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TEM studies of coal tars effects of stresses (filtration and decantation)
Carbon Vol. 28. No. 5, pp. 617-629. Printed in Great Britain.
WM8-6223/W $3.00+ .oO Copyright 8 19w Pergamon Press plc
1990
TEM STUDIES OF COAL TARS EFFECTS OF STRESSES (FILTRATION AND DECANTATION) K. LAFDI,S.BONNAMY,~~~ A.OBERLIN Laboratoire Marcel Mathieu, U.A. 1205 CNRS UniversitC, HClioparc, 2, avenue du President P. Angot, 64000-Pau, France (Received 19 June
1989; accepted in revised form 22 September 1989)
Abstract-In order to understand the role of a, l3, y resins present in a Solmer tar, various techniques of separation (filtration and decantation) were used. The different fractions obtained from ail the processes were studied by Optical Microscopy and by Transmission Electron Microscopy using a thin sectioning method. The p resins have an important role because, under stress, they are able to form flow preferred orientations of aromatic units (edge to edge associations of aromatic ring structures), which constitute a determinant factor in all the processes. Key WordsTEM
coal tar, decantation,
carbon layer orientations.
using a peristaltic pump. At the bottom of the decantation column was a tap (tap opening 2.5 mm) through which the tar could flow down. During the decantation process the temperature of the coal tar was 40°C. The experiment lasted seven days. Samples were picked up at the top of the decantation column, 0.2 m lower, 1.38 m lower, immediately above the tap, and after flowing through the tap. The same experiment was repeated with crude and filtered tar.
1. INTRODUCTION
TEM studies of thin sections of crude Solmer tars have shown the occurrence of multiple anisotropic extraphases, either solid or plastic, insoluble in an-
thracene oil (AOI)[l]. They are included in droplets showing a gradient of solubility. The heaviest part (i.e., p resins, fraction soluble in quinoline and insoluble in toluene) is in contact with the anisotropic phases, whereas the lightest (i.e., y resins, soluble in toltine) is external or alone. In the present paper various ways were tried to eliminate the anisotropic a resin insoluble phases in the same Solmer tar (i.e., cake filtration and decantation). Both the residue and the filtrate (without fractionation) were studied by all modes of TEM using the same thin sectioning methods as in [l]. Additional observations by optical microscopy (OM) were also made. In addition, fractionation trials were performed directly on the microscope grid by using toluene or quinoline to identify y or p resins.
3. RESULTS
The crude tar studied was described in [l]. 3.1 Cake filtration The filtrate, the cake, and the tar in filter pores were studied by thin-sectioning. 3.1.1 Filtrate. The thin sections show clean isotropic droplets uniformly distributed. See Fig. l(a). They are the majority. They are soluble in toluene (y2 resins). However, a very few droplets containing the extraphases previously described (carbon blacks, viscoelastic, and plastic anisotropic phases)[l] can be found, as in Fig. l(b). 3.1.2 Tar in the filter pores. Various morphologies are found:
2. EXPERIMENTAL
2.1 Cake filtration The filtration cell is similar to that described by Clarke and Rantell[2] in their study on the filtration of coal extracts. The coal tar was first heated and was then filtered at constant pressure (5 bar) at 80°C on a metallic disc (pore diameter 20 km). The mass of filtrate was measured automatically using an electrobalance. This process is similar to that used for evaluating impregnation performance of the pitch as filterability index[3].
l Figures 2(a), (b), (c) show the same extraphases already observed in the crude tar[l]. Figure 2(a) shows carbon blacks, Fig. 2(b) shows a radialsphere, and Fig. 2(c) shows a sphere with an alternate texture. They are in the same relative amounts as in tar. However these phases are always entirely devoid of surrounding isotropic droplets. The objective aperture used for the titled dark field imaging had an effective diameter of 2 nm-’ corresponding to an azimuthal opening of +20” if it is centered on the
2.2 Decantation process A column 1.40 m in height and 8 mm in diameter was filled with coal tar. Hot water (60-70°C) was forced to circulate in the double wall of the tube 617
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002 ring of graphite. Because the material is very poorly organized, this theoretical position does not correspond to the maximum brightness of the dark field image. The interlayer spacing being much larger than the graphite one (0.3354 nm), the diameter of the 002 ring is smaller than 2.98 nm-r. The true azimuthal opening is not ?20”, but *30”, as it can be directly evaluated in Fig. 3(a) by the opening of the carbon black bright sectors. l Fig. 3 shows particles containing domains with preferred orientation. Figs. 3(a) and (b) are two orthogonal 002 DF where the direction of aromatic layers is indicated by a double bar. The homogeneously oriented domains are faintly bright. They are distributed at random. They have a size ranging from 170 nm down to isotropic from the core to the external surface (arrow). Such particles are not disturbed by the microtome knife displacement, which indicates a plastic state. These two features are general and characteristic of these particles. When submitted to direct fractionation on the microscope grid[l], these particles are insoluble in toluene but soluble in quinoline. They are thus B resins. l Figs. 4 and 5 also show oriented areas as seen by the two orthogonal 002 DF images. However, these areas are different from those illustrated in Fig. 3, since they are broken by the microtome knife. They are thus much less plastic. They form elongated fragments. They are insoluble in toluene but soluble in quinoline (B resins). In both Figs. 4 and 5 an additional feature is observed. If the total anisotropic area is observed as a whole, Figs. 4(a) and 5(a) show more bright domains than Figs. 4(b) and 5(b). This indicates that a majority of aromatic layers are oriented parallel to the same plane (plane of statistical orientation). In both Figs. 4(a) and 5(a) this plane P is parallel to the average elongation of the area observed. In addition to the large elongated anisotropic areas, others are observed that are very thin (i.e., -100 nm) (Fig. 6). They correspond to cylindrical small pores. All have a radial texture occupying the whole length except in zones where the aromatic layers are not exactly under the Bragg angle. They are not disrupted by the microtome knife; they are thus more plastic than those observed in Figs. 4 and 5. The block prepared for obtaining thin sections is now used as a polished section for OM observation. Under these conditions the surface is perfect (free from scratches). Even very fragile textures are kept intact. The anisotropy is observed by using crossed polarizers with a h first order plate added. All kinds of imperfectly oriented areas already observed in TEM appear as homogeneous anisotropic areas showing sharp colors-blue, yellow and magenta. Fig, 7 shows the elongation of these anisotropic areas (single arrow). Other elongated fragments are mosaic-like (double arrow). All of them are obviously broken. In the sample itself, these areas could belong
to a larger entity seen in optical microscopy as cenosphere-like particles (see Fig. 17 relative to the decantation process). In Figs. 4 and 5 outside the oriented areas, clean isotropic droplets are observed (arrows). They are similar to those observed in the filtrate, but are smaller and floculated. They are also toluene soluble (y resins). 3.1.3 Cake. Carbon blacks, radial, and alternate spheres form the majority of the deposit. All of them are devoid of surrounding droplets (Fig. 8). With them are associated a small amount of particles (Fig. 9) where statistical orientation occurs as in Figs. 4 and 5. The plane of deposit is also parallel to the elongation of the fragment. The plasticity is high; these fragments are not broken by the microtome knife. 3.2 Decantation 3.2.1
Crude tar (after seven days of decantation).
At the top of the decantation column only isotropic clean droplets are observed. 0.2 m lower, the crude tar morphology is exactly recovered (i.e., clean droplets and droplets surrounding carbon blacks and radial spheres are found). The same distribution is found all along the tube down to 1.38 m (i.e., in the immediate vicinity of the conical part of the tube through which the tar will be forced by the opening of the tap). After having opened the tap, the parts of the tar above and below (i.e., having flowed into the cone and through the tap) are recovered. They are observed after thin sectioning and are identical. The clean droplets, either aggregated or not, are replaced by a continuous isotropic film of toluene soluble materials (y resins), often full of empty bubbles (Fig. 10). Some droplets, however, are still visible in the sections. This feature occurs everywhere in Fig. 11 through 14, but is particularly visible in Fig. 13. The inner part of the figure (single arrow) is a very large distorted drop of a continuous bubbled film, whereas the outer part (double arrow) contains individual droplets of y resin trapped in the thin section embedding medium. In Fig. 14 also, the left-hand region is made up of a continuous film, containing bubbles only in the bottom. All the c1 resins initially contained in the crude tar are now included in the film of y resins, although very few of them are dispersed in the embedding medium. In this case they are always clean (i.e., devoid of surrounding droplets). Areas with statistical orientation always occupy the border of the y resin films around which they form more or less thick rims, closely following the deformation contours of the y resins (Fig. 11 through 14). They are very abundant. They are p resins because they are soluble in quinoline and insoluble in toluene. They are not disrupted by the microtome knife, and are thus more plastic than the fragments observed in metallic filter (see Fig. 5). Because the oriented areas are plastic they are sensitive to the
TELMstudies of coal tars: stresses
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Fig. 1. 002 dark field images of droplets: (a) CIean isotropic droplets observed in the filtrate (cake filtration), (b) Droplets including anisotropic phases.
stresses produced by the y resins’ large deformations. Therefore their own orientations are flow orientations of their aromatic ring structures (edge to edge associations). From place to place they adopt either convex shapes (i.e., drop-like shapes) illustrated by Fig. 11 or concave shapes, (filament-like) illustrated by Fig. 12. Many complicated additional distortions occurs as twisting of a drop (Fig. 13) or a combination of all (Fig. 14). Distinct superimposed layers can often be distinguished in a given area by a change of both orientation and size of the small o~entations zones. As an example, in Fig. 12, three layers (numbered 1, 2, 3) are visible on the micrograph. The extension
of the small oriented zones increases from l-2 to 3. Correspondingly, 3 is less plastic since it is sometimes disrupted by the microtome knife. Figs. 11 and 14 show the same feature. The average direction of the aromatic layers able to be imaged in 002 DF are represented by a double bar on the two orthogonal 002 DF of Figs. 11(a) and (b), 12(a) and (b), 13(a) and (b), and 14(a) and (b). From the micrographs (a) to micrographs (b) the small clusters of bright dots disappear almost entirely. This is the feature showing the long range statistical orientation. rotating the aperture relative to the 002 ring allows all the layers fulfilling the Bragg condition to image. When rotating the aper-
Fig. 2. 002 dark field images of morphologies found in the metallic filter. Double bars indicate the orientation of aromatic layers. (a) carbon blacks, (b) radial sphere, (c) sphere with alternate texture.
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Fig. 3. Particles observed in the metallic filter and presenting small domains with preferred orientation. Double bars indicate the orientation of aromatic layers. (a)-(b) Orthogonal 002 dark field images.
ture, the micrographs (a) show much brighter domains than micrographs (b) where most of the image surface appears dark. This shows that most of the layers do not fulfill the 002 Bragg condition. The layers are thus tilted but not twisted. A complete determination of the aromatic ring structure orientations can be deduced from a careful examination of the 002 DF images of Fig. 11 through Fig. 14. It has to take into account the effective diameter of the objective aperture (2 nm-I) and the tilt used for centering it. The azimuthal opening of such an aperture is +30”. As an example, these orientations for Fig. 12 are roughly sketched on Fig. 15. In Fig. 15 the most probable directions of stresses are in heavy solid line whereas the aromatic ring structures’ orientations are represented in dashed lines. This figure shows the flow orientation under
stresses. Likewise, Fig. 16 sketches Fig. 13. It shows how the strong deformation of the y resins induces a strong flow orientation of the B resin rim. The OM (Fig. 17) does not see the deviations of the layers from the 002 Bragg angle. Thus, it only shows large anisotropic areas having sharp colors (yellow in A and blue in B). They correspond to the large imperfectly oriented areas visible in TEM in the rims around the y resins (edge to edge associations of aromatic ring structures). The overall shape of the total entity (of which only the details are emphasized by TEM), can be observed in OM. This entity is a porous cage, the walls of which are the oriented p resins (edge to edge associations already demonstrated as surrounding y resins). The cages are sometimes approximately spherical (Fig. 17). Sometimes they are more or less flattened or elon-
Fig. 4. Metallic filter. Elongated particles made of oriented domains. The aromatic layers are represented by a double bar. The clean isotropic droplets are indicated by a single arrow. (a)-(b) Orthogonal 002 dark field images.
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TEM studies of coal tars: stresses
Fig. 5. Metallic filter. Large oriented domains. The orientation of aromatic layers is indicated by a double bar. The clean isotropic droplets are indicated by a single arrow. (a)-(b) Orthogonal 002 dark field images.
gated; they are cenosphere-like. The term cenosphere is not attributed here to a typical set of experimental conditions; it is used for characterizing a peculiar morphology. More or less deformed porous spherical cages with morphologies highly complicated in their details were observed under various thermodynamic conditions[4,5]. Since they lead to convergent morphologies the term cenosphere-like can be adopted. 3.2.2 Filtered coal tar (after seven days of decantation). Only clean droplets soluble in toluene are found at the top, at the intermediate level, before tap, and after flowing through the tap. No insoluble phase either in toluene or quinoline was found, and no preferred orientation appeared.
4.
DISCUSSION
The initial purpose of this study was to eliminate the a resins (QI or AOI). In a previous paper[l] it was shown that these (Yresins were included in droplets formed by p resins in contact with the central c1 resins particle and y, resins forming the external part of the droplets. The data obtained in the present work suggest that the halo surrounding the a resins is retained in the metallic filter or at the bottom of the decantation tube, to form flow orientations of p resins (edge to edge associations of aromatic ring structures). The progressive compaction of I3 resins in the pores of the filter could produce in a first step the very labile local orientations of Fig. 3. They are
Fig. 6. Metallic filter. Thin elongated anisotropic filament with radial texture. 002 dark field image. The orientation of aromatic layers is indicated by a double bar.
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3Anisotrodc filament
Fig. 7. Metallic filter. Optical microscopy image showing the elongation of the anisotropic areas.
labile because the viscosity is probably too low to remain imprinted with the “memory” of the stresses. In a second step, the extreme compaction inside a filter pore (Figs. 4 and 5) is strong enough to increase the viscosity (the material is disrupted by the microtome knife displacement). This blocks the pore with a highly stress oriented material (orientation is always parallel to the pore axis).
In the same manner, the flow stresses produced by the conical part of the tube and by the opening of the tap in the decantation tube induce deformations in the y resins forming the majority of the tar. Since they are light fractions, their deformation under stress does not induce a permanent orientation. It only creates a progressive compaction from individual droplets to aggregated ones, to porous large drops (appearing as porous films in a thin section), then to compact material (continuous thin film) (see Fig. 13). However, the heavier p, and maybe y, halos separated from their core, are more viscous. They accumulate along the free surfaces of the deformed y resins, forming rims, oriented by stresses as flow orientations (edge to edge associations of aromatic ring structures). Their viscosity is large enough to keep a permanent but imperfect statistical orientation. It is highly probable that various kinds of increasingly heavy fractions can separate by increasing viscosity. They are thus responsible for the occurrence of successive interfaces separating areas 1, 2, and 3 in Fig. 12 (and also in Figs. 11 and 14). The higher the viscosity, the more nearly perfect the induced remaining orientation. Some rough calculation can sustain the hypothesis that attributes to l3 and y, halos the origin of the oriented /3 resins either blocking the filter pores or forming oriented rims around y resins in the decantation tube. By calculating, from the tar, the average
Fig. 8. Cake filtration. Thin section of the cake. 002 dark field image.
TEM studies
Fig. 9. Cake.
Particle
with statistical orientation. The orientation of aromatic double bar. (a)-(b) Orthogonal 002 dark field images.
volumes of these halos (knowing their sizes), one can see that the amount of p resins is about three times larger than the central core of (Yresins, whereas the fraction y, of y resins contained in the drop was five times larger than the (Yfraction. These relative amounts fit well with the fractionation analysis, which gives o. = 0.67% and p = 1.8%; y, is thus 3.3%. yZ being >94%. A first conclusion obtained from the present paper is that it is possible to eliminate at least the fraction (Y + l3 by filtration methods but it is impossible to
Fig. 10. Decantation
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of coal tars: stresses
process.
Continuous
layers
is indicated
by a
know whether y, remains fixed with (Yand p or joins the clean droplets. The ones forming the filtrate are thus either y, + y2 or ‘yZ.The experiments show that they never form any permanent preferred orientation while flowing through the tap. The second and very important conclusion from this work is the occurrence of flow preferred orientations leading to largely developed edge to edge associations of aromatic ring structures. It is clear that the edge to edge associations of aromatic ring structures are entirely different from the germination
isotropic film (y resins) image.
with empty
bubbles.
002 dark field
K. LAFDIet al.
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Fig. 11. Decantation process. Particle with flow orientations (oriented areas formed by stresses with a convex shape). Double bars indicate the orientation of aromatic layers. (a)-(b) Orthogonal 002 dark field images.
growth that initiates nuclei and then leads to liquid crystals (individual not coalesced mesophase spheres)[6,7]. This kind of growth occurs by fixation of individual aromatic ring structures along a nucleus’ layer edges. It is specifically induced by thermal evolution (e.g., soaking or isothermal heat treatment) . When HTT further increases, the spheres coalesce and finally yield a brittle solid made of large optically anisotropic areas (mosaics). Viewed only by OM, anisotropic areas issued from mesophase spheres.are not easy to distinguish from those corresponding to edge to edge associations. Only TEM is able to separate the former from the latter easily. The mesophase based mosaics appear in 002 DF as very large brilliant areas corresponding to very much extended perfect local orientations (91 pm). On the contrary, low orientations appear as very large brilliant areas, but always having a spotty appearance due to many dark regions inside the brilliant area. Most of these defects are due to deviation from the 002 Bragg angle, some of them to azimuthal misorientations. The brilliant areas thus correspond to very imperfect statistical long range orientations. In addition, mesophase based mosaics viewed in TEM or OM show acute grain boundaries between two adjacent rupt changes
anisotropic zones corresponding to abin local orientation. However, the flow
orientations which were flexible enough to acquire internal distortions, responsible for the spotty brilliant images, are also flexible enough to acquire a large radius of curvature introducing a smooth and progressive change of orientation, without any abrupt change (see Figs. 11 through 14 and sketches of Figs. 15 and 16). It is unsuitable to attribute the term mesophase to the edge to edge associations, because they are not liquid crystals issued from a germination growth going through mesophase spheres[6,7]. In some very rare and peculiar cases, a form of growth already found in the crude coal tar[l] and also under pressure[B] was observed. Very thin filaments corresponding to pores of very small diameter in the metallic filter (Fig. 6), or to very thin layers of p resins on the surface of y resins-see triple arrow in Fig. 13(b)-show a perfect radial texture. This could be due to a peculiar growth corresponding to a centripetal progressive organization of the aromatic ring structures beginning at any free interface available. In a filter pore having a very small diameter, such radial growth can occur freely, without being disturbed by stresses, because no flow can be easily produced in a capillary. Growth initiates at the pore wall and ends when reaching the axis. In the same manner it may occur in a thin p
TEM studies of coal tars: stresses
625
Fig. 12. Decantation process. Particle with flow orientations (oriented areas formed by stresses with a concave shape). Double bars indicate the orientation of aromatic layers. (a)-(b) Orthogonal 002 dark field images.
resins film at the contact of a y resins drop; see arrow in Fig. 12(b). This discussion accounts for all the data obtained in the present work. In the filter pores the f3 resins were forced inside pores but cannot reach the filtrate. The resulting statistical orientation is thus mainly parallel to the pore axis (Fig. 5). The extreme compaction of the material due to the strength of the stresses accounts for its relatively low plasticity (it is disrupted by the microtome knife). In the cake, the stresses in a pore are lower. The material is thus more plastic, not disrupted by the knife, but its orientation is still mainly parallel to the pore axis (Fig. 9). The more liquid-like particles still belong to p resins (Fig. 3) cannot keep the imprint of stresses because of their low viscosity. They have thus no statistical orientation, and are the first step of compaction. They are probably very fragile and labile. Such droplets or liquid phases having only random local orientations were first observed in [9] in a coal tar pitch, then[lO] in a coal (rich in p resins). Because the preparations for TEM were not thin sections but dusted samples,
it was not possible to correlate the mo~hology observed with that obtained under stresses or with those really present in the samples. However, in both cases their presence was attributed to p resins. Such particles are found here both in the cake and in the metallic filter but never reach the filtrate. The progressive compaction of p resins in the filter pores progressively blocks the filter pores and, finally, the ~ltration process. In the so-called decantation process the efficient parameter is also the flow stresses induced by opening the tap and forcing the tar to enter the conical part of the tube. The occurrence of edge to edge associations, and the disruption of the radial orientation by stresses inducing a long range orientation are demonstrated here particularly clearly. 5. CONCLUSION The crude coal tar already contains aromatic ring structures distributed at random in the bulk. Upon them are grafted heteroatoms and various functional groups. The morphologies described in this paper are due to the progressive associations between in-
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Fig. 13. Decantation process. Flow orientations forming a rim around a large drop of y resin (single arrow). The double arrow indicates individual droplets of y resins. Triple arrow indicates a radial growth on a thin layer of p resins. (a)-(b) Orthogonal 002 dark field images.
TEM studies of coal tars: stresses
Fig. 14. Decantation process. Combination of distortions due to stresses forming oriented rims of p resins around deformed bulk y resins (left). (a)-(b) Orthogonal 002 dark field images.
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Fig. 15. Sketch of Fig. 12 showing the distortions of the aromatic layer associations due to stresses.
Fig. 16. Sketch of Fig. 13.
TEM studies of coal tars: stresses
Fig. 17. Decantation process. Optical microscopy overall view of an area already observed by TEM.
dividual aromatic ring structures caused by modifications in the chemical composition of the tar. The main feature of this work is the formation of largely extended edge to edge associations between aromatic ring structures (-1 nm in size) as the coal tar precursor reaches the convenient chemical composition (mainly an increase in p resin content). This composition is reached in various ways: formation of cake above and inside a filter or residue of decantation forced through a tap. It is the occurrence of the a resins (plastic radial or alternate spheres) already contained in the initial tar[l] that insured the transformation of the tar residues, as they accumulate in the cake, in the filter, or near the bottom of the decantation tube. Correspondingly, they carry along with them their surrounding satellites of p resins and some y resins (probably heavier than the others). They thus yield the opportunity for forming edge to edge associations. However, these latter are in fact flow orientations due to stresses occurring during either filtration or flow through the tap at the bottom of the decantation tube. They are evidenced only because of the new method of thin sectioning used both through TEM imaging or optical microscopy. Such preferred orientations are more or less labile depending on their molecular weight spectrum. The heavier the p fractions, the less labile are the orientations up to the moment they become stable, though still plastic (heaviest p resins). The fact
629
that the filtrate, when decanted, never gives any edge to edge association suggests that yr (the lightest y fraction) is unable to form such associations. The edge to edge associations, considered as a whole, are similar to gels of increasing rigidity (mineral gels for example), which are often able to “remember” by their flow orientation the stresses to which they have been subjected during increasing gelification[ 111. It seems very probable that this kind of flow orientation can be controlled with suitable stresses, so as to align the aromatic ring stuctures parallel to an axis, following various predetermined transverse configurations. To do so, the ideal composition has to be reached either by making the precursor heavier or lighter, depending on its initial composition. The edge to edge associations can be a leading thread for understanding the behavior of pitches as precursors of pitch based fibres[l2-151. Acknowledgements-The authors would like to acknowledge Dr. J. L. Saint Romin and NORSOLOR Society (H.G.D. department) for partially supporting this study.
REFERENCES
1. K. Lafdi, S. Bonnamy, and A. Oberlin, Carbon, 28, 57 (1990). 2. J. W. Clarke and T. D. Rantell, Fuel 59, 35 (1980). 3. S. Compin, R. Benai‘m, P. Couderc, and J. L. Saint Romain, Fuel 66, 1552 (1987). 4. J. G. Bailey, Report on ICCP working group combustion (1987).
5. C. F. K. Diessel and G. Wolf-Fischer, Report on ICCP working group combustion (1987).
6. S. Bonnamy and A. Oberlin, Extended Abstract, Carbon 88, Internat. Carbon Conference, Newcastle, p. 374 (1988). 7. S. Bonnamy and A. Oberlin, Carbon, to be submitted (1989). 8. J. Ayache, A. Oberlin, and M. Inagaki, Part I, Carbon, in press (1990). 9. D. Auguit, Thesis, Universite d’orleans, (1979). 10. J. N. Rouzaud and A. Oberlin, C. R. Acad. SC., Paris, 757 (1983). 11. H. Van Olphen, In An introduction to clay colloids chemistry, Interscience, London, pp. 89-94 (1963). .. 12. S. Otani, Curbon 3, 31 (1965) 13. L. S. Sineer. Carbon 16. 409 f1978). 14. H. Hondi, &bon 26, 139 (1988). ’ 15. I. C. Lewis and R. T. Lewis, Carbon 26, 757 (1978).
WM8-6223/W $3.00+ .oO Copyright 8 19w Pergamon Press plc
1990
TEM STUDIES OF COAL TARS EFFECTS OF STRESSES (FILTRATION AND DECANTATION) K. LAFDI,S.BONNAMY,~~~ A.OBERLIN Laboratoire Marcel Mathieu, U.A. 1205 CNRS UniversitC, HClioparc, 2, avenue du President P. Angot, 64000-Pau, France (Received 19 June
1989; accepted in revised form 22 September 1989)
Abstract-In order to understand the role of a, l3, y resins present in a Solmer tar, various techniques of separation (filtration and decantation) were used. The different fractions obtained from ail the processes were studied by Optical Microscopy and by Transmission Electron Microscopy using a thin sectioning method. The p resins have an important role because, under stress, they are able to form flow preferred orientations of aromatic units (edge to edge associations of aromatic ring structures), which constitute a determinant factor in all the processes. Key WordsTEM
coal tar, decantation,
carbon layer orientations.
using a peristaltic pump. At the bottom of the decantation column was a tap (tap opening 2.5 mm) through which the tar could flow down. During the decantation process the temperature of the coal tar was 40°C. The experiment lasted seven days. Samples were picked up at the top of the decantation column, 0.2 m lower, 1.38 m lower, immediately above the tap, and after flowing through the tap. The same experiment was repeated with crude and filtered tar.
1. INTRODUCTION
TEM studies of thin sections of crude Solmer tars have shown the occurrence of multiple anisotropic extraphases, either solid or plastic, insoluble in an-
thracene oil (AOI)[l]. They are included in droplets showing a gradient of solubility. The heaviest part (i.e., p resins, fraction soluble in quinoline and insoluble in toluene) is in contact with the anisotropic phases, whereas the lightest (i.e., y resins, soluble in toltine) is external or alone. In the present paper various ways were tried to eliminate the anisotropic a resin insoluble phases in the same Solmer tar (i.e., cake filtration and decantation). Both the residue and the filtrate (without fractionation) were studied by all modes of TEM using the same thin sectioning methods as in [l]. Additional observations by optical microscopy (OM) were also made. In addition, fractionation trials were performed directly on the microscope grid by using toluene or quinoline to identify y or p resins.
3. RESULTS
The crude tar studied was described in [l]. 3.1 Cake filtration The filtrate, the cake, and the tar in filter pores were studied by thin-sectioning. 3.1.1 Filtrate. The thin sections show clean isotropic droplets uniformly distributed. See Fig. l(a). They are the majority. They are soluble in toluene (y2 resins). However, a very few droplets containing the extraphases previously described (carbon blacks, viscoelastic, and plastic anisotropic phases)[l] can be found, as in Fig. l(b). 3.1.2 Tar in the filter pores. Various morphologies are found:
2. EXPERIMENTAL
2.1 Cake filtration The filtration cell is similar to that described by Clarke and Rantell[2] in their study on the filtration of coal extracts. The coal tar was first heated and was then filtered at constant pressure (5 bar) at 80°C on a metallic disc (pore diameter 20 km). The mass of filtrate was measured automatically using an electrobalance. This process is similar to that used for evaluating impregnation performance of the pitch as filterability index[3].
l Figures 2(a), (b), (c) show the same extraphases already observed in the crude tar[l]. Figure 2(a) shows carbon blacks, Fig. 2(b) shows a radialsphere, and Fig. 2(c) shows a sphere with an alternate texture. They are in the same relative amounts as in tar. However these phases are always entirely devoid of surrounding isotropic droplets. The objective aperture used for the titled dark field imaging had an effective diameter of 2 nm-’ corresponding to an azimuthal opening of +20” if it is centered on the
2.2 Decantation process A column 1.40 m in height and 8 mm in diameter was filled with coal tar. Hot water (60-70°C) was forced to circulate in the double wall of the tube 617
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002 ring of graphite. Because the material is very poorly organized, this theoretical position does not correspond to the maximum brightness of the dark field image. The interlayer spacing being much larger than the graphite one (0.3354 nm), the diameter of the 002 ring is smaller than 2.98 nm-r. The true azimuthal opening is not ?20”, but *30”, as it can be directly evaluated in Fig. 3(a) by the opening of the carbon black bright sectors. l Fig. 3 shows particles containing domains with preferred orientation. Figs. 3(a) and (b) are two orthogonal 002 DF where the direction of aromatic layers is indicated by a double bar. The homogeneously oriented domains are faintly bright. They are distributed at random. They have a size ranging from 170 nm down to isotropic from the core to the external surface (arrow). Such particles are not disturbed by the microtome knife displacement, which indicates a plastic state. These two features are general and characteristic of these particles. When submitted to direct fractionation on the microscope grid[l], these particles are insoluble in toluene but soluble in quinoline. They are thus B resins. l Figs. 4 and 5 also show oriented areas as seen by the two orthogonal 002 DF images. However, these areas are different from those illustrated in Fig. 3, since they are broken by the microtome knife. They are thus much less plastic. They form elongated fragments. They are insoluble in toluene but soluble in quinoline (B resins). In both Figs. 4 and 5 an additional feature is observed. If the total anisotropic area is observed as a whole, Figs. 4(a) and 5(a) show more bright domains than Figs. 4(b) and 5(b). This indicates that a majority of aromatic layers are oriented parallel to the same plane (plane of statistical orientation). In both Figs. 4(a) and 5(a) this plane P is parallel to the average elongation of the area observed. In addition to the large elongated anisotropic areas, others are observed that are very thin (i.e., -100 nm) (Fig. 6). They correspond to cylindrical small pores. All have a radial texture occupying the whole length except in zones where the aromatic layers are not exactly under the Bragg angle. They are not disrupted by the microtome knife; they are thus more plastic than those observed in Figs. 4 and 5. The block prepared for obtaining thin sections is now used as a polished section for OM observation. Under these conditions the surface is perfect (free from scratches). Even very fragile textures are kept intact. The anisotropy is observed by using crossed polarizers with a h first order plate added. All kinds of imperfectly oriented areas already observed in TEM appear as homogeneous anisotropic areas showing sharp colors-blue, yellow and magenta. Fig, 7 shows the elongation of these anisotropic areas (single arrow). Other elongated fragments are mosaic-like (double arrow). All of them are obviously broken. In the sample itself, these areas could belong
to a larger entity seen in optical microscopy as cenosphere-like particles (see Fig. 17 relative to the decantation process). In Figs. 4 and 5 outside the oriented areas, clean isotropic droplets are observed (arrows). They are similar to those observed in the filtrate, but are smaller and floculated. They are also toluene soluble (y resins). 3.1.3 Cake. Carbon blacks, radial, and alternate spheres form the majority of the deposit. All of them are devoid of surrounding droplets (Fig. 8). With them are associated a small amount of particles (Fig. 9) where statistical orientation occurs as in Figs. 4 and 5. The plane of deposit is also parallel to the elongation of the fragment. The plasticity is high; these fragments are not broken by the microtome knife. 3.2 Decantation 3.2.1
Crude tar (after seven days of decantation).
At the top of the decantation column only isotropic clean droplets are observed. 0.2 m lower, the crude tar morphology is exactly recovered (i.e., clean droplets and droplets surrounding carbon blacks and radial spheres are found). The same distribution is found all along the tube down to 1.38 m (i.e., in the immediate vicinity of the conical part of the tube through which the tar will be forced by the opening of the tap). After having opened the tap, the parts of the tar above and below (i.e., having flowed into the cone and through the tap) are recovered. They are observed after thin sectioning and are identical. The clean droplets, either aggregated or not, are replaced by a continuous isotropic film of toluene soluble materials (y resins), often full of empty bubbles (Fig. 10). Some droplets, however, are still visible in the sections. This feature occurs everywhere in Fig. 11 through 14, but is particularly visible in Fig. 13. The inner part of the figure (single arrow) is a very large distorted drop of a continuous bubbled film, whereas the outer part (double arrow) contains individual droplets of y resin trapped in the thin section embedding medium. In Fig. 14 also, the left-hand region is made up of a continuous film, containing bubbles only in the bottom. All the c1 resins initially contained in the crude tar are now included in the film of y resins, although very few of them are dispersed in the embedding medium. In this case they are always clean (i.e., devoid of surrounding droplets). Areas with statistical orientation always occupy the border of the y resin films around which they form more or less thick rims, closely following the deformation contours of the y resins (Fig. 11 through 14). They are very abundant. They are p resins because they are soluble in quinoline and insoluble in toluene. They are not disrupted by the microtome knife, and are thus more plastic than the fragments observed in metallic filter (see Fig. 5). Because the oriented areas are plastic they are sensitive to the
TELMstudies of coal tars: stresses
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Fig. 1. 002 dark field images of droplets: (a) CIean isotropic droplets observed in the filtrate (cake filtration), (b) Droplets including anisotropic phases.
stresses produced by the y resins’ large deformations. Therefore their own orientations are flow orientations of their aromatic ring structures (edge to edge associations). From place to place they adopt either convex shapes (i.e., drop-like shapes) illustrated by Fig. 11 or concave shapes, (filament-like) illustrated by Fig. 12. Many complicated additional distortions occurs as twisting of a drop (Fig. 13) or a combination of all (Fig. 14). Distinct superimposed layers can often be distinguished in a given area by a change of both orientation and size of the small o~entations zones. As an example, in Fig. 12, three layers (numbered 1, 2, 3) are visible on the micrograph. The extension
of the small oriented zones increases from l-2 to 3. Correspondingly, 3 is less plastic since it is sometimes disrupted by the microtome knife. Figs. 11 and 14 show the same feature. The average direction of the aromatic layers able to be imaged in 002 DF are represented by a double bar on the two orthogonal 002 DF of Figs. 11(a) and (b), 12(a) and (b), 13(a) and (b), and 14(a) and (b). From the micrographs (a) to micrographs (b) the small clusters of bright dots disappear almost entirely. This is the feature showing the long range statistical orientation. rotating the aperture relative to the 002 ring allows all the layers fulfilling the Bragg condition to image. When rotating the aper-
Fig. 2. 002 dark field images of morphologies found in the metallic filter. Double bars indicate the orientation of aromatic layers. (a) carbon blacks, (b) radial sphere, (c) sphere with alternate texture.
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Fig. 3. Particles observed in the metallic filter and presenting small domains with preferred orientation. Double bars indicate the orientation of aromatic layers. (a)-(b) Orthogonal 002 dark field images.
ture, the micrographs (a) show much brighter domains than micrographs (b) where most of the image surface appears dark. This shows that most of the layers do not fulfill the 002 Bragg condition. The layers are thus tilted but not twisted. A complete determination of the aromatic ring structure orientations can be deduced from a careful examination of the 002 DF images of Fig. 11 through Fig. 14. It has to take into account the effective diameter of the objective aperture (2 nm-I) and the tilt used for centering it. The azimuthal opening of such an aperture is +30”. As an example, these orientations for Fig. 12 are roughly sketched on Fig. 15. In Fig. 15 the most probable directions of stresses are in heavy solid line whereas the aromatic ring structures’ orientations are represented in dashed lines. This figure shows the flow orientation under
stresses. Likewise, Fig. 16 sketches Fig. 13. It shows how the strong deformation of the y resins induces a strong flow orientation of the B resin rim. The OM (Fig. 17) does not see the deviations of the layers from the 002 Bragg angle. Thus, it only shows large anisotropic areas having sharp colors (yellow in A and blue in B). They correspond to the large imperfectly oriented areas visible in TEM in the rims around the y resins (edge to edge associations of aromatic ring structures). The overall shape of the total entity (of which only the details are emphasized by TEM), can be observed in OM. This entity is a porous cage, the walls of which are the oriented p resins (edge to edge associations already demonstrated as surrounding y resins). The cages are sometimes approximately spherical (Fig. 17). Sometimes they are more or less flattened or elon-
Fig. 4. Metallic filter. Elongated particles made of oriented domains. The aromatic layers are represented by a double bar. The clean isotropic droplets are indicated by a single arrow. (a)-(b) Orthogonal 002 dark field images.
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TEM studies of coal tars: stresses
Fig. 5. Metallic filter. Large oriented domains. The orientation of aromatic layers is indicated by a double bar. The clean isotropic droplets are indicated by a single arrow. (a)-(b) Orthogonal 002 dark field images.
gated; they are cenosphere-like. The term cenosphere is not attributed here to a typical set of experimental conditions; it is used for characterizing a peculiar morphology. More or less deformed porous spherical cages with morphologies highly complicated in their details were observed under various thermodynamic conditions[4,5]. Since they lead to convergent morphologies the term cenosphere-like can be adopted. 3.2.2 Filtered coal tar (after seven days of decantation). Only clean droplets soluble in toluene are found at the top, at the intermediate level, before tap, and after flowing through the tap. No insoluble phase either in toluene or quinoline was found, and no preferred orientation appeared.
4.
DISCUSSION
The initial purpose of this study was to eliminate the a resins (QI or AOI). In a previous paper[l] it was shown that these (Yresins were included in droplets formed by p resins in contact with the central c1 resins particle and y, resins forming the external part of the droplets. The data obtained in the present work suggest that the halo surrounding the a resins is retained in the metallic filter or at the bottom of the decantation tube, to form flow orientations of p resins (edge to edge associations of aromatic ring structures). The progressive compaction of I3 resins in the pores of the filter could produce in a first step the very labile local orientations of Fig. 3. They are
Fig. 6. Metallic filter. Thin elongated anisotropic filament with radial texture. 002 dark field image. The orientation of aromatic layers is indicated by a double bar.
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3Anisotrodc filament
Fig. 7. Metallic filter. Optical microscopy image showing the elongation of the anisotropic areas.
labile because the viscosity is probably too low to remain imprinted with the “memory” of the stresses. In a second step, the extreme compaction inside a filter pore (Figs. 4 and 5) is strong enough to increase the viscosity (the material is disrupted by the microtome knife displacement). This blocks the pore with a highly stress oriented material (orientation is always parallel to the pore axis).
In the same manner, the flow stresses produced by the conical part of the tube and by the opening of the tap in the decantation tube induce deformations in the y resins forming the majority of the tar. Since they are light fractions, their deformation under stress does not induce a permanent orientation. It only creates a progressive compaction from individual droplets to aggregated ones, to porous large drops (appearing as porous films in a thin section), then to compact material (continuous thin film) (see Fig. 13). However, the heavier p, and maybe y, halos separated from their core, are more viscous. They accumulate along the free surfaces of the deformed y resins, forming rims, oriented by stresses as flow orientations (edge to edge associations of aromatic ring structures). Their viscosity is large enough to keep a permanent but imperfect statistical orientation. It is highly probable that various kinds of increasingly heavy fractions can separate by increasing viscosity. They are thus responsible for the occurrence of successive interfaces separating areas 1, 2, and 3 in Fig. 12 (and also in Figs. 11 and 14). The higher the viscosity, the more nearly perfect the induced remaining orientation. Some rough calculation can sustain the hypothesis that attributes to l3 and y, halos the origin of the oriented /3 resins either blocking the filter pores or forming oriented rims around y resins in the decantation tube. By calculating, from the tar, the average
Fig. 8. Cake filtration. Thin section of the cake. 002 dark field image.
TEM studies
Fig. 9. Cake.
Particle
with statistical orientation. The orientation of aromatic double bar. (a)-(b) Orthogonal 002 dark field images.
volumes of these halos (knowing their sizes), one can see that the amount of p resins is about three times larger than the central core of (Yresins, whereas the fraction y, of y resins contained in the drop was five times larger than the (Yfraction. These relative amounts fit well with the fractionation analysis, which gives o. = 0.67% and p = 1.8%; y, is thus 3.3%. yZ being >94%. A first conclusion obtained from the present paper is that it is possible to eliminate at least the fraction (Y + l3 by filtration methods but it is impossible to
Fig. 10. Decantation
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of coal tars: stresses
process.
Continuous
layers
is indicated
by a
know whether y, remains fixed with (Yand p or joins the clean droplets. The ones forming the filtrate are thus either y, + y2 or ‘yZ.The experiments show that they never form any permanent preferred orientation while flowing through the tap. The second and very important conclusion from this work is the occurrence of flow preferred orientations leading to largely developed edge to edge associations of aromatic ring structures. It is clear that the edge to edge associations of aromatic ring structures are entirely different from the germination
isotropic film (y resins) image.
with empty
bubbles.
002 dark field
K. LAFDIet al.
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Fig. 11. Decantation process. Particle with flow orientations (oriented areas formed by stresses with a convex shape). Double bars indicate the orientation of aromatic layers. (a)-(b) Orthogonal 002 dark field images.
growth that initiates nuclei and then leads to liquid crystals (individual not coalesced mesophase spheres)[6,7]. This kind of growth occurs by fixation of individual aromatic ring structures along a nucleus’ layer edges. It is specifically induced by thermal evolution (e.g., soaking or isothermal heat treatment) . When HTT further increases, the spheres coalesce and finally yield a brittle solid made of large optically anisotropic areas (mosaics). Viewed only by OM, anisotropic areas issued from mesophase spheres.are not easy to distinguish from those corresponding to edge to edge associations. Only TEM is able to separate the former from the latter easily. The mesophase based mosaics appear in 002 DF as very large brilliant areas corresponding to very much extended perfect local orientations (91 pm). On the contrary, low orientations appear as very large brilliant areas, but always having a spotty appearance due to many dark regions inside the brilliant area. Most of these defects are due to deviation from the 002 Bragg angle, some of them to azimuthal misorientations. The brilliant areas thus correspond to very imperfect statistical long range orientations. In addition, mesophase based mosaics viewed in TEM or OM show acute grain boundaries between two adjacent rupt changes
anisotropic zones corresponding to abin local orientation. However, the flow
orientations which were flexible enough to acquire internal distortions, responsible for the spotty brilliant images, are also flexible enough to acquire a large radius of curvature introducing a smooth and progressive change of orientation, without any abrupt change (see Figs. 11 through 14 and sketches of Figs. 15 and 16). It is unsuitable to attribute the term mesophase to the edge to edge associations, because they are not liquid crystals issued from a germination growth going through mesophase spheres[6,7]. In some very rare and peculiar cases, a form of growth already found in the crude coal tar[l] and also under pressure[B] was observed. Very thin filaments corresponding to pores of very small diameter in the metallic filter (Fig. 6), or to very thin layers of p resins on the surface of y resins-see triple arrow in Fig. 13(b)-show a perfect radial texture. This could be due to a peculiar growth corresponding to a centripetal progressive organization of the aromatic ring structures beginning at any free interface available. In a filter pore having a very small diameter, such radial growth can occur freely, without being disturbed by stresses, because no flow can be easily produced in a capillary. Growth initiates at the pore wall and ends when reaching the axis. In the same manner it may occur in a thin p
TEM studies of coal tars: stresses
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Fig. 12. Decantation process. Particle with flow orientations (oriented areas formed by stresses with a concave shape). Double bars indicate the orientation of aromatic layers. (a)-(b) Orthogonal 002 dark field images.
resins film at the contact of a y resins drop; see arrow in Fig. 12(b). This discussion accounts for all the data obtained in the present work. In the filter pores the f3 resins were forced inside pores but cannot reach the filtrate. The resulting statistical orientation is thus mainly parallel to the pore axis (Fig. 5). The extreme compaction of the material due to the strength of the stresses accounts for its relatively low plasticity (it is disrupted by the microtome knife). In the cake, the stresses in a pore are lower. The material is thus more plastic, not disrupted by the knife, but its orientation is still mainly parallel to the pore axis (Fig. 9). The more liquid-like particles still belong to p resins (Fig. 3) cannot keep the imprint of stresses because of their low viscosity. They have thus no statistical orientation, and are the first step of compaction. They are probably very fragile and labile. Such droplets or liquid phases having only random local orientations were first observed in [9] in a coal tar pitch, then[lO] in a coal (rich in p resins). Because the preparations for TEM were not thin sections but dusted samples,
it was not possible to correlate the mo~hology observed with that obtained under stresses or with those really present in the samples. However, in both cases their presence was attributed to p resins. Such particles are found here both in the cake and in the metallic filter but never reach the filtrate. The progressive compaction of p resins in the filter pores progressively blocks the filter pores and, finally, the ~ltration process. In the so-called decantation process the efficient parameter is also the flow stresses induced by opening the tap and forcing the tar to enter the conical part of the tube. The occurrence of edge to edge associations, and the disruption of the radial orientation by stresses inducing a long range orientation are demonstrated here particularly clearly. 5. CONCLUSION The crude coal tar already contains aromatic ring structures distributed at random in the bulk. Upon them are grafted heteroatoms and various functional groups. The morphologies described in this paper are due to the progressive associations between in-
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Fig. 13. Decantation process. Flow orientations forming a rim around a large drop of y resin (single arrow). The double arrow indicates individual droplets of y resins. Triple arrow indicates a radial growth on a thin layer of p resins. (a)-(b) Orthogonal 002 dark field images.
TEM studies of coal tars: stresses
Fig. 14. Decantation process. Combination of distortions due to stresses forming oriented rims of p resins around deformed bulk y resins (left). (a)-(b) Orthogonal 002 dark field images.
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Fig. 15. Sketch of Fig. 12 showing the distortions of the aromatic layer associations due to stresses.
Fig. 16. Sketch of Fig. 13.
TEM studies of coal tars: stresses
Fig. 17. Decantation process. Optical microscopy overall view of an area already observed by TEM.
dividual aromatic ring structures caused by modifications in the chemical composition of the tar. The main feature of this work is the formation of largely extended edge to edge associations between aromatic ring structures (-1 nm in size) as the coal tar precursor reaches the convenient chemical composition (mainly an increase in p resin content). This composition is reached in various ways: formation of cake above and inside a filter or residue of decantation forced through a tap. It is the occurrence of the a resins (plastic radial or alternate spheres) already contained in the initial tar[l] that insured the transformation of the tar residues, as they accumulate in the cake, in the filter, or near the bottom of the decantation tube. Correspondingly, they carry along with them their surrounding satellites of p resins and some y resins (probably heavier than the others). They thus yield the opportunity for forming edge to edge associations. However, these latter are in fact flow orientations due to stresses occurring during either filtration or flow through the tap at the bottom of the decantation tube. They are evidenced only because of the new method of thin sectioning used both through TEM imaging or optical microscopy. Such preferred orientations are more or less labile depending on their molecular weight spectrum. The heavier the p fractions, the less labile are the orientations up to the moment they become stable, though still plastic (heaviest p resins). The fact
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that the filtrate, when decanted, never gives any edge to edge association suggests that yr (the lightest y fraction) is unable to form such associations. The edge to edge associations, considered as a whole, are similar to gels of increasing rigidity (mineral gels for example), which are often able to “remember” by their flow orientation the stresses to which they have been subjected during increasing gelification[ 111. It seems very probable that this kind of flow orientation can be controlled with suitable stresses, so as to align the aromatic ring stuctures parallel to an axis, following various predetermined transverse configurations. To do so, the ideal composition has to be reached either by making the precursor heavier or lighter, depending on its initial composition. The edge to edge associations can be a leading thread for understanding the behavior of pitches as precursors of pitch based fibres[l2-151. Acknowledgements-The authors would like to acknowledge Dr. J. L. Saint Romin and NORSOLOR Society (H.G.D. department) for partially supporting this study.
REFERENCES
1. K. Lafdi, S. Bonnamy, and A. Oberlin, Carbon, 28, 57 (1990). 2. J. W. Clarke and T. D. Rantell, Fuel 59, 35 (1980). 3. S. Compin, R. Benai‘m, P. Couderc, and J. L. Saint Romain, Fuel 66, 1552 (1987). 4. J. G. Bailey, Report on ICCP working group combustion (1987).
5. C. F. K. Diessel and G. Wolf-Fischer, Report on ICCP working group combustion (1987).
6. S. Bonnamy and A. Oberlin, Extended Abstract, Carbon 88, Internat. Carbon Conference, Newcastle, p. 374 (1988). 7. S. Bonnamy and A. Oberlin, Carbon, to be submitted (1989). 8. J. Ayache, A. Oberlin, and M. Inagaki, Part I, Carbon, in press (1990). 9. D. Auguit, Thesis, Universite d’orleans, (1979). 10. J. N. Rouzaud and A. Oberlin, C. R. Acad. SC., Paris, 757 (1983). 11. H. Van Olphen, In An introduction to clay colloids chemistry, Interscience, London, pp. 89-94 (1963). .. 12. S. Otani, Curbon 3, 31 (1965) 13. L. S. Sineer. Carbon 16. 409 f1978). 14. H. Hondi, &bon 26, 139 (1988). ’ 15. I. C. Lewis and R. T. Lewis, Carbon 26, 757 (1978).