The carbonization and graphitization process of PAN carbon felt

The preparation process of PAN-based carbon fibers is complex, with key steps including polymerization, spinning, pre-oxidation, carbonization, and graphitization. Here, we will introduce the carbonization and graphitization processes of PAN-based carbon felts.

Carbonization process of PAN-based carbon felts:

 High-temperature carbonization is a major stage in the preparation of carbon fibers. Generally, PAN-based carbon fibers are heated to 1100-1600°C under the protection of high-purity inert gas to remove non-carbon atoms and transform them into carbon fibers with a disordered graphite structure.

During the high-temperature carbonization process, the polymer structure of PAN fibers transforms into a polycrystalline carbon structure. Further cross-linking occurs between the trapezoidal polymers. In a sense, the process parameters of the high-temperature carbonization furnace determine the final strength and modulus of the fibers. Carbonization temperature, carbonization time, and drawing force are the main factors affecting the carbonization process of PAN-based carbon fibers.

Generally speaking, within a certain carbonization temperature range, the strength, modulus and density of carbon fibers all increase with the rise of the carbonization temperature. The application of stretching force is beneficial for the graphite layers to align along the fiber axis, thereby enhancing the strength and modulus of the fibers. In addition, during the carbonization process, gases and liquids produced by thermal decomposition are prone to form pore cracks within the fibers, and at the same time, they can also cause adhesion between the fibers. Therefore, how to remove the decomposition products of heat becomes a key technology in carbonization.

Currently, this can generally be achieved by chemically pre-treating the PAN pre-oxidized fibers before carbonization, or by adding reaction reagents during the carbonization process to inhibit the formation of its decomposition products.

The graphitization process of PAN-based carbon felt

 Graphitization is usually carried out in an inert gas atmosphere, with the temperature ranging from 1500 to 3000℃. Since N2 will transform into active molecules above 2000℃ and react with carbon atoms to form cyanide groups, more stable inert gases are generally selected. Research has found that when the same carbonized PAN fibers undergo graphitization, the temperature has the greatest impact on the properties of the carbon fibers. When the processing temperature is 1600 to 1900℃, the tensile strength of the obtained carbon fibers reaches above 3.5 GPa; if the processing temperature exceeds 2500℃, the Young's modulus of the obtained carbon fibers exceeds 300 GPa.

 In addition, adding some metal oxides (such as V2O5, MoO3, etc.) to the carbonization of PAN or introducing boron vapor during graphitization is beneficial to the growth of carbon fiber microcrystals, which can lower the graphitization temperature. There are also reports indicating that passing carbon fibers through a certain current during graphitization can facilitate the regularity of microcrystal growth.