S. Ike, S. Ling, R. Vander Wal
Penn State University,
Keywords: graphene oxide, novolac, oxygen content, templating, graphitic quality
Summary:The effect of an additive on a C-C composite and its interaction with the matrix is essential to the diversity of C-C composites and their potential applications. C-C composites are made up of a carbon additive embedded in a carbon matrix. The matrix precursor are thermosetting resins, and the additives can vary to include nanomaterials like graphene and carbon nanotubes (CNTs) to form carbon-polymer matrix based composite materials. C-C composites exhibit exceptional properties such as lightweight, high electrical conductivity, mechanical strength, and thermal stability. The properties of C-C composites make them very desirable, and they are foreseen to be used demanding applications in the aerospace industry. The role of the additive is as a reinforcing component to provide mechanical strength and stability. The reinforcement ability of the additive and composite function arises from interfacial interactions between the matrix and additive. Interfacial interactions also can impact the graphitization behavior of carbon-carbon composites. Understanding the operative factors by which carbon additives such as graphene oxide (GO) can direct the nano- to micro-structure of the matrix will provide insights into and potentially control of the graphitization trajectory of the composite. In this work, C-C composites were made using novolac⸻ a non-graphitizing polymeric matrix with GO additives. GO additives of varied oxygen content were used to study the role of oxygen on the graphitization of the graphene oxide-novolac composites. Four stages outline this work: first, novolac composites were prepared using formaldehyde and phenol in ratio less than one with 2.5% weight GO additives. Thermogravimetric analysis (TGA) analysis was performed to study the effect of heating rate on the crosslinking of the GO-novolac mixtures. The results show that samples with GO additives exhibited a greater mass loss compared to pure novolac. This suggests the additive is influencing the matrix decomposition chemistry. Secondly, the GO-novolac mixtures were cured at 100℃ in a vacuum oven for 48 hours, and FT-IR analysis was carried out on the cured samples to evaluate crosslinking mechanisms of the GO-novolac mixtures. FT-IR data showed intensity changes for functional groups associated with cross linking reactions compared to pure novolac. This suggests that the GO additive is influencing the crosslinking of the novolac resin matrix. Thirdly, the cured sample mixtures were carbonized at 500℃ for 5 hours in a sand bath. As the final fourth step, the samples were heated to 2500℃ for 1 hour in a graphitization furnace. Heat treatments were performed in inert conditions. The graphitic quality of the graphitized samples was characterized using X-ray Diffraction (XRD), Raman, and Transmission Electron Microscopy (TEM). XRD analysis revealed a high degree of crystallinity in the graphene oxide-novolac C-C composites. This is attributed to the templating ability of GO which changes the trajectory of the material from a non-graphitizing to a graphitizing path. Additionally, the onset of crystallite parameters occurs roughly 500 ℃ lower in the graphene oxide novolac composite compared to pure novolac. This latter observation suggests that high graphitic quality could be obtained at lower process temperatures, netting energy savings and CO2 reduction.