Carbon has stable allotropes, including graphite and diamond, with useful properties such as high strength and melting points. Amorphous carbon and graphite are common, while diamond is prized as a gemstone and for industrial use. Other allotropes include buckyballs and carbon nanotubes, which have high tensile strength and potential as building materials.
Carbon, due to its unique chemical bonding properties, has the most stable allotropes of any element. Some carbon allotropes, such as graphite and diamond, occur naturally; others, such as nanotubes, must be fabricated in laboratories. Carbon’s abundance and versatility make it one of the most studied materials in nanotechnology research, and its allotropes have a wide range of useful properties, such as high tensile strength and high melting points.
Amorphous carbon and graphite are the two most common allotropes and make up the majority of natural carbon compounds such as coal and soot. Amorphous carbon does not have a particular crystalline structure, like most glasses. The carbon atoms in graphite are arranged in layers; each layer has strong bonds holding it together, but the layers don’t bond strongly and can be easily erased from each other. Pure pyrolytic graphite, in which the layers are all arranged in the same plane, is an extremely strong and heat-resistant material used in high-stress and high-temperature environments. Graphite conducts electricity and can be used as a moderator for nuclear reactors.
Diamond, the other natural allotrope of carbon, has a rigid, interlocking crystalline structure. Diamond is one of the hardest substances known and is used in industry to grind and cut materials. The clarity and durability of the diamond make it highly prized as a gemstone, and the jewelry business has turned diamond mining into a multibillion-dollar industry. Diamond can be produced synthetically from other allotropes, but requires extremely high temperatures and pressures to form.
Other more exotic allotropes include buckyballs, nanotubes and carbon nanofoam. These occur naturally in soot and have been found in at least one planetary nebula, although they can also be created synthetically. Buckyballs are closed ball-shaped networks of carbon atoms, forming interconnected pentagons and hexagons. Carbon nanotubes share the same structure as graphite, but instead of having a stack of layers, a nanotube coils and bonds to itself, forming a long cylinder. Carbon nanotubes have very high tensile strength, making them attractive as new building materials, although they are not as strong under compression.
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