Formation of CarbonCarbon Composite Materials by Pyrolytic Infiltration

17 Formation o f C a r b o n - C a r b o n Composite Materials by Pyrolytic Infiltration

Downloaded by CORNELL UNIV on October 10, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch017

J. J. GEBHARDT, E. R. STOVER, W. MUELLER, and J. YODSNUKIS Advanced Materials Development Laboratory, General Electric Co., Re-EntryandEnvironmentalSystemsDivision,Philadelphia,Penn.19101

Carbon-carbon composites comprise a class of high temperature structurally efficient materials useful among other things, for nose tips and thermal shields on high velocity re-entry vehicles. They consist of polycrystalline graphite, the so-called matrix phase, reinforced with a predetermined, prewoven threedimensional array of graphite or carbon fibers, having spacing, packing, composition and geometry selected for optimum response to certain anticipated thermomechanical service loads. Extensive design and development work has led to two major approaches for formation of the matrix phase, i.e., the graphite in the interstices and within the fiber bundles, of prewoven fiber forms so as to achieve maximum, uniform density throughout the structure with minimum distortion of the geometry or damage to the fibers. These are based respectively on multiple pitch impregnation/char/ graphitization cycles and on the pyrolytic decomposition of a carbon precursor vapor at surfaces within the structure. The two approaches can be combined for the purpose of reducing the overall processing time for larger woven billets, by first infiltrating and depositing from the vapor phase to a predetermined extent and then completing the densification by pitch impregnation. The procedures used in a specific case are determined by the properties of the matrix achieved and whether or not the combination of matrix, fiber and geometry can meet the performance requirements of the mission for which the material is intended. Fiber Preforms The graphite fiber preforms which constitute the reinforcing component of carbon-carbon composites are prepared on the basis of previously stipulated geometry, fiber packing ratios, spacings 212

Deviney and O'Grady; Petroleum Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


17.

GEBHARDT ET A L .

Carbon-Carbon Composite Materials

213

and f i b e r s e l e c t i o n . These are s e l e c t e d to provide optimum per formance under d i f f e r e n t s e r v i c e c o n d i t i o n s i n terms of s t r e n g t h , thermal shock r e s i s t a n c e , thermal c o n d u c t i v i t y , and a b l a t i o n r e s i s t a n c e . Considerable p r i o r development has been undertaken to a r r i v e a t a number o f weave geometries which have shown p o t e n t i a l f o r a c h i e v i n g composites i n which the s t r e n g t h and modulus o f the f i b e r s c o n t r i b u t e to the p r o p e r t i e s of the composite. S e v e r a l of these geometric arrangements are shown s c h e m a t i c a l l y i n Figure 1. The manner o f weaving these c o n f i g u r a t i o n s avoids sharp bends and crimping of the f i b e r bundles; e f f e c t i v e l y the f i b e r bundles a r e a l i g n e d along the p r i n c i p a l axes o f the s e l e c t e d con f i g u r a t i o n and compressed to the d e s i r e d spacing. The f i b e r packing i n any d i r e c t i o n can be v a r i e d so t h a t , f o r example, i n a three d i r e c t i o n a l orthogonal a r r a y , the f i b e r bundles which l i e i n the Ζ d i r e c t i o n can c o n t a i n s e v e r a l times the number of f i l a ments placed i n the X or Y d i r e c t i o n . S i m i l a r l y , the f i b e r bundles forming the orthogonale i n the seven d i r e c t i o n a l geometry can be v a r i e d from those comprising the d i a g o n a l d i r e c t i o n s . By such v a r i a t i o n s , the f i b e r volume i n a s o - c a l l e d u n i t c e l l of the repeating geometric p a t t e r n can be v a r i e d to provide a number o f d i f f e r e n t r a t i o s o f reinforcement between Ζ and X-Y d i r e c t i o n s i n the orthogonal case, or between orthogonale and diagonals i n the 7D case. The packing d e n s i t y , gross preform s i z e and geometry are important f a c t o r s i n the e f f i c i e n c y with which the geometric i n t e r s t i c e s and f i b e r bundles can be f i l l e d with a matrix m a t e r i a l , g r a p h i t i c i n most i n s t a n c e s o f i n t e r e s t i n aerospace uses, to achieve a high d e n s i t y composite with the d e s i r e d c o n s t e l l a t i o n of p r o p e r t i e s . The maximum s t r e n g t h o f m u l t i d i r e c t i o n a l r e i n f o r c e d carboncarbon composites l i e s i n the d i r e c t i o n o f the r e i n f o r c i n g f i b e r bundles. In a 3D orthogonal geometry, t h i s i s c l e a r l y i n the X, Y and X d i r e c t i o n s . The 7D s t r u c t u r e i s a combination of 3D and 4D geometries, c o n s i s t i n g of o r t h o g o n a l l y arranged (Χ,Υ,Ζ) bundles with a d d i t i o n a l f i b e r bundles (T,U,V,W) running along a l l four diagonals of the 3D u n i t c e l l cubes. T h i s design provides improved shear s t r e n g t h compared with the 3D m a t e r i a l (1,2). The c u b i c v o i d s of the 3D s t r u c t u r e share common corners, while the v o i d s of the 4D geometry share common s i d e s , as i s shown i n Figure 1. Hence the l a t t e r geometry i s i n h e r e n t l y more permeable than the former. The 7D s t r u c t u r e , a combination of these geometries, would t h e r e f o r e be expected to be more perme able than a p u r e l y 3D orthogonal. Because of the a v a i l a b l e channels i n the 7D s t r u c t u r e , gas can continue to d i f f u s e to inner regions d e s p i t e r a p i d d e p o s i t i o n w i t h i n the f i b e r bundles, which q u i c k l y s e a l s o f f the r e s t r i c t e d access to corner-connected cubic v o i d s i n 3D s t r u c t u r e s . The remainder of t h i s paper i s concerned w i t h d e n s i f i c a t i o n by p y r o l y t i c carbon i n f i l t r a t i o n , of r e l a t i v e l y l a r g e ( V x V x e " ) preforms c o n t a i n i n g g r a p h i t e f i b e r bundles i n a 7D geometry.* Figures 2 and 3 show r a d i o g r a p h i c p o s i t i v e views o f such a


PETROLEUM DERIVED CARBONS


214



Figure 3. 7D graphite fiber preform viewed in one of the diagonal directions. Right view at lower intensity shows center region.


216

PETROLEUM

DERIVED

CARBONS

preform, taken from one end and from one of the diagonal p o s i t i o n s i n which the dark spots are due to f i b e r bundles, viewed end-on, while the dark l i n e s represent f i b e r bundles viewed l a t e r a l l y . The f i b e r s were p o l y a c r y l o n i t r i l e (PAN) precursor m a t e r i a l , two thousand f i l a m e n t s per tow with four tows comprising the orthogonal bundles and three tows i n the d i a g o n a l d i r e c t i o n (2).


Pyrolytic

Infiltration

The customary procedure by which both r e l a t i v e l y porous and q u i t e dense bodies are i n f i l t r a t e d w i t h carbon, u s i n g the thermal decomposition of methane, c o n s i s t s of maintaining the e n t i r e body at a uniform constant temperature, u s u a l l y around 1050-1100°C, i n flowing methane at a pressure of 1 to 100 t o r r (3). In the process, the feed gas undergoes a s e r i e s of vapor phase i n t e r a c t i o n s as i t d i f f u s e s i n t o the preform, forming i n t e r mediate species which f u r t h e r r e a c t to deposit carbon on a l l heated s u r f a c e s . The o v e r a l l d e p o s i t i o n r a t e i s h i g h l y s e n s i t i v e to s u r f a c e area, as w e l l as c o n c e n t r a t i o n , so that c l o g g i n g of outer access paths i s a problem when h i g h i n f i l t r a t i o n d e n s i t i e s are c a l l e d f o r . The d r i v i n g f o r c e which causes the methane to continue to enter the preform from the e x t e r i o r gas phase i s the concentration gradient e s t a b l i s h e d between the outer surface and l o c a t i o n s w i t h i n the preform where decomposition has depleted the gas phase to v a r y i n g extents. Simultaneously, hydrogen, which i s generated by the decomposition r e a c t i o n s , d i f f u s e s outward to the e x t e r i o r gas stream where the hydrogen content i s c l o s e to zero, i f pure methane i s used as the feed gas. Hydrogen a l s o a c t s as a d e p o s i t i o n suppressant by i t s mass a c t i o n e f f e c t on i n t e r mediate r e a c t i o n s which generate i t , and i s sometimes added to the feed mixture. As a r e s u l t , a d e p o s i t i o n gradient i s establ i s h e d w i t h i n the preforms as a f u n c t i o n of d i s t a n c e from the outer s u r f a c e . T h i s i s i n f l u e n c e d by the geometry of the preform as w e l l as the c o n c e n t r a t i o n g r a d i e n t s so that d e p o s i t i o n g r a d i e n t s w i l l vary with geometry under s i m i l a r i n f i l t r a t i o n c o n d i t i o n s . As a r e s u l t , i n order to achieve high i n f i l t r a t i o n d e n s i t i e s , even with f i b e r preforms which have a w e l l d i s t r i b u t e d surface area, such as carbon f e l t , i t i s necessary to operate at minimal d e p o s i t i o n r a t e s f o r long periods of time, with p e r i o d i c mechanical removal of the outer s u r f a c e through which the gas no longer has access. Because of the high c o s t of the preforms of i n t e r e s t i n t h i s work, such a procedure i s not s u i t a b l e f o r a c h i e v i n g high i n f i l t r a t i o n d e n s i t i e s . A second procedure known as the 'thermal gradient approach' (4) c o n s i s t s of maintaining the d e p o s i t i o n region which i s f a r thest away from the gas supply at the highest acceptable temperat u r e , i . e . , below that at which gas phase s o o t i n g occurs. This i s accomplished by p l a c i n g the woven c o n s t r u c t i o n , which i s to be i n f i l t r a t e d , against a heated s u r f a c e , thereby e s t a b l i s h i n g a temperature gradient w i t h i n the s t r u c t u r e . The gas flow i s


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Carbon-Carbon Composite Materiah

217


r e l a t i v e l y high and at approximately one atmosphere w i t h some degree of d i l u t i o n w i t h e i t h e r hydrogen or an i n e r t gas. Deposit i o n occurs on the back or h o t t e s t s u r f a c e f i r s t and proceeds uniformly through the weave to the outer s u r f a c e as the s t r u c t u r e becomes denser and thermal c o n d u c t i v i t y improves. The method i s most u s e f u l f o r r e l a t i v e l y t h i n (e.g., up to 1 i n c h t h i c k ) s t r u c t u r e s . In the case of the l a r g e r three-dimensional s t r u c t u r e s such as the 7D b i l l e t s of i n t e r e s t i n the present work, the d i f f u s i o n paths would be too long and d e p o s i t i o n would occur p r i n c i p a l l y i n the outer r e g i o n s , l e a v i n g a low d e n s i t y core. Furthermore, the l a r g e r c r o s s - s e c t i o n of the specimen causes cons i d e r a b l e gas turbulence and u n c e r t a i n gas phase p y r o l y s i s c o n d i tions · A v a r i a t i o n of the temperature gradient method was developed for the purpose of d e n s i f y i n g woven s t r u c t u r e s of s u i t a b l e s i z e f o r f a b r i c a t i o n of nose t i p s . This approach c o n s i s t s of c o u p l i n g the f i b e r s t r u c t u r e d i r e c t l y w i t h the f i e l d produced w i t h i n a r.f. coil. The densely packed g r a p h i t e f i b e r s conduct s u f f i c i e n t l y w e l l to heat the preform, i n such a manner that the highest temperature produced i s i n the center of the preform. The temperature gradient between the h o t t e s t and the c o o l e s t surfaces of a f i b e r preform depends on the d e n s i t y of the preform, the f i b e r geometry and the thermal c o n d u c t i v i t y of the f i b e r s themselves as w e l l as the v e l o c i t y w i t h which gas i s passed around the preform. In preforms comprised of randomly o r i e n t e d short f i l a m e n t s , such as a carbon f e l t , the temperature g r a d i e n t can be as high as s e v e r a l hundred degrees per i n c h , while i n 3D orthogonal, t i g h t l y packed preforms, i t can be as low as ten to twenty degrees per i n c h , from the center of the preform to the outer surface (5). By u s i n g a short c o i l , i . e . , w i t h only two or three turns, one end of the preform can be heated i n t h i s manner, and the b i l l e t can be drawn slowly through the c o i l so that e v e n t u a l l y i t s e n t i r e length i s heated i n t h i s manner. During t h i s procedure, the process gas i s passed around the p r e form at high v e l o c i t y to p r o v i d e h i g h methane c o n c e n t r a t i o n at preform surface as w e l l as maintain the steepest p o s s i b l e temperature g r a d i e n t . This method was explored i n p r e l i m i n a r y experiments using small (e.g., 0.5" square to 2" square) woven substrates of v a r y ing reinforcement geometry, mounted i n a f i x e d p o s i t i o n w i t h i n the c o i l . The arrangement i s shown s c h e m a t i c a l l y i n Figure 4. The temperature of the h o t t e s t v i s i b l e p a r t of the b i l l e t w i t h i n the c o i l was maintained a t 1075-1100°C w i t h an o p t i c a l pyrometer and manual power supply c o n t r o l . The r e s u l t s of these e x p e r i ments showed that at f i x e d temperature, gas flow and p r e s s u r e , secondary e f f e c t s such as geometry, packing d e n s i t y , and v o i d s i z e and shape govern the degree of d e n s i f i c a t i o n achievable, as w e l l as the u n i f o r m i t y of d i s t r i b u t i o n of the p y r o l y t i c carbon w i t h i n the s t r u c t u r e (5). Figure 5 r e l a t e s some of these f a c t o r s (unbroken l i n e s ) i n terms of i n i t i a l preform d e n s i t y and the


218

PETROLEUM

DERIVED

Specimen Motion


To Vent

Figure 4. Schematic of direct coupling infltration


CARBONS

GEBHARDT E T A L .



17.


219


220

P E T R O L E U M DERIVED

CARBONS

i n c r e a s e i n d e n s i t y achieved by the time d e p o s i t i o n began to occur predominately on the outer s u r f a c e . Several f i b e r geome t r i e s and methods were c o r r e l a t e d i n t h i s manner, and i n general i n d i c a t e d that higher i n f i l t r a t i o n d e n s i t i e s could be achieved with r e l a t i v e l y open a c c e s s i b l e v o i d s t r u c t u r e s , such as random f i b e r o r i e n t a t i o n (e.g., f e l t ) or connected voids (e.g., noncubic) when d i r e c t c o u p l i n g was used (6). I n f i l t r a t i o n o f these geometries by the temperature gradient method y i e l d e d s l i g h t l y lower d e n s i t y i n c r e a s e s than d i d the d i r e c t c o u p l i n g technique. The c l o s e d , cubic v o i d s t r u c t u r e o f the 3D orthogonal s t r u c t u r e s was more d i f f i c u l t t o d e n s i f y by e i t h e r method. P r e l i m i n a r y experiments were conducted with m a t e r i a l s which were about one square i n c h o r s l i g h t l y l a r g e r i n c r o s s - s e c t i o n , and which were drawn through the c o i l manually i n batch f a s h i o n u n t i l the e n t i r e length of the specimen has been i n f i l t r a t e d . For the purpose o f a c h i e v i n g u n i f o r m i t y from end to end with l a r g e r specimens, the 4" x4" χ 6-8" b i l l e t s processed i n the scale-up work, were suspended from a motor d r i v e n l i f t and con t i n u o u s l y r a i s e d through the c o i l during the e n t i r e i n f i l t r a t i o n . In s e v e r a l cases, the feed gas was d i l u t e d with hydrogen to repress premature d e p o s i t i o n i n the outer p o r t i o n s o f the b i l l e t s . The r a t e a t which the b i l l e t was r a i s e d through the c o i l was a l s o v a r i e d to provide specimens which would u l t i m a t e l y contain d i f f e r ent r a t i o s of p y r o l y t i c t o pitch-base graphite a f t e r impregnation and g r a p h i t i z a t i o n . Table 1 summarizes the process data and l i s t s the r e s u l t s i n terms o f bulk d e n s i t y measured a f t e r remov i n g the surface s c a l e and lower d e n s i t y ends of the preforms. R e f e r r i n g to Figure 5, i t i s seen that the i n c r e a s e s i n average bulk d e n s i t y a t t a i n e d , represented by open symbols, agree f a i r l y c l o s e l y w i t h the p r e l i m i n a r y r e s u l t s , which are shown as s o l i d l i n e s . However, a l l o f the f i n i s h e d m a t e r i a l s contained d e n s i t y gradients from center t o o u t s i d e . D e n s i t i e s were estimated r a d i o m e t r i c a l l y to range from 1.2 t o 1.4 gm/cm^ a t the center t o 1.6 to 1.7 gm/cm^ i n the outer f i b e r r e g i o n and a t the corners i n specimens drawn through the c o i l a t .06 in./hour. Specimens i n f i l t r a t e d a t higher draw speeds, e.g., 0.125 or 0.25 in/hour were l e s s dense as i s shown i n Figure 6, but a l s o had n o t i c e a b l e d e n s i t y g r a d i e n t s , ranging from about 0.85 to 1.20 gm/cm^ through the t h i c k n e s s . T h i s i s i l l u s t r a t e d i n the radiograph shown i n Figure 7. Hydrogen was added to the feed gas i n an e f f o r t to repress premature d e p o s i t i o n of carbon i n the outer f i b e r r e g i o n but t h i s was not a wholly s u c c e s s f u l procedure, and radiographs of t h e . i n f i l t r a t e d specimens showed s i g n i f i c a n t d e n s i t y gradients e x i s t e d . Figure 8 shows how the s u r f a c e area s e n s i t i v e nature of the p y r o l y t i c i n f i l t r a t i o n process causes carbon t o concentrate i n the f i b e r bundles. Clogging of the access paths a t i n t e r s e c t i o n s of the f i b e r bundles leaves the l a r g e i n t e r s t i t i a l voids incompletely f i l l e d . In Figure 9 a t higher m a g n i f i c a t i o n , v o i d regions can be seen among i n d i v i d u a l filaments w i t h i n the f i b e r bundles·



709A 709B 709E 709G 721 731 709F 732T 732B 733 703 705 701 707 710 706 720 723A 723B 727 719

Spec. No.

1.52 X 1.41 1.25 χ 1.25 1.30 X 1.30 1.90 X 1.90 1.94 X 1.87 1.20 X 1.19 1.33 X 1.33 1.0 X 1.0 1.0 X 1.0 1.0 X 1.0 3.38 X 3.38 3.33 X 3.33 4.20 X 4.20 3.20 X 3.20 3.40 X 3.40 3.39 X 3.58 3.33 X 3.40 3.40 X 3.38 3.50 X 3.25 3.28 X 3.28 4.27 X 1.21

Cross S e c t i o n (in.)

.05 .06 .066 .057 .06 .06 .066 .06 .125 .06 .05 .05 .06 .057 .06 .125 .250 .250 .250 .250 .250 Conditions: —

2

2

9

3

____ _ __ .39 1.86 1.12 1.26 .43 1.69 .38 1.63 1.25 .41 1.67 1.27 .32 1.20 1.85 . 1.69 .54 1.74 1.37 1.85 1.06 .40 1.46 1.86 .47 1.76 1.29 20 7· H L 1.84 1.67 1.20 .47 1.84 1.70 .35 1.35 1.34 1.82 .46 1.66 .44 1.82 1.54 1.10 1.75 - 1.83 20 % H 1.42 0.93 1.79 .50 1.2 - 1.7 ( a f t e r CVD) 20 % H .53 1.86 1.60 1.07 1.09 1.82 .48 1.57 .53 0.93 1.83 1.45 0.78 .38 1.16 .84 0.47 .37 0.39 .39 .78 .73 0.28 .45 0.41 .38 .79 Max. Temperature Approximately 1075-llOOoc Gas V e l o c i t y Approximately 165 cm/sec Methane-City Gas, P h i l a . Gas Co., 1030 B T U / f t , d r i e d

TABLE I PROCESS DATA FOR INFILTRATION OF 7D WOVEN CONSTRUCTIONS Draw Speed Bulk Density ( g/cm3) (in/hr) I n i t i a l F i n a l I n c r . A f t e r Gas Comp. Density G r a d i e n t (g/cm3) P i t c h C i t y plus


222

PETROLEUM

DERIVED

CARBONS


2.0

Draw Speed

Figure 6.

in'hr.

Correlation between specimen draw speed and density increase


17.

GEBHARDT E T A L .


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PETROLEUM

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224

Figure 8. Scanning electron microscope montage of high density (1.73 em/cm ) i trated 7Ό billet at fracture surface (76X) showing concentration of infiltrated car withinfiberbundles 5



Figure 9. Scanning electron microscope views of pyrolytic carbon deposit within bundles in 7D billet (left, * 850X; right, 2550X)


226

PETROLEUM

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CARBONS

In subsequent p r o c e s s i n g , the i n f i l t r a t e d b i l l e t s were impregnated with c o a l t a r p i t c h at h i g h pressure (1 Kbar), pyrolyzed a t that pressure and then g r a p h i t i z e d a t 2800°C to con v e r t the p y r o l y t i c and pitch-base carbon to g r a p h i t e , * * F i n a l d e n s i t i e s were i n the range o f 1.75 t o 1.83 gm/cm . Examination of the f i n i s h e d b i l l e t s by x-radiography showed that d e n s i t y gradients remained even a f t e r impregnation, and that regions of the preforms which had been i n f i l t r a t e d the most now had the lower f i n a l d e n s i t i e s . T h i s i s due to s e a l i n g o f the v o i d regions w i t h i n f i b e r bundles by the p y r o l y t i c c o a t i n g , as i s shown i n Figure 9, preventing access of the p i t c h to these l o c a t i o n s . F i b e r bundles i n the centers o f the preforms, not being so h e a v i l y coated, permitted g r e a t e r access o f the p i t c h and thus higher f i n a l d e n s i t i e s . B i l l e t s which were i n f i l t r a t e d a t higher draw speeds, i n which f i b e r bundles were not as h e a v i l y coated, were more uniform a f t e r impregnation.


3

Conclusions D e n s i f i c a t i o n o f carbon and g r a p h i t e f i b e r three-dimensional preforms was c a r r i e d out by p y r o l y s i s o f methane gas, with heat i n g e f f e c t e d by d i r e c t i n t e r a c t i o n of the preforms with a r . f . field. The p y r o l y s i s process was i n f l u e n c e d p r i m a n i l y by concen t r a t i o n gradients w i t h i n the preforms and by the g r e a t e r a v a i l able s u r f a c e area w i t h i n f i b e r bundles, as compared t o i n t e r s t i t i a l v o i d w a l l s . Density gradients were s u b s t a n t i a l l y reduced by subsequent impregnation and p y r o l y s i s of l i q u i d c o a l t a r p i t c h , although the i n i t i a l gradient due to i n f i l t r a t i o n became reversed owing to decreased a c c e s s i b i l i t y to p i t c h where f i b e r bundles were h e a v i l y coated. S e l e c t i o n o f t h i s approach to f a b r i c a t i n g carbon-carbon composites would depend on the extent to which property v a r i a t i o n s w i t h i n the f i n i s h e d m a t e r i a l could be t o l e r a t e d i n the u l t i m a t e use o f the composite.

Literature Cited 1. Stover, E.R. and Latva, J.D., Eleventh Biennial Conference on Carbon, Extended Abstracts, June 4-8, 1973, Paper No.FC-9, p. 277. 2. Stover, E.R., Roetling, J.Α., Ruzauskas, E.J., Connell, W. and Hall, K.J., Ibid, Paper No. FC-22, pps. 335-6. 3. Kotlensky, W.V., "Deposition of Pyrolytic Carbon in Porous Solids", in "Chemistry and Physics of Carbon", Vol.9, ed. by P. L. Walker, Jr. and P. A. Thrower, pps. 191-198, Marcel Dekker, N.Y., 1973. 4. Ibid, pps. 198-202. 5. Gebhardt, J.J., Stover, E.R., and Yodsnukis, J.J., Proceed ings of 4th National Technology Conf., Soc. Adv. Mat. Proc. Eng. (SAMPE), October 1972, Palo Alto, CA.


17. GEBHARDT ET AL. 6.

*

Gebhardt, J.J., Tenth Biennial Conference on Carbon, June 1973, Lehigh University, Paper No. FC-33, Summary of Papers, Defense Ceramic Information Center.

Fiber Preforms were fabricated by Fiber Materials Inc., Biddeford, ME, 04005 Performed at the Y-12 Plant, Union Carbide Corporation, Nuclear Div., Oak Ridge, Tenn.


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Formation of CarbonCarbon Composite Materials by Pyrolytic Infiltration

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