Wavy Graphene-Like Network Forming during Pyrolysis of Polyacrylonitrile into Carbon Fiber

Abstract

Carbon fiber (CF) obtained by pyrolysis of polyacrylonitrile (PAN-CF) surpasses metals in properties suitable for diverse applications such as aircraft manufacture and power turbine blades. PAN-CF obtained by pyrolysis at 1200− 1400 °C shows a remarkably high tensile strength of 7 GPa, much higher than pitch-based CF (pb-CF) consisting of piles of pure graphene networks. However, little information has been available on the atomistic structure of PAN-CF and on how it forms during pyrolysis. We pyrolyzed an acrylonitrile 9-mer in a carbon nanotube, monitored the course of the reaction using atomic-resolution electron microscopy and Raman spectroscopy, and found that this oligomer forms a thermally reactive wavy graphene-like network (WGN) at 1200−1400 °C during slow graphitization taking place between 900 and 1800 °C. Ptychographic microscopic analysis indicated that such material consists of 5-, 6-, and larger-membered rings; hence, it is not flat but wavy. The experimental data suggest that, during PAN-CF manufacturing, many layers of WGN hierarchically pile up to form a chemically and physically interdigitated noncrystalline phase that resists fracture and increases the tensile strength�the properties expected for high-entropy materials. pb-CF using nearly pure carbon starting material, on the other hand, forms a crystalline graphene network and is brittle.

Toward three-dimensionally ordered nanoporous graphene materials: template synthesis, structure, and applications

“…We addressed the qualification and quantification of surface defects in NPGs by TPD 86 and Raman spectroscopy, 159–161 whereas Poisson statistics are useful for quantitatively determining the distribution of graphene layers 86,162–164 in continuous 3D graphenes. The complementary and convincing direct observation of defects by electron microscopy techniques 3,165–167 will result in a much deeper understanding of the nanostructure. These insights into the atomic level structure of NPGs including the chemical and topological defects will be conducive to further control of high-quality NPGs.…”