Macroporous polymer-derived SiO2/SiOC monoliths freeze-cast from polysiloxane and amorphous silica derived from rice husk

A freeze-casting route towards macroporous SiOC/SiO₂ ceramic nanocomposites from preceramic polymers was developed. Amorphous SiOC/SiO₂ monolith with pore channels aligned along the freezing direction were obtained from commercially available methyl-phenyl-vinyl-hydrogen polysiloxane (Silres® H62C) and amorphous silica derived from rice husk ash freeze-cast with water or tert-butyl alcohol, crosslinked and pyrolyzed at 1100 °C in nitrogen. The influence of processing parameters such as solvent (tert-butyl alcohol or water), polymer to silica ratio (2:1, 1:1, 1:2), cooling rate (2, 4, 6 °C/min) and pre-crosslinking of polysiloxane on the porosity and structure of the obtained ceramic nanocomposites were assessed by X-ray tomography, XRD, solid state NMR, scanning electron microscopy and mercury porosimetry. The microstructure of SiOC ceramics derived from the Silres H62C polysiloxane was studied as well.