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Graphite

Graphite.jpg
In addition to diamonds and fullerenes, graphites form the third modification of carbon which occurs naturally and, from a technological point of view, it is the most important one. In a monocrystalline form graphite is characterized by a pronounced anisotropy which is caused by its chemical bonding properties. The carbon atoms are arranged in a hexagonal layer structure in the graphite. Within these basal levels or graphene layers there is an extremely strong covalent linkage between the atoms. On the other hand, the layers which are arranged in parallel only have a very weak bond between one another. This leads to a very pronounced anisotropy of the mechanical properties, of the electrical and thermal conductivity or the heat expansion. Moreover, the shearing of the graphene layers against one other, which is well known from pencils, and the good lubricating properties of graphite which are associated with it are also caused by this. However, the extraordinary properties of graphite are not only due to this anisotropy but also to the strength of the bond within the basal levels. This means graphite only decomposes at a temperature of approx. 3800 °C and is, hence, one of the most temperature stable materials available today. Therefore, graphite has an excellent thermal shock resistance in combination with the low thermal expansion. Moreover, graphite has a high chemical resistance against acids, bases and molten materials and can be produced with the highest possible purity.

On account of these special properties the range of applications of graphite and graphite materials is extremely broad. It extends from bearings and packing rings to heating elements and melting pots for the solar cell and semiconductor industry, carbon brushes for electric motors ranging from vacuum cleaners to drill hammers and railways, components for medical, measuring and analytical engineering and to components for reactor technology - just to name a few.

For these applications synthetically manufactured, polycrystalline graphite is primarily used which corresponds closely to the monocrystalline natural graphite in its basic properties but which also greatly differs from it in some respects. For example, at approx. 1.5 to 2.0 g/Cm³ the apparent density of graphites produced industrially is considerably below the theoretical value of 2.26. Moreover, the degree of organization is generally much lower in the polycrystalline material. In addition, the anisotropy which is very pronounced in the monocrystal has a much lower effect macroscopically. This means technological graphites have an almost homogeneous electrical and thermal conductivity and isotropic mechanical properties.

The production of technical graphite materials is carried out on the basis of ceramic process engineering. The raw materials used include (natural) graphite, pet cokes, pitch cokes and soots. These are processed together with binding agents such as pitches or synthetic resins before they are shaped. In this process, different aggregates can be mixed in which means that the properties of the resulting graphite material can be influenced in a targeted manner. Subsequent to that, a one- or two-stage thermal treatment is carried out. In this process, the binder phase is carbonised at a high temperature during the first stage; during this stage, the yield of solid carbon can amount to up to 90 per cent depending on the respective binder. This carbon graphite can be used for different applications, such as e.g. in the field of tribology without any further temperature treatment. However, the graphitic share in this material is still low so that, usually, a second stage of thermal treatment has to be completed. During this carbonisation which takes place at temperatures of more than 2100°C a graphite structure evolves which has an increasingly strong organisation. In this way, the material gets the typical graphitic properties. Depending on the carbonisation temperature and the annealing time the degree of carbonisation and, hence, the properties of the material differ. For example the strength of the material reduces with an increasing degree of carbonisation, whereas thermal and electric conductivity increase.

This means there is a possibility of optimally adjusting the properties of graphite materials in a targeted manner by means of controlling the temperature. Moreover, the limits of the properties of graphite materials can be shifted very much by impregnating them with metals, salts, resins or other organic media. In addition to gas tightness and the mechanical properties electrical and thermal conductivity, tribological properties and radio shielding can be optimised in this way - just to name some of the most important possibilities. The density and corrosion resistance can be increased with the help of chemical vapour infiltration with pyrolitic carbon or graphite.

| Natural graphite

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| Electrographite

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| Impregnated Graphite

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| Carbon Graphite

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