Ordered Mesoporous SiOC and SiCN Ceramics from Atmosphere-Assisted in Situ Transformation

Abstract

An atmosphere-assisted heating process was utilized to transform mesostructured SiC−C nanocomposites in situ into ordered mesoporous SiOC and SiCN ceramics with relatively small structural shrinkage. The mesostructured SiC−C nanocomposites were fabricated using commercially available polycarbosilane (PCS) as a ceramic precursor and mesoporous carbon CMK-3 as a hard template that itself was prepared by a nanocasting procedure from mesoporous silica SBA-15. Reactive gases including air and ammonia were employed to simultaneously incorporate O or N into SiC ceramics and oxidize or reduce the carbon template and excess carbon deposits. The procedure was carried out at 500 °C for 10 h and at 1000 °C for 10 h for air- and ammonia-assisted in situ transformations, respectively. SAXS, XRD, N2 sorption, and TEM analyses revealed that the mesoporous SiOC and SiCN ceramics exhibit open, continuous frameworks similar to that of the primary template ordered mesoporous SBA-15. The ordered mesoporous SiOC and SiCN ceramics have high surface areas (200−400 m2 g-1), large pore volumes (0.4−0.8 cm3 g-1), and narrow pore size distributions (4.9−10.3 nm). The structural shrinkage from mesostructured SiC−C composites to mesoporous SiC-based ceramics decreased with increasing initial pyrolysis temperature for SiC−C nanocomposites owing to the improvement of structural rigidity. This shrinkage was found to be as low as 2.6% from mesostructured SiC−C pyrolysis at 1400 °C to mesoporous SiCN-1400 via ammonia-assisted in situ transformation.