James Alan Rabe

Fine-diameter polycrystalline SiC fibers

Abstract Various organosilicon polymers have been converted to small-diameter, polycrystalline silicon carbide fibers by melt-spinning, cross-linking and pyrolyzing to high-temperature in argon. Several wt% boron was doped into the fibers before pyrolysis. Use of polycarbosilane precursor gave 8–10 μm diameter fibers having up to 2.6 GPa tensile strength, 450 GPa elastic modulus, 3.1–3.2 g/cm3 density. The microstructure consists of >95 wt% β-SiC crystallites of 30–40 nm average crystallite size. Stoichiometric fibers or fibers having excess carbon content have been prepared. Fiber has been thermally aged under inert conditions at 1800°C for 12 h with minimal strength and microstructural change. Stoichiometric fiber maintains higher strength after oxidative aging at 1370°C. Current processing efforts are aimed at preparing the fiber in continuous tow form.

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

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.

Characterization of the Reaction Products of Symmetrical Dimethyltetrachlorodisilane with Hexamethyldisilazane by29 Si NMR Spectroscopy

Abstract Ceramic fiber-based composites are becoming an increasingly important group of structural materials (3). Useful ceramic fibers can be prepared by melt-spinning, cure and pyrolysis of a polymethyldisilylazane polymer precursor (4, 5), which is, in turn, the reaction product of mixtures of the following methylchlorodisilanes (I) with the indicated approximate compositions: This mixture is reacted with excess hexamethyldisilazane (HMDZ) to generate polymeric species with the approximate composition II, along with trimethylchlorosilane [Me3SiC1] and ammonium chloride [NH4C1] as by-products.