Hexagonal graphite structure

Carbon nanotubes are rolled up tubes of carbon made up of a graphitic hexagonal structure. They can either be open or capped at the end by carbon pentagons which give all Fullerenes their closed curvature. 

Carbon blacks are synthetic materials which essentially contain carbon as the main element. The structure of carbon black is similar to graphite (hexagonal rings of carbon forming large sheets), but its structure is tridimensional and less ordered. The layers of carbon blacks are parallel to each other but not arranged in order, usually forming concentric inner layers (turbostratic structure). Some typical properties are density 1.7-1.9 g/cm pH of water suspension 2-8 primary particle size 14-250 nm oil absorption 50-300 g/100 g specific surface area 7-560 m /g. 

Carbon materials which have the closest-packed hexagonal structures are used as the negative electrode for lithium-ion batteries carbon atoms on the (0 0 2) plane are linked by conjugated bonds, and these planes (graphite planes) are layered. The layer interdistance is more than 3.35 A and lithium ions can be intercalated and dein-tercalated. As the potential of carbon materials with intercalated lithium ions is low,... 

Coke materials are generally made by heat-treatment of petroleum pitch or coal-tar pitch in an N2 atmosphere. Coke made from petroleum is called "petroleum coke" and that from coal is called "pitch coke". These materials have the closest-packed hexagonal structures. The crystallinity of coke materials is not so high as that of graphite. The crystallite size of coke along the c-axis (Lc) is small (about 10-20 A) and the interlayer distance (d value about 3.38-3.80 A) is large. 

Molybdenum disulhde (M0S2), graphite, hexagonal boron nitride, and boric acid are examples of lamella materials commonly applied as solid lubricants. The self-lubricating nature of the materials results from the lamella crystalline structure that can shear easily to provide low friction. Some of these materials used to be added to oils and greases in powder forms to enhance their lubricity. Attention has been shifted in recent years to the production and use of nanosize particles of M0S2, WS2, and graphite to be dispersed in liquid lubricants, which yields substantial decreases in friction and wear. 

Carbon atoms in graphite form a hexagonal structure. 

In graphite, a different form of carbon, atoms are bonded to each other in such a way that a hexagonal structure is formed in a plane. Each carbon atom is bonded to three other carbon atoms with an angle of 120° between the bonds. The bonding involves sp2 - sp2 hybrid overlap and this gives rise to layers. 

The process of forming boron-boron bonds is carried on further in aluminum boride, AIB, which has a very simple hexagonal structure, consisting of hexagonal layers of boron a,toms, like the layers of carbon atoms in graphite, with aluminum atoms in the spaces between the layers (Pig. 11-15). The B—B bond length is 1.73 A, corresponding to n = 0.66 that is, two valence electrons per boron atom are used in the B—B bonds, which are two-thirds bonds. 

Hexagonal graphite Rhombohedral graphite Turbostratic structure... 

FIGURE 7.2 The hexagonal and rhombohedral graphite crystal structures. 

Boron nitride (BN) can normally be prepared from the reaction of boric acid and urea or melamine. For example, the pyrolysis of MB can yield hexagonal BN. It is commonly referred to as white graphite because of its platy hexagonal structure similar to graphite. Under high pressure and at 1600°C, the hexagonal BN is converted to cubic BN, which has a diamond-like structure. 

Sub-steps, similar to those in Figure 9.7, have been observed with both methane and ethane (Bienfait, 1980, 1985). It has been possible to construct 2-D phase diagrams for several of these systems (Gay et al., 1986 Suzanne and Gay, 19%). LEED and neutron diffraction have provided information on the 2-D structures. For example, seven different 2-D phases have been reported for ethane on graphite over the temperature range 64-140 K. Thus, three solid commensurate phases were identified at temperatures <85 K, the S3 phase apparently having a close-packed hexagonal structure, with ofCjHj) = 0.157 nm2. 

Figure 8.1 The atomic arrangement of carbon atoms in the hexagonal structure of graphite.

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