CHARACTERIZATION OF LOW THERMAL-CONDUCTIVITY PAN-BASED CARBON-FIBERS

The microstructure and surface chemistry of eight polyacrylonitrile (PAN)-based carbon fibers heat treated at relatively low temperatures to retain low thermal conductivity were determined. Properties of these fibers (hereafter called LTC fibers) were compared with those of PAN-based carbon fibers subjected to higher heat treatment temperatures (HTT's). Wide-angle x-ray diffraction shows that LTC fibers have a turbostratic structure with large 002 d-spacings, small crystallite sizes, and only moderate preferred orientation. Small-angle x-ray scattering results indicate that LTC fibers do not have well-developed pores. Transmission electron microscopy shows that the texture of LTC fibers consists of multiple sets of parallel, wavy, bent layers that interweave with each other, forming a complex, three-dimensional network oriented randomly around the fiber axis. Crystallite size and extent of graphitization both increase with increased HTT. However, scanning electron microscopy indicates that HTT has little effect on the surface texture of PAN fibers. X-ray photoelectron spectroscopy analyses find that concentrations of surface oxygen and nitrogen decrease with increasing HTT, which is consistent with increased volatilization of nitrogen and greater extent of graphitization.