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The catalytic graphitization of naphthalenediol and a urethane foam — a feasibility study
THE CATALYTIC GRAPHITIZATION OF NAPHTHALENEDIOL AND A URETHANE FOAM -A FEASIBILITY STUDY R. L. COURTNEY
and S. F. DULIERE
Polymer Science Division and Electra-Optical Analysis Division, Sandia Laboratories. Albuquerque. New Mexico, U.S.A. (Recezued 26 March 19’71) Abstract-Catalytic graphitization was accomplished by the addition of Cobalt, Nickel, and Iron to the original polymers of a urethane foam and naphthalenediol, respectively. Structural changes were observed using the scanning electron microscope, as well as changes in interplanar spacings (d,,,) and apparent crystallite sizes (L,) by X-ray diffraction. Electron probe and electron diffraction were also used in determming structures present. At lOWC, partial graphitization occurred and what is believed to be the transition phase from amorphous carbon to graphite was detected. In most specimens, after heat treatment for 0.5 hr at 1500%. doozwas 3.36 A and L, was as high as 220 1%. 1. INTRODUCTION Acheson
was the first to produce
artificial
graphite toward the end of the nineteenth century [ 11. The conversion to graphite involved the heating of an amorphous carbon
in an electric furnace to temperatures of 2600-3000°C. This process is still normally employed today whenever attempts are made to prepare artificial graphite from polymeric structures which afford hard carbons upon carbonization. One method of determining a degree of graphitization in these structures consists of measuring, by X-ray diffraction techniques, the interplanar spacing and the crystallite dimension normal to the crystallographic c axis. Heating a hard carbon to a temperature as high as 3000°C does not guarantee its conversion to a well-defined graphitic structure. Many carbons of this type either remain as non-graphitized structures, or are only partially graphitized. One possible explanation for the behavior of these non-graphitizing carbons is that they have previously formed a strong system of crosslinks uniting small packets of parallel layers of carbon
Carbon
V”l
IO No
1 -E
hexagon networks[Z]. ‘The crosslinks do not allow the displacement of the layer planes to the extent required to obtain a good graphite. Presumably, the non-graphitic carbon networks become ‘locked-in-place’ during the early stages of polymerization. Much is known concerning the precipitation of graphite from dilute solutions of carbon in metals, especially iron, but relativel) little is known on the effects of small amounts of metal in carbon[3]. However, evidence is available which shows that the addition of small amounts of various metals enhances graphitization [4.5]. ‘The purpose of this work was to determine the feasibility of graphitizing polymers which norm.llly do not afford graphitic structures, at temperatures significantly lower than the 2600-3000°C range, by the addition of various metal salts to the polymer prior to cure and subsequent heat treatments. 2. EXPERIMENTAL MATERIAIS TECHNIQUES
AND
2.1. Polymeric materials
‘The
polymeric
materials
studied
were
R. L. COURTNEY
66
naphthalenediol (L-8 resin, from Ironsides Resins, Inc., Columbus, Ohio) and a urethane foam system (CPR-727, the Upjohn Company, Terrance, California). CPR-727 is a PAPI” based urethane foam. 2.2. Metallic-salt additives In each polymer, three different metalsalt concentrates were used to determine the effect of concentration on graphitization. Table 1 lists the chemical analysis of the various initial metal-salt concentrations and polymer metal-salt combinations. The metalsalts (from Wilshire Chemical Co., Inc., Gardena, California) did not receive special treatment prior to sample preparation. Initially it was thought that by adding CoF3 and NiF, to the urethane, and FeF, to the naphthalenediol, the metal-salts might form complexes with the organic polymer. The structural changes accompanying the formation of the complexes would possibly facilitate graphitization. However, it was determined that formation of complexes had very little, if any, effect upon graphitization. 2.3. Preparation of samples The samples are identified according to the particular metal incorporated into the they were initially sample, even though *‘Trade name of Upjohn Company, a polyarylpolyisocyanate.
and S. F. DULIERE
prepared using the metal fluoride. The Fe-naphthalenediol samples. The first sample is referred to as the concentrated Fe-naphthalenediol sample. The FeF3 powder was added to an aluminum dish which contained the naphthalenediol polymer. After the metal-salt had settled to the bottom of the dish, the dish was placed upon a hot plate preheated to 150°F. The sample was heated for 0.5 hr at 150, 200 and 250”F, taking approximately 10 min for each 50°F increment increase in temperature. The sample received a final cure of 3 hr at 300°F in an air circulating oven. Because of the tendency to segonly one sample, which initially regate, contained approximately 6.5% FeF3 (3.21 wt. % Fe), was prepared in this way. The second type of sample prepared differed from that of the first in that while the sample was being heated, it was constantly stirred in an attempt to keep the metal-salt dispersed throughout the sample. Three samples were prepared in this manner using the concentrations mentioned in Table 1. Control samples, without the metal-salt additives, were also prepared. 2.4. The Co-, Ni-urethane samples Another type of sample was prepared using both the CoF3 and NiF2 powders. The metalsalt was placed into a cup which already contained the prescribed amounts of the prepolymer and polyol components. The mix
Table 1. Polymer metal-salt combinations
Polymer
Metal-salt*
Naphthalenediolt Urethane
FeF,
CoF, NiF,
Wt. % metal-salt 646,3.23,0.65 2.20, 1.10, 0.22 1.60,0+32,0.16
Wt. % metal
3.20, 1.60, 0.32 1~10,0~55,0~11 1~00.0~50, 0.10
*FeF, - Ferric fluoride CoF, - Cobalt trifluoride NiFz - Nickel difluoride tAttempts to graphitize naphthalenediol with copper (ic) difluoride (CuF,), titanium diboride (TiB*), tantalum diboride (TaB,), tungsten boride (WB), titanium oxide (TiO,), boron (B) and Boron carbide (B&) were unsuccessful.
CATALYTIC
GKAPHITIZATION
was continuously stirred with a spatula until the foam began to rise. The foaming action was sufficient to keep the powders dispersed and suspended throughout the sample. Control samples were also prepared in the same manner. All the urethane samples received the following combined cure and postcure cycles: Temp. (“F)
Time W
200 225 250 300 350 400
16 16 4 4 4 24
2.5. High temperature heat treatment of samples
The Fe-naphthalenediol and the Co-, Niurethane samples were heat-treated under identical conditions. After cure, 4 specimens were cut from each sample, 1 of which was used as a control specimen. The remaining 3 specimens received subsequent heat treatments for 0.5 hr at 850, 1000 and 1500°C. The heat treatment up to 1000°C took place in a graphite crucible under vacuum. The heat-up periods were 30 and 45 min for 850 and lOOo”C, respectively, while the cool-down rime for both was approximately 2 hr. A graphite resistance furnace, with a positive flow of helium, lvas used for heat treating the specimens at 1500°C. An optical pyrometer was used in determining when the furnace had reached temperature. It took approximately 1 hr to heat the specimens to 1500°C. In cooling, the specimens were furnace-cooled for 2 hr, then pulled down into a quench chamber and removed. 2.6. Experimental
measurements
In the systems examined, there was no method established to quantitatively determine the amount of graphitization. There-
67
fore, no attempt was made to assign a value to these properties and only relative comparisons are made. After each heat treatment, the existence of graphitization was established by X-ray diffraction measurements and the microstructure characterized by scanning microscopy and electron probe. Although some of the samples were analyzed by X-ray diffractometry, most of the data were obtained by the Debye-Scherrer powder method. The samples required a 4 hr X-ray exposure using manganese-filtered iron K, radiation (X = 1.9373 A) for samples containing cobalt and iron while nickel-filtered copper K, radiation (h = 1.5418 A) was used for the specimens containing nickel. The purpose of the X-ray diffraction analysis was to define, whenever possible, the interplanar spacing (d002) and apparent crystallite size normal to the d,,,, planes as a function of heat treatment temperature. The relative degree of graphitization can be assessed by changes in the interplanar spacing and crystallite size. The d-spacings were calculated from the data read from the exposed films on a filrn reading device after correcting for film shrinkage. In order to determine the L,. parameter, tracings of the films were obtained from an optical microdensitometer and the full width of the d,,, diffraction profile was measured at onehalf the maximum peak intensity (FWHM). The value of L, was then calculated using the method of Warren and Scherrer [S],
where, K = a shape factor equal to 0.9 for FWHM measurements D or L, = the mean dimension of the crystallites normal to the diffracting planes /3= corrected line breadth b = instrumental broadening 0 = Bragg angle A = wavelength
68
R. L. COURTNEY
The approximate values of L, were included only to represent trends and should not be considered absolute. After each heat treatment, the X-ray diffraction patterns were also examined to determine the temperature at which the diffuse halos representing the amorphous components, initially present, were no longer detectable. A Philips EM-200 electron microscope was used for electron transmission and diffraction analyses, and an Applied Research Laboratories (ARL), Inc., electron probe was used to determine the presence and distribution of metal atoms. The X-ray data obtained at 1000°C for the Co-urethane and the dispersed Fe-naphthalenediol specimens, indicated the presence of two phases: an amorphous phase, depicted by a diffuse halo, and a crystalline phase exhibiting a diffraction line on the outer edge of the diffuse halo. Therefore, the Co-urethane and the dispersed Fe-naphthalenediol specimens containing the highest per cent of additive, which were heat-treated at only lOOo”C, were chosen to be investigated using the SEM in order to study the initiation of a transition from the amorphous carbon to the graphite phase.
3. EXPERIMENTAL
RESULTS
Since graphitization was catalyzed by the presence of molten metals, and not the fluoride salts initially added, the remainder of the report will emphasize the effect of the specific metal upon graphitization.
3.1. X-ray di$raction data The X-ray diffraction data listed in Table 2, were obtained using the Debye-Scherrer powder method. In some instances there is no value of d,,,, listed for the halo at that particular temperature; in these cases the halo was too diffuse to obtain a value for d 0,,2. The values of L, were obtained from measurements on the crystalline phase.
and S. F. DULIERE
3.2. Control samples The naphthalenediol and urethane control samples (Figs. 1 and 2, respectively) show that without the presence of an additive, graphitization did not occur at 1500°C. These SEM photographs illustrate the smooth conchoidal fracturing glasslike surface present throughout both samples. The X-ray data (Table 2) also depict the nongraphitic nature of the resulting structure. 3.3. Zron as a graphitizing agent The concentrated Fe-nuphthalenediol sample. The scanning electron photomicrographs (Figs. 3 and 4) were taken after the specimen had received the final heat treatment of O-5 hr at 1500°C. It is quite obvious from the photomicrographs that a transformation has occurred which produced a layered structure similar to that of graphite. Figure 4 shows a portion of the layered structure. This layered structure was not present on the top surface of the specimen, which was intentionally voided of any FeF3 when preparing the specimen. Since compounds of FeC and graphite have very similar X-ray diffraction characteristics, other techniques were used to verify that the layered structure was not iron carbide but graphite. The sample was analyzed using the electron probe to determine if iron atoms were part of the ‘flake’ material (Figs. 5-7). In Fig. 5, the flake material is as indicated by the arrow, and the lighter spotted areas around the flake are iron. As can be seen, no iron is contained within the flake, but Fig. 6 shows the flake to be composed of carbon. The areas around the flake are composed of both iron and carbon. In order to remove the iron, the specimen was etched with dilute hydrochloric acid and the scanning electron photomicrograph of the resulting structure is as shown in Fig. 8. An electron diffraction pattern (Fig. 9) of the layered, flake material shows the existence of a crystalline graphitetype structure.
Fig. I, ~~phth~llenedi~l
control
specimen
(1500%) - x 1000.
Fig. 3. Concentrated Fe-naphthaienediol specimen (1NO”C) - X 1COO.
I:ig-.
11. L‘wthme
control
qwt-imen
( 1500°c) - x 20,000.
Fig. 4. Concentrated thalenediol specimen 3000.
Fe-naph(1500”(~)-- X
Fig. 5. Electron probe analysis for iron in Rake material.
Fig. 7. Backscatter efectron probe analysis-concentrated Fe-naphthalenediol.
Fig. 6. Electron probe analysis for carbon in Rake material.
Fig. 8. Concentrated Fe-naphthalenediol specimen (lSOO*C) -etched with dilute HCI.
Fig. 9. Electron diffraction profile for the concentrated Fe specimen.
Fig. 11. 140% me-naphtha~e~ediol men (1500°C) - X 10,000.
speci-
Fig. 10. 0.32% Fe-naphthalenediol specimen (IMfO”C) - X lO,(~O.
Fig. 12. 3.20% Fe-naphthaienedioi specimen (1500°C) - X 10.000.
Fig.
13. 3.20% Fe-naphthalenediol specimen (850°C) - X 3000.
Fig. 15. Amorphous phase-3.20% Fe-naphthalenediol specimen ( 1000°C) - x 10,000.
Fig.
14. 390% Fe-na~hthalened~~)~ specimen (1OOO’C)- X 3000.
Fig. 16. Graphite phase-3.20% Fenaphthalenediol specimen (lOOO’C)x 10,000.
Fig. 17. 0.11% Co-urethane specimen (1500°C) - x 10,000.
Fig. 18. 0.55% Co-urethane specimen (1500x)x 10,OON.
Fig.
Fig. 20. 1.10% Co-urethane specimeu (1500°C) particulate loading-x 20,000
19. 1.10% Co-urethane specimen (15OO”C)- x 10,000.
Fig. 21. 1.10% Co-urethane specimen ( looo”c) - x 3000.
Fig. 23. Graphite phaseurethane specimen x 10.000.
1.10% Co(lOOO’C)-
Fig. 22. Amorphous phase-l.1096 specimen ( 1000°C) Co-urethane x 10.000.
Fig. 24. Electron diffraction profile for the 1.10% Co-urethane specimen (1500°C).
Fig. 25. 0~10% Ni-urethane
specimen
(15oo”c) - x 10,000.
Fig. 27. I*OO’Z Xi-urethane
specimen
( 1500°C)- x 10,000.
Fig. 26. O~.SO% Xi-urethane specimen ( 1300%) - x 10,000.
diffraction profile Fig. ‘LX. Electmn for the 1.OO% Si-urethane specimen (1500”(:).
Fig. 29. 1.OO% IQ-urethane loading
(1500°C) particulate
-
specimen X 30,000
CATALYUC
GRAPHITIZATION
Table 2. X-ray diffraction data:*: Debye-Scherrer
Temp. PC)
System Urethane Urethane+
control
1500
0.10% Ni
Urethane + 0.50% Ni
L’rethane-t
1.00% Ni
L’rethane + 0.11% Co
L’rethane + 0.55% Co
Urethane + 1.10% Co
Naphthalenediol control Naphthalenediol+ 0.32%’ Fe
Naphthalenediol-t
3.20%’ Fe$
Cured 850 1000 1500 Cured 850 1000 1500 Cured 850 1000 1500 Cured 850 1000 1500 Cured 850 1000 1500 Cured 850 1000 1500 1.500 Cured 850 1000 1500 (lured 850 1000 1500
powder method
do,, $1 Halo 36 3.8 4.5 3.7 3.7 $ 4.5 3.8 I: 4.5 4 4.5 * 4.5 $ 4.5 I + 3% 3.6 $ 46 $
Line
Approximate (A,
3.36 3.36
255
3.40 3.40 3.36
110
3.40 3.40 3.36
130 I50 155
3.40 3.40 3.40
L,i-
70 100
80 90 110
3.40 3.39 3.37
80 I10 135
3.41 3.36 3.36
70 255 220
3.40 3.40 3.36 3.40 340 3.36
‘.Heat soak of 0.5 hr at each temperature. tL, of the line only. $The heat treatment temperature at which the diIfuse halo. inmallv present, dissappeared or was not measurable. QThe specimen heat treated for a total of 2 hr showed an increase of L,. to 2!!2. while having the same d,,,,. The dispersed Fe-naphthalenediol snmples. In contrast to the specimen mentioned above, these specimens initially contained dispersed Fe at 3 levels of concentration (‘Table 1). The SEM photomicrographs pertaining to the structure of these specimens after 0.5 hr
at 1500°C are Figs. 10-12.
These
represent
the
lowest to highest initial concentrations of Fe. As was observed from the X-ray data, the specimen having the lowest initial concentration of Fe produced a structure with a cI,,,,.. of’ 3.40 Lk and an L, of approximatelv 80 ii. Ita
70
R. L. COURTNEY
lamellar structure was not as well defined as the other 2 specimens initially containing higher percentages of Fe. Also, a diffuse halo was present after heat treating the specimen at 1500°C. The specimen containing the next highest concentration of Fe exhibited a more graphitic structure, as determined from SEM, than the above mentioned specimen. This correlated with the lower dooz (3.36 A) and the higher L, (175 A). The diffuse halo existed up to lOOo”C, and after 1500°C had essentially disappeared. The structure of the specimen (Fig. 12) having the highest concentration of Fe was very similar to that of the specimen just discussed. After heat treating the specimen at 15Oo”C, a d,,02 of 3.36 A and an L, of approximately 140 A were observed and the diffuse halo had disappeared. This same specimen was then heat-treated for an additional O-5 hr at 1500°C; the value for dooz was still 3.36 A, but the value for L, had increased from 140 to 220 A. In none of the above specimens did X-ray diffraction show the presence of FeF3. However, elemental iron was identified. It is interesting to note that all 3 specimens had d-spacings of 3.40 A at lOOO”C, and that after 0.5 hr at 15OO”C, the dooz for 2 of the 3 specimens had decreased to 3.36 A. Also, it was difficult to observe any iron particles after the final heat treatment. However, those which were observed appeared to be discrete spheres, indicating the existence of a once molten phase. Scanning electron photomicrographs of this same system were obtained after the 850°C and the 1000°C heat treatments. The results are shown in Figs. 13-16. The heat treatment at 850°C did not produce a graphitic structure. The particles appeared not as discrete spheres but were very angular. However, after lOOO”C, two distinct phases were present. The one phase was very graphitic in appearance, whereas the other looked as if a molten phase had been ‘frozen’ in place upon cooling the specimen. Figure 15 is an
and S. F. DULIERE
enlargement of the area observed in Fig. 14. The particles are discrete spheres, and a large number of the spheres are enveloped by carbon. This phase is believed to be a transition phase from amorphous carbon to graphite. A close-up of the graphitic structure is shown in Fig. 16. 3.4. Cobalt and nickel as graph&zing agents The Co-urethane specimens. Graphitic sturctures (Figs. 17-19) were obtained from all 3 cobalt specimens. The specimen which contained the least Co behaved much like the naphthalenediol sample which contained the least Fe, in that it attained ad,,, of 3.40 A and an approximate L, of 90 A at the highest heat treatment temperature. The other 2 compositions displayed d-spacings of 3.37 and 3.36 .& along with L, values of 140 and 220 A, respectively. The specimen containing the highest per cent Co also exhibited a 3.36 A diffraction line at 1000°C. Figure 20 shows the discrete spherical metal particles and their distribution in a small area of the specimen heat treated at 1500°C. X-ray diffraction analysis shows the presence of elemental cobalt after the 1500°C heat treatment with no CoF, detected. The series of SEM photomicrographs (Figs. 21-23) once again show the presence of 2 distinct phases. Prior to analysis, the specimen containing the 1.10% cobalt was etched with dilute hydrochloric acid. The electron diffraction pattern (Fig. 24) of this specimen shows its graphite structure (Fig. 23). One phase is graphitic, while the other consists of spherical metallic particles encased within a carbon, finger-like network. It looks as if the fingerlike carbon projections have actually grown together. Again, it is believed that this is a transition from amorphous carbon to graphite. This latter phase is very similar to that which was obtained with the Fe-naphthalenediol specimen; however, this transition phase appears to have taken place to a greater Fe-naphthalenediol extent than in the specimen.
CATALYTIC
GRAPHITIZATION
It is also worthy to note that as the concentration of Co was increased, the value for d,,, decreased while L, increased. Also, the decrease of d,,, as the temperature increased was a more gradual change than that enwith countered the Fe-naphthalenediol specimens. The Ni-urethane specimens. These specimens behaved similarly to the Co-urethane specimens. A marked difference is that the specimen containing the lowest per cent of Ni attained a d,,, of 3.36 A. whereas in the previous systems comparable specimens did not (doe, = 340 A). However, a slight diffuse halo persisted after 1500°C in this specimen. X-ray diffraction showed that NiF, was no longer detected after the 1500°C heat treatment. Elemental xi, however. was observed. The SEhl photomicrographs (Figs. 25-27) illustrate the graphitic structure of these specimens after the 1500°C heat treatment. Figure 28 is the electron diffraction pattern of the specimen containing l*OO% of Ni (after a dilute hydrochloric acid etch). The spherical nature of the particles can be seen m Figure 29. 4. SUMMARY
(1) The control samples (no metal salt additive) showed no signs of being graphitized when heated for 0.5 hr at 1500°C. (2) The Fe-naphthalenediol system underwent graphitization as did the Co-, Niurethane systems. (3) A molten metal phase was observed, as indicated by discrete spherical particles. There was no apparent agglomeration of the particulate loading. (3) The electron proble analysis showed that both carbon and iron were present; however, no iron was present in the graphitic flakes. (5) As observed from scanning electron photomicrographs, local graphitization was obtained in both the Fe-naphthalenediol and Co-urethane samples at 1000°C. A possible transition phase of the amorphous carbon
71
being converted to graphite was observed in both systems. (6) Crystallite growth occurred to a greater extent at 1000°C in the Co-urethane specimen than in the comparable Fe-naphthalenediol specimen. (7) The trend in most all specimens, as the temperature increased, was a decrease in do,,* and increase in L,. (8) With three exceptions, the amorphous carbon phase gradually disappeared and the graphitic phase appeared at 1500°C. The exceptions were the specimens containing the lowest per cent metal. (9) After graphitization, metallic particles were observed on the surface of the urethane samples. The particles were not as evident on the surface of dispersed Fe-naphthalenediol samples. (10) The X-ray powder patterns did not show the existence of any of the initial fluorides after graphitization. However, the elemental metals were detected in each system. (11) If any metal-carbon compounds had been formed in the graphitized areas examined, they were not detectable. (12) The naphthalenediol polymer was not catalvtically graphitized by CuF,, TiB,, TaB,, WB, TiO,, B and B,C within the designated times and temperatures. 5. CONCLUSIONS Fairly recent investigations of Baraniecki, Pinchbeck and Pickering[7] show that graphite nucleates around and within metal particles, and it is postulated that the rapid transport of carbon atoms through the liquid metal is responsible for the increased rate of graphitization. Three ways which graphite might be produced are: (1) the formation of an unstable carbide which decomposes to the metal and graphite; (2) the diffusion of the amorphous carbon phase into the molten metal phase with subsequent precipitation of the less soluble graphite; or (3) a combination of
72
R. L. COURTNEY
(1) and (2). The SEM photomicrographs obtained during this study of the transition phases indicate that the second method of graphite production plays the major role in the systems studied here. Also, since graphitization in this study appears to be time dependent, it is believed that the solutiondissolution process is diffusion controlled, whereby the amorphous carbon is continuously diffusing into the molten metal accompanied by continuous precipitation of graphite. Therefore, the solubility of amorphous carbon in the metal, the eutectoid temperature and the absence of stable carbides, should be critical factors for low temperature graphitization. This could explain why the other polymer-metal salt systems examined in this study did not graphitize at 1500°C. Carbon is not appreciably soluble in copper, and the other systems form eutectoids at temperatures higher than 1500°C. It is also noteworthy that stable carbides of cobalt and nickel do not exist above 800-900°C [8]. Baraniecki, Pinchbeck and Pickering[7] as well as Oberlin and Rouchy[9], were able to obtain graphite when adding iron powder to an already carbonized matrix. Therefore, the interaction which might have taken place at lower temperatures between the metalsalt and the organic material seems to have had no essential role in the catalytic graphitization observed during this study.
and S. F. DULIERE
The study has shown that the hard carbons formed from the urethane foam and the naphthalenediol control samples were not graphitized. However, by the addition of certain metal catalysts, graphitization occurred at temperatures well below those required for non-catalytic graphitization. Acknowledgements-The authors are indebted to Dr. B. Granoff for his helpful suggestions during the course of the study, and to Mr. C. J. Miglionico and Mr. C. R. Hills for their cooperation in the areas of electron scanning microscopy and electron diffraction, respectively.
REFERENCES 1. Acheson, E. G., U. S. Put. 568323, (1896). A. R., and Lewis, F. A., Graphite 2. Ubbelohde, and Its Crystal Compounds, Oxford University Press, Oxford (1960). 3. Van Vlack, L. H., Elements of Material Science (2nd Edition), Addison-Wesley, Palo Alto (1967). 4. Schwartz, A. S., and Bokros, J. C., Carbon, 5,325 (1967). 5. Presland, A. E. B., and Walker, P. L., Jr., Carbon, 7 1 (1969). 6. Taylor, A., X-ray Metallography Chapter 14, John Wiley, New York (1961). Baraniecki, C., Pinchbeck, P. H., and Pickering, F. B., Carbon, 7,213 (1969). Elliot, R. P., Constitution of Binary Alloys (First Supplement), Materials Science and Engineering Series, McGraw-Hill, New York (1965). Oberlin, A., and Rouchy, J. P., Carbon, 9, 39 (1971).
and S. F. DULIERE
Polymer Science Division and Electra-Optical Analysis Division, Sandia Laboratories. Albuquerque. New Mexico, U.S.A. (Recezued 26 March 19’71) Abstract-Catalytic graphitization was accomplished by the addition of Cobalt, Nickel, and Iron to the original polymers of a urethane foam and naphthalenediol, respectively. Structural changes were observed using the scanning electron microscope, as well as changes in interplanar spacings (d,,,) and apparent crystallite sizes (L,) by X-ray diffraction. Electron probe and electron diffraction were also used in determming structures present. At lOWC, partial graphitization occurred and what is believed to be the transition phase from amorphous carbon to graphite was detected. In most specimens, after heat treatment for 0.5 hr at 1500%. doozwas 3.36 A and L, was as high as 220 1%. 1. INTRODUCTION Acheson
was the first to produce
artificial
graphite toward the end of the nineteenth century [ 11. The conversion to graphite involved the heating of an amorphous carbon
in an electric furnace to temperatures of 2600-3000°C. This process is still normally employed today whenever attempts are made to prepare artificial graphite from polymeric structures which afford hard carbons upon carbonization. One method of determining a degree of graphitization in these structures consists of measuring, by X-ray diffraction techniques, the interplanar spacing and the crystallite dimension normal to the crystallographic c axis. Heating a hard carbon to a temperature as high as 3000°C does not guarantee its conversion to a well-defined graphitic structure. Many carbons of this type either remain as non-graphitized structures, or are only partially graphitized. One possible explanation for the behavior of these non-graphitizing carbons is that they have previously formed a strong system of crosslinks uniting small packets of parallel layers of carbon
Carbon
V”l
IO No
1 -E
hexagon networks[Z]. ‘The crosslinks do not allow the displacement of the layer planes to the extent required to obtain a good graphite. Presumably, the non-graphitic carbon networks become ‘locked-in-place’ during the early stages of polymerization. Much is known concerning the precipitation of graphite from dilute solutions of carbon in metals, especially iron, but relativel) little is known on the effects of small amounts of metal in carbon[3]. However, evidence is available which shows that the addition of small amounts of various metals enhances graphitization [4.5]. ‘The purpose of this work was to determine the feasibility of graphitizing polymers which norm.llly do not afford graphitic structures, at temperatures significantly lower than the 2600-3000°C range, by the addition of various metal salts to the polymer prior to cure and subsequent heat treatments. 2. EXPERIMENTAL MATERIAIS TECHNIQUES
AND
2.1. Polymeric materials
‘The
polymeric
materials
studied
were
R. L. COURTNEY
66
naphthalenediol (L-8 resin, from Ironsides Resins, Inc., Columbus, Ohio) and a urethane foam system (CPR-727, the Upjohn Company, Terrance, California). CPR-727 is a PAPI” based urethane foam. 2.2. Metallic-salt additives In each polymer, three different metalsalt concentrates were used to determine the effect of concentration on graphitization. Table 1 lists the chemical analysis of the various initial metal-salt concentrations and polymer metal-salt combinations. The metalsalts (from Wilshire Chemical Co., Inc., Gardena, California) did not receive special treatment prior to sample preparation. Initially it was thought that by adding CoF3 and NiF, to the urethane, and FeF, to the naphthalenediol, the metal-salts might form complexes with the organic polymer. The structural changes accompanying the formation of the complexes would possibly facilitate graphitization. However, it was determined that formation of complexes had very little, if any, effect upon graphitization. 2.3. Preparation of samples The samples are identified according to the particular metal incorporated into the they were initially sample, even though *‘Trade name of Upjohn Company, a polyarylpolyisocyanate.
and S. F. DULIERE
prepared using the metal fluoride. The Fe-naphthalenediol samples. The first sample is referred to as the concentrated Fe-naphthalenediol sample. The FeF3 powder was added to an aluminum dish which contained the naphthalenediol polymer. After the metal-salt had settled to the bottom of the dish, the dish was placed upon a hot plate preheated to 150°F. The sample was heated for 0.5 hr at 150, 200 and 250”F, taking approximately 10 min for each 50°F increment increase in temperature. The sample received a final cure of 3 hr at 300°F in an air circulating oven. Because of the tendency to segonly one sample, which initially regate, contained approximately 6.5% FeF3 (3.21 wt. % Fe), was prepared in this way. The second type of sample prepared differed from that of the first in that while the sample was being heated, it was constantly stirred in an attempt to keep the metal-salt dispersed throughout the sample. Three samples were prepared in this manner using the concentrations mentioned in Table 1. Control samples, without the metal-salt additives, were also prepared. 2.4. The Co-, Ni-urethane samples Another type of sample was prepared using both the CoF3 and NiF2 powders. The metalsalt was placed into a cup which already contained the prescribed amounts of the prepolymer and polyol components. The mix
Table 1. Polymer metal-salt combinations
Polymer
Metal-salt*
Naphthalenediolt Urethane
FeF,
CoF, NiF,
Wt. % metal-salt 646,3.23,0.65 2.20, 1.10, 0.22 1.60,0+32,0.16
Wt. % metal
3.20, 1.60, 0.32 1~10,0~55,0~11 1~00.0~50, 0.10
*FeF, - Ferric fluoride CoF, - Cobalt trifluoride NiFz - Nickel difluoride tAttempts to graphitize naphthalenediol with copper (ic) difluoride (CuF,), titanium diboride (TiB*), tantalum diboride (TaB,), tungsten boride (WB), titanium oxide (TiO,), boron (B) and Boron carbide (B&) were unsuccessful.
CATALYTIC
GKAPHITIZATION
was continuously stirred with a spatula until the foam began to rise. The foaming action was sufficient to keep the powders dispersed and suspended throughout the sample. Control samples were also prepared in the same manner. All the urethane samples received the following combined cure and postcure cycles: Temp. (“F)
Time W
200 225 250 300 350 400
16 16 4 4 4 24
2.5. High temperature heat treatment of samples
The Fe-naphthalenediol and the Co-, Niurethane samples were heat-treated under identical conditions. After cure, 4 specimens were cut from each sample, 1 of which was used as a control specimen. The remaining 3 specimens received subsequent heat treatments for 0.5 hr at 850, 1000 and 1500°C. The heat treatment up to 1000°C took place in a graphite crucible under vacuum. The heat-up periods were 30 and 45 min for 850 and lOOo”C, respectively, while the cool-down rime for both was approximately 2 hr. A graphite resistance furnace, with a positive flow of helium, lvas used for heat treating the specimens at 1500°C. An optical pyrometer was used in determining when the furnace had reached temperature. It took approximately 1 hr to heat the specimens to 1500°C. In cooling, the specimens were furnace-cooled for 2 hr, then pulled down into a quench chamber and removed. 2.6. Experimental
measurements
In the systems examined, there was no method established to quantitatively determine the amount of graphitization. There-
67
fore, no attempt was made to assign a value to these properties and only relative comparisons are made. After each heat treatment, the existence of graphitization was established by X-ray diffraction measurements and the microstructure characterized by scanning microscopy and electron probe. Although some of the samples were analyzed by X-ray diffractometry, most of the data were obtained by the Debye-Scherrer powder method. The samples required a 4 hr X-ray exposure using manganese-filtered iron K, radiation (X = 1.9373 A) for samples containing cobalt and iron while nickel-filtered copper K, radiation (h = 1.5418 A) was used for the specimens containing nickel. The purpose of the X-ray diffraction analysis was to define, whenever possible, the interplanar spacing (d002) and apparent crystallite size normal to the d,,,, planes as a function of heat treatment temperature. The relative degree of graphitization can be assessed by changes in the interplanar spacing and crystallite size. The d-spacings were calculated from the data read from the exposed films on a filrn reading device after correcting for film shrinkage. In order to determine the L,. parameter, tracings of the films were obtained from an optical microdensitometer and the full width of the d,,, diffraction profile was measured at onehalf the maximum peak intensity (FWHM). The value of L, was then calculated using the method of Warren and Scherrer [S],
where, K = a shape factor equal to 0.9 for FWHM measurements D or L, = the mean dimension of the crystallites normal to the diffracting planes /3= corrected line breadth b = instrumental broadening 0 = Bragg angle A = wavelength
68
R. L. COURTNEY
The approximate values of L, were included only to represent trends and should not be considered absolute. After each heat treatment, the X-ray diffraction patterns were also examined to determine the temperature at which the diffuse halos representing the amorphous components, initially present, were no longer detectable. A Philips EM-200 electron microscope was used for electron transmission and diffraction analyses, and an Applied Research Laboratories (ARL), Inc., electron probe was used to determine the presence and distribution of metal atoms. The X-ray data obtained at 1000°C for the Co-urethane and the dispersed Fe-naphthalenediol specimens, indicated the presence of two phases: an amorphous phase, depicted by a diffuse halo, and a crystalline phase exhibiting a diffraction line on the outer edge of the diffuse halo. Therefore, the Co-urethane and the dispersed Fe-naphthalenediol specimens containing the highest per cent of additive, which were heat-treated at only lOOo”C, were chosen to be investigated using the SEM in order to study the initiation of a transition from the amorphous carbon to the graphite phase.
3. EXPERIMENTAL
RESULTS
Since graphitization was catalyzed by the presence of molten metals, and not the fluoride salts initially added, the remainder of the report will emphasize the effect of the specific metal upon graphitization.
3.1. X-ray di$raction data The X-ray diffraction data listed in Table 2, were obtained using the Debye-Scherrer powder method. In some instances there is no value of d,,,, listed for the halo at that particular temperature; in these cases the halo was too diffuse to obtain a value for d 0,,2. The values of L, were obtained from measurements on the crystalline phase.
and S. F. DULIERE
3.2. Control samples The naphthalenediol and urethane control samples (Figs. 1 and 2, respectively) show that without the presence of an additive, graphitization did not occur at 1500°C. These SEM photographs illustrate the smooth conchoidal fracturing glasslike surface present throughout both samples. The X-ray data (Table 2) also depict the nongraphitic nature of the resulting structure. 3.3. Zron as a graphitizing agent The concentrated Fe-nuphthalenediol sample. The scanning electron photomicrographs (Figs. 3 and 4) were taken after the specimen had received the final heat treatment of O-5 hr at 1500°C. It is quite obvious from the photomicrographs that a transformation has occurred which produced a layered structure similar to that of graphite. Figure 4 shows a portion of the layered structure. This layered structure was not present on the top surface of the specimen, which was intentionally voided of any FeF3 when preparing the specimen. Since compounds of FeC and graphite have very similar X-ray diffraction characteristics, other techniques were used to verify that the layered structure was not iron carbide but graphite. The sample was analyzed using the electron probe to determine if iron atoms were part of the ‘flake’ material (Figs. 5-7). In Fig. 5, the flake material is as indicated by the arrow, and the lighter spotted areas around the flake are iron. As can be seen, no iron is contained within the flake, but Fig. 6 shows the flake to be composed of carbon. The areas around the flake are composed of both iron and carbon. In order to remove the iron, the specimen was etched with dilute hydrochloric acid and the scanning electron photomicrograph of the resulting structure is as shown in Fig. 8. An electron diffraction pattern (Fig. 9) of the layered, flake material shows the existence of a crystalline graphitetype structure.
Fig. I, ~~phth~llenedi~l
control
specimen
(1500%) - x 1000.
Fig. 3. Concentrated Fe-naphthaienediol specimen (1NO”C) - X 1COO.
I:ig-.
11. L‘wthme
control
qwt-imen
( 1500°c) - x 20,000.
Fig. 4. Concentrated thalenediol specimen 3000.
Fe-naph(1500”(~)-- X
Fig. 5. Electron probe analysis for iron in Rake material.
Fig. 7. Backscatter efectron probe analysis-concentrated Fe-naphthalenediol.
Fig. 6. Electron probe analysis for carbon in Rake material.
Fig. 8. Concentrated Fe-naphthalenediol specimen (lSOO*C) -etched with dilute HCI.
Fig. 9. Electron diffraction profile for the concentrated Fe specimen.
Fig. 11. 140% me-naphtha~e~ediol men (1500°C) - X 10,000.
speci-
Fig. 10. 0.32% Fe-naphthalenediol specimen (IMfO”C) - X lO,(~O.
Fig. 12. 3.20% Fe-naphthaienedioi specimen (1500°C) - X 10.000.
Fig.
13. 3.20% Fe-naphthalenediol specimen (850°C) - X 3000.
Fig. 15. Amorphous phase-3.20% Fe-naphthalenediol specimen ( 1000°C) - x 10,000.
Fig.
14. 390% Fe-na~hthalened~~)~ specimen (1OOO’C)- X 3000.
Fig. 16. Graphite phase-3.20% Fenaphthalenediol specimen (lOOO’C)x 10,000.
Fig. 17. 0.11% Co-urethane specimen (1500°C) - x 10,000.
Fig. 18. 0.55% Co-urethane specimen (1500x)x 10,OON.
Fig.
Fig. 20. 1.10% Co-urethane specimeu (1500°C) particulate loading-x 20,000
19. 1.10% Co-urethane specimen (15OO”C)- x 10,000.
Fig. 21. 1.10% Co-urethane specimen ( looo”c) - x 3000.
Fig. 23. Graphite phaseurethane specimen x 10.000.
1.10% Co(lOOO’C)-
Fig. 22. Amorphous phase-l.1096 specimen ( 1000°C) Co-urethane x 10.000.
Fig. 24. Electron diffraction profile for the 1.10% Co-urethane specimen (1500°C).
Fig. 25. 0~10% Ni-urethane
specimen
(15oo”c) - x 10,000.
Fig. 27. I*OO’Z Xi-urethane
specimen
( 1500°C)- x 10,000.
Fig. 26. O~.SO% Xi-urethane specimen ( 1300%) - x 10,000.
diffraction profile Fig. ‘LX. Electmn for the 1.OO% Si-urethane specimen (1500”(:).
Fig. 29. 1.OO% IQ-urethane loading
(1500°C) particulate
-
specimen X 30,000
CATALYUC
GRAPHITIZATION
Table 2. X-ray diffraction data:*: Debye-Scherrer
Temp. PC)
System Urethane Urethane+
control
1500
0.10% Ni
Urethane + 0.50% Ni
L’rethane-t
1.00% Ni
L’rethane + 0.11% Co
L’rethane + 0.55% Co
Urethane + 1.10% Co
Naphthalenediol control Naphthalenediol+ 0.32%’ Fe
Naphthalenediol-t
3.20%’ Fe$
Cured 850 1000 1500 Cured 850 1000 1500 Cured 850 1000 1500 Cured 850 1000 1500 Cured 850 1000 1500 Cured 850 1000 1500 1.500 Cured 850 1000 1500 (lured 850 1000 1500
powder method
do,, $1 Halo 36 3.8 4.5 3.7 3.7 $ 4.5 3.8 I: 4.5 4 4.5 * 4.5 $ 4.5 I + 3% 3.6 $ 46 $
Line
Approximate (A,
3.36 3.36
255
3.40 3.40 3.36
110
3.40 3.40 3.36
130 I50 155
3.40 3.40 3.40
L,i-
70 100
80 90 110
3.40 3.39 3.37
80 I10 135
3.41 3.36 3.36
70 255 220
3.40 3.40 3.36 3.40 340 3.36
‘.Heat soak of 0.5 hr at each temperature. tL, of the line only. $The heat treatment temperature at which the diIfuse halo. inmallv present, dissappeared or was not measurable. QThe specimen heat treated for a total of 2 hr showed an increase of L,. to 2!!2. while having the same d,,,,. The dispersed Fe-naphthalenediol snmples. In contrast to the specimen mentioned above, these specimens initially contained dispersed Fe at 3 levels of concentration (‘Table 1). The SEM photomicrographs pertaining to the structure of these specimens after 0.5 hr
at 1500°C are Figs. 10-12.
These
represent
the
lowest to highest initial concentrations of Fe. As was observed from the X-ray data, the specimen having the lowest initial concentration of Fe produced a structure with a cI,,,,.. of’ 3.40 Lk and an L, of approximatelv 80 ii. Ita
70
R. L. COURTNEY
lamellar structure was not as well defined as the other 2 specimens initially containing higher percentages of Fe. Also, a diffuse halo was present after heat treating the specimen at 1500°C. The specimen containing the next highest concentration of Fe exhibited a more graphitic structure, as determined from SEM, than the above mentioned specimen. This correlated with the lower dooz (3.36 A) and the higher L, (175 A). The diffuse halo existed up to lOOo”C, and after 1500°C had essentially disappeared. The structure of the specimen (Fig. 12) having the highest concentration of Fe was very similar to that of the specimen just discussed. After heat treating the specimen at 15Oo”C, a d,,02 of 3.36 A and an L, of approximately 140 A were observed and the diffuse halo had disappeared. This same specimen was then heat-treated for an additional O-5 hr at 1500°C; the value for dooz was still 3.36 A, but the value for L, had increased from 140 to 220 A. In none of the above specimens did X-ray diffraction show the presence of FeF3. However, elemental iron was identified. It is interesting to note that all 3 specimens had d-spacings of 3.40 A at lOOO”C, and that after 0.5 hr at 15OO”C, the dooz for 2 of the 3 specimens had decreased to 3.36 A. Also, it was difficult to observe any iron particles after the final heat treatment. However, those which were observed appeared to be discrete spheres, indicating the existence of a once molten phase. Scanning electron photomicrographs of this same system were obtained after the 850°C and the 1000°C heat treatments. The results are shown in Figs. 13-16. The heat treatment at 850°C did not produce a graphitic structure. The particles appeared not as discrete spheres but were very angular. However, after lOOO”C, two distinct phases were present. The one phase was very graphitic in appearance, whereas the other looked as if a molten phase had been ‘frozen’ in place upon cooling the specimen. Figure 15 is an
and S. F. DULIERE
enlargement of the area observed in Fig. 14. The particles are discrete spheres, and a large number of the spheres are enveloped by carbon. This phase is believed to be a transition phase from amorphous carbon to graphite. A close-up of the graphitic structure is shown in Fig. 16. 3.4. Cobalt and nickel as graph&zing agents The Co-urethane specimens. Graphitic sturctures (Figs. 17-19) were obtained from all 3 cobalt specimens. The specimen which contained the least Co behaved much like the naphthalenediol sample which contained the least Fe, in that it attained ad,,, of 3.40 A and an approximate L, of 90 A at the highest heat treatment temperature. The other 2 compositions displayed d-spacings of 3.37 and 3.36 .& along with L, values of 140 and 220 A, respectively. The specimen containing the highest per cent Co also exhibited a 3.36 A diffraction line at 1000°C. Figure 20 shows the discrete spherical metal particles and their distribution in a small area of the specimen heat treated at 1500°C. X-ray diffraction analysis shows the presence of elemental cobalt after the 1500°C heat treatment with no CoF, detected. The series of SEM photomicrographs (Figs. 21-23) once again show the presence of 2 distinct phases. Prior to analysis, the specimen containing the 1.10% cobalt was etched with dilute hydrochloric acid. The electron diffraction pattern (Fig. 24) of this specimen shows its graphite structure (Fig. 23). One phase is graphitic, while the other consists of spherical metallic particles encased within a carbon, finger-like network. It looks as if the fingerlike carbon projections have actually grown together. Again, it is believed that this is a transition from amorphous carbon to graphite. This latter phase is very similar to that which was obtained with the Fe-naphthalenediol specimen; however, this transition phase appears to have taken place to a greater Fe-naphthalenediol extent than in the specimen.
CATALYTIC
GRAPHITIZATION
It is also worthy to note that as the concentration of Co was increased, the value for d,,, decreased while L, increased. Also, the decrease of d,,, as the temperature increased was a more gradual change than that enwith countered the Fe-naphthalenediol specimens. The Ni-urethane specimens. These specimens behaved similarly to the Co-urethane specimens. A marked difference is that the specimen containing the lowest per cent of Ni attained a d,,, of 3.36 A. whereas in the previous systems comparable specimens did not (doe, = 340 A). However, a slight diffuse halo persisted after 1500°C in this specimen. X-ray diffraction showed that NiF, was no longer detected after the 1500°C heat treatment. Elemental xi, however. was observed. The SEhl photomicrographs (Figs. 25-27) illustrate the graphitic structure of these specimens after the 1500°C heat treatment. Figure 28 is the electron diffraction pattern of the specimen containing l*OO% of Ni (after a dilute hydrochloric acid etch). The spherical nature of the particles can be seen m Figure 29. 4. SUMMARY
(1) The control samples (no metal salt additive) showed no signs of being graphitized when heated for 0.5 hr at 1500°C. (2) The Fe-naphthalenediol system underwent graphitization as did the Co-, Niurethane systems. (3) A molten metal phase was observed, as indicated by discrete spherical particles. There was no apparent agglomeration of the particulate loading. (3) The electron proble analysis showed that both carbon and iron were present; however, no iron was present in the graphitic flakes. (5) As observed from scanning electron photomicrographs, local graphitization was obtained in both the Fe-naphthalenediol and Co-urethane samples at 1000°C. A possible transition phase of the amorphous carbon
71
being converted to graphite was observed in both systems. (6) Crystallite growth occurred to a greater extent at 1000°C in the Co-urethane specimen than in the comparable Fe-naphthalenediol specimen. (7) The trend in most all specimens, as the temperature increased, was a decrease in do,,* and increase in L,. (8) With three exceptions, the amorphous carbon phase gradually disappeared and the graphitic phase appeared at 1500°C. The exceptions were the specimens containing the lowest per cent metal. (9) After graphitization, metallic particles were observed on the surface of the urethane samples. The particles were not as evident on the surface of dispersed Fe-naphthalenediol samples. (10) The X-ray powder patterns did not show the existence of any of the initial fluorides after graphitization. However, the elemental metals were detected in each system. (11) If any metal-carbon compounds had been formed in the graphitized areas examined, they were not detectable. (12) The naphthalenediol polymer was not catalvtically graphitized by CuF,, TiB,, TaB,, WB, TiO,, B and B,C within the designated times and temperatures. 5. CONCLUSIONS Fairly recent investigations of Baraniecki, Pinchbeck and Pickering[7] show that graphite nucleates around and within metal particles, and it is postulated that the rapid transport of carbon atoms through the liquid metal is responsible for the increased rate of graphitization. Three ways which graphite might be produced are: (1) the formation of an unstable carbide which decomposes to the metal and graphite; (2) the diffusion of the amorphous carbon phase into the molten metal phase with subsequent precipitation of the less soluble graphite; or (3) a combination of
72
R. L. COURTNEY
(1) and (2). The SEM photomicrographs obtained during this study of the transition phases indicate that the second method of graphite production plays the major role in the systems studied here. Also, since graphitization in this study appears to be time dependent, it is believed that the solutiondissolution process is diffusion controlled, whereby the amorphous carbon is continuously diffusing into the molten metal accompanied by continuous precipitation of graphite. Therefore, the solubility of amorphous carbon in the metal, the eutectoid temperature and the absence of stable carbides, should be critical factors for low temperature graphitization. This could explain why the other polymer-metal salt systems examined in this study did not graphitize at 1500°C. Carbon is not appreciably soluble in copper, and the other systems form eutectoids at temperatures higher than 1500°C. It is also noteworthy that stable carbides of cobalt and nickel do not exist above 800-900°C [8]. Baraniecki, Pinchbeck and Pickering[7] as well as Oberlin and Rouchy[9], were able to obtain graphite when adding iron powder to an already carbonized matrix. Therefore, the interaction which might have taken place at lower temperatures between the metalsalt and the organic material seems to have had no essential role in the catalytic graphitization observed during this study.
and S. F. DULIERE
The study has shown that the hard carbons formed from the urethane foam and the naphthalenediol control samples were not graphitized. However, by the addition of certain metal catalysts, graphitization occurred at temperatures well below those required for non-catalytic graphitization. Acknowledgements-The authors are indebted to Dr. B. Granoff for his helpful suggestions during the course of the study, and to Mr. C. J. Miglionico and Mr. C. R. Hills for their cooperation in the areas of electron scanning microscopy and electron diffraction, respectively.
REFERENCES 1. Acheson, E. G., U. S. Put. 568323, (1896). A. R., and Lewis, F. A., Graphite 2. Ubbelohde, and Its Crystal Compounds, Oxford University Press, Oxford (1960). 3. Van Vlack, L. H., Elements of Material Science (2nd Edition), Addison-Wesley, Palo Alto (1967). 4. Schwartz, A. S., and Bokros, J. C., Carbon, 5,325 (1967). 5. Presland, A. E. B., and Walker, P. L., Jr., Carbon, 7 1 (1969). 6. Taylor, A., X-ray Metallography Chapter 14, John Wiley, New York (1961). Baraniecki, C., Pinchbeck, P. H., and Pickering, F. B., Carbon, 7,213 (1969). Elliot, R. P., Constitution of Binary Alloys (First Supplement), Materials Science and Engineering Series, McGraw-Hill, New York (1965). Oberlin, A., and Rouchy, J. P., Carbon, 9, 39 (1971).