Characterization of nanoporosity in polymer-derived ceramic fibres by X-ray scattering techniques

Abstract

Ceramic fibres with Si-C-O and Si-N-C-O compositions, prepared by pyrolysis of polymer precursors, generally have densities lower than those calculated from a volume additivity rule. However, techniques often used to detect porosity such as electron microscopy methods, surface area and porosimetry measurements show that little surface-connected porosity is present. X-ray scattering measurements, both wide-angle (WAXS) and small-angle (SAXS), show considerable scattering in the range 1° < 2θ < 10° (CuKα). Treatment of the scattering data by the classical Guinier (low angle limit) and Porod (high angle limit) methods indicate that closed, globular, nanometre-scale porosity (1 to 3 nm diameter) is present in all ceramic fibres examined. X-ray scattering power correlates quantitatively with the volume fraction porosity, as expected if porosity is the dominant facto affecting X-ray scattering. Nano-particles of excess carbon and ofβ-SiC nanocrystallites, though present, are minor contributors to the scattering of X-rays in these ceramic fibres. Fibres are three-dimensional, not of fractal dimension, and are not oriented. As density increases with increasing pyrolysis temperature, average pore size increases and pore volume fraction decreases. This results from a thermodynamically favourable reduction of surface free energy and apparently occurs by a viscous flow process.