Vanadium pentoxide gels V2O5·nH2O are actually hydrous oxides. Water adsorption and dissociation occurs at the surface of the oxide leading to negatively charged oxide particles surrounded by an acid aqueous medium. Ionic conductivity is observed, arising from proton diffusion through the water molecules. This process mainly depends on the nature of adsorbed water molecules. Proton conductivity is strongly related to the water adsorption isotherm.

Ion exchange readily occurs at the oxide-solution interface when the gel is dipped into an aqueous solution of a metal chloride. New vanadium bronzes have been obtained upon heating such gels around 300° C. They exhibit interesting properties as reversible cathodes.

Electrochemical insertion of Li* into the gel phase is quite easy. This is due to the very open structure of the gel and the mixed valence behavior of the vanadium oxide.

Transition metal oxide gels could then be used as thin films or pressed pellets for making micro-ionic devices.

The driving force for syneresis is generally attributed to the same chemical reactions that produce gelation, but it has also been proposed that shrinkage could be driven by interracial energy. The latter possibility is explored and discounted. The kinetics of syneresis depend on the driving force, the mobility of the gel network, and the rate of fluid flow through the contracting gel. A model that allows for viscoelastic behavior of the gel and fluid flow according to Darcy's law is shown to provide a quantitatively accurate representation of the shape of the shrinkage curves and the dependence of the shrinkage rate on sample size.

We used fracture mechanics test techniques to measure the adhesive bond energy formed between hydrated silica glass surfaces and silicate species deposited from solution. In the case of silicate surfaces hydrated in room temperature water vapor, intermolecular bonding between hydrated surfaces can be attributed to either hydrogen bonding interactions between adsorbed water molecules (0.15 J/m2) or electrostatic bonds formed between adsorbed cations and anionie-nonbridging surface oxygen groups (2.0 J/m2). The bonding interaction observed at room temperature depends upon the glass surface composition and the degree of surface hydration. When hydrated silicate solution species are added to the interface and heat treated, adhesion energies as large as the cohesive energy of silica glass can be obtained with heat treatments as low as 200 C. The adhesion of the silicate interfacial film produced by the addition of solution species is greatest for silicate precursors showing a low degree of molecular crosslinking. In addition, the presence of alkali in the silicate solution greatly enhances interface adhesion for heat treatments below 200 C.

The crystallization behavior of chemically prepared and rapidly solidified PbTiO3 was investigated. Chemical methods were (i) polymeric sol-gel processing and (ii) co-precipitation of fine particles. Rapid solidification was obtained by a twin-roller quencher. Details are reported for the processing methods and the characteristics of the materials. X-ray diffraction and electron microscopy confirmed the amorphous nature of the prepared materials. Crystallization was examined as a function of heat-treatment conditions. Non-isothermal DSC was used to determine the kinetics of crystallization. Activation energies and frequency factors were determined. Co-precipitated PbTiO3 crystallized at the lowest temperature of 375 C, followed by sol-gel at 425 C, and rapidly solidified at 475 C. The results are interpreted in terms of processing routes, structural free-energies, available surface areas and fractional free volumes.

Zirconia gels have been synthesized from zirconium butoxide in excess water with HNO3. The structure and characteristics of the gels have been studied as a function of the acid content, and the transformation during heat treatment has been followed up to 1150°C.

From the results of a previous similar study on aluminum hydroxide gels, a comparison is made between the sol-gel processes of zirconia and those of alumina. A discussion of the differences of the aqueous media chemistry of Al and Zr, and gelation mechanisms, is presented.

Ceramic precursor materials in the silica-alumina-phosphous pentoxide system can be retained as solutions under suitably high or low pH conditions. Molecular species in these solutions have been characterized by their sharp, well-defined Raman spectra. Changes in pH or reduced water activity by addition of miscible organic solvents takes the system across sharp precipitation boundaries. Precipitates can take the form of colloidal gels, polymerized gels, or crystallites depending on chemical composition and rate of precipitation. Raman spectra of precipitates immersed in their parent solutions have much less structure and broader bands. Information of the structural characteristics of the precipitates are deduced from band widths and the amount of fine structures related to the coherence length for Raman scattering. Phosphate-rich compositions tend to be more highly ordered with more detailed spectra than the silicate rich compositions.

The effect of various processing parameters during the aging of sol-gel derived silica are evaluated. Improvement in the physical properties of the wet gel-silica are shown to be well correlated to their aging temperatures and time. Changes in the pore structure also show correlations with aging process parameters.

Fundamental issues involving interactions and packing of nanometer-sized particles are being investigated as an extension of previous experimental studies on larger submicron particles and in relationship to general theoretical work on colloidal systems. Relationships between particle interaction energies and packing indicate (1) dense gels can be formed in stable systems by minimization of the hydrodynamic radius and (2) dense clusters can be formed in flocculated systems by the use of weakly attractive particles where particle restructuring occurs. Novel techniques for the formation of nanostructures within polymeric matrices are also introduced to address gel cracking problems.

A new way of producing alumina powder from the alkoxide is reported. Aluminum basic acetate is first synthesized by reactions which allow controlled precipitation. The chemical reactions are followed using IR spectroscopy. Alumina powder is obtained by subsequent thermal decomposition.

Cordierite (Mg2Al4Si5O18) powders prepared from compositionally triphasic nanocomposite gels led to enhanced densification when compared to powders made by the conventional SSG route. The densification of the sintered cordierite pellets was determined by utilizing the Archimedes method and the microstructure of the pellets was examined with a scanning electron microscope (SEM). Bulk densities of monophasic, triphasic nanocomposite (two sols and one solution) and truly triphasic nanocomposite (three sols) pellets sintered at 1300°C for 2 hours were 2.01 g/cm 3 (80% relative density), 2.42 g/cm 3 (96% relative density) and 2.51 g/cm 3 (100% relative density) respectively. The enhanced densification of α-cordierite utilizing triphasic nanocomposite sol-gel processing is, at least in part, due to the contribution of the heat of reaction from the three components.

The oxalate precursor method appears as a new efficient route in order to control the shape and size of particles of various manganites. Thus electrical properties of electronic ceramics (NTC) are optimized through the proposed chemical process.

From the first recorded investigations by Bosetti in 1745 through G.I. Taylor's[2] work on the “Disintegration of Water Droplets in an Electric Field” in 1964, electrostatic atomization of liquids has been put to use in a variety of applications ranging from crop spraying to the propulsion of space vehicles. Here, we have used electrostatic atomization to form micron size droplets of zirconium acetate which pyrolyze to spherical ZrO2 particles for use in sintering studies. Two ZrO2 powders were fabricated. Both are spherical and of similar size distribution, however, in one, the particles are single crystals, while in the other, the particles are polycrystalline. This work discusses the processing of these different particles, and the relative sintering behavior of powders that are composed of either monocrystalline or polycrystalline particles.

Submicrometer-size TiO2 powders were continuously produced by vapor-phase hydrolysis of titanium tetraisopropoxide, TTIP, in a tubular-flow reactor. Particle size increased with increasing inlet TTIP concentration, and with decreasing inlet water concentration and temperature. At high water concentrations, primary particles form by instantaneous chemical reactions and particle growth occur by collision and coalescence of the particles. Brownian collision and coalescence theory could predict the experimental effect of inlet TTIP concentration on particle size. The experimental deviations occurred from the theory with increasing temperature were attributed to the changing particle properties with temperature.

Powders of BaTiO3, SrTiO3, and CaTiO3, were prepared from intermediate catecholate complexes, M'[Ti(C6H4O2], M' = Ba, Sr, Ca. Conversion to oxide was accomplished by heating at 600°C to produce stoichiometric powders of crystallite size, < 0.1 μm, with relatively weak agglomeration. Ultrasonic treatment of aqueous suspensions of the powders resulted in measured agglomerate sizes of less than 2–3 μm. Chemical and physical properties of the new powders are reported, together with information on sintering behaviour.

Sol-gel processes for the synthesis of high purity, ultrafine BaTi4O9 and BaTiO3 powders are described. Hydrolysis of an alkoxide precursor derived from barium metal and titanium (IV) isopropoxide resulted in formation of powders consisting of ultrafine (0.2 to 1.0 μm) spherical particles. These amorphous precursor powders were converted to crystalline BaTi4O9 powders (particle size 2–3 μm) by heat treatment at 1100 C. The sol-gel process for the synthesis of BaTiOj powders utilized chemical polymerization between moisture insensitive and relatively inexpensive barium acetate and titanium (IV) isopropoxide in the presence of acetic acid. The gel to ceramic conversion was achieved by firing the gels at 1000 °C to obtain high purity, stoichiometric BaTiO3 powders. The powders were sintered to obtain relatively dense (90 % relative density) ceramic bodies.

Barium acetate, titanium ethoxide, and the acetates of calcium and magnesium and zirconium-n-propoxide as additives, were dissolved in an acetic acid methanol mixture. By adding water, this sol was converted to a gel.

Extended investigations of the calcination result in a crystalline barium tita-nate without other titanate phases, with a very small amount of carbonate and a negligible contents of free carbon. Aggregates could not be destroyed totally by the milling process. Therefore, the green density of the green bodies reaches only 60 to 65 %. Thus, a minimum sintering temperature of 1200° C was necessary to obtain a density of at least 95 %. In future work, a special treatment to destroy the aggregates must be developed to lower the sintering temperature.

The production of ceramics having high density, low porosity, and fine grains requires the use of high-purity metal oxide powders that consist of particles of uniform shape, size, and composition. Metal alkoxide hydrolysis is an attractive method for the synthesis of high-purity metal oxide particles. The nature of the precipitate formed is affected by the solvent.

This present study investigates the growth kinetics and characteristics of metal oxide particles (i.e., ZrO2 and TiO2) that are produced in various alcohol solvents (ethanol, 1-butanol, 2-butanol, and tert-butyl alcohol). Results are also presented for the synthesis of uniform homogeneous composite metal oxide particles (i.e., Y2O3-ZrO2 and ZrO2-TiO2) by low temperature metal alkoxide hydrolysis.

Sol-gel processes can provide a route to oxide ceramics with controlled microstructure and homogeneity. This feature can be exploited from an understanding of the inter-relationship between the characteristics of concentrated sols and the microstructure of the resultant gels produced on dehydration. Among several characterisation techniques employed (light scattering, incoherent neutron scattering, viscoelastic behaviour) neutron small angle scattering (SANS) has proved particularly powerful in the study of the mechanism of the sol to gel conversion process. Applications of SANS in the characterisation of sols containing (a) discrete particles (charge stabilised), (b) mixed oxide systems, and (c) particle aggregates, with fractal properties are illustrated.

Fractal analysis is used to interpret small-angle X-ray and neutron scattering data on commercial fumed silica powders. Rough primary particles are found in high surface area powders. A model based on ballistic polymerization, sintering, and diffusion-limited aggregation is proposed to explain the observed powder structure.

SiO microspheres have been prepared from TMOS/Methanol/water sols in basic condition (pH ≃ 12) by the Stöber method and studied by TEM static and dynamic light scattering techniques in order to determine the mean size Ṝ size distribution, hydrodynamic radius RH, radius of gyration RG and translational diffusion coefficient Dt; the ratio RH/RG was found less than 0.55 and the aggregates have a fractal geometry with df ∼ 2.3 ± 0.05. The results are compared to previous work on colloidal silica aggregates prepared by other techniques and discussed.