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An electron microscopic study on the turbostratic carbon formed in phenolic resin carbon by catalytic action of finely dispersed nickel
AN ELECTRON MICROSCOPIC STUDY ON THE TURBOSTRATIC CARBON FORMED IN PHENOLIC RESIN CARBON BY CATALYTIC ACTION OF FINELY DISPERSED NICKEL ASAO~YA, MITSURUMOCHIZUKI and SIC~O@ANI Faculty of Technology, Gunma University, Kiryu, Gunma 376, Japan and ISA0 TOhrIZUKA National ResearchInstitute for Metals, Meguro, Tokyo 153,Japan (Received 7 July 1978)
Abstract-Very fine particles of certain substances are known to promote the catalytic graphitization of ~ar~naceous material, resulting in a unique carbon whose X-ray parameters, lying in the range of tu~ostratic carbon, remain almost unchanged even after more severe heat-treatments. To elucidate the actual structure of this type of carbon, its formation mechanism, and the reason for its high thermal-stability, a high resolutionelectron microscopic investigation was carried out on carbons derived from phenolic resin lightly and heavily doped with an organo-nickel compound. The electron microscopic texture of this c?bon was found to consist of entangled lattice fringes of a stack number comp~able with its X-ray L, -value (110A). This texture was analogous to that of some types of the hard carbon heated to very high temperatures, but the former was less sinuous in its lattice fringe and more open in its structure. The formation mechanism appeared to differ from that accepted for conventional nickel-catalyzed graphitization, and to be particularly related with nickel particles in a certain size range. The evidence obtained so far was not enough to suggest a new mechanism, but only to illustrate part of some of the possible, hitherto little known mechanisms which might proceed along with it. Thermal stability of the carbon was understood to be due to its structure being similar to the hard carbon, but the ultimate reason why this type of structure was thermally stable could be made clear only to a limited extent.
I. IN~ODU~ION When certain types of carbon are heated with some kinds of metals or inorganic compounds, a graphitic carbon is formed at a much lower temperature than otherwise. This phenomenon is widely known as “catalytic graphitization“ and an extensive study has been made on it[l, 21. One of the significant recent discoveries is that the resultant carbons are not identical, but depend on the size of the catalyst: when non-graphitizing carbon containing finely despersed metal is heated, the resultant carbon is exclusively turbostratic (the T-component), whereas it is three dimensional graphite (the G-component) when the dispersion is less fine[3,4,.5]. Some of the authors of this paper (A.O. and S.O.), in addition, have revealed that the T-component is characterized by X-ray parameters of 3.40-3.44 A for the interlayer spacing (d,) and W-160 .& for the cryst~lite thickness (L,), varying somewhat with the kind of metal catalyst (61,but almost unchanged after a heat-treatment at 3000°C under normal pressure [3,7] or at 2WC under 5 kbar [8]. In spite of the ~-component exhibiting such characteristic properties, its actual structure has remained almost unrevealed, and it is not yet elucidated why the T-component instead of the G-component is formed when finely dispersed metal catalyst is used. To settle these problems, we doped different amounts of an organo-nickel compound into a phenolic resin and observed with a high resolution electronic microscope a
series of carbons obtained by subsequent heat-treatments. 2. EXPERIMENTAL. Certain amounts of an ethylalcohol~ch~oroform (i : 1)SO~Ution of nickel (II) acetylacetonate (NiAA) were added to a given amount of the phenol-formaldehyde tPF) resin (resol type) reported elsewhere[9]. The mixture was stirred with heating until the solvent had evaporated, then cured at 100°C for one day. The resulting resin block was initially calcined at 800°C for 1hr in argon atmosphere, and then heat-treated at a rate of SO”C/min in argon atmosphere to various heat-treatment temperatures (HTT) not exceeding 18WC, at which almost all nickel disappeared. After the required temperature was attained, the specimen was quenched immediately by dropping it into water. Specimens with 1 and 3Owlo nickel, as specified by the content in the 800°C calcined carbon, were prepared by controlling the amount of NiAA. X-ray diffraction analysis and high resolution electron microscopic observation were performed in the ways reported eisewhere[3, IO].
3. RESULTS 3.1 X-Ray di~ruc~iun analysis Figure I shows variations of the (002) X-ray diffraction profiles of PF carbons with I and 30 w/o nickel, heated to various HTTs. As is evident from comparison with the
ASAO(IYA et al.
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Fig. l(a). Changeof (002)diffractionprofilesof the 1w/o Ni-PF carbon with I-RI (----thePF carbon with no additive).
Fig. l(b). Changeof (002)diffractionprofilesof the 30 w/o NCPF carbon with HTT (----the PF carbon with no additive),
profiles from the nickel-free specimens heat-treated at the same tem~ratures (dotted lines), the effect of nickel is obvious even at 1w/o content. These profiles, notably those above 12OO”C,have a common definite (002) peak situated at about 26.0”(2& CuKJ, which will be called as T-component. The T-component had dooz- 3.42 A and L, - 110A. Furthermore, a qualitative difference exists between the series of 1w/o specimens and the 30 wfo specimens. An additional peak appears at 26.5”on top of that for the T-component for the latter specimens heated above l~l~DC. This peak is well known to be due to the graphitic structure (the G-component) and had do02= 3.36*A and L, = 800 A in the case of the 30 w/o nickel specimen heated at 1800°C.
those specimens, implies strongly that the nickel particle sizes would differ considerably with the added amount of NiAA. To clarify it, the size of the individual nickel particles was measured on the micro~aphs, As can be seen from the size distribution thus obtained (Fig. 2), the particle sizes of nickel widely varied not only with the amount of nickel added but also with the H’IT. By comparison with Fig. 1, it is suggested that the formation of the T- and the G-components could be attributed to nickel particles of around 2OOA and of greater than about 800 A, respectivety, although roles of the particles in sub-microscopic size ranges may need be taken into account. 3.2.2 The textures of the carbon components.The texture of the nickel-free specimen was homogeneous and increasingly, but slightly, better organized with increase of H’IT. Figure 3 shows a microphotograph of the 1800°Cheat-treated carbon. The appearance is similar to those reported for non-graphitizing carbons heated to appropriate temperatures (11).
3.2 Electron microscopicobservations 3.2.1 lke size of nickel particies in the heated carbons. The fact that the G-component was formed in the specimens with the higher nickel content and only in 50
r
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Fig.2. Size distributionsof nickel particles in the PF carbons for various H’ITs.
73
An electron microscopic study on turbostratic carbon
Fig. 3. Typical texture of a nickel-free specimen. (Scale: 100 A, HIT: 18OO”C, time at the HTT: 1 hr, Nickel content: 0 w/o)----, in this order in the captions of the other photographs.
With the addition of a small amount of nickel, the texture was changed remarkably as predicted from the X-ray diffraction profiles (Fig. 1). For all of the 1 w/o nickel specimens heated to lSOO”C,two kinds of textures were observed. The first was the type shown in Fig. 3, and the other was of mixed texture, but commonly of far-better defined lattice images. Since the latter texture was not observed in the nickel-free specimens whatever the HTT was, it is reasonable to attribute the Tcomponent profile in the X-ray diffractograms to this group of the texture. Two representatives of this type are shown in Fig. 4. Here it is evident that the lattice images of the T-component is fairly similar to that of some kinds of the non-graphitizing carbon (e.g. pitch-based carbon fibres) simply heated (i.e. without stretch, pressure or catalyst) to very high HTT[12]. The crystallites, as defined as a stack of lattice fringes, are randomly oriented and tangled in a complicated manner and not extremely large; lattice fringes continue through many crystallites, merging and branching several times. Simultaneously, however, some differences also exist: some lattice fringes, especially long ones, are less sinuous; and the texture is frequently more open. These appearnaces indicate that the T-component is a turbostratic carbon consisting of moderately large crystallites, substantially inheriting the features on the non-graphitizing carbon. In the specimens with 30 w/o nickel, which exhibited composite (002) diffraction profiles consisting of the Gand T-components together with the original, uncatalyzed phenolic resin carbon, three kinds of textures appeared. Two of them were the same to those in the previous specimens. An example of the third, together with its electron diffraction pattern, is shown in Fig. 5. As this is a well graphitized fragment, it must be one of the components which account for the G-component. There may be some other G-components which look more like Fig. 4 than like Fig. 5. However, it is impossible to classify them on the microphotographs, since the distance between the lattice fringes cannot be so precise. It is, however, our view that the lattice fringes
(4
OJ) Fig. 4. Two typical textures of the T-component. (a), Well-ordered region (100A, MOO”C,~ 0 min. 1 w/o); (b), Less-ordered region (100A, 1400°C.0 min 1 w/o).
Fig. 5. An example of a fragment belonging to the G-component together with its selected area diffraction pattern. The diffraction pattern implies a hexagonal structure (5OOA. 18OO”C,Omin, 30 w/o).
74
ASAO ~YA et al.
with a size comparable with the X-ray parameters of the T-component belong almost always to the T-component except the case where the lattice band is firmly fixed on the surface of the nickel particle. This means that a majority of the textures appearing like Fig. 4 belong to the T-component. 4. DISCUSSION
4.1 The processes of Ni-catalyzed gruphitization As described in Section 3.2.1, the G-component in this work was always accompanied by large nickel particles >8OOA in diameter. Actually the present authors frequently observed very thick stacks of the graphitic layers covering nickel particles of that size. The conventional description of this catalytic graphitization process is said to be analogous to an activated solid-state sintering process [ 131:The nickel particle which is mediating the graphitization is unsaturated in terms of poorly graphitized carbon, whereas it is supersaturated in terms of a more highly graphitized carbon; as a result, the former dissolves continuously into the nickel particle, while the latter precipitates continuously from it [ 11.The driving force of this process, which occurs isothermally, is the excess free energy of the poorly graphitized carbon[l4]. This mechanism is considered applicable to the formation of the G-component in this work. Fig. 6 may be an illustration of this mechanism. One of the purposes in this work is to reveal the formation mechanism of the T-component catalyzed by finely dispersed nickel particles. Some investigators have already used organometallic compounds as a source of metal catalysts and observed formation of certain types of carbon equivalent to the T-component in this paper [4,5]. For the catalytic formation mechanism of these carbons, they proposed the conventional ones. However it seems difficult to explain in this way the following observations experienced by us in a series of studies on this subject: (i), Formation temperature of the T-component is not identical for individual metal catalysts. But, provided an appropriate temperature is applied, an identical metal catalyst could produce either the G-component or the T-component at the same temperature simply by making an appropriate choice on the size of the catalyst particles[3,6,9, IS]. This fact cannot be well explained by any of the conventional mechanisms, because, according to them, the product should be the
Fig. 6. Well-ordered f$rly large band off a large nickel particle (100A, IOOOT,0 min, 30 w/o).
G-component. (ii), According to the data on the X-ray diffraction analysis, the resulting T-component more or less retains the structural memories of the raw carbon from which the T-component was formed[7,16]. The conventional carbon dissolution-precipitation mechanism or carbide formation-decomposition mechanism cannot reasonably explain this phenomenon, because neither mechanism permits retaining.any structural memories of raw carbon[l, 21. (iii), It was observed in the catalytic graphitization of PF carbon by finely dispersed nickel that the T-component was formed abundantly in the I-III range where nickel disappears from the carbon[l7]. This suggests that vaporized nickel would be more effective than the fine solid or liquid nickel in Tcomponent formation, although it must be noted here that the gasous species formed from Ca or Mg simply accelerated the homogeneous graphitization process of the entire carbon without any catalytic effect on Tcomponent formation [ 181. Close examination of electron micrographs has revealed many textures that suggest three of complementary mechanisms which could be operating in the specimens. First, the lattice fringes in the vicinity of a nickel particle firmly embedded in matrix carbon without free space (Fig. 7, arrowed) suggest a lattice formation caused by stress concentration around a hard nickel particle. The matrix within a carbon-fibre reinforced carbon heated to an appropriate temperature is reported to be more remarkably graphitized around the carbon fibres than elsewhere, due to stress concentration resulting from the thermal expansion differential between the fibre and the matrix [19]. Second, graphitic bands of mixed sizes in whose vicinity no appropriate nickel particle is observed (Fig. 8) suggest a new mechanism for lattice formation which is likely caused by nickel species of a sub-microscopic size. Although such species are not yet fully confirmed, to assume them is well suitable for
Fig. 7. Small lattice fr@ges (arrowed) off a small nickel particle (100A, lOOO”C, 0 min, 30 w/o).
An electron microscopic
study on turbostratic
7s
carbon
smaller fragments followed by their migration towards the matrix carbon, leaving behind initially the Gcomponent and later the Tcomponent[21], can be well accounted for by either the conventional or any of the newly suggested mechanisms. 4.2 On the high-thermal
Fig 8. Large but less-ordered graphite band likely to be unaffected by large nickel particles (500 A, lOC@C, Omin, 30 w/o).
explaining each of the above observations. Third, lattice images along the boundary of a carbon fragment (Fig. 9) suggest the boundary itself could be a cause of lattice formation. Glassy carbon is reported to be better graphitized on heating when its interface with open space is increased with help of a foaming agent [20]. Yet, none of the three suggested mechanisms can provide a through description of the T-component formation. The first and third describe only the very initial stage of lattice formation and the second is entirely lacking in details. In addition, none of them takes account of positive roles of the nickel particles in the diameter range of around 2OOA, for which, the relationship with T-component formation illustrated in Fig. 2 does not, in our view, seem to be a meaningless coincidence. Actually, other, not yet revealed, significant mechanisms operate besides the already known or suggested mechanisms, because neither the development of the infant lattice shown in Figs. 7-9 to a more complicated texture of the T-component shown in Fig. 4, nor some of the other observations made by us, e.g. a successive disintegration of a bulky nickel particle into
Fig. 9. Lattice fringes along a boundary of a carbon fragment (100 A, lOOOT, 0 min, I w/o).
stability
of the T-component
Another main purpose in this work is to discuss why the T-component is stable against heat-treatments. Four possible courses are considered for a lattice band to increase its stack number. First is a direct conversion of a less-ordered component to a graphitic layer through a mechanism similar to ledge migration[22]. Second is a quantity of thin layers being aligned in a structure which is easily convertible to a thick band. Third is a coalescence of several moderately thick layer stacks to a combined thicker one. Fourth is transportation of free carbon atoms (monatomic or olig-atomic) on to a top layer of a lattice stack already existing. Our experimental evidence has confirmed that none of these courses can work effectively for the T-component to transfrom to the G-component. It will be discussed below. The first course is easily ruled out by the electron microscopic observation. The T-component, as shown in Fig. 4, is so poorly organized that it cannot respond to a migration mechanism which requires a more or less organized atomic network. The second course will be limited. Although we could observe a structure shown in Fig. 10, which is very similar to what Oberlin et al. claimed for graphitizing carbon [23],it was very rare and this sort of texture was never a general feature of the T-component. The third course will be limited as well. Although textures like Fig. 11 were observed fairly frequently, the structure of the T-component is such that an extensive coalescence to comparatively large bands is hardly expected. We have secured evidence to suggest that the fourth course is actuelly operating in the G-component; the arrowed area in Fig. 12 is considered to be a pit in a large graphitic layer being filled by these free atoms. But why this course does not work extensively in the T-component is still a matter of
Fig. 10. Roughly parallel but very poorly ordered lattice fringes presumed to be an infant stage of band development (100 A, lZOO”C,0 min. 1w/o).
ASAO~YA et al.
the crystallites of the T-component must be stacked for ever in the turbostratic way: And the tuibostratic stack formed in this way can grow only to limited thickness, because the well-known dW2-LCrelationsh~p[71 does not allow turbostratic crystallites (whose d, value lie between 3.40 and 3.44 A) to have an L-value exceeding a certain limit. From these arguments, we reach the conclusion that the crystallites of the ~-component cannot develop further because they have only turbostratic layers, too highly entangled to be unfolded to Rat layers. But it is not absolutely clear why this type of structure cannot be reorganized by an entire or partial disintegration of the network. Ac~~ow~e~ge~e~ts~ne of the authors (I. T.) thanks Mr. Ogawa (National Research Institute for Metals) for his technical assistance in electron microscopy.
1. D. B. F~schba~h, In Chemists and Physics of Carbon (Edited by P. L. Walker Jr.), Vol. 7, p. 83. Marcel Dekker, Fig. 11. Small graphitic bapds appearing to be merging to a New York (1972). thicker one (500A. looO”C,0 min, 30w/o). 2. H. Marsh and A. P. Warburton, J. Appl. Chem. 20, 133 (1970). 3. S. btani, A. dya and .I. Akagami, Carbon 13, 353 (1975). 4. E. M. Wewerka and R. P. Imprescia, Carbon 11,289 (1973). 5. R. L. Courtney and S. F. Duliere, Carbon IO,65 (1972). 6. S. &ani and A. 6ya. Tunso No. 79, Ill (1974). 7. S. &ani, A. dya &d M. Nishinow, Tans; No. 76, 2 (1974). 8. M. Inaaaki, Tanso No. 79, 116(1974). 9. A. dyaand S. aani, Carbon 14, 191(1976). 10. D. J. Johnson, I. Tomizuka and 0. Watanabe, Carbon 13,321 (1975). Il. G. M. Jenkins and K. Kawamura, In Polymeric Carbons, p. 70. C~bridge University Press, London (1976). 12. D. J. Johnson, I. Tomizuka and 0. Watanabe~Carbon 13,529 (1975). 13. G. M. Jenkins and K. Kawamura, In Polymeric Carbons, p. 143.Cambridge University Press, London (1976). 14. E. Fitzer and B_Kegel, Carbon 6,433 (196ll). 15. A. Qya and S. Otani, High_Temp.-HighPress. 7,563 (1975). Fig. 12. A pit in a graphitic layer (arr_awed)apparently being 16. A. Oya, A. Yutaka and S. Otani, Carbon, in contribution. filled with imigratingcarbon atoms (500A, 18oo”C,0 min, 30w/o}. 17. S. &ani, A. eya and H. Kakegawa, Tanso No. 91, (1977). lg. A. oya, R. Yamashita and S. &ni, High Temp.-High Press., conjecture. A possible way to understand it may be to in press. assume that “when a graphitic layer is formed layer by 19. Y. Hishiyama, M. Inagaki, S. Kimura and S. Yamada, Carbon 12,249 (1974). layer through transportation of free carbon atoms on the 20. K. Kawamura a_ndT. Tsuzuku, Carbon 12,352 (1974). already stacked graphitic layers, the sequence of the new 21. A. Oya and S. Otani, Carbon 16, 153(1978). layer is the same as that of the substrate layer”. Under this 22. D. R. Clarke and G. Thomas, f. Am. Ceram. Sot. 60, 492 assumption, if this is the only process to add a new layer, all (1977).
Abstract-Very fine particles of certain substances are known to promote the catalytic graphitization of ~ar~naceous material, resulting in a unique carbon whose X-ray parameters, lying in the range of tu~ostratic carbon, remain almost unchanged even after more severe heat-treatments. To elucidate the actual structure of this type of carbon, its formation mechanism, and the reason for its high thermal-stability, a high resolutionelectron microscopic investigation was carried out on carbons derived from phenolic resin lightly and heavily doped with an organo-nickel compound. The electron microscopic texture of this c?bon was found to consist of entangled lattice fringes of a stack number comp~able with its X-ray L, -value (110A). This texture was analogous to that of some types of the hard carbon heated to very high temperatures, but the former was less sinuous in its lattice fringe and more open in its structure. The formation mechanism appeared to differ from that accepted for conventional nickel-catalyzed graphitization, and to be particularly related with nickel particles in a certain size range. The evidence obtained so far was not enough to suggest a new mechanism, but only to illustrate part of some of the possible, hitherto little known mechanisms which might proceed along with it. Thermal stability of the carbon was understood to be due to its structure being similar to the hard carbon, but the ultimate reason why this type of structure was thermally stable could be made clear only to a limited extent.
I. IN~ODU~ION When certain types of carbon are heated with some kinds of metals or inorganic compounds, a graphitic carbon is formed at a much lower temperature than otherwise. This phenomenon is widely known as “catalytic graphitization“ and an extensive study has been made on it[l, 21. One of the significant recent discoveries is that the resultant carbons are not identical, but depend on the size of the catalyst: when non-graphitizing carbon containing finely despersed metal is heated, the resultant carbon is exclusively turbostratic (the T-component), whereas it is three dimensional graphite (the G-component) when the dispersion is less fine[3,4,.5]. Some of the authors of this paper (A.O. and S.O.), in addition, have revealed that the T-component is characterized by X-ray parameters of 3.40-3.44 A for the interlayer spacing (d,) and W-160 .& for the cryst~lite thickness (L,), varying somewhat with the kind of metal catalyst (61,but almost unchanged after a heat-treatment at 3000°C under normal pressure [3,7] or at 2WC under 5 kbar [8]. In spite of the ~-component exhibiting such characteristic properties, its actual structure has remained almost unrevealed, and it is not yet elucidated why the T-component instead of the G-component is formed when finely dispersed metal catalyst is used. To settle these problems, we doped different amounts of an organo-nickel compound into a phenolic resin and observed with a high resolution electronic microscope a
series of carbons obtained by subsequent heat-treatments. 2. EXPERIMENTAL. Certain amounts of an ethylalcohol~ch~oroform (i : 1)SO~Ution of nickel (II) acetylacetonate (NiAA) were added to a given amount of the phenol-formaldehyde tPF) resin (resol type) reported elsewhere[9]. The mixture was stirred with heating until the solvent had evaporated, then cured at 100°C for one day. The resulting resin block was initially calcined at 800°C for 1hr in argon atmosphere, and then heat-treated at a rate of SO”C/min in argon atmosphere to various heat-treatment temperatures (HTT) not exceeding 18WC, at which almost all nickel disappeared. After the required temperature was attained, the specimen was quenched immediately by dropping it into water. Specimens with 1 and 3Owlo nickel, as specified by the content in the 800°C calcined carbon, were prepared by controlling the amount of NiAA. X-ray diffraction analysis and high resolution electron microscopic observation were performed in the ways reported eisewhere[3, IO].
3. RESULTS 3.1 X-Ray di~ruc~iun analysis Figure I shows variations of the (002) X-ray diffraction profiles of PF carbons with I and 30 w/o nickel, heated to various HTTs. As is evident from comparison with the
ASAO(IYA et al.
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Fig. l(a). Changeof (002)diffractionprofilesof the 1w/o Ni-PF carbon with I-RI (----thePF carbon with no additive).
Fig. l(b). Changeof (002)diffractionprofilesof the 30 w/o NCPF carbon with HTT (----the PF carbon with no additive),
profiles from the nickel-free specimens heat-treated at the same tem~ratures (dotted lines), the effect of nickel is obvious even at 1w/o content. These profiles, notably those above 12OO”C,have a common definite (002) peak situated at about 26.0”(2& CuKJ, which will be called as T-component. The T-component had dooz- 3.42 A and L, - 110A. Furthermore, a qualitative difference exists between the series of 1w/o specimens and the 30 wfo specimens. An additional peak appears at 26.5”on top of that for the T-component for the latter specimens heated above l~l~DC. This peak is well known to be due to the graphitic structure (the G-component) and had do02= 3.36*A and L, = 800 A in the case of the 30 w/o nickel specimen heated at 1800°C.
those specimens, implies strongly that the nickel particle sizes would differ considerably with the added amount of NiAA. To clarify it, the size of the individual nickel particles was measured on the micro~aphs, As can be seen from the size distribution thus obtained (Fig. 2), the particle sizes of nickel widely varied not only with the amount of nickel added but also with the H’IT. By comparison with Fig. 1, it is suggested that the formation of the T- and the G-components could be attributed to nickel particles of around 2OOA and of greater than about 800 A, respectivety, although roles of the particles in sub-microscopic size ranges may need be taken into account. 3.2.2 The textures of the carbon components.The texture of the nickel-free specimen was homogeneous and increasingly, but slightly, better organized with increase of H’IT. Figure 3 shows a microphotograph of the 1800°Cheat-treated carbon. The appearance is similar to those reported for non-graphitizing carbons heated to appropriate temperatures (11).
3.2 Electron microscopicobservations 3.2.1 lke size of nickel particies in the heated carbons. The fact that the G-component was formed in the specimens with the higher nickel content and only in 50
r
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Fig.2. Size distributionsof nickel particles in the PF carbons for various H’ITs.
73
An electron microscopic study on turbostratic carbon
Fig. 3. Typical texture of a nickel-free specimen. (Scale: 100 A, HIT: 18OO”C, time at the HTT: 1 hr, Nickel content: 0 w/o)----, in this order in the captions of the other photographs.
With the addition of a small amount of nickel, the texture was changed remarkably as predicted from the X-ray diffraction profiles (Fig. 1). For all of the 1 w/o nickel specimens heated to lSOO”C,two kinds of textures were observed. The first was the type shown in Fig. 3, and the other was of mixed texture, but commonly of far-better defined lattice images. Since the latter texture was not observed in the nickel-free specimens whatever the HTT was, it is reasonable to attribute the Tcomponent profile in the X-ray diffractograms to this group of the texture. Two representatives of this type are shown in Fig. 4. Here it is evident that the lattice images of the T-component is fairly similar to that of some kinds of the non-graphitizing carbon (e.g. pitch-based carbon fibres) simply heated (i.e. without stretch, pressure or catalyst) to very high HTT[12]. The crystallites, as defined as a stack of lattice fringes, are randomly oriented and tangled in a complicated manner and not extremely large; lattice fringes continue through many crystallites, merging and branching several times. Simultaneously, however, some differences also exist: some lattice fringes, especially long ones, are less sinuous; and the texture is frequently more open. These appearnaces indicate that the T-component is a turbostratic carbon consisting of moderately large crystallites, substantially inheriting the features on the non-graphitizing carbon. In the specimens with 30 w/o nickel, which exhibited composite (002) diffraction profiles consisting of the Gand T-components together with the original, uncatalyzed phenolic resin carbon, three kinds of textures appeared. Two of them were the same to those in the previous specimens. An example of the third, together with its electron diffraction pattern, is shown in Fig. 5. As this is a well graphitized fragment, it must be one of the components which account for the G-component. There may be some other G-components which look more like Fig. 4 than like Fig. 5. However, it is impossible to classify them on the microphotographs, since the distance between the lattice fringes cannot be so precise. It is, however, our view that the lattice fringes
(4
OJ) Fig. 4. Two typical textures of the T-component. (a), Well-ordered region (100A, MOO”C,~ 0 min. 1 w/o); (b), Less-ordered region (100A, 1400°C.0 min 1 w/o).
Fig. 5. An example of a fragment belonging to the G-component together with its selected area diffraction pattern. The diffraction pattern implies a hexagonal structure (5OOA. 18OO”C,Omin, 30 w/o).
74
ASAO ~YA et al.
with a size comparable with the X-ray parameters of the T-component belong almost always to the T-component except the case where the lattice band is firmly fixed on the surface of the nickel particle. This means that a majority of the textures appearing like Fig. 4 belong to the T-component. 4. DISCUSSION
4.1 The processes of Ni-catalyzed gruphitization As described in Section 3.2.1, the G-component in this work was always accompanied by large nickel particles >8OOA in diameter. Actually the present authors frequently observed very thick stacks of the graphitic layers covering nickel particles of that size. The conventional description of this catalytic graphitization process is said to be analogous to an activated solid-state sintering process [ 131:The nickel particle which is mediating the graphitization is unsaturated in terms of poorly graphitized carbon, whereas it is supersaturated in terms of a more highly graphitized carbon; as a result, the former dissolves continuously into the nickel particle, while the latter precipitates continuously from it [ 11.The driving force of this process, which occurs isothermally, is the excess free energy of the poorly graphitized carbon[l4]. This mechanism is considered applicable to the formation of the G-component in this work. Fig. 6 may be an illustration of this mechanism. One of the purposes in this work is to reveal the formation mechanism of the T-component catalyzed by finely dispersed nickel particles. Some investigators have already used organometallic compounds as a source of metal catalysts and observed formation of certain types of carbon equivalent to the T-component in this paper [4,5]. For the catalytic formation mechanism of these carbons, they proposed the conventional ones. However it seems difficult to explain in this way the following observations experienced by us in a series of studies on this subject: (i), Formation temperature of the T-component is not identical for individual metal catalysts. But, provided an appropriate temperature is applied, an identical metal catalyst could produce either the G-component or the T-component at the same temperature simply by making an appropriate choice on the size of the catalyst particles[3,6,9, IS]. This fact cannot be well explained by any of the conventional mechanisms, because, according to them, the product should be the
Fig. 6. Well-ordered f$rly large band off a large nickel particle (100A, IOOOT,0 min, 30 w/o).
G-component. (ii), According to the data on the X-ray diffraction analysis, the resulting T-component more or less retains the structural memories of the raw carbon from which the T-component was formed[7,16]. The conventional carbon dissolution-precipitation mechanism or carbide formation-decomposition mechanism cannot reasonably explain this phenomenon, because neither mechanism permits retaining.any structural memories of raw carbon[l, 21. (iii), It was observed in the catalytic graphitization of PF carbon by finely dispersed nickel that the T-component was formed abundantly in the I-III range where nickel disappears from the carbon[l7]. This suggests that vaporized nickel would be more effective than the fine solid or liquid nickel in Tcomponent formation, although it must be noted here that the gasous species formed from Ca or Mg simply accelerated the homogeneous graphitization process of the entire carbon without any catalytic effect on Tcomponent formation [ 181. Close examination of electron micrographs has revealed many textures that suggest three of complementary mechanisms which could be operating in the specimens. First, the lattice fringes in the vicinity of a nickel particle firmly embedded in matrix carbon without free space (Fig. 7, arrowed) suggest a lattice formation caused by stress concentration around a hard nickel particle. The matrix within a carbon-fibre reinforced carbon heated to an appropriate temperature is reported to be more remarkably graphitized around the carbon fibres than elsewhere, due to stress concentration resulting from the thermal expansion differential between the fibre and the matrix [19]. Second, graphitic bands of mixed sizes in whose vicinity no appropriate nickel particle is observed (Fig. 8) suggest a new mechanism for lattice formation which is likely caused by nickel species of a sub-microscopic size. Although such species are not yet fully confirmed, to assume them is well suitable for
Fig. 7. Small lattice fr@ges (arrowed) off a small nickel particle (100A, lOOO”C, 0 min, 30 w/o).
An electron microscopic
study on turbostratic
7s
carbon
smaller fragments followed by their migration towards the matrix carbon, leaving behind initially the Gcomponent and later the Tcomponent[21], can be well accounted for by either the conventional or any of the newly suggested mechanisms. 4.2 On the high-thermal
Fig 8. Large but less-ordered graphite band likely to be unaffected by large nickel particles (500 A, lOC@C, Omin, 30 w/o).
explaining each of the above observations. Third, lattice images along the boundary of a carbon fragment (Fig. 9) suggest the boundary itself could be a cause of lattice formation. Glassy carbon is reported to be better graphitized on heating when its interface with open space is increased with help of a foaming agent [20]. Yet, none of the three suggested mechanisms can provide a through description of the T-component formation. The first and third describe only the very initial stage of lattice formation and the second is entirely lacking in details. In addition, none of them takes account of positive roles of the nickel particles in the diameter range of around 2OOA, for which, the relationship with T-component formation illustrated in Fig. 2 does not, in our view, seem to be a meaningless coincidence. Actually, other, not yet revealed, significant mechanisms operate besides the already known or suggested mechanisms, because neither the development of the infant lattice shown in Figs. 7-9 to a more complicated texture of the T-component shown in Fig. 4, nor some of the other observations made by us, e.g. a successive disintegration of a bulky nickel particle into
Fig. 9. Lattice fringes along a boundary of a carbon fragment (100 A, lOOOT, 0 min, I w/o).
stability
of the T-component
Another main purpose in this work is to discuss why the T-component is stable against heat-treatments. Four possible courses are considered for a lattice band to increase its stack number. First is a direct conversion of a less-ordered component to a graphitic layer through a mechanism similar to ledge migration[22]. Second is a quantity of thin layers being aligned in a structure which is easily convertible to a thick band. Third is a coalescence of several moderately thick layer stacks to a combined thicker one. Fourth is transportation of free carbon atoms (monatomic or olig-atomic) on to a top layer of a lattice stack already existing. Our experimental evidence has confirmed that none of these courses can work effectively for the T-component to transfrom to the G-component. It will be discussed below. The first course is easily ruled out by the electron microscopic observation. The T-component, as shown in Fig. 4, is so poorly organized that it cannot respond to a migration mechanism which requires a more or less organized atomic network. The second course will be limited. Although we could observe a structure shown in Fig. 10, which is very similar to what Oberlin et al. claimed for graphitizing carbon [23],it was very rare and this sort of texture was never a general feature of the T-component. The third course will be limited as well. Although textures like Fig. 11 were observed fairly frequently, the structure of the T-component is such that an extensive coalescence to comparatively large bands is hardly expected. We have secured evidence to suggest that the fourth course is actuelly operating in the G-component; the arrowed area in Fig. 12 is considered to be a pit in a large graphitic layer being filled by these free atoms. But why this course does not work extensively in the T-component is still a matter of
Fig. 10. Roughly parallel but very poorly ordered lattice fringes presumed to be an infant stage of band development (100 A, lZOO”C,0 min. 1w/o).
ASAO~YA et al.
the crystallites of the T-component must be stacked for ever in the turbostratic way: And the tuibostratic stack formed in this way can grow only to limited thickness, because the well-known dW2-LCrelationsh~p[71 does not allow turbostratic crystallites (whose d, value lie between 3.40 and 3.44 A) to have an L-value exceeding a certain limit. From these arguments, we reach the conclusion that the crystallites of the ~-component cannot develop further because they have only turbostratic layers, too highly entangled to be unfolded to Rat layers. But it is not absolutely clear why this type of structure cannot be reorganized by an entire or partial disintegration of the network. Ac~~ow~e~ge~e~ts~ne of the authors (I. T.) thanks Mr. Ogawa (National Research Institute for Metals) for his technical assistance in electron microscopy.
1. D. B. F~schba~h, In Chemists and Physics of Carbon (Edited by P. L. Walker Jr.), Vol. 7, p. 83. Marcel Dekker, Fig. 11. Small graphitic bapds appearing to be merging to a New York (1972). thicker one (500A. looO”C,0 min, 30w/o). 2. H. Marsh and A. P. Warburton, J. Appl. Chem. 20, 133 (1970). 3. S. btani, A. dya and .I. Akagami, Carbon 13, 353 (1975). 4. E. M. Wewerka and R. P. Imprescia, Carbon 11,289 (1973). 5. R. L. Courtney and S. F. Duliere, Carbon IO,65 (1972). 6. S. &ani and A. 6ya. Tunso No. 79, Ill (1974). 7. S. &ani, A. dya &d M. Nishinow, Tans; No. 76, 2 (1974). 8. M. Inaaaki, Tanso No. 79, 116(1974). 9. A. dyaand S. aani, Carbon 14, 191(1976). 10. D. J. Johnson, I. Tomizuka and 0. Watanabe, Carbon 13,321 (1975). Il. G. M. Jenkins and K. Kawamura, In Polymeric Carbons, p. 70. C~bridge University Press, London (1976). 12. D. J. Johnson, I. Tomizuka and 0. Watanabe~Carbon 13,529 (1975). 13. G. M. Jenkins and K. Kawamura, In Polymeric Carbons, p. 143.Cambridge University Press, London (1976). 14. E. Fitzer and B_Kegel, Carbon 6,433 (196ll). 15. A. Qya and S. Otani, High_Temp.-HighPress. 7,563 (1975). Fig. 12. A pit in a graphitic layer (arr_awed)apparently being 16. A. Oya, A. Yutaka and S. Otani, Carbon, in contribution. filled with imigratingcarbon atoms (500A, 18oo”C,0 min, 30w/o}. 17. S. &ani, A. eya and H. Kakegawa, Tanso No. 91, (1977). lg. A. oya, R. Yamashita and S. &ni, High Temp.-High Press., conjecture. A possible way to understand it may be to in press. assume that “when a graphitic layer is formed layer by 19. Y. Hishiyama, M. Inagaki, S. Kimura and S. Yamada, Carbon 12,249 (1974). layer through transportation of free carbon atoms on the 20. K. Kawamura a_ndT. Tsuzuku, Carbon 12,352 (1974). already stacked graphitic layers, the sequence of the new 21. A. Oya and S. Otani, Carbon 16, 153(1978). layer is the same as that of the substrate layer”. Under this 22. D. R. Clarke and G. Thomas, f. Am. Ceram. Sot. 60, 492 assumption, if this is the only process to add a new layer, all (1977).