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New nanosized Si/O/C composites have been synthesized by laser-aerosol interaction. The 29 Si spectra revealed a deep rearrangement of Si bonding in the samples with respect to the original aerosol. From the siloxane (CH 3 ) 3 SiOSi(CH 3 ) 3 (MDSO) and the silane C 2 H 5 -OSi(CH 3 ) 3 (ETMS) aerosols with M environments, solids with a wide variety of SiO x C 4-x environments were produced. These environments were shown to be randomly distributed. On the other hand, the same Q environment from the Si(OC 2 H 5 ) 4 (TEOS) aerosol remained in the powders. This can be rationalized in terms of Si-O and Si-C bond exchanges. After pyrolysis and before crystallization, there was a clear deviation from the random model, indicating that the material started to organize. 13 C and 1 H NMR distinguished Si-C and aromatic free C. Both spectroscopies indicated that there was less C in the Si/O/C phase of the ex-MDSO powder than in the ex-ETMS one. The ex-MDSO Si/O/C was therefore less oxygenated, contained less carbon, and consequently the most condensed of the two. This study illustrates the great versatility of the method. The local environment of the Si in the precursor is modified through a limited bond redistribution and condensation during the synthesis and before pyrolysis. The choice of a Q organosilane, not allowing bond redistribution, leads to a highly porous, nanosized silica, because excess aromatic carbon is burned off by thermal oxidation. On the other hand, a choice of a precursor with a mixed SiO x C 4-x environment leads after pyrolysis under an inert atmosphere to a mixture of Si/ O/C and free aromatic carbon.

Using transmission FT-IR, surface characterizations are performed on ultrafine Sic powders produced by a laser-driven method. The surface species sensitive to thermal treatments (OH, CH,, C = 0, SiH,) are identified on samples evacuated at various temperatures. Absorption bands attributed to overtones of the fundamental Si-C modes are also present in the IR spectra and remain unchanged after treatment. The reaction of Sic with oxygen and water vapor produces a layer of silica on the sample and gaseous CO,; the reaction with ammonia results in a partial nitridation of the surface, with the formation of NH, groups that apparently increase the stability of the Sic against oxidation; and reaction with hydrogen produces methane in the gas phase and causes the disappearance of the bands due to surface CH, groups. [Key words: surface, silicon carbide, infrared, Fourier analysis, spectroscopy.]

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