Synthesis, Characterization And Thermal Decomposition Characteristics Of The The Preceramic - Poly Methyl Silane

SiC ceramic matrix composites could be prepared from organosilicon precursors. Three precursors: polymethylsilane(PMS), polymethylsilane containing nitrogen (N-PMS) and poly-methylsilane containing aluminium(Al-PMS) were designed and synthesized. The effect of synthesis condition on composition, structure and properties of precursors was studied by element analysis, GPC, IR and 1H-NMR. The pyrolytic conversion of three precursors in high temperature was investigated by IR, TG, DTA, GC and XRD. Furthermore, the composition, structure of pyrolytic products were systematically studied by element analysis, IR and XRD.Polymethylsilane was synthesized by dehalo-coupling of MeHSiCb with sodium. The optimum synthesis condition is 100℃, 6 hours. PMS was a highly branched structure, - [MeSiH]0.7 [MeSi]0.3, the molecular weight about 500 Da. PMS were sensitive to oxygen and sometimes pyrophoric. Ceramic yields of PMS was not high because low-molecular-weight volatile species escaped from the system during prolysis. After heat-treated at 280~300℃, the molecular weight increased up to 800 Da and ceramic yields increased to above 60%. The pyrolytic products mostly was beta-SiC microcrystalline, Si/C ratio was 1.23 and had small oxygen.N-PMS which had higher molecular weight than PMS was achieved through ethyldiamine ammonolysis with PMS at 80℃. Furthermore, the Si/C ratio of N-PMS could be controlled by adjusting the ethyldiamine quantum. After heat-treated 3 hours at 250~270℃, N-PMS was changed to solid with melting point between 110~130℃. The ceramic yields of N-PMS was up to about 70% and the pyrolytic products was stoichoimetric SiC which had 1.5wt% nitrogen more or less. The N-PMS pyrolytic conversion to ceramics was detailed investigated and found the major weight loss occurred between 250~850℃ because of Kumada rearrangement reaction.Al-PMS was synthesized from PMS and a small quantity Aluminium acetylacetonate. The reaction at about 240℃ by the condensation of Si-H bonds and the ligands of aluminium acetylacetonate accompanied by the evolution of acetylacetone; the molecular weight then increased by the cross-linking reaction with the formation of Si-Al-Si and Si-O-Al-O-Si bonds. It was found that 6-10wt% Aluminium acetylacetonate reaction with PMS, the product Al-PMS had molecular weight 1000 Da and melted between 100~120℃. The prolytic products had about 0.6wt% Al content and Si/C ratio 1.23.