The influence of the formation of Fe3C on graphitization in a carbon-rich iron-amorphous carbon mixture processed by Spark Plasma Sintering and annealing

A promising fabrication method of bulk porous graphitic materials is based on consolidation of metal-amorphous carbon powder mixtures, in which the metal serves as both a graphitization catalyst and a removable space holder. In this work, iron was evaluated for this purpose. The phase formation and evolution in a carbon-rich iron-amorphous carbon mixture during Spark Plasma Sintering (SPS) and subsequent annealing was studied to reveal the peculiarities of the low-temperature catalytic graphitization process determined by the transformations of the iron catalyst. Mixtures of carbon black with iron of the Fe-20 wt%C composition were ball milled, Spark Plasma Sintered at 600-900 °C for 5 min and further annealed at 800 °C for 2 h. During the SPS, iron carbide Fe3C formed, while the free carbon remained poorly graphitized. In the compact sintered at 900 °C, Fe3C was the only iron-containing phase and metallic iron was not detected. For conducting structural studies of the free carbon by X-ray diffraction and Raman spectroscopy, iron was dissolved from the sintered compacts in HCl solution. It was found that during annealing, the graphitization degree increased only in the compacts that still contained free (metallic) iron. These results suggest that Fe3C does not catalyze graphitization in a carbon-rich mixture of iron and carbon black making the presence of residual (metallic) iron crucial for the advancement of catalytic graphitization during annealing.