Organically modified silica gels were prepared from mixtures of TEOS and methyl-substituted silicon alkoxides. The pore structure of dried gels was investigated in order to determine the effect of the organic additions on gel structure. For base catalyzed gels, surface area, pore volume, and skeletal density all showed dramatic decreases at high methyl contents. The surface area dropped by nearly three orders of magnitude, from over 800 m2/g for normal silica gels to less than 1 m2/g for gels containing over 60 mole percent modified alkoxide. The pore structure of acid catalyzed gels showed less dependence on composition. The nature of the surface of both acid and base catalyzed gels varied with modified alkoxide content. Higher organic contents produced materials that were more hydrophobic. In addition, the mechanical strength of acid catalyzed gels dropped in direct proportion to methyl content.

The first representatives of novel materials that contain five- and six-coordinate anionie silicon are reported. Linear (or cyclic) ionomers incorporating pentacovalent siliconate were synthesized by condensation of 1,2,4,5-tetrahydroxybenzene (THB) with alkyl and aryl trialkoxysilylanes. Network ionomers that incorporate pentacovalent siliconates were formed from THB and alkyl and aryl bistrialkoxysilanes. In addition, network ionomers with hexacoordinate silicon result from condensation of TEOS and THB. Spectroscopie evidence is presented to verify the identity of the novel structural features of these materials.

A new class of (meth)acrylate substituted alkoxysilanes has been developed. These are synthesized by addition of the H-S- or H-Si-unit of thiosilanes (e.g. HS- (CH2)3-SiCH3(OCH32)or hydrosila-nes to one (meth) acrylate C=C bond of commercially available di-, tri-, tetra- or penta-(meth)acrylate compounds. The alkoxysilyl groups allow the formation of an inorganic Si-O-Si-network by a hydrolysis and polycondensation reaction (sol-gel process), and the (meth)acrylate groups are available for thermally or photochemical ly induced organic polymerisation. The elegant procedure for the synthesis of a wide variety of multifunctional alkoxysilanes and the preparation of optical lenses will be discussed.

The development of a new scratch resistant UV-curable coating system is described. The material can be applied to optical data discs by spin coating. The coating shows superior adhesion even without priming and good weathering performance especially on polycarbonate.

The material is prepared from Zr(OPr)4 and 3-Methacryloxi-propyltrialkoxysilanes via sol-gel processing. Complexation of Zr(OPr)4 with methacrylic acid yields complexes which can be copolymlrized with the prehydrolyzed silane. The hydrolysis and self-condensation of the trialkoxysilanes were studied under different conditions. The consumption of water was studied by Karl-Fischer titration and the formation of oligomeric and polymeric species by GPC-measurements.

Multifunctional acrylate alkoxysilanes synthesized from commercial acrylate compounds and mercapto-substituted alkoxysilanes were used as precursors in the development of materials for various purposes. The alkoxysilyl groups are available for the construction of an inorganic backbone by the sol-gel process, and the acrylate groups for building an organic polymeric matrix by thermal or photochemical curing.

Tailoring the material properties is achieved by use of specific molecular structures and functionalities within the monomeric compounds. Linear and branched structural units influence the mechanical properties (modulus of elasticity, abrasion resistance, flexural strength). Rapidly UV-cured materials have been prepared for use as primary or secondary coatings for silica optical fibres with improved adhesion under humid conditions. Further organic modification results in polymers which can be used for manufacturing lenses of high optical quality. Preliminary attempts to draw fibres have also been successful.

Chemical processing routes to advanced ceramic materials are gaining importance as a convenient approach to control the stoichiometry, purity, microstructure and final form of the ceramic products [1]. The pyrolytic conversion of organometallic molecules and polymers is one such chemical processing route that has been widely applied in ceramic fiber technology [1,2], in coating processes [1,2], and in the sintering of bulk ceramic objects [3]. Despite these advances in practical applications, there is a continuing need in this area for a better fundamental understanding of the chemistry involved during the precursor-to-ceramic conversion process and for the development of new precursors which yield the desired ceramic(s) in high yield and purity.

To prepare new polycarbosilane polymer precursors with high solubility and the capability of UV cross-linking, commercial polycarbosilane was modified by a chemical route. These modifications involved AlCl3 catalyzed chlorination reactions of polycarbosilane's Si-H bonds. The resultant Si-Cl bonds were substituted by a reaction with sodium acetylyde to form Si-C=CH ligands. These ligands are suitable for controlled, free radical initiated cross-linking of the polycarbosilane polymers. The increase in molecular weight should allow for increased Tg's and the retention of polymer pre-forms. In this report, the chlorination of the polycarbosilane polymer and the substitution reactions of polycarbosilane were examined with IR, 29Si and 13C NMR spectroscopy. In addition, the retention of polymer pre-forms were analyzed after UV exposure and inert atmosphere pyrolysis.

A commercial polycarbosilane, thermolysis product of polysilastyrene, was spun into fibres. The self-curing character of the polymer permitted both oxidative and thermal curing of the precursor fibres. Pyrolysis of the latter produced ceramic fibres in high yields, which were fully characterised. Optimum tensile strength was attained after heat treatment at 1100°C. Oxidative reactions and crystallisation caused strength degradation above this temperature.

Continuous silicon carbide fibers (”UF fibers”) with low oxygen content (∼2 wt%) were prepared by dry spinning of high molecular weight polycarbosilane solutions and subsequent pyrolysis of the polymer fibers. Room temperature mechanical properties were similar to those of commercially-available Nicalon™ fibers, as average tensile strengths as high as 3 GPa were obtained for some batches with fiber diameters in the range ∼10–15 μm Furthermore, UF fibers showed significantly better thermomechanical stability compared to Nicalon™, as indicated by lower weight losses, lower specific surface areas, and improved strength retention after heat treatment at temperatures up to 1700°C. UF fibers were also characterized by elemental analysis, X-ray diffraction, and scanning Auger microprobe. Strategies were suggested for achieving further improvements in thermomechanical stability.

Vinylsilane polymerizes to form predominantly a carbosilane polymer using dimethyltitanocene catalyst. This is in contrast to alkylsilanes, which afford polysilanes under the same conditions. The mechanism of polymerization of alkenylsilanes has been shown to be fundamentally different from that for the polymerization of alkylsilanes. The silyl substituent apparently activates a double bond to participate in a number of polymerization processes in this system, particularly hydrosilation. Isotopie labeling indicates the involvement of silametallocyclic intermediates, accompanied by extensive nuclear rearrangement. Polymers and copolymers derived from alkenylsilanes have relatively high char yields even for conditions which afford low molecular weight distributions. Formation of crystalline β-SiC is optimum for a copolymer of an alkylsilane and an alkenylsilane having a silane/carbosilane backbone ratio of 85/15 and a C/Si ratio of 1.3/1.

With ceramics prepared from organic precursors, polymers are often formed by thermolysis, and the ceramics are obtained by pyrolysis of the polymer. This paper will cover the NMR methods used to characterize the polymerization as well as the pyrolysis of such polymers. The reticulation rate of such materials is measured on the silicon atoms and on the carbon atoms, leading to a silicon reticulation state and a carbon reticulation state. For the ceramics, the quantification of the SiCxH4−x or SiCxN4−x sites is performed using a priori chemical shift calculation. A measure of the H/C of the free carbon phase at the different temperatures of pyrolysis is presented. This measure uses a absolute determination of hydrogen content in ceramics, more accurate than a classical chemical analysis of hydrogen. The evolution of the reticulation, of the free carbon amount and of the H/C ratio in the free carbon phase will be followed with the temperature.

The stability of silicon oxycarbide glasses has been studied at temperatures up to 1500°C. The silicon oxycarbide glasses were synthesized using a sol/gel process. The pyrolysis treatment in argon influenced the structure and composition of the synthesized glasses, and in turn, their high temperature stability in oxidizing atmosphere. The oxycarbide glasses pyrolyzed at > 1000°C had lower hydrogen concentration and a more polymerized network structure, and thereby were more resistant to oxidation and crystallization at higher temperatures.

Silicon oxycarbide glasses have been prepared from sol-gel precursors containing not only Si-CH3, but also Si-H bonds. Three systems have been chosen containing various Si units but the same C/Si ratio. Their pyrolysis process has been mainly followed by 29Si MAS-NMR and the composition of the final glass extracted. This study shows that a suitable choice of Si-CH3 and Si-H functionnalized silicon alkoxides can lead to a strong decrease in the free carbon content and to an almost pure silicon oxycarbide phase.

Silicon oxycarbide gels and glasses were synthesized using various ratios of methyldimethoxysilane and TEOS. These gels and glasses were compared with those made from methyltrimethoxysilane and TEOS. The effect of the Si-H functionality in the methyldimethoxysilane was of primary interest. Hydrolysis and condensation processes were monitored using 1H and 29Si-NMR spectroscopy. The structures and the oxycarbide fractions of the glasses, obtained after heating the gels to 900°C in flowing argon, were investigated with 29si-MAS NMR. The total carbon and silicon contents of the glasses were determined using chemical analysis. The glasses covered the range of carbon concentration from ∼1 to 15%, while the silicon concentrations were constant at about 40%. The concentration of the oxycarbide species was enhanced in the glasses synthesized with the Si-H functionality in the precursor.

High surface area silicon oxynitrides have been prepared by nitrida- tion of silica with ammonia. Characterization by Fourier-transform infrared spectroscopy has allowed quantitative determination of hydroxyl, amido and imido groups. Data obtained by X-ray photoelectron spectroscopy show that the nitrogen is well distributed in the surface of the materials.

A tailorable viscosity, liquid polysilazane has been developed which thermosets to a solid ceramic precursor for both silicon nitride (Si3N4) and silicon carbide (SiC). Conversion of the thermoset, crosslinked polysilazane to a ceramic material has been examined. Preferential formation of silicon carbide or silicon nitride is controlled by the pyrolysis conditions employed.

Knowing the nitridation mechanism of Si is important to obtain near-net-shape high strength reaction bonded ceramics. In this report, comparison of nitridation mechanism of small spherical Si powder produced by plasma arc method and large faceted Si powder, is discussed on the basis of microstructural analyses by SEM and TEM. From the analyses, nitrogen diffuses through the oxide film of Si powder and forms Si3N4 under the oxide films during the initial stage of sintering. At higher temperature, the oxide film transform into fine Si3N4 grains when heated to the 1350°C. The Si3N4 has morphology of hollow shell and a size, which corresponds to that of the original Si particles.

The reaction of Cl3Si-NH-Si(CH3)3 with BCl3 yields the novel compounds Cl3Si-NH-BCl2 and (Cl3SiNH)2-BCl. The ammonolysis of these monomeric precursors in liquid ammonia gives rise to highly crosslinked borosilicon imides. Pyrolysis of the polymeric imides in a stream of ammonia at 1250°C yields amorphous borosilicon nitrides with formal compositions of Si3B3N7 and Si6B3Nu respectively. MAS-NMR, DTA/TG, SEM and TEM investigations reveal that the resulting materials are single phase.

The reaction of the novel precursors with primary amines yields meltable and soluble polymers suitable for producing fibers or coatings. Subsequent pyrolysis in NH3 leads also to amorphous borosilicon nitride. The physical and chemical properties of the new materials will be discussed.

The design of molecular and preceramic polymer precursors to boron carbide based ceramics is described. The goal was to design nonvolatile, tractable precursors to boron carbide with high ceramic yields. Molecular precursors containing carburane cages and acetylenie groups were synthesized and converted to nonvolatile, soluble preceramic polymers. Pyrolysis of these polymers gave boron carbide based ceramics in 60–70% ceramic yield. A B4C/SiC film has been fabricated from one of the polymers and the film is uniform and crack-free.

The use of alkynyl substituted derivatives of o-carborane as precursors to boron containing ceramics is described. These compounds undergo a thermally or photochemically induced polymerization to afford cross linked polyakynyl-o-carborane derivatives. The increase in molecular weight should allow for increased Tg's and the retention of modelled polymer preforms. In this report, these modification reactions are described. In addition, the retention of molded polymer preforms were analyzed after UV exposure and inert atmosphere pyrolysis.