Sol-gel derived optical composites have been prepared by both the in-situ polymerisation and ORMOSIL methods and characterised by FT-Raman and near infrared (NIR) spectroscopies regarding their chemical structures, optical properties and molecular interactions. A model based on intermolecular and intramolecular hydrogen bondings has been proposed to interpret the observed spectral results.

Thin films of niobium pentoxide were deposited on quartz plates by spin-coating a solution of niobium ethoxide in ethanol. The films were tested as potential selective gas sensors by exposing them to various reducing environments. It was found that they were selective and sensitive to ammonia. This sensitivity was attributed to the formation of donor-acceptor complexes between chemisorbed ammonia and Lewis acid sites on the solid.

The synthesis of sol-gel glasses containing organic and organometallic molecular dopants has been well established as an approach for creating new optical materials. Some of these properties are dependent upon chemical reactions which occur in porous xerogel matrices during the sol-gel process or when encapsulated molecules are exposed to other molecules in solution. In this paper, the study of two different types of chemical reactions in the pores of xerogel matrices is reported. In one case copper phthalocyanine is used to characterize dimerization within the pores. The results show that dimer formation is most likely to occur towards the end of the drying stage as the dye concentration in the pores increases from solvent evaporation. A second example involves the use of a pump-probe technique to determine the rate of proton recombination inside the pores of silica monoliths. The behavior of sols and gels is similar to aqueous solution while recombination of protons in the xerogel seems to be affected by the walls of the pores.

Time-resolved depolarization measurements are used to investigate rotational diffusion and optical energy transfer dynamics of chromophores incorporated into silica and aluminosilica solutions, gels, and glasses. The use of an organically doped sol-gel clad waveguide as a novel intrinsic fiber optic sensor device is also demonstrated, and advantages of time-resolved detection of the fiber optic sensor are illustrated.

Amorphous SiSx:H (x ∼ 2) films have been synthesized from H2S and SiH4 precursors using a remote plasma enhanced chemical vapor deposition apparatus. Structural studies by solid state nuclear magnetic resonance (NMR) and Raman scattering reveal that the atomic environments in these materials are similar to those observed in melt-quenched silicon sulfide glasses, and are characterized by corner- and edge-shared SiS4/2 tetrahedra. Compared to these glasses, however, the films show consistently higher fractions of corner-sharing S1S4/2 tetrahedra. The ratio of corner- to edge sharing tetrahedra and the Si:S ratio can be influenced by the H2S/S1H4 flow rate ratio during deposition. Thus, PECVD opens up wider opportunities for structural tailoring of amorphous silicon sulfide materials than currently possible by means of the melt-quenching method. Preliminary data for the PECVD synthesis of phosphorus sulfide is also presented.

The synthesis and properties of two polycarbosilanes that have essentially a “SiH2CH2” composition is described. One of these polymers is a highly branched hydridopolycarbosilane (HPCS) derived from Grignard coupling of CI3SiCH2CI followed by LiAIH4 reduction. This synthesis is amenable to large scale production and we are exploring applications of HPCS as a source of SiC coatings and its allyl-derivative, AHPCS, as a matrix source for SiC- and C-fiber-reinforced composites. These polymers thermoset on heating at 200-400 °C (or at 100 °C with a catalyst) and give near stoichiometric SiC with low O content in ca. 80% yield on pyrolysis to 1000 °C. The second method involves ring-opening polymerization of 1,1,3,3-tetrachlorodisilacyclobutane and yields a high molecular weight, linear polymer that can be reduced to [SiH2CH2]n (PSE), the monosilicon analog of polyethylene. In contrast to high density polyethylene which melts at 135 °C, PSE is a liquid at room temperature which crystallizes at ca. 5 °C. On pyrolysis to 1000 °C, PSE gives stoichiometric, nanocrystalline, SiC in virtually quantitative yield. The polymer-to-ceramic conversion was examined for PSE by using TGA, mass spec, solid state NMR, and IR methods yielding information regarding the cross-linking and structural evolution processes. The results of these studies of the polymer-to-ceramic conversion process and our efforts to employ the AHPCS polymer as a source of SiC matrices are described.

We report on an alternative route to quaternary ceramics which can be prepared by polycocondensation of commercially available polysilazanes with the appropriate alkyl amides (M(NRR')X with M = B or P and subsequent pyrolysis in argon atmosphere at 1000°C. Ceramic monoliths are obtained if the modified polysilazane is crosslinked at 400°C for 3 h. After ball-milling, sieving and cold isostatic pressing the green body is directly pyrolysed to the appropriate monolith. The obtained material is amorphous to X-ray-diffraction. Investigations by electron spectroscopie imaging (ESI) in TEM indicate a homogeneous distribution of the elements M, Si, C and N (M = B, P) within a resolution of 2 nm. Upon annealing at elevated tempertures in a nitrogen atmosphere crystallization of the appropriate thermodynamically stable phases occurs around 1400°C yielding a micro/nano-, nano/nano- or micro/microcrystalline microstructure with design-tailored properties depending on the element M.

Micro-nano-structured Si3N4/SiC-composites were prepared by liquid phase sintering of either amorphous, polymer derived Si-C-N powder or SiC1+x coated OC-Si3N4 powder, followed by gas pressure sintering. The obtained microstructures were investigated by scanning and transmission electron microscopy (SEM, TEM). Nanosized SiC inclusions embedded in microcrystalline Si3N4 grains were analysed by elemental mapping with electron spectroscopie imaging (ESI).

The thermal stability of amorphous borosilicon nitride (Si3B3N7) and borosilicon carbonitride (SiBN3C) between 1000°C and 2000oC both in air and under inert conditions is reported. Both materials are derived from polymerization and subsequent pyrolysis of a “single source” precursor. On heating in vacuum or nitrogen SiBN3C remains amorphous up to lCWC whereas Si3B3N7 crystallizes at about 1800°C under these conditions. At about 2000^ the SiBN(C)-materials decompose into SiC, BN, B4C and N2.

Oxidation studies performed by TEM- and SEM-investigations of oxidized borosilicon carbonitride grains reveal that an interlayer consisting of B, N, and only little O is formed between the oxide scale on the surface and the inner bulk material. The interlayer does not disappear at temperatures above 1450°C in contrast to the Si2N20-interiayer observed on oxidized silicon nitride. The oxidation kinetics of the new ceramics are established in the range from 1000°C to 1600°C indicating a very high oxidation resistance. Possible applications as matrix materials as well as materials for fibers and coatings are discussed.

Poly(methylsilane) and poly(vinylsilane) were synthesized using a titanocene catalyst, and their pyrolytic conversion to ceramics was followed using a combination of thermal analysis and infrared spectroscopy. The two polymers have distinctly different backbone structures, as determined by 29Si NMR; methylsilane polymerizes to a polysilane, while vinylsilane polymers have a predominately polycarbosilane backbone, with some polysilane structure as well. The pyrolysis path and char yield were dependent primarily on backbone structure, with little influence of polymer molecular weight. The majority of the weight loss on conversion occurs below 650 °C, although bond rearrangement continues to 1400 °C. Poly(vinylsilane) produced a C-rich Si-C ceramic in which the carbon was dispersed on a sufficiently fine level to show resistance to oxidation on heating in air to 1400 °C.

Clear gels have been made from TEOS and the aminofunctional silane under acid-catalyzed conditions and light scattering of the gels has been related to pH and the concentration of fluoride ions in the sol as well as the amount of the aminosilane used. We have succeeded in preparing a series of gels containing Ni2+ or Cu2+ ions immobilized by chelation either before or after the gel formation. Aerogels made from these gels in particular, doped by the method of impregnation, have had a homogeneous microstructure on the scale of only a few nanometers.

Nanosized silica particles dispersed in various solvents were spray dried and the change in size distribution, agglomerate strength, and strength distribution was determined. The effect of solvent surface tension, pH, and particle surface chemistry on strength of agglomerates formed during spray drying was studied for particle sizes between 15 and 500 nm. Alcohol/water mixtures having different surface tension, and water at different pH levels, were employed to separate the effects of capillary pressure and surface hydroxyl condensation reactions. The agglomerate strength was determined using an ultrasonic measurement technique. The particle size was determined using sedimentation. The strength and strength distribution of agglomerates was found to depend on the solvent surface tension, solubility (pH), and primary particle size.

Capillary stresses during drying that result from the contact angle of a liquid solvent to a gel, surface tension, and pore size have an immense effect on the dried gel. The extent to which the gel shrinks is a balance between the capillary stress and the solid matrix strength. The dynamic contact angles and surface tensions of various solvents commonly employed in sol-gel processing on silica gels of various surface chemistries have been evaluated. A thin, dense coating of silica gel was formed by dip coating a standard glass slide in an acid catalyzed silica sol and drying. Some of the sample surfaces were organically modified. Dynamic contact angles were determined using a modified Wilhelmy plate technique. Solvent surface tensions were determined using the De Nouy ring technique. The bulk modulus of wet gels were determined with a three point bend experiment. We have found that contrary to previous investigators, who attributed different bulk densities obtained after drying from various alcohols to contact angles variations, gel shrinkage during drying is actually due to slightly different surface tensions and degrees of depolymerization of the gel network.

Thin films of BaTiO3 were prepared by the sol-gel route using barium titanium methoxypropoxide in methoxypropanol. Sols with water of hydrolysis varying between h=0 and h=2 were spun onto (100) Si and Ge coated (100) Si substrates. XRD and analytical TEM were used to study the microstructure and crystallization behavior of these films. Polycrystalline BaTiO3 was obtained by heat treating the films at temperatures between 600 and 750°C using either conventional furnace annealing or rapid thermal annealing. Films prepared from sols having the highest water content tended to crystallize first. The BaTiO3 thin films exhibited a fine grain size on the order of 25–50 nm. No preferred orientation was observed. The effects of the hydrolysis conditions of the sol, the type of heat treatment, and the choice of substrate on the final microstructure of the films are discussed.

Halogenoalkoxides are promising molecular precursors useable in sol-gel technology. They exhibit a lower sensitivity towards hydrolysis than common alkoxides and so appear to be easier to handle. Some data about the structure of titanium chloroalkoxides, pure and with the addition of a controlled amount of water, were obtained using FT-Raman, while the effect of thermal treatment under nitrogen or oxygen was studied using TGA-IR coupling. Finally the oxide products were characterized by Raman and by ED-XRMA, proving mat chloroalkoxides could be used as molecular precursors of oxide by sol-gel process. We showed that TGA-FTIR coupling is a powerful tool in order to study a thermal treatment and could be used to optimize it.

Various kinds of active alumina supports were obtained in the following steps: (1) preparation of alumina sols by extraction of nitrates with Primene JMT from aluminium nitrate solution, (2) gelation to spherical particles (with diameter < 100 μm) by extraction of water from sol emulsion in 2-ethyl -1-hexanol, (3) sometimes conversion of NO3− stabilized gel to OH− form by reaction with ammonia, (4) calcination of gels to γ-Al2O3 with specific surface area >100 m2/g at 500°C . The powders were then impregnated with chloroplatinic acid and recalcined at 500°C. Catalysts were also prepared by direct gelation of alumina sol containing hydroplatinic acid followed by steps (3) and (4). Pt/Al2O3 catalysts were used for reduction of 1M UO2(NO3-0.5M HCOOH solution with hydrogen at atmospheric pressure. The course of the reduction was controlled analytically and by measuring the U(VI)/U(IV) redox potential. It was concluded that the catalysts obtained by impregnation of supports prepared from nitrate stabilized gels exhibit the best activity and stability.

The Y2O3-La2O3 additive-coated Si3N4 powders of about 1.5 μm in size were prepared by the sol-gel method. X-ray diffraction, X-ray photoelectron spectroscopy, differential thermal analysis, thermogravimetric analysis, electron microscopies, and particle size analysis were used to study the coating on the Si3N4 particles. The results show that properties of the bulk Si3N4 powder are not affected by using the sol-gel coating and the powders are homogeneously coated by a thin layer of the Y2O3-La2O3 additives. The structure of the Y2O3-La2O3 coating layer is amorphous or microcrystalline with a submicron thickness. The coated powders are then more sinterable, and the mechanical properties of the ceramics prepared from such powders are improved.

Host-matrix gels were prepared by hydrolysis of TEOS with the addition of cadmium nitrate, formamide as DCCA (drying control chemical additive) and submitted to a preselected dose of high power ultrasound. The impregnation of the dry “sonogel” with KSeCN solution under vacuum conditions promoted the formation of the CdSe nanocrystals and enhanced the mechanical properties of the matrix. A transparent red xerogel was obtained and characterized by optical spectroscopy, TEM and SAXS measurements. The results support a structural model of a porous matrix containing CdSe nanocrystals exhibiting a bimodal size distribution that depends on the Se content of the impregnating solutions. The blue shift of the optical absorption bands reveal the quantum-size confinement effects of CdSe nanocrystals.

Polymers that can be easily processed into fibers, films, or bodies are excellent precursors for ceramics with useful morphologies. The ideal preceramic polymer not only has proper physical properties (e.g. solubility or fusibility) for processibility but also decomposes at low temperature with high ceramic yield. One possibility for such precursors for yttria are yttrium oxycarboxylates. The preparation of such polymers from YOC1 has been investigated and several compounds with useful physical properties have been discovered. These include a methanol-soluble yttrium oxycarboxylate, (CH3OCH2CH2OCH2CO2YO)n a liquid yttrium tricarboxylate, (CH3OCH2CH2OCH2CO2)3Y, and a method for preparing very concentrated yttrium mixed acetate and formate sols.