M. Singh, R. Vander Wal
Pennsylvania State University,
Keywords: carbon-carbon composite, graphene, TEM
Summary:Characterizing carbon-carbon composite behavior across multiple length scales is essential to comprehending its macro-scale behavior. This work characterizes a graphene-anthracene composite from the nano- to the micron-scale with a variety of analytical techniques brought to bear to understand its mechanical hardness. Three variations of the graphene-anthracene composite are prepared by using graphene sheets of varied sizes as a filler in an anthracene matrix. The graphene materials included 300-800 nm reduced graphene oxide, 1-2 um graphene nano-platelets and 2-5 µm graphene. The composite is prepared by first carbonizing the mix at 500 °C in a sand bath followed by high temperature heat treatment in a graphitization furnace at 2700 °C. The carbon-carbon composite so formed is then characterized at different length scales to understand the graphene additives’ influence on the composites’ bulk properties. Nanostructure, for instance, is visualized using transmission electron microscopy (TEM). The material’s nanostructure relates to its graphitizing (or non-graphitizing) behavior, using bright field and selected area imaging modes. The dominant presence of stacked lamellae confirms the graphitizing nature of the graphene-anthracene composite under study. This observation is consistent across the three graphene varieties, where the differently sized graphene sheets as additives all lead to graphitized (nano) structure. However, the composites vary greatly in their mechanical hardness and TEM alone is unable to explain the reason for the differential. This suggests micron scale characterization data an important piece of the puzzle. Scanning electron microscopy (SEM) enables visualization of filler/matrix (graphene/anthracene) interaction. Across the three composites stark differences are observed at fracture planes. The structure of the intermediate carbonized product is also known to dictate the path to graphitization. Polarized light microscopy (PLM) differentiates carbonization micro-structure based on optical texture developed during carbonization at 500 °C. Both SEM of the final “graphitized” products and PLM of the intermediate carbonized products point to the composites being graphitized to different degrees, an observation not captured via the materials’ nanostructure. This difference in the material’s degree of graphitization is further confirmed using X-ray diffraction (XRD), a technique that is widely used to determine crystal lattice structure. Using macroscopic samples, XRD gives an overall picture of the material’s crystallinity, further confirming that the composites differ – attributed to the filler size playing a role in determining its macroscopic properties. Thus, to analyze carbon-carbon composites, such as the graphene-anthracene composite studied here, it is essential to bring together the pieces of the puzzle using characterization techniques at the various length scales to visualize the material and consequently explain material properties that are sought after for industrial applications.