Ordered mesoporous SiOC and SiCN ceramics from atmosphere-assisted in situ transformation

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 degrees C for 10 h and at 1000 degrees C for 10 h for air- and ammonia-assisted in situ transformations, respectively. SAXS, XRD, N-2 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 m(2) g(-1)), large pore volumes (0.4-0.8 cm(3) 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 degrees C to mesoporous SiCN-1400 via ammonia-assisted in situ transformation.