Energy & Fuels 1987,1, 105-110 have a much higher molecular weight.
Conclusions For a given set of distillate fractions, the following trends can be observed. The refractive index increases as the distillate aromaticity increases, and the refractive index increases as the atomic hydrogen to carbon ratio of the distillate decreases. A decrease in the ratio of atomic hydrogen to carbon with a concomitant increase in aromaticity, boiling point, and molecular weight is a consequence of the increasing degree of aromatization and condensation of the molecular components of the distillates. Equations 1 and 2 lead to estimates of the refractive index of the coal-derived materials studied here that are more accurate than other correlations in the literature. Similarly, eq 3-5 lead to a more accurate estimation of molecular weight of coal-derived distillates than other equations in the literature, including the correlations of Riazi and Dauberty Brul6 et al.,23Gray et al.,24and WilEquation 6 calculates aromaticity from the measured atomic hydrogen to carbon ratio and refractive index with an average absolute deviation of 0.037 f a units. It is generally thought that fa can be determined by NMR to within 0.03 f a units.27 Thus, the values estimated by using eq 6 are as precise as those determined by NMR. The correlations developed indicate that refractive index is an important property of coal-derived materials that is related to a variety of physical and chemical properties. Measurement of refractive index is rapid and precise, and does not require expensive instrumentation or expertise. These qualities combine to make refractive index a potentially attractive on-line monitoring measurement for coal liquefaction processes, possibly in combination with density or some other parameter. The correlations developed may also find application in quality control of analytical data. For example, if the measured refractive index, mid-boiling point, and density lead to a predicted
105
molecular weight significantly different from the measured value, it does not necessarily mean the measured value is incorrect, but it does indicate that the value should be checked and further measurements may be needed. An important aspect of this investigation has been to determine if reliable correlations could be developed that allow the estimation of properties of coal-derived materials formed by different process streams of the same process (Thermal Hydrocracker and Catalytic Hydrotreater), by different processes using bituminous coals (Wilsonville, H-Coal, and SRC-11),and by different ranks of feed coals (bituminous and subbituminous). This investigation shows that reliable correlations with refractive index can be developed that fit the observed properties of distillates from various coal liquefaction products, processes, and feed coals. Nevertheless, the work should be continued and expanded to determine the general applicability of the results. The results presented will be useful in developing other new correlations and in modifying existing correlations.
Acknowledgment. The authors are indebted to M. E. Bott and M. Farabaugh for helping to perform the calculations and to M. Hough and J. Adkins for making the NMR measurements. Special recognition is given to K. Champagne and J. Knoer for distilling the Wilsonville products into narrow-boiling-range distillates. The authors acknowledge helpful discussions of the manuscript with D. Finseth, R. Warzinski, G. Holder, A. Cavanaugh, and G. Gibbon. Supplementary Material Available: Table IV, the experimentally determined refractive indices a t 293 K of the 28 K boiling range distillates from H-Coal and Wilsonville along with the values predicted by both eq 1and 2 as well as the equations of Riazi and Daubert; Table V, average molecular weights determined by VPO, freezing point depression, and LVHRMS for the H-Coal distillates; Table VI, experimental molecular weights and those predicted by using a variety of methods; Table VII, experimental aromaticities and those estimated by eq 6 (8 pages). Ordering information is given on any current masthead page.
Electron Microscope Investigation of the Structures of Annealed Carbons Peter R. Buseck,* Huang Bo-Jun,f and Lindsay P. Keller Departments of Geology and Chemistry, Arizona State Uniuersity, Tempe, Arizona 85287 Receiued August 6, 1986. Reuised Manuscript Receiued Nouember 4 , 1986 High-resolution transmission electron microscope images were obtained of the layers that form when samples of coke, produced in the laboratory by pyrolysis of known organic compounds, become structurally ordered as a function of heating temperature. The images show the progressive changes that occur as amorphous material grows to form isolated layers of carbon. These then grow laterally as well as form clusters of poorly stacked layers. Increased heating changes the subparallel layers to parallel seta, initially with many discontinuities, but then to crystals having few or no such defects. By the use of images obtained with transmission electron microscopy, details of graphite crystallization can be followed in considerable detail.
Introduction Carbonaceous materials are widespread in the natural environment and are, of course, major sources of energy and fuels. During maturation over geological time such Current address: Institute of Geochemistry, Academia Sinica, Guiyang, Guizhou Province, People’s Republic of China.
materials lose heteroatoms (such as H, N, 0, S) and become richer in carbon, finally ending in fully ordered, crystalline graphite. For convenience, following common usage, we shall refer to the carbon-rich materials collectively as carbon, recognizing that they may range from the pure element to carbon-richorganic molecules that contain minor, unspecified amounts of other elements. The
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106 Energy & Fuels, Vol. 1, No. 1, 1987
structure of such carbon directly influences its chemical and physical properties and potentially provides information about modes of origin. Determination of origin is of particular importance for geochemistry and the geological aspects of fuel science. There is great structural diversity among the several forms of carbon, even neglecting high-pressure varieties such as diamond. This diversity is reflected in the use of adjectives such as graphitic, turbostratic, vitreous, amorphous, sooty, black, pyrolytic, polymeric, semicrystalline, poorly crystalline, hard, soft, and glassy to describe various carbons.'P2 Highly averaged parameters such as doozand P, the probability of perfectly periodic stacking sequences, are best determined by X-ray diffraction measurements. However, the details of several stages of carbon ordering are not readily distinguishable by such measurements. We therefore utilized high-resolution transmission electron microscopy (HRTEM) to attempt to understand and characterize the differences among poorly crystalline carbon species. Graphite consists of sheets of carbons that are rigorously stacked with perfect order of type AB (space group P63/mmc). The weak interlayer bonding can give rise to rotation between successive layers, yielding turbostratic stacking. The presence of such rotations cannot be readily detected in HRTEM images, although indications can be obtained from ultrahigh-resolution image^.^ Moreover, the extension of the layers, the number in a stack, and the degree to which they are parallel can be readily observed in HRTEM images, and it is these features that are noted in this paper. Thus, when we refer to terms such as structural order, it is in regard to stacking perfection except for layer rotation. Our long-range goal is to develop the sequence or sequences of order that occur as totally disordered carbon, both natural and synthetic, progresses to well-crystallized graphite. In the present study, we have used synthetic samples of coke that have been heat-treated at different temperatures. The immediate aim is to describe the structural changes, specifically the growth and stacking of carbon planes, that occur during annealing at different temperatures of a few model compounds used to form cokes. The effects of temperature have been monitored for the present set of samples. In order to apply these results to the natural environment, it will also be necessary to consider the effects of pressure and catalytic species. There have been a number of electron microscope studies of carbon that indicate changes in structure with increasing temperature. For example, Ban4 showed that the graphitic layers of heat-treated carbon blacks are commonly arranged concentrically in nearly spheroidal particles. After heat treatment at a sufficiently high temperature (>2700 "C),the layers may stack regularly into platelike domains that constitute the walls of polyhedral particles. the network structure of tangled graphite-like ribbons of glassy carbon prepared from phenolic resin has been convincingly di~played.~?~ The lattice-fringe images obtained from carbons of heat-treated polyvinylidene chloride by Ban et revealed that carbon (1) International Committee. Carbon 1983,5,517. (2) International Committee. Carbon 1983,5, 445.
(3) Barry, J.; Buseck, P. R. Proc.-Annu. Meet., Electron Microsc. SOC.Am. 1986,44th,460-461. (4) Ban, L. L. Spec. Period. Rep.: Surf. Defect Prop. Solids 1972,1, 54-94. (5) Jenkins, G.M.; Kawamura, K.; Ban, L. L. h o c . R. SOC.London, A 1972,327,501-517. (6) Ban, L. L.; Crawford, D.; Marsh, H. J.Appl. Crystallogr. 1975,8, 416-420.
Buseck et al.
a
b
C d Figure 1. Structures of the precursor molecules used for making the cokes used in this study: (a) acenaphthylene;(b) p-terphenyl; (c) fluoranthene; (d) bifluorenyl.
possesses poor crystalline order when heat-treatment temperatures (HTT) for 1-h heat treatments are lower than 1000 "C and that carbon has good crystalline order when the HTT for the same duration are higher than 2000
"C. The feasibility of directly imaging microstructuraldetails in carbons has been demonstrated only relatively re~ently.~ Subsequently,several extensive studies have been reported regarding aspects of graphitization processes of both synthetic and natural carbon materials.&l' The interpretation of fringe images of carbon, as well as the decrease in order upon mesophase formation and the subsequent increase with order at higher temperatures, is aided by multislice calculations.12J3 Since planes of carbon atoms can curl but still appear as fringes when viewed edge-on, rigorous interpretation of two-dimensional images must be done with great caution. Fryer14 examined the micropore structure of disordered carbons as a function of accelerating potential of the TEM. Smith and Buseck15J6studied an acid-insoluble, carbon-rich residue from the Allende meteorite. The carbon consists of a tangled aggregate of somewhat fibrous crystallites, with a prominent latticefringe spacing of 3.4-3.9 A. They showed that the carbon is not carbyne, as had been suggested, but poorly graphitized or "glassy" carbon. The sequence of ordering toward the formation of graphite in metamorphic rocks is discussed and illustrated by Buseck and Huang."
Experimental Section Cokes derived from the coke precursors, acenaphthylene, fluoranthene,bifluorenyl,and p-terphenyl, were examined.I6 The f i h sample is a commercial,premium-grade,petroleum coke that was produced by the Conoco petroleum company (Lake Charles, LA) and is typical of premium-grade petroleum coke.17 Their structures are illustrated in Figure 1. These model compounds were selected because they are typical of the components of carbonaceous materials and thus of the feedstocks of petroleum coke. They also are representative of (7) Millward, G. R.; Jefferson, D. A,; Thomas, J. M. J. Microsc. (Oxford) 1977,113,1-13. ( 8 ) Oberlin, A.; Terriere, G. Carbon 1975,13,367-376. (9)Oberlin, A.; Boulmier, L.; Durand, B. Geochim. Cosmochim. Acta 1974,38,697-650. (10)Oberlin, A. Carbon 1984,22,521-541. (11) Buseck, P. R.; Huang, B. J. Ceochim. Cosmochim. Acta 1985,49, 2003-2016. (12) Marsh, H.; Crawford, D. Carbon 1984,22,413-422. (13) O'Keefe, M. A.; Buseck, P. R. Trans. Am. Crystallogr. Assoc. 1979,15,27-46. (14) Fryer, J. R. Carbon 1981,19, 431-439. (15) Smith, P. P. K.; Buseck, P. R. Science (Washington,D.C.) 1981, 212, 322-324. (16) Smith, P.P. K.; Buseck, P. R. Science (Washington,D.C.)1982, 216,984-986. (17) Strong, S. L. Pap.-London Int. Conf. Carbon Graphite, 4th, 1974 1976. 544-557.
Structures of Annealed Carbons a wide range of compounds that display different reaction paths and propensities for the formation of graphite. The samples were selected from the set prepared by Lewis and were initially studied by using X-ray diffraction (XRD).'6J8*19Considerable work has been reported on the effects of heating of acenaphthylene, and thus it seemed an especially desirable material for high-resolution st~dies.'~*%~~ The samples we examined were previously studied by a wide variety of different techniques.21*23-25 In general, the acenaphthylene is highly graphitizable, fluoranthene and bifluorenyl form more disordered structures, and p-terphenyl is essentially a nongraphitizing compound.24 (See ref 26 for a discussion of graphitizing vs. nongraphitizing carbon.) The starting materials were converted to coke by heating them to 500 "C a t 60 "C/h and then annealing for 4 h. Heating to temperatures up to 1000 "C was done a t the same rate, with annealing for 1h. Heating to temperatures greater than lo00 "C was done more rapidly, but all annealing times were for 1h. For the HRTEM observations, the samples were gently ground in acetone with an agate mortar and pestle and ultrasonically dispersed; a drop of suspension was deposited on a copper TEM grid that had previously been coated with a holey-carbon film. The samples were examined in a JEOL JEM 200CX electron microscope operated a t 200 kV. Fragments that extend over the holes in the supporting film were examined. Optical diffraction patterns were obtained directly from the HRTEM photographic plates through the use of a 3-m optical bench and a laser source. In order to sample the areas of interest, an opaque mask with a circular opening of -2-mm diameter was positioned over the HRTEM, plates, similar to previously published method^.^ Selected-area electron diffraction (SAED) and optical diffraction patterns are included as inserts on most figures. The SAED patterns are obtained directly from the sample by using the beam electrons in the TEM. They are produced from regions several thousands of angstroms in diameter. In contrast, photographs of the images that are obtained with the TEM are used to produce the optical diffraction patterns by using a laser beam and optical bench. They are thus produced from far smaller regions than the SAED patterns. Therefore the optical diffraction patterns tend to have poorer definition, but on the other hand they are restricted to the regions of interest. Some coke samples have diffraction spots that are diffuse and extremely poorly defined. For samples that are poorly crystalline in the aggregate but contain local regions of order, there is a pronounced difference between SAED and optical diffraction patterns. For example, in Figure 5 the SAED pattern, obtained from a large region, consists of rings. In contrast, the optical diffraction pattern obtained from the ordered portion illustrated in the figure displays well-developed spots indicative of considerable local structural order. However, like electron and X-ray diffraction, optical diffraction also smooths out the spread in spacings and irregularities that are evident in the images. We are addressing this problem in studies where the images are digitized and studied by image analysis procedures. In addition to the HRTEM experiments, XRD measurements were performed on each sample. Oriented specimens were prepared from a slurry of powder and ethanol and placed onto a zero-background quartz slide. Data were obtained by using a Rigaku D-Max II-B automated diffractometer. It was operated a t 50 kV and 30 mA, lo00 counts/s full scale, a time constant of 1, and a scan rate of 1/20 28/min. The observed 28 angles have been corrected by comparison to an external standard (NBS SRM-640a silicon metal). (18)Strong, S. L.,personal communication. (19)Ruland, W.Carbon 1965,2,365-378. (20)Edstrom, T.;Lewis, I. C . Carbon 1969,7,85-91. (21)Jones, J. I. J. SOC.Chem. Znd., London 1949,68,225-232. (22)Lewis, I. C.;Edstrom, T. Proc. Conf. Carbon, 5th. 1961 1963, 413-426. (23)Fitzer, E.; Mueller, K. Carbon 1968,6, 234-240. (24)Fitzer, E.; Mueller, K.; Schaefer, W . Chem. Phys. Carbon 1971, 7,237-383. (25)Leung, P.S.;Safford,G. J. Carbon 1970,8,527-544. (26)Lewis, I. C. Carbon 1982,20,519-529. (27)Franklin, R. E. Acta Crystallogr. 1951,4,253-261.
Energy & Fuels, Vol. 1, No. 1, 1987 107 Table I. Coke Samples Used for This Study, the Temperatures at Which They Were Annealed, Their XRD Data, and the Figures Where They Are Illustrated starting Lc,* figure annealing dm2, fwhm: no. temp, "C A den x 28 A material 425 6 0.31 acenaphthylene 2000 3.417 252 5 0.44 1750 3.430 44 4 1.97 1250 3.48 28 3 3.03 3.51 lo00 28 2 3.03 500 3.51 147 9 0.67 3.430 fluoranthene 2000 124 8 0.77 3.431 1750 49 7 1.76 3.46 1500 32 2.66 3.50 525 323 0.37 3.423 petroleum 2000 218 0.49 3.432 1800 88 10 1.04 3.44 1600 43 2.01 3.47 1400 56 1.57 3.44 p-terphenyl 3000 39 11 2.18 3.47 2500 31 2.69 3.52 2000 158 0.63 3.430 bifluorenyl 3000
"Measured full width of the peak at half its maximum height. *Cross-sectionallength of carbon planes; see text.
Figure 2. HRTEM image of acenaphthylene coke heated to 500 "C. The fringes are short, not more than 10 A in length. There are some clusters of between two and four highly subparallel fringes; their lengths are less than 14 A. The insets in this and subsequent figures and SAED patterns (right) and optical diffraction patterns (left). The Scherrer e q ~ a t i o n ' was ~ * ~used to estimate L,. In the Scherrer equation [L, * B cos e)], X is the wavelength of the radiation used (1.541 = OS9 78 for Cu Ka),8 is the Bragg angle, and B is the profile broadening due to the small particle effect. The value of R in the equation was calculated by subtracting the instrumental from observed broadening of the (002) line. The instrumental broadening (0.12" 28) was determined from the (101) reflection of quartz. Table I contains the data for the various samples.
1'
Results Several structural steps that arise during the process of ordering of carbon planes can be distinguished on HRTEM images: (a) development of roughly planar layers of carbon; (b) organization of these layers into stacks, thereby forming precursors of graphite crystallites; (c) removal of defects from these locally ordered regions; (d) increase in the degree to which the fringes are parallel and thus periodically ordered, together with a decrease in the inter(28)Klug, H.P.;Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials; Wiley: New York, 1974;pp 700-708.
108 Energy & Fuels, Vol. 1, No. 1, 1987
Figure 3. HRTEM image of acenaphth lene coke heated to lo00 "C. The fringe lengths average 15-50 l a n d some are up to 100 8, long. Up to 10 fringes occur in stacks in which the fringes are subparallel.
Buseck et al.
Figure 5. HRTEM image of acenaphthylene coke heated to 1750 "C. Most fringes are more than 100 A in length. They are combined to form crystallitesthat are 40-120 A wide and 50-120 8, long. Initiation of individual spots, indicative of single-crystal formation, is evident in both the SAED and the optical diffraction patterns.
:
Figure 4. HRTEM image of acenaphthylene coke heated to 1250 "C. Most fringes are more than 40 A in length, but they are nonparallel. Stacks 10-60 8, thick and 15-40 A long are common. The presence of limited ordering is also revealed by the optical diffraction pattern.
layer spacings. Standard symbols are used for several of these parameters: La,length of layers, L,,stacking height, a3spacing between adjacent layers, and c12,disorder parameter.28.29All of these values have been defined in terms of XRD measurements, and their determination yields highly averaged numbers. Furthermore, Rulandm states that the effects of small crystalsize and structural disorder manifest themselves similarly, and thus interpretation of line widths does not generally yield unique answers. However, in principle, corresponding features can be used to judge the structural order of carbon directly from its high-resolution images; such features include (a) length of fringes, (b) dimensions of fringe groups or stacks, (c) abundance of defects such as dislocations, (d) interfringe spacings, and (e) planarity of the fringes. Figures 2-6 show images of acenaphthylenecoke heated a t various temperatures; the figures illustrate increasing crystallinity with increasing temperature of annealing. Figure 2 shows an image of a sample that was heated a t 500 "C. No structural order is evident. In places there are clusters of between two and four highly subparallel, discontinuous layers, generally less than 14 in length. Except for these crystallites, which could act as nucleii for (29) Ergun, S. Chem. Phys. Carbon 1968; 4,211-288.
..
Figure 6. HRTEM image of acenaphthylene coke heated to 2000 "C. Most fringes are continuous for over lo00 A, and up to 120 parallel fringes are organized into well-ordered arrays.
additional growth, the material appears amorphous. SAED patterns show no discrete spots, and broadened ring patterns are prominent. The acenaphthylene sample that was annealed at lo00 "C (Figure 3) shows the initiation of recognizable layering over distances greater than 100 A Stacks of up to 10 layers occur; the layering is still highly irregular and imperfect, but ordering is appreciably greater than in the sample that was heated a t 500 "C. It is interesting to note that the XRD measurements for the acenaphthylene samples heated a t 500 and lo00 "C are identical whereas there are obvious differences in the HRTEM images. Heating acenaphthylene a t 1250 "C produces appreciable growth in both the lateral dimensions of the layers and, to a lesser extent, the number that combine to form stacks. However, layering is still poorly defined and without fully parallel layers (Figure 4). Heating a t 1750 "C produces a structure that is sufficiently ordered to resemble graphite (Figure 5). Layers are extensive (greater than 300 A long), parallel, and numerous-up to 40 occur in a set. The degree of order is sufficiently well developed to permit recognition of numerous dislocations. By the time acenaphthylenecoke has been heat-treated a t 2000 "C,it has taken on many of the characteristics of graphite. Layers are continuous for over 1000 A, and up
Energy & Fuels, Vol. 1, No. 1, 1987 109
Structures of Annealed Carbons
- 40A
L
Figure 7. HRTEM image of fluoranthene coke heated to 1500 "C. Most fringes are continuous for over 120 A, but they are not parallel to one another. Fringes that are 25 A long are common, and they form in stacks containing roughly 15 fringes. Dislocations are ubiquitous.
Figure 9. HRTEM images of fluoranthene coke heated to ZOO0 "C. The fringes on one image average more than 500 A in length. The other image shows less well-developed material. Fringe stacks up to 120 A wide and 250 A long occur. The lack of homogeneity and crystalline perfection is striking.
Figure 8. HRTEM image of fluoranthene coke heated to 1750 "C. Crystallinityhas improved appreciably relative to the material heated at 1500 "C. However, crystalline perfection has certainly not been achieved, as is indicated by the image as well as the diffraction patterns.
to 120 parallel layers are stacked in well-ordered arrays. The curls in the fringes resemble features seen in sheet ~ i l i c a t e and s ~ ~carbons ~ ~ from meteoritic and terrestrial geological en~ironments.'~J~ However, XRD data suggest that some disorder is still present because the dm spacing of the sample has not yet attained the value of graphite. It has been shown that acenaphthylene does not become graphite until heat treatments above 2500 "C are used.18 Fluoranthene and petroleum cokes show similar features of increasing order when they are subjected to heat treatment a t higher temperatures. The fluoranthene structure heated a t 525 "C is indistinguishable from that in Figure 2. Figures 7-9 illustrate fluoranthene heated a t 1500,1750,and 2000 "C,respectively. They show increased structural organization; some of the material has attained the appearance of graphite by 2000 "C, but XRD measurements show that it is still partially disordered. The petroleum coke is poorly ordered at 1400 "C,corresponding (30) Ruland, W. Chem. Phys. Carbon 1968,4,1-84. (31) Yada, K. Acta Crystallogr. 1967.23, 704-707. (32) Veblen, D. R.; Buseck, P. R. Science (Washington, D.C.) 1979, 206, 1398-1400. (33) Tomeoka, K.; Buseck, P. R. Ceochim. Cosmochim Acta 1985,49, 2149-21 64.
1
Figure 10. HRTEM image of petroleum coke heated to 1600 "C. Note the thick layer stacks and the pronounced waviness of their fringes; dislocations are abundant.
to acenaphthylene that forms between 1000 and 1250 "C. A t 1600 "C it has become relatively well-ordered (Figure lo), although it still retains some nonparallel layers and wrinkled character; these features have disappeared in the sample heated a t 2000 "C. We only have samples of p-terphenyl coke that were heated a t 2000,2500, and 3000 "C. However, an illustration of Millward and Jefferson34 shows that a sample heated a t 800 "C displays no appreciable structural order. The samples heated a t 2500 and 3000 "C resemble each other and are in small but well-developed structures that look like TEM images of graphite (Figure 11). The layers are parallel and either slightly curled or in the polygonal, hexagonal forms typical of graphite. The layers are tangled (34) Millward, G. R.; Jefferson, D. A. Chem. fhys. Carbon 1978,14, 1-82.
110 Energy & Fuels, Vol. 1, No. 1, 1987
-.
Figure 11. HRTEM image of p-terphenyl coke heated to 2500 "C. The fringes are long, and 4-16 fringes are formed in tangled ribbons in a network structure. The small sizes and varied orientations of the crystallites produce the streaked character of the SAED pattern.
f
Figure 12. HRTEM image of well-crystallized natural graphite (from Sri Lanka).
and intertwined in a complex manner similar to that described by Ban4 and Jenkins et al.5 and, for silicates, by Veblen and Buseck.lg In several regions, the graphite layers are continuous from one small crystallite to another, and "loose ends" are sparse or absent. The paucity of terminating layers may explain the apparent inertness of glassy ~ a r b o n .Jenkins ~ et al.5 concluded that glassy carbon has a network structure of tangled graphite-like ribbons. According to Fischbach,= the nongraphitizing carbons, which include glassy carbon, re(35) Fischbach, D.B. Chem. Phys. Carbon 1971,7,1-105.
Buseck et al.
tain an imperfect structure even after prolonged heating a t very high temperatures (e.g., 2 h a t 3200 "C for glassy carbon) a t ambient pressure. p-Terphenyl coke displays the features of glassy carbon in both high-resolution images and XRD patterns. The atom planes are oriented parallel or subparallel to each other to form tangled ribbon molecules. We also studied one sample of bifluorenyl coke heated to 3000 "C. It formed in well-developed layered structures. Figure 12 is an image of natural graphite (from Sri Lanka; ASU No. 349)that can be compared to those of the coke. Graphite crystals, when prepared by crushing, tend to form flat flakes that have their c axes oriented parallel to the electron beam. It is only at an occasional curled-up edge of a flake that the orientation is suitable for viewing lattice-fringe images. The graphitic layers are straight and parallel.
Conclusions A general sequence of steps can be observed during graphitization. In the most poorly ordered state, short (- 10 A) discontinuous fringes are evident; these presumably represent isolated planes of carbon. With heating, these group into highly subparallel clusters, and the planes increase in both length and number in a stack as annealing proceeds. There is a simultaneous, progressive increase in stacking perfection, until the planes are finally parallel and periodic. Another observable effect of heating is a decrease in the interplanar spacing and a concurrent increase in uniformity of spacings within a given crystal and set of crystals. Only the very earliest stages are properly amorphous on the HRTEM scale, even though 002 diffraction spots or rings do not appear until there is evident development of carbon atom layers and layer groups above approximately 1000 "C. Images obtained by electron microscopy are limited in that they show only the projected one- or two-dimensional structural state rather than the three-dimensional order required of graphite. However, in this paper we have used such images to track some of the localized structural details of ordering that occur during the development of graphite from selected precursors.
Acknowledgment. We thank Drs. S. L. Strong and I. C. Lewis of Union Carbide for helpful reviews and Dr. Strong for providing the samples. J. Collins, and B. Miner made useful comments. Financial support was provided by a travel award from the Chinese Academy of Sciences (to H.B.-J.) and Grant EAR84-08168 from the Earth Sciences Division of the National Science Foundation (to P.R.B.). Registry No. graphite, 7782-42-5.
