Yttrium methoxyethoxide, barium methoxyethoxide and various copper(II) alkoxide groups were used as precursors for the formation of YBa2Cu3O7-δ stable sols in a 2- methoxyethanol / methyl ethyl ketone / toluene / diisopropyl ketone solvent system. Sol fractal dimensions were varied with the sol concentration and with the addition of pyridine. Strong correlation was found between the wettability and the fractal dimensions of the various precursors as determined by contact angle measurements. YBa2Cu3O7-δ thin films were deposited on sapphire substrates using a dip-coating technique. The films were fired at 850°C under flowing oxygen or an ozone/oxygen mixture. Although the ozonetreatments helped in the formation of single YBa2Cu3O7-δ phase, they contributed to extensive cracking in the films. The different copper(II) alkoxide precursors had little effect on final film microstructure.

Alkoxide sol-gel processing of Y1Ba2Cu3O7-x was performed using copper(II) ethoxide, barium methoxyethoxide, and yttrium methoxyethoxide precursors in a methoxyethanol/methy ethyl ketone/toluene solvent system. In situ neutron diffraction experiments were performed on individual precursors and mixed component gels during heat treatment. Phase development and consumption was observed for both the calcination and annealing segments of the heat treatment. Formation of Y1Ba2Cu3O7-x was observed as early as 700°C. CuO, Cu2O, BaCuO, YCu2O5, BaCO3, BaCO4, and YlBa2Cu3O7-x were observed during the conversion/ consolidation reactions to 850°C.

For the synthesis of composites on a nano or molecular level, chemical or physical methods have to be used since mechanical procedures cannot be employed for these systems. If organics have to be included, low temperature processing has to be used for the synthesis of the inorganic component in order not to damage the organic component. Sol-gel techniques are an appropriate means to synthesize oxidic networks by soft chemistry, and various systems of organically modified ceramics (ORMOCERs) have been developed so far. It has been shown that the synthesis parameter can be used to tailor the material properties in a wide range. A review over the chemistry and chemical properties of ORMOCERs and several examples of material developments will be given.

The first representatives of a new family of microporous, aryl-bridged polysilsesquioxanes have been prepared by sol-gel processing of bis-1,4-(triethoxysilyl)benzene la, bis-4,4′- (triethoxysilyl)biphenyl 2a, bis-4,4′-(triethoxysilyl)terphenyl 3a, and bis-9,10-triethoxysilyl anthracene 4a. The bis(trichlorosilyl) analogs of la and 2a (lb and 2b, respectively) were also examined. The materials produced by hydrolysis and condensation of the monomers provide an opportunity to fully condense to a network with rigid-rod organic spacers interspaced at regular intervals in the silicate-like framework. The xerogels produced upon subsesquent processing of the gels have extremely high surface areas (256–1100 m2/g; BET) with porosities confined to the micropore domain (< 200 nm). Solid state (CP MAS) 13C and 29Si NMR were used to evaluate the extent of hydrolysis and degree of condensation in the xerogels. The porosity and thermal stability of the aryl bridged polysilsesquioxanes may lead to applications as chromatographic absorbents. The transparent materials may also have optical applications arising from both the gels′ high refractive indices and the covalent incorporation of ultraviolet chromophores.

A new technique which permits the successful preparation of ORMOSILS (Organically Modified Silicates) with rubbery elasticity has been developed. Common alkoxides were reacted with polydimethyl siloxane (PDMS) and the microstructures were carefully controlled. Large monoliths of the porous rubbery ORMOSILS have been prepared. Gelation process was studied by 29Si liquid NMR spectroscopy. A structural model for rubber elasticity of ORMOSILS was proposed.

Non-linear organic dyes, such as 2-methy1–4-nitroaniline (MNA) and p-nitroaniline (pNA), have been incorporated in Polyceram monoliths and thick films prepared by the hydrolysis and co-condensation of tetraethoxysilane and trimethoxysilylpropyl substituted polyethyleneimine. Strong second harmonic generation was observed.

Enhanced integration, faster signal transmission and reduced size of mounting devices in components for microelectronics requires new patternable materials. Inorganic-organic copolymers (ORMOCERs = ORganically MOdified CERamics), prepared by sol-gel techniques have been developed for interconnection technologies in microelectronics. Photopolymerization is enabled by unsaturated hydrocarbon or epoxide substituents and UV-sensitive initiators. Using a frequency doubled Argonion laser at 257 nm for direct laser writing, patterned layers with high edge quality have been realized. In combination with high breakthrough voltages, low permittivity constants and high bulk resistivities they open interesting aspects for very large system integration techniques (VLSI).

We report on the rapid thermal processing (RTP) of YBa2Cu3O7 (Y-123) fibers. Unsintered fibers are densified by RTP. We show that fibers which were originally semiconducting and tetragonal before RTP form normal twinned orthorhombic material after processing for 2–4 seconds above 1000°C with a 3 min. cool down in oxygen. They subsequently show Tc to 90K and magnetization indicative of substantially shielding. We present the effects of varying the RTP parameters on the morphology, phase, and superconducting properties of a number of tetragonal and orthorhombic Y-123 fibers.

A method was developed for preparing silica gels by silicon alkoxide hydrolysis using water-swellable polymers to control the shape. A variety of polymer beads with different water absorbencies and different bead sizes were used. Silica beads in a wide range of sizes were prepared from polymer beads. The final bead size was dependent on the size and water absorbency of the beads, on the extent of reaction, and on the final density. It was shown that porosity could be formed in two regions. The silica beads contained pores of 3–4 nm that were attributed to the intrinsic gel structure. In addition, when large amounts of polymer were removed, additional porosity on the scale of 20–50 nm was formed. It was also shown that surface areas varied dramatically with calcining. For beads containing only small pores, surface areas varied from 400 m2/g to less than 0.2 m2/g as the calcining temperature was increased from 600°C to 1000°C. Beads containing porosity formed by removal of the polymer remained porous even after calcining at 1000°C.

After a brief introduction on how the differing properties of H2O and NH3 may effect the strategies used to synthesize metal nitrides, an overview of our use of azides to produce aluminum nitride thin films will be presented. The effect of changing the nitrogen source to one which contains at least one N - C bond is to increase dramatically the carbon content of the films. Replacing the alkyl groups attached to the aluminum with hydride ligands removes the final carbon source and forms what appears to be a promising new class of precursors. This is demonstrated by the study of the reaction of Me3NGaH3 with NH3 to produce the novel trimer, [H2GaNH2]3 . This fully characterized molecule converts at 150°C into gallium nitride. Surprisingly, it yields GaN having the sphalerite structure instead of the known wurtzite phase. A discussion of the reasons for this unusual route to a new crystalline phase of GaN is included.

Poly(borazinylamines) have been processed in THF and toluene solvents and aerogel forms produced by supercritical drying methods. The polymer aerogels have been pyrolyzed and porous amorphous and crystalline BN monoliths have been produced. These have been characterized and effects of processing factors on surface area and pore structure are described.

Polytitanocarbosilane precursor for SiC/TiC ceramics was characterized by various spectroscopic techniques (29Si and 13C CP MAS NMR, Ti K-edge XANES and EXAFS) in order to follow how titanium ions are incorporated into the polycarbosilane-based polymer. This polymer has been fired in argon atmosphere, and the evolution of the local environments of Si and Ti atoms, during the pyrolysis process, have been followed using 29Si MAS-NMR and Ti K-edge X-ray absorption: this study reveals the formation of Si-O bonds above 500°C, while Ti-C bonds clearly appear around 800°C. Above 1000°C, Si-O bonds are consumed by reaction with excess carbon and the system finally crystallizes into SiC/TiC mixture above 1200°C.

A class of borazene polymers has been developed which consist of a twodimensional array of six-membered borazene rings with the borons of adjacent borazene rings separated by -NH- groups. Pyrolysis of these polymers above ≈1000 C leads to crystalline graphite-like boron nitride (h-BN). The thermal chemistry of thin films of one polymer deposited on KOH etched Al has been examined by thermal decomposition mass spectroscopy (TDMS) and thermal gravimetric analysis (TGA), and the gas evolution chemistry is found to be essentially complete by temperatures of less than 400 C. All products desorb with the same temperature profile and the major desorbing species are NH3 and N2 consistent with a loss of excess nitrogen and hydrogen in the polymer, ang HCI from decomposition of by-products of the synthesis step. Isotope labeling shows that complete exchange occurs below 400 C between the ring and amino bridge nitrogens. Since the formation of ordered crystalline h-BN films requires heating to temperatures of the order of 1000 C, whereas the gas evolution and ring opening chemistry is complete by roughly 400 C, it is concluded that gas evolution chemical Processes are not rate limiting in BN ceramic production.

Alkoxide-derived silica coatings were deposited on fused silica by dip coating. The samples were covered with a metal film to absorb the infrared radiation from an Nd:YAG laser. Coupling the laser power to the coating produced localized surface heating on the samples. By scanning the sample across the beam's path, channels in the coating were formed. The channels varied from 150 μm to 600 μm wide and from 200Å to 1000Å deep depending on the laser power and the translation speed. The size and shape of the channels also depended on the composition of the sol-gel coating. Optical microscopy showed that there were no cracks in either the substrate or in the coating on the micron scale. SEM revealed cracks on the nanometer scale in the laser fired coatings and no cracks in the unfired coatings. Ellipsometry showed that the index of refraction increased with increasing depth of the channels.

Electrochemistry can be used for the atomic-level architecture of ceramic materials. In this report, ceramic superlattices based on the TlaPbbOc/TldPbeOf system were electrodeposited with individual layer thicknesses as thin as 3nm. The superlattices were deposited from a single aqueous solution at room temperature, and the layer thicknesses were galvanostatically controlled. Substitution of Tl2O3 into PbO2 appears to stabilize a face-centered cubic structure with an average lattice parameter of 0.536nm. The lattice parameters for the T1aPbbOc. mixed oxides vary by less than 0.3% when the Pb/Tl ratio is varied from 0.84 to 7.3. Because the modulation wavelengths are of electron mean free path dimensions, this new class of degenerate semiconductor metal-oxide superlattices may show thickness-dependent quantum optical, electronic, or optoelectronic effects.

A new application area for sol-gel processing biomaterials and implant devices, is presented. The concepts of bioactive fixation of implants is reviewed. Conventional processing methods for bioactive materials are compared with low temperature chemical processing. Advantages of sol-gel processing include new bioactive compositional ranges of ultraporous CaO-P2O5-SiO2 biogel-glass powders. Clinical applications of bioactive implants are discussed.

Nuclear Magnetic Resonance (NMR) investigations on the Oak Ridge National Laboratory process for sol-gel synthesis of microspherical nuclear fuel (UO2), has been extremely useful in sorting out the chemical mechanism in the sol-gel steps. 13C, 15N, and 1H NMR studies on the HMTA gelation agent (hexamethylenetetramine, C6H12N4) has revealed near quantitative stability of this adamantane-like compound in the sol-gel process, contrary to its historical role as an ammonia source for gelation from the worldwide technical literature. 17O NMR of uranyl (UO2++) hydrolysis fragments produced in colloidal sols has revealed the selective formation of a uranyl trimer, [(UO2)3 (μ3-O)(μ2-OH)3]+, induced by basic hydrolysis with the HMTA gelation agent. Spectroscopic results will be presented to illustrate that trimer condensation occurs during sol-gel processing leading to layered polyanionic hydrous uranium oxides in which HMTAH+ is occluded as an ‘intercalation’ cation. Subsequent sol-gel processing of microspheres by ammonia washing results in in-situ ion exchange and formation of a layered hydrous ammonium uranate with a proposed structural formula of (NH4)2 [(UO2)8 O4 (OH)10]- 8H2)O. This compound is the precursor to sintered UO2 ceramic fuel.

Hydrolysis and gelation processes in uranyl solutions are observed using the strong sharp uranyl oxygen-17 resonance.

The mechanical properties of silica gels (alcogels and aerogels) are measured by three point bending. The elastic moduli (E,G) and the fracture strength (σ) are investigated as functions of the concentration of silicon precursor, the catalysing conditions, and the aging time.

Elastic moduli and strength follow scaling laws as a function of the bulk density of the solid. The scaling exponents are discussed in terms of percolation theory.

The evolution of the mechanical properties is also followed during the steps of the gel to glass transformation:

- hypercritical drying

- oxidation of the organic groups

-densification heat treatment

At each step of the process the increases of the elastic moduli and σ are related to the gel structure. During the hypercritical drying flexible “dead ends” in the network can condense and form new links. The connectivity is also enhanced by the removal of the organic groups creating siloxane bonds. Finally densification leads to a material having mechanical properties identical to those of conventional vitreous silica.