| Abstract
| - Chemical vapor infiltration of ordered mesoporous silica is used as a method for the fabrication of ordered SiC/silica nanocomposites. The infiltration process enhances the thermal stability of the 3D pore structure, and dissolution of the silica matrix affords high-surface-area SiC.
- Silicon carbide (SiC) was infiltrated into the ordered mesoporous molecular sieves MCM-48 and SBA-15 using chemical vapor infiltration (CVI) of dimethyldichlorosilane (DDS) andhydrogen as the carrier gas. The infiltration process was followed ex situ using nitrogenphysisorption measurements, small- and wide-angle X-ray diffraction, X-ray photoelectronspectroscopy, IR spectroscopy, and transmission electron microscopy. For MCM-48, infiltration at lower temperatures (1023 K) affords a thin, X-ray amorphous, SiC-based coating onthe inner surface of the molecular sieve and the pore size of the mesoporous host decreasesfrom 2.4 nm into the micropore regime (<2 nm). At higher temperature (1163 K), thedeposition of 20−30-nm sized β-SiC particles is observed on the outer surface of themesoporous particles as a process competitive to the pore filling. The crystalline nanoparticlesform a hard protective coating on the outer surface of the larger spherical MCM-48 particlesresembling hedgehog-like core−shell particles composed of an inner ordered mesoporousmatrix and a hard nanosized silicon carbide coating. For SBA-15 it is shown that in theearly stages of the CVI process at 1118 K, an ultrathin coating is produced that mainlyconsists of silicon oxycarbide. Subsequently, X-ray amorphous SiC is formed on top of thiscoating. In SBA-15, along with the formation of the coating, the pore size decreases from5.5 to 3.0 nm, but further deposition leads to inhomogeneous coatings, and pore blockingand crystalline β-SiC particles are detected on the outer surface of the porous matrix bymeans of dark field transmission electron microscopy and wide-angle X-ray diffraction. TheCVI process results in a significant enhancement of the thermal stability of SBA-15 evenfor very small degrees of filling.
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