Showing papers by "Florence Babonneau published in 1990"

Aluminum sec-butoxide modified with ethylacetoacetate: An attractive precursor for the sol-gel synthesis of ceramics

Abstract: Aluminum sec-butoxide can be modified by chelating ligands such as ethylacetoacetate (etac). Al(OBus)2(etac) was characterized by IR, 27Al, 13C and 1H NMR: several oligomeric species containing 4, 5 or 6 coordinated Al atoms could be present. During hydrolysis, butoxy groups are rapidly removed, but the presence of less hydrolyzable ligands slows down the reactivity of the precursor towards water. Precipitation can be avoided. Thus, this precursor appears quite attractive for the preparation of multicomponent ceramics such as α-cordierite. Modified aluminum alkoxide, tetraethoxysilane and magnesium acetate were used to obtain monolithic gels that convert into α-cordierite at 1000°C. The conversion processes, from the solution to the gel and the ceramics, were followed by 27Al and 29Si NMR in both liquid and solid states.

Synthesis and High-Temperature Chemistry of Methylsilsesquioxane Polymers Produced by Titanium-Catalyzed Redistribution of Methylhydridooligo- and -polysiloxanes

Abstract: : The synthetic utility of homogeneous titanium catalyzed redistribution of the cyclomers-(MeHSiO) (x)-(where x=4 or 5) and the linear oligomer -MeHSiO(x) - MeHSiO(x)-(M(n) approx. 2,200 D) was explored as a route to MeSiH3 and a methylsilsesquioxane-methylhydridopolysiloxane copolymer of approximate composition- MeHSiO 0.35 MeSi(O)1.5)0.65-. The high temperature behavior of the titantium derived methylsilsesquioxane copolymer follows closely that of similar polymers prepared by sol-gel processing. Heating under nitrogen to 800-1000 C at a heating rate of 5 C/min gives a black glass with an apparent composition of: SiO2 (70%); SiC (19%) and C (10%). The apparent composition bellies the true nature of the material which is probably a metastable glass, best described by the various Si-X bonding arrangements (X= O, C, H). Thus, MAS NMR and DRIFT spectroscopy show the existence of species containing 4,3 and 2 Si-O bonds with the remaining bonds either Si-C or SiH. At 600 C and below (under N2), the Si-C bonds are almost exclusively Si-H3, which is an indication of the extraordinary thermal stability of silsesquioxane polymers. Above 600 C, the CH3 groups react with Si-O bonds to generate Si-OH bonds, new Si-C bonds and Si-H bonds. This type of reactivity is illustrative of the basic chemistry involved in the carbothermal reduction of SiO2 to Si and/or SiC as well as the degradation mechanisms of the polymer. Keywords: Ceramic materials; Silica glass. (aw)

The pyrolysis process of a polytitanocarbosilane into SiC/TiC ceramics: An XPS study

Abstract: The pyrolysis process of polytitanocarbosilane, precursor for SiC/TiC ceramics, has been followed step by step using x-ray photoclectron spectroscopy. This study shows that Ti–O bonds, present in the precursor, are stable up to 700 °C. Above this temperature, Ti–C bonds, precursor for TiC network, start to form. From a detailed analysis of Ti(2p) and Si(2p) peaks, the formation of intermediate species such as SiCxOy and TiCxOy has been detected.

Mixed Carbides via Polymer Route

Abstract: Several polymetallocarbosilanes, pre-ceramics precursors for Si-M-C-O systems, have been prepared from polycarbosilane and metallic alkoxides, M(OR)n with M = Ti, Zr and Al. Polymers have essentially been characterized by Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). The pyrolysis process has been followed for each system with X-Ray Diffraction (XRD) and MAS-NMR. The role of the metallic element M on the transformation process of these systems will be discussed.