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

Abstract

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.

Graphitization Control Using Additives in Pitch Composites–Part II: Effect of Size, Stacking, Loading, and Composition of Graphene and Graphene Oxide on Crystallite Sizes

“…This phenomenon can be attributed to the dominance of templating effects over mesophase effects. As discussed in the first part of this series, graphenic additives introduce two contrasting effects in pitch composites: the subdivision of the forming mesophase, which reduces crystallite sizes, and a templating effect, which increases domain size. This variance in behavior between Gr composites of ARP and ADP is logical, given that templating effects hold more significance for a poorly graphitizable aliphatic ADP, in contrast to ARP, with a high aromatic content and resulting high graphitizability.…”

“…Eight commercially available graphenic additives presenting a range of structural parameters were obtained, as detailed in Table . A 2.5 wt % additive was mixed with the pitches, and the samples were carbonized at 600 °C and graphitized (designated as “GR”) at 2500 °C using the methodology previously described in the first part of this series . The previously described X-ray diffraction (XRD) methodology was used to analyze the differences in the crystallite parameters within the composites.…”

“…The previously described X-ray diffraction (XRD) methodology was used to analyze the differences in the crystallite parameters within the composites. Raman analysis was performed for validation of the trend observed in XRD analysis using the procedure described in the first part of this series …”

“…This work is performed on the same two commercial pitches (an aromatic rich pitch (ARP) and an aromatic deficit pitch (ADP)) used in the first part of the study . Eight commercially available graphenic additives presenting a range of structural parameters were obtained, as detailed in Table .…”

“…As concluded from part I of the study, Gr additives interact with the pitch matrix by three mechanismschemical bonding at the reactive edge sites, π–π interactions leading to stabilization and physical templating, and self-templating wherein graphene-templated layers extend these effects to subsequent layers. The structural parameters influencing these interactions are the lateral size and stacking height of the Gr.…”