Graphitization Control Using Additives in Pitch Composites─Part I: Effect of Graphene on Temperature-Dependent Crystallite Size Evolution, Carbonization Reactions, and Mesophase Formation

This article presents a systematic work on the graphitization behavior of graphene (Gr)-pitch composites. Two commercial pitches of varying composition and sources (coal tar pitch and petroleum pitch) and their Gr composites were subjected to a broad series of heat treatment temperatures─750, 1000, 1500, 2000, and 2500 °C. The trajectory of the crystallite parameter growth was analyzed using X-ray diffraction (XRD) with both in situ and ex situ heating. Incorporating a small amount of Gr into the pitch yielded remarkable improvements in the interplanar spacing, accompanied by increases of up to 60% and 200% in the in-plane crystallite diameter and stacking height, correspondingly. Thermogravimetric analysis/differential scanning calorimetry indicated that Gr acts as a catalyst to lower the onset temperature of stabilizing polymerization reactions, reduce devolatilization, and improve concatenation reactions during carbonization. A qualitative assessment of the mesophase was performed using scanning electron microscopy imaging. High-resolution transmission electron microscopy (TEM) with in situ heating revealed three mechanisms─chemical templating, physical templating, and self-templating in the Gr-pitch composite. A noteworthy feature of G peak splitting observed in the Raman spectrum for the pitch composite provided supplementary evidence of templating. Raman spectroscopy and TEM have been employed on graphitized samples to acquire supplementary insights in conjunction with XRD-based crystallite analysis.