Petroleum-Derived Carbons - American Chemical Society

23 Growth of Carbon Fibers in Stainless Steel Tubes by Natural Gas Pyrolysis Downloaded by EAST CAROLINA UNIV on December 13, 2017 | http://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch023

G. G. Tibbetts Physics Department, General Motors Research Laboratories, Warren, MI 48090-9055

Formation of uniform, macroscopic carbon fibers by pyrolysis of hydrocarbons occurs in two principal stages: 1) growth of long submicron filaments by the interaction of nanometer-sized transition metal par ticles with a decomposing hydrocarbon gas, and 2) thickening of the filaments to diameters on the order of micrometers by subsequent deposition of pyrolysis products. Stainless steel tubes provide a suitable environment for both of these processes. As the steel begins to carburize, its surface fragments to produce metal particles. These particles can catalyze filament growth because gas phase hydrocarbon concentrations are suitably low due to substantial absorption by the walls. Thickening of the filaments to macroscopic fibers takes place after the walls are saturated with carbon. The concentration in the gas phase increases markedly, depositing pyrolytic carbon on previously-grown filaments to thicken them to the required diameter. Uniform 10 µm fibers as long as 20 cm and of average modulus 1.8 x 10 Pa have been grown by this method. 11

In a 1953 study o f m a t e r i a l d e p o s i t e d on b l a s t f u r n a c e b r i c k w o r k , D a v i s e t a l . Q ) observed t h e presence o f t w i s t e d carbon f i l a m e n t s about 0.01 ym t h i c k . S i n c e t h e n , s i m i l a r m i c r o s c o p i c carbon f i l aments formed d u r i n g t h e d e c o m p o s i t i o n o f CO and h y d r o c a r b o n s have been observed by many o t h e r i n v e s t i g a t o r s . T y p i c a l l y , t h e s e f i l aments a r e formed by the d e c o m p o s i t i o n o f CO o r o t h e r gaseous h y d r o carbons on i r o n subgroup m e t a l c a t a l y s t p a r t i c l e s ( N i , C o , F e , and Cr) ( 2 ) . C o n s i d e r a b l y fewer r e p o r t s o f m a c r o s c o p i c carbon f i b e r s ( i . e . , exceeding 1 ym i n d i a m e t e r ) have been p u b l i s h e d . I l e y and R i l e y ( 3 ) (1948) grew v i s i b l e f i b e r s by decomposing methane, propane, and e t h y l e n e a t 1200°C on q u a r t z s u b s t r a t e s . H i l l e r t and Lang (4) grew a wide v a r i e t y o f g r a p h i t i c f i l a m e n t s , i n c l u d i n g h e l i c a l c o i l s , t w i s t e d t r i a d s , and branched s t r a i g h t t h r e a d s , by decomposing η-heptane i n a 0097-6156/86/0303-0335$06.00/0 © 1986 American Chemical Society

Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

336

PETROLEUM-DERIVED CARBONS

s i l i c a tube at 1000°C. Occasional straight f i b e r s with lengths up to 5 cm and diameters up to 200 ym were also observed. Koyama (5) and l a t e r Koyama and Endo (6) grew fibers by the thermal decomposition of benzene at about 1200°C. Most recently, Katsuki et a l . (7) reported f i b e r growth from the decomposition of napthalene-hydrogen mixtures. Methane, or rather natural gas (which may contain carbon oxides, higher hydrocarbons, and inert gases), i s of great interest as a source of p y r o l y t i c a l l y grown fibers because of i t s r e l a t i v e l y low cost.

Downloaded by EAST CAROLINA UNIV on December 13, 2017 | http://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch023

Fiber Growth General Requirements. At General Motors carbon f i b e r s were accident a l l y grown from the decomposition of natural gas in an apparatus designed to measure the d i f f u s i v i t y of carbon through steel tubes (8). In the apparatus of Figure 1, (a modification of one o r i g i n a l l y designed for diffusion studies by R. P. Smith (9)) the i n t e r i o r surface of a s t e e l tube was saturated with carbon from pyrolyzing gaseous hydrocarbons. Simultaneously, the exterior surface was continuously decarburized by wet hydrogen flowing through a jacket s u r rounding the tube. For a rather broad set of conditions of natural gas flow rate and temperature, i t was found that, after many hours, masses of fibers (Figure 2) grew within a 304 s t a i n l e s s s t e e l (18? C r , 8? Ni) tube of 0.8 mm wall thickness. Most of the experiments u t i l i z e d experimental conditions consisting of a temperature of 970°C, room temperature flow rate of 20 cc/min of natural gas (containing 1.8? ethane and 1.7? oxides of carbon), and a room temperature flow rate of 200 cc/min of wet H through the surrounding jacket. Figure 3 shows video images of f i b e r growth made through a window at the top of the growth tube; each dark c i r c l e defines the inner wall of the tube at a different time. Figure 3a was made after the growth tube had been exposed to the experimental conditions c i t e d above for 9.5 h. Very thin fibers f i r s t became v i s i b l e within 30 minutes (Figure 3b). These fibers continued to thicken with time and thus became more v i s i b l e as the experiment was concluded. Under some conditions one may observe a brightness increase within the volume of the growth tube during f i b e r growth. There are many references in the l i t e r a t u r e to a " f o g " of droplets produced in organic vapors under highly pyrolyzing conditions (K)). This increase in luminosity i s due to thermal radiation emitted or s c a t tered by these droplets. Figure 4 i s a plot of l i g h t i n t e n s i t y in the growth tubes from which the appearance of t h i s fog can be determined. Because the d i r e c t i o n of gas flow in the experiment of Figure 4 was toward the camera, the fog is more apparent than in F i g ure 3, where i t i s not d i s c e r n i b l e in the photographs. The tube i n t e r i o r remained dark for 10.5 h and then substantially brightened just as the fibers appeared. The brightness rapidly increased as the fibers continued to thicken u n t i l the experiment was terminated after 16 h. Also shown in Figure 4 i s the brightness measured within a 1010 s t e e l tube which did not grow fibers under otherwise i d e n t i c a l condit i o n s . No brightness increase was observed in such low-Cr mild s t e e l tubes, and no fibers were grown. In contrast to mild s t e e l , fused quartz tubes showed a high l e v e l of hydrocarbon fog during the entire 2


Carbon Fiber Growth in Stainless Steel Tubes


23. TIBBETTS

Figure 2.

Carbon fibers

produced from natural gas.


337


338


F i g u r e 3. M i c r o g r a p h s o f the i n t e r i o r o f an 11 mm i n s i d e diameter growth tube a t 970 C a f t e r a) 9 - 1 / 2 h , b) 10 h , c ) 1 0 - 1 / 2 h , d) 11 h , e) 1 1 - 1 / 2 h , f ) 12 h . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 8 . C o p y r i g h t 1983 American I n s t i t u t e o f P h y s i c s .

0

Growth Stops

Light Intensity

Gas 304 Stainless Steel 1010 3

7

_1_ 11 Time (h)

Steel

J_

15

F i g u r e 4 . L i g h t i n t e n s i t y i n t h e c e n t e r o f t h e growth tube ( o b t a i n e d by p h o t o - d i o d e measurement from t h e v i d e o t a p e d image) a s a f u n c t i o n o f time f o r 3 0 4 s t a i n l e s s and 1010 s t e e l .



23.

TIBBETTS


339

course of the experiment, yet no f i b e r s could be grown within fused quartz tubes under the same conditions of flow rate and temperature that were successfully used with s t a i n l e s s s t e e l tubes. Even when these tubes were provided with suitable nuclei for filament growth, no fibers could be grown. These observations suggested that the onset of the fog was related to the saturation of the walls of the growth tube by carbon. In order to elucidate t h i s effect i n 304 s t a i n l e s s s t e e l , a set of experiments was performed at 970°C and several d i f f e r e n t natural gas flow rates F to measure τ, the time elapsed before the onset of fogging. The r e s u l t s , plotted in Figure 5, show that τ i s as small as 3 h for rapid gas flow through the system and as long as 17 h for slow flow. The observed linear increase of τ with 1/F corresponds to the behavior expected i f a fixed concentration of carbon atoms [C] were required in order to saturate the s t e e l surface at time τ, [C] « χ . F

.

(1)

Measurements of the temperature dependence of τ yielded an a c t i v a t i o n energy consistent with t h i s p i c t u r e . Thus, the time τ required for the appearance of fogging corresponds to the time necessary to s a t urate the growth tube's surface with carbon. Pyrolysis Regimes. Results of a further experiment show how pyrolysis conditions change after time τ when the tube i n t e r i o r saturates. A number of thick 304 s t a i n l e s s s t e e l wires (0.31 mm in diameter) were supported in the furnace during a f i b e r growth exper iment and allowed to a drop out of the hot zone after d i f f e r e n t periods. Thus, the mass increase of these wires could be determined as a function of the length of time they remained in contact with pyrolyzing natural gas in the growth tube. The measurements were performed under standard conditions except that the flow rate of natural gas was 50 cnH/min. The mass of the wires (top panel of Figure 6) shows an i n i t i a l sharp r i s e as they carburize, followed by a much slower r i s e during the time required to carburize the growth tube of wall thickness 0.8 mm. In a separate experiment, i t was shown (Figure 6b) that, after 4.6 hours, fogging begins. At exactly that time, the mass of the s t a i n l e s s steel wires began to increase rapidly again, corresponding to deposition of an ever thickening layer of p y r o l y t i c carbon. It i s t h i s pyrocarbon deposition which thickens any fibers present within the growth tube after fogging begins. Measurements of the effluent gas from the growth reactor also show s i g n i f i c a n t changes when fogging begins. The effluent from a fused quartz tube showed l i t t l e change with time (Figure 7), in sharp contrast with the effluent from a 304 stainless s t e e l tube. The tubes u t i l i z e d in these experiments were from a d i f f e r e n t l o t of stainless steel which apparently had a somewhat more reactive surface and thus required only 2.7 h to carburize at a flow rate of 50 cnrVmin. During most of this period, the concentration of e t h y l ene and a l l higher hydrocarbons was below 0.1?. However, when fibers f i r s t became v i s i b l e at 2.7 h, the ethylene concentration had risen to h a l f i t s ultimate value. Higher hydrocarbons were beginning to be



340


Ol 0

ι

ι

I ι ι 3 1 / F (sec/cm3)

L 6

F i g u r e 5. Time r e q u i r e d t o grow v i s i b l e carbon f i b e r s u s i n g a f r e s h 0.9 mm w a l l 304 s t a i n l e s s s t e e l growth tube a t 970°C, p l o t t e d as a f u n c t i o n o f r e c i p r o c a l n a t u r a l gas f l o w r a t e .

30

Time (h)

F i g u r e 6. T o p : Mass i n c r e a s e o f s t a i n l e s s s t e e l w i r e s as a function of p y r o l y s i s time. Bottom: L u m i n o s i t y as a f u n c t i o n o f time. A r b i t r a r y u n i t s are used.


23.

TIBBETTS


341


produced in the tube in abundance; from them the complex molecules which produce pyrocarbon were beginning to appear. The conditions appropriate for growing fibers from natural gas may not be successful with other hydrocarbon feedstocks. Using pure ethane during a standard growth run produces merely a tarry sludge. At the other extreme, pure methane w i l l neither grow f i b e r s nor deposit a pyrocarbon layer under the standard conditions at 970°C, but i t can grow fibers near 1100°C. Formation of Catalyst P a r t i c l e s by Surface Fragmentation. The phenomenon of "metal dusting corrosion" by which s t a i n l e s s s t e e l surfaces are fragmented to produce submicron c a t a l y t i c p a r t i c l e s for fiber growth has been described by Bradley (_!_]_). In t h i s process, the strongly reducing and carburizing atmosphere in the growth tube severely corrodes the natural oxide surface of the s t a i n l e s s s t e e l to produce a fine dust of metal, carbide, and oxide p a r t i c l e s . Some of the smaller p a r t i c l e s can then act as nuclei for f i b e r growth. The role of the hydrogen jacket in f i b e r growth w i l l now be discussed. Gas chromatographic studies have shown that s u f f i c i e n t hydrogen from the jacket diffuses through the s t a i n l e s s steel to increase the concentration of hydrogen i n the effluent gas by 2% under standard conditions. In the v i c i n i t y of the s t e e l walls, where the filaments grown, the increase i s even greater. Figure 8 shows the influence of hydrogen jacket pressure on fiber growth. A series of experiments were performed, each under standard conditions but with a different H jacket pressure. The ordinate i s Nc, the number of d i s t i n c t fibers in focus s l i g h t l y below the midplane of the f u r nace 3 hours after fibers f i r s t became v i s i b l e — a quantity that i s proportional to t o t a l fiber y i e l d . It i s clear from these data that the hydrogen in the jacket aids in producing f i b e r s . The top curve refers to growth experiments in which the inside of the tube i s seeded with Fe(NOg)g to provide nuclei suitable for filament growth. In this case only a very weak dependence on hydrogen pressure i s noted. These data imply that wet hydrogen from the jacket aids i n promoting the production of adequate m e t a l l i c nuclei from the s t a i n less s t e e l surface. Furthermore, i f adequate nuclei are already present, the hydrogen jacket i s not useful i n the remainder of the growth process. 2

Fiber Morphology and Properties As Figure 9a shows, the fibers grow nearly p a r a l l e l to each other; they appear to grow approximately along the gas streamlines. The fibers are attached by one end to a laminar pyrocarbon deposit which adheres to the tube w a l l . The "cobbled" appearance of t h i s layer may originate from shorter fibers which have been buried by pyrocarbon. The pyrocarbon deposit i s about 0.3? hydrogen by weight, and contains p a r t i c l e s of a l l the metallic constituents of the tube. Figure 9b shows that the fibers are quite uniform i n diameter. Fibers as long as 20 cm and having diameters up to nearly 1 mm have been produced. Thickening of the fibers by chemical vapor deposition gives them an annular structure (Figure 10a), in contrast to the radial structure of a Thornel Ρ fiber (Figure 10b). The average modulus of the fibers i s 180 GPa (26.1 Mpsi), although peak values of up to 450 GPa (65.3 Mpsi) have been measured


342


100%r

_

Λ

Quartz 304

10.0 1-

CH.

Fibers Visible

a Quartz ο 304 Stainless Steel

Effluent Concentration


(%)

Quartz

Time (h)

F i g u r e 7. CHjj and C H^ e f f l u e n t o b t a i n e d a f t e r p y r o l y z i n g n a t u r a l gas a t 970°C and 5 sec r e s i d e n c e time i n a 304 s t r a i n l e s s s t e e l o r quartz reactor. F i b e r s appeared i n t h e s t a i n l e s s s t e e l r e a c t o r a f t e r 2.8 h . 2

( ) P

1 / 2

1

(Atm) /2

F i g u r e 8. Number o f f i b e r s counted i n t h e camera's f o c a l p l a n e 2.5 cm below t h e f u r n a c e c e n t e r l i n e a f t e r a number o f s t a n d a r d growth e x p e r i m e n t s where the hydrogen j a c k e t p r e s s u r e was h e l d c o n s t a n t a t the v a l u e s shown. Bottom c u r v e r e f e r s t o a s e r i e s o f 304 t u b e s . In t h e top curve 304 tubes w h i c h had p r e v i o u s l y grown f i b e r s and whose s u r f a c e s were t r e a t e d w i t h Fe(NO^)^ were u s e d .


TIBBETTS


343


23.

Figure 9. Scanning electron micrograph of a) fibers growing from matrix layer, b) a bundle of f i b e r s . Copyright 1983 American Institute of Physics.


344



F i g u r e 10. S c a n n i n g e l e c t r o n m i c r o g r a p h s n a t u r a l gas and b) T h o r n e l Ρ f i b e r s .

o f broken ends o f


a)

23.

TIBBETTS


345

f o r f i b e r s s l i g h t l y s m a l l e r than 10 ym i n d i a m e t e r . Fiber tensile s t r e n g t h s average 1.0 GPa (145 k p s i ) . X - r a y d i f f r a c t i o n s t u d i e s show t h a t the AQQ? s p a c i n g f o r f i b e r s grown a t 970°C i s 0.345 nm, compared to 0.335 nm f o r c r y s t a l l i n e g r a p h i t e .


Conclusions S e v e r a l f a c t o r s c o n t r i b u t e t o the growth o f carbon f i b e r s from n a t u r a l gas i n 304 s t a i n l e s s s t e e l tubes surrounded by a j a c k e t c o n t a i n i n g c i r c u l a t i n g wet h y d r o g e n . F i r s t , the c a r b u r i z i n g s t a i n l e s s s t e e l s u r f a c e fragments t o produce m e t a l p a r t i c l e s s u i t a b l e f o r c a t a l y z i n g f i l a m e n t g r o w t h , p a r t i c u l a r l y when hydrogen d i f f u s e s through i n l a r g e q u a n t i t i e s . Second, d u r i n g c a r b u r i z a t i o n the s t a i n l e s s s t e e l absorbs enough hydrocarbons from the gas phase t o p r o v i d e an atmosphere s u f f i c i e n t l y d e p l e t e d i n hydrocarbons t o be s u i t a b l e f o r growth o f m i c r o s c o p i c f i l a m e n t s . T h i r d , a f t e r t h e s t a i n l e s s s t e e l i s s a t u r a t e d , the h y d r o c a r b o n c o n c e n t r a t i o n c l i m b s so t h a t the f i b e r s may be t h i c k e n e d by d e p o s i t i o n o f p y r o c a r b o n . Acknowledgments I would l i k e t o acknowledge h e l p f u l d i s c u s s i o n s w i t h J . R. B r a d l e y , G. W. S m i t h , and W. E . Y e t t e r . C. P . B e e t z , J r . and G. W. Budd made the modulus and s t r e n g t h measurements. I would l i k e t o thank M. G. Devour f o r s k i l l f u l e x p e r i m e n t a l h e l p .

Literature Cited 1.

Davis, W. R.; Slawson, R. J.; Rigby, G. R. Nature 1953, 171, 756. 2. A good recent review i s : Baker, R. T. K . ; Harris, P. S. Chemistry and Physics of Carbon, Walker, P. L.; Thrower, P. Α., Eds.; Dekker: New York, 1978; vol. 14, p. 83. 3. Iley, R.; Riley, H. L. Jour. Chem. Soc. London, II 1948, 1362. 4. H i l l e r t , M.; Lang, Ν Zeit Krist. 1958, 111, 24. 5. Koyama, T. Carbon 1972, 10, 757. 6. Koyama, T.; Endo, M. Oyo Buturi 1973, 42, 690 (in Japanese). 7. Katsuki, H . ; Matsunaga, K . ; Egashira, M.; Kawasumi, S. Carbon 1981, 19, 148. 8. Tibbetts, G. G. Appl. Phys. Lett. 1983, 42, 666. 9. Smith, R. P. Acta. Meta. 1953, 1, 578. 10. Lahaye, J.; Prado, G; Donnet, J . B. Carbon 1974, 12, 27. 11. Bradley, J . R. Extended Abstracts, 16th Conf. on Carbon 1983, 533. RECEIVED

April 24,

1985


Petroleum-Derived Carbons - American Chemical Society

Recommend Documents