Sol-gel methods have been applied to the production of the multicomponent ceramic, Synroc B. These techniques involved the hydrolysis of a mixture of Ti and Zr, alkoxides peptising to form a sol and subsequent sorption of Al3+, Ba2+ and Ca2+ cations under acidic conditions. Powder properties were examined by a variety of techniques, including electron microscopy, x-ray diffraction, N2 sorption, and differential thermal analysis. The effects of processing conditions on the physical properties of the powders are discussed.

Near the gel point, light scattering studies of silica sol-gels reveal fractal clusters whose size diverges as a power law, in accord with the predictions of percolation theory. More surprising is the appearance of a fractal time description of the dynamics of these clusters. This novel dynamics has recently been revealed by quasielastic light scattering from the density fluctuations that occur at the sol-gel transition. Since the relaxation of fluctuations in these branched polymer systems is self-similar, decay processes occur on all time scales (fractal time), and average decay times diverge. An interpretation of this observation will be presented that relies on a length-scale-dependent viscosity and the geometrical self-similarity of the sol-gel transition. The scattering theory is extended to the calculation of time- and frequency-dependent viscoelastic properties, as well as mechanical properties such as the shear modulus, steady state creep compliance, and viscosity. The viscoelastic predictions are found to be in good agreement with experimental data.

The complete condensation kinetics of three silicon alkoxides have been studied by 29 Si NMR in the conditions of rapid hydrolysis (acidic medium, water in excess). The gelation of the tetravalent TEOS takes several weeks, whereas the trivalent MTEOS and VTEOS do not form gels. From a quantitative data analysis, we deduce that the first steps of the condensation proceed by progressive formation of small organized units. This accounts for the occurence of highly condensed agglomerates, the absence of gelation in trivalent systems and the very slow kinetics (logarithmic function of time). For the tetravalent TEOS, this is followed by an aggregation phase, which leads to gelation. After the gel time, the transformation of the sol fraction into gel has been followed by observing the broadening of the static NMR spectra. Moreover, the structures of the aerogels obtained by hypercritical drying and of the densified materials resulting from further sintering are characterized by MAS NMR. In particular, measurements of the nuclear relaxation in samples doped with paramagnetic impurities give directly the fractal dimension D of the silicate lattice. The measured value is in agreement with that obtained by the SAXS technique in aerogels (D≈2.2) and is close to D=3 for the densified materials.

Raman spectra, measured on silica and silicophosphate aerogels in the range of 5 to 70 cm−1 , revealed a “particle band” that arises from the discrete particulate character of the gels at very small size scales. TEM measurements of the nanoscale structure in the gels correlate with the location of the particle band maximum.

The chemical and physical properties of two base-catalyzed silica gels systems were characterized. The sol-gel transition was studied by titration, rheometry, and NMR. Pyrolysis behavior was monitored by TGA and mass spectroscopy. Many characteristics were strikingly similar for the two base-catalyzed systems. However ammonium hydroxide additions resulted in heterogeneities at lower concentrations than ammonium carbonate. Ammonium carbonate enhanced oxidation on pyrolysis.

In the alumina-silica system, the surface area of gels exhibit dependence upon both composition and the fluid in the pores during drying. Under controlled conditions, an anomalous drop in both the surface area and skeletal density of identically prepared gels occurs at a composition of 47% alumina. An effort has been made to understand the reasons for this phenomenon. The effect of various solution precursor systems has been investigated. Precursors that contain boehmite, or other colloidal species, do not exhibit low surface area/density at 47% alumina. Data from light scattering, infrared spectroscopy, and solution NMR will be discussed for the precursor system used to prepare low surface area gels. Data will be interpreted to determine the effect of solution structure on gel characteristics.

The use of NMR relaxation measurements for the in-situ study of pore structure evolution during gel aging and drying is illustrated. The change in the pore size distribution and surface area of both wet and dried gels is examined as a function of aging conditions including temporal aging, thermal aging, changing pH, and changing pore fluid. The effect of pore fluid pH on dissolution/reprecipitation in ordered packings of monodisperse silica spheres is also examined as a model system for particulate gels. As expected, the pore size distribution narrows with increasing time of treatment in high pH pore fluids. Interpretation of high pH results for the wet state is complicated by a microporous layer which forms on colloidal silica resulting in significantly larger wet surface area as compared to the final dried material. Narrowing of the pore size distribution, which is of interest for maximizing drying rates, is maximized in the least time by using either high pH or repeated ethanol washes for the base-catalyzed gel (B2) used.

We have investigated the radiation induced generation of paramagnetic point defects in high surface area sol-gel silicates containing various concentrations of the Raman active 608 cm−1 D2 “defect” band attributed to cyclic trisiloxanes (3 membered rings). Our results indicate that strained silicon-oxygen bonds due to three membered rings are the dominant E΄ (trivalent silicon center) and paramagnetic oxygen center precursors at high irradiation doses for silicates containing large concentrations of the D2 species. These results directly demonstrate that atomic level stress does play a role in the radiation damage process of silicon dioxide.

A water molecule hydrogen bonded to a surface SiOH group produces an IR vibrational transmission peak (ν3) at 2.82 μm. Water was adsorbed into the pores of a metal alkoxide derived silica gel monolith, and the increase in the wavelength of the first vibrational overtone (2ν3) of this peak was measured as a function of the adsorbed water content W (g H2O/g SiO2). The peak shifted from 1.390 to 1.420 μm as W increased by 0.14 g/g. Intermediate Neglect of Differential Overlap (INDO) Molecular Orbital (MO) theory was used to model this process. The effect of a H2O molecule, hydrogen bonded to a hydroxylated tetrasiloxane ring, on the structure of the ring and the water molecule was investigated. The bond length of the O-H group H-bonded to the water molecule increased, as expected from the increase in wavelength of the 2ν3 IR peak.

The aim of this research was to determine the relative effects of three different base catalysts (NaOH, NH4OH, and RbOH) on the rate of gelation of SiO2 from reactive silicon ethoxide solutions in alcohol (the sol/gel method). The time to gelation was determined using a reaction protocol which assured that condensation was the rate controlling step. NMR spectroscopy was used to verify that the nature of the reacting polymers remained similar as the base catalyst was varied. It was determined that under these conditions the condensation reactions avoid diffusional limitations. Furthermore, the base catalyst serves to modulate both the activation energy and also the Arrhenius preexponential factor. This leads to the proposal of a new condensation mechanism.

Ethyl silicate-derived silica gels were synthesized with organic acid and base hydrolysis catalysts. The nature of the organic catalyst was found to strongly affect dry gel structure and carbon content. No consistent relationship between dry gel structure and water content of the calcined gels was identified.

Silica gels are prepared by acidification (pH = 2 − 4) of water glass. SAXS measurements show that the gel consists of fractal aggregates (D = 2.2). Although the fractal dimension is not influenced by addition of cations, Al3+ and Mg2+ retard the growth of the fractal aggregates, while TMA+ has a promoting effect.

The hydrolysis and gelation sequence occurring in the formation of a filament or a rigid coherent gel from an organic silicate requires a mutual solvent for the organicsilicate and water if it is to proceed successfully. The solvent is usually a water-miscible alcohol. This alcohol can also be a reaction product, which makes the sequence very solvent dependent. The best control of both hydrolysis and gelation is obtained when the alcohol solvent and the organic silicate each contain the same alkoxy group. The gel resulting is also the best binder for refractory grain. Suitable systems are ethyl silicate and ethanol also isopropyl silicate and isopropanol. The formation of rigid coherent gels, or filaments which on firing convert to ceramic fibres, from hydrolysates of ethyl silicate and aluminium chlorhydrate species is also very solvent dependent. With glycol solvents, aluminium chlorhydrates do not form filaments nor ceramic fibres. With ethanol as solvent, aluminium chlorhydrate-polyol complexes readily form rigid coherent gels and filaments, hence ceramic fibres, because the polyol complex is formed by displacing coordinated water in the aluminium chlorhydrate. This reduces the amount of water available for reaction, which optimises filament and fibre formation.

Silica sols were produced with various catalysts (NaOH and HF). Dynamic small strain rheometry was used to probe the formation of the silica network from the sol state through the gelation point into the region of high modulus. These results were related to the dried xerogel properties of surface area and porosity.

The alkalizing of aqueous mixtures Fe111 + xFe11 has been investigated at pH ≈ 8 for 0≤x≤0.5. Three composition zones have been observed. At x<O.l goethite was the only stable product. At 0.1≤x≤0.25 the early stage, characterized as mixed-valent protoferrihydrite, spontaneously transformed into nonstoichiometric magnetite. At x>0.25 the system, made of spinel particles, was homogeneous during all the evolution.

Microelectrophoresis Is An Important method for measuring surface properties of colloidal materials. In order to obtain reliable measurements, a good reference colloid must be chosen first, and both the behavior of the reference under a variety of measurement conditions and instrumental factors must be established. Polystyrene latex has proven to be a good reference material. Time-dependent, solids-loading, and electrolyte concentration effects are determined below in order to establish reproducible referencing conditions. Using these referencing conditions, surface properties of silicon nitride as a function of aging time and atmosphere are studied.

The modelling of the aggregation kinetics of iron oxides has been succesful in predicting the increase in aggregate size as determined by dynamic light scattering measurements. The aggregates were found to exhibit fractal behaviour with fractal dimensions obtained from the scattering exponent in static light scattering studies dependent on the aggregation mechanism and ranging from 2.3 for rapid (diffusion limited) to 2.8 for slow (reaction limited) aggregation. Polydispersity and restructuring of aggregates were found not to affect the relationship between scattering exponent and aggregate fractal dimension. Excellent correspondence over a range of temperatures and ionic strengths has been obtained between results of sizing experiments using dynamic light scattering and sizes predicted using a modified Smoluchowski model incorporating fractal dimensions.

Small angle scattering clearly shows that aluminosilicate aerogels, prepared from Si-Al double precursor, form mutually self-similar volume fractals at least in the range of densities 40-160kg/m3. The structure, insensitive to the pH of the added water, consists of primary homogeneous rough units attached into volume fractal clusters (D=2.1) .Heating at 800°C oxidizes residual organic groups and decreases the roughness of the unit particles. Densification above 900°C both implies increasing of the size of units and decreasing of the fractal domain.

A stable mixture of partially condensed polysilicates is formed from the substoichiometric, acid catalysed hydrolysis of tetramethoxysilane [Si(OCH3)4] and the removal of the reaction- generated methanol by distillation. This mixture has been used as a precursor to make silica aerogels that show microstructural differences from base catalysed TMOS aerogels. In this paper, we report the analysis of this mixture for a determination of the type and distribution of the polysilicate species formed, and we include measurements of some physical properties of this fluid.

Silica aerogels having densities in the range of 0.003 to 0.05 g/cm3, have been prepared from a condensed silica formulation involving a two-step hydrolysis/condensation process with tetramethoxysilane (TMOS). This work describes the preparation of these unique ultralow-density aerogels, and compares several of their properties with those of aerogels made by a single step, base catalysed process.