Rheological properties, structural and thermal elucidation of coal-tar pitches used in the fabrication of multi-directional carbon-carbon composites

Abstract

Two types of coal-tar pitches varying by softening points and quinolone insoluble (QI) contents, used in the fabrication of carbon-carbon composites (C/C's), were studied using various techniques regarding their physical and chemical transformations during pyrolysis and graphitization. These pitches were highly graphitizable and were found the best suited for fabricating high-density multi-directional C/C's. Raman micro-spectroscopy (RMS) of the pitches showed a gradual decrease in the width and frequency of the G band as pyrolysis and graphitization proceeded, corresponding to the decrease of non-aromatic C–C bonds and increase in the ordered layered graphite network. The pitches showed non-Newtonian behavior because of the presence of QI particles, and their viscosity decreased with increasing shear rate and temperature. The decrease in viscosity with increasing shear rates was found constant at all measured temperatures. RMS and elemental analysis showed that structural transformation in the pitch during pyrolysis took place most noticeably beyond 800 °C, resulting in an abrupt increase in the carbon/hydrogen ratio, due to the vanishing of mesophase structure and extension of the distorted carbon network. Sigmoidal curve fitting using the Boltzmann equation was found the best fit for the weight loss of these pitches during pyrolysis. Mesophase formation was found to take place at 425 °C in both pitches and its contents increased with increasing the retention time. The mesophase contents in the pitches were directly related to their thermal stability, coke yield, and ease of graphitization. Taken together, these coal-tar pitches seem to be very promising carbon matrix precursor to produce high density and easily graphitized multidirectional C/C's for aerospace applications.