recombination coefficient (cm3/sec) 2.5 X 10 -7 2.9 X 10 -7 2 . 8 X 10 -7

a Oxidizer, air. P l a t i n u m probe; radius, 0.007 em. b Equivalence ratio, ~b, is the stoichiometric airfuel ratio divided by the actual air-fuel ratio. An examination of flame intermediates shows a number of species which are electronegative. Of these the OH radical seems to be the most likely to attach an electron and become a negative ion. Although there seems to be some disagreement as to the actual electron affinity of this radical, it does seem to be fairly high. Page and Sugden have suggosted a value of 65 kcal/mole. If we assume this

63A~

FUNDAMENTAL FLAME PROCESSES

value, a calculation of equilibrium concentrations shows that O H - may actually outnumber the free electrons. Furthermore, all hydrocarbon-air flames contain fairly large amounts of OH. Thus O H - seems the most likely candidate for the negative ion. In a recent article we suggested recombination in hydrocarbon-air flames probably occurs between the H~O + ion, already identified in flames, and the O H - ion. Although mass spectrometer studies have thus far failed to verify the presence of negative ions in these flames, it is extremely difficult to account for the observed electron deficiency in any other way. Diffusive losses are undoubtedly responsible for some of the electron deficiency, especially at the lower pressures, but it is difficult to account for the entire loss in this manner. A dissociative recombination process, where the H30 + ion picks up an electron and dissociates into H:O and H, or similar products, has also been suggested. This process would also account for the loss of some electrons. However, under these conditions we would expect the positive ion and electron concentrations to be equal. This has not been found to be the case. Furthermore, the effects of temperature and pressure on recombination rates are not indicative of a dissociative process. Experimental results show that the recombination coefficient, a, varies inversely with pressure and shows a slight increase with increasing temperature. Theory says a should be independent of pressure and should vary approximately as T ~l. Also, according to Loeb, a dissociative process has only been observed in inert and pure nonehctron-attaching gases. Thus it seems rather unlikely that a dissociative recombination process is active in these flames. All of the above evidence tends to verify the presence of negative ions in hydrocarbon-air flames. However, much remains to be done before a complete understanding of the ionization and recombination processes occurring in flames is obtained. CaIcote has stressed the need for detailed profiles of ions, electrons, temperature, stable species, and intermediate species through the flame front of a number of different flames as a logical step in solving the riddle. Although this approach presents many problems, it does seem to be the most logical plan of attack for such a complicated system. PROF. A. VAN TIGGELEN (University of Louvain) : I would like to confirm the presence of O H - ions in the hydrocarbon flames. We have recently identified a very weak peak at mass 17. Concerning a primary process for ion formation I would suggest C H -t- O~ --* CO~H + -{- e- (=1=15 kcal?) as more probable. A lower apparent activation energy (as observed) would correspond to this process as compared to CH -t- 0 --~ COH + ~- e-. Furthermore, it is supported by the fact that the OH emission varies linearly with ion concentration

as we have observed C0~H + + s/ CH + O2 x~ CO + OH* Both processes occur simultaneously in an almost constant ratio. DR. H. F. C~.LCOTE (AeroChem Research Laboratories): While I agree~. 2 with King's interpretation that the ion recombination process in flames may be an ion-ion recombination as opposed to an ionelectron recombination, I cannot agree with some of his arguments nor with his interpretation of Langmuir probe experiments. He states that "the order of magnitude of the recombination coefficient is also indicative of an ion-ion process." True, three-body ion-ion recombination coefficients and mutual neutralization ion-ion recombination coefficients may be of the magnitude observed in flames, but so are ionelectron dissociative recombination coefficients (see reference 1 of this discussion for details). When ambipolar diffusion is neglected, as King does, in computing the effect of pressure on recombination coefficients calculated from Langmuir prove curves, an inverse pressure relationship i s obtained which is difficult to explain. When the correction for ambipolar diffusion is made the recombination coefficient is independent of pressure as the following data 2 shows:

Pressure (mm Hg) 33 66 520 760 760 (Green and Sugden, this symposium)

Recombination coefficient (a, 10 -~ co/see) 1.6 2.4 1.6 2 2.2

Actually we agree that the recombination process is second order; this is consistent with a recombination coefficient which is independent of pressure. A pressure dependence of a for a second order process would require some explaining. King further argues against a dissociative recombination process on the basis that his results for a show " a slight increase with increasing temperature" and he continues that "theory says a should be independent of pressure and should vary ap-

ION AND ELECTRON PROFILES IN FLAMES proximately as T-l. '' I t is a common error to interpret Bates' theory of dissociative recombination as giving a T - t dependence.a.4 It may be this, or conceivably a positive temperature dependence. The theoretical treatment is hardly sufficient in its present state of development to eliminate from consideration a specific recombination as being dissociative because it does not conform to a particular temperature dependence. In fact the slight temperature dependence observed by King 6 may be dependent upon the means he uses to reduce his probe data or on his neglect of the diffusion correction. The argument from Loeb's book published in 1955 is of little weight because most of the work on dissociative recombination has been done subsequent to the time that book was prepared. Thus I see no reason, arising from the presence of negative ions, to abandon the dissociative recombination concept for ion losses in flames. In fact, the original proposal that the loss mechanism was dissociative recombination is completely consistent with negative ions. In reference 1 of this discussion it is stated: "The expected recombination process might be: H30 + + O H - --* 2H20

(1)

or --* H30 + OH + H etc. or H~O + + e - ~ H20 + H

(2)

or --* OH + 2H". The consistent observation of an excess of positive ions and negative ions over electrons and the observation that the electron-concentration-distance curves are relatively flat certainly favors reaction (1). REFERENCES ]. CALCOTE,H. F.: Proceedings of the Third Biennial Gas Dynamics Symposium, p. 36. Northwestern University Press, Evanston, Ill., 1960. 2. CALCOTE, H. F.: Eighth Symposium (International) on Combustion, p. 184. Williams and Wilkins, 1962. 3. BATES, D. R.: Phys: Rev. 78, 492 (1950). 4. BATES, D. R. and DELOARNO, A.: Atomic and Molecular Processes, p. 262, edited by D. R. Bates. Academic Press, 1962. 5. KINQ, J. R.: J. Chem. Phys. 35, 380 (1961). DR. P. F. KNEWSTUEB (University of Cambridge): In view of the remarks which have been made regarding the occurrence of large numbers of negative

635

ions in flames, I should like briefly to recount some experiments which we did, seeking to extract negative ions into our mass spectrometer and to analyze their masses. Negative ions were not detected at all unless the flame was seeded with sodium or potassium to provide a considerable concentration of free electrons. When this was done, negative ions were readily detectable in the upper parts of the flame, but not in or near the reaction zone. Details of the numerous types detected are to be published. The suggestion is strong that the heavy negative ions are formed only in the cooled parts of the flame, and this is supported by the mobility experiments of Kinbara and Nakamura {Seventh Symposium on Combustion). There is at first sight a possibility that the sampling of negative ions might be prevented by the retarding potential at the boundary of the plasma, until this falls with decreasing electron and flame temperatures. However, it was found that on introducing various amounts of iodine into the system, I - ions could be found throughout the flame in concentrations satisfactorily close to those expected. Thus it does not seem that the sampling of negative ions from flames is seriously impeded by any wall potential in this apparatus. DR. P. J. PADLEu {University of Cambridge): Dr. Calcote mentions the difficulty of interpreting the observation in gas chromatography that the rate of ion formation in a hydrogen-air flame is directly proportional to the concentration of hydrocarbon introduced. Flame ionization detectors are usually operated under such conditions that, once an ion is formed in the flame, it is drawn out and detected as a current reading. Thus the method unambiguously measures the rate of ion production directly, giving it, for this purpose, certain advantages over the mass spectrometric or cyclotron resonance type of observation presented at this Symposium which, at best, have to infer rates of ion production from steady state considerations and from rates of ion disappearance. Now the work of Bulewicz and Padley (this Symposium) has shown that the concentration of single carbon atom species involved in the primary ionization step is directly proportional to the fuel concentration, for all hydrocarbon type fuels. An interpretation of the gas chromatography observation thus immediately follows. We have interpreted our results (this Symposium) as implying an electron attachment process predominantly responsible for disappearance of electrons in both hydrocarbon-oxygen and cyanogenoxygen flames; these m a y therefore lend support to the view that H30 + probably reacts with an O H species rather than directly with an electron. How

636

FUNDAMENTAL FLAME PROCESSES

ever, it seems rather unlikely t h a t the observed discrepancy between positive ion concentration and electron concentration can be explained satisfactorily also in ~ r m s of this same species, OH-. Taking a figure of 50 kcal/mole for the electron affinity of OH, the negative ion is most likely to be produced by direct ..ttachment, in the pseudoequilibrium e TOH

~ X ~- O I t - - l - X

rather than by a process of the type e -t- H_.O ~-~ O H - ~ tI.

Thus the Saha equation can still be used, even though both [e-] and [OI[] are well out of equilibrium. Such calculations suggest t h a t the observed discrepancy could, indeed, be accounted for in terms of O H - a t atmospheric pressure, b u t not, however, at lower pressures, since the ratio [OH-]/ [e-] decreases as the pressure decreases. T h e discrepancy does not seem to decrease as the pressure is reduced, and to account for it in the lowest pressure flames the flame gases would have to consist purely of OH. DR. I-I. F. C&IM3OTE: Knewstubb has pointed out t h a t he does not find O i l - with a mass spectrometer, although he has looked for it in 1 arm pressure flames. Padley has raised the question of equilibrium a t t a c h m e n t of electrons to OH in low pressure flames and has pointed out t h a t this would require an unreasonably large concentration of OH radicals. Both of these observations are inconsistent with our observation t h a t n: > n , and thus negative ions must be present in large concentrations. I n spite of the uncertainties involved in electron determinations by Langmuir probes, it is difficult to concede t h a t the probe results might be off by more t h a n an order of magnitude which these two observations would require. Possibly Knewstubb has worked in different flames because in our 1 atmospheric flame (Table 3 of the paper) a reasonable excess of O i l over the equilibrium value would account for the observed negative ion concentration. Such excesses of O t t radicals have been reported by several authors in this and in previous symposia. I think the essential problem is the rate of a t t a c h m e n t and we plan to address ourselves to this subject in the near future. Williams (Seventh and Eighth Combustion Symposia) has already shown t h a t electron a t t a c h m e n t rates are i m p o r t a n t in some flames. I recoguize the dilliculty which Padley raises for low pressure flames; part of the explanation of low electron concentrations in these flames is certainly related to the high diffusion coefficient for electrons. We have already pointed out t h a t diffusion is the dominant loss mechanism for both electrons and

ions at low pressures (reference 25 of the paper). We are pursuing the problem of negative ion formation in flames further with both Langmuir probes and mass spectrometry; this paper represents only a progress report. PnoF. T. KINn^nA (Sophia University, Tokyo): We must be very careful in applying the Langmuir probe method to the measurement of ion concentration in a flame. I would like to ask about these points. 1. A probe, except for its end, should be protccted b y a tube of some kind which is very resistive to the electric current through it. However, it is almost hopeless, I believe, to find a material which is completely resistive to an electric current at high temperatures in a flame. Such materials can be resistive t)ut are porous and electrons can easily pass through them. The surface area of the protecting tube is quite large compared with t h a t of the wire exposed to the flame, and this leakage current cannot be neglected. How does one correct the actually observed current and get the pure ion current which flows out the naked end of the probe wire? 2. I suppose platinum was used as a probe wire. Can one make sure t h a t the catalytic action of platinum does not give any effect on the ion concentration in a flame? 3. In studying the ion concentration using a Langmuir probe wire of cylindrical form, it is necessary to know the surface area of the sheath around the probe wire. The area changes according to the current, and we need some assumptions for this relation. Results depend upon the assumption adopted. I would like to know how this problem was dealt with and I would like also to mention t h a t the temperature of the probe is, in general, lower t h a n t h a t of the flame. This difference should be taken into account. DR. H. F. CALCOTE: I certainly agree, the use of Langmuir probes in flame.s is fraught with difficulties. We tolerate its idiosyncracies because it is essentially the only means of obtaining local ion and electron concentrations through a flame front. In its defense, it should be pointed out t h a t the rates of ion recomt)ination and the mechanisms first deduced from probe measurements are being continually reconfirmed by other techniques as we have observed in this meeting. In answer to the specific questions raised by Professor Kinl)ara. 1. We did not use only the end of the probe to measure currents but part of the cylindrical area. In some eases the probe was stretched completely across the flame so no insulators were involved. Data obtained by this technique and with the in-

ION AND ELECTRON PROFILES IN FLAMES sulated probe (Fig. 3 of the paper) gave the same result. Leakage current was maintained at a negligible level by the silver (or sometimes copper) cooling jacket. 2. Platinum-40% rhodium has been found to be the best probe material. Platinum is much more catalytic. Sometimes catalytic action, usually indicated by abnormal probe temperatures, effects the ion concentration but often it does not. Probes of different substances, e.g, rhodium, nickel, palladium, and stainless steel have been used at various times to verify the lack of influence of catalytic action. 3a. The difficulty of knowing the sheath area is avoided by extrapolating the positive ion current (at negative probe voltage) to the plasma potential

637

where the sheath thickness is zero. Then the probe area is used. As can be seen in Fig. 1 of the paper this extrapolation is reasonable because the negative portion of the curve is essentially linear. 3b. The temperature of the probe is always lower than the flame temperature, the difference increasing with decreasing pressures because radiation cooling is independent of pressure and convective heating decreases with pressure. I do not believe this temperature difference is very important so long as small probes are used which do not greatly cool the surrounding gas. A quantitative analysis of this problem could be made by comparing the distance the probe cooling extends into the flame and the relaxation distance of the plasma properties as they diffuse through this cooled gas.