Abstract
The mechanical properties of carbon fibers (CFs) can be controlled by their internal structures such as the distribution of π-orbital-oriented domains, as well as the diameter and cross-sectional shape of the fiber. In this study, we investigated the carbon chemical structure maps of commercial polyacrylonitrile (PAN)- and pitch-based CFs using scanning transmission X-ray microscopy to evaluate the differences in the distribution of π-orbital-oriented domains. The graphene sheets in the PAN-based CFs have a fiber texture that is aligned along the fiber direction and randomly oriented within the cross section. The domain sizes within the cross section are less than the resolution limit (i.e., 50 nm). By contrast, the graphene sheets in the pitch-based CFs are aligned parallel to each other and form aggregates with a size ranging from approximately 100 nm to 1 μm within the cross sections. They form 200–300-nm stripes along the CF axis in the longitudinal sections.
Highlights
The microtexture of carbon fibers (CFs) plays a dominant role in controlling their physical properties [1,2,3]
The results showed that the distribution of the π-orbital-oriented domains in the CFs varied considerably, depending on the raw material
With respect to the PAN-based CFs, the graphene sheet structure was distributed along the fiber axis direction, and the cross section consisted of randomly distributed domains with sizes less than approximately 50 nm, as reported in a previous study [19]
Summary
The microtexture of carbon fibers (CFs) plays a dominant role in controlling their physical properties (e.g., tensile strength, tensile modulus) [1,2,3]. Useful information on the nanoscopic structure cannot be obtained using conventional elemental mapping techniques (e.g., electron probe micro analyzer) because CFs are composed entirely of carbon. The spatial resolutions of the conventional techniques that analyze chemical structures, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) spectroscopy [4,5], range from μm to mm. This makes it difficult to observe detailed information on the inner texture of CFs. The spatial resolution of electron energy loss spectroscopy in a transmission electron microscope (TEM-EELS) is much higher (less than 1 nm) than that of conventional spectroscopic techniques.
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