Graphite

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is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large scale for uses in pencils, lubricants, and electrodes. Under high pressures and temperatures it converts to diamond. It is a good conductor of both heat and electricity.

Types and VarietiesNatural GraphiteCrystalline: small flakes of graphite occurs as isolated, flat, plate-like particles with hexagonal edges if unbroken.

Amorphous graphite: very fine flake graphite.

Lump graphit: occurs in fissure veins or fractures and appears as massive platy intergrowths of fibrous or acicular crystalline aggregates, and is probably hydrothermal in origin.

Highly ordered pyrolytic graphite refers to graphite with an angular spread between the graphite sheets of less than 1°.

Graphite fiber is sometimes used to refer to carbon fibers or carbon fiber-reinforced polymer.Synthetic Graphite

Synthetic graphite is a material consisting of graphitic carbon which has been obtained by graphitizing of non-graphitic carbon, by CVD from hydrocarbons at temperatures above 2500 K, by decomposition of thermally unstable carbides or by crystallizing from metal melts supersaturated with carbon

Graphite StructureGraphite consists of sheets of trigonal planar carbon. The individual layers are called graphene.In each layer, the carbon atoms are arranged in a honeycomb lattice with a bond length of 0.142 nm, and the distance between planes is 0.335 nm.Bonding between layers is relatively weak van der Waals bonds and are often occupied by gases, which allows the graphene-like layers to be easily separated and to glide past each other.Electrical conductivity perpendicular to the layers is consequently about 1000 times lower.The two forms of graphite are called alpha (hexagonal) and beta (rhombohedral). Their properties are very similar.They differ in terms of the stacking of the graphene layers: stacking in alpha graphite is ABA, as opposed to ABC stacking in energetically less stable and less common beta graphite.Graphite Properties

The acoustic and thermal properties of graphite are highly anisotropic, since phonons propagate quickly along the tightly bound planes, but are slower to travel from one plane to another. Graphite’s high thermal stability and electrical and thermal conductivity facilitate its widespread use as electrodes and refractories in high temperature material processing applications. However, in oxygen-containing atmospheres graphite readily oxidizes to form carbon dioxide at temperatures of 700 °C and above. Graphite is an electrical conductor, hence useful in such applications as arc lamp electrodes. It can conduct electricity due to the vast electron delocalization within the carbon layers (a phenomenon called aromaticity). These valence electrons are free to move, so are able to conduct electricity. However, the electricity is primarily conducted within the plane of the layers. The conductive properties of powdered graphite allow its use as pressure sensor in carbon microphones.



Graphite and graphite powder are valued in industrial applications for their self-lubricating and dry lubricating properties. There is a common belief that graphite’s lubricating properties are solely due to the loose interlamellar coupling between sheets in the structure. However, it has been shown that in a vacuum environment (such as in technologies for use in space), graphite degrades as a lubricant, due to the hypoxic conditions. This observation led to the hypothesis that the lubrication is due to the presence of fluids between the layers, such as air and water, which are naturally adsorbed from the environment. 



The use of graphite is limited by its tendency to facilitate pitting corrosion in some stainless steel, and to promote galvanic corrosion between dissimilar metals (due to its electrical conductivity). It is also corrosive to aluminium in the presence of moisture. For this reason, the US Air Force banned its use as a lubricant in aluminium aircraft, and discouraged its use in aluminium-containing automatic weapons. Even graphite pencil marks on aluminium parts may facilitate corrosion. Another high-temperature lubricant, hexagonal boron nitride, has the same molecular structure as graphite. It is sometimes called white graphite, due to its similar properties.

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