Polymer Architecture Controls Energetic Stabilization of Polymer Derived Ceramics

1. Polymer Architecture Controls Energetic Stabilization of Polymer Derived Ceramics Alexandra Navrotsky, University of California-Davis, DMR 0907792 GM35 HN1 Polymer derived ceramics (PDC) are lightweight silicon-oxygen-carbon-nitrogen materials suitable for high temperature applications. Though amorphous to X-ray diffraction, they contain nanodomains of different compositions and structures. In a recent collaborative study of SiCN PDCs from the research groups of Navrotsky and Sen at the UC Davis and Riedel and Kleebe at TU Darmstadt, Germany, funded by the Materials World Network program, two polysilylcarbodiimides with branched (GM35) and linear (HN1) geometries were synthesized, pyrolyzed, and characterized using solid state NMR and oxide melt solution calorimetry. After pyrolysis to 800 oC, the GM35 ceramic contains sp2 C-N-Si bridges (mixed bonding) that connects Si3N4 and C nanodomains together, while there are no connecting elements between the nanodomainsin the HN1 ceramic. NMR results have also shown that there is hydrogen present in the GM35 ceramic and that it is situated at the interfaces of the Si3N4 and C nanodomains, while the HN1 ceramic has a lower concentration of H. Calorimetryshows that the GM35 ceramic is more energetically stable than the HN1 ceramic. Both are energetically stable relative to a mixture of silicon nitride, silicon carbide, and graphite.

2. Mixed Bonding Stabilized via Hydrogen Bonding at Interfacial Regions Alexandra Navrotsky, University of California-Davis, DMR 0907792 After further pyrolysis to 1100 oC, NMR results showed that the relative concentration of mixed bonding between the nanodomains decreased substantially. This change is concurrent with a drop in hydrogen content and the ceramic becomes less energetically stabile. Based on NMR studies which locate much of the hydrogen in the mixed bond region, it is hypothesized that the mixed bonding is stabilized by the presence of hydrogen bonding at the interfacial regions between the Si3N4 and C nanodomains. The loss of hydrogen at higher pyrolysis temperatures provides the entropic driving force for the destruction of mixed bonding. It is concluded that the hydrogen may play a defining structural and thermodynamic role in PDC formation and can not be ignored, even when present at seemingly low levels.