Aerogels are highly porous solids derived from the hypercritical drying of covalently crosslinked gels. SEM, TEM and scattering studies reveal that these materials have an open-cell morphology with a solid matrix composed of interconnected colloidal-like particles (30–200 A) Processing variables and synthetic conditions largely determine the microstructure and morphology. In this study, the structure-mechanical property relationships of silica, resorcinolformaldehyde, and carbon aerogels are examined.

Positronium lifetimes inside porous silica are measured and discussed in terms of surface interactions, oxygen and nitrogen induced quenching and pore sizes. Surface interactions are studied for silica gels having surfaces covered solely with hydroxyl or methoxy groups. A rapid decay of positronium inside the pores filled with gaseous or liquid oxygen is reported and explained in terms of quenching mechanisms. An increase in the measured lifetimes for pores filled with nitrogen is attributed to the change in the pore distribution function.

Titania-silica gels containing up to 30% titania have been obtained from silicon and titanium alkoxides and from silicon alkoxides and TiCl4. These gels were then processed by normal drying and by hypercritical methods. The thermal evolution of these gel/glass systems, specifically with respect to crystallization behavior has been followed using x-ray diffraction and Raman spectroscopy. From these studies, it was found that not only the different chemistries, but also the different processing methods significantly affect the microstructure of these titania-silica systems.

Lead titanate was prepared by the sol-gel processing of metal alkoxide solutions. The effects of hydrolysis level (i.e., moles H2O/mole PbTiO3) and acid and base additions (HNO3 or NH4OH) on the properties of desiccated gels, and the gel-to-ceramic conversion, were studied. Microstructural, structural, and physical properties were characterized at three stages of the processing cycle: (i) the desiccated gel state; (ii) the amorphous state, following organic pyrolysis; and (iii) the crystalline state. Differences in the physical and structural properties for the desiccated gels, which were induced through manipulation of the hydrolysis conditions, persisted in the amorphous state after organic pyrolysis. Minor differences remained after crystallization. Variations in material properties, with low temperature processing (e.g., the gel-to-glass transformation), were considered from the standpoint of hydrolysis and additive effects on the gel network structure and consolidation behavior. Data are reported for the densification and crystallization behavior for the gels. Through proper control of hydrolysis conditions, relatively dense ceramics were obtained at temperatures as low as 700°C.

Hydrolysis, condensation and thermal reactions of silica gels derived from TEOS with and without 1,3,5-trioxane (C3H6O3) were studied. Effects of trioxane on viscosity and gelation time of solutions were determined. Specific surface, density, porosity and microhardness of gels were measured. The behavior of gels at elevated temperature was clarified using DTA, TGA, and dilatometry. The properties and structure of trioxane containing sol/gels may be explained by the role trioxane played in the sol-gel transition. The results of this investigation concerning sol-gel/gel-glass transitions, structure and properties of gels are presented.

Zirconium silicate gels containing small amounts of foreign cations were found to exhibit different crystallization behavior as compared to undoped gels. The reaction to form the zircon phase was enhanced by the presence of these dopants, with zinc oxide additions causing the largest decrease in crystallization temperature. The crystallization behavior of these gels is compared to that of zircon prepared from alternative processing techniques.

The morphological evolution and SnO2 crystallization of various gel-derived materials, composed of SiO2-SnO2 in the range from 34 to 5 wt % SnO2, are studied in function of the thermal treatment in the interval 200-800°C. SnO2 phase separation occurring above 400°C affects the density, the apparent porosity and mechanical properties like hardness and Young's modulus. At the lowest SnO2% a continuous density increase is observed on heating at variance with high tin dioxide contents for which the apparent density is almost constant. The results are discussed on the basis of SnO2 crystallization, which occurs by two different mechanisms, evidenciated by X-ray analyses. The data will be compared with those of pure SiO2.

Pure SiO2, ZrO2 and SiO2−ZrO2 gels have been prepared by hydrolysis and gelation of tetraethylorthosilicate (TEOS) and zirconium acetate solution. The crystallization and phase transformation of SiO2 and ZrO2 in these gels have been followed up to 1450°C using differential thermal analysis (DTA), thermal gravimetric analysis (TGA) and X-ray diffraction analysis (XRD). The crystallization and phase transformation behavior of SiO2 or ZrO2 is quite different in different gel systems. It was found that in pure SiO2 gel, the cristobalite phase was precipitated out at about 1000°C. In the pure ZrO2 system, tetragonal (t−) phase zirconia was crystallized at about 430°C, t-ZrO2 crystals grew three dimensionally and the activation energy for growth was calculated as about 365 kJmol−1. But in the SiO2-ZrO2 system, the presence of ZrO2 suppressed the crystallization of SiO2 , and SiO2 increased the initial crystallization temperature and the stability of t-ZrO2.

A new method for preparation of porous mixed oxide with SiO2 -ZrO2 -Na2O composition was investigated by heating the sol-gel derived SiO2 microsphere gel in the presence of ZrOCl2 and NaCI. Surface of the -products was composed of sponge-like structure, similar to the porous glasses manufactured by the phase separation method. Their pore volumes ranged from 1.0 to 1.5 cc/g(pore radius 100–lO00Å) and their pore distributions were sharp, with peak tops of about 450–600Å in radius.

Barium and calcium aluminate powders have been elaborated using chemical methods like coprecipitation, alkoxide polymerization and metalloorganic decomposition. These powders have been successfully used for impregnation of porous tungsten cathodes for use in electronic tubes. Spectroscopic studies on the modification of aluminum s-butoxide reveal that carboxylic acid substitutes OR groups forming preferentially bidentate chelating complexes.

SiO2-TiO2 sol-gel solutions and synthesis techniques have been developed to produce thin films of proper index, thickness and quality for use as planar waveguides. The composition of the synthesized films ranged from 50 to 68 mole % TiO2 and produced layers with indices ranging from 1.7 to 1.8. Final film thicknesses ranged from 1000 to 3000 A and varied as a function of the coating chemistry and conditions. Also, changes in film shrinkage and index with heat treatment were determined and compared to literature data on similar films used as antireflective coatings. Finally, confirmation of the waveguiding qualities of these films was made through prism coupler measurements conducted by the University of Arizona Optical Sciences Center.

The shrinkage behavior and changes in refractive index of sol-gel derived SiO2 -TiO2 thin films, with a composition of 50 mole %, were investigated. Two regions of shrinkage rate were observed, a rapid initial stage and a slower second stage. The refractive index of the film was found to be dependent upon the heat treatment. Samples prebaked to 100°C and processed at temperatures below 500°C were found to have lower indices than films processed without prebaking. Films heated to 700°C contained small crystallites of anatase uniformly distributed throughout the film.

Optical interference filters have been synthesized by sol-gel. The selected filter is a multilayer of alternating TiO2 and SiO2 films. In transmission, the filter edge depends on the angle of incident light, which can be tailored through control of thickness and refractive index of the individual films. Theoretical modeling of the filter with film thicknesses obtained by Rutherford backscattering spectrometry is in good agreement with the experimental optical response.

Two techniques for thin film measurement were compared: an optical method combining ellipsometry and reflectance spectroscopy, and cross-sectional transmission electron microscopy. These techniques were used to measure the absolute thicknesses of titania/silica sol-gel films in the size range 0.1 to 0.8 microns. The relative advantages and disadvantages of these methods will be described in this study.

Lead(II) oxides are involved in various advanced materials. Their high volatility as compared to that of other elements makes chemical routes to such materials especially attractive. Various synthetic routes to lead(II) alkoxides have therefore been estimated. They display a strong tendency to undergo spontaneous condensation reactions, giving oxoalkoxides Pb4O(OR)6 (R = tBu, iPr, Et). Reaction between Pb4O(OEt)6 and [Nb(OEt)5l2 does not proceed by simple addition of the alkoxides, but the lead tetranuclear oxoalkoxide is transformed to a hexanuclear one, Pb6O4 (OEt)4 , whose oxo ligands bear Nb(OEt)5 moieties, the overall formula being Pb6O4 (OEt)4 [Nb(OEt)5]4 1. The stoichiometry between the two metals corresponds to that of the PNM ceramic, and 207Pb NMR has been used as a tool to study the reactivity of 1 in the presence of various modifiers and magnesium ethoxide.

Hydrolysis of niobium and tantalum pentaethoxides in ethanol and in the presence of basic (ammonia, tetramethylammonium hydroxide, di and triethylamine) or acidic (nitric or hydrochloric acids) additives was investigated. Triethylamine gives monodispersed, small particles of amorphous M2O5,nH2O (M = Nb, Ta). These colloidal suspensions were used to obtain thin films by spin-coating techniques. The coatings display a thickness of 100–300 nm and a refractive index around 1.7. Laser damage tests at 1064 nm wavelength with a pulse length of 3 ns were carried out on the single layer systems. The threshold values (one-onone) are in favor of the Ta2O5 coatings with an average of 14.5 ± 2.1 J/cm2 , by comparison with 8.3 ± 1.6 J/cm2 for the Nb2O5 films.

Microwave energy has been used in a novel processing method. The relationship between processing, structure and properties of sol-gel-derived potassium niobate sol as well as thin films has been investigated. A comparison has been established between microwave and conventional heating for drying, calcining and crystallizing sol-gel-derived samples. Structural and phase changes were monitored by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and x-ray diffraction (XRD).

Polycerams are an emergent class of hybrid, multifunctional materials which combine the properties of organic and inorganic materials. Films have been prepared from silicon alkoxides and reactive, functionalized polymers such as triethoxysilyl modified polybutadiene (MPBD), (N-triethoxysilylpropyl)O polyethylene oxide urethane (MPEOU) and trimethoxysilylpropyl substituted polyethyleneimine (MPEI). Characterization of dielectric constant and tan δ of the films has been carried out over a range of frequency from 500 Hz to 100 kHz; and the results are used to consider the potential of Polycerams as dielectric materials.

We have explored the effects of various processing parameters on the dielectric and electronic integrity of sol-gel derived silicate thin films and have identified several factors that strongly affect the thin film electronic properties. We find that sol-gel dielectrics can exhibit excellent dielectric integrity: viz., low interface trap densities and fairly good insulating properties approaching those of a thermally grown SiO2 film on Si.

Lead titanate fibres for use in piezoceramic-polymer composites have been prepared by extrusion of polymeric gels. The gels are formed from solutions of lead acetate, titanium isopropoxide and 2-methoxyethanol. The concentration of the solution, and amount of water and acid added, proved to have a strong influence on the suitability of the gel for extrusion purposes. The drying and firing conditions have been altered to obtain dense, fully crystalline PbTiO3 fibres of 100 μm diameter.