Carbon-Carbon Composites - ACS Symposium Series (ACS

25 Carbon-Carbon Composites Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 14, 2018 | https://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch025

Matrix Microstructure and Its Possible Influence on Physical Properties

R.A.Meyer andS.R.Gyetvay Materials Sciences Laboratory, The Aerospace Corporation, P.O. Box 92957, Los Angeles, CA 90009 Processing conditions required to attain desirable composite properties can be defined more easily i f the factors controlling composite microstructure are understood. Such factors include type of pre cursors employed and the composite's processing history. The microstructure of the matrix may contribute to the performance of the fibers and influence the properties of the composite. Re viewed are experiments to determine matrix micro structural features, how microstructural variations are achieved, and ways in which thermal expansion and fracture behavior relate to microstructure. C a r b o n - c a r b o n ( C - C ) composites w i t h a v a r i e t y o f unique p r o p e r t i e s can be f a b r i c a t e d by a l t e r i n g t h e combinations o f t h e type and d i s t r i b u t i o n o f f i l a m e n t s and t h e bonding m a t r i x u s e d . Many e n g i n e e r i n g a p p l i c a t i o n s can be s a t i s f i e d w i t h a composite m a t e r i a l whose d e n s i t y i s j u s t 70% t h a t o f aluminum and 25% t h a t o f s t e e l , but f o r which t h e s p e c i f i c s t r e n g t h and s t i f f n e s s v a l u e s a r e f o u r or f i v e times those o f s t e e l . To a t t a i n the d e s i r e d p r o p e r t i e s f o r such a p p l i c a t i o n s r e q u i r e s an u n d e r s t a n d i n g o f t h e i n t e r r e l a t i o n s h i p o f t h e f i b e r s and the m a t r i x t h a t h o l d s them. The newest advances i n C-C composite m a t e r i a l s have r e s u l t e d from f i b e r improvements, such as modulus i n c r e a s e s o f two t o t h r e e times and diameter decreases of more than 50% i n t h e l a s t t e n years. Weaving t e c h n i q u e s have a l s o been i m p r o v e d , so t h a t i n c r e a s e d f i b e r content and r e p r o d u c i b l e d i s t r i b u t i o n of f i l a m e n t s and yarns (down t o 0.75 mm c e n t e r - t o - c e n t e r s p a c i n g between y a r n s i n 3D c o m p o s i t e s ) a r e p o s s i b l e . However, because t h e i n t e r r e l a t i o n s h i p of t h e f i l a m e n t s and y a r n s w i t h t h e bonding m a t r i x i s n o t w e l l u n d e r s t o o d , t h e improved f i b e r p r o p e r t i e s have n o t y e t been f u l l y t r a n s l a t e d i n t o a c o r r e s p o n d i n g magnitude o f improvement i n C-C c o m p o s i t e s . T h i s paper i s i n t e n d e d t o s t i m u l a t e more i n t e n s e study o f t h e m a t r i x ' s i n f l u e n c e on the b e h a v i o r o f C-C c o m p o s i t e s . Experimental 0097-6156/86/0303-0380$06.00/0 © 1986 American Chemical Society

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

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o b s e r v a t i o n s are p r e s e n t e d t o e x e m p l i f y how the m a t r i x and i t s m i c r o s t r u c t u r e may c o n t r i b u t e t o v a r i a t i o n s i n some p h y s i c a l p r o p e r t i e s ; reasons f o r those v a r i a t i o n s are a l s o p r o p o s e d .

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 14, 2018 | https://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch025

Background A s t r u c t u r e woven of carbon f i b e r (a preform) can be d e n s i f i e d i n a number of ways t h a t w i l l r e s u l t i n d i f f e r e n t types of m a t r i x m i c r o structures. The two u s u a l d e n s i f i c a t i o n methods are c h e m i c a l vapor d e p o s i t i o n (CVD) and i m p r e g n a t i o n w i t h p i t c h or r e s i n i n the l i q u i d state. The c h o i c e of d e n s i f i e r can determine whether the m a t r i x w i l l be g r a p h i t i z a b l e . P i t c h e s and CVD are c o n s i d e r e d g r a p h i t i z a b l e , whereas most r e s i n s do not g r a p h i t i z e except under g r e a t stress. Composites can a l s o be made u s i n g c l o t h t h a t has been "prepregged" with a resin, then l a i d up i n t o the desired configuration. A f t e r a s e r i e s of f o r m i n g s t e p s , the r e s i n - c l o t h layup is carbonized, possibly g r a p h i t i z e d , and then f u r t h e r d e n s i f i e d by CVD or l i q u i d - i m p r e g n a t i o n methods. The m i c r o s t r u c t u r e of the m a t r i x depends to a l a r g e e x t e n t on the composite c o n f i g u r a t i o n and p r o c e s s i n g c o n d i t i o n s . I d e n t i f y i n g those f a c t o r s t h a t i n f l u e n c e the m a t r i x p r o p e r t i e s s h o u l d a i d i n d e f i n i n g the c o n d i t i o n s n e c e s s a r y to a t t a i n d e s i r e d composite p r o p e r t i e s . S t u d i e s reviewed by F i t z e r and G k o g k i d i s (1) on composite samples c a r b o n i z e d to 1000°C i n d i c a t e t h a t c a r b o n i z i n g the m a t r i x c o n t r i b u t e s to h i g h s t r e n g t h and low s t r a i n , changes the f r a c t u r e b e h a v i o r , and enhances the e f f e c t i v e f i b e r s t r e n g t h . In c o n t r a s t , lower composite s t r e n g t h s , h i g h e r s t r a i n s t o f a i l u r e , and lower u t i l i z a t i o n of f i b e r s t r e n g t h s have been observed a f t e r the m a t r i x is t r a n s f o r m e d to a more g r a p h i t i c s t a t e by a d d i t i o n a l h i g h temperature heat treatment ( H T ) . I n comparative f l e x u r e tests performed on s m a l l 2D f l e x u r e samples b e f o r e and a f t e r HT ( 2 ) , we have observed decreases of the l o a d t o f a i l u r e of the order of 15%; however, the s t r a i n - t o - f a i l u r e v a l u e s were i n c r e a s e d by 100%. E v a n g e l i d e s has noted t h a t the more g r a p h i t i c m a t r i x ( d e r i v e d from c o a l t a r p i t c h ) i n composites p r e f e r s to f r a c t u r e a l o n g the weaker d i r e c t i o n p a r a l l e l to the a-b p l a n e s ( 3 ) . This f r a c t u r i n g e f f e c t i s r e a s o n a b l e because the m e c h a n i c a l p r o p e r t i e s of c r y s t a l l i n e g r a p h i t e (4_) are h i g h l y a n i s o t r o p i c ; those measured i n the d i r e c t i o n of the b a s a l l a y e r p l a n e s ( " a - d i r e c t i o n " ) v a r y by o r d e r s of magnitude from the p r o p e r t i e s measured a c r o s s the layers ("c-direction"). The a / c a n i s o t r o p y r a t i o s of t e n s i l e s t r e n g t h and modulus are about 20 and 150, r e s p e c t i v e l y ( 5 ) . The b a s a l l a y e r p l a n e s are h e l d t o g e t h e r i n the c - d i r e c t i o n by weak Van der Waals forces. C o n s e q u e n t l y , the l a y e r p l a n e s are r e a d i l y d i s p l a c e d by shear f o r c e s p a r a l l e l t o the a - d i r e c t i o n and e a s i l y s e p a r a t e d or c r a c k e d by t e n s i l e f o r c e s i n the c - d i r e c t i o n , and the p r e f e r r e d d i r e c t i o n f o r f r a c t u r e l i e s i n the a-b p l a n e . Therefore, the f r a c t u r e p a t h i n the m a t r i x depends on the l a y e r o r i e n t a t i o n on the microstructural scale. If the p r o p e r t i e s of c a r b o n - c a r b o n composites are i n p a r t determined by the p r o p e r t i e s of the m a t r i x , then a l t e r i n g the m a t r i x and o b s e r v i n g the r e s u l t i n g changes i n the composite's t h e r m a l - m e c h a n i c a l p r o p e r t i e s would be i m p o r t a n t f o r u n d e r s t a n d i n g the b e h a v i o r of C-C c o m p o s i t e s . A f i r s t step toward t h i s g o a l i s



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t o c h a r a c t e r i z e the m a t r i x i n the y a r n bundles and the preform p o c k e t s w i t h r e s p e c t to i t s unique f r a c t u r e and t h e r m a l e x p a n s i o n characteristics· The m a t r i x pockets w i t h i n a C-C composite are f i l l e d w i t h variously oriented g r a i n s whose f r a c t u r e characteristics are comparable to those of b u l k g r a p h i t e . P r e v i o u s s t u d i e s have shown t h a t the s i z e , o r i e n t a t i o n , and d i s t r i b u t i o n of c r y s t a l l i t e s i n the g r a i n s w i t h i n a volume of b u l k g r a p h i t e determine i t s s t r a i n and s t r e n g t h p r o p e r t i e s , because the mode of f r a c t u r e i s c o n t r o l l e d by the m i c r o c r a c k i n g of the i n d i v i d u a l g r a i n s and the r e d i r e c t i o n of c r a c k s by the b a s a l p l a n e o r i e n t a t i o n i n a d j a c e n t g r a i n s and by pore d i s t r i b u t i o n s . V e r i f i c a t i o n of t h i s concept was made f o r b u l k g r a p h i t e by a m i c r o m e c h a n i c a l model t h a t uses m i c r o s t r u c t u r a l i n f o r m a t i o n to p r e d i c t the s t r e s s - s t r a i n c h a r a c t e r i s t i c s of g r a p h i t e (type ATJ-S) ( 6 ) . The same approach s h o u l d be u s e f u l to approximate the performance of the m a t r i x i n p o c k e t s i n C-C com p o s i t e s , p r o v i d e d d e t a i l s of the m i c r o s t r u c t u r e , e s p e c i a l l y the c r y s t a l l i n i t y of the m a t r i x , have been d e t e r m i n e d . S e v e r a l procedures a r e a v a i l a b l e to d e f i n e the degree of c r y s t a l l i n i t y of carbon and the o r i e n t a t i o n of l a y e r p l a n e s i n the matrix. X - r a y d i f f r a c t i o n can be used to measure the b a s a l l a y e r s p a c i n g , but f i b e r cannot be d i s t i n g u i s h e d from m a t r i x i n the c o m p o s i t e s . The o r i e n t a t i o n and a l i g n m e n t of b a s a l p l a n e s i n the m a t r i x of p o l i s h e d samples of C-C composites can be mapped by p o l a r i z e d microscopy ( 7 , 8 ) . S i m i l a r i n d i c a t i o n s can be o b t a i n e d by c a t h o d i c a l l y e t c h i n g the p o l i s h e d samples w i t h i n e r t g a s , which r e v e a l s the l a m e l l a r s t r u c t u r e by s p u t t e r i n g o f f carbon atoms i n lower-density regions. The l a m e l l a e are p a c k e t s of s t a c k s of nearly p a r a l l e l basal layer planes. Lamellar structure i s relevant to the d i r e c t i o n of c r a c k p r o p a g a t i o n ( 9 ) . The h i g h l y a n i s o t r o p i c c h a r a c t e r i s t i c s of the g r a p h i t i c m i c r o s t r u c t u r e p e r m i t HT, i n a d d i t i o n to changing the f r a c t u r e b e h a v i o r , to a l t e r the t h e r m a l e x p a n s i o n . In s i n g l e - c r y s t a l form, the e x p a n s i v i t y i s a p p r o x i m a t e l y 25 * 10~ °C'" i n the c - d i r e c t i o n and 0.5 x 10~"" C~* i n the a - d i r e c t i o n , g i v i n g an a n i s o t r o p y factor, c / a , of about 50 to 60. T h e r e f o r e , w i t h such a h i g h degree of a n i s o t r o p y , the expansion of the m a t r i x w i t h i n a pocket between y a r n s or between f i b e r s would be expected to depend on the d i s t r i b u t i o n and s i z e of g r a i n s , t h e i r o r i e n t a t i o n s , and the v o i d s t r u c t u r e s u r r o u n d i n g the g r a i n s . 6

1

o

Matrix Microstructures The m a t r i x m i c r o s t r u c t u r e can be i n t e n t i o n a l l y v a r i e d by m a n i p u l a t i n g such f a c t o r s as f i b e r s p a c i n g , h e a t - t r e a t m e n t r a t e and t e m p e r a t u r e , m a t r i x p r e c u r s o r , and p r o c e s s i n g p r e s s u r e . From m i c r o s c o p i c e x a m i n a t i o n s , a number of i n f e r e n c e s about the importance of the d i s t a n c e of s p a c i n g between f i b e r s have been drawn. O p t i c a l m i c r o scopy has r e v e a l e d , f o r example, t h a t w i t h p i t c h i m p r e g n a t i o n p r o c e s s i n g at ambient p r e s s u r e , the m a t r i x a l i g n s i n sheaths around f i l a m e n t s , even below g r a p h i t i z a t i o n t e m p e r a t u r e s . The a l i g n m e n t of the l a m e l l a e as r e v e a l e d by i o n e t c h i n g i n d i c a t e s t h a t l a r g e molecules of p i t c h p r e f e r e n t i a l l y o r i e n t themselves p a r a l l e l t o any surface (Figure l a ) . I t has been r e p o r t e d t h a t as the s p a c i n g



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between f i b e r s i n c r e a s e s , the alignment or " s h e a t h " e f f e c t around the f i b e r s d i m i n i s h e s and the m a t r i x can c o n t a i n d i s c l i n a t i o n s and various grain orientations. The p r e f e r r e d o r i e n t a t i o n of the m a t r i x p a r a l l e l to the f i b e r bundles appears to be a f f e c t e d by p r o c e s s i n g pressure during pyrolysis (400-500°C), becoming more random as impregnation p r e s s u r e s are i n c r e a s e d . Even t r a n s v e r s e l a m e l l a r o r i e n t a t i o n s can r e s u l t (Figure l b ) . Because p i t c h mesophase c o a l e s c e n c e has been i n h i b i t e d by the p r e s s u r e , g r a i n s of m a t r i x may be t r a n s v e r s e l y o r i e n t e d to the f i l a m e n t s . Heat treatment above 2000°C i s known to i n c r e a s e the degree of g r a p h i t i z a t i o n , e . g . , as evidenced by the f o r m a t i o n of the l a m e l lae. A t around 2000 to 2500°C, the frequency of l a m e l l a e i n c r e a s e s as the degree of g r a p h i t i z a t i o n i n c r e a s e s . As F i g u r e 2 i l l u s t r a t e s f o r i s o t h e r m a l HT, when the time i s i n c r e a s e d , the boundaries (lines) between lamellae become more d i s t i n c t and numerous. F u r t h e r m o r e , the l a m e l l a e tend t o s t r a i g h t e n o u t , then b u c k l e , c a u s i n g the m i c r o s t r u c t u r e to appear segmented or p o l y g o n i z e d ( 1 0 ) . X - r a y d e t e r m i n a t i o n s of the s p a c i n g between b a s a l l a y e r p l a n e s show d e c r e a s i n g v a l u e s as the degree of g r a p h i t i z a t i o n i s i n c r e a s e d . From h i g h - m a g n i f i c a t i o n o b s e r v a t i o n s w i t h the s c a n n i n g e l e c t r o n microscope (SEM), these boundaries do not appear to be c r a c k s ( e . g . , Mrozowski c r a c k s ) . I t i s t h o u g h t , a l t h o u g h not c o n c l u s i v e l y p r o v e n , t h a t such boundaries are the r e s u l t of i n t e r s t i t i a l carbon atoms and d e f e c t s d i f f u s i n g to these areas as the b a s a l l a y e r s between the boundaries become more o r d e r e d and g r a p h i t i c . Longrange l a m e l l a r alignment i n m a t r i x g r a i n s p e r m i t s the f o r m a t i o n of l a r g e r c r y s t a l l i t e s than f o r the more randomly o r i e n t e d or d i s c l i n ated shorter l a m e l l a r s t r u c t u r e s . I n c o n t r a s t , u n g r a p h i t i z e d m a t r i x or f i b e r t h a t has o n l y been c a r b o n i z e d (HT < 1900°C) does not e x h i b i t s i g n i f i c a n t l a m e l l a r m i c r o s t r u c t u r e . X - r a y s t u d i e s c o n f i r m the d i f f e r e n t degrees of c r y s t a l l o g r a p h i c o r d e r of the c a r b o n i z e d and g r a p h i t i z e d m a t r i x . The degree to which g r a p h i t i c p e r f e c t i o n can be a t t a i n e d i s l i m i t e d by the morphology of the m a t r i x d u r i n g p y r o l y s i s , because the a l i g n m e n t of the g r a i n s i s set i n the m a t r i x when i t i s i n the liquid crystal state. But t h i s alignment i s not transformed i n t o crystalline structure unless sufficient time is provided at temperatures >2000°C. The p r e c u r s o r used f o r i n f i l t r a t i o n can a l s o i n f l u e n c e the microstructure that i s generated. G e n e r a l l y , r e s i n s do not g r a p h i t i z e u n l e s s s u b j e c t e d to s t r e s s and h i g h - t e m p e r a t u r e heat treatment (11 ) . P i t c h p r e c u r s o r s , which are r e a d i l y g r a p h i t i z e d , appear to have t h e i r g r a i n s i z e reduced by an i n c r e a s e i n q u a n t i t y of q u i n o l i n e - i n s o l u b l e p a r t i c l e s (_7_), by heat t r e a t i n g the p i t c h p r i o r to i n f i l t r a t i o n , or by the use of h i g h p r o c e s s i n g p r e s s u r e s d u r i n g the i m p r e g n a t i o n c y c l e . V a r i a t i o n s of P h y s i c a l P r o p e r t i e s by A l t e r i n g the M i c r o s t r u c t u r e The exact r o l e of m a t r i x m i c r o s t r u c t u r e i n d e t e r m i n i n g the p h y s i c a l p r o p e r t i e s of C-C composites i s d i f f i c u l t to determine p r e c i s e l y because the m i c r o s t r u c t u r a l v a r i a t i o n s and f i b e r - m a t r i x i n t e r a c t i o n s are h i g h l y complex. Some i n s i g h t i n t o the importance of



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F i g u r e 1. Scanning e l e c t r o n m i c r o g r a p h s of (a) s h e a t h of a l i g n e d m a t r i x produced by l o w - p r e s s u r e i m p r e g n a t i o n , and (b) transversely oriented matrix produced by high-pressure impregnation.

Figure 2. Scanning electron micrographs of bulk matrix g r a p h i t i z e d a t 2400°C f o r (a) 2100°C to 2500°C o c c u r s , which i s a t t r i b u t e d to c r y s t a l l i z a t i o n of the m a t r i x i n t o more g r a p h i t i c s t r u c t u r e s whose o r i e n t a t i o n i s p a r a l l e l t o the u n i d i r e c t i o n a l f i b e r s and thereby i n c r e a s e s t h e i r e f f e c t i v e s t i f f n e s s or dynamic modulus. I f the m a t r i x can become more g r a p h i t i c , as the above examples i n d i c a t e , more shear p l a n e s become a v a i l a b l e ; hence more m i c r o c r a c k i n g can o c c u r , r e s u l t i n g i n g r e a t e r s t r a i n at lower s t r e s s levels. Thus the apparent or e f f e c t i v e modulus of the 2D composite materials is reduced, and more energy i s r e q u i r e d to cause f a i l u r e — a n outcome i n d i c a t e d by the d i f f e r e n c e of a r e a under the a s - r e c e i v e d and h e a t - t r e a t e d l o a d - d e f l e c t i o n curves i n F i g u r e s 7 and 9. Summation Examples have been p r e s e n t e d t h a t suggest the f i b e r s and m a t r i x have an i n t e r r e l a t i o n s h i p t h a t appears to have i n f l u e n c e d the t h e r m a l expansion and m e c h a n i c a l p r o p e r t i e s of s e v e r a l types of C-C composites. The e f f e c t s can be a t t r i b u t e d to the d e n s i f i c a t i o n methods, p r e c u r s o r s , and subsequent heat t r e a t m e n t s , which a l t e r the c r y s t a l l i n i t y of the r e s u l t i n g m a t r i x and i t s c o n t a i n e d v o i d age, i n c l u d i n g p o r e s , gaps, and c r a c k s . The e x t e n t and d i s t r i b u t i o n of c r y s t a l l i n i t y of the m a t r i x appear to be i m p o r t a n t because of the v e r y a n i s o t r o p i c p r o p e r t i e s of the g r a p h i t e c r y s t a l . T h e r e f o r e , i f the degree and d i s t r i b u t i o n of c r y s t a l l i n i t y can be c o n t r o l l e d , the t h e r m a l e x p a n s i v i t y and m e c h a n i c a l c h a r a c t e r i s t i c s of the m a t r i x can be changed. Such changes w i l l , i n t u r n , i n t e r a c t w i t h the yarns by s t r e s s i n g or i n i t i a t i n g and p r o p a g a t i n g m a t r i x c r a c k i n g around them. The m e c h a n i c a l response of c o m p o s i t e s , as shown i n these e x p l o r a t o r y s t u d i e s , i n d i c a t e s dependence on the ease w i t h which f r a c t u r e can occur between f i b e r s , y a r n s , and p l i e s . Poorly c r y s t a l l i z e d m a t r i c e s r e s u l t i n composites t h a t a r e s t r o n g and s t i f f but w i t h l i t t l e y i e l d so t h a t f a i l u r e o c c u r s c a t a s t r o p h i cally. In c o n t r a s t , more c r y s t a l l i n e m a t r i c e s seem to be not q u i t e as s t r o n g and to have a lower e f f e c t i v e modulus, but t h e i r i n c r e a s e d s t r a i n c a p a b i l i t y ensures t h a t f a i l u r e i s not c a t a s t r o p h i c ; the c o m p o s i t e ' s s t r e n g t h decays g r a d u a l l y as f u r t h e r s t r a i n i s a p p l i e d . Thus, the energy r e q u i r e d f o r t o t a l f a i l u r e i s i n c r e a s e d , and the composite w i t h more c r y s t a l l i n e m a t r i x i s more t o l e r a n t of d e f e c t s or s t r e s s r i s e r s . F i n a l l y , we note t h a t v o i d s , c r a c k s , and gaps can absorb t h e r m a l e x p a n s i o n and p r o v i d e s i t e s a t w h i c h f r a c t u r e may be i n i t i a t e d or means by which the p r o p a g a t i o n of c r a c k s may be interrupted.


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P r o c e s s i n g c o n d i t i o n s have been i m p l i c a t e d as c o n t r i b u t o r s t o some v a r i a t i o n s i n p h y s i c a l p r o p e r t i e s . P o s s i b l e e x p l a n a t i o n s have been proposed i n o r d e r t o s t i m u l a t e f u r t h e r research. More d e t a i l e d i n f o r m a t i o n must be o b t a i n e d b e f o r e the c a u s e - e f f e c t r e l a t i o n s h i p s can be more f u l l y u n d e r s t o o d .


Acknowledgment We e x p r e s s a p p r e c i a t i o n t o our c o l l e a g u e s , M . B u e c h l e r , J . E v a n g e l i d e s , and J . L . W h i t e , f o r t h e i r h e l p f u l c o n t r i b u t i o n s and d i s c u s s i o n s about t h i s p a p e r .

S. to

Literature Cited 1. 2. 3.

Fitzer, E . ; Gkogkidis, A. This volume. Buechler, M.; Meyer, R. A. Unpublished results. Evangelides, J . S. "Microstructure and Fracture of Carbon– Carbon Composites"; TOR-0075(5626)-2; The Aerospace Corp.: El Segundo, California, 1974. 4. Reynolds, W. N. In "Physical Properties of Graphite"; Elsevier Publishing Co.: New York, New York, 1968; p. 1. 5. Ibid.; p. 33. 6. Meyer, R. Α.; Zimmer, J . E. "Final Report, Failure Criteria in Graphite"; ATR-74(7425)-3; The Aerospace Corp.: E l Segundo, California, 1974. 7. Dubois, J.; Agace, C.; White, J . L. Metallography 1970, 3, 337. 8. Zimmer, J . E . ; White, J . L. Carbon 1983, 21, 323. 9. Meyer, R. Α.; Zimmer, J. E.; Almon, M. C. "Micromechanics of Failure in Carbon System"; ATR-74(7408)-2; The Aerospace Corp.: E l Segundo, California, 1974; p. 21. 10. Meyer, R. Α.; Gyetvay, S. R.; Chase, A. B. Ext. Abstr., 16th Conf. Carbon, 1983, p. 505. 11. Hishiyama, Y . ; Inagaki, M.; Kimura, S.; Yamada, S. Carbon 1974, 12, 249. 12. Feldman, L. A. Ext. Abstr., 16th Conf. Carbon, 1983, p. 499. 13. Feldman, L. Α.; Gyetvay, S. R.; Meyer, R. A. Ext. Abstr., 17th Conf. Carbon, 1985, p. 385. RECEIVED December 12, 1985


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