C

The polymeric route to silicon-based ceramics was developed by Yajima et al., who used polycarbosilane (-[MeH-SiCH2]x-) as a precursor to SiC [502, 516-518]. One of the drawbacks of this method was the initial Si:C ratio of 1:2, which resulted in excess carbon in the final product. The nonstoichiometric product offers mechanical properties inferior to those of bulk stoichiometric SiC [519]. To overcome these problems, pure and modified polymethylsilane (PMS) precursors with a Si:C ratio of 1:1 were successfully used by Laine et al. to obtain stoichiometric SiC [520]. Unmodified PMS, -[MeSiH]- [521, 522], with the stoichiometry required to form SiC, offers the potential to be the SSP to SiC. The only gaseous species that should be released in the pyrolytic conversion of PMS is H2 with the retention of the Si:C ratio. However, unmodified PMS undergoes a rearrangement upon heating [520, 523] and loses 7-8 wt.% by releasing CH4 and H2 (Scheme 23). As a consequence, pyrolysis of unmodified PMS gives a mixture of SiC and excess Si, which shows the importance of molecular architecture and its inherent chemical behavior for the composition of the final material. The functionalization of PMS by vinyl groups was shown to compensate for the carbon loss by introducing additional carbon content. Depending upon the functionality of the vinyl groups, it is possible to adjust the composition from silicon rich to stoichiometric SiC and to carbon-rich ceramic.

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