A variety of silica aerogels have been prepared by the hydrolysis of TEOS, and dried using supercritical CO2. The shrinkage which occurs during the drying process is dependent on the gel formulation and the extent of aging of the gels in their pore liquor. Such aging normally results in an increased density, modulus and pore size of wet gels. Upon drying the corresponding aerogels show the opposite behavior for modulus and density, which decrease with the extent of aging. Both drying and aging shrinkage were not observed for base-catalyzed gels, and were very small for HF-catalyzed gels. The use of formamide resulted in reduced drying shrinkage and a slightly larger amount of syneresis. Drying shrinkage is associated with the presence of micropores. Shrinkage during drying has been observed using a high pressure view cell and it was found that most of the shrinkage occurred during depressurization. An explanation consistent with the above is proposed.

Low density aerogels have numerous unique properties which suggest a number of applications such as ultra high efficiency thermal insulation. However, the commercial viability of these materials has been limited by the high costs associated with drying at high pressures (supercritical), low stability to water vapor, and low mechanical strength. Normally, critical point drying is employed to eliminate the surface tension and hence, the capillary pressure, of the pore fluid to essentially zero. However, we show that by employing a series of aging and surface derivatization steps, the capillary pressure and gel matrix strength may be controlled such that gel shrinkage is minimal during rapid drying at ambient pressure. The properties (density, surface area, pore size, SAXS) of aerogel monoliths prepared from base catalyzed silica gels using this technique, supercritical CO2 drying, and supercritical ethanol drying are compared. An additional advantage of this approach is that the final gels are hydrophobic.

Monolithic aerogels and aerogel coatings of hydrated V2O5 containing 0 to 10 mol% GeO2 were prepared from metal alkoxides by hydrolysis and autoclave drying. The bulk aerogels of high porosity, more than 90 %, were obtained. On the other hand, the porosity of the aerogel coatings, about 50 nm in thickness, was about 50 %. The aerogels consisted of micro fibrils, less than 20 nm in diameter. Diffraction peaks of crystalline vanadium oxides were not observed for the as-dried aerogels. Dynamic microhardness of the bulk aerogels was very low, but increased with addition of GeO2· Dc electrical conductivity of the bulk aerogels was lower than that of the bulk xerogels by one order of magnitude, and was isotropie. The aerogels were surface fractal with Ds ≅ 2.7.

Novel ceramic compositions can be produced from carefully chosen, modified, and processed zeolites. The use of zeolites as precursors for leucite-pollucite glass ceramics having adjustable thermal-expansion behavior is described. The facile sintering properties of the zeolite-derived precursors allow production of these high-temperature materials at several hundred degrees below the solidus temperatures. A representative leucite-pollucite ceramic was studied by high temperature x-ray powder diffraction and synchrotron powder diffraction to determine the nature of the crystal structure and its relationship to the leucite and pollucite end members of the family.

Oxide-based Al2O3/SiC nanocomposites, in which the nanometer-sized nonoxide SiC particles are dispersed mainly within the matrix Al2O3 grains, were fabricated by the normal powder metallurgical methods. High resolution transmission electron microscopic observation showed that the nanometer sized SiC particles were directly bonded to the matrix Al2O3 grain without any reacting layers. The nano-sized SiC dispersions gave the significant improvement of mechanical properties such as fracture strength over 1000 MPa and no degradation of its strength up to 1000°C for Al2O3/SiC nanocomposites, was observed. The sintering process and the correlation between mechanical properties and micro/nanostructure will be discussed for this nanocomposite.

A novel, low temperature and pressureless bonding technique has been developed to fabricate Al2O3 coated Nb composite. The nature of the interface bond has been characterized by extensive electron microscopic investigation, and a phenomenological model has been developed to explain the mechanism of bond formation. In the present technique a suspension (in distilled water) of fine Al2O3 powder (mean particle size of 0.2μm) was applied on polished and cleaned Nb blocks. The coatings were air dried and the coated metals were heat treated at 1100°C to achieve bonding between the ceramic and the metal. The process was carried out in an argon atmosphere. To reduce the oxygen impurity content in argon, the gas was passed over Mg turnings at 300°C before its entry into the bonding apparatus. Control of oxygen impurity in the bonding atmosphere in conjunction with the use of fine ceramic particles led to enhancement of interfacial diffusion. Scanning electron microscopy of the bonded specimens showed a consolidated layer in the ceramic phase close to the metal-ceramic interface. Transmission electron microscopy of the metal-ceramic interfacial region rOevealed a distribution of Al2O3 particles in an amorphous phase containing varying amount of Al, Nb, and 0. Prior treatment with Mg was to reduce the oxygen impurity content below the equlibrium oxygen partial pressure (at 1100°C) of Al2O3. This would result in loss of oxygen from the surface of the ceramic particles. Thus a region, with high concentration of 0= vacancy would form along the surfaces of the particles. As a result of excessive oxygen depletion, further continuation of this process would lead to degeneration of the crystal structure of the regions along the surfaces of the Al2O3 particles. These regions along the Al2O3 particle surfaces may then act as easy diffusion paths for Nb. Extensive diffusion of the metal into the ceramic phase along these paths would lead to the formation of an amorphous phase along the ceramic particle surfaces. Eventually, this phase will occupy the voids in between the ceramic particles, that are close to the metal-ceramic interface, and thus bond them together with the metal.

A parabolic rate kinetics model has been developed and verified by experiments for the pressure infiltration of textile ceramic matrix composites using sol-gel processing with added solid particles. Darcy's law and a global permeability of the whole system were adopted in the model. The permeability of the whole system consisting of fiber preform, compaction, and filter was derived from the individual permeabilities of the fiber preform and particle compaction inside the preform which were based on the Kozeny-Carman equation. Experiments were conducted using 3-D angle interlock woven carbon fiber preforms and silica sol containing silica particles of 0.25μm and 0.5μm sizes. The total infiltration time was inversely proportional to the constant processing pressure and increased significantly with reducing the solid particle size by a factor of two. This is because the permeability of particle compaction layer is the dominant term and it is proportional to the square of particle size.

Macro-defect-free cement is a composite material made by processing a hydraulic cement with a water-soluble polymer under high shear roll-milling. This paper examines the principles of processing and the development of microstructure that results in a material with ceramic-like properties. The nature of the chemical interaction between the cement and the polymer is also considered in some detail.

Alkoxides of the type (RO)nE-X-A in which a functional organic group A is connected to the alkoxide moiety via an inert and hydrolytically stable spacer X, are used to bind metal complex moieties. Sol-gel processing of the resulting metal complexes [(RO)nE-X-A]yMIm under standard conditions results in the formation of metal complex containing gels. If the metal complex moiety MLm is catalytically active, this method results in the heterogenization of homogeneous catalysts. Another application of the metal complex substituted gels is the preparation of composites containing uniform, nanometer-sized metal particles homogeneously dispersed in a ceramic matrix.

The hydrolysis of H2N(CH2)2NH(CH2)3Si(oMe)3 with a stoichiometric amount of water results in loss of the -OMe groups after two days and formation of a mixture of species mainly with empirical formula, [H2N(CH2)2NH(CH2)3Si(O)3]. The reaction of representative examples of metal salts, e.g. Cu(NO1.5)2, and metal-organic compounds, [(NBD)RhCl]2, with the amino-group of the amine functionalized polysiloxanes [H2N(CH2)2NH(CH2)3Si(O)1.5] has been demonstrated. These species undergo thermal decomposition in air, typically at 200°C, with complete loss of the organic supporting ligands. On heating to 450°C in O2, sintering occurs to form large, ∼1μm, sized silica particles which contain highly dispersed, ∼4nm sized metal-containing particles. The surface areas range from 228 to 583 m2/g. After reduction, the rhodium species is active towards catalytic hydrogenation of pyrene.

The sol-gel process provides new opportunities for the synthesis of photonic materials. Sol-gel chemistry is mainly based on inorganic polymerization reactions performed in solution at lower temperatures than conventional chemical methods. Homogeneous doping by mixing components at a molecular level and synthesis of mixed organic-inorganic materials can then be performed. Luminescent properties of dyes inside silica, aluminosilicates or transition metal oxide based gels have been studied extensively. Much less work has been devoted to the study of energy transfer (ET) inside sol-gel matrices. Sol-gel materials provide the opportunity to investigate dye-dye ET, as well as dye-ion ET. This communication presents an optical study of a variety of sol-gel matrices (SiO2, Al2O3, ZrO2 based gels) codoped with rhodamine 6G(Rh6G) and europium. Different types of ET are observed among Rh6G, Eu3+ and Eu2+. The Eu2+/Eu3+ ratio is found to be strongly dependent on the host matrices. The reduction of Eu3+ t· Eu2+ in dried gels may relate to the presence of Rh6G.

The diffusion of carbon monoxide into thin, transparent, porous film media is studied by doping the film with an iron porphyrin and spectroscopically monitoring the chemical interaction between the reduced iron porphyrin and the diffusing solute. The films are spin-cast from metal alkoxide carboxylates. Both optical absorption and resonance Raman spectroscopies have been used to study the porphyrin-doped film and the diffusion process.

The studies yield information on the effective pore size and homogeneity of the materials, and show that redox chemistry can be performed on incorporated metallo-porphyrins. This investigation also demonstrates the usefulness of porphyrin-doped films as chemical sensors, showing a sensitivity below 70 ppb to aqueous carbon monoxide.

The sol-gel derived ORMOSIL (organically modified silicate) composed of copolymerized tetramethylorthosilicate (TMOS), methylmethacrylate (MMA), and 3-(trimemoxysilyl)-propylmethacrylate (TMSPM) is a stable host for organic molecules. Due to the ORMOSILs utility as an optical host matrix, it is important to obtain a more detailed understanding of the changes that occur during gelation, aging and drying. A useful probe molecule for this purpose is ReCl(CO)3-2,2'-bipyridine. The emission band of this molecule shows a large blue shift as the medium is changed from fluid to rigid. In this paper the changes that occur during the gelation, aging, and drying of a sol-gel derived ORMOSIL are studied. The emission maximum shifts gradually from 615 ran in the sol to 575 nm in dried gels. The final wavelength is indicative of an environment that is not fully rigid, and suggests that the molecule is in a flexible organic site. The properties of the ORMOSIL are contrasted with those of silicate and aluminosilicate sol-gel matrices.

The sol-gel process was used to incorporate organic dyes including Rhodamine 6G (Rh6G), 2-(4-pyridyl)-5-(4-phenyl) oxazole (4PyPO) and the n-methyl tosylate salt of 2-(4-pyridyl)-5-(4-methoxy phenyl)oxazole (4PyMPO-MePTS) in silica gel. Thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) analysis of the dye doped gels showed that the gel structure loses the adsorbed water molecules from room temperature to 150°C and decomposition of the dye molecules followed at the higher temperature. Absorption and emission of the dyes in the sol-gel glass matrix were also studied and compared with the results of the dyes in alcohol solution. The environments of the dye in silica were different than in alcohol solution.

Optically clear gels doped with organic molecules are prepared in the zirconia-silica system by hydrolysis of metal alkoxides in a wet atmosphere. After drying, dense xerogels are obtained which exhibit a closed porosity. By using the same route, organic-inorganic hybrid xerogels are also produced as films.

The molecule-matrix interactions are evaluated from the absorption recovery times of the S0→S1 transition for triphenylmethane dyes. Concerning doped xerogels with other organic molecules having polar groups, the application of the strong polarized electric field of an ultrashort optical pulse allows to locally create a birefringence with a memory effect. This type of sample could be used for optical storage and treatment of information.

The chemical control of hydrolysis-condensation reactions of transition metal alkoxides can be performed through the modification of the transition metal coordination sphere by using strong complexing ligands (SCL). Complexing organic groups can be bonded to the transition metal oxide network in two different ways, as network modifiers or network formers. Different illustrations of the role of complexing ligands on Ti(IV) and Zr(IV) alkoxides are presented. As a network modifier, SCL act as termination agents for condensation reactions allowing a control of particle growth. The complexing ligands being located at the periphery of the oxo core open many opportunities for colloid surface protection. SCL carrying organofunctional groups which exhibit non linear optical (NLO) properties have also been used as probes to study sol-gel transformations. SCL functionalized with organic polymerizable functions act as network formers. Hybrid organic-inorganic copolymers intimately interpenetrated on a nanometer size scale were synthesized from zirconium oxo polymers chemically bonded to polymeric methacrylate chains via a complexing function.

The mechanical properties corresponding to the different structures of organically modified silicates (Ormosils) prepared by the sol-gel process were studied. Tetraethoxysilane (TEOS) and polydimethyl siloxane (PDMS) were used as the inorganic and polymeric components, respectively. Large variations in the microstructure of the Ormosils were found by changing the reaction temperature, and the content of PDMS, acid catalyst (HCI),and water. The structure of the Ormosils and the mechanical properties were correlated by the edge connectivity. Modulus of elasticity and fracture strength were used to study their relations Ormosils structures.

This paper emphasizes on the mechanical characterization of heteropolysiloxane (HPS) derived materials using rheology and acoustic microscopy methods. Dynamic rheological measurements have been performed at the sol and the gel stages with two different precursors. An organic gel-like behaviour has been brought into evidence related to the fact that several microns thick coated layers can be obtained with these materials. On the contrary acoustic analysis revealed a glass-like behaviour for the cured layers. These results confirm the interest of organic/inorganic polymers to design new materials with tailored properties.

Ultrasound has been applied to organically modified silicates (ORMOSILs) solutions during gelation. A series of silica and polydimethylsiloxane (PDMS) composites have been prepared by the sol-gel method. Tetraethoxysilane (TEOS), PDMS, water and a HC1 catalyst were mixed and exposed to 20 kHz ultrasonic radiation under different conditions. Ultrasonic radiation made it possible to obtain wet gels in 5 to 30 min. without solvent (e.g. alcohol, and THF). And exothermic reaction was observed due to hydrolysis and polycondensation of TEO S. The reaction process was examined by liquid state 29Si NM R. Densities of obtained sono-ORMOSILs were observed to be higher than those produced by ordinary processes. Some sono-ORMOSIL samples demonstrated extremely low porosity and low specific surface areas.

We report the preparation of a new class of organic/inorganic hybrid aerogels from aryl-bridged polysilsesquioxanes [1]. l,4-Bis(triethoxysilyl)benzene and 4,4'-bis(triethoxysilyl)-biphenyl were sol-gel processed to form phenyl- and biphenyl-bridged polysilsesquioxane gels (Figure 1, structures 1 and 2, respectively). The gels were then dried using supercritical carbon dioxide extraction. It was discovered that aryl-bridged polysilsesquioxane aerogels are indeed formed, but with a pronounced influence on surface area from the reaction conditions used in preparing the initial gels. Specifically, high surface aerogels ( up to 1750 m2/g) are obtained from gels prepared with either acid or base catalysts. With reduced concentrations of base catalyst, however, supercritical processing afforded phenyl-bridged xerogel-like materials, The materials were characterized by nitrogen sorption surface analysis, and by transmission electron microscopy.