Study On Synthesis Of SiC Precursor And Processing Of Fiber Formation

Continuous SiC fibers are critical for high-performance composites. The strategy to prepare SiC fibers is in four steps: (1) synthesis of a precursor polymer, usually polycarbosilane(PCS), consisting of Si and C elements; (2) spinning of the precursor polymer into "green fibers"; (3) curing the fibers to make the polymer chains crosslinked and (4) pyrolysis of the cured fibers at above 1300℃in inert atmosphere. Although SiC is famous in being durable in rigorous environment, in practice, the temperature limitation of general grade SiC fibers is under 1200℃due to high content of oxygen, free carbon and coarseness of crystalline. This work is focusing on preparing SiC fibers with higher temperature resistance.Oxygen content of SiC fiber was high by adopting melt-spinning method, which limits the improvement of strength and high temperature endurance properties. Furthermore, its application was limited. However, acquired high molecule weight(HMW-PCS) fiber can be sintered straightly, which reduces its oxygen content and increase the high temperature resistance. This research not only helps to improve the formation characteristic of the PCS fiber, but also provides a new method to study the high-softening point HMW-PCS fiber.In this paper, emphasis was laid on combination of HMW-PCS powders, the rheological properties of different HMW-PCS solution systems, as well as the formation of HMW-PCS fibers. It was found that changes of reaction conditions could regulate molecular weight distribution. The component and structure of HMW-PCS powders were studied by means of IR, GPC and NMR. The rheological property of HMW-PCS solution system and influence on dry-spinning were systematically studied. Influence of temperature, pressure and winding velocity on spinning of HMW-PCS solution was also studied. It was for the first time to acquire the fine diameter HMW-PCS fiber felts by electrospinning technology.It was found that the branching degree of HMW-PCS molecule chain rose as the increase of reaction temperature. Its branching degree increased from 29.8% to 46% as the reaction temperature changed from 435℃to 470℃. HMW-PCS/Xylene solution was shear stress diluting fluid and its flow index was about 0.9, whose properties are close to the characteristics of Newton fluid as the concentration increased. The viscous activation energy of HMW-PCS/Xylene solution increased from 15.38kJ/mol to 80.16kJ/mol as the concentration increased from 38% to 68%. In the electrospinning technology, the surface of HMW-PCS fiber became smooth by adopting mixed solvent system.