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Tuesday, 19 December 2017
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Graphitization Print E-mail
Some applications, such as graphite electrodes for the electric arc furnace require a higher thermal and electrical conductivity than that of baked carbon materials. These synthetic graphites normally follow a production process similar to that of baked carbon (forming, impregnation, rebaking) but require an additional process step, that of graphitization where temperatures of around 3000°C are achieved.
The final step in graphite manufacture is a conversion of baked carbon to graphite, called graphitizing, i.e. heat-treating the material at temperatures in the region of 2600°C –3300°C. During the graphitizing process, the more or less pre-ordered carbon (turbostratic carbon) is converted into a three-dimensionally ordered graphite structure. Depending on the raw materials and the processing parameters, various degrees of convergence to the ideal structure of a graphite single crystal are achieved.
Since graphitization increases the lattice order and produces smaller layer distances, it simultaneously leads to a considerable growth of ordered domains. However, the degree of order that can be reached depends largely on the crystalline pre-order of the solid used. These reduced lattice layer distances are macroscopically noted as a contraction in volume. This graphitization-shrinkage is approximately 3 to 5%. Due to this shrinkage, density of the graphite increases.
In general the graphitization process includes the stages as given below:
•Amorphous or baked carbon is converted to electrographite by a thermal treatment at approximately3000°C.
•Essentially any amorphous carbon material can be graphitized. The potential crystallite growth and ordering are latent within the baked carbon structure.
•Under the influence of temperature the crystallites grow and rearrange in an ordered pattern of stacked parallel planes. This transformation is accompanied by a change in the physical properties of the material
(See change during graphitization).
•The greater the degree of crystallite growth during heating up, the better the graphitability (graphitization degree), which effects the final resistivity achieved.
•There is a variation among different needle cokes concerning the graphitability.
•The graphitization degree depends on the structure of the basic material (graphitability) and the applied graphitization temperature. It is determined by x-ray measurements.
The graphitizing process is also accompanied by a purification of the material treated, normally reducing the content of impurities to considerably less than 1000ppm. For many applications, this purity is insufficient, so that a thermal purification at higher temperatures up to 3100°C with longer residence times is carried out to reduce the impurities to a concentration of less than 200ppm. If still lower ash values are required, a thermo-chemical purification is necessary.
Even after graphitization at around 3000°C most graphites contain small amounts of metallic impurities. If the ash values in the material have to be below 200ppm, thermal purification is applied. By adding gaseous halogens or halogen compounds, all hetero-atoms forming stable carbides are transferred into volatile halogen compounds and thus removed. By means of this procedure, impurities may be lowered to less than 1ppm.

 
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