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

Chemical Formula

C

Topics Covered

Background

Graphite Classifications

Natural Graphite

Amorphous Graphite

Flake Graphite

Crystalline Graphite

Synthetic Graphite

Key Properties

Applications

Refractory Materials

Chemical Industry

Nuclear Industry

Electrical Applications

Mechanical Applications

Other Applications

Background

Carbon has two natural crystalline allotropic forms: graphite and diamond.  Each has its own distinct crystal structure and properties. 

Graphite derives its name from the Greek word "graphein", to write. The material is generally greyish-black, opaque and has a lustrous black sheen.  It is unique in that it has properties of both a metal and a non-metal.  It is flexible but not elastic, has a high thermal and electrical conductivity, and is highly refractory and chemically inert. Graphite has a low adsorption of X-rays and neutrons making it a particularly useful material in nuclear applications.

The unusual combination of properties is due its crystal structure. (Figure 1.) The carbon atoms are arranged hexagonally in a planar condensed ring system.  The layers are stacked parallel to each other.  The atoms within the rings are bonded covalently, whilst the layers are loosely bonded together by van der Waals forces.  The high degree of anisotropy in graphite results from the two types of bonding acting in different crystallographic directions.  For example, graphite's ability to form a solid film lubricant comes from these two contrasting chemical bonds. The fact that weak Van der Waals forces govern the bonding between individual layers permits the layers to slide over one another making it an ideal lubricant. World production of graphite was estimated to be about 602,000 tons in 2000, with China being the biggest producer followed by India, Brazil, Mexico and then the Czech Republic.

AZoM - Metals, ceramics, polymers and composites - Structures of graphite

Figure 1. Crystal structure of graphite.

Graphite Classifications

There are two main classifications of graphite, natural and synthetic.

Natural Graphite

Natural Graphite is a mineral consisting of graphitic carbon. It varies considerably in crystallinity.  Most commercial (natural) graphites are mined and often contain other minerals. Subsequent to mining the graphite often requires a considerable amount of mineral processing such as froth flotation to concentrate the graphite. Natural graphite is an excellent conductor of heat and electricity. It is stable over a wide range of temperatures. Graphite is a highly refractory material with a high melting point (3650°C.)

Natural graphite is subdivided into three types of material:

         Amorphous

         Flake

         High Crystalline

Amorphous Graphite

Amorphous graphite is the least graphitic of the natural graphites.  However, the term "amorphous" is a misnomer since the material is still crystalline.  Amorphous graphite is found as minute particles in beds of mesomorphic rocks such as coal, slate or shale deposits.  The graphite content ranges from 25% to 85% dependent on the geological conditions. 

Amorphous graphite is extracted using conventional mining techniques and occurs primarily in Mexico, North Korea, South Korea and Austria. 

Flake Graphite

Flake graphite is found in metamorphic rocks uniformly distributed through the body of the ore or in concentrated lens shaped pockets.  Carbon concentrations vary between 5% and 40%. Graphite flake occurs as a scaly or lamella form in certain metamorphic rocks such as limestone, gneisses and schists. 

Flake graphite is removed by froth flotation.  "As floated" graphite contains between 80% and 90% graphite.  Flake graphite is produced with >98% through chemical beneficiation processes. Flake graphite occurs in most parts of the world. 

Crystalline Graphite

Crystalline vein graphite is believed to originate from crude oil deposits that through time, temperature and pressure have converted to graphite.  Vein graphite fissures are typically between 1cm and 1 m thick, and are typically > 90% pure.  Although this form of graphite is found all over the world, it is only commercially mined in Sri Lanka by conventional shaft or surface mining techniques. 

Synthetic Graphite

Synthetic graphite can be produced from coke and pitch. It tends to be of higher purity though not as crystalline as natural graphite. There are essentially two types of synthetic graphite. The first is electrographite, which is pure carbon produced from calcined petroleum coke and coal tar pitch in an electric furnace. The second type of synthetic graphite is produced by heating calcined petroleum pitch to 2800°C. On the whole synthetic graphite tends to be of a lower density, higher porosity and higher electrical resistance. Its increased porosity makes it unsuitable for refractory applications

Synthetic Graphite consists mainly of graphitic carbon that has been obtained by graphitisation, heat treatment of non-graphitic carbon, or by chemical vapour deposition from hydrocarbons at temperatures above 2100K.

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

Due to its high temperature stability and chemical inertness graphite is a good candidate for a refractory material. It is used in the production of refractory bricks and in the production of “Mag-carbon” refractory bricks (Mg-C.) Graphite is also used to manufacture crucibles, ladles and moulds for containing molten metals. Additionally graphite is one of the most common materials used in the production of functional refractories for the continuous casting of steel. In this application graphite flake is mixed with alumina and zirconia and then isostatically pressed to form components such as stopper rods, subentry nozzles and ladle shrouds used in both regulating flow of molten steel and protecting against oxidation. This type of material may also be used as shielding for pyrometers.

In the production of iron, graphite blocks are used to form part of the lining of the blast furnace. Its structural strength at temperature, thermal shock resistance, high thermal conductivity, low thermal expansion and good chemical resistance are of paramount importance in this application.  

The electrodes used in many electrical metallurgical furnaces are manufactured from graphite such as the electric arc furnaces used for processing steel. 

Chemical Industry

There are many high temperature uses for graphite in the chemical industry such as in the production of phosphorus and calcium carbide in arc furnaces. Graphite is used as anodes in some aqueous electrolytic processes such as in the production of halogens (chlorine and fluorine.) 

Nuclear Industry

High purity electrographite is used in large amounts for the production of moderator rods and reflector components in nuclear reactors. Their suitability arises from their low absorption of neutrons, high thermal conductivity and their high strength at temperature.

Electrical Applications

The main application for graphite as an electrical material is in the manufacture of carbon brushes in electric motors. In this application the performance and lifetime of the component is very dependent on grade and structure. 

Mechanical Applications

Graphite is used widely as an engineering material over a variety of applications. Applications include piston rings, thrust bearings, journal bearings and vanes. Carbon based seals are used in the shafts and fuel pumps of many aircraft jet engines.

Other Applications

Amorphous graphite has applications in:

         Metallurgy

         Pencil production

         Refractories

         Coatings

         Lubricants

         Paint production

Crystalline graphite is used in:

         Batteries

         Lubricants

         Grinding wheels

         Powder metallurgy.

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

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

Synthetic graphites are used in:

         Aerospace applications

         Batteries

         Carbon brushes

         Graphite electrodes for electric arc furnaces for metallurgical processing

         Moderator rods in nuclear power plant.

Due to its increased porosity synthetic graphite tends not be used in refractory applications.

 

Source: CERAM Research

 

For more information on this source please visit CERAM Research Ltd

 

Date Added: Sep 10, 2002 | Updated: Jul 12, 2013
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