Backscattering spectrometry has been used to determine the density of TiO2 sol-gel films. The density of the as-deposited films relative to that of anatase varies slightly with respect to deposition conditions and is approximately 0.5. Annealing at temperatures as high as 750°C increases the relative density to 0.7 with concomitant decreases in the O:Ti ratio and H content but it does not result in complete densification. Film densities are consistently higher for films annealed under dynamic vacuum compared to those in air.

The structure of self-supported SiO2 gel-films prepared from acid and basic TEOS solutions is analysed by high energy transmission electron diffraction method. The reduced radial distribution function (RDF) curves show that all the films are already well dense despite the low drying temperature (≤50°C) and short drying time (≤30 s). The Si-O bond length of the gelfilms prepared from highly acid and basic solutions is about 1.58 Å; it is smaller than that of bulk vitreous silica (1.61 Å) but similar to that of BO Å thick evaporated a-SiO2 film.

A sol-gel method was used to prepare bulk, closed pore, amorphous alumina-silica. Films prepared from this 47wt% A12O3-SiO2 composition were examined by SAW, ellipsometry and electrical measurements. The films were found to have a surface area of 1.1 cm2/cm2, a refractive index of 1.44 at 633 nm, and a relative permittivity of 6.2 at 200 KHz. These properties indicate potential applications as hermetic seals, barrier coatings, dielectric layers for capacitors and passivation coatings for electronic circuits.

Using sol-gel technology, thin organic/ceramic (ceramer) coatings have been applied to metal surfaces in order to enhance such surface properties as adhesion promotion and corrosion prevention. Isotropic coatings have been found to be effective in certain applications such as corrosion prevention, but the formation of anisotropic coatings permits greater flexibility over the resulting properties. Isotropic coatings derived from tetraethoxysilane, for example, effectively inhibit corrosion while being only 100 to 1000 Å thick. These coatings do not, however, promote adhesion. Thin coatings made from traditional silane adhesion promoters alone are unable to prevent corrosion of metallic substrates.

Using monomers with appropriate reactivities permits the single-step synthesis of anisotropic coatings that can both promote adhesion and prevent corrosion. These types of anisotropic coatings allow the physical and chemical properties of a coating to be varied as a function of distance from the substrate and confer properties to the substrate that would not be possible from a single isotropic coating. The principle behind the construction of these anisotropic coatings is general enough that it can be used in many applications where microengineering of surface structures is important.

The hydrolysis of tetraethoxysilane much used in the sol-gel approach to ceramics can be studied in a matrix isolation technique in which the silica thus formed precipitates within a polymeric material. This technique was applied here in the in-situ precipitation of silica particles in an elastomeric matrix of poly(dimethylsiloxane). Transmission electron microscopy and mechanical property measurements were carried out as a function of time over a period of 60 days. The equilibrium amount of silica precipitated was obtained relatively quickly, after approximately one day, but reorganization of the particles continued for several days thereafter. This “aging” process resulted in particles that are better defined and more uniform in size, but their ability to reinforce the elastomeric materials was only marginally increased. Scattering techniques are also being used to obtain additional information on this interesting process.

We describe a dip-coating method of depositing thin electronic ceramic coatings on various substrates from sol-gel solutions. The solutions we use are alkoxide-based, although the specific formulations depend on the system being deposited. We are working with two electronic ceramic systems: barium titanate on glass, and alumina, and yttrium barium copper oxide on alumina. We discuss methods used to reduce and remove processing defects such as crazing, cracking and pin-holing. We are also studying the effects of varying the processing conditions (dipping, hydrolysis and firing) on the electronic properties of the films. Initial results on BaTiO3 indicate that it is better to deposit thin films (∼0.1 – 0.2μm) and build up the coating thickness by repeated dipping.

The synthesis and characterization of several polymeric titanates and their conversion to carbon deficient TiC is described. The physical properties of one of these titanates allows it to be drawn into fibers and applied to substrates as thin films. Pyrolysis of these fibers and films to carbon deficient TiC is described.

Thin coatings of Al2O3, and Y2O3, have been prepared on desized Nicalon yarn. The layers were deposited by dip coating in homogeneous, alcohol solutions of partially hydrolyzed metal alkoxides. The coating thicknesses could be controlled by multiple dipping.

The uniformity of the coatings was affected by the alkoxide concentration. Solutions containing greater than 1 wt % equivalent oxide resulted in oxide bridging the void between the fibers.

Transmission Electron Microscopy (TEM) studies on alumina coatings formed by dip coating 300 mesh nickel grids, have shown that either eta or gamma alumina is formed after heating at temperatures as low as 700 °C. The rate of grain growth increases as the temperature increases. In addition, increased grain growth is observed for films produced from solutions with higher water:metal ratios.

The strength of coated fibers heat treated in air after coating has been determined. Alumina, and yttria coatings reduce the strength of the fiber after heating at temperatures above 900 °C. Scanning Auger analysis suggests that the coating is reacting with the fiber at these high temperatures.

Different multicomponent solutions have been developed by which building materials were coated. After drying and annealing, the coatings convert into silicate based glassy films. TEOS or fumed silica were used as silica carriers and by adding lead compounds also low melting compositions were prepared. In some cases it proved to be convenient to adjust the viscosity of the solutions by additions of suitable organic polymers. The heat treatment was done either in an electric furnace or by means of a special acetylene burner. The produced films can protect building materials against corrosion, may reduce the absorption and permeation of water or improve abrasion resistance.

Two multi-component sol-gel compositions were developed and compared to several commercially available high-temperature glasses. All were then used and characterized as protective coatings for intermetallic titanium aluminide.

The sol-gels were studied as thin film coatings and the commercial glasses were studied as enameled coatings. Attention was given to (1) the effect of the application temperature on the original microstructure of the metal, and (2) the role of interfacial conditions between the glass and metal in cyclic and isothermal thermal cycles between ambient temperature and 760°C (1400°F).

In this study ceria stabilized zirconia was used to obtain an uniform and adherent film on iron and 304 S.S.. Coatings were prepared by dipping in solutions of organometallic precursors of zirconium and cerium. Structure and morphology of samples were investigated using SEM and XRD techniques before and after thermal treatments. The oxidation behaviour in air of coated samples was followed through thermogravimetrical analysis in a range of temperature from 773 K to 1123 K. The preliminary results show, for the samples coated and pretreated in high vacuum at 923 K, a good decrease in oxidation rate.

Lead titanate fibers were formed from lead acetate and titanium isopropoxide by sol-gel methods. Additions of acids and chelating agents were used to influence hydrolysis and condensation reactions. A variety of solvents were examined, and the effect of conditioning treatments at elevated temperatures were investigated. Data are reported for the rheological behavior of viscous sols, fiber drawing ability, pyrolysis and crystallization of gels and fibers. Features of the ceramic microstructure are reported. A pyroelectric effect was measured for PbTiO3 fibers.

Several mechanisms have been proposed for the growth of silica spheres by the controlled hydrolysis of silicon alkoxides, the limiting cases of which are the conventional and aggregative growth models. The evolution of surface area predicted from the two models is substantially different at early times into the reaction. In order to probe the change in surface area during growth, 1H NMR measurements of the solvent were made during the growth process. It has been demonstrated that the spin-spin relaxation time.(T2) for an absorbed phase is less than that of the bulk solvent. Using this principal, the change in surface area can be followed in situ during the reaction. The experimental results were compared to the predictions of the conventional model and found not to be in agreement.

The cracking behavior of acidic silica sol-gel films during drying and low temperature heat treatment was studied. Films that cracked during drying exhibited a variety of unusual crack morphologies including sinusoidal cracks and parallel crack pairs. The effects of the water content of the sol and temperature on the critical thickness above which cracking of the films occurred were determined. The critical thickness decreased with increasing water content, most likely because the surface tension of the liquid in the pores of the gel increases with water content and because sols with low water concentrations did not gel until the solvent had evaporated. To determine why the critical thickness decreased with temperature, thermal analysis and shrinkage measurements of both constrained and unconstrained films were performed. Thermogravimetric analysis indicated that rapid weight loss occurred in the temperature region where the rapid decrease in critical thickness occurred. A small amount of shrinkage of the films also occurred in this region. Finally, shrinkage measurements of films debonded from the substrate indicated that shrinkage was very anisotropic with nearly all of the shrinkage occurring in the plane of the film and very little in the thickness direction.

The effects of inclusions on densification are discussed, including their influence on the development of stresses during drying, and on the kinetics of sintering. Special attention is given to the problem of crystallization during sintering.

The controlled nucleation of phase transformations by seeding is an established technique for influencing transformation kinetics and sintered microstructures in ceramics. Previous studies have focused on seeding with ultrafine, solid particles having the requisite crystal characteristics for either homo- or heteroepitactic nucleation of the desired phase. Size separation of particulate seed crystals is not an efficient process and thus more recent efforts have concentrated on chemical approaches to nucleating solid phase transformations. Hybrid gels, in which two or more gels are combined to capitalize on the benefits of each, have been reported for the homoepitactic nucleation of mullite. In principle, the molecularly-mixed gel crystallizes to mullite at ∼1000°C and, in turn, acts to nucleate the colloidal gel component's transformation to mullite at higher temperatures. However, the transformation sequence and kinetics are profoundly affected by the interfacial reaction between the two gels comprising the hybrid. This paper discusses how the physical distribution and chemistry of the gel components can be manipulated for the control of mullite nucleation, crystallization and microstructure development.

Chemical synthesis of titanate and zirconate based multicomponent electronic ceramics using molecularly modified alkoxide precursors is discussed. Controlled chemical polymerization of the modified alkoxides was conducted to prepare multicomponent gels which on calcination at moderate temperatures (700 – 1000°C) formed high purity, ultrafine powders (e.g. BaTiO3, ZrTiO4, LiTaO3). In the sol-precipitation technique modified precursors were used to prepare ultrafine (< 100 nm) and crystalline powders of such ceramics as BaTiO3 and BaZrO3 at low temperatures (< 100°C), under highly alkaline (pH > 13) conditions.

Lithium silicate gels of composition 15 mole % lithia - 85 mole % silica were prepared by soaking highly porous silica gels in aqueous LiNO3 or methanolic LiOH solutions. The fracture surfaces of these samples were studied by X-ray photoelectron spectroscopy (XPS). High resolution XPS core level spectra were collected such that both chemical shift and atomic concentration could be determined. While the average lithium content was slightly higher in LiNO3 gels, only some of the lithium was associated with the silica network. In contrast, most of the lithium in LiOH gels was associated with the network.

Transparent sols and gels in the ZrO2-SiO2 system containing 25, 50 and 75 mole % ZrO2 were prepared using tetraethyl orthosilicate (TEOS) and Zr n-propoxide solution in the presence of acids. Heating the gels to 600°C yielded an amorphous product which at higher temperature crystallized with formation of tetragonal and monoclinic ZrO2 . Zircon crystallized slowly at 1600°C. The results are explained by metastable liquid-liquid phase separation in the studied system.

Silica and alumina gels were mixed to make a hybrid of a mullite precursor gel and a silicon carbide precursor gel. The crystallization of these above 1400 °C, to form in-situ composites of silicon carbide whiskers in a matrix of mullite, cristobalite and a glassy phase, was studied for different processing conditions.