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14.4: Allotropes of Carbon

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    34158
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    Carbon is capable of forming many allotropes in addition to the well known diamond and graphite forms. The physical properties of carbon vary widely with the allotropic form. For example, diamond is highly transparent, but graphite is opaque and black. Diamond is the hardest naturally-occurring material known, while graphite is soft enough to form a streak on paper (hence its name, from the Greek word "γρáφω" which means "to write"). Diamond has a very low electrical conductivity, while graphite is a very good conductor. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials.

    All carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen. The system of carbon allotropes spans a range of extremes:

    Synthetic nanocrystalline diamond is the hardest material known. Graphite is one of the softest materials known.
    Diamond is the ultimate abrasive. Graphite is a very good lubricant, displaying superlubricity.
    Diamond is an excellent electrical insulator, and has the highest breakdown electric field of any known material. Graphite is a conductor of electricity.
    Diamond is the best known naturally occurring thermal conductor Some forms of graphite are used for thermal insulation (i.e. firebreaks and heat shields), but some other forms are good thermal conductors.
    Diamond is highly transparent. Graphite is opaque.
    Diamond crystallizes in the cubic system. Graphite crystallizes in the hexagonal system.
    Amorphous carbon is completely isotropic. Carbon nanotubes are among the most anisotropic materials ever produced.


    14.4: Allotropes of Carbon is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Peter Bird.

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