Transport of Carbon in Nonisothermal Low Pressure Nitrogen Plasma

Folge,

Transport

of

Carbon

in

Nonisothermal

Low Pressure

_Nitrogen

Plasma

By

S. Veprek

Institute ofInorganic Chemistry,

_University

ofZürich Rämistrasse76, Zürich, Switzerland

With 3figures (Received January 29, 1973)

Zusammenfassung

Kohlenstoffkonnte mit Stickstoff in einem nichtisothermen

Niederdruck-plasma einer

_{Hochfrequenzentladung}

transportiert werden. Der Transport

erfolgte

in

_Richtung

abnehmender

_{Plasmaenergie}

bei

_{Translationstemperaturen}

vonetwa1000°C.Eine Diskussion der

_{Elementarprozesse}

und einVergleichmit

theoretischen

_{Vorstellungen}

liefert tiefere Einblicke in die

_Vorgänge

bei der

chemischenVerdampfungund

_Abscheidung

fester Stoffeunterden

_Bedingungen

einesnicht isothermenPlasmas.

Abstract

Carbon has been

_transported

with

_nitrogen

ina nonisothermal

_low-pressure

plasmaofa

_{highfrequency discharge.}

Thetransportproceedsin the direction of

decreasing plasma energy at a translational temperature of about 1000°C.

A discussion of

_elementary

_{processes and}a comparisonwith

theory

leadsto a

deeper insight

into_{the processes of chemicalevaporation}anddepositionofsolids under nonisothermal plasmaconditions.

1. Introduction

Chemical

_transport

reactions1 are well known methods for the

growth

of

_crystals

and thin films from _{the gas}

_phase.

The use of

nonisothermal

_plasma*

makes

_transport

_possible

also in

_systems

in

* _{For further information about this kind of}

plasma

see 2 and references

mentioned there.

1_{H. Schäfer, Chemische}

_{Transportreaktionen, Verlag}

_Chemie,

_Weinheim/

S. Veprek

which it is otherwise hindered

_kinetically

or

thermodynamically

(without

a

plasma).

In an earlier

paper2

we have demonstrated the

kineticeffect ofa weak

discharge

by

the

_transport

ofcarbon

according

to the Boudouard's reaction and the

_{thermodynamic}

effect of an

intense

_{highfrequency low-pressure discharge}

in

_systems

which are

strongly

exothermic without the

_{plasma (such}

as

TiN/Cfa, AIN/Cl23

and

_{C/H2, C/O22).}

In this _paper we shall show how a nonisothermal

plasma having

a

high

_energy content can shift chemical

equilibrium

of a

strongly

endothermic reaction in such a _{way that} chemical

transport

becomes

_possible

at a

_temperature

ofthe solid at which it

would be

_{quite impossible}

under

_{thermodynamical equilibrium}

con-ditions. The

_system

_{carbon-nitrogen}

seems to be _very suitable for this _{purpose in view} of its

_simplicity,

sufficient

_knowledge

of the

elementary

processes involved and the

possibility

of a

comparison

with

_transport

in other

_systems

like

_{carbon-hydrogen}

and

carbon-oxygen2.

Several results of this

_study

have been

_already

mentioned in our

previous papers2-4.

In this

_article,

a short

description

of the

experi-ments

_along

with a discussion of

elementary

_{processes in the}

system

under consideration will be

_given.

The

_{carbon-nitrogen}

_system

has been studied under

_plasma

con-ditone

_by

a numberof authors5-18. Because ofinsufficient information 2 _{S.Veprek, Second Int. Conf.VapourGrowth &}

_Epitaxy,

_{Jerusalem 1972,}

J.Cryst. Growth 17 (1972) 101.

3 _{S.Veprek, C. Brendeland H. Schäfer,J. Cryst. Growth 9 (1971) 266.} 4 _{S.Veprek,J. chem.}

_Physics

_{57 (1972) 952.}

5_M.Möhren,

Compt.

Rend. (Paris)48(1859) 342.

6_{Modern Aspects of}

Graphite Technology,

ed. L.Blackman, Academic Press, London 1970.

7 _{F. K.McTaggart, Plasma Chemistry in Electrical}

_Discharges,

_Elsevier,

Amsterdam 1967.

8_{A. Wright and C. Winkler,Active}

_Nitrogen,

_{AcademicPress,New York}

1968.

9_{R. Baddour and R.Timmins,} The

_Application

of Plasmas to Chemical

Processing,

M.I.T. Press,

Cambridge

(Massachusetts) 1967.

10_{G. Glocker and S. Lind, The}

_{Electrochemistry}

of Gases and other

Dielectrics,J.

_Wiley,

New York 1939.

11_{C. Fink andD.Wroughton,Trans, electrochem. Soc. 88 (1945)25.} 12_{H.Goldstein, J.}

_physic.

_{Chem. 68 (1964) 39.}

13_H.v._Wartenberg, . anorg. Chem. 52 (1907) 299.

"_{C. Stokes and W.Knipe,}Ind.

_Engng.

Chem. 52 (1960) 287.

is_{T.Wallis,Ann. Chem. 345 (1906) 353.}

about the

_experimental

conditions

_(basic

_plasma

_parameters)

a

com-parison

of different

_experiments

is difficult or

quite

impossible

_in

most cases. There are two

_important

kinds of

_experiments

which have to be

_{distinguished:}

_Experiments

in a

high

temperature

plasma

(arc-discharge, plasma jet)

and in a "cold"

plasma (glow-

and

high-frequency low-pressure discharge).

The

_present

_{paper deals with}

a nonisothermal

low-pressure plasma

of an intense

highfrequency

discharge.

Itwillbe

_shown,

that,

when the inner energy of the

plasma

reaches

a value of about 100

kcal/mole

and the translational

temperature

lies

close to 1000°

C,

the

_transport

of carbon takes

_place

in the direction of lower

_plasma

_energy, 2 ->

(see

sect.

3).

Under these

conditions,

the energy in the translational

_(and

also rotational and

_vibrational)

degrees

of freedom of

_{heavy particles}

_(about

5

_kcal/mole)

is much lower than the energy in the electronical ones

(about

100

_kcal/mole).

Therefore,

the usual chemical

_{thermodynamics}

as well as the linear

irreversible

_{thermodynamics}

are not valid here. Also the

general

thermodynamic theory

of

_stability

cannotbe used in its

_present

form19

asthe

assumption

of local

equilibrium

is violated

already

in individual

molecules.

_Only

a

simple

statistical model of chemical reaction under

nonisothermal

_plasma

conditions4 can be used for

prediction

of

chemical

_transport

_today.

2.

_Experiments

The

_experimental

_arrangement

is

_{schematically}

shown in

_Fig.

1. The

_{highfrequency discharge}

takes

_place

between theouterelectrodes 2 and 2' in a silica tube 1

(inner

diameter R = 37

mm).

The

high-frequency

output

of

_generator

3

_(type

_Philips

PH

_1202/00)

_{may be}

adjusted

between 0 and 1.2kW at the

_frequency

of 80MHz. The h.f. current is measured

_by

a

special

amperemeter

14. The gas flow

and the total pressure in the

discharge

tube are

adjustable by

means

ofneedlevalve

5,

vacuumvalve 6 and

rotary

oil_{pump 4.} The_pressure

in tube 1 is measured

_by

McLeod- and oil-manometers 7. Cold

_trap

8

prevents

a

penetration

of oil- and _{mercury-vapour into} the

plasma.

Pure

_nitrogen

with a total _{oxygen- and} water-traces below 0.5_ppm

has been usedin all

_experiments.

« _M.Berthelot,

_Compt.

_{Rend. (Paris) 95 (1882) 955; 145 (1907) 354.} 18 _{K.Peters,Naturwissenschaften 19 (1931) 402.}

19_{P. Glansdorf and I.Prigogine,}

_{Thermodynamic Theory}

_{of Structure,}

StabilityandFluctuations,

Wiley,

London 1971.

98 S. Vepbbk

Fig.1. _{Experimental arrangement} for the _transport of carbon in nitrogen plasma. Description see text. The length scale to be

applied

only for the

dis-charge

tube

Spectral

pure

graphite

tube 9

(Ringsdorff Company,

diameters

15/8

mm)

is

_placed

in nickel

_inlay

10. The

_temperature

ofthe

_graphite

surface inside tube 9 is measured

_by

an

optical

_pyrometer

through

orifice 11. Because of the

_{non-equilibrium}

radiation of

_plasma

under these

conditions,

direct measurement of surface

_temperature

is not

possible

in the active

_discharge.

_Comparable

measurements of the

temperature

ofthe

_graphite

surface inside tube 9 and in borehole 11 after

_switching

off the

_discharge

show that the difference between both values are smaller than 40° at a

temperature

of about 900°C.

Experiment

No. I

50—100_{mg carbon} are

transported

_{per hour from the inside of}

graphite

tube 9 into the silica

_inlay

12 at a total pressure of 1

Torr,

a _{gas flow of about} 100Torr

cm3/sec

and a h.f. current between

2 and 4A. The

_temperature

of the

_graphite

surface reaches a value

between 950 and 1000°C in the tube 9 and about 800°C in zone a and b of silica

inlay

12.

Discharge

tube 1 is isolated with asbestos

(heat-isolation)

15 on the left of electrode 2'.

Fig.

2 shows the carbon

_deposited

inzone of

inlay

12

(cf.

Fig.

1).

Thereis

_large

amount ofsoot

_deposited

inzone a and a small amount

in zone c on the left of the electrode 2'. No paracyanogen

(CN)X

has

been found in the carbon

_deposited

under these conditions

_(IR-

and

mass

spectrometry).

The amount of

_transported

carbon increases with

_increasing

h.f.

Fig.2. Carbon

_deposited

in thezone6 of

Fig.

1.A) surface in contactwith the

substrate _{(silica), B) growth} surface, C) a cut through the

_{layer. Scanning}

electron

_microscope

(stereoscan) picture; 1500X

temperature

of the

_plasma

increase

_{simultaneously3'20.}

The variations of the

_temperature

of the carbon surface in tube 9 are smaller than

the accuracy of its measurements.

Experiment

No. II

The

_graphite

tube 9 and the nickel

_inlay

10are _shiftedto electrode 2' and heat isolation 15 is removed

_(cf.

_Fig.

1).

The distance between electrode 2' and nickel

_inlay

10 is about 5 mm. The total _pressure,

gas flow and h.f. current are

kept

at the same value as before. There

is a

mingling

ofcarbon

(soot)

and paracyanogen

deposited

onthe left

of electrode 2'. The material

_deposited

in zone c contains more

pa-racyanogen than material of zone d

(IR-

and mass

spectrometry,

habitus).

Experiment

No. Ill

The

_arrangement

of the

_discharge

tube isshownin

_Fig.

3.

_Graphite

rod 17

_(diameter

13 mm,

_length

35

_mm)

is fixed with silica stick 16

20_{S.Veprbk, Dissertation,}

University

ofZurich, 1972.

100 S. Vepèek

Fig.

3. Experimentalarrangement for

_experiment

No. III.

_Description

see text

in the middle of

_discharge

tube 1. The h.f.

_discharge

takes

_place

between electrodes 2 and 2'. The inner diameter of the

_discharge

tube around the

_graphite

rod _{may be varied}

_by

_inserting

silica tubes 18. The_{pressure, gas} flow and h.f. currentare thesame asin

experiments

No. Iand II. Neither

_transport

of

_carbon,

nor

deposition

of

paracyano-gen takes

place

without

inlay

18. If the

inlays

18 are mounted into

tube

_1,

_deposition

of carbon in zone d and of paracyanogen in zone c is observed. The smallest inner diameter of the

inlay

used around

graphite

rod 17 was 25 mm.

Experiment

No. IV

The

_graphite

rod from the

_{previous experiment}

isnowfixed in the

middle of a

discharge

tube with the diameter of 100 mm. The

tem-perature

of

_graphite

is measured with a _gas thermometer. _A

Wrede-Harteck _gauge

_(cf.

_e.g.

_7,

_p.

₇₆₎

is used formeasurement of the

_degree

of

_nitrogen

dissociation in the

_discharge.

_{At the total pressure of}

1.35 Torr and h.f. current of 6.3

_A,

a

graphite

_temperature

of 590°C

anda

degree

ofdissociationof about

50%

arefound20. The

_temperature

ofthetube wall lies between 200 and 300°C.

Under these

_conditions,

about 1.5 _{mg carbon per}hour reacts with

nitrogen,

and paracyanogen is

deposited

on the wall of the

discharge

tube between the electrodes. No

_deposition

ofcarbon canbe observed

in this

_experiment.

The reaction rate increases with

_increasing

h.f.

current. At the same time also the

_temperature

of

graphite

in-creases

(e.g.

_up to 630°C at the h.f. current of 10

A).

The reaction is not observable when the

_temperature

of

_graphite

is too low

(e.g. 300°C)

although

a

degree

of dissociation

higher

than

_20%

_is

found20.

In all

_experiments

careful efforts have been made to exclude

traces of

_humidity

from the

_reacting

_system.

No HCN has been detected in carbon and _paracyanogen

_deposited

in these

_experiments

3. Discussion

A

_comparison

of

_experiments

No.

I,

III and IV shows that both

high

energy content and

high

translational

temperature (resp.

the

temperature

of carbon in the

_starting

_zone)

are _necessary for the

formation of cyanogen.

Experiment

No. II indicates that the "carbon

carrying species"

during

the

_transport

_{is the cyanogen} monomer CN

(cf.

also 21

_).

_The

_largest

part

of the _{inner energy} ofthe

_plasma

cor-responds

to the dissociation _energy when the

_degree

ofdissociation is

higher

than about

_50°/o.

Thus,

the

_degree

ofdissociation _{may be used}

as a

rough

measure of the

plasma

_energy under these conditions.

The dissociation of

_nitrogen

increases with an

increasing

ratio h.f.

current vs. tube

diameter,

I/R3'20.

Because of the

continuity

of the

electric

_current,

the

_I/R

ratio is

_larger

in

_graphite

tube 9

_(see

_Fig.

₁₎

than in silica

_inlay

12. Carbon is then

_transported

in the direction of lower

_plasma

_energy 2-* _.

We shall now discuss the

elementary

_processes

occuring

in the

plasma.

The reaction of carbon with molecular

_nitrogen

Gs

+

Ì

2 ß

^

_CHg

₍₁₎

is

_strongly

endothermic

_{( °2}

= 109

kcal/mole

22)

and a

significant

concentration of cyanogen arises under

thermodynamical equilibrium

condition in the gas

phase only

at _very

_high

_temperature.

It has been

suggested

that therate

_determining

_step

inthe

_{carbon-nitrogen}

reac-tion is the

_adsorption

of

V2

ontwo

adjacent

carbon atoms

_(23,

_p.

_194).

Because of the

_high

_{dissociation energy} of the

_nitrogen

_molecule,

the reaction

_proceeds

with a

significant

rate

_only

above 2000°C. There is also some evidence that the CH molecule

leaving

the carbon surface

is

_{electronically}

excited

_{( 2}

_state)24.

The

_transport

of carbon

_proceeds

in the

_{discharge according}

to

the reaction

_(2).

0.

+

_{1/ ·, * ß.}

(2)

Here,

_N*>g

stands for

_nitrogen

in a

plasma

state with a

high

_energy

content. The _necessary

_energies

of the

_N*>g

_species

_{participating}

in

21_{C. Haggartand C.Winkler, Cañad. J. Chem. 38 (1960) 329.} 22_{JANAF Thermochemical Tables, US Nat. Bur. Stand., second}

edition,

NSRDS-NBS 37 (1971).

23_{Gmelins Handbuch der}

anorg. Chemie, Bd. 14, Dl,VerlagChemie,

Wein-heim/Bergstr.

1971.

102 S. VepSek

reaction

₍₂₎

have to be

_equal

to or

higher

than AH° of reaction

(1).

Moreover,

for the chemical

_equilibrium

of reaction

₍₂₎

tobe

_established,

thereaction has to be fast.

We will now discuss the nature of the

_{particles participating}

in reaction

_(2).

Consider

_starting

from the

_plasma

state with a low

con-centration of cyanogen which does not disturb the

_composition

of

discharge

in pure

nitrogen

significantly.

This low concentration can

originate

either from a low value ofthe

plasma

_energy or from a low

translational

_{temperature (see later).}

Of the molecular

_{species arising}

in the

_{nitrogen low-pressure}

dis-charge

with a

significant

concentration there are

predominantly

mole-cules in the

_ground

_{( 1}

₊₎

and the metastable

_{( 3}

₊₎

states. The

ground

statecan be

vibrationally

excitedtoseveral

_eV,

butthe

_higher

levels have short relaxation times. The

_{relatively high}

concentration of vibrational excited molecules in the

_{nitrogen afterglow}

of the microwave

_discharge

_(about

_{30%25 Corresponds predominantly}

to

the first vibrational level

_{( 1}

_ß+,

= 1

26)

having

a mean life time

of about 50 msec at 1 Torr under the

experimental

conditions. The

energy released

by

the deactivation of the

vibrationally

excited

species

was found to be

_only

about 2

_kcal/mole

25.

_{Highfrequency}

discharges produce

quite

small amounts of vibrational excited mole-cules and also the _{vibrational deexcitation may be}

_expected

to be fasterin active

_discharge

than in the

_afterglow

_(e.g.

the deactivation of

_{vibrationally}

excited

02

in collisions with atoms can be two orders

of

_magnitude

faster than in collisions with molecules 27

_{). Then,}

_the

ground

electronic state ofmolecules canbe assumednot to

_participate

inthe reaction

_(2).

The metastable

N2

( 3

,+)

can_{arise in}

relatively high

concentra-tionsinthe

_plasma,

but itsenergy

(6.17

eV for =

0)

isnot sufficient

for_{the formation of GN molecules. This process may}

_perhaps

be fea-sible

_through

some surface reaction under

participation

of _more of

such

_species,

but because of a short life time of the

( 3

+)

state

(radiative

life time between 10~4 and 10~2 sec

8,

_pp.

96—97)

it seems

tobe

_improbable.

Theothermolecular

_species

_{have very}shortradiative life times

_{(10~6—10~8 sec)}

and will not

_play

_any

_significant

role.

25 _{F.Kaufman and J.Kelso, J. chem.}

_Physics

_{28 (1958) 510.} 26 _{K.Dressler, J. chem. Physics30 (1959)} 1621.

27 _R.

Gordon, W.Klemperer and J. Y.Steinfeld, in Annual Review

of

_Phys.

Chem., Vol. 19, ed. H.Eyring, G. Banta

_Comp.

Inc., Palo Alto

Nitrogen

Generally,

it seems

likely

that any molecular excited

species

will be

deactivated at the surface of a _{solid rather than}

undergo

a chemical

reaction withit.

The situationbecomes

_quite

different ifatomic

_{species participate}

in reaction

_(2).

Thereare the

ground

state

A(4£)

and the metastables

N(2D)

and

_N(2P)

which have

_long

radiative life times of 26hours and 12 seconds

_respectively

_(8,

_p.

_82—83).

The deactivation of the metastable

_{species proceeds predominantly through}

collisions with the wall or other

species.

Reaction

(2)

is

weakly

exothermic

[AH°W&

=

—4

_kcal/mole

_22),

with atomic

_nitrogen

in the

_ground

state and it becomes much more exothermic when the metastables

participate.

In the active

_discharge

in

_nitrogen

a _very

high degree

of dissociation canbe obtained at _{the pressure of} 1 Torr

_(cf.

_3·20).

At a lower

degree

of dissociation of

_nitrogen,

the

_ground

state

_(*8)

will

_predominate

but the concentrationof metastables

_[mainly

_N(2D)]

becomes

signifi-cant ata

degree

of dissociation closeto one

_[e.g.

only 1/500

of

nitrogen

atoms are metastable in the

afterglow,

_{but about}

5—20°/o JV(4$)

atoms occur in the

discharge

in a mixture of

nitrogen

_{with inert}

gas

8,

p.

_82]

At 1

_Torr,

the atoms

_disappear

either

_by

recombination on the

wall and to a lower extent _{in the gas}

phase

or

by

reaction

(2).

The

value of the recombination coefficient of

_nitrogen

on

_quartz

_{is of the}

order of 10~3—10~4

_(8,

_p.

₁₇₀₎

and leads to a mean life time of the

atoms of the order of lO^1—10^2 sec. The recombination coefficient

changes relatively

little with

_{temperature (activation}

_{energy between}

0 and 2

_kcal/mole).

We do not _{know any value} for the recombination coefficient of

_nitrogen

on

graphite.

Measurements of surface

tempera-tures of different solids in

_{nitrogen plasma}

indicate that the

recom-bination of

_nitrogen

atoms on the carbon surface is slower than on

quartz.

Especially

at

_higher

_{temperatures (900}

_°K)

the

_catalytic

heating

of the carbon becomes lower and it

_approaches

the value of

a

quartz

surface

"poisoned"

with

HPO3

20.

The reactionrateofatomic

_nitrogen

with

_graphite

canbe calculated

from the values of the _{activation energy} and

_entropy

of about

18.5

_kcal/mole

and —2.7

_{cal/mole deg}

_respectively

as measured

by

Goldstein12. These data

_give

the

_probability

of reaction

_Cs

_-\-Ng-+CNg

for a

nitrogen

atom

striking

the carbon surface of about 2.7 · 10~5 at

890° and 1.8 · 10~4 at 1275

°K,

which

seems to be _very low for

setting

upthe chemical

equilibrium

of the reaction

(2). However,

two factors have to be accounted for when the reaction

_proceeds

in an

104 S. Veprek

active

_discharge*

:

First,

the excited atoms _may be

responsible

for the

higher

reaction rate in

_comparison

with the reaction outside the

plasma.

Second,

for the

_setting

_{up of the} chemical

_equilibrium

of reaction

₍₂₎

the ratio of the reaction

_probability

to the recombination coefficient is more decisive than the absolute values are. The surface

recombination takes

_place

between an atom

striking

the surface and

anotherone

being

adsorbed there

(e.g.28·30).

Thus,

ifthe

desorption

of

CW-radicals from the surface

_increases,

the surface recombination of

atoms will decrease.

_(A

similar effect has been observed also

_by

re-combination of

_nitrogen

atoms on a nickel surface at a

_temperature

ofabout

_1000°C31).

The measurements of the surface

_{temperature (see above)}

and the

_comparison

ofthe amount of

_transported

carbon with the value calculated from Goldstein's data

_(see

4,

Tab.l)

show that the reaction ofatomswith carbon is faster than its surfacerecombination

at a

high

_temperature.

What is therate

_determining

_step

in the reaction

_Cs

_-+-

_Ng

-*

CNgl

At a carbon

_temperature

_{up to} 800°C this reaction

_{proceeds slowly}

(cf.

4

1)

in pure

nitrogen,

but the formation of HCN

_proceeds

quickly

when asmall amountof

hydrogen

is added32·33. The decrease

of the

_{catalytic efficiency}

of the carbon surface for the

_nitrogen

recombination at

_higher

_temperature

in the

_plasma

_suggests

that

the

_nitrogen

atoms are

strongly

chemisorbed on _{the surface}

and the slowest

_step

is the

_desorption

of the CN

_species

from the surface.

This molecule canleave the surface

by

being electronically

excited

into the

_{( 2}

₎

or

( 2

+)

states

(cf.

24 and

34),

but in view of their

short radiative life times

(3.5

· 10~6_sec and 8.5 10~8_sec

respective-ly23),

the

_ground

state

_{( 2}

₊₎

of CN will

_predominate

in the gas

* _{Goldsteinsmeasurementhas been done under conditions hotsolid-cold}

gas. Due to the thermal accomodation of the _gason the hotsurface, the

re-action rate is lower undersuch conditions than inour case ofequal

tempera-tures.

28 _{Handbuch der}

Katalyse,

Vol.IV, ed. G.Schwab,Wien 1943.

29_{Chemical Reactions in Electrical}

Discharges,

ed. .Blaustein, Adv. Chem. Ser. 80,Amer. Chem. Soc,

Washington

D.C. 1969.

3»_{F.Kaufman, in 29,}

p. 29—47.

si _{N.Buben and A.Schechter, Acta}

physicochim.

URSS 10 ₍₁₉₃₉₎ 371.

32_{W.Zinman, J. Amer. chem. Soc. 82 (1960) 1262.} 33_{M. Coulon and L.Bonnetain,}

Compt.

Rend. C 269 (1969) 1469.

Nitrogen

phase.

Also a _very fast collisional

quenching

of these states hasto be

accounted for

_(cf.

_quenching

of Bfi

_{+ 35).}

The other

_{species occuring}

in the

_plasma,

_{e.g. short lived}

_high

excited atoms and molecules as well as

ions,

can influence the overall

reaction

rate,

but

_they

do not contribute to _{the energy} balance of the

system

significantly

and can be

neglected

in this consideration.

The_{CN molecule in the gas}

_phase

caneither

decompose

or

undergo

collisions with other CN

_species

and

_polymerize.

These _{processes will} be considerednow. The

decomposition

can

proceed

either

by

electron

impact

or

by

collisions with other

heavy

species

or on the wall.

A

_simple

assessment shows that in the

_plasma

under

consideration,

the mean life time of the CN molecule

being decomposed only by

electron excitation is 10~2—10~3 sec or

longer

4.

(The

dissociation

energy of CN molecule is found to be between 7.86eV 23 p.

11,

and

8.1 eV

_36.)

Because of the low vibrational

_temperature,

the

_{decomposition}

of CN _{in the gas}

_phase

can

proceed only by

_reaction such_as*

CNg

+

Ng^ N2,g

+

Cg (AH

= -47

kcal/mole).

(3)

Using

the value of 4 1013 cm3 mole^1 for the reaction rate at a

tem-perature

ofabout8000°K37 andanactivation energy of about25

kcal/

mole

_(lower

_limit),

a mean life time of more than 10~2 sec can be

expected

for CN in the above reaction. The

_{decomposition}

_{may be} fasteratthe wall of the

_discharge

tubeat

_high

_{temperature (23,}

_pp.65 to

66)

aswellas

by

ion-molecule reactions in the gas

phase

(homogeneous

condensation).

These estimations lead to the conclusion that CN is

a stable

species

in the gas

phase

under

plasma

conditions and its

decomposition by

electrons is slower than its formation on the wall.

This is in

_agreement

with our observation that

only

_paracyanogen was

deposited

onthe wall in the

discharge

zone at

_temperatures

lower

than 800°C

_(see

_exp. No.

_IV).

No

_{decomposition}

to the

_elements,

but

only polymerization

was observed also

by

other authors in the

low-* _{The dissociative deexcitation}

CNg

-f Xg*->

Cg

+ Ng + Xg has been discussed in38.

35_{W. Jackson and J.Faris, J. chem.}

Physics

56 ₍₁₉₇₂₎ 95.

36_{B.Darwbnt, Bond Dissociation}

_Energies

_in

_Simple

_{Molecules, US Nat.}

Bur.Stand., NSRDS NBS 31 (1970).

37_{M. Slack andE.}

Fishburne, J. chem.

_Physics

56 (1972) 95.

38_{S.Veprek,H. R. Oswald andW.Peier,Z. angew.Math.}

_Physik

_[Basel]

106 S. Vefrek

pressure

discharge

in cyanogen

(CN)2

(23,

p.

67).

The

polymerization

of CN to

_(CN)X

occurs in the gas

phase

and can be accelerated

by

ions

_(23,

_pp. 22 and

_67).

The reverse reaction

(CN)2,g-+ 2CNg

has a

high

activation energy of 95—125

kcal/mole

(23,

_p.

5)

and _may be

omittedhere.

From the

_point

of view of the above discussion the

_transport

of carbon in

_{nitrogen plasma proceeds by}

the

_following

_steps

:

g>

+

nb

_{'2> °}

~* cn 2 > <800° >1200°

(CN)*,,

Cs

+

\N2,g

T is the translational

_temperature

and the

_plasma

_{energy. The}

reac-tion

Cs

+

N(*S)

^

_CN(X*

₊₎

₍₅₎

AHlm

= —4

kcal/mole

is

_weakly

exothermic and the chemical

_equilibrium

is shifted to the

right (log Kp

âî3 at 13000

_22),

but its

_setting

_up

_requires

a

high

temperature

of solid carbon in zone 2.

This is illustrated

_by

a

comparison

of the amountofcarbon

trans-ported

at different translational

_temperatures

with theoretical

pre-dictions

_(see

_4,

Table

_1).

If the

_temperature

is too low

_{(e.g. 890°K),}

the formation of CN on the

graphite

surface

according

to _eq.

(5)

proceeds slowly,

whereas at a

higher

temperature

(e.g.

1275°K)

it is

fast. The theoretical considerations4 are based on the

assumption,

that the chemical reactions under

_plasma

conditions are faster than

other processes of energy

dissipations.

Onecan seefrom the

_comparison

mentioned

above,

that the theoretical

_predictions

andthe

_experiments

agree better at

higher

translational

temperature.

This is an

important

limitationof

_{applicability}

of thestatistical model4aswellasthe method

of

_simple

_energy

_diagrams2,

and it is a

subject

ofour further

paper38.

4. Conclusions

Transport

ofcarbon takes

_place

inanonisothermal

nitrogen plasma

along

a

gradient

of

decreasing plasma

_{energy when the}

degree

of

dissociation and the translational

_temperature

reach

_high

values in the

_"starting

zone". The discussion of

_elementary

_processes shows that the formation of _gaseous CN radicals occurs

by

the reaction of

Nitrogen

nitrogen

atoms with solid carbon. The translational

_temperature

of the

_{plasma plays}

an

important

kinetic role inthis reaction.

We have shown in an earlier

paper2

how it is

possible

to

predict

the direction of chemical

_transport,

as well as the relative amount of

plasma

energy necessary for chemical

transport

to take

_place

in

a

plasma.

According

to the statistical

model4,

simple

_energy

level-dia-grams can be used for this

prediction (cf.2).

The exact

_plasma

param-eters _{necessary for chemical}

_transport

to take

_place

in a

_system

of

interest havetobe found

_{experimentally.}

According

to our

opinion,

this

procedure

can be a

relatively

favourable

_approach

in

_planning

further

_experiments

for

_crystal

growth

and

_preparation

of thin films

_by

the use of a nonisothermal

low-pressure plasma.

Acknowledgment

The

_experimental

_part

of this work has been done

_by

the author

at the Institute of

_{Inorganic Chemistry,}

the

_University

of Münster. The author is indebted to Professor H. Schäfer for

_standing

_support

during

this work _{and many valuable} discussions and advices. He thanks also to Professor H. R. Oswald for his

_helpful

remarks in the theoretical

_part

of this work.