Pitch spheres stabilized by HNO₃ oxidation and their carbonization behavior

Pitch spheres were stabilized by HNO₃ oxidation at room temperature using HNO₃ solutions of 10, 20, 30, and 40% mass fraction. The chemical structure and morphology changes of the oxidized pitch spheres and their carbonized samples were investigated by FT-IR, XPS, elemental analysis, and SEM. Results showed that the pitch spheres oxidized by 30% HNO₃ for 12 h could be carbonized at 900 ℃ to produce carbonized spheres without any change in shape. The successful carbonization can be ascribed to the contribution of nitrogen functional groups in the form of -NO₂, which were introduced in the HNO₃ oxidation process. In the early stage of the heat treatment around 400 ℃, the transformation of -NO2 groups to amine groups occurred, which promoted the cross-linking of pitch molecules. As the pitch spheres were carbonized at 900 ℃, cyclization and aromatization reactions led to the formation of pyridine and quaternary nitrogen groups.