Evolution of microstructure and properties of phenolic fibers during carbonization

The microstructure evolution of phenolic fibers carbonized at different temperatures and the influence of the resulting microstructure on the mechanical and electrical behaviors were investigated using a combination of techniques including thermogravimetric analysis-mass spectroscopy (TG-MS), Fourier transform infrared spectroscopy (FT-IR), laser Raman spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 physical adsorption, tensile strength and electrical conductivity measurements. The results showed that below 500 °C aromatic union was crosslinked by aliphatic bridges; above 500 °C with increasing temperature the polymer network was destroyed, hexagonal carbon layers were gradually formed; above 850 °C the content of the hexagonal carbon layers was further increased due to the transformation of the amorphous carbons in the fibers; above 1500 °C slow evolution toward ideal graphite was occurred. The accessible micropores (<1.2 nm, 200-1040 m2/g) could be detected in the range of 600-950 °C. The tensile strength of the carbon fibers exhibited a maximum value of 640 MPa at 550 °C. The electric conductivity increased with increasing temperature and reached a maximum value of 2.45 × 104 S/m at 2800 °C.