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Graphite (C) - Classifications, Properties and Applications of Graphite

Miriam Doerr Martin Frommherz / Shutterstock

Carbon can be found in two natural crystalline allotropic forms, diamond and graphite, each with its own individual crystal structure and properties.

The term graphite comes from the Greek word “graphein”, meaning to write. The material is normally grayish-black in color, opaque and possesses a lustrous black sheen. It is a unique material because it exhibits the properties of both a metal and a non-metal. Although flexible, it is not elastic and has high thermal and electrical conductivity. It is chemically inert and extremely refractory. Since graphite exhibits low adsorption of neutrons and X-rays, it is highly useful in nuclear applications.

This rare combination of properties is because of its crystal structure. The carbon atoms are arranged hexagonally in a planar condensed ring system. The layers are piled parallel to each other. The atoms inside the rings are bonded covalently, while the layers are loosely connected by van der Waals forces. The high degree of anisotropy in graphite is due to the two kinds of bonding acting in various crystallographic directions.

For instance, the ability of graphite to form a solid film lubricant is the result of these two contrasting chemical bonds. Since weak Van der Waals forces dominate the bonding between each layer they can slide against one another, making graphite a perfect lubricant. Global graphite production was estimated to be about 602,000 tons in 2000, with China the biggest producer followed by India, Brazil, Mexico and the Czech Republic.

Graphite Classifications

Graphite can be classified into two primary types: natural and synthetic.

Natural Graphite

Natural graphite is a mineral made up of graphitic carbon. It differs significantly in crystallinity. A majority of the commercial (natural) graphites are mined and mostly contain other minerals. After graphite is mined, it often requires a substantial amount of mineral processing such as froth flotation to concentrate the graphite. Natural graphite is an exceptional conductor of electricity and heat, steady over a wide range of temperatures and an extremely refractory material with a high melting point (3650 °C).

Natural graphite is subdivided into three types:

  • Amorphous
  • Flake
  • High crystalline

Amorphous Graphite

Among the natural graphites, amorphous graphite is the least graphitic. However, the term “amorphous” is inaccurate as the material is still crystalline. Amorphous graphite can be found as tiny particles in beds of mesomorphic rocks such as slate, coal or shale deposits. The graphite content ranges from 25% to 85% based on the geological environment.

Amorphous graphite is extracted using conventional mining methods and can be found mainly in Mexico, South Korea, North Korea and Austria.

Flake Graphite

Flake graphite occurs in metamorphic rocks evenly distributed through the body of the ore or in concentrated lens-shaped pockets. Carbon concentrations range from 5% to 40%. Graphite flake can be found as a scaly or lamella form in specific metamorphic rocks such as gneisses, limestone and schists.

Flake graphite is extracted through froth flotation. “Floated” graphite has 80–90% graphite content. More than 98% of flake graphite is produced using chemical beneficiation processes. Flake graphite can be found in several places across the globe.

Crystalline Graphite

Crystalline vein graphite is said to originate from crude oil deposits that have changed to graphite through time, pressure and temperature. Vein graphite fissures usually measure between 1 cm and 1 m in thickness and are generally more than 90% pure.

Although this kind of graphite can be found worldwide, only Sri Lanka commercially mines it by conventional shaft or surface mining methods.

Synthetic Graphite

Synthetic graphite can be made from pitch and coke. Although not as crystalline as natural graphite, it tends to have higher purity. There are fundamentally two kinds of synthetic graphite. One is electrographite, pure carbon made from coal tar pitch and calcined petroleum coke in an electric furnace. The second is synthetic graphite, made by heating calcined petroleum pitch to 2800 °C. In general, synthetic graphite is lower in density and higher in porosity and electrical resistance. Its improved porosity makes it inappropriate for refractory applications.

Synthetic graphite comprises of mostly graphitic carbon that has been acquired by heat treatment of non-graphitic carbon, graphitization or by chemical vapor deposition from hydrocarbons at temperatures beyond 2100 K.

Key Properties

Property Commercial graphite
Bulk Density (g/cm3) 1.3-1.95
Porosity (%) 0.7-53
Modulus of Elasticity (GPa) 8-15
Compressive strength (MPa) 20-200
Flexural strength (MPa) 6.9-100
Coefficient of Thermal Expansion (x10-6 °C) 1.2-8.2
Thermal conductivity (W/m.K) 25-470
Specific heat capacity (J/kg.K) 710-830
Electrical resistivity (Ω.m) 5x10-6-30x10-6

Applications

Refractory Materials

Thanks to its chemical inertness and high-temperature stability, graphite is an ideal candidate for refractory material. It is used in the manufacture of refractory bricks and “Mag-carbon” refractory bricks (Mg-C). Graphite is also used to make ladles, crucibles and molds for holding molten metals. Furthermore, graphite is one of the most standard materials used in the manufacture of functional refractories for the continuous casting of steel. Here, graphite flake is blended with zirconia and alumina and then isostatically pressed to develop components such as subentry nozzles, stopper rods and ladle shrouds used not only for controlling the flow of molten steel, but also for guarding against oxidation. This kind of material could also be used as shielding for pyrometers.

In the manufacture of iron, graphite blocks are used to form a portion of the lining of the blast furnace. Its structural strength at high temperature, low thermal expansion, high thermal conductivity, thermal shock resistance and good chemical resistance are of utmost significance in this application.

The electrodes used in a number of electrical metallurgical furnaces are mass-produced from graphite, such as the electric arc furnaces used for processing steel.

Chemical Industry

In the chemical industry, graphite finds a number of high-temperature uses such as in the making of calcium carbide and phosphorus in arc furnaces. Graphite is used as an anode in certain aqueous electrolytic processes such as the manufacture of halogens (fluorine and chlorine).

Nuclear Industry

High-purity electrographite is used in large quantities for the manufacture of reflector components and moderator rods in nuclear reactors. Their suitability comes from their high thermal conductivity, low absorption of neutrons and high strength at higher temperatures.

Electrical Applications

Graphite’s main application as an electrical material is in the production of carbon brushes in electric motors. Here, the component’s performance and lifetime are very dependent on structure and grade.

Mechanical Applications

Graphite is extensively used as an engineering material across a range of applications such as piston rings, journal bearings, thrust bearings and vanes. Carbon-based seals are employed in the fuel pumps and shafts of a number of aircraft jet engines.

Other Applications

Amorphous graphite is used in:

  • Coatings
  • Metallurgy
  • Refractories
  • Pencil production
  • Paint production
  • Lubricants

Areas in which crystalline graphite is used include:

  • Powder metallurgy
  • Batteries
  • Grinding wheels
  • Lubricants

Flake graphite is used mainly in refractory applications mostly in secondary steel making. Besides this, it may also be used in powder metallurgy, pencils, lubricants and coatings.

A majority of the sources of natural graphite are also used in the fabrication of graphite foil.

Areas in which synthetic graphites are used include:

  • Carbon brushes
  • Batteries
  • Aerospace applications
  • Moderator rods in nuclear power plants
  • Graphite electrodes for electric arc furnaces for metallurgical processing

The increased porosity of synthetic graphite makes it inapplicable to refractory applications.

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