What is Graphite?- Definition, Types, and Uses

What is Graphite?

Graphite, archaically referred to as plumbago, is a soft, crystalline form of carbon with its atoms arranged in a hexagonal structure. It occurs naturally in metamorphic rocks such as marble, schist, and gneiss, the most stable form of carbon under standard conditions. Under high pressures and temperatures, it converts to diamond.

It is gray to black, opaque, and has a metallic luster, it is flexible but not elastic. It exhibits the properties of a metal and a nonmetal, which make it suitable for many industrial applications. The metallic properties include thermal and electrical conductivity. The nonmetallic properties include inertness, high thermal resistance, and lubricity.

Graphite is used in pencils and lubricants. It is a good conductor of heat and electricity. Its high conductivity makes it useful in electronic products such as electrodes, batteries, and solar panels.

Who discovered graphite?

Graphite was first accidentally synthesized by Edward G. Acheson when he was doing high-temperature experiments with carborundum. He found that at around 4,150°C (7,500°F) the silicon in the carborundum evaporates and the carbon remains in graphitic form.

Acheson was granted a patent for graphite production in 1896, and commercial production began in 1897. Since 1918, petroleum coke, small and imperfect graphite crystals surrounded by organic compounds, has been the primary raw material for the production of 99 to 99.5 percent pure graphite.

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Graphite, archaically referred to as plumbago, is a soft, crystalline form of carbon with its atoms arranged in a hexagonal structure.
Graphite

What color is graphite?

Graphite ranges in color from gray to black and is both opaque and metallic in appearance. It is composed of carbon atoms and can be considered coal in its highest grade, though it is not typically used as a fuel.

Graphite is a medium gray color. As a color name, it is used interchangeably with “charcoal”. It refers to the “lead” in wooden pencils, which are actually made from a graphite-based pigment and a clay binder.

Graphite, a hue of gray, is a color of basically no emotion. Detached, neutral, and indecisive, it is a transition between white and black.

Structure of Graphite

Graphite has a layered structure that consists of rings of six carbon atoms arranged in widely spaced horizontal sheets. Graphite thus crystallizes in the hexagonal system, in contrast to the same element crystallizing in the octahedral or tetrahedral system as diamond.

Graphite consists of layers of carbon atoms arranged in 6-membered, hexagonal rings. These rings are connected to one another at their edges. Layers of fused rings can be modeled as an infinite series of fused benzene rings (without hydrogen atoms).

Graphite has a layered structure that consists of rings of six carbon atoms arranged in widely spaced horizontal sheets. Graphite thus crystallizes in the hexagonal system, in contrast to the same element crystallizing in the octahedral or tetrahedral system as diamond.
The Structure of Graphite

Carbon atoms in these ring arrangements are in the sp2 hybridized state. In the sp2 molecular orbital model, each carbon atom is bound to three other species, in the case of graphite three more carbon atoms. In this bond mode, the bond angle between adjacent carbon atoms is 120.

These “ring arrays” are arranged in large layers of carbon atoms, and individual layers are called graphene layers. The carbon-carbon bond length in a layer plane is 1.418. Graphite layers are stacked on top of each other parallel to the crystallographic “C” axis of the hexagonal 4-axis system in which graphite crystallizes.

Properties of Graphite

Graphite is an allotrope of carbon that is used for making moderator rods in nuclear power plants. Its properties are as follows:

  • A greyish black, opaque substance.
  • Lighter than diamond, smooth and slippery to touch.
  • A good conductor of electricity (Due to the presence of free electrons) and good conductor of heat.
  • A crystalline solid
  • Very soapy to touch.
  • Non-inflammable.
  • Soft due to weak Vander wall forces.
  • The conductor of electricity.

physical properties of graphite

Graphite has a high melting point, similar to that of a diamond. To melt graphite, it is not enough to loosen one sheet from another. You have to break the covalent bond throughout the structure.

It is soft and slippery to the touch and is used in pencils and as a dry lubricant for things like locks. You can think of graphite more like a pack of cards – each card is strong, but the cards slide on top of each other or even fall off the pack altogether. When you use a pencil, the sheets will rub off and stick to the paper.

Also, it has a lower density than diamond. This is due to the relatively large amount of space that is “wasted” between the sheets.

It is insoluble in water and organic solvents – for the same reason that diamond is insoluble. The force of attraction between solvent molecules and carbon atoms will never be strong enough to overcome the strong covalent bonds in graphite.

Graphite conducts electricity. The delocalized electrons can move freely through the sheets. When a piece of graphite is tied into a circuit, electrons can fall off one end of the sheet and be replaced with new ones at the other end.

Chemical properties of Graphite

ColorIron-black to steel-gray; deep blue in transmitted light
Chemical ClassificationNative element
StreakBlack
LusterMetallic, sometimes earthy
DiaphaneityOpaque
CleavagePerfect in one direction
Mohs Hardness1 to 2
Specific Gravity2.1 to 2.3
Diagnostic PropertiesColor, streak, slippery feel, specific gravity
Chemical CompositionC
Crystal SystemHexagonal

Types of Graphite

The Different types of graphite:  

  1. Natural Graphite
    1. High Crystalline graphite
    2. Amorphous graphite
    3. Flake graphite
  2. Synthetic Graphite.

1. Natural Graphite

Natural graphite is a mineral form of graphitic carbon. It varies considerably in crystallinity. Most of the commercial graphite is mine and usually contains other minerals. After graphite is mine, it usually requires a considerable amount of mineral processing like froth flotation to concentrate the graphite.

Natural graphite is an excellent conductor of heat and electricity that is stable over a wide temperature range and is a high-strength material with a high melting point of 3650 ° C. It is mostly used for refractories, batteries, steelmaking, expanded graphite, brake linings, foundry facings, and lubricants.

Natural graphite is available in three forms, all of which are processed from naturally sourced graphite material. The three forms each have unique properties that make them well-suited for certain applications.

1.1 Crystalline Graphite

It is said that crystalline vein or lump graphite is considered to be the rarest, most valuable, and highest quality form of natural graphite. It is formed by direct deposition of solid graphitic carbon from underground high-temperature fluids such as crude oil and has been converted to graphite by time, temperature and pressure.

Sri Lanka is the only jurisdiction currently producing crystalline vein graphite. Crystalline venous graphite is easy to shape and can be formed into solid shapes without the aid of a binder additive, resulting in significant cost savings over lower-quality raw materials.

Crystalline vine graphite cracks typically have a thickness between 1 cm and 1 m and usually have a purity of more than 90% Cg. It has a purity of 95-99% carbon without refining. It is suitable for many uses as flake graphite and provides a distinct competitive advantage in terms of market prices and product applications such as lubricants, batteries, grinding wheel, and powder metallurgy.

1.2 Amorphous Graphite

Amorphous graphite is commonly formed by contact metamorphosis between an anthracite coal seam and a metamorphic agent. The result is microcrystalline graphite, commonly known as amorphous graphite. Amorphous graphite is the most limited graphite among natural graphite. The graphite content varies between 25% and 85% depending on the geological environment.

It was found as extremely small, crystal-like particles in beds of mesomorphic rocks such as coal, slate, and slate. The comparatively low carbon purity varies between 70 and 85% carbon after refining. It is not visible unless viewed under magnification.

It is used for low-quality graphite products such as pencils, lubricants, refractories, paint making, metallurgy, coatings, brake pads, and rubber additives. It’s the cheapest form of graphite. Large deposits of amorphous graphite are found in China, Mexico, and the United States.

1.3 Flake Graphite

Natural flake graphite is formed when carbon material is exposed to high pressure and temperature. The carbon source material can be either organic or inorganic, although most commercially available flake graphite is derived from organic deposits. The pressure required is usually greater than 1 gigapascal and the temperature required is usually greater than 750 degrees Celsius.

Flake graphite can be found in metamorphic rocks that are evenly distributed throughout the ore body or in concentrated lens-shaped pockets. The range of carbon concentrations varies between 5% and 40%. Flake graphite can be found as a lamellar or flaky shape in certain metamorphic rocks such as limestone, gneiss, and schists.

Foam flotation is used to extract flake graphite. “Floating” graphite has a graphite content of 80% -90%. More than 98% of the layer was made using graphite chemical preparation processes. Flake graphite can be found in many places around the world.

2. Synthetic Graphite

Synthetic graphite can be generated from coke and pitch. Although this graphite is not as crystalline as natural graphite. It is possible to have highly ordered pyrolytic graphite or highly oriented pyrolytic graphite(HOPG) refers to graphite with an angular spread between the graphite sheets of less than 1°.

There are basically two types of synthetic graphite. One is electro graphite, pure carbon produced from coal tar pitch, and calcined petroleum coke in an electric furnace. The second is synthetic graphite, created by heating calcined petroleum pitch to 2800 °C.

Basically, synthetic graphite has higher electrical resistance and porosity, and lower density. Its enhanced porosity makes it unsuitable for refractory applications.

Synthetic graphite contains mainly graphitic carbon that has been attained by graphitization, heat treatment of non-graphitic carbon, or chemical vapor deposition from hydrocarbons at temperatures over 2100 K.

Application of synthetic graphite aerospace applications, carbon brushes, graphite electrodes, batteries, and moderator rods in nuclear power plants. The high level of porosity of synthetic graphite makes it unsuitable in refractory applications.

Uses of Graphite

Graphite has been used since ancient times. It has a wide range of applications in the modern world too.

Let’s look at some common uses of graphite below:

  • Writing Materials
  • Lubricants
  • Refractory
  • Nuclear Reactors
  • Batteries
  • Graphene Sheets

1. Writing Materials

The word graphite is from the Greek language which translates as ‘to write’. So the most common use of graphite is in making the lead in pencils. This lead is a mixture of clay and graphite which is in an amorphous form.

2. Lubricants/Repellents

Graphite is one of the main ingredients in lubricants like grease, etc. This mineral reacts with atmospheric water vapor and creates a thin film or layer over the surface applied and thus reduces friction. Graphite is also used in car brakes and clutches.

The powdered form of lump graphite is also used in paints. Why? Well, graphite by nature is water-repellent. So it offers a protective coating on wood and other surfaces.

3. Refractories

Due to its high tolerance to heat and unchangeability, Graphite is a widely used refractory material. It finds its use in the manufacturing industry and it helps in the production of glass and steel as well as the processing of iron.

4. Nuclear Reactors

Graphite can absorb fast-moving neutrons. As a result, it is used in reactors to stabilize nuclear reactions.

5. Electrical Industry

Crystalline flake graphite is used in the manufacturing of carbon electrodes, brushes, and plates needed in dry cell batteries and the electrical industry. Interestingly, natural graphite is also processed into synthetic graphite. This type of graphite is useful in lithium-ion batteries.

6. Graphene Sheets

Graphite can be used to make graphene sheets. These sheets are said to be 100 times stronger and 10 times lighter than steel. This derivative of graphite is further used in making lightweight and strong sports equipment. Many are considering future applications in the field of the medical and aerospace industry.

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Reference:

Graphite materials: usgs.gov