5 Carbon Nanoparticles

In addition to diamond and amorphous films, nanostructural forms of carbon may also be formed from the vapour phase. Here, stabilisation is achieved by the formation of closed shell structures that obviate the need for surface heteroatoms to stabilise dangling bonds, as is the case for bulk crystals of diamond and graphite. The now-classical example of closed-shell stabilisation of carbon nanostructures is the formation of C^ molecules and other Fullerenes by electric arc evaporation of graphite [38] (Section 2.4).

5.1 Carbon nanotubes

Carbon nanotubes are discussed in more detail in Chapter 2 and so only a brief account of them is given here. In early studies of Fullerenes [85] it was observed that carbon nanotubes, also called carbon tubules, were formed at the cathode in the electric arc apparatus. Carbon nanotubes may be viewed as a cylindrical structure formed from graphene sheets and closed by Fullerenoid end-caps, Fig. 8A. There are single wall carbon nanotubes and multiwall carbon nanotubes, consisting of several nested co-axial single wall tubules. Typical dimensions of multiwall nanotubes are: outer diameter, 2-20 nm, inner diameter, 1-3 nm, lengths ~ 1 |im. The intertubular distance is 340 pm, which is slightly larger than the interplanar distance in graphite. Carbon nanotubes can also be grown from the vapour phase [86]. Whether produced from the arc method or from the vapour phase, carbon nanotubes are usually mixed with other forms of carbon; however, methods for producing substantial quantities of carbon nanotubes in bundle form have been published [87, 88].

Armchair Zig-Zag SpiraJ

Fig. 8. A, Structure of a single wall nanotubc; B, schematic illustration of arm-chair, zigzag and spiral forms of single wall nanotubes; the arrows denote the tubule axis.

Armchair Zig-Zag SpiraJ

Fig. 8. A, Structure of a single wall nanotubc; B, schematic illustration of arm-chair, zigzag and spiral forms of single wall nanotubes; the arrows denote the tubule axis.

The structure of carbon nanotubes depends upon the orientation of the hexagons in the cylinder with respect to the tubule axis. The limiting orientations are zigzag and arm chair forms, Fig. 8B. In between there are a number of chiral forms in which the carbon hexagons are oriented along a screw axis, Fig. 8B. The formal topology of these nanotube structures has been described [89]. Carbon nanotubes have attracted a lot of interest because they are essentially one-dimensional periodic structures with electronic properties (metallic or semiconducting) that depend upon their diameter and chirality [90,91], (Note. After this section was written a book devoted to carbon nanotubes has been published [92], see also [58].)

5.2 Carbon nanocones and multiwall carbon spheres

Carbon nanocones have been observed as free-standing structures among the products of the carbon arc method for producing Fullerenes and carbon nanotubes [93, 94], The hemispherical end-caps on carbon nanotubes contain six pentagons, i.e., half of the number of pentagons in C(l0. A conical nanostructure occurs when there are less than six pentagons in the end cap. For example, an end-cap containing one pentagon has a cone angle of 19.2° and for an end cap with three pentagons the cone angle is 60°. [94], Carbon nanocones with geometries conforming to these rules have been observed using STM [94] and carbon nanotubes with conical end caps (cone angle ~20 °) have been observed using transmission electron microscopy, TEM [95]. Fig. 9A shows a molecular model for the apex of a carbon nanocone incorporating one pentagon and with a cone angle of 19.2° [94].

armcha>f armcha>f

ilgrag

Fig. 9. A, Model for the apex of a carbon nanocone with a cone angle of 19.2° [94]; B, polyhedral and spherical forms of a multiwall carbon particle formed from C^, C240 and CS40 [98], ilgrag

Fig. 9. A, Model for the apex of a carbon nanocone with a cone angle of 19.2° [94]; B, polyhedral and spherical forms of a multiwall carbon particle formed from C^, C240 and CS40 [98],

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 [96-98]. These have been called carbon onions or 'Buckyonions'. The shells have external diameters up to ~30 ran and hollow centres with diameters similar to that of the C60 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 C240 about Qo 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 C540 about C240 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 C60, C240, and C540 [98],

6 Engineering Carbons

6.1 Introduction

There are many applications for diamonds and related materials, e.g., diamondlike 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-

important carbons obtained from solid, liquid and gaseous organic precursors is presented in Table 5.

Table 5. Precursors for engineering carbons

Primary precursor

Secondary precursor

Example products

Hydrocarbon gases

Petroleum

Coals

Polymers pyrocarbons, carbon blacks, vapour grown carbon fibres, matrix carbon3 delayed coke, calcined coke needle coke, carbon fibers, binder and matrix carbon2

mesophase pitch mesocarbon microbeads, carbon fibers semi-coke, calcined coke activated carbons premium cokes, carbon fibers, binder and matrix carbons"

mesophase pitch mesocarbon microbeads, carbon fibers polyacrylonitrile PAN-based carbon fibers phenolic and glassy carbons, binder and matrix carbons" furan resins polyimides petroleum pitch coal chars coal tar pitch

Biomass graphite films and monoliths activated carbons a. precursor for binder m polygranular carbons and graphites, precursor tor 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 [100], 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 (primary 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 O, 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 followed the initial application of X-ray diffraction and subsequent application of electron and neutron diffraction, and high resolution electron microscopy, supplemented by a wide range of other analytical techniques.

Kig. 10. Mcchanism of carbonisation of acenaphthylcne [100]. I, acenaphthylene; II, polyaccnaphthylenc; III, biaccnaphthylidene; IV, fluorocyclcnc; V, dinaphthylcncbutadiene; VI, decacyclene; VII, zethrenc. Reprinted from [100] courtesy of Marcel Dekker Inc.

6.2 X-ray studies of engineering carbons

In the 1930s Hoffman and Wilm [101] found only (hkO) graphite 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 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 [1021 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 Lc 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 Lc = 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 Lc values. High resolution electron microscopic studies lend some support to this view (see Section 6.4).

A notable advance was made by Franklin [103] 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, [104] Franklin classified low temperature carbons into graphitising carbons which develop three-dimensional graphitic 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 Noda and co-workers [105 ] obtained radial distribution functions 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

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 radial distribution functions can be satisfactorily indexed to a hexagonal arrays of carbon atoms. A similar conclusion was reached in a recent neutron diffraction study of activated carbons by Gardner et al [108],

Fig. 11. Schematic models for the structure of: A, graphitising carbons, and B, non-graphitising carbons [104].

Fig. 11. Schematic models for the structure of: 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 [109], 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.

Fig. 12. A, Schematic representation of parallel arrays of polynuclear aromatic hydrocarbon molccules in a mesophase sphere. B, a) isolated mesophasc spheres in an isotropic fluid pitch matrix; b) coalescence of mesophase; c) structure of scmi-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, Lc, 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 graphite, 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, 113].

0 0

Post a comment