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On the formation and composition of pyrolytic carbon
Carbon
1967, Vol. 5, pp. 201-203.
ON THE
Pergamon Press Ltd.
Printed in Great Britain
FORMATION
,4ND COMPOSITION
OF PYROLYTIC
CARBON
K. I. SYSKOV and E. I. JELIKHOVSKAYA U. I. Mendeleev Chemico-Technological
Institute,
Moscow,
U.S.S.R.
(Received 23 September 1966)
structure of globular formations of pyrolytic carbon at magnifications from 200 to 500 times are shown. The effect of the processes taking place in gaseous phase on the formation of globular structures of pyrocarbon has been ascertained. The size of the globules increases with the pyrolysis of methane, with the decrease of the rate of flow of the reaction gas and the increase of its concentration. The scheme for the growth of the globules is given. It is suggested that the pyrolytic carbon represents a mixture of hydrocarbons of aromatic nature having very high molecular weight, a part of which is present as free-radicals.
Abstract-The
1. INTRODUCTION
VARIOUS hypotheses have been put forward on the question of formation of pyrolytic carbon. Some authors(‘,3) consider that the decomposition of hydrocarbons on heated surfaces takes place as a direct decomposition of hydrocarbon molecules into carbon and hydrogen. Other investigators(4*5) suggest a mechanism as hydrogenation of hydrocarbons with the formation of complex polymer aromatic compounds. According to the opinion of GRISDALE and PFISTER’~) the process of hydrogenation proceeds in gaseous phase, the high-polymer compounds formed being deposited on the surface of the substrate. The process of formation of pyrolytic carbon has been discussed in a comprehensive review by FIALKOV, BAVER and others(‘) as splitting of hydrocarbons into radicals and into hydrogen atoms, all reacting with the red-hot surface and forming low-molecular hydrocarbons and their radicals on the surface. DAVID@) explains the occurrence of globules by the presence of “embryos” representing the smallest drops of more or less carbonized tars. Layers of oriented carbon are deposited on these drops and globules are formed. The authors think that the oriented carbon is a product of conversion of high-molecular compounds, which form the globules. Dehydrogenation and polycondensation cause the formation of hydrocarbon compounds of even higher molecular weight, of which pyrolytic
carbon is composed. The authors believe that decomposition of high-molecular compounds with the liberation of carbon is very unlikely. The present work is an attempt to find some answers to the above questions. 2. EXPERIMENTAL
AND RESULTS
Pyrolytic carbon was obtained by pyrolysis of methane in the temperature range 850”-1200°C and was deposited on a carbon surface. The temperature, the concentration and the residence time of methane in the reaction zone should affect the formation of molecular compounds which give rise to the globules. Therefore the size of the globular formations depending on these factors was studied. The data on the size of globules, determined with the help of microscope are presented in Table 1. It can be seen that the value of the radius of curvature and the height of the ball-like protrusions vary in proportion to each other. This seems to be an experimental proof of the fact that initial formation of pyrolytic carbon is in the form of drops. It means that formation of pyrolytic carbon passes through a stage of molten intermediate products a transition stage which might exist for a very short time. It is seen from Fig. 1 that the higher the temperature of pyrolysis, the bigger the size of the globules obtained. 201
K. I. SYSKOV
202
and E. I. JELIKHOVSKAYA
TABLE 1. SIZE OF GLOBULARFORMATIONS (Time of stay of CHrmolecules in reaction zone--O,8 Condition of experiment Temperature Concentration (“Cl of CHI (%I
Diameter
less than 1
-
100
900
100
393
2-3
30
2,O 18,0
100
2
23-15
20
100
430 36,0
31
28
30
20,o
23-l 5
20
10
490
30 1000
Figure 2 shows that the increase in concentration of methane leads also to the increase of the size of the globules. The same thing happens when the residence time of methane in the reaction zone is increased (see Fig. 3). To study the finer structure of the globules the method of gradual oxidation of pyrolytic carbon films was applied. The pyrocarbon films were deposited on artificial graphite at a temperature of 1200°C. The oxidation was carried out in oxygen for 1-23 min in an electric resistance furnace at 750°C. The oxidized surface of globular formations was examined under microscope at magnification up to 450 times. In case of oxidation of the films up to 14 min no noticeable destruction of the globular formations was observed. After oxidation of the films for more than 17 min [Fig. 4(a-c)] a finer structure of border-line regions around the large globular formations appeared. These regions as is evident from the figures, represent chains of fine globules. Their further growth apparently leads to consolidation of the pyrocarbon films. T~LE~.ELEM~TARYCO~POSI~~ON
The growth of bali-like formations is shown graphically on the scheme Fig. 5. The elementary composition and parameters of the crystal lattice
FIG. 5. Mechanism of growth of globular-conical ture of pyrolytic carbon.
ANDPA~M~E~OFCRYSTAL
LATTICE OF PYROLYTIC Parameters
Temperature of pyrolysis of CHa W
C
Height of ball-like protrusions
-
850
950
set)
Average size of globules, micron Radius of curvature
Elementary composition H
(%) Impurities
LCO A
LIZ, A
850
99,22
0,16
0,62
23
19
900 950
99,38 99,45
0,14 0,08
0,48 0,47
20 19
20 35
1000
99,62
0,07
0,31
18
45
struc-
CARBON
of crystal lattice Number of layers in the crystallite along the axis c
approx. 7
950°C
Iooo”c
1066°C
I 2oo”c
FIG. 1. Growth of globular formations for various temperatures of pyrolysis of methane. (Concentration of CH, lOO%, rate of passage CHt 0,04 m/set; magnification X200.
[facing p. 202
FIG. 2. Effect of concentration of methane on the size of globules. (Temperature of pyrolysis lOOO”C, magnification X 450).
u=O4 t/set f =02 set
u =O-01 t/set r=3
set
FIG. 3. Effect of residence time of molecules of methane in the reaction zone on the size of globules. (Temperature of pyrolysis 95O”C, magnification X 4.50).
OX THE FORMATIOS
AND CO~MPOSITION
were determined for films of pyrolytic carbon deposited at temperatures from 850 to 1000°C. The average data of 445 determinations are given in Table 2. If one calculates the elementary composition of a high-molecular hydrocarbon which could have been formed during condensation of ovaline and would have the same dimensions of the crystallites as the pyrolytic carbon, one finds that the content of hydrogen in such hydrocarbon is S-10 times higher than for the pyrolytic carbon obtained. This according to the authors view does not contradict the assumption that a part of pyrolytic carbon is present as free radicals. Ilut the basic mass of pyrolytic carbon apparently represent a mixture of hydrocarbons of aromatic nature having a very high molecular weight. 3. CONCLUSIONS
The formation of pyrolytic carbon occurs through a stage of formation of hydrocarbons having medium size of the molecules present in the molten state. These deposited intermediate cornpounds in their tendency to have a minimum
OF P1’ROI,YTlC
203
CARBON
surface energy acquire the form of drops in a very short time of their existence. Further development of this process leads to the formation of globular structure of pvrolytic carbon. It is believed that the main part of pyrolytic carbon is composed of a mixture of high-molecular hydrocarbons and their free radicals. I. 2, 3,
SL-\TEH w. WHEELIV~ TESNER
E:., y. I.
S.,
P.
A.
REFERENCES (‘Iwm. sot. 109, 160 (1916). 10, 175 (1931).
Puel and
Ecmrs~ou.~
X.
1 ,
Ilokl. Aknd.
.Vnuk. SSSR 37, 1029 (1952).
4.
LANETI. J. E. and EGI.OI.I,(G., Ind. Eyq. (‘hem. 9, 350 (1917). 5. Co?i~tou J. S., SLYSH R. S., &I~-RPHY 11. U. and KINNEY C. R., Proceedings of the Third Corzference on ~‘arhon, p. 395. Pergamon Press, Oxford (1959).
6.
GR1SDAI.E IT.,
7.
R.
o.,
I?WI.EH
A.
c.
and
v.4h’
~iOESBROEC!K
SystemTech. J. 30, 271 (1951).
FIALKOV M.
8,
Bell
A.
I. and
S.,
BAVER A.
R,WINOWCH
I., S.
SIDOHO~
M.,
.7.
XORIO!.
A.
i%. AI.,
1 :sprhi
CHAICUN
Himii 1, 132
(1965). I)AvID
C.,
Cm&m
2, 131 (1964).
SUELET
I’.,
md
R~,PPENEAI
J.,
1967, Vol. 5, pp. 201-203.
ON THE
Pergamon Press Ltd.
Printed in Great Britain
FORMATION
,4ND COMPOSITION
OF PYROLYTIC
CARBON
K. I. SYSKOV and E. I. JELIKHOVSKAYA U. I. Mendeleev Chemico-Technological
Institute,
Moscow,
U.S.S.R.
(Received 23 September 1966)
structure of globular formations of pyrolytic carbon at magnifications from 200 to 500 times are shown. The effect of the processes taking place in gaseous phase on the formation of globular structures of pyrocarbon has been ascertained. The size of the globules increases with the pyrolysis of methane, with the decrease of the rate of flow of the reaction gas and the increase of its concentration. The scheme for the growth of the globules is given. It is suggested that the pyrolytic carbon represents a mixture of hydrocarbons of aromatic nature having very high molecular weight, a part of which is present as free-radicals.
Abstract-The
1. INTRODUCTION
VARIOUS hypotheses have been put forward on the question of formation of pyrolytic carbon. Some authors(‘,3) consider that the decomposition of hydrocarbons on heated surfaces takes place as a direct decomposition of hydrocarbon molecules into carbon and hydrogen. Other investigators(4*5) suggest a mechanism as hydrogenation of hydrocarbons with the formation of complex polymer aromatic compounds. According to the opinion of GRISDALE and PFISTER’~) the process of hydrogenation proceeds in gaseous phase, the high-polymer compounds formed being deposited on the surface of the substrate. The process of formation of pyrolytic carbon has been discussed in a comprehensive review by FIALKOV, BAVER and others(‘) as splitting of hydrocarbons into radicals and into hydrogen atoms, all reacting with the red-hot surface and forming low-molecular hydrocarbons and their radicals on the surface. DAVID@) explains the occurrence of globules by the presence of “embryos” representing the smallest drops of more or less carbonized tars. Layers of oriented carbon are deposited on these drops and globules are formed. The authors think that the oriented carbon is a product of conversion of high-molecular compounds, which form the globules. Dehydrogenation and polycondensation cause the formation of hydrocarbon compounds of even higher molecular weight, of which pyrolytic
carbon is composed. The authors believe that decomposition of high-molecular compounds with the liberation of carbon is very unlikely. The present work is an attempt to find some answers to the above questions. 2. EXPERIMENTAL
AND RESULTS
Pyrolytic carbon was obtained by pyrolysis of methane in the temperature range 850”-1200°C and was deposited on a carbon surface. The temperature, the concentration and the residence time of methane in the reaction zone should affect the formation of molecular compounds which give rise to the globules. Therefore the size of the globular formations depending on these factors was studied. The data on the size of globules, determined with the help of microscope are presented in Table 1. It can be seen that the value of the radius of curvature and the height of the ball-like protrusions vary in proportion to each other. This seems to be an experimental proof of the fact that initial formation of pyrolytic carbon is in the form of drops. It means that formation of pyrolytic carbon passes through a stage of molten intermediate products a transition stage which might exist for a very short time. It is seen from Fig. 1 that the higher the temperature of pyrolysis, the bigger the size of the globules obtained. 201
K. I. SYSKOV
202
and E. I. JELIKHOVSKAYA
TABLE 1. SIZE OF GLOBULARFORMATIONS (Time of stay of CHrmolecules in reaction zone--O,8 Condition of experiment Temperature Concentration (“Cl of CHI (%I
Diameter
less than 1
-
100
900
100
393
2-3
30
2,O 18,0
100
2
23-15
20
100
430 36,0
31
28
30
20,o
23-l 5
20
10
490
30 1000
Figure 2 shows that the increase in concentration of methane leads also to the increase of the size of the globules. The same thing happens when the residence time of methane in the reaction zone is increased (see Fig. 3). To study the finer structure of the globules the method of gradual oxidation of pyrolytic carbon films was applied. The pyrocarbon films were deposited on artificial graphite at a temperature of 1200°C. The oxidation was carried out in oxygen for 1-23 min in an electric resistance furnace at 750°C. The oxidized surface of globular formations was examined under microscope at magnification up to 450 times. In case of oxidation of the films up to 14 min no noticeable destruction of the globular formations was observed. After oxidation of the films for more than 17 min [Fig. 4(a-c)] a finer structure of border-line regions around the large globular formations appeared. These regions as is evident from the figures, represent chains of fine globules. Their further growth apparently leads to consolidation of the pyrocarbon films. T~LE~.ELEM~TARYCO~POSI~~ON
The growth of bali-like formations is shown graphically on the scheme Fig. 5. The elementary composition and parameters of the crystal lattice
FIG. 5. Mechanism of growth of globular-conical ture of pyrolytic carbon.
ANDPA~M~E~OFCRYSTAL
LATTICE OF PYROLYTIC Parameters
Temperature of pyrolysis of CHa W
C
Height of ball-like protrusions
-
850
950
set)
Average size of globules, micron Radius of curvature
Elementary composition H
(%) Impurities
LCO A
LIZ, A
850
99,22
0,16
0,62
23
19
900 950
99,38 99,45
0,14 0,08
0,48 0,47
20 19
20 35
1000
99,62
0,07
0,31
18
45
struc-
CARBON
of crystal lattice Number of layers in the crystallite along the axis c
approx. 7
950°C
Iooo”c
1066°C
I 2oo”c
FIG. 1. Growth of globular formations for various temperatures of pyrolysis of methane. (Concentration of CH, lOO%, rate of passage CHt 0,04 m/set; magnification X200.
[facing p. 202
FIG. 2. Effect of concentration of methane on the size of globules. (Temperature of pyrolysis lOOO”C, magnification X 450).
u=O4 t/set f =02 set
u =O-01 t/set r=3
set
FIG. 3. Effect of residence time of molecules of methane in the reaction zone on the size of globules. (Temperature of pyrolysis 95O”C, magnification X 4.50).
OX THE FORMATIOS
AND CO~MPOSITION
were determined for films of pyrolytic carbon deposited at temperatures from 850 to 1000°C. The average data of 445 determinations are given in Table 2. If one calculates the elementary composition of a high-molecular hydrocarbon which could have been formed during condensation of ovaline and would have the same dimensions of the crystallites as the pyrolytic carbon, one finds that the content of hydrogen in such hydrocarbon is S-10 times higher than for the pyrolytic carbon obtained. This according to the authors view does not contradict the assumption that a part of pyrolytic carbon is present as free radicals. Ilut the basic mass of pyrolytic carbon apparently represent a mixture of hydrocarbons of aromatic nature having a very high molecular weight. 3. CONCLUSIONS
The formation of pyrolytic carbon occurs through a stage of formation of hydrocarbons having medium size of the molecules present in the molten state. These deposited intermediate cornpounds in their tendency to have a minimum
OF P1’ROI,YTlC
203
CARBON
surface energy acquire the form of drops in a very short time of their existence. Further development of this process leads to the formation of globular structure of pvrolytic carbon. It is believed that the main part of pyrolytic carbon is composed of a mixture of high-molecular hydrocarbons and their free radicals. I. 2, 3,
SL-\TEH w. WHEELIV~ TESNER
E:., y. I.
S.,
P.
A.
REFERENCES (‘Iwm. sot. 109, 160 (1916). 10, 175 (1931).
Puel and
Ecmrs~ou.~
X.
1 ,
Ilokl. Aknd.
.Vnuk. SSSR 37, 1029 (1952).
4.
LANETI. J. E. and EGI.OI.I,(G., Ind. Eyq. (‘hem. 9, 350 (1917). 5. Co?i~tou J. S., SLYSH R. S., &I~-RPHY 11. U. and KINNEY C. R., Proceedings of the Third Corzference on ~‘arhon, p. 395. Pergamon Press, Oxford (1959).
6.
GR1SDAI.E IT.,
7.
R.
o.,
I?WI.EH
A.
c.
and
v.4h’
~iOESBROEC!K
SystemTech. J. 30, 271 (1951).
FIALKOV M.
8,
Bell
A.
I. and
S.,
BAVER A.
R,WINOWCH
I., S.
SIDOHO~
M.,
.7.
XORIO!.
A.
i%. AI.,
1 :sprhi
CHAICUN
Himii 1, 132
(1965). I)AvID
C.,
Cm&m
2, 131 (1964).
SUELET
I’.,
md
R~,PPENEAI
J.,