Showing papers by "Florence Babonneau published in 1995"

The Evolutionary Process during Pyrolytic Transformation of Poly(N-methylsilazane) from a Preceramic Polymer into an Amorphous Silicon Nitride/Carbon Composite

Abstract: The pyrolytic evolution of poly(N-methylsilazane), –[H2SiN-Me]x–, from preceramic polymer to ceramic product is followed by heating samples of the partially cross-linked polymer, in 200°C increments, from ambient temperature to 1400°C. The intermediate products are characterized by chemical analysis, diffuse reflectance Fourier transform IR spectroscopy (DRIFTS), Raman spectroscopy, and 29Si and 13C magic-angle spinning (MAS) solid-state NMR. Spectro-scopic characterization indicates that the 1400°C pyrolysis products are amorphous silicon nitride mixed with amorphous and graphitic carbon (as determined by Raman spectroscopy), rather than silicon carbide nitride, as expected based on the presence of up to 20 mol% retained carbon. Efforts to crystallize the silicon nitride through heat treatments up to 1400°C do not lead to any crystalline phases, as established by transmission electron microscopy (TEM) and small-area electron diffraction (SAD). It appears that the presence of free carbon, along with the absence of oxygen, strongly inhibits crystallization of amorphous silicon nitride. These results contrast with the isostructural poly-(Si-methylsilazane), –[MeHSiNH]x–, which is reported to form silicon carbide nitride on pyrolysis.

Characterization of methyl-substituted silica gels with Si–H functionalities

Abstract: Gels have been prepared via hydrolysis–condensation reactions of triethoxysilane (TREOS) and methyldiethoxysilane (MDES) in various ratios. The hydrolysis–condensation reactions of the precursors have been investigated by 29Si and 17O solution NMR. Both precursors present a very high reactivity towards hydrolysis and condensation reactions, and formation of co-condensed species will be discussed. These highly reactive precursors lead to transparent gels over a large composition range, 0.1 ⩽ MDES:TREOS ⩽ 10. Their structure was investigated by 29Si MAS NMR, FTIR and DSC: the gels appear as highly condensed homogeneous systems in which the difunctional and trifunctional units are randomly distributed. The microstructure of the gels was also investigated through the measurement of specific surface area and skeletal density. Finally, some physical properties have also been measured such as thermal stability in O2 and inert atmosphere (Ar), coefficient of thermal expansion (α) and elastic modulus (E).