Graphite

Graphite (named by Abraham Gottlob Werner in 1789, from the Greek γραφειν: "to draw/write", for its use in pencils) is one of the allotropes of carbon. Unlike diamond, graphite is a conductor, and can be used, for instance, as the material in the electrodes of an electrical arc lamp.

Detailed properties and uses

Each carbon atom is covalently bonded to three other surrounding carbon atoms. The flat sheets of carbon atoms are bonded into hexagonal structures. These exist in layers, which are not covalently connected to the surrounding layers.

Related Topics:
Carbon - Atom - Covalently - Bonded - Hexagonal structures

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The unit cell dimensions are a = b = 245.6 picometres, c = 669.4 pm. The carbon-carbon bond length in the bulk form is 141.8 pm, and the interlayer spacing is c/2 = 334.7 pm.

Related Topics:
Unit cell - Picometre - Bond length

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Each carbon atom possesses an sp2 orbital hybridisation. The pi orbital electrons delocalized across the hexagonal atomic sheets of carbon contribute the graphite's conductivity. In an oriented piece of graphite, conductivity parallel to these sheets is greater than that perpendicular to these sheets.

Related Topics:
Orbital hybridisation - Pi orbital electrons - Conductivity

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The acoustic and thermal properties of graphite are also highly anisotropic, since phonons propagate very quickly along the tightly-bound planes, but are slower to travel from one plane to another.

Related Topics:
Acoustic - Thermal - Phonons

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Graphite is able to conduct electricity due to the unpaired fourth electron in each carbon atom. This unpaired 4th electron forms delocalised planes above and below the planes of the carbon atoms. These electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted within the plane of the layers.

Related Topics:
Conduct - Electricity - Electron - Delocalised

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The loose coupling among the sheets in graphite contributes to another industrially important property -- graphite powder is used as a dry lubricant. Recent studies suggest that an effect called superlubricity can also account for this effect. When a large number of crystallographic defects bind these planes together, graphite loses this property and becomes known as pyrolytic carbon, a useful material in blood-contacting implants such as prosthetic heart valves.

Related Topics:
Loose coupling - Lubricant - Superlubricity - Pyrolytic carbon - Prosthetic - Heart valve

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Natural and crystalline graphites are not often used in pure form as structural materials due to their shear-planes, brittleness and inconsistent mechanical properties.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In its pure glassy (isotropic) synthetic forms, pyrolytic graphite and carbon fiber graphite is an extremely strong, heat-resistant (to 3000 °C) material, used in reentry shields for missile nosecones, solid rocket engines, high temperature reactors, brake shoes and electric motor brushes.

Related Topics:
Pyrolytic graphite - Carbon fiber - Solid rocket - High temperature reactors - Brake - Electric motor

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Carbon fiber and carbon nanotubes are also used to graphite reinforced plastics, and in heat-resistant composites such as reinforced carbon-carbon (RCC)). They have also successfully reinforced concrete. The mechanical properties of carbon fiber graphite-reinforced plastic composites and grey cast iron are strongly influenced by the role of graphite in these materials.

Related Topics:
Carbon fiber - Carbon nanotube - Graphite reinforced plastic - Reinforced carbon-carbon - Reinforced concrete - Cast iron

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Graphite also finds use as a matrix and moderator within nuclear reactors. Its low neutron cross section also recommends it for use in proposed fusion reactors. Care must be taken that reactor-grade graphite is free of neutron absorbing materials such as boron, widely used as the seed electrode in commercial graphite deposition systems-- this caused the failure of the German's World War II graphite-based nuclear reactors. Since they could not isolate the difficulty they were forced to use far more expensive heavy water moderators.

Related Topics:
Moderator - Nuclear reactor - Neutron - Cross section - Fusion - Boron - World War II - Heavy water

~ ~ ~ ~ ~ ~ ~ ~ ~ ~