Sol-gel derived molecular composites exhibiting intense luminescence, induced from efficient energy transduction processes, have been prepared. The composites are comprised of an Eu3+⊂ 2.2.1 cryptate complex or native Eu3+ ion embedded in sol-gel derived titania glass films. The titania glasses contain interconnected porous networks that permit the diffusion of exogenous substrates, such as the salts of benzoic and 4-tert-butylbenzoic acids, through the film. Interaction of the substrate with the embedded lanthanide complex is indicated by enhanced luminescence from the lanthanide ion. The carboxylic acid salts whose electronic excited states are produced upon capture of incident photons, undergo facile transfer of their electronic energy to the lanthanide ion. By monitoring europium ion luminescence, the diffusion constants of the benzoate and 4-tert-butylbenzoate salts have been measured. Although the diffusion of the 4-tert-butylbenzoate is slower than that of benzoate, the overall higher sensitivity of the former is consistent with hydrophobic guest-host interactions. These new molecular composites relying on the immobilization of an absorption-energytransfer- emission molecular assembly in porous, optically transparent ceramic glasses may be useful in the design of practical sensing devices.

The newly emerging field of Nonlinear Optics and Photonics offers tremendous opportunities for material scientists. In this paper, chemical processing using the sol-gel method is described for the preparation of new composite materials of both a silica glass and a V2O5 gel with a wπ conjugated polymer poly-p-phenylene vinylene up to 5d% by weight. The composite films show highly improved optical quality with large third-order nonlinear optical coefficients, the latter from the conjugated polymer. Optical waveguiding through the film has been achieved. Nonlinear optical studies using femtosecond degenerate four wave mixing and power dependent waveguide coupling have been successfully performed. Also, to investigate the use of such films for optical recording, a two-dimensional grating structure has successfully been produced.

The sol-gel process has been used to incorporate a conjugated polymer, polyaniline, in silica gel. The emeraldine base form of the polymer is partially solubility in aprotic polar solvents which are compatible with the sol. The effect of sol environment on the emeraldine base form of polyaniline was investigated. The soluble emeraldine base form is maintained at pH ≥ 2.4 enabling good optical quality gels to be synthesized. The type of alkoxysilane and the water ratio used in preparing the sol influence the solubility of the emeraldine base.

Vanadium dioxide thin films have been grown from vanadium tetrakis (t-butoxide) by the sol-gel process. A new method for the synthesis of the vanadium precursor was also developed. Films were deposited by dipcoating glass slides from an isopropanol solution, followed by post-deposition annealing of the films at 600°C under nitrogen. The properties of these films, to a high degree, were a function of crystalline boundaries and crystalline grain size. These gel-derived VO2 films undergo a reversible semiconductor-to-metal phase transition near 72°C, exhibiting characteristic resistive and spectral switching comparable with near stoichiometric VO2 films prepared on non-crystalline substrates by other techniques. Films were doped with hexavalent transition metal oxides to demonstrate lowering of the transition of the transition temperature.

Cryogenic transmission electron Microscopy (cryo-TEM) and rheological characterization were conducted in order to understand structural development of vanadium pentoxide gels during processing. Sols were prepared by ion exchange from sodium metavanadate solutions. Cryo-TEM revealed that fine threads about 1.5nm wide initially form and grow into ribbons approximately 25nm wide and at least 1000nm long. The threads appear to self assemble into the ribbons. During this structural development, the dynamic viscosity increased. Upon steady shearing of the sols, the system exhibited thixotropy, i.e. the viscosity decreased with time under constant shear stress and subsequently rheopexy, the viscosity increased with time. Comparison of the structure before and after shearing indicated that during the rheological experiments aggregation of small particles or fragments was occurring.

Thin films (1–10μ thick) can be spin coated on a variety of substrates from formulations containing polyphenylsiloxanes and alkylsilicates. These films may be cured either thermally or photochemically, yielding films with good adherence to the substrate. The combination of photolysis and thermolysis results in changes in the film hardness characteristics. The use of a mask during photocuring results in films which may be developed in either a positive or negative fashion. The preparation of various formulations along with film curing processes will be discussed.

Sol-gel techniques are readily applicable to the preparation of Polycerams. These hybrid materials uniquely combine organic and inorganic functionalities at the molecular level. Polycerams have here been prepared from tetraethoxysilane and modified, functionalized polybutadiene, polyethylene oxide urethane and polyethyleneimine. The structures and physical characteristics of the materials are here in reported.

Oxidative coupling of primary alkylsilanes catalyzed by Group IVB metallocene complexes leads predominantly to linear polysilanes, as first reported by Harrod. We have investigated the polymerization of ethylsilane and vinylsilane using dimethyltitanocene in order to determine the suitability of such polymers as precursors to Si-C based ceramics for application as coatings or composite matrices. A photochemical procedure for initiation of polymerization is described. Ethylsilane forms polysilanes (which contain a -Si-Si-Si-Si- backbone) by a step growth mechanism. Vinylsilane shows some Si-Si formation; however, polymerization by several different routes, including formation of polycarbosilanes (which contain a -Si-C-SI-C- backbone) by hydrosilation reactions and crosslinking via metathesis, predominate. The carbosilane polymer has high char yield (60-75%) and appears advantageous for conversion into silicon carbide, as determined by X-ray diffraction.

A base catalyzed silica aerogel and three base catalyzed resorcinol-formaldehyde (RF) aerogels were imaged with a new vertical platinum-carbon (Pt-C) replication technique. In deep gels single molecular chains can be visualized with transmission electron microscopy (TEM) with 9–10 Å thick Pt-C replicas backed with 110–130 Å of carbon [2]. The three RF aerogels produced with [Resorcinol]/[Catalyst] ratios (R/C) of 50, 200, and 300 had densities of 0.103, 0.063, and 0.065 gm/cc and had surface areas of 905, 580, and 390 m2/gm, respectively. The gel connectors and junctions in RF 300 and RF 200 were composed of 210±30 Å and 140±10 Å beads which were joined by polymeric chains covering an area less than half the bead's diameter and within a bead diameter's distance. The gel connectors and junctions in RF 50 were frequently composed of beads of about 120±10 Å joined over an area larger than half the bead's diameter. Similar to the silica aerogel (0.065 gm/cc, 113±10 Å beads), the beads were frequently connected over several bead diameter's distance from each other.

The aqueous, sol-gel polymerization of melamine with formaldehyde, followed by supercritical extraction, leads to the formation of a new type of organic aerogel. Synthetic conditions (e.g. reaction time, pH) affect the density, transparency, and microstructure of the resultant aerogels. Unlike previous organic aerogels based upon resorcinol-formaldehyde, the melamine-formaldehyde aerogels are both colorless and transparent. Low densities (0.1–0.8 g/cc), high surface areas (∼1000 m2/g), and optical clarity are only a few of the promising characteristics of this new material.

Polysilazanes are inorganic polymers which convert to a ceramic material when pyrolyzed. Initial pyrolysis results in a solid, amorphous material. Further heating transforms the amorphous structure to a crystalline material. In addition to the Si-N linkages in the polymer backbone, polysilazanes generally contain appreciable amounts of carbon in side groups. Consequently, pyrolysis can result in mixed Si3N4 - SiC crystalline materials. It will be shown that either Si3N4 or SiC could be selectively crystallized depending upon processing conditions.

Metal-organic films were produced at ambient temperatures and pressures by the controlled hydrolysis of carboxylic acid and titanium isopropoxide mixtures. Pre-resonance Raman spectroscopy was used to study the effect of the organic acid chain length upon the nature of the resulting film. Propionic, butyric, valeric, hexanoic, and octanoic acids were studied, resulting in a proposed correlation between film quality and certain spectroscopic features of the alkoxide carboxylates. The choice of the carboxylic acid and the presence of an appropriate amount of water were shown to be critical in the development of a good film.

When BN is synthesized via polymeric precursors and applied to ceramic substrates, tough adherent coatings of hexagonal-BN (h-BN) are obtained after annealing at 1200°C in N2. The study of these coatings is facilitated by using nonporous oxide powders containing single crystal particles of submicron size (e.g. cubes of MgO) as model ceramic substrates. These oxide powders permit high resolution TEM examination of the BN coatings with no further sample preparation. In this study, samples of BN/MgO cubes containing 50 wt% BN were heated in air at elevated temperatures for 16 hours to study the oxidation resistance of BN coatings. The BN coating was found to be stable at 600°C, but the 700°C-treated sample showed evidence for partial amorphization of the coating and reaction with MgO. A significant fraction of the MgO in the 800°C-treated sample had transformed to Mg2B2O5. The reaction of MgO with the BN coating under oxidizing ambients leads to loss of the cubic morphology in the precursor powder.

Polymer precursors for Si-C, Si-Ti-C-O and Si-Al-C-O systems have been obtained from polycarbosilane and the corresponding metal alkoxides. X-ray Photoelectron Spectroscopy (XPS) has been used to follow the structural evolution of these preceramic compounds during the pyrolysis process.

Several polymetallocarbosilanes, pre-ceramics precursors for Si-M-C-O systems, have been prepared from polycarbosilane and metallic alkoxides, M(OR)n with M = Ti, Zr and Al. Polymers have essentially been characterized by Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). The pyrolysis process has been followed for each system with X-Ray Diffraction (XRD) and MAS-NMR. The role of the metallic element M on the transformation process of these systems will be discussed.

A three-dimensional transient model for heat conduction in silica glass is developed. The model simulates a three-dimensional temperature distribution in a silica glass irradiated by a moving CO2 laser. Both the reflectivity of the glass surface and the strong attenuation of the laser energy in the glass medium are accounted for by a detailed radiation analysis. The energy absorbed by the glass is determined to be confined in a 10 μm thickness; the laser irradiation is thus treated as a boundary condition. The heat diffusion equation is solved by an alternating direction-implicit method.

The use of several soluble metal polychalcogenides to prepare solid state materials is presented. Thermal gravimetric analysis (TGA) data, under inert atmosphere, of the molecular compounds (Ph4P)2[Cd(Se4)2], (Ph4P)4[Cu2Se14], (Ph4P)2[Cu4Se12], (Ph4P)4[In2Se21] and (Et4N)3[M3Se15], (M=In, TI), show that the corresponding binary solid state compounds are formed as single phases at temperatures as low as 530°C. We have grown films of CdSe, Cu2-xSe, β-In2Se3, TISe and CuInSe2 using these complexes as precursors. The films were prepared by pyrolysis of green precursor films, cast from DMF solutions. CuInSe2 was prepared by co-thermolysis of Cu/Sex and In/Sex complexes. The chemical, X-ray, spectroscopic and electron microscopic characterization of these molecular precursor-derived films as well as their charge transport characterization is reported.

Yttria-stabilized zirconia powders were prepared by three chemical routes utilizing emulsion hydrolysis techniques. Metal alkoxides, acetates and mixtures of alkoxides and acetates were used as starting materials. Under controlled hydrolysis conditions, particles with different morphology and size were obtained. Spherical or granular particles in the range of 20–60u in diameter were formed. The grain size varied from 0.1–0.2u. The powders were applied successfully as plasma spray coatings. The chemical uniformity was demonstrated by the presence of 100% non-transformable tetragonal zirconia in the powder and in the coating.

Zirconia and yttria-stabilized zirconia (YSZ) powders were prepared from water/oil emulsions followed by high temperature calcination. The morphology and particle size were correlated with several variables such as composition of emulsions, concentration of starting materials and surfactants, stirring rate, hydrolysis rate and calcination temperature. The powders were characterized according to crystalline phase, particle size distribution, and flow rate. Emulsion precipitation typically yielded spherical zirconia particles which were either solid or hollow. Particle size could be controlled from submicron to a few hundred microns depending primarily on the composition of the emulsion and stirring rate during hydrolysis. SEM/EDX results showed that yttria was uniformly distributed throughout the zirconia particles. The crystalline phase of the powders after calcination at 800°C was entirely non-transformable tetragonal. The flow rate increased with increasing particle size and with increasing sphericity.

A novel use of sol-gel derived porous Type VI silica for high performance rocket guidance system windows is evaluated. The samples produced for this study were optically transparent hydrofluoric and/or nitric acid catalyzed tetramethylorthosilicate (TMOS) xerogel monoliths with average pore radii of 1.2, 5.0, and 8.Onm. Maximum He transpiration velocities of up to 3 cm/sec, 3 times the velocity needed for transpiration cooling, are measured for a 5.0 nm sample at 3.2 MPa. Transpiration velocities of 0.6 cm/sec result in cooling effects as large as 44°C from 160°C.