Sol-Gel Processing of a Glycolated Cyclic Organosilane and Its Pyrolysis to Silicon Oxycarbide Monoliths with Multiscale Porosity and Large Surface Areas

摘要

This study is related to the preparation of silicon oxycarbide monoliths comprising a hierarchical network build-up from the pyrolysis of monolithic organosilica gels. A novel glycol-modified 1,3.5-trisilacyclohexane-carbosilane "[Si(OCH2CH2OH)2CH2]3" was processed via a polymerization-induced phase separation process in hydrochloric acid solution containing the Pluronic P123 block copolymer and potassium chloride. Highly porous organosilica monoliths with interconnected macropores and a large amount of uniformly sized polymer-lemplated mesopores within the macroscopic framework domains were obtained after supercritical fluid extraction. The monoliths were pyrolyzed in argon atmosphere at 1000 °C to yield silicon oxycarbide monoliths by maintaining the hierarchical porosity of the organosilica gel. Both the organosilica gels and the silicon oxycarbide monoliths were thoroughly investigated with standard characterization techniques. If no salt was used for the preparation of organosilica gels, no distinct macroporous structures were obtained. Instead, the gels show the characteristics of typical aerogels with surface areas of 600-900 m~2 g~(-1) and mesopores with pore sizes > 10 nm. A different behavior was observed if potassium chloride was added to the sols. Then, the system showed the tendency for a polymerization-induced phase separation leading to gels exhibiting multiscale porosity. The onset of the phase separation as well as the macropore/domain size thereby strongly depends on the concentration of KCl. These gels with a surface area of about 1070 m~2 g~(-1) and pore diameters of about 11.6 nm were subjected to pyrolysis at 1000 °C under inert gas atmosphere. Although volume shrinkages of 54% were observed, the monoliths maintained their shape and structural features. The surface areas remained rather high with 531 m~2 g~(-1), and the diameter of the mesopores dropped to 9.1 nm. From solid state NMR measurements and elemental analysis of the pyrolyzed sample, the formation of true silicon oxycarbide monoliths with multiscale porosity and a composition of SiC_(0.23)O_(1.53) + 0.58C_(free) were proven. From resonant ultrasound spectroscopy measurements, a Young's modulus value of 1.42 GPa was obtained.