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Graphite (/rfat/) 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 a large scale (1.3 million metric tons per year in 2022) for uses in pencils, lubricants, and electrodes. Under high pressures and temperatures it converts to diamond. It is a good (but not excellent) conductor of both heat[6] and electricity.[7]

Synthetic graphite (or artificial graphite) is a material consisting of graphitic carbon which has been obtained by graphitizing of non-graphitic carbon, by chemical vapor deposition from hydrocarbons at temperatures above 2500 K, by decomposition of thermally unstable carbides or by crystallizing from metal melts supersaturated with carbon.[11]

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Graphite occurs in metamorphic rocks as a result of the reduction of sedimentary carbon compounds during metamorphism. It also occurs in igneous rocks and in meteorites.[5] Minerals associated with graphite include quartz, calcite, micas and tourmaline. The principal export sources of mined graphite are in order of tonnage: China, Mexico, Canada, Brazil, and Madagascar.[12]

In meteorites, graphite occurs with troilite and silicate minerals.[5] Small graphitic crystals in meteoritic iron are called cliftonite.[9] Some microscopic grains have distinctive isotopic compositions, indicating that they were formed before the Solar System.[13] They are one of about 12 known types of minerals that predate the Solar System and have also been detected in molecular clouds. These minerals were formed in the ejecta when supernovae exploded or low to intermediate-sized stars expelled their outer envelopes late in their lives. Graphite may be the second or third oldest mineral in the Universe.[14][15]

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.[21] The alpha form can be converted to the beta form through mechanical treatment and the beta form reverts to the alpha form when it is heated above 1300 C.[22]

The equilibrium pressure and temperature conditions for a transition between graphite and diamond is well established theoretically and experimentally. The pressure changes linearly between 1.7 GPa at 0 K and 12 GPa at 5000 K (the diamond/graphite/liquid triple point).[23][24]However, the phases have a wide region about this line where they can coexist. At normal temperature and pressure, 20 C (293 K) and 1 standard atmosphere (0.10 MPa), the stable phase of carbon is graphite, but diamond is metastable and its rate of conversion to graphite is negligible.[25] However, at temperatures above about 4500 K, diamond rapidly converts to graphite. Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at 2000 K, a pressure of 35 GPa is needed.[23]

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.[26]

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[27] 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.[28] 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.[29] 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. This hypothesis has been refuted by studies showing that air and water are not absorbed.[30] Recent studies suggest that an effect called superlubricity can also account for graphite's lubricating properties. The use of graphite is limited by its tendency to facilitate pitting corrosion in some stainless steel,[31][32] 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,[33] and discouraged its use in aluminium-containing automatic weapons.[34] Even graphite pencil marks on aluminium parts may facilitate corrosion.[35] 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.

During the 19th century, graphite's uses greatly expanded to include stove polish, lubricants, paints, crucibles, foundry facings, and pencils, a major factor in the expansion of educational tools during the first great rise of education for the masses. The British Empire controlled most of the world's production (especially from Ceylon), but production from Austrian, German, and American deposits expanded by mid-century. For example, the Dixon Crucible Company of Jersey City, New Jersey, founded by Joseph Dixon and partner Orestes Cleveland in 1845, opened mines in the Lake Ticonderoga district of New York, built a processing plant there, and a factory to manufacture pencils, crucibles and other products in New Jersey, described in the Engineering & Mining Journal 21 December 1878. The Dixon pencil is still in production.[41]

Historically, graphite was called black lead or plumbago.[9][44] Plumbago was commonly used in its massive mineral form. Both of these names arise from confusion with the similar-appearing lead ores, particularly galena. The Latin word for lead, plumbum, gave its name to the English term for this grey metallic-sheened mineral and even to the leadworts or plumbagos, plants with flowers that resemble this colour.

Abraham Gottlob Werner coined the name graphite ("writing stone") in 1789. He attempted to clear up the confusion between molybdena, plumbago and black lead after Carl Wilhelm Scheele in 1778 proved that these were at least three different minerals. Scheele's analysis showed that the chemical compounds molybdenum sulfide (molybdenite), lead(II) sulfide (galena) and graphite were three different soft black minerals.[45][46][47]

The use of graphite as a refractory (heat-resistant) material began before 1900 with graphite crucibles used to hold molten metal; this is now a minor part of refractories. In the mid-1980s, the carbon-magnesite brick became important, and a bit later the alumina-graphite shape. As of 2017[update] the order of importance is: alumina-graphite shapes, carbon-magnesite brick, Monolithics (gunning and ramming mixes), and then crucibles.

Crucibles began using very large flake graphite, and carbon-magnesite bricks requiring not quite so large flake graphite; for these and others there is now much more flexibility in the size of flake required, and amorphous graphite is no longer restricted to low-end refractories. Alumina-graphite shapes are used as continuous casting ware, such as nozzles and troughs, to convey the molten steel from ladle to mold, and carbon magnesite bricks line steel converters and electric-arc furnaces to withstand extreme temperatures. Graphite blocks are also used in parts of blast furnace linings[49] where the high thermal conductivity of the graphite is critical to ensuring adequate cooling of the bottom and hearth of the furnace.[50] High-purity monolithics are often used as a continuous furnace lining instead of carbon-magnesite bricks.

Natural graphite in steelmaking mostly goes into raising the carbon content in molten steel; it can also serve to lubricate the dies used to extrude hot steel. Carbon additives face competitive pricing from alternatives such as synthetic graphite powder, petroleum coke, and other forms of carbon. A carbon raiser is added to increase the carbon content of the steel to a specified level. An estimate based on USGS's graphite consumption statistics indicates that steelmakers in the US used 10,500 tonnes in this fashion in 2005.[48]

Natural amorphous and fine flake graphite are used in brake linings or brake shoes for heavier (nonautomotive) vehicles, and became important with the need to substitute for asbestos. This use has been important for quite some time, but nonasbestos organic (NAO) compositions are beginning to reduce graphite's market share. A brake-lining industry shake-out with some plant closures has not been beneficial, nor has an indifferent automotive market. According to the USGS, US natural graphite consumption in brake linings was 6,510 tonnes in 2005.[48]

A foundry-facing mold wash is a water-based paint of amorphous or fine flake graphite. Painting the inside of a mold with it and letting it dry leaves a fine graphite coat that will ease the separation of the object cast after the hot metal has cooled. Graphite lubricants are specialty items for use at very high or very low temperatures, as forging die lubricant, an antiseize agent, a gear lubricant for mining machinery, and to lubricate locks. Having low-grit graphite, or even better, no-grit graphite (ultra high purity), is highly desirable. It can be used as a dry powder, in water or oil, or as colloidal graphite (a permanent suspension in a liquid). An estimate based on USGS graphite consumption statistics indicates that 2,200 tonnes were used in this fashion in 2005.[48] Metal can also be impregnated into graphite to create a self-lubricating alloy for application in extreme conditions, such as bearings for machines exposed to high or low temperatures.[55] e24fc04721