Fundamentals of graphite

Graphite, along with diamond and fullerene, is a modification of carbon and a naturally occurring mineral, though it is relatively rare. It typically crystallizes in a hexagonal structure, though a rhombohedral crystal system is also possible. With a hardness of 1 to 2, graphite is a very soft material. It has a black color and a gray-black streak, and it sublimates at a temperature of 3,825°C.

Structure

Graphite crystallizes in parallel layers, known as basal planes. Within these planes, the bonding energy between carbon atoms is 4.3 electron volts, whereas the bonding energy between the planes is only 0.07 electron volts. This extreme directional dependence of bonding forces results in a distinct anisotropy of graphite’s mechanical, electrical, and thermal properties:

Easy cleavage of pure graphite along the basal planes, while showing significantly higher strength along the crystal layers.
Thermal and electrical insulation perpendicular to the basal planes, but almost metallic conductivity along the planes.

If the basal planes have no fixed correlation to one another, the structure is referred to as “turbostratic” carbon. Transmission electron microscopy reveals the stacking of basal planes in graphite. The overlap of tilted stacks creates Moiré patterns, though the 0.34-nanometer basal plane spacings are not always clearly resolved.

Hexagonal (hcp) Lattice Structure of Graphite

In glass-like carbon, the planes are not arranged parallel like the pages of a book, but rather randomly crumpled like crushed paper. This carbon is hard and isotropic, similar to glass, hence its name. Through specific processing techniques, such as stretching polymer precursors and subsequent graphitization, it is possible to orient the layers in the fiber direction, resulting in high-strength carbon fibers.

Fullerenes and nanotubes consist of just one basal plane, which is curved into a sphere (fullerenes) or a tube (carbon nanotubes). These forms are closely related to graphite, and additional layers can grow in an onion-like structure, creating soot-like powders.

Electron Microscopy Image of Graphite Basal Plane Stacking in Cross-Section

Occurrence

Graphite occurs naturally as isolated flakes and grains in carbon-rich metamorphic rock and in veins within pegmatite formations. The primary sources of graphite mining are China, Korea, Madagascar, Zimbabwe, Brazil, and India, with both open-pit and underground mining operations. Annual global graphite production is approximately 600,000 tons. In Europe, there are currently three active graphite mines: one in Kropfmühl, Bavaria (Germany) and one each in Sweden and Norway. These three sites extract macrocrystalline natural graphite, where individual graphite crystallite flakes are clearly visible. Additionally, microcrystalline natural graphite is mined in Kaisersberg, Austria.

Production

Graphitized carbon materials are produced by coking carbon-containing substances such as lignite, hard coal, petroleum, and pitches, as well as plastics. Artificially produced graphite is also known as Acheson graphite.

Applications

Graphite is widely used in various industries, including:

Pencil leads
Black printing inks (as carbon black)
Rubber filler in tires (as carbon black)
Lubricants
Bearings and seals
Enhancing electrical conductivity
Carbon brushes in electric motors
Compressed electrodes for electrical power transmission
Electrodes in arc lamps
Crucibles for high-temperature applications
High-temperature furnace linings (in reducing atmospheres)
Nuclear reactor control
Activated carbon for gas, liquid, and particle adsorption
Monochromators in X-ray diffractometers

Graphite materials come in various forms, including graphite plates, fiber bundles with approximately 5,000 carbon fibers, cylindrical electrodes, and graphite foil. A 1-cent coin is often used for scale comparisons.

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