Pyrolytic carbon coating
The graphite contains trace elements, the composition and contents of which is based on the composition of source raw materials. Most of the impurities are removed during the final production operation in the production of graphite – graphitization process. Amorphous or baked carbon is converted to electrographite by a thermal treatment at temperatures of about 2600-3000 °C. Under the influence of the very high temperature most of the impurities sublime, and are removed from the hot zone in gaseous phase. Normally, the content of impurities of most electrographites, is well below 1000 ppm.
However, for many applications much higher purity is required. The residuous impurities are typically metals and their compounds, which is not possible to remove by the high temperature during graphitization process.
Therefore, a thermo-chemical purification method is applied. By use of gaseous halogens, that are very reactive (chlorine, flourine) and high temperature, metal impurities and their compounds are converted into volatile halogen compounds, and removed from the graphite. The final ash content can be <5 ppm.