Generation of meso- and microporous structures by pyrolysis of polysiloxane microspheres and by HF etching of SiOC microspheres

The porosity of polysiloxane microspheres obtained by emulsion processing of variably modified polyhydromethylsiloxane (PHMS) and subjected to pyrolysis in an Ar atmosphere at 450-650 °C was studied. Materials having micro- and mesopores with specific surface areas (SSAs) of up to 786 m2/g and pore volumes of up to 0.35 cm3/g were obtained. A high porosity was displayed by the microspheres heated at 600 °C that underwent deep depolymerization processes. Some polysiloxane microspheres were ceramized at temperatures of 1200-1500 °C and were subjected to etching by 35% aqueous HF. The microspheres heated to 1200-1400 °C were free of microcracks, whereas those ceramized at 1500 °C showed microcracks and macropores, although they preserved their spherical structure well. All of the microspheres ceramized at temperatures of 1200-1400 °C had low porosity. HF etching granted high micro- and mesoporosity to the materials ceramized at 1300-1500 °C. Microspheres heated at 1500 °C showed specific surface areas above 1000 m2/g after etching. These microspheres had low oxygen contents and were mostly composed of silicon carbide. Since they also showed macroporosity, HF etching of the polysiloxane microspheres ceramized at 1500 °C could be used to obtain hierarchically mesoporous-macroporous ceramic microspheres.