Porous SiOC beads by freeze-drying polycarbosilane emulsions

Porous silicon oxycarbide (SiOC) beads were prepared by freeze-drying of water-in-oil (w/o) emulsion, containing water and polycarbosilane (PCS) dissolved p-xylene in the presence of sodium xylenesulfonate (SXS) as an emulsifier. The emulsion was frozen by being dropped onto a liquid N₂ bath, which resulted in 1-2 mm sized beads. After curing at 200 °C for 1 h in air and subsequent pyrolysis at 800°C for 1 h in an Ar gas flow, porous SiOC beads were obtained. Freeze-dried beads showed lamellae-shaped macro-pore structures at a moderate freezing rate due to a phase separation behavior of PCS during freezing, while no lamellae pores were formed at a very high freezing rate. Lamellae-shaped pores developed along the freezing direction. Water droplets that were formed in w/o emulsion converted to spherical pores after drying. The combined processes of producing PCS emulsion and freeze-drying of emulsion resulted in two types of macro-pores: lamellae-shaped and spherical pores. Meso-pores, of which specific surface area and average pore size were 71.5 m² g^-1 and 4.85 nm, respectively, were formed inside the SiOC strut.