SiCI4 was tested in both the liquid and vapor state for its suitability as a monolithic gel precursor. Samples with molar ratios of H2O/SiCl4 ranging from 4 to 400 were investigated. Several advantages of using SiCl4 over the classic organometallics such as tetraethylorthosilicate and tetramethylorthosilicate were observed. Gelation of the SiCl4-H2O sol is more rapid. Also, greater purity is achieved with SiCl4 vapor as the starting material. In addition to the SiCl4 experiments, GeCl4 was also investigated as both a dopant and in the pure state for its gelation characteristics. Higher dopant concentrations were achieved using this technique than are possible with other monolithic gel methods, but pure GeO2 gels were not formed. The gel made by the vapor phase method has been named vapogel.

This paper describes important process considerations in the preparation of super-critically evacuated alcogels called aerogels. Aerogels are fine grained, open pore, low density materials that possess a variety of unusual properties and have a number of diverse applications. Factors influencing the microstructure and uniformity of alcogel monoliths are discussed. The effects of hydrolysis rate, diffusion, condensation, dispersion medium, and electrostatic interactions on particle formation and gelling are outlined. These effects are illustrated with the preparation of zirconia, mullite precursors, and silica alcogels. A theoretical model of particle interaction based on the combination of van der Waals and electrostatic double layer forces is used to interpret gelation and homogeneity. Solvent substitution and supercritical drying processes are discussed.

The classical sol-gel process for synthesizing SiO2 aerogels involves the hydrolysis/condensation of tetraethyoxysilane (TEOS) and/or teramethyoxysilane (TMOS) to produce a gel which can then be super-critically extracted to a low density, highly porous aerogel glass. Controlled hydrolysis of TEOS and TMOS leads to partially hydrolyzed compounds that can be subsequently water processed to form silica aerogels in the density range from .020 to .500 gms/cc. The partially hydrolyzed compounds are stable when sealed from moist air and can be stored for future use.

We discuss the controlled conditions used to obtain these compounds and present data that characterize their structure. We detail the procedures for preparing the wide range of aerogel densities. We also report on their use as an adhesive.

Small-angle X-ray scattering is used to characterize the structure of aerogels prepared by two-stage polymerization processes. Second-stage catalysis controls the resistance to collapse during drying with the base-catalyzed system being most resistant. Base catalysis in the second stage leads to compaction of the polymer network on short length scales. This short-scale rigidity makes the networks sufficiently robust to withstand the surface tension forces present during solvent extraction and re-exposure to the atmosphere. Aging in solution also improves aerogel quality. In this case, a dissolution-repolymerization process leads to short length scale circuits and improved rigidity.

The Young's modulus of silica aerogels is measured using the three points flexural technique. Various parameters have been investigated such as the catalyting conditions used to elaborate the initial alcogel and the heat treatment performed to density the material. In a given sample porosity the elastic behaviour of aerogels depends on the conditions of gel preparation. This different elastic behaviour is related to the different spatial arrangement of the particles pointed out by fractal features.

Two optical methods are described for mapping the local variations of refractive index within monoliths of porous silica aerogel. One is an interferometrie measurement that produces “iso-index” fringes in a two dimensional image; an orthogonal view gives the third dimension information. The other method uses the deflection of a He-Ne laser beam to map the gradient index within a sample. The quantification of the measurements is described and the accuracy of the results is discussed.

The feasibility of using supercritical fluids to impregnate carbon/carbon composites with a ceramic precursor was successfully demonstrated. Improvements in mechanical properties and oxidation resistance were correlated with the distribution of silicon within the microstructure of the composites. In addition, supercritical fluids were also used to fractionate silicon carbide precursors and thereby control the physical properties and char yield.

Dip coating is a simple old way of depositing onto a substrate, especially small slabs and cylinders, a uniform thin film of liquid for solidification into a coating. The basic flow is steady, and in it film thickness is set by the competition among viscous force, capillary (surface tension) force and gravity. Thickness and uniformity can be sensitive to flow conditions in the liquid bath and gas overhead. The faster the substrate is withdrawn, the thicker the film deposited. This can be countered by using volatile solutes and combining rapid enough drying with the basic liquid flow. Then the physics grows more complicated, theoretical prediction of process performance more difficult, and control of the process more demanding. Outside product R&D labs it is far less often used in precision coating manufacture than a variety of premetered coating methods.

Spin coating is a more recently developed way of getting onto piecemeal substrates, especially small flat disks, a uniform thin liquid film for the same end. The basic flow is unsteady radial drainage in which centrifugal and viscous forces so compete that ordinary (Newtonian) liquid of constant viscosity tends toward a uniform film that grows ever thinner ever more slowly. Volatile solvents are commonly used because conditions can often be found that adequately separate thinning by spin-off from later thinning and solidification by drying. Thickness and uniformity, today theoretically predictable, are sensitive to speed, gas conditions, and rheology of concentrating, solidifying liquid. For the rheology of photoresist coating in microelectronics, spin coating works well. For that of suspension coatings in magnetic disk technology the process demands more careful control; actually it is often modified.

Although controlled dip-coating is an established way to apply high quality uniform coatings, the details of the coating and drying process have not been deeply studied. Depending on the physical and chemical state of the sol and the parameters of dipping (dipping angle and speed), a variety of thicknesses and porosities can be achieved [1]. For optical coatings, the refractive index and optical thickness can thereby be controlled.

We have developed a method to view the drying front of a dip-coated film using broad beam ellipsometry [2], or “ellipsometric imaging.” In dip-coated films we take full advantage of the fact that a steady state is quickly reached where the drying line velocity matches the withdrawal velocity; however, the technique might also be used in unsteady situations such as spin coating. Imaging ellipsometry makes it possible to measure the refractive index and thickness profiles of both wet and dry films point-by-point in an entire image at once. These profiles provide important clues as to the relative importance of gravity, evaporation and other phenomena.

Film formation by sol-gel techniques is an advantageous method to obtain inorganic coatings. The sol-gel process allows the preparation of prepolymers with suitable viscosities to use various coating techniques like dipping, spraying and spin-on coating. The main disadvantage is the shrinkage of the gel layer during drying or sintering which leads to cracks with layer thicknesses more than some micrometers.

The introduction of organics, causing a decrease of the network connectivity and increasing relaxation ability can help to overcome densification problems. Moreover, in some cases, hardness or abrasion resistance are not too much affected by organics. In these cases, the organics can be left within the inorganic network. The possibilities of inorganic-organic processing techniques will be discussed and examples for film formations will be given.

The objective of this Small-Bus i ness Innovative Research (SBIR) effort is to expand on work reported previously [1] and further develop a new class of microcomposite materials which combine the outstanding properties of ordered polymers and the excellent compressive strength of glass. In Phase I we demonstrated adverse property changes in poly(p-phenylene benzobisthiazole) (PBT) ordered polymer films by infiltrating sol-gel glass reagents. During Phase II we have addressed the following areas: sol-gel infiltration process; effect of sol-gel glass composition; tensile strength characterization, microcomposite UV resistance, lamination process, compressive strength determination. The following subsections present the work conducted to date.

Small crystals of zeolites (500–1000 nm) with two- and three-dimensional channel systems (faujasite and ZSM-5 structures) were embedded in amorphous thin films derived from TEOS hydrolyzed in alcoholic solution. Scanning electron microscopy studies show that the zeolites can be quite evenly dispersed in the membrane, resulting in single layers of zeolite crystals protruding out of the amorphous matrix. In situ FT-IR studies with a series of probe molecules revealed that in most membranes the zeolites were 100% accessible from the gas phase. The membranes excluded molecules which are larger than the pore openings of the zeolite embedded in the composite.

Polymeric sols of SiO2 or siloxane polymers in n-propanol were investigated for use as dielectric and planarization layers on patterned silicon wafers for device application. Spin coating of the sol (SOG) resulted in rapid gelation through solvent evaporation but cracking often occurred on heating. This study examined the influence of SOG composition, deposition process and heating rate on film planarization and cracking. Planarization was found to be enhanced by layer thickness, but cracking stability was reduced. Sequential drying of the applied SOG layers and use of trlethano-lamine as a complexing base was found to significantly reduce cracking tendency in the films.

Sol-gel processing was used for the fabrication of LiNbO3 thin-layers on semiconductors. Two different alkoxide systems were investigated (i.e., ethoxide vs. methoxyethoxide) to determine the role of solution chemistry on the evolution of structure in LiNbO3 layers. An alcohol exchange reaction, which produced lithium niobium methoxyethoxide, proved advantageous with respect to the amount of hydrolysis water content which could be added to give stable solutions. Spin-casting was used for the deposition of thin-layers which crystallized at 500 C on Si. The refractive index of sol-gel derived LiNbO3 was determined to be similar to sputtered or epitaxially grown films. Data are reported for the processing route and the material characteristics. Methoxyethanol based systems appeared to be more forgiving than ethanol based solutions for the fabrication of high-quality thin-layer devices.

Semiconducting V2O5 gels containing up to 30 mol% of TiO2 were prepared through simultaneous hydrolysis of alkoxides in ethanol solution. Coatings of these gels about 5 nm to 20 μm,in thickness were made by dip coating. The coatings were yellowish green and transparent. The fiber-like structure of the gels was observed by TEM. The gels were about 10 nm or less in diameter, and the diameter decreased with increasing TiO2 content. The refractive indices, n, and the thickness, d, of the coatings were determined by ellipsometry. d was approximately proportional to the square root of the pulling rate. When d was less than 10 nm, the refractive indices parallel to the pulling direction were lower than those perpendicular to the pulling direction. The dc conductivity parallel to the surface of the coatings, σ//, of V2O5 gel coating was higher than that of crystalline V2O5. The dc conductivity perpendicular to the surface of the coatings, σ/, was about 3 to 4 orders of magnitude lower than σ//. With increasing Tio2 content, both σ// and σ/ decreased.

Multicomponents silicon based ceramic thin films were prepared by ion beam irradiation of organosilicon polymer films. Our approach involved dissolving organosilicon polymers in toluene and using spinning technique to deposit the polymer films. Four organosilicon polymers were used to prepare polymer films with various thickness (2000 Å to 5000 Å). These polymer films were then irradiated with high energy ion beams, 6.0 MeV F beam, 2.0 MeV He beam, 1.0 MeV Au beam, and 50 KeV N beam. Amorphous, crack-free, hard ceramic films of up to 4000 Å may be produced in this manner. The presence and concentrations of each elememt were monitored with proton recoil detection and Auger analysis. The in-situ loss of gases was monitored by quadrupole mass spectrometry. Cross-sectional TEM was used to determine the overall homogeneity and thickness of these films. These ceramic films provide significant protection of silicon substrates from high temperature surface oxidation and chemical corrosion.

The preparation of eutectics in the system BaO-Al2O3-SiO2 by the conventional technique is carried out by fusions with intermittent crushing and grinding. Hydrolysis and polycondensa-tion of alkoxides in a suitable solvent provide a simple way to obtain the homogeneous baria-alumina-silica glasses. The physical and chemical changes which occur during the gel-to-glass conversion were investigated on gel powders by thermal gravimetric analyses, differential thermal analyses, mass spectroscopy, nitrogen sorption analyses, and dilatometry.

By means of the sol-gel route it is also possible to prepare coating solutions in the Y-Ba-Cu-system. Coatings of composition YBa2Cu3O7−x were deposited on substrates using dip coating and characterized by scanning electron microscopy and measurements of superconducting temperature.

ZrO2 and Ba2YCU3O7−x metal oxide films were prepared from carboxylate precursors. The zirconia films were prepared from Zr octoate and the Ba-Y-Cu-0 compound synthesized from a mixture of the metal neodeca-noates. Dense, adherent ZrO2 films were obtained on Si. Dense, superconducting Ba-Y-Cu-0 films were also fabricated utilizing the octoate-derived ZrO2 as a bonding surface and diffusion buffer

We have investigated the electronic properties of sol-gel derived films on silicon substrates. Our investigation involves SiO2, aluminosilicate and borosili cate oxides on silicon. Seme sol-gel oxides are excellait insulators; seme sol-gel films on silicon also exhibit quite low oxide/silicon interface trap densities. We have also subjected sol-gel films on silicon to 4 Mrad(SiO2) of radiation and have found that these structures appear to be radiation hard (very little radiation induced oxide space charge or interface trap generation). Our results strongly suggest that sol-gel processing could provide insulating films for a variety of microelectronic device applications.

The preparation and characterization of organic-modified and organic/ iorganic-modified silica sol-gel spin-coated thin films is presented as a function of thermal processing. The ability to alter the film's final refractive index, thermal expansion coefficient and the polar contribution to the film's surface energetic term is reported. The stability of the coating solution at elevated temperatures was studied by analyzing changes in molecular weight, viscosity and particle size during the sol-to-gel transition. Control of the sol-gel thin film properties allows the design of a film/substrate interface where both thermal mismatches and surface energetic differences are minimized. Decreased stresses should be induced during processing, resulting in increased adhesion between the coating and the substrate.