Synthesis And Properties Of Polycarbosilane As Precursor With High Ceramic Yeild

Precursor polycarbosilane containing acetylenic and Si-h group (PCAS) has been successfully prepared by the reaction of dilithioacetylene with methyldichlorosilane, using methylhydrogen dichlorosilane, trichlorethylene and n-butyl lithium as starting material, in this paper. By orthogonal experimental analysis of the raw material content on PCAS heat performance. The composition, structure and properties were investigated by the characterization of IR,1H NMR,13C NMR, elemental analysis, DSC, TGA, EDS. The curing reaction behaviors of a novel polycarbosilane were investigated by nonisothermal DSC method, FT-IR and rheological analysis. The kinetic parameters of this system were calculated by Kissinger, Ozawa and Crane equations:the apparent activation energy E=122.5kJ/mol (by Kissinger and Ozawa methods), the reaction order n=0.95(by Crane equation), the frequent factor A=9.74X108(by Kissinger method). The kinetic model of the curing process based on order reaction equation was established, which was used for modeling the curing reaction characteristics. The Diels-Alder reaction and the hydrosilylation reaction were investigated by FT-IR and rheological analysis. The viscosity was from7.12X103Pa·s to1.52×107Pa· s at220℃. TGA test indicated that PCAS resin had excellent thermal stability. Under nitrogen, the temperature of5%mass loss (Td5) was613℃and the residual of mass at1000℃was89%. Under air, the Td5was601℃and the highest mass residual ratio was87.5%based on the initial mass at1000℃.Pyrolysis behavior and structure evolution of the cured PCAS were studied by means of XRD, as well as Raman, SEM-EDS, TEM (SAED and HRTEM) and elemental analysis. The polymer to ceramic conversion was completed at1600℃and the results revealed that the ceramic consisted of β-SiC and a-SiC. The composition of the ceramic was near-stoichiometric with molar ratio of Si/C (1.02:1) except rare and localized free carbon inclusions. The SiC ceramics exhibited high thermo-oxidation resistance at elevated temperatures in air atmosphere.