Graphite

Graphite :

Graphite is a highly anisotropic solid. Structurally, its interplanar spacing (3.35 Å) is quite large compared to the in plane interatomic spacing (1.42 Å). Physically, its stiffness along the plane is quite large because of strong π bonds and in the perpendicular direction; it is weak because of the Van der Waal’s forces. The planes can be cleaved easily making graphite quite a soft material. Also electronically it is anisotropic, because of π and π^* bands overlap. This has metallic conductivity along the plane and semi-conducting perpendicular to the plane.

The anisotropy in conductivity is about 10³. The sp² hybridization forms a planar structure. It is made of strong fibers composed of series of stacked parallel layers as shown in Fig. It is black and lustrous, optically opaque, unaffected by weathering. Its greasy friction- resistant properties allow for applications in lubricating oils and greases, dry-film lubricants, batteries, conductive coatings, electrical brushes, carbon additives and paints. The graphite is the most stable form of pure carbon structures, stacked with an offset between neighbouring sheets of 1/2 of a lattice constant in an A-B-A-B layering. The sheets are separated by a larger distance (3.35 Ä) and interact weakly with a mixture of covalent and Van der Waals bonding that gives graphite strong lubricating properties, as pieces of graphite planes can glide along each other almost frictionless. The weak bonding of graphite in the perpendicular direction to the planes (Pierson, 1993) makes the synthesis of monocrystalline graphite almost impossible since the planes can shift with respect to each other, and they can also rotate. This gives the so-called turbostatic graphite, which has very crystalline planes, but with orientational disorder between them. The best commercial graphite is highly-oriented pyrolitic graphite (HOPG) which has a reduced disorder, but still remains polycrystalline. The theoretical interest in being able to play with true monocrystals of graphite is immense