Solid state NMR and TG/MS study on the transformation of methyl groups during pyrolysis of preceramic precursors to SiOC glasses

摘要

The sol-gel method was used to prepare two different starting gels containing SiCH_3-groups for the preparation of SiOC ceramics. Tounderstand the role of Si-H bonds in the incorporation of carbon into the SiOC network,gels prepared from a 1:2 mixtured of triethoxysilane and methyldiethoxysilane (T~HD~H2) and slely methyltriethoxysilane (T~Me) were investigated. Thermogravimetric analysis coupled with mass spectroscopy (TG-MS) in inert atmosphere was performed to attainan insight into the decomposition reactions involved during gel-glass tansformation. Samples calcined at diffeent temperatures up to 1000degC were characterized by ~(29)Si and ~(13)C magic anlge spinning nuclear magnetic resonance (MAS-NMR)spectroscopy. The presence of SiH groups in the starting gel allows an efficient conversion of Si-CH-3 groups into CSi_4 sites at lower temperatures. As a result,despite a much lower amount of carbon in the starting T~HD~D2 gel (C/Si=0.330 compared tothe T~(Me) gel (C/Si=1),the amount of carbon inserted into the SiOC network of both glasses is equivalent,but the T~(Me) sample contains the 10 fold amount of free carbon.