Low-temperature graphitization of cokes and binder-filler artifacts

Carbon,

1972, Vol. 10, pp. 409-415.

Pergamon

Press.

Printed

m Great

Britain

LOW-TEMPERATURE GRAPHITIZATION OF COKES AND BINDER-FILLER ARTIFACTS G. B. ENGLE Gulf General Atomic, San Diego, California 92112, U.S.A. (Received 11 March 1971) Abstract-A series of metal-doped cokes or compacts was prepared with aluminum or titanium and heat treated at 1700-2300°C. Aluminum-doped cokes and compacts readily converted to graphite at 2300°C. The percentage of graphitic structure produced exceeded the original aluminum content by a factor of from 3 to 13 depending on the original metal content in the baked material. When carbon structure is converted to graphite, the Al& evolves from the material. Graphite compacts fabricated with aluminum were highly crystalline but had low bulk densities due to the evolution of the aluminum during heat treatment. Titanium was not as effective in converting disordered carbon to graphite below 2300°C. When aluminum and titanium were present together, the conversion to graphite was more complete than when aluminum alone was used. 1. INTRODUCTION The

high-temperature

of carbons

and

graphites

irradiation

metal-doped crudes were carbonized slowly to 900°C. Compacts were prepared from the various metal-doped cokes by mixing with a coal-tar pitch binder (approximately 25 wt %) and molding the mixture at 4500 psi while heating to 800°C in 1.5 hr. The metal content is quoted for cokes and compacts after carbonization.

behavior

is sensitive

to the

size L,[l]; the stability of the materials increases with increasing L, values when irradiated above 500°C. If the crystallinity of carbonaceous reactor materials could be significantly improved, then considerable improvement may be expected in their irradiation behavior. The use of metallic additives to enhance graphitization below 2800°C is attractive for this purpose, provided the additive can be removed and does not constitute a nuclear poison. Aluminum is of particular interest because of its low capture cross section for neutrons and the low decomposition temperature of Al,&. apparent

crystallite

3. EXPERIMENTAL The

degree

using X-ray

TECHNIQUES

of graphitization

techniques.

was measured

The apparent

crystal-

size L, was measured with a standard X-ray diffractometer, using powdered samples. The lattice parameter Co was determined from the location of the peak of the (002) reflection. Corrections were made for instrument line broadening and doublet broadening according to methods described in Ref. lite

2. PREPARATION

OF COKES AND COMPACTS

PI.

The distribution of the various structural states of carbon in the graphitized compacts was determined by a densimetric technique described in Ref. [3]. The compacts were cornminuted to particles of O-1-38 pm and the particles were separated in a densitygradient column according to their apparent

Metal-doped cokes were prepared by adding very fine metal particles (< 40 pm) to a reduced crude feed stock prior to carbonization. The reduced crude was distilled prior to carbonization to improve the carbon yield and to produce a viscous vehicle for the dispersion of the fine metallic particles. The 409

G. B. ENGLE

410

densities. Density fractions were recovered and the degree of graphitization of each fraction was determined by the X-ray diffraction method described above. 4. EXPERIMENTAL

RESULTS

Two series of coke specimens and two series of compacts were prepared with aluminum or a combination of aluminum and titanium. The specimens were heated at 1700-2300°C for up to 32 hr to study the effect of the metal additives on the degree of graphitization of the carbon structure. In the first coke series the control sample (metal-free coke) and the metal-doped samples, which ranged from 4.7 to 7.3 wt% aluminum or titanium, developed about the same unit cell dimensions (6.71-6.72 A) and apparent crystallite sizes (230-460 A) when heated at 2300-2700°C. The control coke from this batch of distilled crude progressed to an intermediate state of graphitization upon heating in the above range, and the addition of small amounts of aluminum or titanium did not catalyze the graphitization process further.

The second series of coke samples and the first series of compacts were prepared with higher metal content and heat treated at 1700-2300°C (Table 1). The control compact and the coke sample with 7.3 wt % aluminum did not graphitize. Samples with 14.3 wt% aluminum developed traces of graphite structure below 23OO”C, as shown by the shading in Table 1, and then developed closer layer plane spacing at 2300°C. Samples with 25 wt % aluminum developed predominantly graphite peaks on their X-ray profiles at 1900-2100°C and converted to a highly graphitic structure after heat treatment at 2300°C. A comparable sample with 24 wt % titanium developed small graphite peaks throughout the range 1900-23OO”C, but the bulk of the material remained unconverted. These samples appeared to consist of a carbon structure matrix with a small volume of graphitic structure present. When aluminum and titanium were present together, the X-ray patterns for samples heated at 17002100°C were similar to the samples containing only 24 wt % titanium heated at 1700-2300°C. After heat treatment at 2300°C they were

Table 1. X-ray diffraction data on metal-doped

carbons

LOW

TEMPERATURE

converted to graphite. The bimetal sample with 6.8 wt % aluminum was comparable to the 14.3 wt% aluminum sample, and the bimetal sample with about 12.0 wt % aluminum fell between the 14.3 and 25 wt% aluminum samples. Metallographic examination of samples containing 7.3 and 14.3 wt % aluminum and 12.1 wt % Al, 24.2 wt % Ti after heating to 2300°C reveals that graphite crystals have grown in all samples; these crystals appear as large well-defined regions in a carbon matrix (see Fig. 1). The X-ray diffraction profiles do not distinguish the graphite crystals when they are not in abundance. A spot check of the metal content after heat treatment showed that aluminum evolved almost completely, while the titanium remained within the sample after heat

411

GRAPHITIZATION

treatment at 2300°C. Weight losses, measured on the compacts in Table 1, showed samples containing titanium to have weight losses of 5.3-7.0 wt % above that of the control specimen at 1900-23OO“C, while the weight losses of the aluminum-containing samples increased with increasing temperature until after 3.0 hr at 2100-2300°C the weight losses exceeded the combined values of the weight loss of the control and that of the original aluminum content by 3-10 wt 70. The second series of compacts was heated for 6 hr at 2300°C (Table 2). The C,, values for the specimens containing 4.1 to 10.3 wt % Al were somewhat improved over the C,, value for the control specimen, but the I,, values were not much different. The speccontaining 16.7-17 wt% Al were imens graphitized. The aluminum content of all

Table 2. Metallic content, densities, and X-ray data of Al-doped compacts heated at 2300°C for 6 hr Aluminum content (wt %)

Specimen number 3726-24-B 3726-27-A B C D 3726-27-10A B C 3726-27-PO& B C D 3726-26-l

Density (g/cm3)

Petroleum asphalt 0 5

10

20

20

900°C

2300°C

X-ray data

Immersion in liquid

Compacts Bulk

Bulk

(A)

Particles

Lf

L,.t

c,,

0 4.2 4.1 4.2 4.2 10.2 10.0 10.3

0 0.0045

1.83 1.63

2.16 2.08

2.14 2.10

270 238

245 295

6.77 6.75

0.0038

1.33

2.02

1.975

290

310

6.76

16.7 10.3 16.8 16.9 16.8 - 17.0

0.0038

1.18

2.17

2.165

1160

1370

6.72

2.19

2000

3100

6.71

-0.0040

0.895

*L, obtained from aggregate specimen. tSpecimens were comminuted to - 10 pm average particle size and separated in a density column. L, = 3 L,ifi, wherefi is the volume fraction of material of given density p, and Lri is the apparent crystallite size of each fraction of a given density p[6]. Data calculated from Table 3. *Thermal expansivity: parallel, 3.1 X lO@‘C-‘; perpendicular, 2.2 X 106”C-I.

-II:!

(;.

B.

compacts was reduced by heat treatment to - 0.0040 wt %. The bulk density of the compacts decreased with increasing metal content due to the loss of aluminum, but the apparent density measured by immersion in liquid decreased up to 10 wt 5% and then increased at the highest metal content. This increase corresponded with the conversion to a graphitic structure and reflects the higher density of the graphite crystals over that of the unconverted carbon particles. The orientation factor (ratio of longitudinal coefficient of thermal expansion to radial coefficient of thermal expansion) of the specimen with 20 wt % aluminum was 1.40. While this is not considered very isotropic in comparison with materials fabricated with isotropic cokes such as Gilsocoke, it was considerably more isotropic than materials such as compacted natural-flake graphite, which is its equivalent with respect to crystallite size and perfection. The compacts were comminuted to small particles (0.1-38 pm, average - 10 pm) and the particles were separated in a densitygradient column as described in Refs. [3] and 141. The particles were recovered incrementally and each density fraction was examined to determine L,. and C,, values. The data are shown in Table 3. This technique provides a means whereby the carbon and graphitic structures can be physically separated and examined independently. The data in Table 3 reveal considerably more details of the distribution of carbon structures within each compact than the X-ray data in Table 1, which were obtained on aggregates. The X-ray data on the density fractions from the zero and 4.2 wt % samples are essentially the same as the data on the aggregates; i.e., no conversion has occurred. About 10 wt 5%of the structure of the TO-2 wt c/Csample has been converted to graphite, whereas the data on the aggregate do not reveal this detail. Nearly all of the 16*8-17.0 wt%. samples has been converted to a struc-

ENGLE Table

Specimen number

3. Distribution of carbon structures Al-doped graphites Al content in baked sample (wt %I 0

4.2

10.2

l&X

17.0

wt %, of tvtal sample

AFWage density WW

L,

c,,

(A)

I!\)

in

Fig. 1. Photomicrographs showing crystal growth in petroleum coke. (a) 7.3 wt % Al, 2300°C for 32 hr; (b) 14.3 wt % Al, 2300°C for 32 hr; (c) 12.1 wt % Al, 24.2 wt % Ti, 2300°C for 8 hr.

Fig. 2. I’hotomicrographs of molded compacts heated for 6 hr at 2300°C. Aluminum content (wt %) in the baked compact: (a) 0, (b) 4.2, (c) 10.2, (d) - 17.0.

Fig. 1. Photomicrographs showing crystal growth in petroleum coke. (a) 7.3 wt % Al, 2300°C for 32 hr; (b) 14.3 wt % Al, 2300°C for 32 hr; (c) 12.1 wt % Al, 24.2 wt % Ti, 2300°C for 8 hr.

Fig. 2. I’hotomicrographs of molded compacts heated for 6 hr at 2300°C. Aluminum content (wt %) in the baked compact: (a) 0, (b) 4.2, (c) 10.2, (d) - 17.0.

LOW

ture that is equivalent to natural-flake graphite. The photomicrographs in Fig. 2 confirm the X-ray data. The structure in Fig. 2(d) consisted of large volumes of graphite crystals, which appear to have formed around the remnants of metallic particles as in the coke specimens. When the data from Table 3 are arbitrarily clivided according to L, values of <500, 500-1000, and > 1000 A and C,, values greater than and less than 6.750, a semiquantitative analysis of the conversion of the carbon structure to graphite emerges (Table 4). The specimens that contained 17 wt% Al converted almost completely to a graphitic structure, and about 70 wt % of the structure of these specimens was equivalent to naturalAake graphite. The specimen containing 10.2 wt % Al had about 10 wt % of its structure converted to graphite, whereas the 4.2 wt % sample did not convert but remained in a semigraphitic state as did the control sample. 5. DISCUSSION

OF LITERATURE

A number of investigators have added a variety of metals or metallic compounds to carbons and observed the reaction products after heating at temperatures below 2600°C [5-141. The conclusion drawn from these papers is primarily that many metallic additives can enhance the transformation of carbon structure to graphite at low or intermediate temperatures. Gillot et al. [15] showed by Table 4. Semiquantitative

Specimen number

Aluminum content in sample (wt %‘c)

24-B A-5 A-10 A-20 26-l

0 4.2 10.2 16.8 - 17.0

100 100 90 12 12

all Al&

decomposed

*Assuming

413

GRAPHITIZATION

TEMPERATURE

microscopy that compounds of chromium entered coke particles by diffusion after heating to only 1100°C. Upon heating t6 1600” to 1800°C graphite crystals were formed and their production increased rapidly with increasing temperature, but evaporation of the chromium limited the conversion above about 1800°C. Gillot et al. [16] have also worked in the same manner with Ti and V. In these studies they observed that carbide particles, 2-5 wt % metal, entered the grains of compacts prepared with furfuryl coke (nongraphitizing coke) at 1800-2800°C and left a trail of well-graphitized carbon. Oberlin and Rouchy [17] studied the transformation of hard carbons to graphite at 1600°C by adding about 10% pure iron powder. Examination by electron microscopy showed complete transformation of the hard carbon particles to graphite leaving hollow spheres. Fitzer and Kegel[l8] worked with V additives in disordered carbons and concluded that the driving force for graphitization in the presence of a carbide is the difference in free energy between disordered carbon and graphite in the carbide. They demonstrated this point experimentally by showing the disintegration of a disordered carbon when exposed to VC in contrast to the inertness of natural-flake graphite. They found similar evidence for Ti, Zr, and carbon-saturated melts of iron and nickel. Sixty-five per cent of a glassy carbon compact was converted to

analysis of Al-doped compacts

Wt QZof sample with L, values of: Measured <500

500-1000

>I000

0 0 0 14 14

0 0 10 74 74

Calculated* > 1000 0 1.6 4.1 7.3 7.3

in situ. to form near-perfect

Wt % of sampie with C,,valuesof: >6.75

~6.75

100 99 80 0 0

0 1 20 100 100

graphite crystals.

G. B. ENGLE

414

graphite in about 2 min using relatively small amounts of carbonyl nickel. In this case the nickel was molten. In later work Weisweiler [19] and Weisweiler and Subramanium[20] found more evidence to support the mechanisms for catalytic graphitization by Fitzer and Kegel. Yokokawa et al. [21] heat treated carbons with copper. They concluded that copper accelerated the graphitization of hard carbons at low temperatures but soft carbons were not affected. The above investigations clarify that metallic constituents, usually in the molten state, will penetrate coke structures under certain conditions and convert them to graphite at temperatures from 1400 to 2700°C. However, the degree to which these mechanisms operate is sensitive to the carbon structure, the additive, and time-temperature conditions. When the experiments described above involved constituents in the molten state, the conversion to graphite was usually rapid. 6. DISCUSSION

OF PRESENT

RESULTS

The choice of aluminum or titanium as the metal additives and the temperature range of 1700-2300°C precludes the formation of molten carbide phases in the samples. Al&, dissociates at 2200-2500°C [22] and TIC melts above 3000°C. If the graphitic structure in the samples where aluminum was added resulted from the simple dissociation of Al&, as described by Foster et al. [22] with all of the aluminum subsequently evolving and leaving graphite crystals in the sample, then one would expect the small graphitic yields such as those shown in Table 4 (calculated values for L, > 1000 A). However, yields of extremely graphitic structure, considerably higher (by a factor of about 3 and 13 for aluminum contents of 10.2 and 16.8 wt%, respectively) than the calculated values, were observed experimentally. The graphite crystals observed (see Figs. 1 and 2) resemble the graphite crystals formed from the dissociation of Al&, at 2400°C [22]. The

decomposition of Al& may begin below 2000°C as described by Foster et al. [22], and this could account for the presence of graphite crystals as evidenced by graphite peaks at 1900°C. There appears, however, to be some mechanism operating to produce graphite either by reuse of the available aluminum to form additional Al&, as it passes out of compacts or by some other unexplained mechanism. If dissociation of the Al&, is responsible for the conversion to graphite, then the available aluminum would be required to recombine with excess carbon structure to reform Al&~, which would subsequently dissociate again to form additional graphite as it passed out of the compact. This mechanism seems unlikely unless the carbide was thermally cycled above and below its decomposition temperature while evolution is taking place. Where titanium was present, traces of graphitic structure were detected at 170023OO”C, but the bulk of the carbon structure remained untransformed. These data are in agreement with those of Price and Bokros [23] who found graphitization of titaniumdoped pyrolytic carbon to begin at 2300°C and to proceed rapidly at 2300-2500°C. When aluminum and titanium were present together, graphitization was enhanced over that for samples of aluminum or titanium alone; i.e., it required less aluminum to promote graphitization at 2300°C if titanium was abundant. This observation deserves a more detailed investigation. 7. CONCLUSIONS Carbon compacts prepared with aluminumdoped cokes will graphitize completely at 2300°C. The transformation of the carbon structure to graphitic structure exceeds the amount expected from a simple decomposition of Al&, by a factor of 3-13 as the aluminum content is increased from about lo-17 wt % in the baked article. Carbon compacts containing titanium did not graphitize

LOW TEMPERATURE

at 23Oo”C, but showed traces of graphitic structure. When aluminum and titanium were present together, conversion to graphite was more complete at lower aluminum contents in comparison with aluminum alone. Graphite compacts graphitized at 2300°C or below with the aid of additives had low bulk densities due to the loss of Al&,. Acknowledgements-The author gratefully acknowledges the help of J. Pontelandolfo and L. Bailey with sample preparation and experimental measurements. This work was supported by the U.S. Atomic Energy Commission, Contract AT(04-3)-167, Project Agreement No. 12. REFERENCES 1. Bokros J. C., Guthrie G. L. and Schwartz A. S., Carbon 6,55 (1968). 2. Klug H. P. and Alexander L. E., X-Ray DF fraction Procedures. John Wiley, New York (1954). Engle G. B., Morris W. H. and Bailey L., Carbon 8,393 (1970). Engle G. B., Carbon 8,485 (1970). Yokokawa C., Hosokawa K. and Takegami Y., Carbon 4,459 (1966). Yoshizawa S. and Ishikawa T., Symposium on Carbon, Nippon Toshi Center, p. 111-20-l. Carbon Society of Japan, Tokyo (1964). 7. Ishikawa T. and Yoshizawa S., Kogyo Kagaku

GRAPHITIZATION

415

Zasshi 66,929 (1963).

8. Noda T. et al., Bull. Chem. Sot. Japan 42, 1738 (1969). 9. Yamada I., Symposium on Carbon, Nippon Toshi Center, p. 111-21-l. Carbon Society of Japan, Tokyo (1964). 10. Baraniecki C. and Pinchbeck P. H., Carbon 7, 213 (1969). 11. Noda T. and Tanaka M., Kogyo Kogaku Zasshi 65, 1329 (1962). 12. Nishihawa T. and Kiegawa T., Japanese Patent 20,354 (1963). 13. Gillot J. and Lux B., French Patent 1,491,497 (1967). 14. Parker W. E., Marek R. W. and Woodruff E. M., Carbon 2,395 (1965). 15. Gillot J. et al., Ninth Biennial Conf on Carbon, Boston College, Chestnut Hill, ‘Mass., June (1969). 16. 1-1;:). J. et ul., Ber. Deut. Keram. Ges. 45, 224 17. Oberlin A. and Rouchy J. P., Ninth Biennial ConJ on Carbon, Boston College, Chestnut Hill, Mass., June (1969). 18. Fitzer E. and Kegel B., Carbon 6,433 (1968). 19. Weisweiler W., High Temperatures-High Pressures 2, 187 (1970). 20. Weisweiler W. and Subramanium N., High Temperatures-High Pressures 2,411 (1970). 21. Yokokawa C., Hosokawa K. and Takegami Y., Carbon 5,475 (1967). 22. Foster 1~. M., Long G. and Stumpf H. C., Am. Mineralogist 43,285 (1958). 23. Price R. J. and Bokros J. C., Carbong, 205 (1971).