C.J. Brinker

Hydrolysis and condensation of silicates: Effects on structure

The hydrolysis and condensation reactions of monomeric alkoxysilanes and organylalkoxysilanes utilized in sol-gel processing are reviewed. Both reactions occur by acid or base-catalyzed bimolecular displacement reactions. The acid-catalyzed mechanisms are preceded by protonation of OH or OR substituents attached to Si, whereas under basic conditions hydroxyl or silanolate anions attack Si directly. Many of the observed structural trends are understood on the basis of the pH and [H2O] dependence of the hydrolysis, condensation, and dissolution reactions.

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.

Sol → gel → glass: I. Gelation and gel structure☆

Abstract The mechanisms of gel formation in silicate systems derived from metal alkoxides were reviewed. There is compelling experimental evidence proving, that under many conditions employed in silica gel preparation, the resulting polysilicate species formed prior to gelation is not a dense colloidal particle of anhydrous silica but instead a solvated polymeric chain or cluster. The skeletal gel phase which results during desiccation is, therefore, expected to be less highly crosslinked than the corresponding melted glass, and perhaps to contain additional excess free volume. It is proposed that, during gel densification, the desiccated gel will change to become more highly crosslinked while reducing its surface area and free volume. Thus, it is necessary to consider both the macroscopic physical structure and the local chemical structure of gels in order to explain the gel to glass conversion.

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.

Organic “template” approach to molecular sieving silica membranes

Abstract We demonstrate a “template” approach to prepare microporous inorganic membranes exhibiting high flux combined with high selectivity, overcoming limitations inherent to both conventional inorganic (sol-gel, CVD) and organic membrane approaches. Hybrid organic-inorganic polymers prepared by co-polymerization of tetraethoxysilane (TEOS) and methyltrie-thoxysilane (MTES) were deposited on commercial asymmetric alumina supports. Heat treatments were employed to densify the inorganic matrix and pyrolyze the methyl ligands, creating a continous network of micropores. Resulting membranes exhibited very high CO2 permeance values (2.57×10−3 cm3/cm2 s cm Hg) combined with moderate CO 2 CH 4 selectivities (12.2). Subsequent derivatization of the pore surfaces with monomeric TEOS significantly increased CO 2 CH 4 separation factors (71.5) with only a modest reduction in CO2 permeance (2.04×10−4 cm3/cm2 s cm Hg). Combined CO 2 CH 4 selectivity factors and CO2 fluxes exceeded those of all known organic membranes.

NMR confirmation of strained “defects” in amorphous silica☆

Abstract Solid state 29 Si magic angle sample spinning nuclear magnetic resonance spectroscopy and Raman spectroscopy were used to investigate the local silicon environment and siloxane ring vibrations in amorphous silica gels. Our results unambiguously relate the 608 cm −1 Raman “defect” in a-SiO 2 with reduced SiOSi bond angles indicative of strained 3-membered rings of silicate tetrahedra.

Sol-gel derived antireflective coatings for silicon☆

Abstract TiO 2 -SiO 2 films containing from 30 to 95 mol% TiO 2 were prepared by a sol-gel process. These films were applied to polished silicon and heat treated at temperatures less than 450°C to convert the applied films to amorphous oxide films ranging in refractive index from approximately 1.63 to 2.17. The films were evaluated for antireflectance as a function of conversion temperature. It was found that the sol-gel films applied at 400°C showed broad regions of antireflectance compared to other titanium-based films.

Sol → gel → glass: II. Physical and structural evolution during constant heating rate experiments

Abstract Porous, multicomponent gels were converted to dense glasses at temperatures less than 700°C and at heating rates ranging from 0.5 to 40°C/min. The results of shrinkage, weight loss and differential scanning calorimetry experiments were used to elucidate mechanisms responsible for gel densification. We propose that: (1) capillary contraction, (2) condensation polymerization, (3) structural relaxation, and (4) viscous sintering are the principal gel densification mechanisms. Condensation-polymerization and structural relaxation result in skeletal densification, the magnitude of which closely accounts for all the observed shrinkage between 150 and 525°C. Viscous sintering is the predominant shrinkage mechanism above 525°C. Due to the complex interdependency of the densification mechanisms, the kinetics of gel densification depend strongly on thermal history and, therefore, general constant heating rate analyses are inappropriate for deriving meaningful kinetic information regarding the gel → glass conversion.