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Further graphite modifications

In addition to graphite, diamond, fullerene, activated carbon, glassy carbon and carbon nanotubes represent further modifications of carbon. Activated carbons are the most important group in this respect, their enormously expansive interior surface making them ideally suitable as substrates or filter material. Many systems for waste air or waste water cleansing use this material which is manufactured from vegetable, animal, mineral or petrochemical raw materials. Activated carbon thus acts as an adsorption agent in process gas and waste water treatment, ventilation and air conditioning and ABC protection technology.

Diamond is utilised less frequently as a technical material, chiefly because of its price. Applications here concentrate on the extraordinarily high degree of hardness (Mohs' hardness = 10), one of the highest levels of thermal conductivity of all known substances and simultaneously excellent electrical insulating properties (in undoped form). Diamond is therefore chiefly utilised for hard machining (drilling, milling, grinding, polishing), in composite materials as heat sinks in microelectronics or in the form of wear-resistant coatings. Doped diamond is used in high-frequency technology, synthesis chemistry or waste water treatment, due to its electrical conductivity.

Nanotubes made of carbon have only been of minor technical importance to date, but their excellent intrinsic material properties indicate very promising perspectives and possibilities for their use. With a density of only approx. 1.3 g/cm3, they have a tensile strength greater than 50 GPa and an E-module of more than 1 GPa. By way of comparison: steel achieves at best a tensile strength of 2 GPa, and that at a density 6-8 times greater. These extremely high specific mechanical properties could be of particularly great advantage in relation to composite materials, either to supplement CFC or CFRP composites or as independent reinforcing components. Of particular interest to the electronics industry are the (in an ideal case) current carrying capacity, which is around 1000 times higher than copper wire, and the thermal conductivity of 6000 W/m•K at room temperature, this being almost double that of diamond (3320 W/m•K). When utilised as a semiconductor material, nanotubes can withstand higher voltages and temperatures (and consequently a higher clock frequency) than silicon transistors. They are therefore theoretically a perfect substitute material for the production of chips. Contrary to the aforementioned areas, nanotubes are already used in different applications as an additive in polymers. A considerably lower percolation threshold when compared to carbon black means that only a low percentage (3-7) of nanotubes are required to increase the conductivity of a plastic by considerable orders of magnitude. The characteristics of the polymer are retained as a result, and processing is considerably simplified.

Glassy carbon differs from graphite in its extremely low degree of order (hence the name). It possesses an extremely high degree of hardness, excellent corrosion resistance, remains stable under inert gas up to very high temperatures and is also characterised by a high degree of strength. Coatings of glassy carbon are used in electrochemistry and chemical process engineering, due to the resulting high resistance to wear and corrosion and good electrical conducting characteristics.

Fullerenes are the third stable modification of carbon occurring in nature. These carbon clusters with 60 to 100 carbon atoms were first discovered in 1985.They form highly-symmetrical spatial bodies, of which the Buckminster fullerene is the most commonly known. Their technical usefulness is still in the early stages.

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