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 wt%. 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 (C 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 (C, 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 (C, cyclization and aromatization reactions led to the formation of pyridine and quaternary nitrogen groups.