Carbon material is a non-metallic material obtained by a series of processing processes mainly using organic substances such as coal, petroleum or their processed products as the main raw material. Diamond, graphite, carbene, graphene, carbon nanotubes, and carbon/carbon composite materials are all carbon materials.
Depending on the type of the organic compound precursor, the generated carbon material can be gaseous, liquid or solid. Figure 1 shows the products obtained by heat-treating these precursors. The carbon obtained will vary with the raw materials (gas, liquid, solid, etc.), intermediates and heat treatment temperature. Each carbonization method will be described below:
① Gaseous carbonization.
Through the thermal decomposition of volatile organic compounds or hydrocarbons, followed by aromatization based on free radical polymerization to obtain gaseous precursors, and then products can be obtained from them. Examples of carbon materials obtained by gaseous carbonization include carbon black, pyrolytic carbon, and carbon whiskers.
②Liquid phase carbonization.
In the liquid phase carbonization process, low-molecular compounds in the raw materials such as fusible organic compounds or coal volatilize when heated. When the temperature rises, the aroma will be generated through various reactions such as viscosity reduction, thermal decomposition, polymerization, dehydrogenation, and aromatization. hydrocarbon. When the components of the carbon compound are aromatized or the molecular weight is increased, the viscosity of the liquid will increase and eventually become solid carbon. The solidification of the material is accompanied by the formation of pores, and the release of hydrogen, hydrocarbons and other impurities when the temperature rises. The whole process is accompanied by thickening. When the temperature exceeds 1500°C, the graphite crystal structure is also formed. Impurities (S, N, etc.) in carbon materials are released during the graphitization process.
During liquid carbonization, the viscosity starts to increase around 400°C and reaches a maximum value, and then drops to a minimum value before it increases again due to solidification. The change of viscosity with temperature varies with the type of carbon material and the carbonization conditions. The organic compound contains an isotropic liquid and an anisotropic liquid crystal, and this liquid crystal is gradually transformed into an anisotropic solid coke. Because the solid structure does not need to rearrange a large number of atoms, the order of the hexagonal planes in the solid phase determines the progress of graphitization. The degree of graphitization is affected by the grain size (La and Lc) of graphitizable carbon. However, the difference between graphitizable carbon and non-graphitizable carbon lies in the regularity of the arrangement rather than the size of the crystal grains. Since the crystal grains do not grow with the heat treatment, the hardly graphitized carbon exhibits an amorphous form.
③Solid phase carbonization.
When the raw materials (such as thermosetting polymers) are heat-treated, they do not produce liquid, and even remain solid during the pyrolysis reaction, and solid-phase carbonization occurs. The initial solid structure determines the microstructure of carbon because the movement of molecules is restricted in the carbonization reaction. When the fusible organic matter becomes insoluble after the oxidation reaction and heat treatment, the solid-state carbonization reaction occurs in the process. Assuming that the material has a special shape before carbonization, that shape is maintained in the carbon material product. However, if there is no additional thickening step, volatile components will be released during the carbonization process, so it is very possible to obtain porous carbon materials. Typical solid carbonized materials are fiber, phenolic resin, furfuryl alcohol resin and furan resin. The carbon material products obtained from solid-phase carbonization include carbon fiber and activated carbon.