Graphite

Graphite (named by Abraham Gottlob Werner in 1789, from the Greek "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.

Occurrence
Associated minerals include: quartz, calcite, micas, iron meteorites and tourmalines. Notable occurrences include New York and Texas in the USA, Russia, Mexico, Greenland, and India.

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.

Graphite ore[edit]
Detailed properties and uses
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.

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.

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.

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. Graphite is an excellent computer fan lubricant. Two or three drops into the ball-bearings of the fan will greatly improve the efficiency of the fan and quieten it.

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 3000C) material, used in reentry shields for missile nosecones, solid rocket engines, high temperature reactors, brake shoes and electric motor brushes.

Pyrolytic graphite has excellent biocompatibility, and is used in medical equipment to prevent adhesion of blood clots.

Carbon fiber and carbon nanotubes are also used to reinforce plastics, and in heat-resistant composites such as reinforced carbon-carbon (RCC)). They have also successfully reinforced concrete.

The mechanical properties of carbon fiber composites and grey cast iron are strongly influenced by the role of graphite in these materials.

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.

The paintings are the excellent portrayal of the events and scenes that we see around us. The painters are the best cameras of the world. They reproduce many different types of pictures. They even draw imaginary pictures that do not exist in this world. We tend to use both thinned oil paints and dense oil paints. Masterpieces can be dyed more than once, but each time it may be different from the existing paintings.h

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