Quantitative Microstructure Study of PAN and Pitch-based Carbon Fibers Through TEM and X-Ray Scattering Analysis

Abstract

Carbon fiber is widely used reinforcement material for composite applications. Modulus and strength of carbon fiber can be tailored depending on the precursor materials and processing conditions. However, microstructure-property relationships and defects on the crystal scale induced by processing still need to be explored. We studied microstructures of two typical carbon fibers from polyacrylonitrile (PAN) and pitchbased precursor and compared with its X-ray scattering properties and preliminary simulation results including crystal size, orientation and spacing. For this purpose, transmission electron microscopy (TEM) sample was prepared using focused-ion beam (FIB) for both longitudinal and transverse directions of carbon fiber. Pitch-based carbon fiber shows large graphitic structures with triangular defects, meanwhile PAN-based carbon fiber shows smaller graphitic structures with random distribution. In the case of pitch-based carbon fiber, crystalline size ~40 nm can be observed from the low magnification TEM image. X-ray scattering pattern also exhibit the differences between two fibers. The wide-angle X-ray scattering (WAXS) pattern of each carbon fiber shows graphitic peak at 26 and that is sharper for the pitch-based carbon fiber for both radial (2) and azimuthal () direction. This corresponds to the large graphitic crystalline size and high alignment degree of fiber structures. Full-width half maximum (FWHM) of the azimuthal angle for pitch-based and PAN-based carbon fiber is 5.9 and 31.6, respectively. This confirms the high alignment degree of pitch-based carbon fiber ~ 0.99. Precise measurements of the crystal size, orientation and TEM imaging of graphitic packing allows for further comprehension of associations between processing and final material properties which will enable customization of microstructures for property targets.