wed

NATIONAL BUREAU OF STANDARDS REPORT

6999

PLASMA EXTINCTION BY FLAME INHIBITORS

by

Paul Burrow

Physicist

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 201 5.

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NATIONAL BUREAU OF STANDARDS REPORT

ms PROJECT

1002 - 11-10121 October 28, 1960

NBS REPORT

6999

PLASMA EXTINCTION BY FLAME INHIBITORS

by

Paul Burrow

Physicist

Participating in

Junior Scientist and Engineer

Program

1960

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<NBS>

U, S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

PLASMA EXTINCTION BY FLAME INHIBITORS

by

Paul Burrow

ABSTRACT

For a few compounds on which data are available, there

is a parallel between the ability of the compound to

capture and more-or-less permanently remove electrons

from a flame and its efficiency as an extinguishing

agent . Because a gaseous discharge plasma is a much

simpler system than a flame, a preliminary study was made of the ability of fire extinguishing agents to

extinguish a low pressure discharge in argon

„

PLASMA EXTINCTION BY FLAME INHIBITORS

by

Paul Burrow

1 . INTRODUCTION

Many compounds used in the inhibition of flames have been shown, to

have rather large cross-sections for dissociative electron attachment,

that is, processes _such_as CHgBr j- e~ —> CH3 j- Br“ . Papers by Dibeler,

Reese, and Mohler ]_ 1,2/ give data indicating the relative abundance of

the negative ion formed. These abundances, measured by means of a mass spectrograph, must give an indication of the ability of the neutral gas

molecules to attach electrons.

If a list of various compounds on which attachment data are available is formed in decreasing order of negative ion abundance, it is

found that the list is identical, entry for entry, to the list formed

by ordering the same compounds in decreasing effectiveness as extinguishing agents. There is no apparent correlation between the actual

magnitudes on both lists, but the coincidence in crude ordering is

enough to motivate more detailed study of the role of electron attachment in flame inhibition.

2 . FLAME INHIBITION

The effectiveness of many gaseous compounds in flame extinguishment

has been found empirically in experimen£s_done for the Army Engineers

Research and Development Laboratories J_ 3/. These experiments measured,

for many gaseous compounds, the maximum percentage (by volume) necessary

to extinguish a flame composed of variable, amounts of n-heptane and air.

The fuel, air, and inhibitor were mixed together in a combustion tube

containing a spark gap. The mixture was considered flammable if, when

a high voltage was applied, ignition occurred and the flame front

propagated to the end of the combustion tube.

There are three important things to note along with the data given

in the report. 1) "The relative flame inhibiting effectiveness of any

of the agents investigated does not vary appreciably with variation in

the type of fuel (hydrocarbon or oxyhydro carbon) , or of the temperature

of the atmosphere in which the flame is propagated." 2) Most of the

data were taken at pressures of 400-500 mm Hg. "Little variation in the

flammable limits was observed with changes in pressure from about 200 mm

Hg. to atmospheric pressures." 3) Concerning temperature dependence,

tests were made at -78C, 27C, and 145C. It was found that although the

percentage (for extinguishment) of a given inhibitor changed, the rat io

between any two gases did not vary appreciably.
'
-2 -

3 . PROPOSAL

A flame, which is a mixture of free electrons, neutral air and fuel

molecules, free radicals, and many varieties of ions, may be considered

to be a very complicated type of plasma. Its complexity is several magnitudes higher than that of a gas discharge in one species, for example,

an inert gas. It is made more complex not only because of the different

species present, but because of the chemical reactions between different

ions which are usually not inert. Free radicals are present also, along

with the chains of free radical reactions used to explain flame propagation.

To avoid some of these complications, it was decided to determine the

effect of flame inhibitors on the parameters of an argon plasma. These

parameters, such as voltage, current, and pressure, have the advantage of

being simple and easily measurable.

4 . EQUIPMENT

A gas discharge, tube was constructed from the design given in

figure 3, „ It was necessary to have platinum electrodes (made from .004"

platinum foil) to prevent corrosive action by some of the inhibitor gases.

It was also desirable to make one electrode movable to give more variation

in the values of electric field strength between the electrodes. A schematic of the electrical circuitry used is given in figure 2a. Details of

the gas flow system are given in figure 2b „ To facilitate the changing

of the inhibitor to argon ratio, a dynamic system was used, with flowmeters selecting the desired amounts of gases. Most of the mixture was bubbled through water into the atmosphere; only a small portion was bled

off and sent to the discharge tube. The vacuum pump operated continuously

and maintained a constant pressure in the tube.

5 o EXPERIMENTAL PROCEDURE

The discharge, tube was evacuated to the limit of the mechanical pump

(about 200 microns), filled to atmospheric pressure with Argon, and

evacuated again. This process was repeated for several hours. Following

this, argon alone was pumped continuously through the system for one day.

The pressure remained at about 1 mm Hg.

Because an oil diffusion pump was not available, the pressure, could

not be lowered below a few hundred microns. This means that there was still a rather large concentration of impurities remaining both as free,

gases and substances on cne walxs of the tube. For this reason (and con- venience in obtaining gases) no special effort was made to use ultra-pure

gases or extremely "clean" procedure. This necessarily made the experiment

somewhat crude but since it was preliminary work, extreme accuracy was not

required.
, * ,
-3-

With the pressure at 1 mm, the electrodes spaaed 1.5 cm apart, a discharge was generated in argon. Under normal conditions the voltage

was about 250 volts, the discharge current, from 30*60 ma, The discharge exhibited the normal characteristics of a glow discharge, such

as dark spaces and striations. Variable amounts of the inhibitor gas were then fed into the tube.

In the first, experiment, the discharge current was plotted as a function of the percentage (by volume) of the. inhibitor gas. This was

an attempt to see. if the inhibitor ' would extinguish the discharge,

regardless of the voltage fluctuations across the tube (due to the

changing discharge current) , Because of the resistor in series with

the tube, there were two competing effects present. The constraint,

of course, was that the total voltage across the resistor and the tube

must remain constant (the constant output voltage of the power supply)

„

Now, as the inhibitor gas was introduced, the discharge current (hence,

the total current in the. circuit) dropped. This decreased the voltage, across the resistor and therefore, increased the voltage, across the. discharge tube. This "feedback" voltage increased the amount of ionization and tended to oppose, the drop in current caused by the inhibitor.

The net effect is shown in figure 3, As the. voltage, across the tube

increased, the glow region was constricted to a very bright narrow beam

between the electrodes. At 25% inhibitor, the discharge current had

dropped to less than 10 ma. The current flow, however, was constricted

to such a small area, that the plates were overheating. At this point,

the experiment was discontinued’, the only conclusion being that the

introduction of the inhibitor gas caused an immediate drop in discharge

current

,

In the second experiment, the. voltage across the discharge tube was held constant, regardless of the current flow. This, of course, is

necessary to maintain a constant average electron energy. With the

introduction of approximately ,5% CF 3

.Br, the. discharge current dropped,

the. voltage started rising and on being manually returned to the initial

value, the discharge was extinguished. After changing the flowmeters so that a smaller amount of inhibitor could be. admitted, the experiment was again attempted , This time, the. discharge current remained constant

(within 1 or 2 ma) until ,11% C.F^Br was reached. The voltage started to

rise, but was held constant by lowering the supply output voltage. This

caused the current to drop about 10 ma from the initial value. Apparently,

at this point, an equilibrium state existed. As long as the percentage

of inhibitor was not changed, the voltage and current, remained the same. With the slightest increase of inhibitor, however (to somewhat less than

, 12%), the. discharge was extinguished.

4-

On each attempt, it was found that the extinction point was reached

very abruptly. The discharge current remained constant until a certain

concentration was reached or a rate of production exceeded. The current

was then driven to zero.

The amount of inhibitor is very small, being slightly more than one molecule of inhibitor per thousand molecules argon. This ratio is con- siderably smaller than that required to extinguish a flame of n-heptane.

CF^Br, for example, is 50 times more effective in plasma extinction than

in flame.

The same experiment was then tried with two other inhibitor gases,

Cf^Br and CCI

2

F

2

. In each case, the same sharp cutoff was reached. The

current as a function of percent inhibitor curves for three gases is

shown in figure 4.

Above, it was mentioned that the effectiveness of the inhibitors in

plasma was much higher than in flame. The connection between these observations is not obvious. It is, however, interesting and important to

compare the ratio of the effectiveness of any two given inhibitors in

plasma with the ratio of the same two gases in flame extinction. This

information for three gases is given below.

Inhibitor gas Amount by Vol.

to extinguish

flame

Ratio Amount to

extinguish

Plasma

Ratio

CF3Br

CH3 Br

6.1%

9.7%

i

.63

.120

.178 .676

CH3Br

CC

2 F2

9.7%

14.9% .65

£

.178

.264 . 675

£

The agreement of the ratios is quite close, which is all the more unusual

considering the crudeness of the data.

Incidentally, a fourth inhibitor gas (GH.CIF2 ) was attempted. Only

a crude idea of the extinction percentage was found though. It was in

the proper order with respect to the three inhibitors mentioned above.

Before the percentage was measured accurately, water was accidentally fed

into the discharge tube. Time did not permit making any more measurements

after cleaning tne tube.
.
-5-

6 . DISCUSSION

Assuming now that the ratios are the same, as the evidence indicates,

the following conclusions may be drawn. Since the effectiveness of any

inhibitor bears the same fixed relation to those of the other inhibitors,

in both flame and plasma, it is implied that the mechanism for extinction

is the same in both flame and plasma. The increased efficiency of an inhibitor in a plasma then must be attributed to a larger amount (or

higher collision rate) of some vital constituent, for instance, more electrons per inhibitor molecule. This, however, is not yet clear.

One thing of importance follows almost immediately. Inhibition does

not occur by interruption of free radical chains which have been proposed

to explain flame propagation. This is true since the extinction mechanism

is indicated to be the same in both plasma and flame and there are no free

radical chain-type reactions in plasmas.

It has also been suggested that inhibition occurs by interrupting

some form of energy feedback. In flames, this feedback energy is directed

to the reacting gases. The energy could be either radiant or thermal in

nature. It is probable that neither of these forms is important in a plasma, which receives energy continuously from the voltage source con- nected across the electrodes. No feedback energy, then, is necessary to

maintain the glow discharge. As a point of interest, certain halogenated

hydrocarbons, similar in nature to the inhibitors used here, have been

used as "quenching agents" in Geiger counter tubes. Supposedly, their

purpose is to absorb infrared radiation in the tubes to prevent secondary

signals. Evidently these compounds have wide absorption bands in the

infrared region. Some information on this is available in Brown's "Basic

Data of Plasma Physics". If we rule out radiant energy feedback in a plasma, interruption of this chain as a mechanism for inhibition must be

discounted as a consequence.

It seems clear also that the mechanism is not one of simple dilution.

An experiment which should have been performed is the introduction of a

^'durnmy" gas, inert in nature, during the discharge. Since argon has such

a high ionization potential, the actual discharge should be carried out

in another inert gas, with argon used as the "dummy inhibitor". This

would verify that dilution is not important.

It is -important to note the loss or decay mechanisms in a plasma.

Calcote ]_ 4/ mentions the three principal mechanisms; ion recombination,

diffusion to the walls with recombination, and electron temperature decay.

To these, in relevant situations, he adds charge exchange and electron

attachment. Now, the inhibition mechanism must be connected in some manner,

with at least one of these mechanisms. In view of the mass spectrogiaphic

data showing the high cross-sections for electron attachment of the
’

.
- 6 -

inhibitors, it would be very tempting to state that electron attachment

is the sole mechanism. However, there will be interaction between these

modes of decay. Calcote says, "...when electro-negative gases are present, electron attachment will greatly affect the rate of ion recombination

and will greatly reduce the ambipolar diffusion coefficient." These other

side effects, as opposed to attachment which is most likely the major loss,

may account for the large difference in efficiency of an inhibitor in

plasmas and flames; the latter having a much greater variety of ions,

neutrals and free radical.

7, SUMMARY

1. The inhibition mechanism used by the gases that were tested appears

to be the same in both plasma and flame.

2. Since there are no free radical reactions in a simple plasma,

inhibition does not occur through interruption of free radical chains.

3. Inhibition does not operate through simple dilution.

4. In view of the limited number of loss mechanisms in a plasma,

all dealing with electrons, inhibition must occur through direct loss

of electrons through attachment and/or modification of the other loss

mechanisms as rate of ion recombination and ambipolar diffusion.

8. SUGGESTED MODIFICATIONS IN EQUIPMENT

1. A discharge tube with permanent electrodes spaced about 1.5 cm

apart should be constructed. The plates should be smaller in diameter

(about .5 or .75 inches in diameter) and the glass envelope, much larger

(2 inches in diameter) to prevent heating of the glass near the rim of

the plate.

2. Three inlets should be provided, a vacuum indicator, a lead to

the vacuum pump, and a lead to the entering gases. Both of the latter

should be provided with double stop cocks.

3. A high vacuum system should be used consisting of a roughing

pump, oil diffusion pump and liquid N

2 cold trap.

-7-

9. SUGGESTED EXPERIMENTS

1. All experiments listed previously should be repeated with as many additional inhibitors as possible.

2. The experiment should be repeated using several "dummy" inhibitors such as other inert gases.

3. All of the experiments above should be run in various inert gases

in place of argon. The average electron energy in each gas will be different. This should produce somewhat different efficiency ratios since

the capture cross-section of each inhibitor will vary differently with

energy.
V

>

'
- 8 -

10. REFERENCES

fli Dibeler, V. H., Reese, R. M. , and Mohler, F. L. J. Res. NBS 57

113 ( 1956}

£2i Dibeler, V. H., and Reese, R. M. J. Res. NBS 54 127 (1954).

J_ £j "Final Report on Fire Extinguishing Agents for the Period September 1,

1947 to June 30, 1950, Covering Research Conducted by Purdue Research

Foundation and Dept, of Chemistry" under Contract W44-009-eng-507

with Army Engrs. Res. and Dev. Labs., Ft. Belvoir, Va. Also Contained

in Engrs. Res. and Dev. Labs. Report 1177, "Interim Report on Vaporizing Fire Extinguishing Agents" by J. E. Malcolm.

]_ 4/ Calcote, H. F. "Relaxation Processes in Plasma". A paper presented

at the Amer. Rocket Society - Northwestern Gas Dynamics Symposium,

Northwestern University, Evanston, 111., Aug. 24-26, 1959.

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INHIBITOR

U.S. DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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