Structure and pyrolysis of neutron irradiated polycarbosilane ceramic precursor fibers

Silicone carbide (SiC) ceramic fiber is an attractive reinforcing material in advanced composites because of its excellent performances at elevated temperatures. Polycarbosilane is the most popular ceramic precursor polymer to fabricate SiC ceramic fiber. In this process, the precursor is synthesized, melt-spinned, cured, and pyrolyzed to ceramic fibers. Curing the polymer fibers before pyrolysis is a very important step to make the fibers infusible and increase the ceramic yield, with oxygen content introduced as low as possible to enhance the mechanical properties of the products. Irradiating PCS precursor fibers by electron beams or γ-rays under inert atmosphere is an effective curing method. However, the required dose for infusible PCS is about 10-15MGy, which is a tough task, and high cost, with conventional irradiators. In this study, polycarbosilane (PCS) ceramic precursor fibers were irradiated to various neutron fluxes in a nuclear reactor in N2 gas atmosphere. The chemical structure, thermal degradation behavior and gel content of the precursor fibers before and after irradiation were investigated by FT-IR, TGA, and extracting soluble component with xylene, respectively. The curing effect was evaluated, and the reaction mechanism was discussed. The FT-IR spectra showed that the absorption band at 2100cm-1 assigned to v(Si-H) decreased apparently in intensity with increasing neutron flux, while the absorption band of v(Si-C) in Si-CH2-Si at 734cm-1 increased with the neutron flux. This indicates that the neutron irradiation caused rupture of Si-H bonds of PCS fibers and formation of Si-CH2-Si bridge structure. There is no absorption band v(Si-O) at 1080cm-1, suggesting that oxygen content introduced in this process was very low, a favorable condition for synthesizing SiC ceramic fibers with excellent properties at high temperatures. The TGA curves indicated that the residual mass ratio at 1273 K was about 95% for the irradiated PCS fibers, compared with only 67% for the un-irradiated ones. The pyrolysis products were dusty for un-irradiated PCS fibers, blocky for PCS fibers irradiated by low neutrons (≤8.7 x 1016cm-2), and fibrous for PCS fibers irradiated by high neutron fluxes (≥2.6 x 1017cm-2). The gel content of PCS precursor fibers increased with neutron flux. It is up to 80%, which meets the need of infusibility, with a flux of 2.6 x 1017cm-2. In conclusion, after irradiation of a neutron flux of 2.6 x 1017cm-2, PCS ceramic precursor fibers are unsolvable and infusible, and fibrous pyrolysis products are obtained. Curing PCS fibers by reactor neutron irradiation, which has not been reported in literature, is a novel and effective method. (authors)