Porous and high surface area silicon oxycarbide-based materials-A review

Silicon oxycarbide (SiOC)-based materials are a class of polymer-derived ceramics that enables the formation of a homogeneous structure at the molecular level starting from polymer precursors. In this system, oxygen and carbon atoms share bonds with silicon atoms in the amorphous network structure while elemental carbon, and possibly nanosized SiO2 and SiC nanodomains may co-exist. Because of the flexibility of molecular level composition and microstructure designs, the systems can be made porous with high specific surface areas by changing the precursor compositions and the ceramization conditions. In this review, two strategies of creating porous SiOCs are discussed: conventional approach of using fugitive fillers, as well as pore formation and selective removal of certain SiOC matrix compositions (such as carbon, SiO2, or SiC) at the molecular level. For the former, it includes ceramic replication of an organic template, direct foaming, and sacrificial pore formers. For the latter, it includes molecular level pore formation, molecular level species removal, and SiOC porous network creation through molecular templates. Direct pore formation can be achieved by changing processing conditions, using different precursor architectures, and using different hydrosilylation agents. For SiOC porous network creation through molecular level species removal, it includes molecular level free carbon removal, molecular level SiO2 nanocluster removal, and molecular level carbon removal from SiC (and possibly BCx for SiOBC). To understand single nanometer (<10 nm) pore formation and phase separation for selective species removal, SiOC nanostructure models and composition descriptions after the pyrolysis are explained.