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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 an electrical conductor, and can be used, for instance, as the material in the electrodes of an electrical arc lamp. Graphite holds the distinction of being the most stable form of solid carbon ever discovered. It may be considered to be the highest grade of coal, just above anthracite, although it is not normally used as fuel because it is hard to ignite.
Occurrence
Structure
Detailed properties The acoustic and thermal properties of graphite are highly anisotropic, since phonons propagate very quickly along the tightly-bound planes, but are slower to travel from one plane to another. Graphite can conduct electricity due to the vast electron delocalization within the carbon layers. These electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted within the plane of the layers. Graphite powder is used as a dry lubricant, although it might be thought that this industrially important property is due entirely to the loose interlamellar coupling between sheets in the structure, in fact in a vacuum environment (such as in technologies for use in space), graphite was found to be a very poor lubricant, leading to the discovery that in fact lubrication is due to adsorbed air and water between the layers, unlike other layered dry lubricants such as molybdenum disulfide. Recent studies suggest that an effect called superlubricity can also account for this effect. The use of graphite is also limited by its tendency to facilitate pitting corrosion in some stainless steels, and to promote galvanic corrosion between dissimilar metals. It is also corrosive to aluminium in presence of moisture. The US Air Force banned its use as a lubricant in aircraft * and its use for aluminium-containing automatic weapons is discouraged as well.* Even graphite pencil marks on aluminium parts may facilitate corrosion. * A structural analog of graphite, hexagonal boron nitride, is used as a high-temperature lubricant as well, and due to its similarity to graphite is sometimes called white graphite. When a large number of crystallographic defects bind these planes together, graphite loses its lubrication properties and becomes what is known as pyrolytic carbon, a useful material in blood-contacting implants such as prosthetic heart valves. Natural and crystalline graphites are not often used in pure form as structural materials due to their shear-planes, brittleness and inconsistent mechanical properties. Uses Graphite is the substance used as the writing material (lead) in common pencils. In its pure glassy (isotropic) synthetic forms, pyrolytic carbon and carbon fiber, graphite is an extremely strong, heat-resistant (to 3000°C) material, used in reentry shields for missile nosecones, solid rocket engines, pebble bed reactors, brake shoes, electric motor brushes and as electrodes in EDM electrical discharge machines. Intumescent or expandable graphites are used in firestops, particularly plastic pipes and gaskets, fitted around the perimeter of a fire door. During a fire, the graphite expands and chars to resist fire penetration and reduce the likelihood of the spread of fire and fumes. A typical start expansion temperature (SET) is between 150° and 300° Celsius. Carbon fiber and carbon nanotubes are also used in carbon fiber reinforced plastics, and in heat-resistant composites such as reinforced carbon-carbon (RCC). Products made from carbon fiber graphite composites include fishing rods, golf clubs, and bicycle frames, and have been successfully employed in 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. Graphite also finds use as a matrix and neutron 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 Germans' 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. Graphite used for nuclear reactors is often referred to as Nuclear Graphite. Graphite has been used in at least three radar absorbent materials. It was mixed with rubber in Sumpf and Schornsteinfeger, which were used on U-boat snorkels to reduce their radar cross section. It was also used in tiles on early F-117 Nighthawks. Media See also Reference | ||||||||||||
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