GRAPHITE

'Graphite' (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): "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.
The name "graphite" is also sometimes used to refer to carbon fibre or carbon fibre reinforced plastic.

Contents

Occurrence

Structure

Detailed properties

History

Uses

Graphite milling

Media

See also

References

External links

Occurrence

Associated minerals include: quartz, calcite, micas, iron meteorites, and tourmalines. In 2005, China was the top producer of graphite with about 80% world share followed by India and Brazil.
Other characteristics: thin flakes are flexible but inelastic, mineral can leave black marks on hands and paper, conducts electricity, and displays superlubricity. Best field indicators are softness, luster, density and streak.
According to the USGS, world production of natural graphite in 2005 was 1.05 million tonnes, of which the following major exporters produced: China produced 720,000 tonnes, Brazil 76,500 tonnes, Canada 30,000 tonnes, and Mexico (amorphous) 11,143 tonnes. In addition, there are two specialist producers: Sri Lanka produced 3,000 tonnes in 2005 of lump or vein graphite, and Madagascar produced 15,000 tonnes, a large portion of it "crucible grade" or very large flake flake graphite. Some other producers produce very small amounts of "crucible grade".
According to the USGS, U.S. (synthetic) graphite electrode production in 2005 was 146,000 tonnes valued at $391 million, and high-modulus graphite (carbon) fiber production in 2005 was 7,020 tonnes valued at $134 million.

Structure

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. Instead, different layers are connected together by weak forces called van der Waals forces much like those of mica.
The unit cell dimensions are ''a'' = ''b'' = 2.456 ångströms = 245.6 picometers, ''c'' = 6.694 Å = 669.4 pm. The carbon-carbon bond length in the bulk form is 1.418 Å (141.8 pm), and the interlayer spacing is ''c''/2 = 3.347 Å (334.7 pm).

Each carbon atom possesses an sp² orbital hybridisation. The pi orbital electrons delocalized across the hexagonal atomic sheets of carbon contribute to graphite's conductivity. In an oriented piece of graphite, conductivity parallel to these sheets is greater than that perpendicular to these sheets.
The bond between the atoms within a layer is stronger than the bond of diamond, but the force between two layers of graphite is weak. Therefore, layers of it can slip over each other making it soft.

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 and graphite powder, is valued for industrial applications, for its 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 is a very poor lubricant. This observation led to the discovery 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 molecular property is unlike other layered, dry lubricants such as molybdenum disulfide. 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 steels, and to promote galvanic corrosion between dissimilar metals (due to its electrical conductivity). It is also corrosive to aluminium in presence of moisture. For this reason, the US Air Force banned its use as a lubricant in aluminium aircraft [1], and discouraged its use in aluminium-containing automatic weapons [2]. Even graphite pencil marks on aluminium parts may facilitate corrosion [3]. Another high-temperature lubricant, hexagonal boron nitride, has the same molecular structural as graphite. It is sometimes called ''white graphite'', due to its similar properties.
When a large number of crystallographic defects binds these planes together, graphite loses its lubrication properties and becomes what is known as pyrolytic carbon. This material is useful for blood-contacting implants such as prosthetic heart valves. It is also highly diamagnetic, thus it will float in mid-air above a strong magnet.
Graphite forms intercalation compounds with some metals and small molecules. In these compounds, the host molecule or atom gets "sandwiched" between the graphite layers, resulting in compounds with variable stoichiometry. A prominent example of an intercalation compound is potassium graphite, denoted by the formula KC₈.
Natural and crystalline graphites are not often used in pure form as structural materials, due to their shear-planes, brittleness and inconsistent mechanical properties.

History

Some time prior to 1565 (some sources say as early as 1500), an enormous deposit of graphite was discovered at the site of Seathwaite Fell near Borrowdale, Cumbria, England. The locals found that it was very useful for marking sheep. This particular deposit of graphite was extremely pure and solid, and it could easily be sawn into sticks. This was and remains the only deposit of graphite ever found in this solid form. ^[1]

Uses

According to the USGS, U.S. consumption of natural graphite in 2004-05 averaged 43,800 tonnes in end uses such as refractories, steelmaking, expanded graphite, brake linings, foundry facings-lubricants. GAN (Graphite Advocate News) import-export statistics for 2006 and 2007 indicate the consumption will continue at that level unless steelmaking carbon raiser takes a drastic drop.
Refractories: This end-use begins before 1900 with the graphite crucible used to hold molten metal; this is now a minor part of refractories. In the mid1980s, the carbon-magnesite brick became important, and a bit later the alumina-graphite shape. Currently 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 brick requiring not quite so large flake graphite; for these and others there is now much more flexibility in 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 mould, and carbon magnesite bricks line steel convertors and electric arc furnaces to withstand extreme temperatures. High-purity monolithics are often used as a continuous furnace lining instead of the carbon-magnesite bricks. The U.S. and European refractories industry had a crisis in 2000-2003, with an indifferent market for steel underlying firm buyouts and many plant closings. Many of the plant closings resulted from the RHI acquisition of Harbison-Walker Refractories; some plants had their equipment auctioned off. Since much of the lost capacity was for carbon-magnesite brick, graphite consumption within refractories area moved towards alumina-graphite shapes and monolithics, and away from the brick. The major source of carbon-magnesite brick is now imports from China. According to the USGS, 2005 natural graphite consumption in refractories was 11,800 tonnes.
Steelmaking: Natural graphite in this end use mostly goes into carbon raising in molten steel, although it can be used to lubricate the dies used to extrude hot steel. Supplying carbon raiser is very competitive, therefore subject to cut-throat 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 the specified level. A GAN consumption estimate based on USGS graphite consumption statistics indicates that 10,500 tonnes was used in this end-use in 2005.
Expanded Graphite (including foil and packings): Expanded graphite is made by immersing natural flake graphite in a bath of chromic acid, then concentrated sulfuric acid, which forces the crystal lattice planes apart, thus expanding the graphite. The expanded graphite can be used to make graphite foil or used directly as "hot top" compound to insulate molten metal in a ladle or red-hot steel ingots and decrease heat loss, or as firestops fitted around a firedoor (During a fire, the graphite expands and chars to resist fire penetration and spread.), or to make high-performance gasket material for high-temperature use. After being made into graphite foil, the foil is used to make bipolar plates in fuel cells, heat sinks ifor laptop computers, and to make a foil laminate that can be used in valve packings or made into gaskets. Old-style packings are now a minor member of this grouping: fine flake graphite in oils or greases for uses requiring heat resistance. A GAN estimate of current natural graphite consumption in this end use is 8,000 tonnes.
Brake Linings: Natural amorphous and fine flake graphite are used in brake linings for heavier (nonautomotive) vehicles, and became important with the need to substitute for asbestos. This use has been important for quite some time, but newer organic compositions are beginning to cost graphite market share. A brake-lining industry shake-out with some plant closings has not helped either, nor has an indifferent automotive market. According to the USGS, natural graphite consumption in brake linings was 6,510 tonnes in 2005.
Natural graphite is the substance used as the marking 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 electrical discharge machines (EDM).
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. In this context, the term "(100%) graphite" is often loosely used to refer to a pure mixture of carbon reinforcement and resin, while the term "composite" is used for composite materials with additional ingredients.
Synthetic 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.

Graphite milling

One industrial form of processing the mineral graphite is through the milling process. In that process graphite is ground to a fine powder for use as a slurry in oil drilling; in zirconium silicate, sodium silicate and isopropyl alcohol coatings for foundry molds; and for calcium petroleum coke, which is used as a recarbonizer in the steel industry (Earth Metrics, 1989). Rough graphite is typically ground and packaged at a graphite mill; often the more complex formulations are also mixed and packaged at the mill facility. Environmental impacts from graphite mills consist of air pollution including fine particulate exposure of workers and also soil contamination from powder spillages leading to heavy metals contaminations of soil. Dust masks are normally worn by workers during the production process to avoid worker exposure to the fine airborne graphite and zircon silicate.

Media

See also

★ Carbon fiber

★ Intumescent

★ Passive fire protection

★ Pyrolytic graphite

★ Diamond

★ Lonsdaleite

★ Graphene

★ Carbon nanotube

★ Fullerene

★ Pencil

References

★ C.Michael Hogan, Marc Papineau et al., ''Phase I Environmental Site Assessment, Asbury Graphite Mill, 2426-2500 Kirkham Street, Oakland, California'', Earth Metrics report 10292.001, December 18, 1989

★ Klein, Cornelis and Cornelius S. Hurlbut, Jr. (1985) ''Manual of Mineralogy: after Dana'' 20^th ed. ISBN 0-471-80580-7

External links

★ The Graphite Page

★ Mineral galleries

★ Webmineral

★ Mindat w/ locations

★ giant covalent structures

★ USGS Minerals Yearbook: Graphite

★ html Graphite Advocate News (GAN)