Advanced High Pressure Graphite Processing Technology - ACS

14 Advanced High Pressure Graphite Processing Technology

Downloaded by UNIV OF NEW ENGLAND on February 12, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch014

WILLIAM CHARD, MICHAEL CONAWAY, and DALE NIESZ Battelle, Columbus Laboratories, Columbus, Ohio 43201

Since the early 1800's when Sir Humphrey Davy utilized carbonized-wood rods for generating an electric arc, the carbon and graphite industry has grown steadily. Today the industry supplies materials ranging from 55-in.-diameter graphite cylinders for large metallurgical crucibles and arc-melting electrode applications to 1/8-in.-diameter high-purity electrodes for arc lights. In general, the materials supplied for these applications are fabricated by conventional extrusion techniques. Special molding and hot-working techniques have been developed for fabricating more homogeneous, refined graphite materials for certain specialized applications such as ballistic-missile nose tips and rocket nozzles. However, companies in the industry are slow to accept new processing technology or to go outside of their own organizations for advanced developments. The term "Black Art" appropriately fits the industry, as considerable art and experience are typically required to guide the manufacturing practices. This paper describes recently developed high-pressure processing techniques utilizing hot-isostatic-pressing (HIP) equipment and procedures for use in fabricating unique graphites and carbon-carbon composites. This process is by no means a laboratory curiosity. Several manufacturers are producing this equipment and equipment systems commercially and autoclave or pressure vessels with working areas as large as 4 ft in diameter by 10 ft long are currently available at Battelle's Columbus Laboratories for processing studies. This paper briefly reviews the current graphite-processing technology, discusses the HIP process, and summarizes some of the advantages of utilizing this process to fabricate carbon and graphite structures. The graphite structures of most interest here are considered to be specialty graphites. These are fine-grained, reasonably isotropic, high-density, homogeneous, multigranular materials. These materials are typically considered for use as aerospace and reentry materials, bearing seals, special electrodes, nuclear 155

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

P E T R O L E U M DERIVED CARBONS

156

r e a c t o r components, semiconductor f i x t u r e s , and other a p p l i c a t i o n s where high-performance standards are r e q u i r e d . Other s p e c i a l t y carbon-base m a t e r i a l s such as advanced carbon-carbon r e i n f o r c e d s t r u c t u r e s , f i b e r - b a s e g r a p h i t e s , and m u l t i p a r t i c u l a t e composites are a l s o i n c l u d e d .


Conventional

Graphite-Processing

Techniques(1,2,3)

C e r t a i n inherent p h y s i c a l and chemical l i m i t a t i o n s have been imposed on the organic presursors used t o produce carbon and graphite structures,which i n t u r n impose r e s t r i c t i o n s on the method of f a b r i c a t i n g these m a t e r i a l . The major areas o f d i f f i c u l t y are (1) the nature and amount o f v o l a t i l e outgassing c o n s t i t u e n t s evolved during the p y r o l y s i s o f organic p r e c u r s o r s , (2) the complex chemistry and wide v a r i a b i l i t y o f organic-base m a t e r i a l s used i n processing carbon and graphite and t h e i r s e n s i t i v i t y t o p y r o l y s i s environments, and (3) the l i m i t a t i o n i n equipment a v a i l a b l e f o r processing these m a t e r i a l s a t temperatures up to 2800 C. Bulk graphites are, i n f a c t , composites o f carbon p a r t i c u l a t e s bonded together by a second carbon-binder phase. A d d i t i o n a l carbon phases may be introduced by impregnation of an organic m a t e r i a l i n t o the pore s t r u c t u r e o f a t y p i c a l l y subdense m u l t i granular bulk carbon o r g r a p h i t e . Fiber-base graphites are composites that have a chopped f i b e r - f i l l e r s u b s t i t u t e d f o r a particulate grain f i l l e r . Carbon-carbon composites are m a t e r i a l s that possess a continuous f i b e r s t r u c t u r e which dominates o r d e f i n e s the p r o p e r t i e s o f the composites and the m a t e r i a l s are t y p i c a l l y i d e n t i f i e d as " r e i n f o r c e d " s t r u c t u r e s because of the c a r b o n - f i b e r phase. A wide range o f p r o p e r t i e s and c h a r a c t e r i s t i c s can be a t t a i n e d i n carbon-base m a t e r i a l s through the s e l e c t i o n of f i l l e r , binder, and impregnant precursor m a t e r i a l s ; coking procedures; method o f c o n s o l i d a t i n g o r forming ( e x t r u s i o n , molding, hydropressing); processing-temperature sequences and u l t i m a t e temperatures reached; manufactured-product s i z e and shape and other m a t e r i a l and processing f a c t o r s . These complex combinat i o n s are not simple. Table 1 l i s t s s e v e r a l t y p i c a l commercial carbon products with r e p r e s e n t a t i v e p r o p e r t i e s . The conventional methods o f processing commercial bulk graphite are by an e x t r u s i o n and molding technique with s e v e r a l subsequent baking ( p y r o l y s i s ) and impregnation steps, as i l l u s t r a t e d i n F i g u r e 1. H y d r o s t a t i c p r e s s i n g and hot^working methods have seen l i m i t e d development w i t h i n the l a s t 20 years and are a p p l i e d b a s i c a l l y t o the manufacture o f s p e c i a l t y ,

(1) Carbon and Graphite Handbook (1968), Interscience Publishers, Charles Mantell. (2) Carbon and Graphite (1965), Academic Press, E. I. Shobert. (3) The Industrial Graphite Engineering Handbook. Union Carbide.



Hot worked or recrystallized

Extruded, impregnated

Hydrostatic pressed

Molded Resin

2DC/C cloth layup

CVD C/C f e l t

ZTA

Graphitita G

AXF-5Q

Glassy Carbon (GC-20)

Carbitex 700

RPG

(a)

1.47

1.42

1.8

—

—

— 1.85

1.85

1.89

1.95

1.85

1.73

0.001

0.03

0.006

0.006

0.006

2.25

Grain Density, Size, i n . g/cc

W - With the grain (a-b direction) A - Against the grain (c direction).

Woven 3DC/C, impregnated

Molded, Impregnated

ATJS

2.2.3

Molded, impregnated

ATJ

b

CVD

Pyrolytic

Designation

Forming Method 14,000 1,500 4,200 3,000

W A W A

(b) (c)

z

W A

W A

_

27,000

— —

8,000

—

9,000

10,500

12,000 12,000

0.2 1.4 -0.30 -0.20 8.0 15.0

0.4 1.6

2.1

3.2 3.2

0.6 1.9

0.5 3.8

1.2 1.6

1.2 1.9

0.2 10.8

Thermal Expansion at 0-212 F, 10"°/in.

1.9 1.0

1.7 0.4

3.8

1.7 1.7

1.4 1.2

2.5 0.9

1.5 1.1

1.4 1.1

4.4 1.7

6

Young*s Modulus, 10 p s i

Chemical vapor deposit. Fiber orientation, Z indicates longitudinal.

18,000 35,000

6,000 2,600

5,500 300

6,100

10,000 10,000

2,800 1,800

W A W A

5,200 2,500

4,400 1,500

V A 3,200 2,600

5,400 5,100

4,100 3,600

21,000 1,500

Flexure Strength, psi

5,000 4,200

W A

Tensile Strength, psi

DirectionU) of Property Measurement

TABLE 1. TYPICAL PROPERTIES OP SEVERAL COMMERCIAL CARBON-BASE MATERIALS


I

?

3


158

Petroleum coke filter

Coal tar pitch binder

Process additives


Calcine

Melt

Size Mixer

I

Forming extrude or mold

Green carbon body

Baked carbon body

Baking furnace

I

Gas or oil fired

£

Sold as carbon grades Graphitizing furnace Impregnate with coat tar pitch

Graphitizing furnace

ZZI

Sold as impregnated graphite grades

Sold as graphite grades Sold as graphite grades

Figure 1.

Typical graphite manufacturing process



14.

CHARD E T A L .

Graphite Processing Technology

159

r e f i n e d g r a p h i t e s . The f a b r i c a t i o n process have s e v e r a l m o d i f i c a t i o n s which are employed to produce m a t e r i a l of c e r t a i n d e s i r a b l e c h a r a c t e r i s t i c s . A l l of these processes depend on c o n s o l i d a t i n g a thermoplastic carbon mass at e i t h e r low temperatures, 150 to 250 C, or high temperatures, 1100 to 2600 C. These processes t y p i c a l l y r e q u i r e mixing of a coke or graphite f l o u r with an organic binder such as c o a l - t a r p i t c h . A d d i t i v e s such as lamp or carbon b l a c k are mixed i n to o b t a i n c e r t a i n d e s i r a b l e p r o p e r t i e s . The composition i s mixed to blend the batch c o n s t i t u e n t s before h e a t i n g to about 150 C, at which time the thermoplastic binder allows the m a t e r i a l to be c o n s o l i d a t e d by molding or extusion. These green products are cooled and then baked or coked at about 950 C. T h i s heat treatment allows the binder phase to l o s e i t s v o l a t i l e c o n s t i t u e n t s and y i e l d s an extremely hard and porous, s o l i d , carbon mass. T h i s o p e r a t i o n may take an e n t i r e month to complete. M u l t i p l e impregnation operations are commonly performed with i n t e r m i t t e n t heat treatments to i n c r e a s e the d e n s i t y of the m a t e r i a l or decrease i t s p e r m e a b i l i t y . The f i n a l thermal t r e a t ment, g r a p h i t i z a t l o n , converts the hard n o n c r y s t a l l i n e carbon a r t i c l e to s o f t e r c r y s t a l l i n e graphite and i s performed at around 2800 C. T h i s conversion i s very much dependent on the organic precursor and the u l t i m a t e temperature a t t a i n e d . The c y c l e f o r t h i s o p e r a t i o n may be 1 to 5 weeks. Hydrostatic-press-formed and h o t - w o r k e d - r e c r y s t a l l i z e d graphites have been developed on a very l i m i t e d b a s i s to y i e l d certain specialized properties.(4) Hydrostatic-press-formed graphites are reasonably I s o t r o p i c , whereas moderately dense, h o t - w o r k e d - r e c r y s t a l l i z e d graphites are a n i s o t r o p i c and of h i g h d e n s i t y . T y p i c a l l y , graphite m a t e r i a l s r e c e i v e s e v e r a l thermal treatments and impregnations during t h e i r manufacture and t o t a l p r o c e s s i n g times of up to 5 months may be experienced. The advent of h o t - i s o s t a t i c - p r e s e i n g (HIP) o f f e r s c o n s i d e r a b l e p o t e n t i a l f o r f a b r i c a t i n g a wide range of carbon-base m a t e r i a l s with unique r e f i n e d and uniform m i c r o s t r u c t u r e s i n short p r o c e s s i n g times. Advanced Processing Techniques B a t t e l l e ' s Columbus L a b o r a t o r i e s i n i t i a t e d the development of the h o t - i s o s t a t i c - p r e s e i n g (HIP) process i n the mid-1950 s f o r f a b r i c a t i n g metal-clad n u c l e a r - f u e l elements. (5) Since that time, 1

(4)

(5)

"Research and Development on Advanced Graphite M a t e r i a l s , Volume VII-High Density R e c r y s t a l l i z e d Graphite by HotForming", WADD-TR 61-72 (June, 1962). Proceedings of the Second United Nations I n t e r n a t i o n a l Conference on the P e a c e f u l Uses of Atomic Energy (September, 1959), S. J . P a p r o c k i , E. S. Hodge, and C. B. Boyer.



160

PETROLEUM

DERIVED CARBONS

the process has developed i n t o a commercially v i a b l e (6) technique f o r f a b r i c a t i n g s p e c i a l t y m a t e r i a l s . The process i s used commercially to manufacture high-performance powder-metallurgy m a t e r i a l s with c o n t r o l l e d homogeneous m i c r o s t r u c t u r e s and outstanding mechanical p r o p e r t i e s . The p r o c e s s i n g equipment c o n s i s t s of a l a r g e water-cooled gas-pressure v e s s e l w i t h an i n t e r n a l furnace. A schematic of the HIP p r o c e s s i n g system i s shown i n F i g u r e 2, which t y p i c a l l y c o n s i s t s of a gas storage f a c i l i t y , high-pressure gas t r a n s f e r l i n e s , a compressor, a high-pressure v e s s e l or autoclave, and an autocalve furnace. The m a t e r i a l to be processed i s t y p i c a l l y encapsulated i n a h e r m e t i c a l l y sealed metal c o n t a i n e r and p l a c e d i n the autocalve f o r p r o c e s s i n g as shown s c h e m a t i c a l l y i n Figure 3. A schematic i l l u s t r a t i o n of a h i g h pressure autoclave i s shown i n F i g u r e 4. As the sample i s heated, gas pressure i s a p p l i e d to the sample c o n t a i n e r which c o n s o l i d a t e s or presses the sample w i t h i n . The gas i s normally argon or helium, most of which i s reclaimed and reused (approximately 80 volume p e r c e n t ) . At e l e v a t e d temperatures, the metal e n c a p s u l a t i n g c o n t a i n e r becomes extremely p l a s t i c and a c t s l i k e a "rubber bag", t r a n s m i t t i n g the autoclave gas pressure to the sample and thus causing d e n s i f i c a t i o n . The d i f f u s i o n r a t e of the autoclave gas through the encapsulating metal c o n t a i n e r i s normally very low and the hermetic s e a l prevents gas from e n t e r i n g i n t o the sample. The b a s i c advantages of the process are summarized as f o l l o w s : (1) E f f e c t i v e c o n s o l i d a t i o n u t i l i z i n g high i s o s t a t i c gas pressure f o r d e n s i f i c a t i o n (2) The c o n t r o l of f a b r i c a t e d m a t e r i a l s ' m i c r o s t r u c t u r e with the use of h i g h - i s o s t a t i c - p r e s s u r e c o n s o l i d a t i o n and low s i n t e r i n g temperatures. (3) P r e c i s e c o n t r o l of process parameters (4) V e r s a t i l i t y i n the a b i l i t y t o process a wide range of m a t e r i a l s at a wide range of p r o c e s s i n g parameters (5) The a b i l i t y to process l a r g e and unusual geometric parts (6) The c a p a b i l i t y to process l a r g e p a r t s w i t h l e s s concern f o r aspect r a t i o s (length/diameter) or f o r s i z e l i m i t a t i o n s found w i t h u n i d i r e c t i o n a l hot p r e s s i n g (7) Production c a p a b i l i t y through large-volume c a p a c i t i e s and short process turn-around times.

(6)

" S o l i d - S t a t e Bonding and C o n s o l i d a t i o n of Powders Under H o t - I s o s t a t i c Pressure", C. Boyer, H. Hanes, K. Meiners, F. O r c u t t , ASME Paper 71-WA Prod-20, J u l y 15, 1971.



161


14. CHARD E T A L .

COOLING JACKET

Figure 3.

Schematic of high pressure process



Figure 4. Schematic of HIP processing equipment



14.

CHARD E T A L .


163

A v i d e range of autoclave s i z e s are a v a i l a b l e , as i n d i c a t e d i n Table 2, which b r i e f l y o u t l i n e s the c u r r e n t c a p a b i l i t i e s of B a t t e l l e * s Columbus L a b o r a t o r i e s ' high-pressure autoclave facility. The l a r g e s t autoclave has a working area or p r o c e s s i n g zone 4 - f t i n diameter by 10 f t long with o p e r a t i o n a l c a p a b i l i t i e s of 15,000 p s i and temperatures up to 1260 C. The temperature u n i f o r m i t y w i t h i n t h i s working volume has been measured to be b e t t e r than +10 C at 1260 C. The v e s s e l s a t Battelle-Columbus are being modified to p l a c e the heater, pressure and instrumentat i o n feed throughs i n the bottom of the autoclave i n a simple automatic p l u g - i n , plug-out arrangement as shown i n F i g u r e 3. T h i s arrangement e l i m i n a t e s the n e c e s s i t y t o manually connect and disconnect a l l feed throughs, w i t h s i g n i f i c a n t time savings i n l o a d i n g and unloading sequences. Encapsulated m a t e r i a l s to be HIP c o n s o l i d a t e d are t y p i c a l l y loaded i n t o the autoclave w i t h i n the heater assembly. The autoclave l i d i s merely screwed i n t o p l a c e and the p r o c e s s i n g c y c l e i s s t a r t e d . Although t h i s process i s a batch-type o p e r a t i o n , techniques have been developed to make maximum e f f e c t i v e use of the high-pressure autoclave working volume and in-use time. Automatic handling and o p e r a t i n g equipment have a l s o been developed to r e f i n e the process f o r production a p p l i c a t i o n s . Computer systems are now being used to c o n t r o l the pressure/temperature c y c l e sequence and f o r c o n t i n u a l processing-data a c q u i s i t i o n . S p e c i a l procedures, such as dynamic outgassing or v e n t i n g of e n c a p s u l a t i n g c o n t a i n e r s and high-pressure h y d r o s t a t i c e x t r u s i o n have been performed which f u r t h e r i l l u s t r a t e the v e r s a t i l i t y of the process. Three b a s i c approaches have been examined to date i n developing HIP p r o c e s s i n g of g r a p h i t e m a t e r i a l s and two appear to o f f e r c o n s i d e r a b l e p o t e n t i a l f o r f a b r i c a t i n g improved homogeneous carbon and g r a p h i t e s t r u c t u r e s . The three b a s i c approaches are: (1) C o n s o l i d a t i o n of f i b r o u s and p a r t i c u l a t e m a t e r i a l s without any binder phase (2) D e n s i f l c a t i o n of p r e g r a p h i t i z e d or carbonized s o l i d s (3) High-pressure impregnation/carbonization. P a r t i c u l a t e C o n s o l i d a t i o n . Coke (graphite) powder or f i b e r s are p l a c e d i n s e l e c t e d metal p r o c e s s i n g c o n t a i n e r s and processed at temperatures up to 2650 C and 17,000 p s i . ( 7 , 8 ) The p a r t i c u l a t e powders or f i b e r s are t y p i c a l l y pre-formed i n t o a s o l i d shape by

(7) (8)

"Hot I s o s t a t i c Compacting of Graphite", D. C. Carmichael, P. D. Ownby, E. S. Hodge, BMI-1746 (October, 1965). "Dense I s o t r o p i c Graphite F a b r i c a t e d by Hot I s o s t a t i c Compaction", D. C. Carmichael, W. C. Chard, M. C. Brockway, BMI-1796 (March 31, 1967).


164



TABLE Π BATTELLE-COLUMBUS HIP PROCESSING CAPABILITY^) Processing Zone, in. Diameter Length 2.5 3.5 4

5 8 18 3 3 13 48 1

8 12 36 15 30 20 36 15 72 16 3 50 120 3

Pressure, psi 10,000 10,000 10,000 15,000 15,000 15,000 50,000 20,000 30.000 15,000 150,000

Temperature C_ JL 1650 1260 1260 1650 1260 1650 1260 1650 1200 1760 2700 1200 1260 1540

3000 2300 2300 3000 2300 3000 2300 3000 2200 3200 4900 3000 2300 2800

(a) Spécule capabilities are frequently extended and modified as required to meet research and development program needs.



14.

CHARD E T A L .


165

h y d r o s t a t i c p r e s s i n g . To date, the m a t e r i a l s have been processed without a binder phase, hence the term " b i n d e r l e s s " g r a p h i t e s . The highest s t r e n g t h s t r u c t u r e s have been a t t a i n e d u t i l i z i n g coke f l o u r r a t h e r than a g r a p h i t e f l o u r as the powder p r e c u r s o r . The r e l a t i v e t e n s i l e s t r e n g t h of some of these m a t e r i a l s i s shown i n Table 3. The coke f l o u r i s considered to be much more " a c t i v e " than the graphite f l o u r and i s more e a s i l y " s i n t e r a b l e " . Bonding between adjacent coke p a r t i c l e s i s thought to occur during HIP c o n s o l i d a t i o n as a r e s u l t of i n t e n s i f i e d i n t e r p a r t i c l e pressure e f f e c t s between the a v a i l a b l e carbon l a t t i c e bond s i t e s i n adjacent carbon p a r t i c l e s . Graphite f l o u r s have been processed to higher d e n s i t i e s owing to the s o f t n e s s and l u b r i c i t y of the g r a p h i t i z e d m a t e r i a l , but lower strengths are a t t a i n e d due to the l a c k of a v a i l a b l e i n t e r p a r t i c l e carbon bonding s i t e s . B i n d e r l e s s m a t e r i a l has been processed at temperatures from 1800 to 2650 C and at pressures between 30,000 and 17,000 p s i . The highest s t r e n g t h graphites have been obtained when processed at around 2600 C. Tantalum containers are used because of the high tantalum carbide-tantalum e u t e c t i c (2830 C). Reaction between s u l f u r i n the carbon m a t e r i a l s and the tantalum c o n t a i n e r at temperatures above 1500 C has caused premature f a i l u r e of the tantalum cans. Such f a i l u r e should be reduced by lowering the temperature of p r o c e s s i n g to 1000 to 1500 C and u t i l i z i n g lows u l f u r cokes. Less expensive encapsulating m a t e r i a l s can be used at these lower temperatures and much l a r g e r p r o c e s s i n g c a p a c i t i e s would be a v a i l a b l e . To date, the HIP process has been used most e x t e n s i v e l y f o r f a b r i c a t i n g p a r t i c u l a t e g r a p h i t e s t r u c t u r e s , but considerable p o t e n t i a l e x i s t s f o r f a b r i c a t i o n fiber-base structures. The m i c r o s t r u c t u r e s of s e v e r a l unique b i n d e r l e s s g r a p h i t e m a t e r i a l s are shown i n F i g u r e s 5, 6, and 7. F i g u r e 7 i l l u s t r a t e s a u n i d i r e c t i o n a l graphite f i b e r s t r u c t u r e with no binder phase. A l l of these m a t e r i a l s are w i t h i n 95 percent of t h e o r e t i c a l density. Two b a s i c approaches are considered a p p l i c a b l e f o r b i n d e r l e s s - t y p e f a b r i c a t i o n of carbon-base m a t e r i a l s , high temperature and low temperature p r o c e s s i n g . The high-temperature process (1800-2600 C) i s considered more f o r experimental s t u d i e s and does not c o n s i d e r a b l e manufacturing p o t e n t i a l . Both the temperature l i m i t a t i o n s (furnace design and s i z e ) and r e a c t i o n problems are eminent. A l o w - s u l f u r (

Advanced High Pressure Graphite Processing Technology - ACS

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