Carbon Materials for Advanced Technologies Episode 2 ppt

20 A B Fig. 9. A, Model for the apex of a carbon nanocone with a cone angle of 19.2" [94]; E, polyhedral and spherical forms of a multiwall carbon particle formed from C,, C, and c, 1981. Ugarte has shown that faceted carbon particles with structures similar to graphitised carbon black are converted to spherical carbon shell structures under intense electron beam irradiation [96-981. These have been called carbon onions or 'Buckyonions'. The shells have external diameters up to -30 nm and hollow centres with diameters similar to that of the C,, molecule. Ugarte has suggested that the concentric carbon shells are formed about a central C, molecule. Theoretical calculations of the stability of a concentric duplet formed by C,, about C, yield a stabilisation energy of 14 MeV per C atom and an optimal interlayer spacing of 352 pm, close to the value for graphite [99]. Other calculations on the concentric structure formed by CH0 about CZa show that a spherical conformation of the two layers is more stable than the analogous polyhedral duplet [98]. Fig. 9B shows a model for a triple wall carbon particle in spherical and polyhedral forms constructed from C6,, Ca0, and C,,, [98]. 6 Engineering Carbons 6. I Introduction There are many applications for diamonds and related materials, e.g., diamond- llke carbon films, and there are potential applications for Fullerenes and carbon nanotubes that have not yet been realised. However, the great majority of engineering carbons, including most of those described in this book, have graphitic microstructures or disordered graphitic microstructures. Also, most engineering carbon materials are derived from organic precursors by heat- treatment in inert atmospheres (carbonisation). A selection of technically- 21 important carbons obtained from solid, liquid and gaseous organic precursors is presented in Table 5. Table 5. Precursors for engineering carbons Primary Secondag 1 Example products precitrsor precursor Hydrocarbon gases Petroleum petroleum pitch mesophase pitch Coals coal chars coal tar pitch mesophase pitch Polymers polyacrylonitrile phenolic and furan resins pol yimides Biomassb pyrocarbons, carbon blacks, vapour grown carbon fibres, matrix carbonn delayed coke, calcined coke needle coke, carbon fibers, binder and matrix carbon" mesocarbon microbeads, carbon fibers semi-coke, calcined coke activated carbons premium cokes, carbon fibers, binder and matrix carbons' mesocarbon microbeads, carbon fibers PAN-based carbon fibers glassy carbons, binder and matrix carbons" graphite films and monoliths activated carbons a. precursor for binder in polygranular carbons and graphites, precursor for matrix in carbon-carbon composites; b, especially wood and nutshells During carbonisation the organic precursor is thermally degraded by heat- treatment at temperatures in the range -450-1000 "C to form products that undergo either condensation or volatilisation reactions, the competition between these processes determining the carbon yield. Fig. 10 provides examples of the chemical processes that occur during carbonisation of the model precursor acenaphthylene [ 1001, Some of the volatilised products produced during carbonisation may be recovered to produce useful secondary precursors for carbons. For example, petroleum pitch and coal tar pitch are secondary precursors that are produced during carbonisation of petroleum and coal, Table 5. Carbons formed after heating up to -1000 "C (pnmary carbonisation) are low-temperature carbons. They are usually disordered without any evidence for three-dimensional graphitic order and they may also retain significant concentrations of heteroelements, especially 0, H, and S, and mineral matter. It is beyond the scope of this chapter to review structure and bonding in each class of engineering carbons listed in Table 5. Instead, a generic description of microstructure and bonding in these materials will be attempted. The evolution in understanding of the structure of engineering carbons and graphites has foIlowed the initial application of X-ray diffraction and subsequent application 22 of electron and neutron diffraction, and high resolution electron microscopy, supplemented by a wide range of other analytical techniques. further condensation - u Fig. 10. Mechanism of carbonisation of acenaphthylene [ 1001. I, acenaphthylene; 11, polyacenaphthylene; 111, biacenaphthylidene; IV, fluorocyclene; V, dinaphthylenebutadiene; VI, decacyclene; VII, zethrene. Reprinted from [ 1001 courtesy of Marcel Dekker Inc. 6.2 X-ray studies of engineering carbons In the 1930s Hoffman and Wilm [loll found only (hk0) graphte reflections in an x-ray diffraction study of a carbon black. The absence of graphitic (hkl) reflections led them to propose a structure consisting of graphitic carbon layer 23 planes in parallel array but without any three-dimensional order. They also noted from the position of the [002] line that the interlayer spacing, d, was greater than that for the graphite crystal (d = 0.3354 nm). This early concept of the microstructure of an engineering carbon forms the basis of the more refined models that have been developed in subsequent years. Biscoe and Warren [lo21 coined the term 'turbostratic' to describe a parallel stack of carbon layer planes with random translation about the a-axis and rotation about the c-axis. Turbostratic carbon is therefore without three-dimensional order and the turbostratic value of the interlayer spacing d, 0.344 nm, is greater than that for graphite. The dimensions of the turbostratic stack in the a and c crystallographic directions are characterised from the pronounced X-ray line broadening by the width and height, La and L, respectively, as well as the interlayer spacing, d. Values found by Hoffmann and Wilm [101] for a range of technical carbons ranged from La = 2.1-12 nm and L, = 0.9-18 nm; the latter values imply stacks containing from 3 to about 50 layer planes. The broadening of X-ray lines is also influenced by imperfections in the carbon layer planes so that the dimensions of stacks, particularly the width, may be larger than is indicated by La and L, values. High resolution electron microscopic studies lend some support to this view (see Section 6.4). A notable advance was made by Franklin [ 103 J in an X-ray diffraction study of polymer chars. She found that for a low-temperature PVDC char that 65% was in the form of turbostratic carbon and the remainder was an unspecified form of disordered carbon. Subsequently, [ 1041 FrankIin classified low temperature carbons into graphitising carbons which develop three-dimensional graphtic order on heat-treatment above 2000 "C and non-graphitising carbons which do not. The structure of graphitising carbons was envisaged an array of turbostratic carbon units that were oriented in near-parallel (pre-graphitic) array; non- graphitising carbons contained turbostratic units in random array that were cross-linked by disorganised carbon, Fig. 11. Franklin's classification is now recognised as oversimplified, since there is a near-continuum from graphitising to non- graphitising microstructures. Nevertheless, the concepts of graphitising and non-graphitising carbons are useful and they have been retained. Amorphous carbon films of the type a-C and a-C:H produced by physical or chemical vapour deposition from the gas phase contain varying amounts of sp2 and sp3 bonded carbon atoms, see section 4.1. The possibility of both sp2 and sp3 bonded atoms in carbons produced by carbonisation of organic precursors has been considered by a number of workers. The presence of sp3 bonded carbon, particularly in the disorganised carbon that links the carbon layer planes in non-graphitising carbons, seems reasonable in principle. In an X-ray study No& and co-workers [ 105 ] obtained radial distribution hnctions for a glassy carbon and proposed that some sp3 carbon atoms were present. However, a later high resolution X-ray study of a high temperature glassy carbon by Wignall and 24 Pings [106], and a neutron diffraction study by Mildner and Carpenter [107], both concluded that there is no clear evidence for sp3 carbon and that the rachal distribution functions can be satisfactorily indexed to a hexagonal mays of carbon atoms. A similar conclusion was reached in a recent neutron diffraction study of activated carbons by Gardner et al [ 1081. A B Fig. 11. Schematic models for the structure oE A, graphitising carbons, and B, non- graphitising carbons [104]. 6.3 The carbonaceous mesophase It is now known that the development of graphitising carbons depends upon the formation of a liquid crystal phase called the carbonaceous mesophase during a fluid stage in carbonisation. The mesophase appears initially as small, optically anisotropic spheres growing out of an optically isotropic fluid pitch. The mesophase spheres contain polynuclear aromatic hydrocarbons (molecular weight - 2000) in parallel arrays [l09], Figs. 12A, 12Ba). As carbonisation proceeds, higher molecular weight hydrocarbons are formed by condensation and these are incorporated into the mesophase. With growth and coalescence of the mesophase, there is eventually a phase inversion when the coalesced mesophase becomes the dominant phase, Fig. 12Bb). Condensation and polymerisation proceed as the carbonisation temperature is raised until eventually the material solidifies into a semi-coke, Fig. 12Bc). The relics of the coalesced mesophase in the semi-coke have complex anisotropic structures that contains disclinations that can be used to deduce their molecular orientation [110]. The essential point is that the coalesced mesophase generates a pre- graphitic structure that can be developed into graphite on high temperature heat- treatment. The carbonisation of polyacenaphthylene, Fig. 10, is an example of a process that involves the formation of mesophase. By contrast, the carbonisation of precursors of non-graphitising carbons does not involve the formation of mesophase. Either, the non-graphitising precursor is extensively cross-linked, as in the case of phenolic resins, or cross-linking reactions occur in the early stages of carbonisation. 25 Fig. 12. A, Schematic representation of parallel arrays of polynuclear aromatic hydrocarbon molecules in a mesophase sphere. B, a) isolated mesophase spheres in an isotropic fluid pitch matrix; b) coalescence of mesophase; c) structure of semi-coke after phase inversion and solidification. Carbon layer planes in low temperature carbons are highly defective and they have heteroelements bound to their edges. Heat treatment of graphitising carbons brings about an improvement in microstructural order, elimination of heteroelements and eventually the development of a three-dimensional graphite crystal structure. Abundant X-ray studies of a wide range of graphitising carbons, Fig. 13, show that the stack width, La, for graphitising carbons increases almost exponentially with heat-treatment temperature, HTT, from -5 nm at HTT -1500 "C to -35-65 nm at HTT = 2800 "C; the stack thickness, L,, increases in a similar fashion from -2-6 nm at HTT -1400 "C to -15-60 nm at HTT = 3000 "C [112]. At the same time the interlayer spacing d decreases from the turbostratic value, 0.344 nm, towards the value for graphte, 0.335 nm. By contrast, the stack dimensions of non-graphitising carbons increase only slightly with HTT accompanied by small decreases in interlayer spacings [ 104, 1 131. 26 30 Fig. 13. Increase in stack width parameter, La, with heat treatment temperature, HTT, for some graphitising cokes, [Adapted from 1121. 6.4. Electron microscopical studies of engineering carbons The microstructural model for disordered carbons has been greatly elaborated following the application of high resolution transmission electron microscopy. The early work by Ban [ 1 141 and Jenkins et a1 [ 1 151 lead to the development of the ribbon model for glassy carbon, Fig. 14, which envisages the non-graphitic structure as a network of twisted and folded carbon layer planes. Interestingly, this microstructural model for carbons was perhaps the first to depart from the flat graphite layer model and introduce concepts of curvature that can now be rationalised using microstructural elements borrowed from Fullerenes and nanotubes. However, the Jenkins model is essentially intuitive and later workers [ 1 161 have cautioned against the use of such simplistic readings of electron microscopical images. Perhaps the most elaborate and extensive electron microscopical studies of carbonaceous materials were carried out by Agnes Oberlin and her group [ 1 161 who showed that a great deal of microstructural information on carbons can be obtained using a combination of selected area diffraction and dark field and light field imaging. For all carbons, Oberlin defines a basic structural unit, BSU, as a parallel stack of two to four layer planes each containing less than 10-20 aromatic rings. A related concept is local molecular ordering, LMO, which consists of an array of BSU with a near-common orientation, Fig. 15. In non- 27 graphitising carbons there is a high degree of misorientation of BSU so that LMO is small or non-existent, whereas in graphitising carbons the misorientation between adjacent BSU is small and consequently there is extensive LMO extenlng to the order of microns. - L.& Fig. 14. The ribbon model for the microstructure of a glassy carbon [ 1 151. Fig. 15. A schematic model illustrating the concepts of basic structural unit, BSU, and local molecular ordering, LMO [e.g., 1161. 28 The Oberlin group have elaborated the mechanism of graphitisation as shown in Fig. 16. Earlier work on graphitisation mechanisms has been reviewed on several occasions [117-1191. In stage 1, up to HTT = 1000 "C, the carbons contains flat BSU with a high degree of misorientation. Between 1000 and 1500 "C (stage 2) the BSU grow &cker and columnar arrays of BSU (like stacks of coins) develop with misoriented BSU trapped between them. In stage 3, between HTT = 1500 to 2000 "C the misorientation between the columns of BSU decreases, so that extensive, but distorted, carbon layer planes can form by coalescence of adjacent BSU. The fmal stage, above HTI' = 2000 "C, involves the annealing out of defects within the distorted carbon layer planes, so that perfect flat carbon layer planes are produced that allow the formation and growth of graphite crystallites. - @ flat layers f STAGES 44 torted layers rted columns Fig. 16. The mechanism of graphitisation (Reprinted from [ 1161 by courtesy of Marcel Dekker Inc. 7 Concluding Remarks The majority of engineering carbon materials have more-or-less disordered microstructures that are based on that of graphite and in which, therefore, sp2 carbon bonding is dominant. The degree of graphitic order varies widely from very low values for glassy carbons derived from polymer resins [ 1 131 to highly graphitic microstructures, e.g., in HOPG [14]. Engineering carbons are also 29 manufactured in an astounding range of physical forms: powders, granules, beads, films, foams, fibers, textiles, composites, and monoliths, and in sizes that range from sub-micron carbon aerogels to arc furnace electrodes with dimensions of several metres. The steady development of graphtic carbon materials over many years has been complemented by recent developments in amorphous carbon films with mixed sp' and sp3 bonding and, especially rapid developments in CVD diamond films with sp3 carbon bonds. However, the discoveries of Fullerenes and related materials represent the most exciting new developments in carbon science. Indeed, these discoveries have resulted in a paradigm shift in om perception of chemical bonding and microstructure in carbon materials and have helped to stimulate further advances in various areas of carbon science and technology that are discussed elsewhere in this book. 8 Acknowledgements I thank Marcel Dekker Inc. for permission to reprint Figures 10 and 16. 9 References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. Atkins, P.W., Physical Chemistry, 5" Edition, Oxford 'university Press, Oxford, 1994, Chapter 14. Handbook of Chemistry and Physics, 74th edition, ed., D.R. Lide, CRC Press, London., 1994, pp 9-2 to 9-5. Davies, G., Diamond, Hilger, Bristol, 1984. Wilks, J. and Wilks, E., Properties and Applications ofDiamonds, Buttenvorth- Heinemann, Oxford, 199 1. The Properties of Natural and Svnthetic Diamond, ed. J.E. Field, Academic Press, London, 1992. Bundy, F.P., Hall, H.T., Strong, H.M. and Wenthof, R.H., Nature (London), 1956, 176, 51. 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T., Proc Roy SOC.(London) A 4 42, 1993, 129 Prassides, K., Kroto, H.W., Taylor, R., Walton, D.R.M., David, W.I.F., Tomkinson, J., Haddon, R.C., Rosseinsky, M.J and Muphy, D.W., Carbon, 19 92, 30, 127 7 Kroto, H.W., Nature (London) 1987, 329 , 529 Schmalz, T.G., Seitz, W.A., Klein, D.J and Hite, G.E J Amer Chem SOC., 1988,110, 113 Diederich F and Whetten R.L Acc Chem Res., 19 92, 25 , 119 Saito, S., Sawada, S.I.,... compounds (see 52. 6 .2) further spurred research activity in this field of (26 0-relatedmaterials Regarding a historical perspective on carbon nanotubes, very small diameter (less than 10 nm) carbon filaments were observed in the 1970’s through synthesis of vapor grown carbon fibers prepared by the decomposition of benzene at 1100°C in the presence of Fe catalyst particles of -10 nm diameter [ l l , 121 However, . M., J. Mater. Sci., 1988, 23 , 327 2. 16. 17. 18. 19. 20 . 21 . 22 . 23 . 24 . 25 . 26 . 27 . 28 . 29 . 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. Coles, B.F., Hitchcock,. activated carbons premium cokes, carbon fibers, binder and matrix carbons' mesocarbon microbeads, carbon fibers PAN-based carbon fibers glassy carbons, binder and matrix carbons". pyrocarbons, carbon blacks, vapour grown carbon fibres, matrix carbonn delayed coke, calcined coke needle coke, carbon fibers, binder and matrix carbon& quot; mesocarbon microbeads, carbon