Flow behavior of a liquid or suspension depends on how stress varies with strain rate, strainrate rotation, and strain history, as well as the progress of evaporation, extent of reaction, and degree of aggregation in sol-gel systems. Rheological methods suitable for measuring flow behavior are summarized. Examples of measurements and microstructural observations by transmission electron microscopy made during gelation of four sol-gel systems are presented. The relation of rheological response to microstructure is discussed.

The gel fiber formation was studied for the spinnable silica sols prepared from a mixture of TEOS, H2O, EtOH and HNO3 ( 1 : 2 : 2 : 0.01, in mole ratio). The effect of polysiloxane molecular size in the spinning sol on spinnability indicated that the tough skeleton of gel, which is preferable for fabrication of high-strength fiber without breakage, was formed by the selective connection of large size polymers (about 106 in M.W.) during the spinning. It was also found that the smaller size polymers in gel fiber continued to react with water in the atmosphere after the spinning, and attached to the skeleton to strengthen the fiber.

The physical aspects of sol-gel film formation are discussed, including the steady state film profile during dip coating, evaporation, and capillary phenomena. It is argued that, since the evaporation rate increases singularly near a sharp boundary (analogous to an electric field singularity near a sharp conductor), the film profile near the drying line falls off precipitously, following the inverse form of the evaporation singularity. Finally, the large tensile pressures in the solvent during the final stage of drying of a porous film are discussed from the point of view of controlling the degree of capillary collapse.

We have investigated the use of porous oxide coatings, formed using sol-gel chemistry routes, as the discriminating elements of acoustic wave (AW) chemical sensors. These coatings provide several unique advantages: durability, high adsorption capacity based on large surface areas, and chemical selectivity based on both controlled pore size and acid/base, ion exchange or chelation chemistry. The porosity of these coatings is determined by performing nitrogen adsorption isotherms using the AW device response to mass changes to monitor the uptake of nitrogen at 77 K. These studies demonstrate how sol-gel chemistry and film deposition can be combined to tailor the microstructure of thin oxide coatings. The chemical sensitivity and selectivity obtained with this class of coatings will be demonstrated using several examples: hydrous titanate ion exchange coatings, zeolite/silicate microcomposite coatings, and surface-modified silicate films.

New glass-composites with ion exchange properties have been developed. Ammonium 12-molybdophosphate (AMP) (NH4)3PMo12O40, and ammonium 12-tungstophosphate (AWP) (NH4)3PW12O40, known for their ion exchange capabilities, are included either in preformed aerogels with defined pore size, or are added to sol-gel mixtures during the process of gel formation. Characterization is carried out by FTIR, Raman and EXAFS spectroscopy. Ion exchange capacities for the oxometalate precursors are determined for silver and rubidium and are compared to those of the glass composites. Glass composites show high ion exchange capacity, but some portion of the metalate complexes leaches from the glass during the procedure. This is in contrast to thin composite films, which have almost no porosity and do not show loss of metalate. EXAFS spectroscopy demostrates that the oxometalate microstructure is maintained in glass composites and that rubidium ions after ion exchange in glasses occupy similar cation positions as in the precursor compounds.

The structure of sol-gel derived silica coatings was investigated using nanoindentation. chemical etching. and cross-sectional TEM. For coatings heated at 100°C. the surfaces were harder, etched slower, and appeared less porous than the interior of the coatings. For samples fired at higher temperatures (400–800°C). the surfaces remained harder and etched slower than the interior. even though the porosity appeared to be evenly distributed throughout the thickness of the coating. It is proposed that this difference is due to inhomogeneities on two scales in the coatings: bulk porosity and skeletal strain.

Monolithic gels of V2O5, Ta2O5 , and Nb2O5 have been prepared from VO(C2H5O)3 , Ta(C2H5O)5 , and Nb(C2H5 O)5 througg hydrolysis in ethanol Solutions. The conditions for gel formation have been determined. Thin coatings of these gels, less than 100 nm in thickness, have been prepared by dip coating. Changes in the thickness, d, and refractive indices, n, have been measured by ellipsometry during drying. In the first stage of drying, d decreases and n increases rapidly. Densification of the coatings takes place due to evaporation of solvent. In the following stage, increase in d and decrease in n have been observed for thin V2O5 gel coatings, about 30 nm or less in thickness. These results suggest that coarsening of pores and/or increase in surface roughness take place due to release of the capillary pressure during drying. Both d and n have increased with increasing number of dipping. After heat treating, decrease in d and increase in n have been observed.

This paper discusses the possible advantages to be gained from a bio-mimetic approach to ceramics and reviews the work now in progress that will lead us to be able to mimic biological structures with synthetic materials.

Biomimetic design and processing of laminated B4C-Al cermets, based on knowledge gained from the microstructure-property characterization of abalone shells, is described. In the nacre section of the shell, the microstructure is highly organized as CaCO3 (aragonite) crystals, with a thickness of 0.25 μm, separated by a layer of organic matter 300–500 Å thick. This organization forms a miniature “brick and mortar” microstructure. The resultant strength and fracture toughness of the nacre, i.e., 180 MPa and 7 MPa-m1/2 , are many orders of magnitude higher than those of monolithic CaCO3. The processing of laminated B4C-Al cermets, based on the microstructure of the nacre, was performed by a combination of tape casting of the ceramic and infiltration of the metal. The resultant cermets displayed a 40% increase in both fracture toughness and strength over monolithic B4C-Al cermets.

Nanometer sized magnetic particles have been fabricated by aqueous phase coprecipitation inside single compartment vesicles. These vesicles were generated by sonicating egg yolk phosphatidylcholine molecules in an appropriate ionic solution containing the reactant cations. Using cobalt and ferric nitrate as the starting solution, the reaction product was the desired cobalt ferrite; direct formation of the oxide is potentially a very important step. For the barium and ferric nitrate systems, the reaction product could not be identified easily. However, we have not been able to produce barium ferrite directly. In an attempt to compare the effect of drastic changes in the microenvironment, we have also completed aqueous phase precipitation in multilamellar vesicles. For the aluminum nitrate system, precipitation takes place only in the outermost “annular” space. Multiple particles are formed, and the resultant diffraction pattern shows a close match with an aluminum hydroxide chloride complex. This differs significantly from the product in single compartment vesicles, where β-Al2O3 was formed, and in free precipitation which resulted in Al(OH)3.

Wet chemical methods have been used with success to synthesize films for a variety of electrical applications. After first reviewing the important microstructural features of films, the present paper focuses attention on ferroelectric (FE) films. The present state of and critical issues facing wet chemically derived FE films are considered. The needs for more extensive microstructural characterization and for more systematic exploration of electrical behavior are emphasized.

PZT films on fused silica substrates were prepared by a multiple dipping-firing process from a stable solution of titanium isopropoxide, zirconium n-propoxide and lead 2- ethylhexanoate. The crystallization kinetics of the amorphous PZT films was studied by quantitative X-ray analysis. The crystallization of the amorphous film to perovskite phase involved a transitional pyrochlore structure phase. The entire crystallization was described by a consecutive phase transformation process in which nucleation dominated.

Structural rearrangements during the sol-gel processing of lithium niobate were investigated by FTlR and Raman spectroscopic methods. The reaction of lithium ethoxide with niobium ethoxide resulted in the formation of a bimetallic alkoxide, LiNb(OEt)6 , which could be crystallized from solution. Single crystals were comprised of helical polymeric units consisting of niobium octahedra linked by lithium in tetrahedral (distorted) coordination. Successive crystallizations from solution allowed for the enhanced purification of the alkoxide precursor. Hydrolysis of the bimetallic alkoxide resulted in the formation of an amorphous network structure, which contained niobium-oxygen octahedral units modified by lithium. Heat-treatment facilitated structural rearrangements for the niobium environment, which allowed for the formation of the lithium niobate crystal structure. Further heat-treatment above 700°C resulted in structural changes associated with lithium oxide volatility.

Single-phase KNbO3 was prepared using bimetallic alkoxides. Potassium-niobium ethoxide, KNb(OC2H5)6, and potassium-niobium propoxide, KNb(OC3H7)6, were synthesized and subsequently hydrolyzed using several water concentrations. Potassium-deficient particles were rapidly precipitated when higher water concentrations were used and this resulted in the formation of a multiphase material after calcination. In contrast, single-phase KNbO3 powders could be prepared by two methods: (1) hydrolysis of KNb(OC3H7)6/propanol solutions using 1 mole water (per mole of propoxide) added as a water/propanol solution and (2) hydrolysis of KNb(OC2H5)6,/ethanol solutions using 1 mole of water (per mole of ethoxide) added as a water/methanol solution. The latter method was also used to form thin films of KNbO3.

Intercalation of layered solids is used as a means to manipulate a variety of molecular and polymeric species into well-ordered multilayer films with an architecture controllable at the molecular level. Dielectric, conductivity and optical measurements demonstrate the potential of developing new families of materials with new functionalities by exploiting the synergistic effect of guest/host interactions.

The hydrolysis of copper 2–(2-ethoxyethoxy)ethoxide solutions and barium-yttrium copper 2–(2-ethoxyethoxy)ethoxide solutions were investigated. Copper sols with particle sizesof 3–10 nm have been prepared from THF solutions. Sols from barium-yttrium-copper alkoxides were also prepared in THF with particle sizes of 4–10 nm as determined by transmission electron microscopy (TEM) and 50–80 nm as determined by photon correlation. Compositional analysis showed that the sols had a similar composition to the parent solution. Choice of atmosphere and solvent system played an important role in the progress of the hydrolysis reaction. Powders obtained from barium-yttrium-copper solutions can be converted to Ba2YCu3O7−δ.

Silicon and phosphorus ions were implanted into Spin-On-Glass (Sol-Gel) SiO2 films and were found to modify the material properties. Two SOG types, polysiloxane andsilicate, were ion-implanted with doses in the 5 × 1014 − 1.65 × 1016cm−2range and energies of 40keV - 190keV. The implanted SOG on silicon samples were characterized by ellipsometry, infra-red spectroscopy, etching and capacitance measurement of aluminum/SOG/silicon devices. The results indicate that the polysiloxane type SOG shrinks due to the implant and its effective refractive index increases. The ion-implant of the polysiloxane SOG also changed its structure and composition as seen by the variations of the infra-red transmission spectrum, etching characteristics, and dielectric constant. Silicate SOG exhibits less shrinkage due to the implant but its other characteristics show dependence on the dose similar to that of the polysiloxane SOG.

An organoaluminum precursor was synthesized by a low temperature reaction and characterized by NMR, FTIR and mass spectrometry. This compound was pyrolyzed under several experimental conditions to obtain aluminum nitride powders. The pyrolyzed products were subsequently characterized by XRD and SEM. The mechanisms of precursor development and subsequent pyrolysis are discussed.

A gradient-index glass rod of about 13 mm in diameter and 20 mm in length with the refractive index difference of about 0.05 between center and perimeter has been prepared by the solgel process from a precursor solution consisted of tetramethoxysilane, tetraethoxysilane, boron ethoxide and aqueous solution of lead acetate. A bubble free wet gel of about 35 mm in diameter and 50 mm in length was obtained by adding acetic acid to the precursor solution as a buffering agent. The liquid in the micropores of the wet gel was totally replaced with acetone. Then the compositional gradient of lead was formed in radial direction of the gel by soaking in an ethanolic solution of potassium acetate. A transparent r-GRIN rod of about 10 mm in diameter was obtained by the sintering of the gel at 580°C.

Optical quality radial gradient-index (GRIN) glass can be made by first leaching a TiO2-Al2O3-SiO2 gel in aqueous sulfuric acid solution, then drying and firing the gel. In this paper, we present details of a study on the reproducible production of GRIN glasses by this sol-gel process. Physical and optical data on the intermediate gels and resulting gradient-index glass will be discussed.