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The formation of graphitizing carbons from the liquid phase
Carbon
1965,
Vol. 3, pp. 185493.
Pergamon Press Ltd.
THE FORMATION
Printed in Great Britain
OF GRAPHITIZING
CARBONS
FROM THE LIQUID PHASE J. D. BROOKS and G. H. TAYLOR Division of Coal Research, CSIRO, P.O. Box 175, Chatswood, New South Wales, Australia (Received 23 June 1965)
Abstract-The formation of graphitizing low-temperature carbons by solidification from a liquid phase proceeds via the separation of a mesophase having properties similar to those of liquid crystals. Initially, the mesophase, consisting of planar aromatic compounds of high molecular weight, separates in the isotropic liquid as spherical droplets having a considerable degree of molecular order, with the aromatic sheets stacked in parallel array. The sheets are arranged perpendicularly along one diameter of a sphere, but are curved so that they are normal to the surface. On prolonged heating the spheres coalesce and extended regions of uniform orientation may be formed; eventually a solid semi-coke is obtained. It is the generally lamellar arrangement of the molecular structure in these regions which favours the formation of graphite carbon at high temperatures. When isolated spheres are heated to graphitizing temperatures contraction occurs in the direction perpendicular to the preferred orientation. The bodies become elliptical in section and each is converted to a mass of small graphite crystals.
1. INTRODUCTION
2. PREVIOUS WORJS
TEIE structure of carbons ultimately obtained by the pyrolysis of organic materials in the condensed phase is established at a very early stage of thermal decomposition. These carbons are graphitizing or non-graphitizing depending usually upon whether they are formed from the liquid or solid phases respectively; if fusion does not occur the carbons are, in general, non-graphitizing. Substances which form graphitizing carbons have relatively high atomic ratios of hydrogen to noncarbon elements, and include vitrinite of highvolatile coking coals, coal tar pitches, petrofeum polyvinyl chloride and polynuclear bitumen, aromatic compounds such as naphthacene and
dibenzanthrone. Such graphitizing carbons are coke-like in appearance and have an anisotropic microscopic structure which persists on heating to lOOO“C,the degree of anisotropy increasing with rise in temperature. This structure also influences the properties of the graphite which forms on further heating to 2500°C or above. The nature of the solidification process by which graphitizing carbons are formed is little understood, and its investigation is the subject of this paper. 185
The optical anisotropy of coke was first described by RAMDOHR(') in 1928. MARSHALL@) (thin sections of natural coke), STACH(~)(polished surfaces of natural coke), ABRAMSKI~~~ MACKOWSKY’~) (polished surfaces of manufactured coke) and others have described the optical structures of various cokes and coke-like carbons. None of these workers appears to have recognized intermediate stages in the development of the characteristic anisotropic mosaic structure. Such stages were described by TAYLOR(‘)and by BROOKSand TAYLOR@), who reported the appearance and growth of spherical bodies in the plastic carbonaceous material. The spheres, some of the properties of which were described, were observed to grow at the expense of the plastic material until a three-dimensional mosaic structure was formed, which persisted in the soiidified semi-coke. Other workers, e.g. STA&~), had observed spheres associated with carbonized coal but had reported these to be “grapbitic spherulites” which exhibited the so-called Brewster or stationary extinction cross, indicating a radial structure. As has been described(‘), the spheres, which represent an intermediate stage in coke formation, have optical properties which preclude a radial structure. Two
186
J. D. BROOKS and G. H. TAYLOR
alternative structures, both essentially lame&r, were proposed. 3. EXPERIMENTAL
Specimens were prepared by partial carbonixation for examination by microscopy, electron diffraction and chemical analysis. Initially, a horizontal silica retort within a furnace having automatic temperature control was used to carbonize the samples. It was found that very critical temperature control was necessary, especially with substances having a short plastic range. Temperature control and reproducibility to at least +Z”C was essential and + 1°C was desirable if various intermediate stages of carbonization were to be recognised. An inert atmosphere, usually nitrogen, was provided in these experiments. Later carbonizing runs were carried out using vertical tubes under refluxing conditions. The tubes were immersed in a solder bath of controlled temperature. The tubes were capped with aluminium foil; under these conditions of reflux it was not necessary to exclude air rigorously. Microscopic observations were made of the partly carbonized material. With minor exceptions (hot-stage electron and light microscopy) the material was chilled or allowed to cool to room temperature. For light microscopy, thin sections and polished surfaces were prepared using Carborundum papers and polishing on cloth with chrome and magnesium oxides. Ultra-thin sections were cut with a diamond knife. A Leitz Ortholux microscope with xenon arc illumination and a JEM-SY electron microscope were used for light and electron microscopy respectively.
For the class of substances (listed below) being considered, the general pattern of behaviour has been found to be as follows: The substance melts on heating and becomes an isotropic pitch-lie material of plastic or liquid consistency. With rising temperature, spheres appear in the pitchlie mass (Fig. 1). These were first detected at O-1 p diameter but no doubt exist at smaller sizes. Even at O-1 p they are difficult to detect in the electron microscope except with the aid of selectedarea electron diffraction. (The bodies give a characteristic pattern, while the pitch gives
extremely diffuse scattering}. With increase of temperature, and to a certain extent with increase of time at a fixed temperature, the spheres grow larger. When they are of micron size they may be readily seen in the electron microscope (they are slightly more electron-dense than the pitch) and can just be detected with the light microscope. At still larger sizes they are easily observed in polished surfaces with the light microscope, or even, in some cases, with the naked eye. At some stage in the heating process the spheres will have replaced a large part of the pitch-like material and begun to interfere with one another’s enlargement. At this stage a ‘mosaic’ begins to form by coalescence. When no more isotropic pitch-like material remains and only anisotropic material or mesophase* is present the mosaic structure is complete, and then, or shortly afterwards, the mesophase solidifies into semi-coke. The substances which exhibit this behaviour are all materials which become plastic when heated and later solidify to form a semi-coke which is readily graphitizable. The substances listed below have been found to exhibit the behaviour described. The temperatures in brackets are those at which spheres and pitch-like material were present under the particular experimental conditions. Coke-oven pitch (425°C); Vertical-retort pitch (425°C); Petroleum-ether-soluble fraction (“crystalloids”) of coke-oven pitch (425°C); Toluene-soluble, petroleum ether-insoluble fraction (“resinoids”) of coke-oven pitch (425°C); Pyridine-soluble, toluene-insoluble fraction of coke-oven pitch (425°C) ; ONIA-GEGI pitch (~*~~; Carburetted-water-gas pitch (430°C); Petroleum bitumen 2801320 (425°C); Vitrinites from bituminous coals (460-47O”C); Naphthacene (425°C); Polyvinyl chloride (400°C) ; Dibenzauthrone (520°C). In addition, the following substances yield a *The term ‘Lmesophase” (Gr. mesos=intennediate) is used for the substance of which the spheres and the mosaic--before solidification-are formed, cf. GRAY@.
FIG. 1. Spheres at an intermediate stage of development. The grey matrix is pitchlike material (P). The spheres are in random orientations. A few show almost total absorption of polarized light (arrowed). Nearly all spheres have accurately circular outlines. Reflected polarized light.
FIG. 2. A later stage than Fig. 1 in the carbonization sequence. The bodies are mostly no longer spherical but become distorted through mutual interference. Reflected polarized light.
[facing
p. 186
FIG. 3. Mesophase bodies in pitch (P) which contains fine insoluble matter (dark spots). Some bodies are spherical or nearly so, but the majority have less regular shapes. Reflected polarized light.
FIG. 4. Ultra-thin section of part of two spheres showing regularity of sphere outline with insoluble matter at margins (arrowed). The whole field is striated due to scoring during cutting by these insoluble particles. Electron micrograph.
FIG. 5. Complex structure resulting from stirring pitch containing spheres. The pitch (P) and mesophase both behave as liquids. The sphere (arrowed) of mesophase contains tiny spherical inclusions of pitch and is itself contained within an elongate body of pitch wholly surrounded by mesophase. Reflected polarized light.
FIG. 6. Electron-diffraction pattern from part of a single sphere where lamellae are parallel to beam. The 002 and 004 arcs are indicated. The continuous rings are from the internal aluminium standard.
FIG. 7. Electron-diffraction pattern from part of a single sphere where lamellae are slightly oblique to the beam. The 002 and 004 arcs are indicated, also the 10 spacing.
FIG. 10. Polished surface of pitch containing spheres. The sphere (arrowed) has been sectioned with lamellae parallel to polished surface and slightly above the plane of symmetry of the sphere. It shows an extinction cross which remains stationary when the stage is rotated. Spheres in other orientations show transient extinction crosses only. Reflected polarized light.
FIG. 11. A sphere in which the lamellae lie perpendicular to the polished surface which is the median plane of the sphere. Note the two “poles” (arrowed) which correspond to the ends of the major axis of the sphere. Reflected polarized light.
FIG. 12. Large spheres (one nearly complete, one partial) cracked and fragmented after prolonged extraction. Reflected polarized light.
FIG. 13. Residue after prolonged extraction of pitch with benzene and pyridine. The small spheres (arrowed) remain apparently unchanged together with a cellular residue from the pitch. Reflected polarized light.
FIG. 15. Pitch containing insoluble inclusions (dark spots) in which spherical bodies have grown. Note association between spheres and insolubles. Reflected polarized light.
FIG. 16. Pitch containing fine insoluble inclusions. Where margin of sphere is not in contact with insolubles it is smooth and regular. Where insolubles are present (e.g. arrowed) the growth of the ‘sphere’ is restricted and its shape becomes irregular. Reflected polarized light.
FIG. 17. Association of mesophase (M) and graphite (G). All the graphite in the sample is associated with mesophase, and only small spheres exist in the pitch away from graphite. The lamellar direction of the mesophase parallels that of graphite even when the graphite crystals are folded. Reflected polarized light.
FIG. 18. Association of mesophase (M) and mica (Mi). Although these two are strongly associated, the mesophase tends to stay in hemispherical droplets on the mica rather than form a continuous film as with graphite. Reflected polarized light.
FIG. 20. Sphere (arrowed) and part of a large sphere showing complex internal structure. Such spheres have formed by the coalescence of two or more smaller spheres. Reflected polarized light.
FIG. 21, Mesophase having near-parallel orientation over a considerable area. The difference in tone arises from differences of attitude af lamellae to the polished surface, Reflected paluized light.
FIG. 22, Network of insolubles (dark) in mesophase. Each of the cellular arsils represents a sphere which prev with in.solubles at its mar&t until coalescence. Reflected polarized Ii&.
FIG. 23. Pitch containing spheres which WdS extracted and later graphitized at 2600°C. The spheres present shrink to form ellipsoidal bodies which are a mass of small graphite crystals arranged as indicated. Reflected polarized light.
THE
FORMATION
OF G~HITIZING
CARBONS
mosaic structure, especially when carbonized rapidly, and while intermediate stages have not been observed in all cases they may reasonably be inferred to occur by analogy with the others which have been studied in detail: isodibenzanthrone, pyranthrone, indanthrone, dibenzathronyl, dibenzanthrone, l-l dianthrimide. 4.2 ~~~~~t~r~ of the @ieres The outline of the spheres in section is for the most part accurately circular (Fig. 1). Deviations from spheric@ occur-generally as the proportion of spheres to pitch locally becomes large, say more than 1: 1 (Fig. Z), and particularly in some Sited regions in the pyridine-soluble, tolueneinsoluble fraction of coke-oven pitch (Fig. 3). This appears to be due to the presence of very fine solid particles which impede the symmetrical growth of spheres. In the general case, however, the bodies at an intermediate stage of their growth are very close to perfect spheres. Even in the electron microscope the edges of spheres appear sharp (Fig. 4), although owing to low contrast it has so far been difficult to use the highest magnifications. If the pitch-like material containing the spheres is stirred and then cooled, a much more complex structure results (Fig. 5). The mesophase and pitch become intimately mixed, although each retains its identity. Droplets of pitch may be wholly enclosed by mesophase and these pitch droplets may in turn include minute spheres of mesophase. Thus the mesophase as well as the pitch behaves as a liquid at the temperatures where the spheres form and develop.
Seven selected from whfch electron
FIG.
8.
OFWJS over diffraction
disk of potterns
sphere obtofned
FROM
THE
LIQUID
PHASE
187
At the earliest detectable stage of their growth, the spheres in ultra-thin section exhibit characteristic electron diffraction patterns. These vary with the orientation of the sphere in the section-from a pattern as in Fig. 6 (lamellae parallel to beam) to that in Fig. 7 (lamellae oblique to beam). Such patterns are clearly not those of crystalline materials, yet a considerable degree of order is indicated. The kind of order giving rise to the paired arcs in Figs. 6 and 7 must be either lamellar or fibrous, but since other characteristics of fibre aggregates are absent the structure must be of lamellar type. The position of the arcs corresponds to an interlamellar spacing of 3.4711, *O*Ol. Ten and twenty haloes, indexed by analogy with graphite, have been observed (for ten, see Fig. 7). Sections of spheres where the lamellae were parallel to the beam, or nearly so, provided much information about the structure. The central area of such a section gave an approximately constant pattern, but as the selected diffraction area was moved towards the edge of the disk the paired arcs rotated about the undiffracted beam as indicated in Fig. 8. Since the structure is lamellar, and the preferred orientation varies as shown, the structure of a sphere must be as indicated in Fig. 9. Ultra-thin sections of pitch containing spheres were heated in the electron microscope to the temperature at which the spheres had formed (42O”C), and diffraction patterns were obtained from the ultra-thin sections of spheres. The patterns were indistin~ishable from those obtained at room temperature.
Electron obtained
diffraction from
arec1s
patterns indicated
above
Variation in electron diffraction patterns over disk of sphere, oriented with Iamellae parallel to beam.
188
J. D. BROOKS
and G. H. TAYLOR
In a high-temperature pitch fraction which had been carbonized under these conditions until sphere formation was well advanced (30% conversion was usually aimed for) it was found that the spheres were essentially insoluble in solvents. Small spheres were often apparently unaltered. Large spheres tended to crack and fragment after prolonged extraction (Fig. 12). Whether this was due only to mechanical handling or to a small amount of extraction was difficult to judge, but certainly no extensive extraction of spheres FIG. 9. Structure of sphere inferred from electron diffraction and polarized-light absorption. Sphere occurred. There was no indication of channelling sectioned parallel to main axis of symmetry. or marginal solution. The pitch was largely soluble in solvents such as pyridine and toluene. There The phenomena observed in the light microwas, however, always some insoluble residue from scope have already been described by TAYLOR(~). the pitch itself (Fig. 13). These observations, like the diffraction results, The U.V. spectra of benzene-soluble and pyristrongly suggest a lamellar structure of the type dine-soluble fractions of partly carbonized cokeproposed here. Further confirmation is obtained oven pitch showed envelopes with an absorption when serial sections of spheres are made in a plane edge between 370 and 450 rnp. The average parallel to the lamellar structure. The surfaces molecular weights of these fractions were 395 and above and below the median plane (Fig. 10) show 470 respectively, and if composed of single weak illumination of the four sectors in crossed aromatic units, an average unit would contain at nicols, whereas the median plane shows complete least 8 to 10 condensed rings. Most compounds extinction (see Fig. 1). of this size would be expected to have pronounced It can also be observed in some sections that absorption much farther into the ultraviolet“poles” are present (Fig. 11) ; such poles represent visible region, and probably the components in the the intersection of the axis of symmetry of the pitch from which the spheres are formed consist sphere (perpendicular to lamellae) with the plane partly of two or more smaller ring systems joined of the section. Strain effects, formerly thought to in a non-planar fashion, presumably by diarylbe significantt5), are now believed to be of little type linkages or by methylene bridges of dihydroimportance. aromatic structures. 4.3 Chemistry of spheres and sphere formation The ultimate analyses of partly carbonized cokeoven pitch (C100H5s0) differed little from that of The chemical constitution of these pitch-like the solvent-insoluble residue, which consisted materials in which spheres form is very complex. mainly of spherical bodies (CrsoH4sOr.4). The i.r. The compounds present in the original pitch are spectrum of this residue showed strong (95%) mainly polynuclear aromatic hydrocarbons together absorption to 3500 cm-’ (Fig. 14). LEWIS and with some oxygen- and nitrogen-containing compounds. Proton magnetic resonance spectra of EDSTROM@)classified polynuclear hydrocarbons as or reactive depending on solvent-soluble fractions show that over 90% of either non-reactive whether they vaporized unchanged or underwent the protons are attached to aromatic rings. chemical condensation to form a carbonaceous It would be expected that in the specimens residue on heating to 750°C. They calculated the heated in open boats, many compounds with boilionization potential of the reactive hydrocarbons ing points up to the temperatures attained in carfrom the position of the para-band in the u.v.bonization would be removed by distillation or visible spectra and showed that this was usually less sublimation. However, where a pitch was heated than about 7-l eV (p-band at about 387 mp). under reflux conditions, yields of SO-9Oo/oof mesoThe U.V. spectra of the benzene- and pyridinephase were obtained, and a large proportion of the extracts of a partly carbonized coke-oven pitch aromatic structures originally present took part in fraction showed absorption envelopes with log the formation of the spheres.
I F-Y,
cm-’
Moo
1400
FIG. 14. In&a-red spectra of coke-oven pitch fractions.
lmm
no0
Upper spectrum: 1 yOreduced carbonized toluene-soluble coke-oven-tar pitch Lower spectrum: 1% carbonized ~luene-soluble coke-oven-tar pitch extracted with benzene and pyridine
3ooo
coo
700
190
J. D. BROOKS and G. H. TAYLOR
e values at 387 rnp of about 4.2, similar to that of several of the reactive hydrocarbons listed by LEWISand EDSTROM@). Probably under the conditions of carbonization the more reactive hydrocarbons in the pitch phase decompose via a radical mechanism(9) involving little loss of hydrogen to form larger, more complex species, which are planar, and therefore highly absorbing through the u.v.-visible region. These separate from the pitch as the mesophase, in partly ordered lamellar molecular assemblages. The coke-oven pitch fraction, which had been partly carbonized and contained about 30% by volume of spheres, was reduced with lithium in erhylene diamine. The hydrogen content increased, the empirical composition changing from CiooHaa to CicoHr ia. The U.Y.spectra of the reduction product showed low absorption and the i.r. spectrum indicated only weak aromatic features (Fig. 14). The n.m.r. spectrum in chloroform showed less than 10% aromatic protons. Thus the reduction consists mainly of the conversion of existing aromatic CH to alicyclic CH2, with a small conversion of aromatic C to alicyclic CH groups as is the csse with known polynuclear hydro~rbons(“). Measurement of molecular weights showed that the unreduced pitch contained: 26% benzenesoluble fraction, average molecular weight 395; 46% pyridine-soluble fraction, average molecular weight 468; and 27% insoluble sphere-cont~ning fraction. After reduction the molecular weight of the total reduction product (now completely soluble in chloroform) was 790. Assuming that only reduction occurred, it follows that the average molecular weight of the reduction product of the insoluble fraction was approximately 1690.
When coke-oven pitch containing insoluble particles was carbonized, it was found that there was a marked association between the insoluble particles and spheres (Pig. 15). In the earlier stages of sphere formation no spheres formed except in regions adjacent to insoluble particles. The insoluble particles did not become incorporated within spheres and in fact no solid particles have ever been observed within them. The insoluble particles aggregated around the surface of the sphere, causing it to have a somewhat irregular margin (Fig. 16).
Introduced insoluble material (e.g. magnesium oxide) was found to behave in the same way as the naturally occurring insolubles. Finely divided natural graphite was added to the toluene-soluble fraction of a coke-oven pitch, which was then carbonized. There was a marked association between mesophase and graphite, with pronounced orientation of the mesophase on the graphite surfaces (Fig. 17). A mixture of toluene-soluble pitch and finely divided mica was carbonized in the same way. Again there was a marked association between the mesophase and the mica, and the mesophase tended to be ordered parallel to the mica. However, the effect was weaker than with graphite (Fig. 18), and the mesophase formed hemispheres on the surface rather than being distributed evenly, as with the graphite. Powdered aluminium and carbon black had no obvious effect on rhe nucleation and growth of spheres, but the surfaces of glass vessels in which insoluble-free pitches had been carbonized were a preferred site for mesophase oriented parallel to the surface. The general conclusions on nucleation were that any solid surface appeared to be a preferred site for mesophase growth and that the nucleating effect of solids increased with their specific surface area. (However, this refers to available surface area; the total surface area of the carbon black was very large, but most of this was inaccessible to the viscous mesophase.) When a pitch free of solids and the same pitch containing solids were carbonized under the same conditions, the pitch with the solids tended to contain more and smaller spheres than the other, but there was little difference in the overall proportion of mesophase present. When the pitch was heated without agitation at a constant temperature, the spheres formed sank slowly through the pitch, the mesophase being of slightly higher density. This effect appeared to cause the removal of nuclei from the supernatant pitch, and may account to some extent for the growth in the later stages of comparatively large spheres.
The two most important factors in the rate of growth of spheres are temperature and time. In general, the lower the rate of carbonization the
THE FORMATION
OF G~HITIZING
fewer and larger were the spheres which formed. This was most readily observed when no insoluble particles were present. There appears to be a limiting temperature below which no sphere formation occurs even over periods of 24 hr or more. This is about 400°C for the coke-oven pitch used. At slightly higher temperature spheres form, and the longer the time the greater the conversion of pitch to mesophase. However, the increase with time seems to be asymptotic: the increase in mesophase after 5 hr at 425°C as compared with 1 hr at 425” was small, although definite. The increase after longer periods at 425” was almost imperceptible. At higher tempera~res, prolonged heating leads to complete conversion to mesophase, and the higher the temperature, the shorter the time required for complete conversion. Both stirring and the presence of fine solid particles appear to accelerate slightly the formation of mesophase. 4.6 Coalescence of spheres and formation of the mosaic The formation of the mosaic begins with the coalescence of two sphere& This occurs as in Fig. 19. In general, the two spheres will have different orientations. When they cohere, surface rearrangements occur so that a continuous link between the two is formed. The fact that all spheres appear to have radially arranged planar molecules at their surfaces would tend to promote initial fusion. The external margins of the composite of two spheres
Before
Just
contact
after
Short
Type
contact
time
Of
after
complex
Structure
formed
composite
of
spheres
two
COntrOctS
contact
internal when or
to
a
more one
iorge
sphere
Fxc. 19. Rearrsngements which sppesr to occur when two spheres coalesce. G
CARBONS
FROM THE LIQUID
PHASE
191
retain a lamellar orientation perpendicular to the surface, and given sutlicient time such a composite will contract to form a sphere with a complex internal structure as in Fig. 20. This change of form takes a significant time, since intermediate stages are frequently found in the cooled samples. This is unde~tandable in view of the viscosity of the mesophase, which, from probe tests and heating-microscope measurements, is known to be high. If, however, coalescence of spheres becomes general, many new connections are formed as described, and merging to form larger spheres cannot occur. This process goes to completion with the conversion of interstitial pitch to mesophase. Since coalescence is occurring in three dimensions, a structure is formed in which mesophase as seen in any plane comprises areas in which the orientation is fairly uniform connected to one another by zones of mesophase in which the lamellae curve around sharply to conform to the orientation of the next fairly uniform area. Thus although the term “mosaic” has been employed the structure is not composed of sharply bounded units. The sample, after total conversion to mesophase, remains a very viscous liquid which can be deformed by mechanical pressure. If held for long periods in the liquid condition the ordered regions become larger, and regions up to a millimetre across having almost constant parallel orientation have been prepared in this way (Fig. 21). This behaviour, which occurs only when mesophase is locally free from unconverted pitch, is further evidence of the role of the pitch-mesophase interface in controlling the structure of spheres. At a slightly higher temperature the meaophase changes to solid semi-coke without obvious change of structure. It is difficult to say at what temperature this effect occurs-indeed, it is probably a case of the mesophase gradually becoming more viscous. With some substances these events follow each other more closely than with others. Whereas with vitrinite and especially dibenzanthrone the event‘s described are telescoped to within a few degrees, with pitch the range is some tens of degrees. With pitch containing fine insoluble solids, the coalescence of spheres is modified. As described above, the insoluble solids do no’t become incor-
192
J. D. BROOKS and G. H. TAYLOR
porated within the growing sphere but aggregate on its surface. When such insolubles are abundant they remain in the mosaic at the interstitial positions where spheres adjoin, forming a threedimensional network through the coke (Fig. 22). Such deposition at mosaic area boundaries would be expected to modify considerably the mechanical properties of the coke. This in fact may well be the basis of a patent(“). 4.7 Gaphitization When semi-coke, formed as described, is heated, the mosaic texture and lamellar orientation remain, but the perfection of order within any unit increases, until at temperatures above 2500°C diffraction patterns indicate the formation of the full three-dimensional graphite structure. This development of order is accompanied by anisotropic shrinkage which is the cause of shrinkage cracks in some high-temperature cokes. Pitch containing spheres was extracted with toluene, and the sphere-containing residue heated first at 1OOO“Cand subsequently at 2800°C for 1 hr. The spheres retained their identity but changed shape in that they shrank preferentially in a direction perpendicular to the lamellae (Fig. 23). The graphite so formed consisted of many small crystals oriented as shown in Fig. 23. This shrinkage was not merely due to the reduction in the interlamellar spacing, which was less than 5 per cent. The specific gravity of the mesophase was 1.4, and since some of the carbon and all of the other atomic species are removed during graphitixation a given volume of mesophase can yield little more than half its volume of graphite. Although the electron diffraction patterns indicate a considerable degree of crystallographic order in the direction perpendicular to the lamellae, the patterns and also the low specific gravity show that the molecular packing in the lamellar direction must be poor. It is known that even in crystalline polynuclear aromatic hydrocarbons the nonbonded distance between hydrogen atoms is large(r2) and in the case of the mesophase in which the planar molecules probably exhibit a range of size and shape, the average intermolecular distances in the lamellae would be greater still. During graphitixation the loss of peripheral atoms from the aromatic structures creates vacancies and voids in the planes, and since the
isolated spheres contract preferentially in the direction perpendicular to the lamellae the voids in any one plane must be filled by migration of carbon atoms from planes above and below until the perfect three-dimensional crystalline graphitic structure is established. 5. CONCLUSION
These studies suggest that the spheres consist of polynuclear aromatic molecules of the order of some tens of aromatic rings in average size. The molecules appear to be stacked one upon another with fairly good local parallelism but in general, no three-dimensional order. The orientation of the structure so formed varies regularly through the sphere from lamellar at the centre to radial at the surface. This structure is the result of a number of forces. The spherical shape is presumed to be a response to surface-tension forces and further evidence of the effects of surface tension can be seen in the coalescence of two spheres and their subsequent reversion to spherical shape. The spheres appear to exhibit a characteristic molecular orientation at their surfaces and in this and some other respects they have many of the characteristics of liquid crystalsc7). The boundary condition always appears to obtain in small, large, and composite spheres and at the interface between pitch and mosaic mesophase, the surface having a uniform molecular arrangement in which the edges of aromatic planes are perpendicular to the interface. The other major factor determining sphere structure appears to be the tendency of large planar molecules to pack together. But for this, there would Seem to be no reason why the structure of the spheres should not be wholly radial, at least as seen in one plane. But in fact the radial arrangement appears to be imposed on moleculesor perhaps on swarms of molecules-only near the surface of the sphere. As the sphere grows, the molecules originally at the surface must rotate to give an approximately lamellar structure in the interior. Coalescence of spheres may lead to the formation of larger regions of extended order. It is the generally lamellar arrangement of the molecular structure in these regions which favours the formation of graphitic carbon on heating to elevated temperatures.
THE
FORMATION
OF
GRAPHITIZING
CARBONS
Acknowledgements-The authors wish to thank Mr. H. R. Brown, Chief of the Division of Coal Research, CSIRO, for his support and encouragement. They are also grateful to Dr. M. F. R. Mukahy for helpful suggestions and to Mr. J. W. Smith for experimental assistance. Dr. J. S. Shannon provided some of the carbon&d poiynuclear compounds. Mr. J. F. Corcoran and Mr. W. Zeidler made the preparations far electron and light microscopy respectively.
5. 6.
8. 9.
1.
F~AMDOHR
10.
P., Eisenk&tenwesen 1 (1928).
2. MARSHALLC. E., Fuel 24,120
(1945). 3. STACH E., Mikroskopie naturlicker Kokse, Hamibttck der Mikroskopie in der Technik (Ed. H. FREUND), Vol. II, Pt. 1, p. 411. Verlag Umschsu, (1952).
Frankfurt
THE
LIQUID
PHASE
193
4. AEIRAMSKI C. and MACKOWSKYM. T., Methoden und Ergebnisse der angewandten Koksmikroskopie, Handbrsckdet Mikroskopie in der Technik (Ed. H. F-),
7.
REFERJINCES
FROM
11.
12.
Vol. II, Pt. 1, p. 311. Verlag Umschau, Frankfurt (1952). TAYLOR G. H., FueZ 40,465 (1961). B~ooxs J. D. and TAYLOR G. H., paper to be published in the Proceedings of the American Coal Science Conference held in 1964. G-Y G. W., Molecub Structure and the Properties of L&n&dCrvstaZs. Academic Press (19621.
1965,
Vol. 3, pp. 185493.
Pergamon Press Ltd.
THE FORMATION
Printed in Great Britain
OF GRAPHITIZING
CARBONS
FROM THE LIQUID PHASE J. D. BROOKS and G. H. TAYLOR Division of Coal Research, CSIRO, P.O. Box 175, Chatswood, New South Wales, Australia (Received 23 June 1965)
Abstract-The formation of graphitizing low-temperature carbons by solidification from a liquid phase proceeds via the separation of a mesophase having properties similar to those of liquid crystals. Initially, the mesophase, consisting of planar aromatic compounds of high molecular weight, separates in the isotropic liquid as spherical droplets having a considerable degree of molecular order, with the aromatic sheets stacked in parallel array. The sheets are arranged perpendicularly along one diameter of a sphere, but are curved so that they are normal to the surface. On prolonged heating the spheres coalesce and extended regions of uniform orientation may be formed; eventually a solid semi-coke is obtained. It is the generally lamellar arrangement of the molecular structure in these regions which favours the formation of graphite carbon at high temperatures. When isolated spheres are heated to graphitizing temperatures contraction occurs in the direction perpendicular to the preferred orientation. The bodies become elliptical in section and each is converted to a mass of small graphite crystals.
1. INTRODUCTION
2. PREVIOUS WORJS
TEIE structure of carbons ultimately obtained by the pyrolysis of organic materials in the condensed phase is established at a very early stage of thermal decomposition. These carbons are graphitizing or non-graphitizing depending usually upon whether they are formed from the liquid or solid phases respectively; if fusion does not occur the carbons are, in general, non-graphitizing. Substances which form graphitizing carbons have relatively high atomic ratios of hydrogen to noncarbon elements, and include vitrinite of highvolatile coking coals, coal tar pitches, petrofeum polyvinyl chloride and polynuclear bitumen, aromatic compounds such as naphthacene and
dibenzanthrone. Such graphitizing carbons are coke-like in appearance and have an anisotropic microscopic structure which persists on heating to lOOO“C,the degree of anisotropy increasing with rise in temperature. This structure also influences the properties of the graphite which forms on further heating to 2500°C or above. The nature of the solidification process by which graphitizing carbons are formed is little understood, and its investigation is the subject of this paper. 185
The optical anisotropy of coke was first described by RAMDOHR(') in 1928. MARSHALL@) (thin sections of natural coke), STACH(~)(polished surfaces of natural coke), ABRAMSKI~~~ MACKOWSKY’~) (polished surfaces of manufactured coke) and others have described the optical structures of various cokes and coke-like carbons. None of these workers appears to have recognized intermediate stages in the development of the characteristic anisotropic mosaic structure. Such stages were described by TAYLOR(‘)and by BROOKSand TAYLOR@), who reported the appearance and growth of spherical bodies in the plastic carbonaceous material. The spheres, some of the properties of which were described, were observed to grow at the expense of the plastic material until a three-dimensional mosaic structure was formed, which persisted in the soiidified semi-coke. Other workers, e.g. STA&~), had observed spheres associated with carbonized coal but had reported these to be “grapbitic spherulites” which exhibited the so-called Brewster or stationary extinction cross, indicating a radial structure. As has been described(‘), the spheres, which represent an intermediate stage in coke formation, have optical properties which preclude a radial structure. Two
186
J. D. BROOKS and G. H. TAYLOR
alternative structures, both essentially lame&r, were proposed. 3. EXPERIMENTAL
Specimens were prepared by partial carbonixation for examination by microscopy, electron diffraction and chemical analysis. Initially, a horizontal silica retort within a furnace having automatic temperature control was used to carbonize the samples. It was found that very critical temperature control was necessary, especially with substances having a short plastic range. Temperature control and reproducibility to at least +Z”C was essential and + 1°C was desirable if various intermediate stages of carbonization were to be recognised. An inert atmosphere, usually nitrogen, was provided in these experiments. Later carbonizing runs were carried out using vertical tubes under refluxing conditions. The tubes were immersed in a solder bath of controlled temperature. The tubes were capped with aluminium foil; under these conditions of reflux it was not necessary to exclude air rigorously. Microscopic observations were made of the partly carbonized material. With minor exceptions (hot-stage electron and light microscopy) the material was chilled or allowed to cool to room temperature. For light microscopy, thin sections and polished surfaces were prepared using Carborundum papers and polishing on cloth with chrome and magnesium oxides. Ultra-thin sections were cut with a diamond knife. A Leitz Ortholux microscope with xenon arc illumination and a JEM-SY electron microscope were used for light and electron microscopy respectively.
For the class of substances (listed below) being considered, the general pattern of behaviour has been found to be as follows: The substance melts on heating and becomes an isotropic pitch-lie material of plastic or liquid consistency. With rising temperature, spheres appear in the pitchlie mass (Fig. 1). These were first detected at O-1 p diameter but no doubt exist at smaller sizes. Even at O-1 p they are difficult to detect in the electron microscope except with the aid of selectedarea electron diffraction. (The bodies give a characteristic pattern, while the pitch gives
extremely diffuse scattering}. With increase of temperature, and to a certain extent with increase of time at a fixed temperature, the spheres grow larger. When they are of micron size they may be readily seen in the electron microscope (they are slightly more electron-dense than the pitch) and can just be detected with the light microscope. At still larger sizes they are easily observed in polished surfaces with the light microscope, or even, in some cases, with the naked eye. At some stage in the heating process the spheres will have replaced a large part of the pitch-like material and begun to interfere with one another’s enlargement. At this stage a ‘mosaic’ begins to form by coalescence. When no more isotropic pitch-like material remains and only anisotropic material or mesophase* is present the mosaic structure is complete, and then, or shortly afterwards, the mesophase solidifies into semi-coke. The substances which exhibit this behaviour are all materials which become plastic when heated and later solidify to form a semi-coke which is readily graphitizable. The substances listed below have been found to exhibit the behaviour described. The temperatures in brackets are those at which spheres and pitch-like material were present under the particular experimental conditions. Coke-oven pitch (425°C); Vertical-retort pitch (425°C); Petroleum-ether-soluble fraction (“crystalloids”) of coke-oven pitch (425°C); Toluene-soluble, petroleum ether-insoluble fraction (“resinoids”) of coke-oven pitch (425°C); Pyridine-soluble, toluene-insoluble fraction of coke-oven pitch (425°C) ; ONIA-GEGI pitch (~*~~; Carburetted-water-gas pitch (430°C); Petroleum bitumen 2801320 (425°C); Vitrinites from bituminous coals (460-47O”C); Naphthacene (425°C); Polyvinyl chloride (400°C) ; Dibenzauthrone (520°C). In addition, the following substances yield a *The term ‘Lmesophase” (Gr. mesos=intennediate) is used for the substance of which the spheres and the mosaic--before solidification-are formed, cf. GRAY@.
FIG. 1. Spheres at an intermediate stage of development. The grey matrix is pitchlike material (P). The spheres are in random orientations. A few show almost total absorption of polarized light (arrowed). Nearly all spheres have accurately circular outlines. Reflected polarized light.
FIG. 2. A later stage than Fig. 1 in the carbonization sequence. The bodies are mostly no longer spherical but become distorted through mutual interference. Reflected polarized light.
[facing
p. 186
FIG. 3. Mesophase bodies in pitch (P) which contains fine insoluble matter (dark spots). Some bodies are spherical or nearly so, but the majority have less regular shapes. Reflected polarized light.
FIG. 4. Ultra-thin section of part of two spheres showing regularity of sphere outline with insoluble matter at margins (arrowed). The whole field is striated due to scoring during cutting by these insoluble particles. Electron micrograph.
FIG. 5. Complex structure resulting from stirring pitch containing spheres. The pitch (P) and mesophase both behave as liquids. The sphere (arrowed) of mesophase contains tiny spherical inclusions of pitch and is itself contained within an elongate body of pitch wholly surrounded by mesophase. Reflected polarized light.
FIG. 6. Electron-diffraction pattern from part of a single sphere where lamellae are parallel to beam. The 002 and 004 arcs are indicated. The continuous rings are from the internal aluminium standard.
FIG. 7. Electron-diffraction pattern from part of a single sphere where lamellae are slightly oblique to the beam. The 002 and 004 arcs are indicated, also the 10 spacing.
FIG. 10. Polished surface of pitch containing spheres. The sphere (arrowed) has been sectioned with lamellae parallel to polished surface and slightly above the plane of symmetry of the sphere. It shows an extinction cross which remains stationary when the stage is rotated. Spheres in other orientations show transient extinction crosses only. Reflected polarized light.
FIG. 11. A sphere in which the lamellae lie perpendicular to the polished surface which is the median plane of the sphere. Note the two “poles” (arrowed) which correspond to the ends of the major axis of the sphere. Reflected polarized light.
FIG. 12. Large spheres (one nearly complete, one partial) cracked and fragmented after prolonged extraction. Reflected polarized light.
FIG. 13. Residue after prolonged extraction of pitch with benzene and pyridine. The small spheres (arrowed) remain apparently unchanged together with a cellular residue from the pitch. Reflected polarized light.
FIG. 15. Pitch containing insoluble inclusions (dark spots) in which spherical bodies have grown. Note association between spheres and insolubles. Reflected polarized light.
FIG. 16. Pitch containing fine insoluble inclusions. Where margin of sphere is not in contact with insolubles it is smooth and regular. Where insolubles are present (e.g. arrowed) the growth of the ‘sphere’ is restricted and its shape becomes irregular. Reflected polarized light.
FIG. 17. Association of mesophase (M) and graphite (G). All the graphite in the sample is associated with mesophase, and only small spheres exist in the pitch away from graphite. The lamellar direction of the mesophase parallels that of graphite even when the graphite crystals are folded. Reflected polarized light.
FIG. 18. Association of mesophase (M) and mica (Mi). Although these two are strongly associated, the mesophase tends to stay in hemispherical droplets on the mica rather than form a continuous film as with graphite. Reflected polarized light.
FIG. 20. Sphere (arrowed) and part of a large sphere showing complex internal structure. Such spheres have formed by the coalescence of two or more smaller spheres. Reflected polarized light.
FIG. 21, Mesophase having near-parallel orientation over a considerable area. The difference in tone arises from differences of attitude af lamellae to the polished surface, Reflected paluized light.
FIG. 22, Network of insolubles (dark) in mesophase. Each of the cellular arsils represents a sphere which prev with in.solubles at its mar&t until coalescence. Reflected polarized Ii&.
FIG. 23. Pitch containing spheres which WdS extracted and later graphitized at 2600°C. The spheres present shrink to form ellipsoidal bodies which are a mass of small graphite crystals arranged as indicated. Reflected polarized light.
THE
FORMATION
OF G~HITIZING
CARBONS
mosaic structure, especially when carbonized rapidly, and while intermediate stages have not been observed in all cases they may reasonably be inferred to occur by analogy with the others which have been studied in detail: isodibenzanthrone, pyranthrone, indanthrone, dibenzathronyl, dibenzanthrone, l-l dianthrimide. 4.2 ~~~~~t~r~ of the @ieres The outline of the spheres in section is for the most part accurately circular (Fig. 1). Deviations from spheric@ occur-generally as the proportion of spheres to pitch locally becomes large, say more than 1: 1 (Fig. Z), and particularly in some Sited regions in the pyridine-soluble, tolueneinsoluble fraction of coke-oven pitch (Fig. 3). This appears to be due to the presence of very fine solid particles which impede the symmetrical growth of spheres. In the general case, however, the bodies at an intermediate stage of their growth are very close to perfect spheres. Even in the electron microscope the edges of spheres appear sharp (Fig. 4), although owing to low contrast it has so far been difficult to use the highest magnifications. If the pitch-like material containing the spheres is stirred and then cooled, a much more complex structure results (Fig. 5). The mesophase and pitch become intimately mixed, although each retains its identity. Droplets of pitch may be wholly enclosed by mesophase and these pitch droplets may in turn include minute spheres of mesophase. Thus the mesophase as well as the pitch behaves as a liquid at the temperatures where the spheres form and develop.
Seven selected from whfch electron
FIG.
8.
OFWJS over diffraction
disk of potterns
sphere obtofned
FROM
THE
LIQUID
PHASE
187
At the earliest detectable stage of their growth, the spheres in ultra-thin section exhibit characteristic electron diffraction patterns. These vary with the orientation of the sphere in the section-from a pattern as in Fig. 6 (lamellae parallel to beam) to that in Fig. 7 (lamellae oblique to beam). Such patterns are clearly not those of crystalline materials, yet a considerable degree of order is indicated. The kind of order giving rise to the paired arcs in Figs. 6 and 7 must be either lamellar or fibrous, but since other characteristics of fibre aggregates are absent the structure must be of lamellar type. The position of the arcs corresponds to an interlamellar spacing of 3.4711, *O*Ol. Ten and twenty haloes, indexed by analogy with graphite, have been observed (for ten, see Fig. 7). Sections of spheres where the lamellae were parallel to the beam, or nearly so, provided much information about the structure. The central area of such a section gave an approximately constant pattern, but as the selected diffraction area was moved towards the edge of the disk the paired arcs rotated about the undiffracted beam as indicated in Fig. 8. Since the structure is lamellar, and the preferred orientation varies as shown, the structure of a sphere must be as indicated in Fig. 9. Ultra-thin sections of pitch containing spheres were heated in the electron microscope to the temperature at which the spheres had formed (42O”C), and diffraction patterns were obtained from the ultra-thin sections of spheres. The patterns were indistin~ishable from those obtained at room temperature.
Electron obtained
diffraction from
arec1s
patterns indicated
above
Variation in electron diffraction patterns over disk of sphere, oriented with Iamellae parallel to beam.
188
J. D. BROOKS
and G. H. TAYLOR
In a high-temperature pitch fraction which had been carbonized under these conditions until sphere formation was well advanced (30% conversion was usually aimed for) it was found that the spheres were essentially insoluble in solvents. Small spheres were often apparently unaltered. Large spheres tended to crack and fragment after prolonged extraction (Fig. 12). Whether this was due only to mechanical handling or to a small amount of extraction was difficult to judge, but certainly no extensive extraction of spheres FIG. 9. Structure of sphere inferred from electron diffraction and polarized-light absorption. Sphere occurred. There was no indication of channelling sectioned parallel to main axis of symmetry. or marginal solution. The pitch was largely soluble in solvents such as pyridine and toluene. There The phenomena observed in the light microwas, however, always some insoluble residue from scope have already been described by TAYLOR(~). the pitch itself (Fig. 13). These observations, like the diffraction results, The U.V. spectra of benzene-soluble and pyristrongly suggest a lamellar structure of the type dine-soluble fractions of partly carbonized cokeproposed here. Further confirmation is obtained oven pitch showed envelopes with an absorption when serial sections of spheres are made in a plane edge between 370 and 450 rnp. The average parallel to the lamellar structure. The surfaces molecular weights of these fractions were 395 and above and below the median plane (Fig. 10) show 470 respectively, and if composed of single weak illumination of the four sectors in crossed aromatic units, an average unit would contain at nicols, whereas the median plane shows complete least 8 to 10 condensed rings. Most compounds extinction (see Fig. 1). of this size would be expected to have pronounced It can also be observed in some sections that absorption much farther into the ultraviolet“poles” are present (Fig. 11) ; such poles represent visible region, and probably the components in the the intersection of the axis of symmetry of the pitch from which the spheres are formed consist sphere (perpendicular to lamellae) with the plane partly of two or more smaller ring systems joined of the section. Strain effects, formerly thought to in a non-planar fashion, presumably by diarylbe significantt5), are now believed to be of little type linkages or by methylene bridges of dihydroimportance. aromatic structures. 4.3 Chemistry of spheres and sphere formation The ultimate analyses of partly carbonized cokeoven pitch (C100H5s0) differed little from that of The chemical constitution of these pitch-like the solvent-insoluble residue, which consisted materials in which spheres form is very complex. mainly of spherical bodies (CrsoH4sOr.4). The i.r. The compounds present in the original pitch are spectrum of this residue showed strong (95%) mainly polynuclear aromatic hydrocarbons together absorption to 3500 cm-’ (Fig. 14). LEWIS and with some oxygen- and nitrogen-containing compounds. Proton magnetic resonance spectra of EDSTROM@)classified polynuclear hydrocarbons as or reactive depending on solvent-soluble fractions show that over 90% of either non-reactive whether they vaporized unchanged or underwent the protons are attached to aromatic rings. chemical condensation to form a carbonaceous It would be expected that in the specimens residue on heating to 750°C. They calculated the heated in open boats, many compounds with boilionization potential of the reactive hydrocarbons ing points up to the temperatures attained in carfrom the position of the para-band in the u.v.bonization would be removed by distillation or visible spectra and showed that this was usually less sublimation. However, where a pitch was heated than about 7-l eV (p-band at about 387 mp). under reflux conditions, yields of SO-9Oo/oof mesoThe U.V. spectra of the benzene- and pyridinephase were obtained, and a large proportion of the extracts of a partly carbonized coke-oven pitch aromatic structures originally present took part in fraction showed absorption envelopes with log the formation of the spheres.
I F-Y,
cm-’
Moo
1400
FIG. 14. In&a-red spectra of coke-oven pitch fractions.
lmm
no0
Upper spectrum: 1 yOreduced carbonized toluene-soluble coke-oven-tar pitch Lower spectrum: 1% carbonized ~luene-soluble coke-oven-tar pitch extracted with benzene and pyridine
3ooo
coo
700
190
J. D. BROOKS and G. H. TAYLOR
e values at 387 rnp of about 4.2, similar to that of several of the reactive hydrocarbons listed by LEWISand EDSTROM@). Probably under the conditions of carbonization the more reactive hydrocarbons in the pitch phase decompose via a radical mechanism(9) involving little loss of hydrogen to form larger, more complex species, which are planar, and therefore highly absorbing through the u.v.-visible region. These separate from the pitch as the mesophase, in partly ordered lamellar molecular assemblages. The coke-oven pitch fraction, which had been partly carbonized and contained about 30% by volume of spheres, was reduced with lithium in erhylene diamine. The hydrogen content increased, the empirical composition changing from CiooHaa to CicoHr ia. The U.Y.spectra of the reduction product showed low absorption and the i.r. spectrum indicated only weak aromatic features (Fig. 14). The n.m.r. spectrum in chloroform showed less than 10% aromatic protons. Thus the reduction consists mainly of the conversion of existing aromatic CH to alicyclic CH2, with a small conversion of aromatic C to alicyclic CH groups as is the csse with known polynuclear hydro~rbons(“). Measurement of molecular weights showed that the unreduced pitch contained: 26% benzenesoluble fraction, average molecular weight 395; 46% pyridine-soluble fraction, average molecular weight 468; and 27% insoluble sphere-cont~ning fraction. After reduction the molecular weight of the total reduction product (now completely soluble in chloroform) was 790. Assuming that only reduction occurred, it follows that the average molecular weight of the reduction product of the insoluble fraction was approximately 1690.
When coke-oven pitch containing insoluble particles was carbonized, it was found that there was a marked association between the insoluble particles and spheres (Pig. 15). In the earlier stages of sphere formation no spheres formed except in regions adjacent to insoluble particles. The insoluble particles did not become incorporated within spheres and in fact no solid particles have ever been observed within them. The insoluble particles aggregated around the surface of the sphere, causing it to have a somewhat irregular margin (Fig. 16).
Introduced insoluble material (e.g. magnesium oxide) was found to behave in the same way as the naturally occurring insolubles. Finely divided natural graphite was added to the toluene-soluble fraction of a coke-oven pitch, which was then carbonized. There was a marked association between mesophase and graphite, with pronounced orientation of the mesophase on the graphite surfaces (Fig. 17). A mixture of toluene-soluble pitch and finely divided mica was carbonized in the same way. Again there was a marked association between the mesophase and the mica, and the mesophase tended to be ordered parallel to the mica. However, the effect was weaker than with graphite (Fig. 18), and the mesophase formed hemispheres on the surface rather than being distributed evenly, as with the graphite. Powdered aluminium and carbon black had no obvious effect on rhe nucleation and growth of spheres, but the surfaces of glass vessels in which insoluble-free pitches had been carbonized were a preferred site for mesophase oriented parallel to the surface. The general conclusions on nucleation were that any solid surface appeared to be a preferred site for mesophase growth and that the nucleating effect of solids increased with their specific surface area. (However, this refers to available surface area; the total surface area of the carbon black was very large, but most of this was inaccessible to the viscous mesophase.) When a pitch free of solids and the same pitch containing solids were carbonized under the same conditions, the pitch with the solids tended to contain more and smaller spheres than the other, but there was little difference in the overall proportion of mesophase present. When the pitch was heated without agitation at a constant temperature, the spheres formed sank slowly through the pitch, the mesophase being of slightly higher density. This effect appeared to cause the removal of nuclei from the supernatant pitch, and may account to some extent for the growth in the later stages of comparatively large spheres.
The two most important factors in the rate of growth of spheres are temperature and time. In general, the lower the rate of carbonization the
THE FORMATION
OF G~HITIZING
fewer and larger were the spheres which formed. This was most readily observed when no insoluble particles were present. There appears to be a limiting temperature below which no sphere formation occurs even over periods of 24 hr or more. This is about 400°C for the coke-oven pitch used. At slightly higher temperature spheres form, and the longer the time the greater the conversion of pitch to mesophase. However, the increase with time seems to be asymptotic: the increase in mesophase after 5 hr at 425°C as compared with 1 hr at 425” was small, although definite. The increase after longer periods at 425” was almost imperceptible. At higher tempera~res, prolonged heating leads to complete conversion to mesophase, and the higher the temperature, the shorter the time required for complete conversion. Both stirring and the presence of fine solid particles appear to accelerate slightly the formation of mesophase. 4.6 Coalescence of spheres and formation of the mosaic The formation of the mosaic begins with the coalescence of two sphere& This occurs as in Fig. 19. In general, the two spheres will have different orientations. When they cohere, surface rearrangements occur so that a continuous link between the two is formed. The fact that all spheres appear to have radially arranged planar molecules at their surfaces would tend to promote initial fusion. The external margins of the composite of two spheres
Before
Just
contact
after
Short
Type
contact
time
Of
after
complex
Structure
formed
composite
of
spheres
two
COntrOctS
contact
internal when or
to
a
more one
iorge
sphere
Fxc. 19. Rearrsngements which sppesr to occur when two spheres coalesce. G
CARBONS
FROM THE LIQUID
PHASE
191
retain a lamellar orientation perpendicular to the surface, and given sutlicient time such a composite will contract to form a sphere with a complex internal structure as in Fig. 20. This change of form takes a significant time, since intermediate stages are frequently found in the cooled samples. This is unde~tandable in view of the viscosity of the mesophase, which, from probe tests and heating-microscope measurements, is known to be high. If, however, coalescence of spheres becomes general, many new connections are formed as described, and merging to form larger spheres cannot occur. This process goes to completion with the conversion of interstitial pitch to mesophase. Since coalescence is occurring in three dimensions, a structure is formed in which mesophase as seen in any plane comprises areas in which the orientation is fairly uniform connected to one another by zones of mesophase in which the lamellae curve around sharply to conform to the orientation of the next fairly uniform area. Thus although the term “mosaic” has been employed the structure is not composed of sharply bounded units. The sample, after total conversion to mesophase, remains a very viscous liquid which can be deformed by mechanical pressure. If held for long periods in the liquid condition the ordered regions become larger, and regions up to a millimetre across having almost constant parallel orientation have been prepared in this way (Fig. 21). This behaviour, which occurs only when mesophase is locally free from unconverted pitch, is further evidence of the role of the pitch-mesophase interface in controlling the structure of spheres. At a slightly higher temperature the meaophase changes to solid semi-coke without obvious change of structure. It is difficult to say at what temperature this effect occurs-indeed, it is probably a case of the mesophase gradually becoming more viscous. With some substances these events follow each other more closely than with others. Whereas with vitrinite and especially dibenzanthrone the event‘s described are telescoped to within a few degrees, with pitch the range is some tens of degrees. With pitch containing fine insoluble solids, the coalescence of spheres is modified. As described above, the insoluble solids do no’t become incor-
192
J. D. BROOKS and G. H. TAYLOR
porated within the growing sphere but aggregate on its surface. When such insolubles are abundant they remain in the mosaic at the interstitial positions where spheres adjoin, forming a threedimensional network through the coke (Fig. 22). Such deposition at mosaic area boundaries would be expected to modify considerably the mechanical properties of the coke. This in fact may well be the basis of a patent(“). 4.7 Gaphitization When semi-coke, formed as described, is heated, the mosaic texture and lamellar orientation remain, but the perfection of order within any unit increases, until at temperatures above 2500°C diffraction patterns indicate the formation of the full three-dimensional graphite structure. This development of order is accompanied by anisotropic shrinkage which is the cause of shrinkage cracks in some high-temperature cokes. Pitch containing spheres was extracted with toluene, and the sphere-containing residue heated first at 1OOO“Cand subsequently at 2800°C for 1 hr. The spheres retained their identity but changed shape in that they shrank preferentially in a direction perpendicular to the lamellae (Fig. 23). The graphite so formed consisted of many small crystals oriented as shown in Fig. 23. This shrinkage was not merely due to the reduction in the interlamellar spacing, which was less than 5 per cent. The specific gravity of the mesophase was 1.4, and since some of the carbon and all of the other atomic species are removed during graphitixation a given volume of mesophase can yield little more than half its volume of graphite. Although the electron diffraction patterns indicate a considerable degree of crystallographic order in the direction perpendicular to the lamellae, the patterns and also the low specific gravity show that the molecular packing in the lamellar direction must be poor. It is known that even in crystalline polynuclear aromatic hydrocarbons the nonbonded distance between hydrogen atoms is large(r2) and in the case of the mesophase in which the planar molecules probably exhibit a range of size and shape, the average intermolecular distances in the lamellae would be greater still. During graphitixation the loss of peripheral atoms from the aromatic structures creates vacancies and voids in the planes, and since the
isolated spheres contract preferentially in the direction perpendicular to the lamellae the voids in any one plane must be filled by migration of carbon atoms from planes above and below until the perfect three-dimensional crystalline graphitic structure is established. 5. CONCLUSION
These studies suggest that the spheres consist of polynuclear aromatic molecules of the order of some tens of aromatic rings in average size. The molecules appear to be stacked one upon another with fairly good local parallelism but in general, no three-dimensional order. The orientation of the structure so formed varies regularly through the sphere from lamellar at the centre to radial at the surface. This structure is the result of a number of forces. The spherical shape is presumed to be a response to surface-tension forces and further evidence of the effects of surface tension can be seen in the coalescence of two spheres and their subsequent reversion to spherical shape. The spheres appear to exhibit a characteristic molecular orientation at their surfaces and in this and some other respects they have many of the characteristics of liquid crystalsc7). The boundary condition always appears to obtain in small, large, and composite spheres and at the interface between pitch and mosaic mesophase, the surface having a uniform molecular arrangement in which the edges of aromatic planes are perpendicular to the interface. The other major factor determining sphere structure appears to be the tendency of large planar molecules to pack together. But for this, there would Seem to be no reason why the structure of the spheres should not be wholly radial, at least as seen in one plane. But in fact the radial arrangement appears to be imposed on moleculesor perhaps on swarms of molecules-only near the surface of the sphere. As the sphere grows, the molecules originally at the surface must rotate to give an approximately lamellar structure in the interior. Coalescence of spheres may lead to the formation of larger regions of extended order. It is the generally lamellar arrangement of the molecular structure in these regions which favours the formation of graphitic carbon on heating to elevated temperatures.
THE
FORMATION
OF
GRAPHITIZING
CARBONS
Acknowledgements-The authors wish to thank Mr. H. R. Brown, Chief of the Division of Coal Research, CSIRO, for his support and encouragement. They are also grateful to Dr. M. F. R. Mukahy for helpful suggestions and to Mr. J. W. Smith for experimental assistance. Dr. J. S. Shannon provided some of the carbon&d poiynuclear compounds. Mr. J. F. Corcoran and Mr. W. Zeidler made the preparations far electron and light microscopy respectively.
5. 6.
8. 9.
1.
F~AMDOHR
10.
P., Eisenk&tenwesen 1 (1928).
2. MARSHALLC. E., Fuel 24,120
(1945). 3. STACH E., Mikroskopie naturlicker Kokse, Hamibttck der Mikroskopie in der Technik (Ed. H. FREUND), Vol. II, Pt. 1, p. 411. Verlag Umschsu, (1952).
Frankfurt
THE
LIQUID
PHASE
193
4. AEIRAMSKI C. and MACKOWSKYM. T., Methoden und Ergebnisse der angewandten Koksmikroskopie, Handbrsckdet Mikroskopie in der Technik (Ed. H. F-),
7.
REFERJINCES
FROM
11.
12.
Vol. II, Pt. 1, p. 311. Verlag Umschau, Frankfurt (1952). TAYLOR G. H., FueZ 40,465 (1961). B~ooxs J. D. and TAYLOR G. H., paper to be published in the Proceedings of the American Coal Science Conference held in 1964. G-Y G. W., Molecub Structure and the Properties of L&n&dCrvstaZs. Academic Press (19621.