Graphites Fabricated from Green Petroleum Pitch Cokes

12 Graphites Fabricated from Green Petroleum Pitch Cokes O. J. HORNE* and C. R. KENNEDY Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830

Downloaded by UNIV OF ROCHESTER on January 17, 2018 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch012

**

Conventional graphite bodies are normally fabricated by combining carbonized or graphitized filler materials with hydrocarbon binders such as coal tar or petroleum pitches. During the subsequent heat treatment to produce the f i n a l graphite body, the filler undergoes little or no shrinkage, while pronounced shrinkage of the binder material occurs, resulting i n a f i n a l product which contains a network of angular voids. This pore structure weakens the overall structure and i t s a b i l i t y to withstand stressing. The effect of pore morphology i s particularly significant i n obtaining graphites with structures yielding high mechanical integrity. These types of structure are very important when considering the resistance to d i f f e r e n t i a l c r y s t a l l i t e growth during nuclear irradiation and to resisting the high strains produced by thermal stress or shock. Both types of problems can presumably be solved by strengthening the interparticle boundaries and reducing the role of pore structures to act as stress risers. Thus, the desired structure i s one having a monolithic character with the pore texture smaller or less angular than the pores inherently present i n the coke structure. One approach to obtain a monolithic-type structure with smaller or less angular pores i s to employ green coke as the filler material. This approach offers at least two advantages. F i r s t , the coke particle still retains some chemical reactivity and w i l l readily unite with the binder. Secondly, the volume shrinkage of the green coke filler more nearly matches the binder shrinkage during pyrolysis. Work reported by Kennedy and Eat her l y (l 2) has shown that graphite based on green cokes does have the necessary monolithic structural characteristics and does exhibit dramatic improvements i n both irradiation and thermal shock resistance. Highly acicular green cokes have been employed i n efforts to obtain graphites with low thermal expansion and high thermal 9

^Present Address: Decatur, TX

Poco Graphite, Inc., P.O.

Box

2121,

76234.

129

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

130

PETROLEUM DERIVED CARBONS

conductivity, which i s essential for thermal shock resistance. The search for highly acicular f i l l e r s led to the preparation of green cokes from Ashland O i l Company's A-2U0 petroleum pitch. Properties of graphites fabricated with the A-2U0-derived green cokes were then compared with properties of graphites fabricated using high quality acicular cokes graciously supplied by Union Carbide Corporation, Carbon Products Division (UCC) and Great Lakes Carbon Corporation (GLCC).


Experimental A schematic view of the treatment and fabrication procedures u t i l i z e d i n this study can be seen i n Fig. 1. As indicated, both the UCC and GLCC green cokes were f i r e d for 1 hr at 500°C, p r i marily to reduce moisture and/or v o l a t i l e content to an acceptable level for fabrication. The A-2U0 cokes were prepared by f i r i n g approximately l800 g of A-2U0 pitch i n carbon boats for 1 hr at 575, 600, 6 5 Ο , and 675°C under an inert atmosphere. A heating rate of approximately 5°C/min was used. Weight losses after f i r i n g the pitch to the heat treatment temperature were U2.5, ^3.9, hk.l, and 1*5.5$, respectively. An additional f i l l e r was prepared by heating A-2U0 pitch at U25°C for 2k hr with an i n i t i a l room temperature argon pressure of 2000 p s i . The pressure-coked material was then f i r e d at 500°C for 5 hr. After f i r i n g , the cokes were ground and appropriate quanti t i e s of the A-2U0 binder pitch were dry-mixed with the f i l l e r and subsequently slurry blended using benzene u n t i l reasonably dry. After grinding, the mix was warm,-molded (approximately 95°C) at a pressure of 2000 p s i . The 1.6-in.-diam blocks were then placed in a graphite sleeve and force-fitted into an Inconel outer sleeve with graphite end plugs held t i g h t l y i n place by a threaded Inconel end cap. Before the end plugs were set i n place, a mix ture of coarse-grain graphite f i l l e r and natural flake graphite was placed between the end plug and the molded body. The end plugs were then securely held i n place with the threaded steel end caps. An unassembled, along with a completed assembled, restraining holder can be seen i n Fig. 2. The restrained blocks were then carbonized to 1000°C using a controlled three-day cycle. After carbonization, the samples were removed from the restraining holders and heated to 2800°C on a one-day cycle. Specimens were then machined from the graphi t i z e d block for the various property measurements. Results and Discussion Typical microstructures for the graphites fabricated during this study can be seen i n Fig. 3. These graphites were typically very uniform with a maximum particle size of approximately 120 um.


12.

HORNE AND

KENNEDY

Graphites

GLCC, UCC

131

A-210 PITCH COKED 1 HR

GREEN COKE

575,

600,

650

A-240 PITCH COKED 24 HR 125"C, 2000 PSI

675.*C 500 C, 5 HR


500*C, 1 HR

e

GROUND 10,

12,

U , 16 PPH

A-2A0 PITCH BINDER 2 PPH NITROBENZENE

I

t SLURRY MIXED IN HOBART MIXER USING BENZENE

I DRIED

i 1

GROUND MOLDED, 95 C, 2000 PSI AND HELD UNTIL COOLED TO ROOM ^TEMPERATURE e

PLACED IN RESTRAINTS CARBONIZED GRAPHIΤI ZED Figure 1.

Process description for the fabrication of the experimental graphites


132



Figure 3. Typical microstructure of graphites fabricated using the experimental cokes



12.

HORNE AND

KENNEDY

Graphites

133

In general, properties of the graphites fabricated from the UCC-, GLCC-, and A-2U0-derived cokes were quite similar and were directly dependent upon the f i n a l graphitized density, which, i n turn, was dependent upon the i n i t i a l green density. The greatest difference between the A-2U0 cokes and the UCC and GLCC cokes was the achievement of higher green densities of bodies fabricated from the A-2U0 cokes prepared at the lower heat treatment temperatures. As indicated i n Fig. k> graphitized densities appeared to be a linear function of green densities over the region studied. As i l l u s t r a t e d i n this figure, as well as the rest of the figures, the samples are identified according to f i l l e r . Thus, A-575 would denote the graphite fabricated from A-2^0 pitch heated at 575°C for 1 hr. As shown i n Fig. 5, graphitized densities for samples f a b r i cated from the A-2U0-derived f i l l e r s reached an apparent minimum as the heat treatment temperature of the pitch approached the 650-675°C range. Though a dotted line i s drawn below 600°C, data seem to indicate that significant binder content effects are present i n samples fabricated from f i l l e r s prepared at tempera tures greater than 625°C. The graphitized densities and, thus, the properties of the graphites can be improved by either single or multiple impregnations. As mentioned previously, the properties of the graphites were directly dependent upon the f i n a l graphite density and the next four figures emphasize this relationship. Figure 6 shows that e l e c t r i c a l r e s i s t i v i t y values decrease with increasing graphite density. The use of the term "normal" refers to the with-grain direction. Though the relationships are represented as linear functions, i n actuality, an exponential relationship exists. Highest r e s i s t i v i t i e s resulted from A-65O and A-675 as expected from their low graphitized densities. Resistivities of samples fabricated from the pressure-coked A-2U0 (I-A2U0) showed a similar type relationship, but yielded much lower r e s i s t i v i t i e s than were obtained for comparable graphite density. For example, graphitized blocks fabricated from the I-A2U0 with f i n a l densities of 1.63 and I.67 g/cm yielded with-/ across-grain r e s i s t i v i t i e s of 8l0/l2U0 and 7^0/1090 μΩ-cm, respectively. As shown i n Fig. 7, b r i t t l e ring strength increased linearly over the region studied as the graphitized density increased. B r i t t l e ring strength i s correlatable to U-point beam loading and tensile test values through a stress volume relationship. Because of the lower stress volume, higher strengths are obtained from b r i t t l e ring data than from either the U-point loading or tensile testing. Figure 8 shows a b r i t t l e ring specimen under compression. B r i t t l e ring data, modulli of e l a s t i c i t y , and fracture strains for the I-A2U0 specimens show no significant deviation from the density/property relationships exhibited by the other specimens. 3


134


rj-.

ι


•

Figure 4. The effect of green density on final density of graphites fabricated from green cokes

S

A-65Q

Β

A-475 Filler

Filler

ι·ι

1

2

Green Demiry (g/cm )

Figure 5. The effect of the A-240 pitch heattreatment temperature on the density of graphites fabricated from A-240'derived cokes


3

Graphites

HORNS AND K E N N E D Y

12.

1 ν

-

1

1

a

1

Ο

U C C Filler

Δ

G L C C Filler

•

A-575 Filler

•

A-400 Filler

S

A-650 Filler

Β

A-675 Filler

B ™ \

(oxiol)


ft

(normal)

1

1

I

1

Graphite Derwity (a/cm ) 1

Figure 6. The effect offinalgraphite density on the electrical resistance of graphites fabricated from green cokes

,/o y °

Ο

U C C Filler

Δ

G L C C Filler

•

1.5

A-575 Filler

•

A-600 Filler

S

A-650 Filler

Β

A-675 Filler

J 1.6

L

1.7

Graphite Oentity (a/cm ) 1

Figure 7. The effect offinalgraphite density on the strength of graphites fab ricated from green cokes




136

Figure 8. Brittle ring sample under test

/ χ

I

1.0|

0.8

Ο Δ • • Figure 9. The effect offinalgraphite density on the modulus of elasticity of graphites fabricated from green cokes

UCC Filler G L C C Filler A-575 Filler A-600 Filler

S

A-650 Filler

a

A-675 Filler

1.5 1.6 Graphite Density (g/cm ) 3

1.7 9



HORNE AND KENNEDY

Graphites

τ

• A-600 Filler EB A-650 Filler Β A-675 Filler

J 1.4

I

I

L

1.5 1.6 Graphite Density (g/cm )

1.7

3

Figure 10. The effect offinalgraphite density on the fracture strain of graphites fabricated from green cokes


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P E T R O L E U M DERIVED CARBONS

Figures 9 and 10 indicate that both the modulus of e l a s t i c i t y and fracture strain, respectively, increase as the graphite density increases.


Conclusions In conclusion, graphites fabricated from A-2U0-derived green cokes show similar properties to graphites fabricated from highquality acicular cokes. Graphite densities equal to or surpassing the densities of bodies fabricated using either the UCC or GLCC f i l l e r s were obtainable by the choice of the appropriate A-2U0 coke preparation temperature and the appropriate binder content. Properties of the graphites fabricated from the A-2U0-derived cokes were dependent upon achievement upon heat treatment temperature. Graphites fabricated from pressure-coked A-2U0 pitch yielded much lower e l e c t r i c a l r e s i s t i v i t i e s than expected from density/resistivity relationships. However, other properties were similar to those obtained from the UCC, GLCC, and other A-2U0derived cokes. Thus, high-quality acicular cokes can be easily prepared from A-2U0 pitch and these cokes exhibit similar properties to other high-quality acicular cokes. Literature Cited 1.

2.

Kennedy, C. R., and Eatherly, W. P., 11th on Carbon, Gatlinburg, Tennessee, June 4, p. 131. Kennedy, C. R., and Eatherly, W. P., 11th on Carbon, Gatlinburg, Tennessee, June 4, p. 304.

Biennial Conference 1973, CONF-730601, Biennial Conference 1973, CONF-730601,

This work was funded by the U.S. Naval Surface Weapons Center under the REVMAT Program, P.O. 4-0208 . **0perated for the Energy Research and Development Administration by Union Carbide Corporation, Nuclear Division.


Graphites Fabricated from Green Petroleum Pitch Cokes

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