SiC/MCM-48 and SiC/SBA-15 Nanocomposite Materials

Silicon carbide (SiC) was infiltrated into the ordered mesoporous molecular sieves MCM-48 and SBA-15 using chemical vapor infiltration (CVI) of dimethyldichlorosilane (DDS) and hydrogen as the carrier gas. The infiltration process was followed ex situ using nitrogen physisorption measurements, small- and wide-angle X-ray diffraction, X-ray photoelectron spectroscopy, IR spectroscopy, and transmission electron microscopy. For MCM-48, infiltration at lower temperatures (1023 K) affords a thin, X-ray amorphous, SiC-based coating on the inner surface of the molecular sieve and the pore size of the mesoporous host decreases from 2.4 nm into the micropore regime (<2 nm). At higher temperature (1163 K), the deposition of 20−30-nm sized β-SiC particles is observed on the outer surface of the mesoporous particles as a process competitive to the pore filling. The crystalline nanoparticles form a hard protective coating on the outer surface of the larger spherical MCM-48 particles resembling hedgehog-like core−shell particles composed of an inner ordered mesoporous matrix and a hard nanosized silicon carbide coating. For SBA-15 it is shown that in the early stages of the CVI process at 1118 K, an ultrathin coating is produced that mainly consists of silicon oxycarbide. Subsequently, X-ray amorphous SiC is formed on top of this coating. In SBA-15, along with the formation of the coating, the pore size decreases from 5.5 to 3.0 nm, but further deposition leads to inhomogeneous coatings, and pore blocking and crystalline β-SiC particles are detected on the outer surface of the porous matrix by means of dark field transmission electron microscopy and wide-angle X-ray diffraction. The CVI process results in a significant enhancement of the thermal stability of SBA-15 even for very small degrees of filling.