Pyrolytic carbon coating
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.