Carol S. Ashley

Sol-gel transition in simple silicates II☆

Abstract Silicate gels were prepared under a range of conditions in which the rate of hydrolysis was varied from fast to slow with respect to the rate of condensation. When hydrolysis was fast, larger, more highly condensed polymers were formed during gelation. Conversely, for slow hydrolysis, smaller, less highly condensed polymers were formed. These gels dried to low density coarse textured and high density fine textured gels, respectively. High temperatures, (>800°C) were required to densify the coarse gels by viscous sintering. Lower temperatures were sufficient to densify fine gels by a process which was postulated to consist of polymer relaxation followed by condensation and pore collapse.

Fundamentals of sol-gel dip coating

Abstract During sol-gel thin film formation via dipping, polymeric or particulate inorganic precursors are concentrated on the substrate surface by a complex process involving gravitational draining with concurrent drying and continued condensation reactions. The structure of films deposited from polymeric precursors depends on such factors as size and structure of the precursors, relative rates of condensation and evaporation, capillary pressure, and substrate withdrawal speed. Using polymeric silicate precursors, the porosity and refractive index of the deposited films may be varied as follows: volume percent porosity (0%–56%); pore radius (0–3.1 nm); surface area (1.2–263 m 2 g -1 ); refractive index (1.18–1.45). For repulsive, monosized particulate precursors, higher coating rates promote ordering of the particles as manifested by a reduction in porosity from 36% (random dense packing) to about 25% (f.c.c. or h.c.p.).

Review of sol-gel thin film formation

Abstract Sol-gel thin films are formed by gravitational or centrifugal draining accompanied by vigorous drying. Drying largely establishes the shape of the fluid profile, the timescale of the deposition process, and the magnitude of the forces exerted on the solid phase. The combination of coating theory and experiment should define coating protocols to tailor the deposition process to specific applications.

Sol-gel strategies for controlled porosity inorganic materials

The porosity (i.e., pore volume, pore size, and surface area) of ceramic materials prepared by sol-gel processing depends on the size and structure of primary particles or polymers formed by condensation reactions, the organization of these structures, often by aggregation, to form a gel, and the collapse of the gel by drying. This paper reviews these ideas in the context of the formation of thin films suitable for inorganic membranes and introduces a number of specific strategies designed to control pore sizes in the range appropriate for gas separation: ( 1) aggregation of fractals; (2 ) management of capillary pressure, (3) control of condensation rate, and (4) the use of organic or microporous templates in composite thin film structures. These strategies are contrasted with the more traditional particle packing approach to preparing controlled porosity materials.

Sol-gel transition in simple silicates: III. Structural studies during densification☆

Abstract Raman spectroscopy and DSC were used to determine the structure and average heat of formation of siloxane species formed during the gel-to-glass conversion. Based on structural studies of cyclic model compounds and MO calculations of ring strain energies, we conclude that 3 and 4-fold siloxane rings are responsible for the strong Raman bands at 608 and 490 cm−1, respectively.

Sol-gel thin film formation

Abstract The overlap between the drying stage and the aggregation/gelation and aging stages of sol-gel film formation establishes a brief time for further condensation reactions to occur. For this reason, the structures of films are often considerably more compact than those of the corresponding bulk gels or xerogels prepared from identical precursors. Experimental techniques to study film formation and the structure of the deposited film in situ have been developed. These techniques include imaging ellipsometry, infrared microscopy and gas sorption on surface acoustic wave substrates.

Sol-gel double-layer antireflection coatings for silicon solar cells

Abstract Double-layer thin films of SiO2 and TiO2, applied using the sol-gel process, were utilized as antireflection coatings on silicon solar cells. When coated with these films, the efficiency of a solar cell was increased by 44%, which agrees well with the measured increase in cell solar absorbance of 47%. Modeling of the reflectance properties of the coated cells, using thickness and index of refraction values determined from ellipsometric measurements, was in excellent agreement with the spectral reflectance properties measured from 300 to 1100 nm. Transmission electron microscopy on sectioned samples revealed sharp interfaces between the sol-gel films and thicknesses in good agreement with the ellipsometric values.

Sol-gel processing of controlled pore oxides

Abstract In sol-gel processing the pore structure of the dry gel (xerogel) or film is a consequence or the sequential (or overlapping) gelation, aging, and drying stages. This paper demonstrates how the surface area, pore volume, and pore size are influenced by such physical and chemical factors as the size, structure and composition of the inorganic polymers, the magnitude of the capillary pressure, conditions of aging, and choice of pore fluid.

Porous Optical Composites

Previous studies have shown that sol-gel matrices are excellent low temperature hosts for various optically-active materials, both organic and inorganic. Optical properties of these composites depend upon such factors as the structure of the matrix and size, shape, and degree of dispersion of the optically-active phase. We discuss factors that control the shrinkage and clarity of silicate aerogel host matrices and report on novel composites in which the optical properties are controlled by solid-vapor and/or solid-liquid reactions within the host matrix.

Sol—gel derived ceramic films — fundamentals and applications

Sol-gel processing begins with a colloidal dispersion, or sol, of particles or polymers in a liquid. Through subsequent chemical cross-linking, electrostatic destabilization, evaporation or some combination thereof, the fluid sol may be transformed into a rigid gel, which is a substance containing a continuous solid skeleton enclosing a continuous liquid phase. This sol-to-gel transition allows the solid phase to be shaped into films, fibers, microspheres or monoliths. Of these various forms, amorphous (or partially crystalline) thin films represent the earliest commercial application of sol-gel technology [1]. Thin films (normally less than 1 μm in thickness) use little in the way of raw materials and may be processed without cracking, overcoming the major disadvantage of sol-gel processing of bulk materials. Early applications of sol-gel coatings as optical films were reviewed by Schroeder [2]. Since then, many new uses of sol-gel films have appeared in electronic, protective, membrane and sensor applications [3–15]. Most often the as-deposited films are amorphous, but depending on composition and thermal history, they may subsequently crystallize: ferroelectric PLZT (lead lanthanum zirconate titanate) and nonlinear optic LiNbO3 are excellent examples of crystalline films derived from amorphous precursors [16–18].