M. Singh, R.L. Vander Wal
Penn State University,
Keywords: carbon, composite, graphitization, graphene
Summary:Carbon-carbon composites, originally developed for aerospace applications, are materials that consist of a carbonaceous matrix with an embedded carbon filler providing the required reinforcement for thermal and mechanical stability1,2. An important aspect of composites is the interdependence of the matrix and additive phases to strengthen one another and the material as a whole with the interfacial stress transfer being key to this reinforcement3. This work characterizes carbon-carbon composites from the nano- to 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 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 °C4. 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), as shown in Fig. 1a. The material’s nanostructure relates to its graphitizing (or non-graphitizing) behavior, using bright field and selected area imaging modes5. 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) and polarized light microscopy (PLM) are therefore used to understand the causative factor for the observed variation in the materials’ strength6. SEM enables visualization of filler/matrix (graphene/anthracene) interaction. across the three composites stark differences are observed at fracture planes, as shown in Fig. 1b. The structure of the intermediate carbonized product is also known to dictate the path to graphitization5. Polarized light microscopy 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 structure7. 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.