Song Yongcai

Defects of polymer-derived Si−C−O fibers and their originations

Defects of polymer-derived Si−C−O fibers were intensively studied by the SEM and TEM techniques and their originations were also discussed on the basis of factors experiments. The defects were found mainly in the form of strumaes, pits and splits on surfaces as well as microflaw networks, porosity clusters and inclusions in the bulk. Factors experiments reveal that a nonuniform or an insufficient curing would result in larger-sized strumaes or interior microflaws. Gas evolution rates due to different firing rates have a great influence on the formation of internal microflaws or porosity clusters and some oxidation-induced pits or splits may be formed on surfaces because of a trace of oxygen or water vapor accumulated from the flowing inert atmosphere during pyrolysis.

Evolution of molecular composition of polycarbosilane and its effect on spinnability

The molecular composition of polycarbosilane (PCS) and its evolution in the synthesis process, as well as the contribution of each component to the spinnability of PCS, were explored by combining distillation separation with gel permeation chromatogram peak-splitting. The results show that PCS is composed of four kinds of molecules with different structures, namely ML, MM, MH, and MSH (number-average molecular weights of 531, 1207, 2731, and 10 977, respectively). In the synthesis process, PCS changed from containing two components to three components, and finally to four components. The PCS can be spun when it is composed of ML, MM, and MH, but is not suitable for spinning when it is composed of one, two, or four components. For a PCS having three components, increasing the amount of MH and restricting ML to within a certain range can enhance the spinnability of PCS. The underlying mechanism is controlled by the linear structure of MH and its low melting point by there being an appropriate ML content, which ensures melting, de-foaming, and stability in spinning. The final optimum ratio is found to be an MH content of 30–45%, an MM content of 30–51%, and an ML content of 18–25%.