Francis Taulelle

Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Novel polycarbosilazanes (PCSZs) were prepared by stepwise synthesis and thermal crosslinking of polysilasilazane (PSSZ) copolymers. Their pyrolysis under inert gas, producing Si-C-N ceramics, was investigated up to 1600 °C by analyses performed on the solids (elemental analysis; EPMA; TGA, density; 1H, 13C and 29Si solid state NMR, i.r. XRD, electrical conductivity) and analyses of the evolved gases (gas chromatography and mass spectrometry). From 250 to 450 °C, a first strong weight loss was observed, which was due to the formation and elimination of low-boiling-point oligomers. The weight loss closely depends on the cross-linking degree of the ceramic precursor resulting from the PSSZ/PCSZ conversion. Then, the organic/inorganic transition took place between 500 and 800 °C, proceeding via evolution of gases (mainly H2 and CH4) and yielding a hydrogenated silicon carbonitride. This residue remained stable up to 1250 °C although it progressively lost its residual hydrogen as the temperature was raised. Then, crystallization occurred between 1250 and 1400 °C, yielding β-SiC crystals surrounded by free-carbon cage-like structures. Finally, above 1400 °C, the remaining amorphous Si-C-N matrix underwent a decomposition process accompanied by nitrogen evolution and a second substantial weight loss. At 1600 °C, the pyrolytic residue was a mixture of β-SiC and free carbon. So, the amorphous silicon carbonitride resulting from the pyrolysis of PCSZ precursors was found to be appreciably more thermally stable than the previously reported Si-C-O ceramic obtained by pyrolysis of polycarbosilane precursors.

SOL-GEL SYNTHESIS OF PHOSPHATES

Phosphate and silicate materials exhibit similar structures but their solution chemistry is very different. Polyphosphates cannot be synthesized under ambient conditions from PO(OR)3 or PO(OH)3 solutions. More convenient PO(OH)3−x(OR)x precursors have been obtained via the dissolution of P2O5 in alcohols. Their reactivity towards hydrolysis and condensation depends on x and the alkyl group. They have been used as precursors for the sol-gel synthesis of titanium phosphates. 31P NMR experiments show the formation of P-O-Ti bonds during the course of the reactions.

Sol-gel synthesis of SiO2P2O5 glasses

The synthesis of P2O5ue5f8SiO2 glasses from solutions has been studied through gelation kinetics, Raman and nuclear magnetic resonance (NMR) spectroscopies. Different preparation methods are compared. The effect of the phosphorus precursor has been investigated. Phosphoric acid esters are not hydrolyzed and they do not yield condensation with silica. Reaction of anhydrous H3PO4 with Siue5f8OR groups leads to the formation of Siue5f8Oue5f8P bonds which are easily hydrolyzed. Only a few remain after addition of water and gelation. Therefore, Pue5f8Si homogeneity is difficult to achieve after gelation and the following drying and heat-treatment procedures. This absence of homogeneity is observed by solid-state magic angle spinning (MAS)-NMR and X-ray diffraction studies performed on gel samples heated at 300, 500 and 800°C in the case of xerogels and 500, 950 and 1140°C for aerogels.

Novel polycarbosilazanes with Si−C and Si−N bonds in the main chain

Novel polycarbosilazanes possessing Si−CH2 Si N linkage in their backbone are described. Their preparation involves thermal cross-linking of poly[(dimethylsilylene)-co-(1,3-dimethyl-1,3-disilazane)]s at 300 350 C. Thus, under controlled conditions, soluble and fusible preceramic polymrs were obtained. Upon pyrolysis, these materials gave good yields of silicon carbonitride-based ceramics containing low free carbon percentages and controlled nitrogen proportions.

Catalyst design by

Hierarchical zeolites are a class of superior catalysts which couples the intrinsic zeolitic properties to enhanced accessibility and intracrystalline mass transport to and from the active sites. The design of hierarchical USY (Ultra-Stable Y) catalysts is achieved using a sustainable postsynthetic room temperature treatment with mildly alkaline NH4OH (0.02 m) solutions. Starting from a commercial dealuminated USY zeolite (Si/Al = 47), a hierarchical material is obtained by selective and tuneable creation of interconnected and accessible small mesopores (2–6 nm). In addition, the treatment immediately yields the NH4+ form without the need for additional ion exchange. After NH4OH modification, the crystal morphology is retained, whereas the microporosity and relative crystallinity are decreased. The gradual formation of dense amorphous phases throughout the crystal without significant framework atom leaching rationalizes the very high material yields (>90%). The superior catalytic performance of the developed hierarchical zeolites is demonstrated in the acid-catalyzed isomerization of α-pinene and the metal-catalyzed conjugation of safflower oil. Significant improvements in activity and selectivity are attained, as well as a lowered susceptibility to deactivation. The catalytic performance is intimately related to the introduced mesopores, hence enhanced mass transport capacity, and the retained intrinsic zeolitic properties.