Gradient pore size distributions in porous silicon oxycarbide materials

Hybrid materials have been obtained using a given concentration of tetraethylorthosilicate (TEOS) and polydimethylsiloxane (PDMS). Different size and shape monolithic samples have been achieved by using containers of both different evaporation areas and heights. The hybrid materials have been pyrolyzed under nitrogen atmosphere to yield porous silicon oxycarbide glasses, consisting of interconnected spherical particles and a continuous open-pore channel. The texture of the glasses shows a gradient particle size distribution (PtSD) from the top (2.88 μm) to the bottom (7.83 μm) of the samples. This gradient in the particle size leads also to a gradient distribution of porosities in the oxycarbide glasses. Thus, pore sizes increases from 6 μm at the top evaporation face to 13 μm at the bottom of the samples, and porosities in the corresponding regions decrease from 77% to 63%, respectively.The gradient distribution has been assigned to a gradient concentration of PDMS in the hybrid preceramic material as determined by the elemental analysis. The carbon concentration (free and covalently bonded carbon) in the hybrid materials increases with the sample thickness or height. However, the lower concentration of PDMS molecules found at the bottom of the hybrid samples leads, after pyrolysis, to higher values of retained carbon and pore sizes in the SiOC glasses. An interesting finding is that the gradient PDMS distribution also provokes a gradient nanodomain size of the segregated free carbon as a form of graphite, typical in SiOC glasses.